Modulation of cyclic AMP production by strial marginal cells from gerbil in culture

Expression and localization of α2A-adrenergic receptor in the rat post-natal developing cochlea

Lots of adrenergic receptors (ARs) are widely present across the auditory pathways and are positioned to affect auditory and vestibular functions. However, noradrenergic regulation in the cochlea has not been well characterized. In this study, a rat model of noise-induced hearing loss was developed to investigate the expression of α2A-adrenergic receptor (AR) after acoustic trauma, then, we investigated the expression of α2A-AR in the developing rat cochlea using immunofluorescence, qRT-PCR, and Western blotting. We found that the expression of α2A-AR significantly increased in rats exposed to noise compared with controls. Immunofluorescence analysis demonstrated that α2A-AR is localized on hair cells (HCs), spiral ganglion neurons (SGNs), and the stria vascularis (SV) in the postnatal developing cochlea from post-natal day (P) 0 to P28. Furthermore, we observed α2A-AR mRNA reached a maximum level at P14 and P28 when compared with P0, while no significant differences in α2A-AR protein levels at the various stages when compared with P0. This study provides direct evidence for the expression of α2A-AR in HCs, SGNs, and the SV of the cochlea, indicating that norepinephrine might play a vital role in hearing function within the cochlea through α2A-AR.

Sympathetic Nervous System Regulation of Auditory Function

BACKGROUND

The auditory system processes how we hear and understand sounds within the environment. It comprises both peripheral and central structures. Sympathetic nervous system projections are present throughout the auditory system. The function of sympathetic fibers in the cochlea has not been studied extensively due to the limited number of direct projections in the auditory system. Nevertheless, research on adrenergic and noradrenergic regulation of the cochlea and central auditory system is growing. With the rapid development of neuroscience, auditory central regulation is an extant topic of focus in research on hearing.

SUMMARY

As such, understanding sympathetic nervous system regulation of auditory function is a growing topic of interest. Herein, we review the distribution and putative physiological and pathological roles of sympathetic nervous system projections in hearing. Key Messages: In the peripheral auditory system, the sympathetic nervous system regulates cochlear blood flow, modulates cochlear efferent fibers, affects hair cells, and influences the habenula region. In central auditory pathways, norepinephrine is essential for plasticity in the auditory cortex and affects auditory cortex activity. In pathological states, the sympathetic nervous system is associated with many hearing disorders. The mechanisms and pathways of sympathetic nervous system modulation of auditory function is still valuable for us to research and discuss.

Effects of vasopressin on gene expression in rat inner ear

Vasopressin regulates water excretion from the kidney by increasing water permeability of the collecting duct as a hormone secreted from the posterior pituitary. A clinical study reported that plasma levels of arginine vasopressin were significantly higher in patients suffering from Meniere's disease. It was histologically confirmed that chronic administration of vasopressin induced endolymphatic hydrops in guinea pigs. However, the mechanism of endolymphatic hydrops induced by vasopressin is still unclear. We use cDNA microarray to study the effects of vasopressin on gene expression profiles in rat inner ear to elucidate the possible mechanism of the induced hydrolabyrinth. Wistar rats were intraperitoneally injected with 50 microg/kg arginine vasopressin once a day for one week. Hydrolabyrinth in rat inner ear induced by administration of vasopressin was detected by HE stain. The bullae were dissected out for total RNA extraction. cDNAs were synthesized by reverse transcription and labeled with Cyanine3 (Cy3) or Cyanine5 (Cy5). The BiostarR-40s cDNA microarray was hybridized with the above cDNAs and the changes of mRNA expression intensity were showed by data analysis. Furthermore, the changes of aquaporins expression level were measured by reverse transcription polymerase chain (RT-PCR). Endolymphatic hydrops were present in rats intraperitoneally injected with vasopressin. 226 known differentially expressed genes were screened out in rat inner ear induced by vasopressin injection. Of the 226 genes, 18 transcripts were increased by 5-fold or more, and 7 transcripts were decreased to 0.2-fold or less. Ten differentially expressed genes were identified that associate with cell signal transduction, 14 differentially expressed genes were identified that relate to ion transport, 7 differentially expressed genes were involved in vesicle-mediated transport, and 2 differentially expressed genes were aquaporin 2 (AQP2) and aquaporin 7 (AQP7). The expression level of AQP2 was significantly higher and AQP7 was significantly lower. These results suggest that there are obvious differences in gene expression profiles in inner ear between vasopressin injected rats and normal control rats. Vasopressin may disturb fluid homeostasis in inner ear by way of signal transduction, ion transport, vesicle-mediated transport and aquaporins. It is likely that up-regulated expression of AQP2 mRNA and down-regulated expression of AQP7 mRNA in the rat inner ear caused by vasopressin induce an increased production and a decreased absorption of endolymph, resulting in endolymphatic hydrops.

K(+) cycling and its regulation in the cochlea and the vestibular labyrinth

Potassium (K(+)) plays a very important role in the cochlea. K(+) is the major cation in endolymph and the charge carrier for sensory transduction and the generation of the endocochlear potential. The importance of K(+) handling in the cochlea is marked by the discovery of several forms of hereditary deafness that are due to mutations of K(+) channels. Deafness results from mutations of KCNQ4, a K(+) channel in the sensory hair cells, as well as from mutations of the gap junction proteins GJB2, GJB3 and GJB6 that may facilitate cell-to-cell movements of K(+). Deafness results also from mutations of KCNQ1 or KCNE1, subunits of a K(+) channel that carries K(+) from strial marginal cells and vestibular dark cells into endolymph. Further, deafness results from mutations of KCNJ10, a K(+) channel that generates the endocochlear potential in conjunction with the high K(+) concentration in strial intermediate cells and the low K(+) concentration in the intrastrial fluid spaces. This review details recent advances in the understanding of K(+) transport and its regulation in the cochlea and the vestibular labyrinth.

Functional beta2-adrenergic receptors are present in nonstrial tissues of the lateral wall in the gerbil cochlea

Recently, we have demonstrated that functional beta1-adrenergic receptors are the dominant beta-adrenergic receptor subtype in the stria vascularis and that beta1-adrenergic receptors stimulate K+ secretion in strial marginal cells. The goal of the present study was to determine whether nonstrial tissues in the cochlear lateral wall contain beta-adrenergic receptors and if so which subtype is present. Pharmacological tools were used to identify receptors in functional studies where cAMP production was measured. Further, receptors were identified as transcripts by cloning and sequencing of reverse-transcriptase polymerase chain reaction (RT-PCR) products. Experiments were performed on gerbil nonstrial lateral wall tissues. Tissues stimulated with 10(-5) M isoproterenol produced 0.42 +/- 0.22 pmol cAMP per ear within 12 min (n = 14). The selective beta-adrenergic receptor agonist isoproterenol stimulated cAMP production with an EC50 of (2 +/- 3) x 10(-7) M (n = 7). Isoproterenol-stimulated cAMP production was inhibited by the beta2-adrenergic receptor antagonist ICI 118551 with an IC50 of (7 +/- 7) x 10(-6) M, which corresponds to an affinity constant of 1 x 10(-7) M (pK(DB) = 6.89 +/- 0.23, n = 3). Isoproterenol-stimulated cAMP production was not inhibited by the highly selective beta1-adrenergic receptor antagonist CGP 20712A. The IC50 and the affinity constant for CGP 20712A were estimated to be >3 x 10(-4) and >6 x 10(-6) M, respectively. RT-PCR of total RNA isolated from nonstrial lateral wall tissues using primers specific for beta1-, beta2- and beta3-adrenergic receptors revealed products of the predicted sizes for the beta1- and beta2- but not for the beta3-subtype. Sequence analysis confirmed that amplified cDNA fragments encoded gene-specific nucleotide sequences. These results demonstrate that nonstrial lateral wall tissues contain transcripts for beta1- and beta2- but not for beta3-adrenergic receptors and that the beta2-adrenergic receptor is the dominant functional receptor subtype. The cellular localization and function of the beta2-adrenergic receptors remains to be determined.

Immunohistochemical localization of adenylyl cyclase isoforms in the lateral wall of the rat cochlea

The enzymatic activity of adenylyl cyclase (AC) is attributable to nine isoforms with individual pharmacology and tissue distribution. Polyclonal antibodies for AC isoforms I-IV, VII and VIII were applied to sections of cochlear lateral wall, a tissue involved in ion transport contributing to the unique ion content of endolymph and electrical potential of scala media. Within the stria vascularis, immunoreactivity primarily to Ca(2+)/calmodulin-independent isoforms II, IV and VII was localized to sites consistent in position to the basolateral extensions of marginal cells. Little immunoreactivity was observed in the stria vascularis for Ca(2+)/calmodulin-dependent isoforms I, III and VIII. Within the spiral ligament, type II and type IV fibrocytes exhibited moderate staining for ACII, IV and VII, less staining for VIII and little for I and III. Immunoreactivity to ACII, IV, VII and VIII was observed in type I fibrocytes. The outer sulcus cells and root processes were highly immunoreactive for isoforms I and VIII, but not for III or the Ca(2+)/calmodulin-independent isoforms. The differential pattern of immunoreactivity in the lateral wall overall appears to reflect subfamily-specific expression with Ca(2+)/calmodulin-independent isoforms expressed in the stria vascularis and Ca(2+)/calmodulin-dependent isoforms expressed in the outer sulcus cells and root processes. cAMP-mediated modulation of ion transport by marginal cells is predicted to exhibit, in the microenvironment of basolateral membrane infoldings, pharmacological characteristics of the AC type II subfamily (II, IV and VII), including activation by protein kinase C (II and VII).

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.

Differentiation of cyst-forming stria vascularis tissues in vitro

The marginal cells of the stria vascularis possess distinctive morphological characteristics associated with their role in endolymph production. Interestingly, when stria-derived epithelial cells are grown in association with the underlying mesenchyme, the final differentiation of these cell types does not occur. Beyond the rudimentary polarity that is established, similar to that shown in epithelial monolayers, cells in culture bear only a slight resemblance to their marginal cell counterparts in vivo. The ultrastructural features that typify these epithelia, extensive cytoplasmic invaginations, with an abundance of mitochondria, and darkly stained cytoplasm, are not evident under standard culture conditions. In order to determine whether fluid transport, a key function of the stria vascularis, has an effect on the ultrastructural morphology, we examined de novo stria vascularis tissues that formed a fluid-filled cyst in vitro. We found that only cells associated with the luminal structure demonstrated dark cytoplasmic staining and amplification of the basolateral membrane of the marginal cells. Additionally, other epithelial features, such as mitochondria-rich and microvilli-rich cells, were observed in cyst-forming tissues. The enhancement of the marginal cell specializations was not as robust as that observed in vivo; however, they were clearly more extensive when compared to cells in the same culture that were not associated with a fluid-filled lumen. Thus it appears that fluid transport may be necessary to maximize differentiation of stria vascularis tissues in vitro.

cAMP mediates transepithelial K+ and Na+ transport in a strial marginal cell line

Because cytoplasmic cAMP has been reported to be the secondary messenger mediating K+ transport in marginal cells of freshly isolated stria vascularis, the possible role of cAMP in ion transport processes of an immortalized marginal cell line (MCPV-8) showing evidence of K+ and Na+ reabsorption was evaluated in this study. Confluent MCPV-8 monolayers were mounted into Ussing chambers and perfused on both sides with perilymph-like Ringer's solution. Transepithelial short-circuit current (I(SC)), resistance (R(T)) and open-circuit voltage (V(T)) were measured using voltage clamp technique. The following results were obtained. (1) Addition of forskolin (10(-4) M) to the basolateral perfusate increased I(SC) to 311 +/- 42%; no significant change in RT was observed. Addition of BaCl2 (2 mM) to the apical perfusate at the maximal response of forskolin blocked 50-60% of I(SC) and subsequent addition of amiloride (10(-5) M) to the apical perfusate further blocked I(SC) to a value close to 0. (2) To evaluate the effect of cellular cAMP on Ba2+-sensitive K+ current, amiloride-sensitive Na+ current was blocked first by addition of amiloride (10(-5) M) to the apical perfusate; subsequent addition of 3-isobutyl-1-methylxanthine (IBMX, 1 mM) or N6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (dbcAMP, 1 mM) to the basolateral perfusate increased I(SC) to 175 +/- 13 and 411 +/- 32%, respectively. The stimulated I(SC) was blocked to close to 0 by addition of BaCl2 (2 mM) to the apical perfusate. N2,2'-O-Dibutyrylguanosine 3',5'-cyclic monophosphate (dbcGMP, 1 mM) had no effect on I(SC). (3) To assess the effect of cellular cAMP on amiloride-sensitive Na+ current, Ba2+-sensitive K+ current was blocked in advance by addition of BaCl2 to the apical perfusate; subsequent addition of IBMX or dbcAMP to the basolateral perfusate increased I(SC) to 219 +/- 21% and 388 +/- 39%, respectively. The stimulated I(SC) was blocked to close to 0 by addition of amiloride to the apical perfusate. dbcGMP had no effect on I(SC). Hence, these results suggest that cellular cAMP is the secondary messenger that mediates the transepithelial transport of both K+ and Na+ in MCPV-8 monolayers.

Establishment and characterization of a strial marginal cell line maintaining vectorial electrolyte transport

E6/E7 genes of human papilloma virus type 16 were used to immortalize a primary culture of marginal cells (MC) from gerbils. One of the cloned lines was selected which demonstrated preservation of the main characteristics of the MC, both morphologically and physiologically. Electron microscopic examination showed well-developed junctional complexes and apical microvilli which suggested its epithelial origin. Polymerase chain reaction (PCR) demonstrated the incorporation of E6/E7 genes with the genome. Reverse transcription PCR revealed the existence of mRNA of the IsK channel, a unique marker of MC among the inner ear cells, in this clone. Flow cytometric analysis of this cell line's DNA content was diploid. Numerous large domes formed after confluence of the cell monolayer. Electrophysiologic studies displayed evidence of apical K+ and Na+ channels which were blocked by Ba2+ (2 mM) and amiloride (10(-5) M), respectively. Existence of basolateral Na,K-ATPase and Na+/Cl-/K+ cotransporter was shown by blockage by ouabain (10(-3) M) and bumetanide (50 microM), individually. Injection of the cell line to nude mice failed to induce growth of tumors. This cell line was serum-, density- and anchorage-dependent when cultured in plastic dishes. In conclusion, this cell line shows characteristics of well-differentiated MC maintaining the major ionic transport processes, and provides us a good model to study the possible mechanisms and regulating factors of endolymph production.

cAMP increases K+ secretion via activation of apical IsK/KvLQT1 channels in strial marginal cells

In the cochlea, K+ is secreted by electrodiffusion across the apical membrane of strial marginal cells via the IsK/KvLQT1 ('IsK') channel. This channel complex has been reported to be activated in other systems by adenosine 3',5'-cyclic monophosphate (cAMP). Since several reports had suggested that cAMP is a second messenger in the cochlea, the effect of the cAMP pathway on transepithelial K+ secretion by strial marginal cells of the gerbil was studied. Both the transepithelial current (Isc) and K+ flux (JK) across strial marginal cell epithelium were measured; Isc in a micro-Ussing chamber and JK as the gradient of K+ concentration near the apical membrane. The apical membrane current (IIsK) and conductance (gIsK) of IsK channels were recorded with the on-cell macro-patch and the nystatin-perforated whole-cell patch clamp techniques. It has previously been shown that the apical IsK channel constitutes the primary pathway for K+ secretion. Cytoplasmic cAMP was elevated by applying dibutyryl cyclic-AMP (dbcAMP) or the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) at 37 degrees C. dbcAMP (1 mM) increased Isc by 51 +/- 4% and IIsK in on-cell and whole-cell recordings increased by 214 +/- 63% and 390 +/- 61% above the control value, respectively. IBMX (1 mM) caused transient increases of Isc by 53 +/- 3% and IIsK in on-cell recordings by 177 +/- 75% above the control value. The leak conductance due to all non-IsK channel sources did not change in the presence of dbcAMP or IBMX. dbcAMP (1 mM at 24 degrees C) increased JK by 53 +/- 16% and Isc by 18 +/- 4%. IBMX (1 mM at 24 degrees C) had no effect, suggesting reduced activity of adenylate cyclase at this temperature. Our results demonstrate that the cAMP pathway is constitutively active in strial marginal cells and that the cAMP pathway stimulates transepithelial K+ secretion by increasing IsK channel current rather than by altering another transport pathway.

Stria vascularis morphogenesis in vitro

Explants of neonatal murine stria vascularis were maintained in vitro to evaluate the process of morphogenesis in cochlear tissue. Immunohistochemical and electron microscopic studies showed that the relatively undifferentiated cells in culture attained morphological features characteristic of the stria vascularis cell types in vivo (marginal, intermediate and basal cells). The three kinds of cells formed a trilaminated tissue, with the epithelial cells bordering the culture medium, basal-like cells resting on the culture substrate, and the melanocytes layered between. Furthermore, approximately 20% of these cultures displayed a unique alignment of melanocytes which formed elongated bands along the contour of the tissue edge. However, only limited cell extensions were formed between different cell types and interdigitation amongst these processes was abbreviated. Thus, cells from different embryological origins divided, migrated and reestablished appropriate cell-to-cell associations to form a layered tissue similar to the stria vascularis in vivo.

Establishment of primary cell culture from stria vascularis explants. Morphological and functional characterization

To provide the prerequisite for long-term study of the inner ear related to structural and functional integrity, tissue of stria vascularis with spiral ligament was isolated from Wistar rat cochleas and cultured using the explant-culture technique. The following culture media were used: EMEM with Hepes buffer, hydrocortisone (400 ng/ml), transferrin (5 micrograms/ml). triiodothyronine (10(-9) M), cholera toxin (10(-10) M), insulin (5 micrograms/ml), and epidermal growth factor (10 ng/ml). To characterize the cells growing out from the explant, immunofluorescence with cytokeratin (cytokeratin 18) and ultrastructural examination with SEM and TEM were performed. The marginal cell function was investigated by expression of Na+, K(+)-ATPase antisera against beta 2 subunit of rat Na+, K(+)-ATPase and P-NPPase. We were able to maintain the cultured cells for 3 weeks or more. Monolayered marginal cells were observed beyond 14 days in vitro and the expression of cytokeratin 18 was especially enhanced. The cultured marginal cells were almost identical to in vivo cells both as regards ultrastructural features and Na+, K(+)-ATPase activity. The present results suggest that the primary explant culture technique is a reliable in vitro model of strial marginal cells. However, establishment of the cell line is needed for long-term study.

Luminal non-selective cation and outwardly rectifying chloride channels in cultured strial marginal cells from gerbil

Ionic channels located on the luminal side of strial marginal cells (MCs) of gerbil in culture were investigated using the patch-clamp technique. Two types of channels were identified. The most frequently recorded single-channel activity corresponded to a non-selective cation (NSC) channel with a conductance of 23.7 +/- 0.2 pS (n = 18) in symmetrical NaCl conditions. The channel was activated by internal Ca2+ and inhibited by internal adenine nucleotides and flufenamic acid. Spontaneous activity of NSC channels was found in 16% of the cell-attached patches and with a very high density (9 +/- 2 levels/patch, n = 28) in 100% of the excised patches. An outwardly rectifying chloride (ORC) channel was also identified in 14% of the patches but only after excision. The channel exhibited at 0 mV a unit conductance of 26.8 +/- 1.3 pS (n = 8) and a strong outward rectification in symmetrical NaCl conditions, and the open probability increased with depolarization. The luminal NSC channel and the ORC channel evidenced in this study might participate in the production of endolymph. Although extrapolation of the presents results to the in vivo situation should be made with caution, this study suggests that culture of strial MCs may be a suitable model for investigation of endolymph physiology.