Cloning and Expression of Vacuolar Proton-Pumping ATPase Subunits in the Follicular Epithelium of the Bullfrog Endolymphatic Sac

In an investigation aimed at clarifying the mechanism of crystal dissolution of the calcium carbonate lattice in otoconia (the mineral particles embedded in the otolithic membrane) of the endolymphatic sac (ELS) of the bullfrog, cDNAs encoding the A- and E-subunits of bullfrog vacuolar proton-pumping ATPase (V-ATPase) were cloned and sequenced. The cDNA of the A-subunit consisted of an 11-bp 5'-untranslated region (UTR), a 1,854-bp open reading frame (ORF) encoding a protein comprising 617 amino acids with a calculated molecular mass of 68,168 Da, and a 248-bp 3'-UTR followed by a poly(A) tail. The cDNA of the E-subunit consisted of a 72-bp 5'-UTR, a 681-bp ORF encoding a protein of 226 amino acids with a calculated molecular mass of 26,020 Da, and a 799-bp 3'-UTR followed by a poly(A) tail. Western blot and immunofluorescence analyses using specific anti-peptide antisera against the V-ATPase A- and E-subunits revealed that these subunits were present in the ELS, urinary bladder, skin, testes, and kidneys. In the ELS, positive cells were scattered in the follicular epithelium which, as revealed by electron microscopy, corresponds to the location of mitochondria-rich cells. These findings suggest that V-ATPase, including the A- and E-subunits, exists in mitochondria-rich cells of the ELS, which might be involved in dissolution of the calcium carbonate crystals in the lumen of the ELS.

Co-expression of pendrin, vacuolar H+-ATPase alpha4-subunit and carbonic anhydrase II in epithelial cells of the murine endolymphatic sac

The endolymph in the endolymphatic sac (ES) is acidic (pH 6.6-7). Maintaining this acidic lumen is believed to be important for the normal function of the ES. The acid-base regulation mechanisms of the ES are unknown. Here we investigated the expression patterns of acid-base regulators, including vacuolar (v)H+-ATPase (proton pump), carbonic anhydrase (CA) II, and pendrin in the murine ES epithelium by immunohistochemistry (IHC) and compared their expression patterns by double immunostaining. We found that pendrin and vH+-ATPase were co-localized in the apical membrane of a specific type of ES epithelial cell. Pendrin- and vH+-ATPase-positive cells also expressed cytoplasmic CA II. Co-expression of pendrin, vH+-ATPase, and CA II in the same subgroup of ES cells suggests that this specific type of ES cell is responsible for the acid-base balance processes in the ES and pendrin, vH+-ATPase, and CA II are involved in these processes.

Ultracytochemical localization of Ca2+-ATPase activity in the endolymphatic sac of the chick

The localization of Ca2+-ATPase activity was examined ultracytochemically in the endolymphatic sac (ES) of the 1-day-old chick. The reaction products showing Ca2+-ATPase activity were localized along the microvilli and the apical cell membranes of the epithelial cells of the ES. The lateral cell membranes also showed mild activity. These reactions were completely abolished when either Ca2+ or ATP was omitted from the incubation medium. The results suggest that Ca2+-ATPase plays a significant role as a Ca2+ pump for regulation of the Ca2+ concentration in the endolymph of the ES.

The endolymphatic sac: a scanning and transmission electron microscopy study

A recent investigation has suggested that the chief cells of the endolymphatic sac produce an endogenous inhibitor of sodium resorption in the kidneys, tentatively named saccin. In the current study, the ultrastructure of the endolymphatic sac and in particular the chief cells are described to demonstrate that this organ fulfils the morphological criteria of a potential endocrine gland. Accordingly, the chief cells are shown to exhibit all the organelles and characteristics of cells that simultaneously synthesize, secrete, absorb and digest proteins.

[Expression of alpha and beta subunit isoforms of Na, K-ATPase in the guinea pig endolymphatic sac]

OBJECTIVE

To investigate the expression and significance of alpha and beta subunit isoforms of Na, K-ATPase in the guinea pig endolymphatic sac.

METHODS

The distribution of alpha and beta subunit isoforms of Na, K-ATPase in endolymphatic sac of guinea pig was identified by immunocytochemistry and in situ hybridization.

RESULTS

The expression of Na, K-ATPase alpha and beta subunit isoforms varied among different cell regions of the endolymphatic sac. Epithelial cells of the endolymphatic sac were observed to contain the alpha 1, beta 1 and beta 2 subunit isoforms, and the expression of beta 2 was stronger than that of beta 1. Subepithelial cells contained alpha 1 and alpha 2 subunit isoforms and the expression of alpha 1 was stronger than that of alpha 2. No expression of alpha 3 subunit isoform was observed in endolymphatic sac.

CONCLUSION

Na, K-ATPase of endolymphatic sac consists of different alpha and beta subunit isoforms which, working in concert, serve to maintain homeostasis in inner ear.

[mRNA expression of hyaluronan synthase in the endolymphatic sac cells of guinea pigs]

OBJECTIVE

To detect the mRNA expression of hyaluronan synthase in the endolymphatic sac cells of guinea pigs.

METHODS

After consulting gene bank, we analyzed conservative sequence of hyaluronan synthases in different species, detected the mRNA expression of hyaluronan synthase in the endolymphatic sac cells of guinea pigs with oligonucleotide probe by hybridization in situ.

RESULTS

mRNA of hyaluronan synthase was strongly expressed in some epithelial cells of endolymphatic sac, coupled with negative expression in negative control groups.

CONCLUSION

It confirms that endolymphatic sac cells can synthesize hyaluronan.

Presence of plasma membrane-bound Ca(2+)-ATPase in the secretory epithelia of the inner ear

The Ca2+ concentration of the endolymph is low, around 0.023 mM. Yet, because of the positive endocohlear potential, Ca2+ must be actively transported into the endolymphatic space. The mechanisms responsible for the active Ca2+ transport into the endolymph are not known. In this study, the presence of plasma membrane-bound Ca(2+)-ATPase (PMCA ATPase) in the endolymph-producing, secretory epithelia of the inner ear from guinea pig was investigated with immunoblotting and immunohistochemistry. The antibody used was a monoclonal antibody which recognizes an epitope shared by all four known isoforms of PMCA ATPase. With immunoblotting, a band corresponding to PMCA ATPase was found in the stria vascularis, the ampullary tissue, the utricle and the endolymphatic sac in assays from at least three different batches of tissue. With immunohistochemistry, a strong positive staining reaction for PMCA ATPase could be seen in the stria vascularis and the dark cells of the ampullary tissue and the utricle. The epithelial cells in the endolymphatic sac showed a moderate positive staining reaction. Accordingly, in this study the presence of PMCA ATPase was shown in all the endolymph-producing, secretory epithelia of the inner ear. These results indicate that PMCA ATPase plays a role in the regulation of the Ca2+ concentration in the endolymph.

Localization of nitric oxide synthase isoforms (I, II and III) in the endolymphatic sac of the guinea pig

The localization of nitric oxide (NO) synthase (NOS) isoforms was examined in the endolymphatic sac (ES) of the pigmented guinea pig by indirect immunohistochemistry. The cytoplasm of the epithelial cells in the ES showed both NOS-I and -III immunoreactivity, whereas their nuclei appeared negative. NOS-III staining was also observed in the endothelial lining of the blood vessels. These findings support the hypothesis that NO in the epithelial cells may play an important role for the active intracellular transport of the endolymph and ion. NO may also be crucially involved in the regulation of ES blood flow. Immunostaining for NOS II revealed no reactivity in general, while in lipopolysaccharide-inoculated animals, intense reactivity was found in the cytoplasm of the ES epithelial cells as well as macrophages in the lumen. Thus it has been indicated that NO also may play an important role in the immunodefensive mechanisms in the ES.

Development of melanosomes and cytochemical observation of tyrosinase activity in the inner ear

The ultrastructure and tyrosinase activity of melanocytes in the inner ear of pigmented guinea pigs were observed. Melanocytes/melanocyte-like cells were seen in the stria vascularis, the vestibular dark cell area and the endolymphatic sac. In the stria vascularis, melanosomes in several stages of maturation were seen in the cytoplasm of the intermediate cells and melanin-laden endosomes existed in the basal cells. Only the intermediate cells contained tyrosinase-positive cytoorganelles; Golgi sacs and neighboring small vesicles. Numerous melanocytes containing many melanosomes were observed under the epithelium of the vestibular dark cell area, and they showed tyrosinase activity. Melanocytes were seen in the endolymphatic sac, and they also showed tyrosinase activity. However, the epithelial cells of the endolymphatic sac, which had vacuoles containing melanin, did not show tyrosinase activity. Based on these findings, it can be said that (1) most of the intermediate cells of the stria vascularis must be melanocytes, (2) melanogenesis is vigorous in melanocytes of the inner ear, and (3) melanin in the epithelial cells of the endolymphatic sac is transferred in from melanocytes and is never synthesized in the epithelial cells.

Na,K-ATPase subunit isoform expression in the guinea pig endolymphatic sac

Na,K-ATPase subunit isoform expression was studied by immunocytochemistry in the guinea pig endolymphatic sac, using subunit isoform-specific polyclonal antibodies. Epithelial cells of the guinea pig endolymphatic sac were observed to contain the alpha 1- and beta 2-subunit isoforms, and to a lesser extent the beta 1-subunit isoform, of Na,K-ATPase. The alpha 1- and beta 2-subunit isoforms of Na,K-ATPase have been observed previously in other ion and fluid transporting regions of the membranous labyrinth, e.g., stria vascularis and vestibular dark cells. Combined data indicate that the alpha 1-, beta 2-form of Na,K-ATPase plays a role in the microhomeostasis of endolymph. The alpha 1 beta 2 Na,K-ATPase subunit isoform combination is different from typical ion and fluid transporting tissues, e.g., kidney and colon, and may reflect distinctive characteristics of inner ear Na,K-ATPase.

Immunohistochemical localization of carbonic anhydrase in the endolymphatic sac of the mouse and guinea pig

The localization of carbonic anhydrase (CA) in mouse and guinea pig endolymphatic sac (ES) was determined by immunohistochemistry. In the distal and intermediate portions of the ES of both species, epithelial cells reacted with anti-CA antiserum. All epithelial cells in the distal portion stained for CA. In the intermediate portion, immunoreactivity was prominent in about half the epithelial cells, but less intense in the remaining epithelial cells. The proximal portion of the ES lacked immunoreactivity. CA may play a crucial role in ES electrolyte transport.

Hyaluronan synthesis by in vitro cultured endolymphatic sac cells

The endolymphatic sac (ELS) has been the subject of investigation for many years and yet its overall function remains unclear. It is believed mainly to be involved in the regulation of endolymph through fluid resorption and secretion of osmotically active substances. The present study was performed using in vitro cultured, fetal ELSs of 18 to 19 day gestational mice, to assess whether the ELS cells can synthesize the osmotically active polysaccharide, hyaluronan (HA). The ELS and portions of the membranous labyrinth were dissected from whole otocyst specimens and placed in 14C glucose-enhanced tissue culture media. A light microscopic (LM), autoradiographic study was performed to assess whether 14C glucose could be incorporated by the tissue into HA. Both the ELS cells and the adjacent cartilage demonstrated radiolabel incorporation within 4 hours of incubation in tissue culture medium, with increased radiolabel density in ELS cells after 24 hours of incubation. HA-specific hyaluronidase (HAase) resulted in removal of HAase-sensitive compounds in the ELS in both 4-hour and 24-hour cultured specimens when compared to adjacent cartilage cells (p = 0.001). Approximately 43 percent of the radiolabel was incorporated into HA in ELS specimens, as compared to a 22 percent HA synthesis in the adjacent cartilage tissue, suggesting preferential synthesis by ELS cells. The dissected murine otocysts demonstrate viability in vitro as measured by their ability to incorporate 14C glucose from tissue culture medium. Under these conditions the cultured ELS demonstrates an ability to synthesize HA. A theory of ELS function is proposed.

Changes in hyaluronan synthesis by in vitro cultured endolymphatic sac cells

The endolymphatic sac (ELS) has been implicated in the maintenance of endolymph volume and pressure in the membranous labyrinth through fluid resorption and secretion of osmotically active substances, known as glycosaminoglycans (GAGs). To assess whether the ELS cells synthesize the GAG, hyaluronan (HA), and to further elucidate the secretory function of the ELS, a series of experiments were carried out on in vitro tissue-cultured, fetal murine ELSs. In phase 1 of the investigation, the ELSs that were attached to a small portion of the posterior labyrinth, were resected from whole otocyst specimens and studied in tissue culture. This model was chosen to determine whether a change in endolymph homeostasis affects ELS activity. Radiolabeled 14C glucose incorporation was used to evaluate HA synthesis by ELS cells when cultured in vitro. Approximately 43 percent of the incorporated 14C glucose radiolabel was digested by Streptomyces hyaluronidase (an HA-specific hyaluronidase), suggesting HA synthesis by sac cells. In phase 2 of our experiments, the ELSs were not resected from the otocysts. Instead, they were left attached to intact membranous labyrinths, and whole otocysts were cultured. Studies analogous to those of phase 1, assessing the ability of the ELS cells to incorporate 14C glucose into HA, were performed on these specimens. Streptomyces hyaluronidase treatment of these ELS specimens resulted in a reduction in the removal of radiolabel. Therefore, the ELS cells of intact otocysts incorporated less 14C glucose into HA when compared to the ELS cells of the resected specimens.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of carbonic anhydrase isoenzymes in the developing endolymphatic sac of the human fetus and the mouse embryo

The distribution of carbonic anhydrase isoenzymes (CA) was analyzed in the developing endolymphatic sac (ES) of the mouse embryo and human fetus using immunohistochemical method. The primordial ES epithelium was labelled with CA I and CA II, but was weakly labelled with CA III and CA V. In the thirteenth and fifteenth gestational day (GD) mice, the ES epithelium was positive for CA I and CA II. After seventeenth GD, the ES epithelium was however weakly positive for CA I and CA II. In the 11 and 12 week old human fetus, the ES epithelium was strongly labelled with CA I and CA II. In the 16 week old human fetus, the ES epithelium was however weakly positive for CA I and CA II. These results suggest that the fetal ES has an activity of CA and plays a role in the otoconial formation especially in the early stage during evolution.

Cell membrane polarity of the epithelial cells in the endolymphatic sac of the guinea pig

The epithelial cells of the endolymphatic sac (ES) were studied in order to characterize their glycocalyx composition. Colloidal thorium and cationized ferritin were used as electron-dense markers to visualize the glycocalyx as well as the basement membrane. In addition, enzymatic digestions were performed to identify the reactive components of the glycocalyx responsible for colloidal thorium labeling. The results indicate that the glycocalyx of the ES epithelial lining is very rich in sialic acid. Furthermore, a marked polarity in glycocalyx reactivity is present between the apical and basolateral membranes, whereas difference in glycocalyx reactivity between the light and the dark cells are not obvious. We speculate that the specific composition and the apparent polarity of the ES epithelial cell glycocalyx are of importance to the equilibrium of electrolytes in the ES lumen.

Immunohistochemical localization of Na+, K+-ATPase in the human endolymphatic sac

The presence and distribution of the transport protein complex Na+,K+-ATPase in the human endolymphatic sac (ES) was demonstrated immunohistochemically using a monoclonal antibody directed toward the denatured catalytic subunit of Na+,K+-ATPase from lamb kidney medulla. Our findings support an active transcellular ion exchange by the ES epithelium with subsequent passive transcellular and paracellular outflow of water. The possible role of the lateral intercellular spaces in the outflow of endolymph at the level of the ES is discussed.

Cytochemical localization of Na-K ATPase in the guinea pig endolymphatic sac

The light microscopical and ultracytochemical localization of Na-K ATPase (ouabain-sensitive, potassium-dependent p-nitrophenylphosphatase) in the guinea pig endolymphatic sac was studied by a newly developed lead citrate one step method. The cytochemical reaction product was detected in the epithelial cell layer of the endolymphatic sac at the light microscopical level. At the electron microscopical level, the reaction product was located to the basolateral plasma membrane of both the dark and the light cells. The results suggest that Na-K ATPase may play a role in the ion transport mechanism in the endolymphatic sac.

Ultrastructural localization of carbonic anhydrase and its possible role in the endolymphatic sac

Carbonic anhydrase was detected histochemically in the guinea pig endolymphatic sac. The enzyme reaction was positive in both light and dark epithelial cells. In the former, the reaction product was found in the cytoplasm, especially around the intracytoplasmic vesicles and vacuoles. Reaction product was found in the basolateral infoldings as well. The dark epithelial cells also displayed carbonic anhydrase activity in the cytoplasm as well as in the lysosome-like bodies. It is suggested that this enzyme may be involved in (1) ionic or fluid regulation of the endolymph, (2) otoconia metabolism, and (3) phagocytotic activities.

Ultrastructural changes of statoconia after segmentation of the otolithic membrane

The chick vestibule transformed from a homogeneous epithelial layer at day 2 (stage 15) into a pseudo-stratified epithelial layer at day 4 (stage 24). The apical columnal appearance of sensory cells was evident by day 6 (stage 29). In the supporting cells of the saccule and utricle large rough endoplasmic reticulum cisterns filled with material similar to the primitive organic matrix. Fibrillar material of the otolithic membrane remained attached to the supporting cells and accumulated over the saccule and utricle. The primitive otolithic membrane acquired stress-like lines and statoconial units emerged from the upper surface without a central core. Statoconia thickened at the periphery and a central core formed. Calcium was deposited between the fibrils of older statoconia which were located on top of the segmenting membrane. DIAMOX inhibited statoconia formation and/or prevented calcium and the matrix from associating. Large statoconia (100-200 microns diameter) were formed in embryos injected with this carbonic anhydrase inhibitor. Gel electrophoresis of immature statoconial complexes yielded at least 5 major protein bands between 25 and 210 kDa. Ouabain-sensitive potassium-dependent p-nitrophenylphosphatase activity was demonstrated in the endolymphatic sac of newly hatched chicks.

Histochemical localization of carbonic anhydrase in the inner ear

Carbonic anhydrase was histochemically located in chinchilla inner ear tissues. A strong carbonic anhydrase reaction was observed in the spiral ligament cells, Boettcher's cells, the external sulcus cells, and the stria vascularis (intermediate and/or basal cells). The enzymatic reaction was also positive in the supporting cells of all vestibular sensory epithelia, as well as in the dark cells and transitional cells of the utricle and saccule. Some epithelial cells of the endolymphatic sac were also positive. It is speculated that this enzyme may be involved in: 1) ionic or fluid regulation of the endolymph, 2) removal of CO2 from the inner ear tissue near the sensory cells, and 3) otoconia formation and maintenance.