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Originally published In Press as doi:10.1074/jbc.M609872200 on January 5, 2007

J. Biol. Chem., Vol. 282, Issue 10, 7450-7456, March 9, 2007
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FXYD6 Is a Novel Regulator of Na,K-ATPase Expressed in the Inner Ear*

Benjamin Delprat{ddagger}, Danièle Schaer{ddagger}, Sophie Roy{ddagger}, Jing Wang§, Jean-Luc Puel§, and Käthi Geering{ddagger}1

From the {ddagger}Department of Pharmacology and Toxicology, University of Lausanne, Rue du Bugnon 27, 1005 Lausanne, Switzerland and §Institut National de la Santé et de la Recherche Médicale U583, Institut des Neurosciences de Montpellier and University of Montpellier 1, 34091 Montpellier, France

Received for publication, October 20, 2006 , and in revised form, December 21, 2006.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
The exquisite sensitivity of the cochlea, which mediates the transduction of sound waves into nerve impulses, depends on the endolymph ionic composition and the endocochlear potential. A key protein in the maintenance of the electrochemical composition of the endolymph is the Na,K-ATPase. In this study, we have looked for the presence in the rat inner ear of members of the FXYD protein family, recently identified as tissue-specific modulators of Na,K-ATPase. Only FXYD6 is detected at the protein level. FXYD6 is expressed in various epithelial cells bordering the endolymph space and in the auditory neurons. FXYD6 co-localizes with Na,K-ATPase in the stria vascularis and can be co-immunoprecipitated with Na,K-ATPase. After expression in Xenopus oocytes, FXYD6 associates with Na,K-ATPase {alpha}1-beta1 and {alpha}1-beta2 isozymes, which are preferentially expressed in different regions of the inner ear and also with gastric and non-gastric H,K-ATPases. The apparent K+ and Na+ affinities of {alpha}1-beta1 and {alpha}1-beta2 isozymes are different. Association of FXYD6 with Na,K-ATPase {alpha}1-beta1 isozymes slightly decreases their apparent K+ affinity and significantly decreases their apparent Na+ affinity. On the other hand, association with {alpha}1-beta2 isozymes increases their apparent K+ and Na+ affinity. The effects of FXYD6 on the apparent Na+ affinity of Na,K-ATPase and the voltage dependence of its K+ effect are distinct from other FXYD proteins. In conclusion, this study defines the last FXYD protein of unknown function as a modulator of Na,K-ATPase. Among FXYD protein, FXYD6 is unique in its expression in the inner ear, suggesting a role in endolymph composition.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
The mammalian inner ear is composed of two distinct sensory organs, the cochlea (the organ of hearing) and the vestibule (the organ of equilibrium). Proper functioning of the sensory hair cells depends on the ionic composition of endolymph and perilymph. Whereas perilymph resembles most other interstitial fluids in containing low K+ and high Na+, endolymph has an ionic composition characterized by high K+ and low Na+, similar to the intracellular fluid (1–3). K+ is central to the inner ear physiology, as it is the charge-carrying ion for the sensory transduction. In the cochlea, K+ is secreted by the marginal cells of the stria vascularis to maintain a very high concentration (150 mM) in the endolymph, the extracellular fluid bathing the stereocilia of the sensory hair cells. This results in a positive potential (+90 mV), which provides a large driving force for K+ entry into the sensory hair cell. The unique ionic composition of endolymph, along with a highly positive endocochlear potential, are thus essential to mechanoelectric transduction (by hair cells) and to normal hearing (4–6).

Maintenance of both the ionic composition of endolymph and the endocochlear potential depends on the activity of Na,K-ATPase (5, 6). The Na,K-ATPase is an ubiquitous enzyme consisting of an {alpha}- and a beta-subunit, which is responsible for the creation and maintenance of the Na+ and K+ gradients across the cell membrane by transporting three Na+ out of and two K+ into the cell. This function is crucial for cell survival and body homeostasis, because the Na+ gradient is used as an energy source to transport ions or solutes and is at the origin of the vectorial Na+ reabsorption in the kidney and of action potentials in excitable tissues. Regulation of the activity and expression of Na,K-ATPase is tight and governed by a variety of mechanisms. Short term regulation involves protein kinases and results in modulation of the cell surface expression of the Na,K-ATPase, whereas long term regulation, mediated by mineralocorticoid or thyroid hormones, leads to a change in the total number of Na,K-ATPase units (for review, see Refs. 7 and 8). Moreover, the existence of multiple {alpha} and beta isoforms permits the production of isozymes with different transport properties (9). Finally, recent experimental evidence shows that members of the FXYD protein family specifically associate with and modulate the transport properties of Na,K-ATPase (for review, see Refs. 10 and 11).

The mammalian FXYD family contains seven members that share a common signature sequence encompassing the transmembrane and adjacent regions (12). Most characterized FXYD proteins exhibit a similar structure with a single transmembrane domain and a type I orientation that is achieved, in some but not all cases, by the cleavage of an N-terminal signal peptide (for references, see Ref. 10). So far, six of the seven members of the FXYD protein family have been shown to regulate Na,K-ATPase in a tissue- and isozyme-specific way. Four FXYD proteins (FXYD1, FXYD3, FXYD4, and FXYD7) affect, in a distinct way, the apparent affinity for extracellular K+ of the Na,K-ATPase, which in the case of the brain-specific FXYD7 may be physiologically relevant in neuronal excitability (13). In addition, FXYD1 (phospholemman) (14), FXYD2 ({gamma}-subunit) (15–17), FXYD3 (18), and FXYD4 (19, 20) affect the apparent affinity for internal Na+ of Na,K-ATPase in a way that is consistent with the physiological demands of the tissues in which they are expressed. These results suggest that at least one common function of all FXYD proteins could be the regulation of Na,K-ATPase transport properties.

Given the key role of the Na,K-ATPase in the maintenance of the electrochemical composition of the endolymph and the modulatory role of FXYD proteins on its transport properties, we have looked in this study for the expression of several FXYD proteins in the inner ear. The only FXYD protein that we were able to detect by Western blot was FXYD6. We, therefore, characterized FXYD6, the last member of the FXYD family of unknown function. FXYD6, also known as phosphohippolin, was previously identified as a phospholemman-like protein expressed at high levels in brain and at a lesser level in lung, testis, and colon (21).

In this study, we investigated the association of FXYD6 with Na,K- and H,K-ATPases, the modulation of Na,K-ATPase function, and its cellular distribution in the inner ear and in PC12 cells. Our results show that FXYD6 has functional characteristics that are distinct from other FXYD proteins and that the protein may have a crucial role in endolymph homeostasis.


   EXPERIMENTAL PROCEDURES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
RESULTS
 DISCUSSION
 REFERENCES
 
cDNAs—Mouse FXYD6 cDNA, subcloned between the 5' and 3' domains of Xenopus beta-globin, was kindly provided by H. Garty (Weizmann Institute of Science). Cloning of rat Na,K-ATPase {alpha}1, {alpha}2, {alpha}3, beta1, and beta2 cDNAs has been described previously (9). cDNAs for the rabbit gastric H,K-ATPase {alpha}- and beta-subunits were kindly provided by G. Sachs (UCLA) and cDNA for rat colonic H,K-ATPase {alpha}-subunits by Fréderic Jaisser (INSERM, U478, Paris, France). All cDNAs were introduced into the pSD5 vector, and cRNAs were prepared by in vitro translation (22).

Protein Expression in Xenopus laevis Oocytes—Stages V and VI oocytes were obtained from X. laevis as described previously (23). cRNAs coding for mouse FXYD6, rat Na,K-ATPase {alpha}1-, {alpha}2-, {alpha}3-, beta1-, and beta2-subunits, or rabbit gastric or rat colonic H,K-ATPase {alpha} and gastric H,K-ATPase beta-subunits were injected into oocytes in different combinations, as described in the figure legends. To study protein expression and association, oocytes were incubated in modified Barth's solution in the presence of 0.7–1 mCi/ml [35S]methionine (Easy Tag Express [35S]Protein labeling kit, PerkinElmer Life Sciences). Oocytes were subjected to a 6-h pulse and to 24–48-h chase periods in modified Barth's solution containing 10 mM cold methionine. After the pulse and chase periods, microsomes were prepared as described previously (23) and subjected to immunoprecipitations with an FXYD6 antibody under non-denaturing conditions (see below).

Preparation of Extracts of Rat Inner Ear—Rat cochleae were isolated and ground in a mortar. The tissue was homogenized manually in a phosphate-buffered saline (pH 7.4) containing antiproteases (Roche Applied Science). The homogenate was diluted 1:1 with a lysis buffer containing 200 mM NaCl, 2% digitonin, 10 mM phenylmethylsulfonyl fluoride, 10 mM EDTA, 40 mM Tris-HCl, pH 7.4, for 1 h at 4°C and centrifuged at 10,000 x g for 5 min. The supernatants were used for immunoprecipitation and Western blotting.

Antibodies, Immunoprecipitation, and Western Blotting—A polyclonal FXYD6 antibody directed against a C-terminal glutathione S-transferase fusion peptide of mouse FXYD6 (62-CSFNQKPRAPGDEEAQVENLITTNAAEPQKAEN) was produced by Pascal Béguin. The corresponding cDNA region was cloned in-frame into the pGEX-4T1 vector. After purification using glutathione beads and dialysis against phosphate-buffered saline (PBS),2 the fusion protein was used for the immunization of rabbits (Cocalico Biologicals). This antibody was used for co-immunoprecipitation experiments of metabolically labeled Na,K- and H,K-ATPase expressed in Xenopus oocytes under non-denaturing conditions (24). Immunoprecipitates were resolved on 5–13% SDS-polyacrylamide gels or SDS-Tricine gels and revealed by fluorography. FXYD1 (14), FXYD2 (16), FXYD3 (18), FXYD4 (19), FXYD6, and FXYD7 (13) antibodies were used in Western blots to test the expression of FXYD proteins in the inner ear.

To demonstrate the association of FXYD6 with Na,K-ATPase in rat cochlea, 50 µg of the protein of cochlear extracts were incubated with a Na,K-ATPase {alpha}1-subunit antibody (Bioreagents) overnight at 4 °C. Immunoprecipitates were migrated on a 5–13% SDS-polyacrylamide gel and transferred onto nitrocellulose. FXYD6 was revealed with the FXYD6 antibody and chemiluminescence detection (ECL, Amersham Biosciences).

Electrophysiology—Electrophysiological measurements were performed 3 days after oocyte injection with rat Na,K-ATPase {alpha}1-beta1 or {alpha}1-beta2 cRNAs alone or together with FXYD6 cRNA by using the two-electrode voltage clamp technique. Measurements of the apparent external K+ affinity were carried out as described previously (16) in the presence of 1 µM ouabain, which inhibits the endogenous oocyte Na,K-pumps but not the expressed ouabain-resistant rat Na,K-ATPase isozymes. The maximal Na,K pump current and the apparent K+ affinity (K1/2 K+) measured in the presence of 90 mM external Na+ were obtained by fitting the Hill equation to the data with a Hill coefficient of 1.6 (25). Measurements of the apparent Na+ affinity of Na,K-ATPase in intact cells were performed as described previously (26) by co-expressing rat {alpha}1 with rat beta1 or beta2 cRNAs together with rat epithelial Na+ channel {alpha}-, beta-, and {gamma}-subunit cRNAs in the presence or absence of FXYD6 cRNA. The maximal current (Imax) and the apparent Na+ affinity (K1/2 Na+) were fitted by using the Hill equation and a Hill coefficient of 3 (26). Statistical analysis was performed by Student's t test.

Immunocytochemistry of FXYD6—The rats were deeply anesthetized and then fixed intra-aortically with 4% paraformaldehyde in 0.1 M phosphate buffer, pH 7.4. Cochleae were dissected, the oval and round windows were opened, and a hole was drilled at the apex before an overnight post-fixation at 4 °C. Cochleae were then transferred to a phosphate buffer containing 20% sucrose for cryoprotection and frozen. Fourteen-micrometer-thick sections were cut with a Reichert-Jung 2800 cryostat microtome and stored at -20 °C until use. The immunohistochemical procedures were similar to those described previously (27). The sections were rinsed (3–5 min) in PBS, preincubated 1 h in 30% normal goat serum with 0.3% Triton X-100, and incubated overnight at 4 °C with FXYD6 (1:500), Na,K-ATPase {alpha}1-subunit (1:100), and parvalbumin (Swants, 1:750) antibodies diluted in PBS with 1% normal goat serum. They were then rinsed in PBS (3–8 min) and incubated for 2 h with goat anti-rabbit IgGs conjugated to Alexa 488 (Molecular Probes) diluted 1:2000 in PBS, anti-mouse-conjugated CY3 (Molecular Probes), and anti-goat-conjugated CY5 (Molecular Probes). The sections were then rinsed in PBS (3–8 min) and mounted in Mowiol. For co-localization experiments of FXYD6 and barttin in the stria vascualris, a chicken barttin antibody (28) (kindly provided by Shinishi Uchida) was used at a dilution of 1:100. The sections were observed with a ZEISS LSM510 Meta laser-scanning confocal microscope. No immunostaining was observed when the primary antibody was omitted in the first incubation step or when preimmune sera were used.

PC12 Cell Culture—Rat pheochromocytoma (PC12) cells (kindly provided by Bernard Thorens, CIG, Lausanne, Switzerland), a cell model commonly used to study neurite formation, were maintained in 100-mm2 culture dishes in Dulbecco's modified Eagle's medium (Invitrogen) supplemented with 6% horse serum, 6% fetal bovine serum, 20 mM Hepes, 2 mM glutamine, 1 mM sodium-pyruvate, and 100 µg/ml penicillin/streptomycin at 37 °C, 5% CO2. To induce differentiation, culture medium was supplemented with nerve growth factor (10 µg/ml) for 1 week. For immunocytochemistry, cells were plated at 4 x 105 cells/well on 6-well glass slides. All samples were washed in PBS, fixed in 4% paraformaldehyde for 10 min, and rinsed with PBS. Cells were permeabilized using 0.3% Triton X-100, rinsed with PBS, and blocked with 3% bovine serum albumin in PBS for 1 h. Cells were then co-incubated with an FXYD6 antibody and Na,K-ATPase {alpha}1-subunit monoclonal antibody in 1% bovine serum albumin for 1 h at room temperature, washed with PBS, and incubated with fluorescence-labeled secondary antibodies (Alexa 488-conjugated goat anti-rabbit IgG, 1:100, and Cy3-conjugated goat anti-mouse, 1:150) for 1 h. Samples were rinsed with PBS and coverslipped with Fluorsave (Calbiochem). Immunolabeling was examined using a Zeiss LSM 510 Meta laser-scanning confocal microscope. Cellular extracts of PC12 cells were prepared in a lysis buffer containing 100 mM NaCl, 1% digitonin, 5 mM phenylmethylsulfonyl fluoride, 5 mM EDTA, 20 mM Tris-HCl, pH 7.4, and used for co-immunoprecipitation assays.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
DISCUSSION
 REFERENCES
 
Location of FXYD6 in the Cochlea and Interaction with Na,K-ATPase in Situ—Previously, expression of several FXYD RNAs has been reported in the inner ear of mice (29, 30). We performed Western blot analysis, which revealed that, of 6 FXYD proteins tested, only FXYD6 could be detected in extracts of rat inner ear (Fig. 1A). FXYD6 expression was low at developmental stage P0 but increased at P12 and P30 (Fig. 1A, lanes 17–19; Fig. 1B). FXYD6 was revealed as a doublet with a major band at ~20 kDa corresponding to the molecular mass of FXYD6 expressed in brain (21) and a minor band at ~18 kDa of unknown origin.


Figure 1
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  		FIGURE 1.
FXYD6 is associated with Na,K-ATPase in the inner ear. A, expression of FXYD6 in the inner ear. Five micrograms of the protein of inner ear extracts from developmental stage P0 (lanes 1, 5, 9, 13, 17, and 21), P12 (lanes 2, 6, 10, 14, 18, and 22), or P30 (lanes 3, 7, 11, 15, 19, and 23) or 10 µg of the protein of oocyte microsomes (lanes 4, 8, 12, 16, 20, and 24) were tested by Western blotting for the expression of FXYD proteins with specific antibodies to FXYD1, FXYD2, FXYD3, FXYD4, FXYD6, and FXYD7. B, aliquots (5 µg of protein) from inner ear extracts at different postnatal stages (P0, P12, and P30) were used in Western blots and probed with an FXYD6 antibody. C, rat inner ear slices (14 µm) were used for triple immunostaining with an FXYD6 antibody (green), and a polyclonal parvalbumin antibody (blue) to specifically immunolabel inner hair cells and radial dendrites (41) and a monoclonal Na,K-ATPase {alpha}1-subunit antibody (red). After merge, colocalization is indicated in yellow. Magnification is 10x. Scale bar = 10 µm. SG, spiral ganglion; IC, interdental cells; D, dendrites; OSC, outer sulcus cells; RP, root process; SV, striavascularis. D, aliquots (10 µg of protein) from extracts of inner ear at P0 were directly loaded on a Tricine gel (Input). Other samples of the inner ear at P0 or P12 were first immunoprecipitated (IP) (50–100 µg of protein) with an antibody against Na,K-ATPase {alpha}-subunit (IP {alpha}) or a preimmune serum (IP PI) (lower panel) under non-denaturing conditions. Proteins were transferred onto nitrocellulose membranes, and Western blot analysis was performed using an FXYD6 antibody.

 


Figure 2
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  		FIGURE 2.
FXYD6 is co-localized with Na,K-ATPase in the stria vascularis. A, rat inner ear slices were used for double immunostaining with a FXYD6 antibody (green) plus a monoclonal Na,K-ATPase {alpha}1 antibody (red). After merge, co-localization is indicated in yellow. Magnification is 63x. B, rat inner ear slices were used for double immunostaining with an FXYD6 antibody (red) and a chicken barttin antibody (green). After merge, co-localization is indicated in yellow. Magnification is 40x.

 
The cellular localization of the FXYD6 protein in the cochlea was investigated by immunohistochemistry. In the adult rat cochlea, FXYD6 was expressed in various epithelia bordering the endolymph (the interdental cell, the outer sulcus cells and their root process, and the stria vascularis) (Fig. 1C, green). Moreover, the protein was expressed in somas and dendrites of the spiral ganglion neurons. FXYD6 co-localized with Na,K-ATPase (Fig. 1C, red and merged) in several regions including the stria vascularis. A Na,K-ATPase {alpha}-subunit antibody was able to co-immunoprecipitate FXYD6 in the inner ear at developmental stages P0 and P12 (Fig. 1D), indicating that FXYD6 is associated with Na,K-ATPase.

Location of FXYD6 in the Stria Vascularis—FXYD6 is expressed in the stria vascularis where it co-localizes with Na,K-ATPase (Fig. 2A). A strong signal was diffusely detected in the middle part of the stria vascularis where basolateral membranes of marginal cells and apical membranes of intermediate cells are localized (see Fig. 6). The resolution of confocal microscopic analysis was not sufficient to precisely determine the localization of FXYD6, because the basolateral membrane of marginal cells and the apical membrane of intermediate cells are associated in close vicinity of ~150 –~200 Å and prominently invaginated (31, 32). Barttin is a beta-subunit of chloride channels specifically expressed in the basolateral membrane of marginal cells (33) (see Fig. 6). Co-immunolabeling of FXYD6 and barttin showed co-localization of the two proteins (Fig. 2B), suggesting the presence of FXYD6 in the basolateral membrane of marginal cells but not excluding expression in intermediate cells.

Subcellular Localization of FXYD6 in PC12 Cells—In addition to brain, Kadowaki et al. (21) have shown that FXYD6 is expressed in non-differentiated pheochromocytoma (PC12) cells. To verify whether FXYD6 co-localizes with Na,K-ATPase in these cells, we performed co-immunostaining with FXYD6 and Na,K-ATPase antibodies, which revealed co-localization of FXYD6 and Na,K-ATPase at the plasma membrane of non-differentiated PC12 cells (Fig. 3A, upper panels). Moreover, in differentiated PC12 cells of neuronal phenotype, FXYD6 colocalized with Na,K-ATPase in dendrites and the cell body (Fig. 3A, lower panels). Both in non-differentiated (Fig. 3B, left panel) and differentiated (Fig. 3B, right panel) PC12 cells, FXYD6 was associated with Na,K-ATPase, because it could be co-immunoprecipitated with a Na,K-ATPase {alpha}-subunit antibody.


Figure 3
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  		FIGURE 3.
Subcellular localization of FXYD6 in PC12 cells. A, confocal microscope analysis of non-differentiated (upper panel) and differentiated (lower panel) PC12 cells using double immunostaining with a FXYD6 antibody (green) and a monoclonal Na,K-ATPase {alpha}1 antibody (red). After merge, colocalization is indicated in yellow. B, extracts from non-differentiated (ND, left panel) or differentiated (D, right panel) PC12 cells were directly loaded on a SDS-polyacrylamide gel (10 µg of protein; Input) or first immunoprecipitated (100 µg of protein) with a Na,K-ATPase {alpha}1-subunit antibody (IP {alpha}) or with preimmune serum (IP PI) under non-denaturing conditions. Proteins were transferred onto nitrocellulose membranes, and Western blot analysis was performed using an FXYD6 antibody.

 
Association of FXYD6 with Na,K-ATPase and H,K-ATPase in Xenopus Oocytes—The association of FXYD6 with Na,K-ATPase was investigated after co-expression of FXYD6 in Xenopus oocytes together with Na,K-ATPase {alpha}1-beta1, {alpha}2-beta1, {alpha}3-beta1, or {alpha}1-beta2 isozymes. After metabolic labeling and various pulse-chase periods, microsomes were prepared and subjected to immunoprecipitations under nondenaturing conditions with an FXYD6 antibody. The FXYD6 antibody immunoprecipitated the 20-kDa form of FXYD6 (Fig. 4A). The intensity of the FXYD6 band was intrinsically low, because FXYD6 contains only 1 methionine, which can be metabolically labeled. Western blot analysis confirmed the efficient expression of FXYD6 and also revealed the 18-kDa form of FXYD6 observed in the inner ear (data not shown). Na,K-ATPase {alpha}- and beta-subunits of all {alpha}-beta1 isozymes could be co-immunoprecipitated with FXYD6 after the pulse and prolonged chase periods (Fig. 4A, lanes 1–21). Moreover, FXYD6 antibodies co-immunoprecipitated Na,K-ATPase {alpha}1-beta2 isozymes (Fig. 4B, lanes 1–6), which is the prominent Na,K-ATPase isozyme in the stria vascularis (34). Similar experiments were performed with gastric and non-gastric H,K-ATPase, and they showed that both H,K-ATPases co-expressed with FXYD6 could be co-immunoprecipitated with the FXYD6 antibody over prolonged chase periods (Fig. 4C).


Figure 4
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  		FIGURE 4.
Association of FXYD6 with P-type ATPases in Xenopus oocytes. Oocytes were injected with rat Na,K-ATPase {alpha}1-, {alpha}2-, or {alpha}3- and beta1(A)- or {alpha}1- and beta2(B)-subunit cRNAs (10 ng of {alpha} and 1 ng of beta cRNAs) in the absence or presence of 8 ng of FXYD6 cRNA. After cRNA injection, oocytes were labeled with [35S]methionine for 6 h followed by a 24- and 48-h chase period, and digitonin extracts were prepared. Non-denaturing immunoprecipitations were performed with an FXYD6 antibody, and the immunoprecipitates were resolved by SDS-PAGE and revealed by fluorography. C, oocytes were injected with rat colonic H,K-ATPase (c HKA) {alpha}-subunit (10 ng) and rabbit gastric H,K-ATPase beta-subunit (1 ng) cRNAs in the absence or presence of 8 ng of FXYD6 cRNA. Non-denaturing immunoprecipitations with an FXYD6 antibody were performed on digitonin extracts of metabolically labeled oocytes.

 
Functional Effects of FXYD6 on Na,K-ATPase Transport Properties—The functional effects of FXYD6 on Na,K-ATPase transport properties were investigated by electrophysiological measurements in Xenopus oocytes expressing rat Na,K-ATPase {alpha}1-beta1 or {alpha}1-beta2 isozymes with or without FXYD6.

As shown in Fig. 5A, Na,K-ATPase {alpha}1-beta2 isozymes expressed without FXYD6 had a significantly lower apparent K+ affinity than {alpha}1-beta1 isozymes, and the voltage dependence of the K1/2 K+ values was more pronounced at negative membrane potentials. FXYD6 produced a small but significant decrease in the apparent K+ affinity of the Na,K-ATPase {alpha}1-beta1 isozyme at slightly negative and positive membrane potentials. In contrast, FXYD6 produced a significant increase in the apparent K+ affinity of Na,K-ATPase {alpha}1-beta2 isozymes at negative membrane potentials (Fig. 5A). FXYD6 also had significant and distinct effects on the apparent affinity for internal Na+ of the two Na,K-ATPase isozymes and produced a 2-fold decrease in the apparent Na+ affinity of the {alpha}1-beta1 isozyme without significantly affecting maximal Na,K pump currents and a slight increase in the apparent Na+ affinity of the {alpha}1-beta2 isozyme (Fig. 5B).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
REFERENCES
 
In this paper, we provide evidence that FXYD6, the last uncharacterized FXYD protein, may also be a tissue-specific regulator of Na,K-ATPase, similar to other members of the FXYD protein family. Indeed, our results show that FXYD6 is associated with Na,K-ATPase in the inner ear and that it modifies the transport properties of different Na,K-ATPase isozymes after co-expression in Xenopus oocytes.

Expression of FXYD6 in the Inner Ear—RNA expression of several members of the FXYD family in the inner ear has been reported previously (29, 30). In this study, we show that, of 6 FXYD proteins, only FXYD6 was detected in this organ at the protein level. Thus, the other FXYD proteins are either not expressed or at much lower levels than FXYD6.


Figure 5
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  		FIGURE 5.
Effects of FXYD6 on the transport properties of Na,K-ATPase. A, three days after injection of rat Na,K-ATPase {alpha}1 (10 ng)- and beta1 (1 ng)- or {alpha}1 (10 ng)- and beta2 (1 ng)-subunit cRNAs in the absence or presence of FXYD6 cRNA (8 ng), K+-activation constants (K1/2K+) of the Na,K-ATPase were determined in the presence of 90 mM external Na+. Closed triangles, {alpha}1 + beta1 alone; open triangles, {alpha}1 + beta1 plus FXYD6; closed circles, {alpha}1 + beta2 alone; open circles, {alpha}1 + beta2 plus FXYD6. Shown are means ± S.E. of 20 oocytes from two different batches. * refers to statistical significance between {alpha}1 + beta1 alone and {alpha}1 + beta1 plus FXYD6 (p < 0.05) or {alpha}1 + beta2 alone and {alpha}1 + beta2 plus FXYD6 (p < 0.01). B, two days after injection of rat {alpha}1 and beta1 or {alpha}1 and beta2 cRNAs in the absence or presence of FXYD6 cRNA, epithelial Na+ channel subunit cRNAs ({alpha}, beta, and {gamma}; 0.3 ng of each) were injected. After overnight incubation, Na+ activation constants (K1/2 Na+) were determined at -50 mV. Shown are means ± S.E. from 9–10 oocytes from five different batches. ***, p < 0.001. Maximal Na,K-pump currents of Na,K-ATPase {alpha}1 + beta1 alone were 286 ± 27 mV and of Na,K-ATPase {alpha}1 + beta1 plus FXYD6 were 274 ± 18 mV. Maximal Na,K-pump currents of Na,K-ATPase {alpha}1 + beta2 alone were 196 ± 27 mV and of Na,K-ATPase {alpha}1 + beta2 plus FXYD6 were 140 ± 18 mV. For all electrophysiological measurements, endogenous, oocyte Na,K-ATPase was inhibited by the presence of 1 µM ouabain.

 
Interaction of FXYD6 with P-Type ATPases—A common feature of all FXYD proteins so far characterized is their interaction with Na,K-ATPase, both in expression systems and in situ (for review, see Refs. 10 and 11). FXYD6 shares this property but shows a broader specificity than other FXYD proteins. Similar to mouse FXYD3 (18), FXYD6 stably interacts also with both gastric and non-gastric H,K-ATPases after expression in Xenopus oocytes. In the cochlea, FXYD6 is localized in neurons and, interestingly, in the stria vascularis, which co-expresses Na,K-ATPase and gastric H,K-ATPase at the basolateral side of marginal cells (28) (Fig. 6). Further experiments are needed to confirm the potential association of FXYD6 and H,K-ATPase in this organ.


Figure 6
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  		FIGURE 6.
Schematic model of the cochlear stria vascularis, K+ secretion into endolymph, and distribution of Na,K-ATPase and FXYD6. For a detailed description, see "Discussion."

 
Putative Physiological Relevance of the Regulation of Na,K-ATPase by FXYD6 in the Maintenance of the Endolymph Electrochemical Composition—Compared with other FXYD proteins studied, association of FXYD6 modulates the transport properties of Na,K-ATPase in a distinct way. In view of the cellular localization of FXYD6 in the cochlea, we could predict at least two sites of action, which preferentially express either Na,K-ATPase {alpha}1-beta1 or {alpha}1-beta2 isozymes. Interestingly, our study shows that these two rat Na,K-ATPase isozymes have different functional properties (Fig. 5A). These results confirm our previous observation that beta isoforms can differentially modulate the transport properties of Na,K-ATPase (9, 25).

The first site of action of FXYD6 might be the auditory neurons, mainly expressing Na,K-ATPase {alpha}1-beta1 isozymes. Intracellular Na+ concentrations in dendrites increase significantly during neuronal activity, passing from 11 to 50 mM (35). The nearly 2-fold decrease in the apparent Na+ affinity, produced by the association of FXYD6 with Na,K-ATPase {alpha}1-beta1 isozymes, may be necessary for the efficient extrusion of Na+ during neuronal activity. Upon a rise in intracellular Na+ to 50 mM, Na,K-ATPase {alpha}1-beta1 isozymes with a K1/2 Na+ of ~10 mM (Fig. 5B) would transport at maximal rates. On the other hand, Na,K-ATPase with a K1/2 Na+ of 20 mM, due to association with FXYD6, would have the capacity to increase its transport rate at increased intracellular Na+ concentrations. A similar decrease in the apparent Na+ affinity of {alpha}1-beta1 isozymes is produced by FXYD1 (phospholemman) expressed in skeletal and heart muscle and has been related to efficient extrusion of increased intracellular Na+ concentrations during action potentials in contractile tissues (14). Of course, at present, we cannot exclude that, alternatively or in addition to the observed Na+ effect on {alpha}1-beta1 Na,K-ATPase isozyme, FXYD6 may be involved in other regulatory mechanisms of the expression or function of Na,K-ATPase isozymes in neuronal cells.

The second site of action of FXYD6 might be the stria vascularis. The stria vascularis is made up of three layers of cells, epithelial marginal cells, intermediate cells, and epithelial basal cells (Fig. 6). Marginal cells interdigitate with intermediate cells and are separated by the intrastrial space. K+ coming from the spiral ligament enter basal cells and intermediate cells via a dense network of gap junctions. Thereafter, it is secreted into the intrastrial space by potassium channels and taken up by marginal cells via Na,K-ATPase and other K+ transporters, e.g. H,K-ATPase. Finally, K+ is secreted into the scala media to form the endolymph. FXYD6 is strongly expressed in the marginal cells and possibly in the intermediate cells, which mainly express {alpha}1-beta2 Na,K-ATPase isozymes (34). {alpha}1-beta2 Na,K-ATPase complexes associated with FXYD6 with a high apparent affinity for Na+ might be necessary to maintain the low intracellular Na+ concentrations in marginal cells that have been reported by Ikeda et al. (36).

The intrastrial space is critical for the generation of the endocochlear potential. Indeed, the endocochlear potential is essentially a K+ equilibrium potential that is generated by the conjunction of two K+ channels, Kcnj10 (Kir 4.1) and MERG1a located in the intermediate cells of the stria vascularis in conjunction with the very low K+ concentration in the intrastrial fluid and a normally high K+ concentration in the cytosol of intermediate cells (37–39). Intermediate cells might act as glial cells in siphoning the extracellular K+ (40). Na,K-ATPase and, as suggested by Nie et al. (39), Kcnj10 may be implicated in this process, which would permit the secretion of K+ by the MERG1a channel. At 1 mM K+ in the intrastrial space (37), the {alpha}1-beta2 isozyme with a K1/2 K+ of ~1.7 mM (Fig. 5) would not efficiently transport K+ at a membrane potential of -110 mV characteristic of the intermediate cells (37). However, an FXYD6-associated {alpha}1-beta2 complex with a higher apparent affinity for K+ would be able to transport at a more efficient rate and keep a low K+ concentration in the intrastrial space.

In conclusion, in this study, we have characterized the last FXYD protein of unknown function. Our results show that FXYD6 associates with Na,K-ATPase as other FXYD proteins and has a distinct functional effect on Na,K-ATPase, indicating that all mammalian FXYD proteins associate with and modulate the transport properties of Na,K-ATPase in a tissue-specific way. The localization of FXYD6 in the inner ear suggests an important role in endolymph production.


   FOOTNOTES
 
* This study was supported by Grant 3100A0-107513 (to K. G.) from the Swiss National Science Foundation. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Back

1 To whom correspondence should be addressed: Department of Pharmacology and Toxicology, University of Lausanne, Rue du Bugnon 27, 1005 Lausanne, Switzerland. Tel.: 41-21-692-54-10; Fax: 41-21-692-53-55; E-mail: Kaethi.Geering{at}unil.ch.

2 The abbreviation used is: PBS, phosphate-buffered saline. Back


   ACKNOWLEDGMENTS
 
We thank Stéphanie Bibert and Jean-Daniel Horisberger for helpful discussion in the preparation of the manuscript. We also thank the Cellular Imaging Facility of the University of Lausanne, which permitted us to realize immunohistochemistry acquisitions. Anti-barttin antibody was kindly provided by Dr. Shinichi Uchida (Tokyo Medical and Dental University).



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 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
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