Protein phosphorylation in the organ of Corti: differential regulation by second messengers between base and apex

Histopathologic changes in the cochlea associated with diabetes mellitus--a review

BACKGROUND

The pathologic changes that occur as a result of diabetic microangiopathy have been well described for the kidneys and the eyes. Although many studies suggest an association between diabetes mellitus and hearing loss, the pathologic changes in the cochlea in association with the diabetic state remain to be clarified.

AIM/OBJECTIVE

The aim of this review is to determine the effects of diabetes mellitus on cochlear morphology.

METHOD

A comprehensive search for relevant articles was carried out on electronic databases of Ovid Medline, Ovid Medline in Process, PubMed, Ovid Embase,or Biosis Preview, The Cochrane Library, ISI Web of Science, and Scopus. Articles published in English between 1940 and June 2010 were eligible to be reviewed. Using predefined inclusion criteria, published articles on histologic changes occurring in the cochlea due to diabetes mellitus were selected and reviewed, and their findings were synthesized.

RESULTS

Changes were observed in the basement membrane of the capillaries of the stria vascularis and in the basilar membrane, which was remarkably thickened, giving rise to diabetic microangiopathy. Loss of spiral ganglion neurons, organ of Corti cells, and atrophic changes in the stria vascularis were varied and infrequent.

CONCLUSION

There seems to be variable vulnerability of different cochlear cell types to the DM state. Further studies are required to determine the factors responsible for the differences in the histopathologic observations of cochlear tissues.

Proteomic analysis of cisplatin-induced cochlear damage: Methods and early changes in protein expression

To identify early changes in protein expression associated with cisplatin ototoxicity, we used two dimensional-difference gel electrophoresis (2D-DIGE) and matrix-assisted laser desorption-time-of-flight (MALDI-TOF) mass spectrometry to analyze proteins from P3 rat cochleae that were cultured for 3h with or without 1mM cisplatin. Replicate analysis of fluorescent images from six gels revealed significant (p<0.01) cisplatin-induced changes (greater than 1.5-fold) in expression of 22 cochlear proteins. These include increases in the expression of five proteins, four of which were identified as nucleobindin 1, a nuclear calcium signaling and homeostasis protein (2.1-fold), heterogeneous nuclear ribonucleoprotein C, an RNA processing protein (1.8-fold), a 55 kDa protein that is either endothelial differentiation-related factor 1 or alpha-6 tubulin (1.7-fold), and calreticulin, a calcium binding chaperone of the endoplasmic reticulum (ER, 1.6-fold). The expression of 17 proteins was significantly (p<0.01) decreased by greater than 1.5-fold. These include ribonuclease/angiogenin inhibitor 1 (1.6-fold), RAS-like, family 12 (predicted), ras association (RalGDS/AF-6) domain family 5 (4.5-fold), homologous the RAS family of GTPase signaling proteins (2.4-fold), and Protein tyrosine phosphatase domain containing 1 (predicted, 6.1-fold). We identified seven cochlear proteins with either smaller (1.2-1.5-fold) or less significant (p<0.05) cisplatin-induced changes in expression. Notably, heat shock 70 kDa protein 5 (Hspa5, Grp78, and BiP), an ER chaperone protein involved in stress response, decreased 1.7-fold. We observed changes consistent with phosphorylation in the level of isoforms of another ER stress-induced protein, glucose-regulated protein Grp58. Changes in cisplatin-induced protein expression are discussed with respect to known or hypothesized functions of the identified proteins.

A PI3K pathway mediates hair cell survival and opposes gentamicin toxicity in neonatal rat organ of Corti

Gentamicin is well known to promote hair cell death in inner ear, but it also appears to activate opposing pathways that promote hair cell survival. In combination with others, our previous work has indicated that a K-Ras/Rac/JNK pathway is important for hair cell death and an H-Ras/Raf/MEK/Erk pathway is involved in promoting hair cell survival (Battaglia et al., Neuroscience 122(4):1025-1035, 2003). However, these data also suggested that a Ras-independent survival pathway for activation of MEK might be stimulated by gentamicin. To investigate alternatives to the Ras/Raf/MEK/Erk pathway in promoting hair cell survival, cochlear explants were exposed to gentamicin combined with several inhibitors of alternative pathways (LY294002, calphostin C, SH-6, U73122). When exposed to gentamicin with the PI3K inhibitor LY294002 (10, 50 microM), the protein kinase C (PKC) inhibitor calphostin C (50, 100 nM) or the PKB/Akt inhibitor SH-6 (5, 10 microM), hair cell damage was significantly increased compared to gentamicin alone. By Western blotting, strong PKB/Akt activation was observed in the organ of Corti following exposure to 50 microM gentamicin for 6 h. In addition, PKC activation by 12-O-tetradecanoylphorbol-13-acetate protected outer hair cells from gentamicin induced cell death. In contrast, the phospholipase C-gamma (PLCgamma) inhibitor U73122 (2, 5 microM) did not affect hair cell damage when combined with gentamicin. Also, phosphorylation of PLCgamma was not increased in the organ of Corti following gentamicin treatment, as evaluated by Western blot. The results indicate that PI3K promotes hair cell survival via its downstream targets, PKC and PKB/Akt. This suggests that both Ras-dependent and Ras-independent survival pathways are involved during gentamicin exposure. In contrast, PLCgamma activation of PKC does not appear to play a role.

A BAD link to mitochondrial cell death in the cochlea of mice with noise-induced hearing loss

Acoustic overstimulation induces calcium overload and activation of mitochondria-mediated cell death pathways in outer hair cells (OHC) of the cochlea. However, it is not known whether these events are interrelated or independent. We have recently reported that the calcium-dependent phosphatase calcineurin is activated in OHC following noise exposure and now postulate that calcium overload triggers mitochondria-mediated death pathways through activation of Bcl-2-associated death promoter (BAD) by calcineurin. CBA/J mice were exposed to broadband noise (2-20 kHz), causing a permanent threshold shift of about 40 dB at 12 and 20 kHz, corresponding to damage in the middle and basal turns of the cochlea. Loss of OHC in the basal region was evident in surface preparations. BAD immunostaining in control animals had a cytoplasmic distribution in the cells of the organ of Corti. Five hours after acoustic overstimulation, mitochondria and BAD redistributed to the perinuclear region of OHC in the basal and middle turns but not in the apical turn. The nonapoptotic phospho-BAD (Ser 112) was up-regulated in cells undamaged by noise (supporting cells and inner hair cells) but not in OHC. These data establish a connection between calcium overload and mitochondria-mediated death pathways in OHC and also suggest a dual role for BAD. The translocation of BAD to the mitochondria in degenerating cells is indicative of the activation of its proapoptotic capacity, whereas up-regulation of phospho-BAD is consistent with a nonapoptotic role of BAD in less vulnerable cells.

Phosphorylation mediates the influence of acetylcholine upon outer hair cell electromotility

Isolated guinea-pig outer hair cells (OHCs) (n = 52) were inserted into a partitioning microchamber and electromotility was measured by a calibrated optoelectronic apparatus. Acetylcholine (ACh), and ACh together with different protein kinase inhibitors, were applied to OHCs through a puffer pipette. ACh produced a magnitude increase of electromotility. This magnitude increase was inhibited by co-application of KN-62, a calcium/calmodulin-dependent protein kinase II (CAMKII) inhibitor. Simultaneous application of ACh and H-89, a selective protein kinase A (PKA) inhibitor, did not antagonize the ACh response. Further support for the CAMKII-mediated ACh influence on electromotility is that the magnitude increase is also inhibited by the calmodulin antagonist trifluoperazine (TFP) and by the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) inhibitor thapsigargin. The results suggest an essential role of calcium in the ACh-mediated increase of the magnitude of electromotility. Elevation of the intracellular calcium concentration apparently activates CAMKII which, in turn, phosphorylates membrane or cytoskeletal substrate(s). This molecular modification probably leads to reduced axial cell stiffness and subsequent increase of the electromotile response.

Cyclic GMP and outer hair cell electromotility

The aim of this study is to examine the effect of phosphorylation pathways on the electrically evoked fast motile response of isolated outer hair cells (OHCs). Transcellular electrical stimulation was applied in the microchamber to guinea pig OHCs and motility was measured before and after drug application. Forskolin (adenylate cyclase activator), phorbol 12-myristate 13-acetate (PMA, protein kinase C activator) and dibutyryl 3',5'-cyclic guanosine monophosphate (cGMP agonist) were studied. As controls, L15 medium and dimethyl-sulfoxide (DMSO) were used. In each group, 12 cells were measured. Forskolin and PMA were dissolved in 0.1% DMSO to render them membrane permeable. DMSO by itself caused a statistically significant electromotility magnitude decrease. Forskolin and PMA could not reverse the motility decrease due to DMSO, the effects seen in their presence were the same as observed with DMSO alone. Thus, neither 3',5'-cyclic AMP-dependent protein kinase nor calcium/phospholipid-dependent protein kinase appear to have modulatory effects on electromotility. Dibutyryl cGMP (DBcGMP), in concentrations of 200 microM, elicited a significant electromotility magnitude increase. The DBcGMP effect could be inhibited by co-application of 200 microM DBcGMP and 100 microM 8-Rp-pCPT-cGMPS (8-4-chlorophenylthio-guanosine 3',5'-cyclic monophosphothioate, Rp isomer, a cGMP antagonist). Our results suggest that OHC electromotility is modulated by a cGMP-dependent pathway.

Expression pattern of adenylyl cyclase isoforms in the inner ear of the rat by RT-PCR and immunochemical localization of calcineurin in the organ of Corti

Most studies concerning adenylyl cyclases in the inner ear were carried out before the advent of molecular biology. In a PCR approach using cDNAs of six inner ear tissues (stria vascularis, endolymphatic sac, organ of Corti, vestibulum, cochlear and vestibular nerve) we found tissue specific expression of adenylyl cyclase isoforms. Adenylyl cyclases types 2 and 4 are predominant in the fluid controlling tissues, i.e. in the stria vascularis and endolymphatic sac. In the organ of Corti and vestibulum the Ca2+-modulated isoforms types 1, 6 and 9 were expressed. The regulation of adenylyl cyclase 9, which is the major isoform expressed in the organ of Corti, proceeds via the Ca2+-activated protein phosphatase 2B (calcineurin, PPP3). PCR with specific primers for calcineurin demonstrated its abundant expression in the organ of Corti. Using a monoclonal antibody we localized calcineurin immunochemically to the cochlear nerve, the nerve fibers and the inner hair cells. In the cochlear and vestibular nerves a characteristic neuronal expression pattern of adenylyl cyclase isoforms was observed, i.e. adenylyl cyclases types 2, 3 and 8. The functional consequences of the adenylyl cyclase expression pattern in the inner ear are discussed in conjunction with its unique sensory performance.

Cyclic GMP-dependent protein kinase-I in the guinea pig cochlea

Recent studies have begun to characterize the nitric oxide/cyclic GMP/protein kinase G pathway in the mammalian cochlea by demonstrating the presence of both the enzyme that produces nitric oxide (NO), nitric oxide synthase, and the NO receptor, soluble guanylate cyclase. The present study investigated protein kinase G (cyclic GMP-dependent protein kinase-I, cGK-I), the downstream enzyme of this pathway that frequently mediates its physiological effects. A commercial antibody to a human cGK-I sequence recognized a protein of appropriate molecular weight in Western blots of guinea pig aorta. Immunostaining of guinea pig aorta was consistent with the expected distribution of cGK-I. In lateral wall tissues of the cochlea, pericytes lining the blood vessels of the spiral ligament were strongly immunoreactive. In the organ of Corti, cGK-I was detected in Hensen's, Deiters', and pillar cells, but not in inner and outer hair cells. This distribution coincides with the localization of soluble guanylate cyclase activity and suggests that cGK-I mediates the effects of the NO/cyclic GMP pathway in the cochlea. It reinforces the hypothesis that the NO/cyclic GMP/cGK-I pathway is involved in regulation of cochlear blood flow and supporting cell physiology.

Acetylcholine-induced phosphorylation in isolated outer hair cells

Two groups of isolated, surviving outer hair cells (OHCs) of guinea pig cochleas (n = 20, for each group) were treated with 10 microM acetylcholine or acetylcholine plus strichnine (an alpha9 nAChR antagonist), respectively, under short-term tissue culture conditions. The protein content of the cell homogenates was separated by SDS-polyacrylamide gel electrophoresis, Western blotted and labelled with an antibody against phosphoserine residues. Signals were detected using the ECL system. Acetylcholine challenge of the OHCs resulted in a difference in the pattern of phosphorylated proteins from those of strichnine pretreated cells. A 220 kDa and a 120 kDa protein expressed a more intense phosphorylated state in the ACh group compared with the ACh plus strichnine group. The 220 kDa phosphoprotein is in the range of the cytoskeletal protein beta-fodrin, whereas the 120 kDa fraction is similar to alpha-fodrin or an ankyrin isoform. Phosphorylation of proteins due to activation of the AChR by agonist can play a role in the signalling mechanism between receptor activation and increase in the electromotile capability of isolated OHCs.

OCP2 exists as a dimer in the organ of Corti

OCP2 is one of the most abundant proteins in the organ of Corti (OC), comprising approximately 5% of the total protein in the supporting cell population. Although the very close homolog, Skp1p, has been implicated in regulating cell-cycle progression, the function of OCP2 in the terminally differentiated cochlea is presently unknown. We have purified recombinant OCP2 from Escherichia coli and examined the protein by analytical ultracentrifugation. Interestingly, sedimentation equilibrium data collected at 20 degrees C unequivocally indicate that, at the concentrations present in the OC, free OCP2 would exist as a dimeric species. The apparent sedimentation coefficient is independent of concentration at loading concentrations between 10 and 100 microM, indicating the absence of a significant monomer-dimer equilibrium in this concentration range. The functional significance of this finding is discussed.

Development of cochlear sensitivity to aminoglycoside antibiotics

This study examined the temporal relationship between aminoglycoside ototoxicity and the onset of auditory function in the rat. A single dose of gentamicin sulfate (200 mg/kg) and furosemide (100 mg/kg) was administered on postnatal day 6 (P6), P7, P8, P9, or P10, just before the onset of auditory function. Ototoxicity was assessed by the elevation of auditory brain stem response (ABR) thresholds, recorded once the rats had matured. The ABRs were evoked by acoustic clicks and tone pips. The thresholds of control and P6- and P7-treated animals did not differ significantly from each other. Thresholds of some P8- and all P9-treated animals were elevated. The P10-treated animals were deafened, according to these ABR criteria. These data suggest that the potential for aminoglycoside ototoxicity develops rapidly between P8 and P10, just before the onset of auditory function.

Inhibition of calcium-dependent motility of cochlear outer hair cells by the protein kinase inhibitor, ML-9

The calcium ionophore ionomycin has been shown to induce length increases of guinea pig outer hair cells (Dulon et al., 1990). We have demonstrated that these length increases can be inhibited by a 30 min preincubation of the cells with the protein kinase inhibitor ML-9. At either 30 or 60 s after ionomycin application, the effect of ML-9 was dose-dependent with a half maximal response at approximately 0.3 microM. No effect on cell length was detected after 30 min incubation with 0.5 and 5 microM ML-9 alone. However, with 50 and 500 microM ML-9, significant contraction in cell length was observed. 50 microM ML-9 did not interfere with the ability of ionomycin to elevate fluorescence of the calcium indicator Fluo-3, nor did it alter the ability of cells to exclude propidium iodide from their nuclei. Treatment with 500 microM ML-9 resulted in impaired cell morphology. The data support the hypothesis that protein kinase activity regulates calcium-dependent processes that affect shape changes of outer hair cells. They are consistent with the involvement of the calcium/calmodulin-dependent enzyme, myosin light chain kinase, a known target of ML-9, but do not preclude the possibility of another intracellular target for ML-9.

Expression of adenylyl cyclase type I in cochlear inner hair cells

Expression of calcium/calmodulin-activated adenylyl cyclase type I (ACI) mRNA has been determined in the cochlea and in an organ-of-Corti subdissected tissue fraction by reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. Amplification products of predicted size were obtained from the mouse cochlea and rat organ of Corti with nucleotide sequences corresponding to respective ACI brain transcripts. In addition, ACI template was detected in a rat inner hair cell cDNA library by PCR. Immunoreactivity to ACI has been localized within the organ of Corti to the inner hair cell, with diaminobenzidine staining found in both the cell body and in the stereocilia. Evidence, thus, has been obtained that both ACI transcript and protein are expressed in the inner hair cell, the primary mechanosensory receptor cell of the cochlea. We hypothesize that ACI is activated by calcium influx through a calcium/calmodulin interaction and that this adenytyl cyclase isoform may have a role in modulation of receptoneural afferent transmission and/or mechanosensory transduction in the cochlea.

Specific proteins of the organ of Corti

The mammalian organ of Corti has achieved a degree of perfection unequaled in other hair cell systems. Although cellular metabolism requires the coordinated action of thousands of proteins, the physical processes underlying auditory transduction in the OC are undoubtedly mediated by a much smaller subset of these. OCP1, OCP2, and CBP-15-identified by 2D-PAGE-are apparently members of this elite class. OCP1 and OCP2 are restricted to the supporting cells of the organ of Corti and adjacent epithelia. Their distribution closely parallels the boundaries of the epithelial gap junction system, implying a role in cochlear potassium and pH homeostasis. CBP-15 was recently shown to be identical to oncomodulin, the mammalian beta-parvalbumin, heretofore documented only in the placenta and neoplasms. Expression of this small calcium-binding protein in the OC is restricted to the outer hair cells, where it may function as a calcium-dependent regulatory protein.

Protein kinase C may be involved in synaptic repair of auditory neuron dendrites after AMPA injury in the cochlea

A suitable model of sudden deafness occurring after acoustic trauma or ischemia, is obtained in guinea pigs by an acute intracochlear perfusion of 200 microM alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), a glutamate analog. By overloading the AMPA/kainate receptors, located post-synaptically to inner hair cells (IHCs), it induces a massive swelling of primary auditory neuron dendrites, which disconnects the IHCs. This synaptic uncoupling and the resulting hearing loss are followed by a progressive regrowth of dendrites, which make new synapses with IHCs, leading to a functional recovery of auditory responses that is completed after 5 days. Knowing the role of protein kinase C in neuroplastic events, we studied the expression of its isoforms alpha,beta(I,II) and gamma, respectively pre- and post-synaptic, in auditory neurons at various times after AMPA administration. In untreated cochleas, we observed an expression of PKC alpha,beta(I,II) and gamma in cell bodies of primary auditory neurons. After the intracochlear administration of AMPA, both isozymes were transiently overexpressed, with a peak at 3-6 h, followed by a decrease after about 24 h. At this point in time immuno-electron microscopy revealed some regrowing dendrites immunoreactive for PKCgamma. Five days after AMPA, when the auditory responses were restored, PKCgamma levels were still elevated in ganglion cell bodies.

Effect of acetylcholine and GABA on the transfer function of electromotility in isolated outer hair cells

Outer hair cells (OHC) from high- and low-frequency regions were separately isolated from guinea pig cochleas. The cells were inserted with their ciliary pole first into a partitioning microchamber so that only 20-50% of the cell length was excluded. Somatic length changes due to transcellular electrical stimulation were measured at the cuticular plate in the inserted portion of the cells. Transfer curves of electromotility of the OHCs were obtained by both a series of brief (2.5 ms) and longer (30 ms) square pulses with opposite polarity and linearly increasing size from 40 to 280 mV in both negative and positive directions. Alterations in the transient and steady-state electromotility transfer curves were examined by application of acetylcholine (ACh) and gamma-aminobutyric acid (GABA) to the synaptic pole. ACh, in the concentration range of 10-30 microM, evoked a significant magnitude and gain increase of electromotility in both transient and steady-state responses without a measurable shift in the operating point of the displacement-voltage transfer curve. A tonotopic response magnitude difference is found for ACh challenge. Basal turn OHCs responded with greater magnitude increase (+90% increase from control) than apical turn OHCs (+40%). GABA exerted an opposite effect, again in a location-dependent manner. Magnitude response decreased about 30% for long cells and 14% for short ones. Atropin, a muscarinic receptor antagonist, completely blocked the increase in electromotility response due to ACh. However, D-tubocurarine, a nicorinic receptor antagonist, while not blocking the ACh effect, altered the cell's apparent operating point. Bicuculline methiodide, a GABAA-receptor antagonist, completely arrested GABA influences on the electromotility response. These results suggest that both ACh and GABA can change the electromotile activity of OHCs, in a tonotopically biased manner. ACh challenge evokes greater magnitude responses in basal turn OHCs, whereas GABA induces greater motility response decrease in apical turn OHCs. The control of the gain and magnitude of electromotility by the transmitter substances appear to involve at least two mechanisms. One is probably related to conformational changes of the voltage-to-movement converter molecules and a change in their number in an effective operational pool, the other operates via changing the electrical resistance of the basolateral cell membrane.

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).

Use of the teleost saccule to identify genes involved in inner ear function

The vertebrate inner ear sensory epithelia contain different types of hair cells and supporting cells. The teleost saccule is anatomically similar to the mammalian saccule and is primarily involved in the detection of translational acceleration and orientation with respect to gravity. To facilitate molecular studies of the teleost saccule cDNA libraries were constructed from microdissected Lepomis macrochirus (bluegill sunfish) saccular maculae. To our knowledge, this is the first report of cDNA libraries constructed from the saccule. In one instance, a non-polymerase chain reaction-based method of amplifying a mRNA population from limited amounts of starting tissue was employed that allowed construction of cDNA libraries from nanogram amounts of tissue mRNA. Conventional cDNA libraries were constructed from the sunfish saccular maculae as well. These cDNA libraries enriched in hair cell and supporting cell transcripts should facilitate molecular biological studies of inner ear sensory epithelia. As an example of their utility, efforts to identify tyrosine kinases expressed in the saccular endorgan using low-stringency hybridization screening of these cDNA libraries and the partial sequence of a cDNA found to encode an erbB-2-related tyrosine kinase are also reported.

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.

The effect of protein kinase C stimulator and inhibitor on cochlear potentials in the guinea pig

To determine a possible role of protein kinase C (PKC) in the cochlear, the effects of a PKC stimulator (phorbol-12-myristate-13-acetate; PMA), an inactive analogue of PKC stimulator (4 alpha-phorbol-12,13-didecanoate; 4 alpha-PDD) and a PKC inhibitor (D-sphingosine) on cochlear potentials were examined in the guinea pig. The perilymphatic perfusion with PMA (3 x 10(-6) M) produced an increase in compound action potential (CAP) amplitude and no change in N1 latency, the amplitudes of negative summating potential (-SP), cochlear microphonics (CM) and endocochlear potential (EP). The perfusion with 4 alpha-PDD (3 x 10(-6) M) did not change the sound-evoked cochlear potentials and the EP. The perfusion with D-sphingosine (10(-5) M) produced a decrease in CAP amplitude and no change in N1 latency and the amplitudes of -SP, CM and EP. The results suggest that PKC may be involved in the mechanism underlying the CAP generation.

Second messengers in the cochlea

Second messengers are vital to the regulation of nearly every aspect of cellular physiology. Evidence is reviewed for the role of second messengers in cochlear fluid homeostasis, hair cell adaptation, motility and synaptic activity. We suggest that the elucidation of the role of second messengers in the regulation of cochlear physiology will be important for the therapeutic management of otopathologies and the side effects of ototoxic agents.

Detection and characterization of nitric oxide synthase in the mammalian cochlea

The messenger molecule nitric oxide (NO) is involved in blood flow regulation, cytotoxicity, and neural signalling, processes that are important in the physiology and pathophysiology of the mammalian cochlea. However, neither the presence of NO nor its synthetic enzyme, NO synthase, has been established in the peripheral auditory system. NO synthase activity, measured as the enzymatic conversion of radioactive arginine to citrulline, was predominantly soluble in the auditory nerve, lateral wall, vestibule and cochlear neuroepithelium. N-methyl-L-arginine and trifluoperazine inhibited NO synthase activity in the lateral wall and auditory nerve. Histochemical staining by NADPH-diaphorase localized NOS activity to the lateral wall and the neuronal elements of the organ of Corti. Based on these results, the predominant NO synthase isoform in the cochlea is the neuronal type-I isoform.

Calcium and calmodulin inhibit phosphorylation of a novel auditory nerve protein

The growing use of cochlear prosthetic devices and demonstrations of direct ototoxic insult to spiral ganglion neurons make it imperative to gain an understanding of intracellular biochemical regulation in primary sensory neurons. Calcium and calmodulin regulate many aspects of neuronal cellular physiology through stimulation of protein kinase activity. We have previously demonstrated the presence of calmodulin-dependent protein kinase substrates in the guinea pig modiolus and, additionally, the presence of two proteins (12 kDa and 81 kDa, designated as p12 and p81) whose phosphorylation is blocked by calcium and calmodulin (Coling and Schacht, 1991). Here, we investigate three models for this unusual regulatory mechanism. The effects of calcium, calmodulin and trifluoperazine on dephosphorylation of both proteins suggests that calmodulin inhibits protein kinase activity. P81 was identified by immunoprecipitation as the myristoylated alanine-rich C kinase substrate (MARCKS), a ubiquitous actin-binding protein. Two observations indicate that MARCKS may be regulated differently in acoustic nerve than in cerebral cortex. 32P incorporation was significantly higher in acoustic nerve than in brain. The calmodulin-dependent block of MARCKS phosphorylation was observed only in acoustic nerve. p12 shares several characteristics with myelin basic protein (MBP). We used a double label assay with 32P autoradiography and immunoblotting to show that p12 is in fact distinct from MBP. We suggest that either p12 or p12 kinase may be either specific to the peripheral auditory system or novel marker proteins for that tissue.

Immunohistochemical localization of S-100 protein in the saccule of the rainbow trout (Salmo gairdnerii R.)

The distribution of S-100-like immunoreactivity in the trout saccule (a presumed organ of hearing in fish) has been determined by means of immunohistochemistry. Within the sensory epithelium of the saccular macula, hair cells and myelinated saccular nerve fibers were found to be immunoreactive. Hair-cell immunoreactivity was relatively uniform throughout the macula except at the extreme periphery (rostral, caudal, ventral and dorsal), where staining was either decreased or absent. The immunoreactivity associated with myelinated nerve fibers was greatest at the peripheral edges of the nerve processes, a position corresponding to the location of Schwann cells. However, the nerve processes themselves (within and subjacent to the sensory epithelium), as well as cell bodies within the saccular nerve, were also immunoreactive. Thus, the immunoreactivity of the saccular nerve observed above the basal lamina can be attributed to the saccular nerve processes as well as to nerve-associated Schwann cells. Overall, the immunoreactivity displayed by hair cells was less intense than that associated with myelinated saccular nerve, as evidenced by a disappearance of signal in hair cells first, upon serial dilution of antibody. No S-100-like immunoreactivity was observed in supporting cells within the sensory epithelium or in epithelial cells in non-sensory regions. A concentration of S-100-like immunoreactivity in hair cells and saccular nerve is suggestive of the presence of S-100 calcium-binding protein-mediated activities in these cell types.

Cholinergic regulation of the phosphoinositide second messenger system in the guinea pig organ of Corti

The effect of cholinergic agents on the phosphoinositide second messenger system was investigated in the cochlea of the adult guinea pig in vivo and in vitro. In vivo, phospholipids were labeled with [32P]-orthophosphate by perilymphatic perfusion and their hydrolysis assayed in 'chase' experiments with non-radioactive orthophosphate. Carbachol (1 mM) reduced the content of 32P-labeled phosphatidylinositol 4,5-bisphosphate in the organ of Corti from 31% to 21% of total 32P-lipids, indicating stimulated hydrolysis. The pharmacology of this effect was studied in detail in vitro via the release of inositol phosphates from phosphoinositides pre-labeled with 3H-inositol. Release was increased 2-fold by 1 mM carbachol, 1.6-fold by 1 mM muscarine, but was unaffected by dimethylphenylpiperazinium; the stimulation was blocked by 1 microM atropine but not mecamylamine. These responses indicate the coupling of phosphoinositides to a muscarinic receptor. Furthermore, stimulated inositol phosphate release was higher in the base of the organ of Corti than in the apex which correlates with the increased cholinergic efferent innervation of outer hair cells in the basal region. These results suggest that muscarinic-stimulated inositol phosphate release occurs at the level of the outer hair cell and thus may have an important modulatory role in auditory transduction.