Structural and functional properties of two human FXYD3 (Mat-8) isoforms

Na+/K+-ATPase: More than an Electrogenic Pump

The sodium pump, or Na+/K+-ATPase (NKA), is an essential enzyme found in the plasma membrane of all animal cells. Its primary role is to transport sodium (Na+) and potassium (K+) ions across the cell membrane, using energy from ATP hydrolysis. This transport creates and maintains an electrochemical gradient, which is crucial for various cellular processes, including cell volume regulation, electrical excitability, and secondary active transport. Although the role of NKA as a pump was discovered and demonstrated several decades ago, it remains the subject of intense research. Current studies aim to delve deeper into several aspects of this molecular entity, such as describing its structure and mode of operation in atomic detail, understanding its molecular and functional diversity, and examining the consequences of its malfunction due to structural alterations. Additionally, researchers are investigating the effects of various substances that amplify or decrease its pumping activity. Beyond its role as a pump, growing evidence indicates that in various cell types, NKA also functions as a receptor for cardiac glycosides like ouabain. This receptor activity triggers the activation of various signaling pathways, producing significant morphological and physiological effects. In this report, we present the results of a comprehensive review of the most outstanding studies of the past five years. We highlight the progress made regarding this new concept of NKA and the various cardiac glycosides that influence it. Furthermore, we emphasize NKA's role in epithelial physiology, particularly its function as a receptor for cardiac glycosides that trigger intracellular signals regulating cell-cell contacts, proliferation, differentiation, and adhesion. We also analyze the role of NKA β-subunits as cell adhesion molecules in glia and epithelial cells.

FXYD3 functionally demarcates an ancestral breast cancer stem cell subpopulation with features of drug-tolerant persisters

The heterogeneity of cancer stem cells (CSCs) within tumors presents a challenge in therapeutic targeting. To decipher the cellular plasticity that fuels phenotypic heterogeneity, we undertook single-cell transcriptomics analysis in triple-negative breast cancer (TNBC) to identify subpopulations in CSCs. We found a subpopulation of CSCs with ancestral features that is marked by FXYD domain-containing ion transport regulator 3 (FXYD3), a component of the Na+/K+ pump. Accordingly, FXYD3+ CSCs evolve and proliferate, while displaying traits of alveolar progenitors that are normally induced during pregnancy. Clinically, FXYD3+ CSCs were persistent during neoadjuvant chemotherapy, hence linking them to drug-tolerant persisters (DTPs) and identifying them as crucial therapeutic targets. Importantly, FXYD3+ CSCs were sensitive to senolytic Na+/K+ pump inhibitors, such as cardiac glycosides. Together, our data indicate that FXYD3+ CSCs with ancestral features are drivers of plasticity and chemoresistance in TNBC. Targeting the Na+/K+ pump could be an effective strategy to eliminate CSCs with ancestral and DTP features that could improve TNBC prognosis.

A Monoclonal Human Alveolar Epithelial Cell Line ("Arlo") with Pronounced Barrier Function for Studying Drug Permeability and Viral Infections

In the development of orally inhaled drug products preclinical animal models regularly fail to predict pharmacological as well as toxicological responses in humans. Models based on human cells and tissues are potential alternatives to animal experimentation allowing for the isolation of essential processes of human biology and making them accessible in vitro. Here, the generation of a novel monoclonal cell line "Arlo," derived from the polyclonal human alveolar epithelium lentivirus immortalized cell line hAELVi via single-cell printing, and its characterization as a model for the human alveolar epithelium as well as a building block for future complex in vitro models is described. "Arlo" is systematically compared in vitro to primary human alveolar epithelial cells (hAEpCs) as well as to the polyclonal hAELVi cell line. "Arlo" cells show enhanced barrier properties with high transepithelial electrical resistance (TEER) of ≈3000 Ω cm2 and a potential difference (PD) of ≈30 mV under air-liquid interface (ALI) conditions, that can be modulated. The cells grow in a polarized monolayer and express genes relevant to barrier integrity as well as homeostasis as is observed in hAEpCs. Successful productive infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in a proof-of-principle study offers an additional, attractive application of "Arlo" beyond biopharmaceutical experimentation.

FXYD3 facilitates Na+ and liquid absorption across human airway epithelia by increasing the transport capacity of the Na/K ATPase

Na/K ATPase activity is essential for ion transport across epithelia. FXYD3, a γ subunit of the Na/K ATPase, is expressed in the airway, but its function remains undetermined. Single-cell RNA sequencing and immunohistochemistry revealed that FXYD3 localizes within the basolateral membrane of all airway epithelial cells. To study FXYD3 function, we reduced FXYD3 expression using siRNA. After permeabilizing the apical membrane with nystatin, epithelia pretreated with FXYD3-targeting siRNA had lower ouabain-sensitive short-circuit currents than control epithelia. FXYD3-targeting siRNA also reduced amiloride-sensitive short-circuit currents and liquid absorption across intact epithelia. These data are consistent with FXYD3 facilitating Na+ and liquid absorption. FXYD3 may be needed to maintain the high rates of Na+ and fluid absorption observed for airway and other FXYD3-expressing epithelia.

Structure and Function of Na,K-ATPase-The Sodium-Potassium Pump

Na,K-ATPase is an ubiquitous enzyme actively transporting Na-ions out of the cell in exchange for K-ions, thereby maintaining their concentration gradients across the cell membrane. Since its discovery more than six decades ago the Na-pump has been studied extensively and its vital physiological role in essentially every cell has been established. This article aims at providing an overview of well-established biochemical properties with a focus on Na,K-ATPase isoforms, its transport mechanism and principle conformations, inhibitors, and insights gained from crystal structures. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

Integrated Analysis of Microarray and RNA-Seq Data for the Identification of Hub Genes and Networks Involved in the Pancreatic Cancer

Pancreatic cancer (PaCa) is the seventh most fatal malignancy, with more than 90% mortality rate within the first year of diagnosis. Its treatment can be improved the identification of specific therapeutic targets and their relevant pathways. Therefore, the objective of this study is to identify cancer specific biomarkers, therapeutic targets, and their associated pathways involved in the PaCa progression. RNA-seq and microarray datasets were obtained from public repositories such as the European Bioinformatics Institute (EBI) and Gene Expression Omnibus (GEO) databases. Differential gene expression (DE) analysis of data was performed to identify significant differentially expressed genes (DEGs) in PaCa cells in comparison to the normal cells. Gene co-expression network analysis was performed to identify the modules co-expressed genes, which are strongly associated with PaCa and as well as the identification of hub genes in the modules. The key underlaying pathways were obtained from the enrichment analysis of hub genes and studied in the context of PaCa progression. The significant pathways, hub genes, and their expression profile were validated against The Cancer Genome Atlas (TCGA) data, and key biomarkers and therapeutic targets with hub genes were determined. Important hub genes identified included ITGA1, ITGA2, ITGB1, ITGB3, MET, LAMB1, VEGFA, PTK2, and TGFβ1. Enrichment analysis characterizes the involvement of hub genes in multiple pathways. Important ones that are determined are ECM–receptor interaction and focal adhesion pathways. The interaction of overexpressed surface proteins of these pathways with extracellular molecules initiates multiple signaling cascades including stress fiber and lamellipodia formation, PI3K-Akt, MAPK, JAK/STAT, and Wnt signaling pathways. Identified biomarkers may have a strong influence on the PaCa early stage development and progression. Further, analysis of these pathways and hub genes can help in the identification of putative therapeutic targets and development of effective therapies for PaCa.

FXYD proteins and sodium pump regulatory mechanisms

The sodium/potassium-ATPase (NKA) is the enzyme that establishes gradients of sodium and potassium across the plasma membrane. NKA activity is tightly regulated for different physiological contexts through interactions with single-span transmembrane peptides, the FXYD proteins. This diverse family of regulators has in common a domain containing a Phe-X-Tyr-Asp (FXYD) motif, two conserved glycines, and one serine residue. In humans, there are seven tissue-specific FXYD proteins that differentially modulate NKA kinetics as appropriate for each system, providing dynamic responsiveness to changing physiological conditions. Our understanding of how FXYD proteins contribute to homeostasis has benefitted from recent advances described in this review: biochemical and biophysical studies have provided insight into regulatory mechanisms, genetic models have uncovered remarkable complexity of FXYD function in integrated physiological systems, new posttranslational modifications have been identified, high-resolution structural studies have revealed new details of the regulatory interaction with NKA, and new clinical correlations have been uncovered. In this review, we address the structural determinants of diverse FXYD functions and the special roles of FXYDs in various physiological systems. We also discuss the possible roles of FXYDs in protein trafficking and regulation of non-NKA targets.

FXYD-3 expression in relation to local recurrence of rectal cancer

Purpose

In a previous study, the transmembrane protein FXYD-3 was suggested as a biomarker for a lower survival rate and reduced radiosensitivity in rectal cancer patients receiving preoperative radiotherapy. The purpose of preoperative irradiation in rectal cancer is to reduce local recurrence. The aim of this study was to investigate the potential role of FXYD-3 as a biomarker for increased risk for local recurrence of rectal cancer.

Materials and Methods

FXYD-3 expression was immunohistochemically examined in surgical specimens from a cohort of patients with rectal cancer who developed local recurrence (n = 48). The cohort was compared to a matched control group without recurrence (n = 81).

Results

Weak FXYD-3 expression was found in 106/129 (82%) of the rectal tumors and strong expression in 23/129 (18%). There was no difference in the expression of FXYD-3 between the patients with local recurrence and the control group. Furthermore there was no difference in FXYD-3 expression and time to diagnosis of local recurrence between patients who received preoperative radiotherapy and those without.

Conclusion

Previous findings indicated that FXYD-3 expression may be used as a marker of decreased sensitivity to radiotherapy or even overall survival. We were unable to confirm this in a cohort of rectal cancer patients who developed local recurrence.

FXYD5: Na(+)/K(+)-ATPase Regulator in Health and Disease

FXYD5 (Dysadherin, RIC) is a single span type I membrane protein that plays multiple roles in regulation of cellular functions. It is expressed in a variety of epithelial tissues and acts as an auxiliary subunit of the Na⁺/K⁺-ATPase. During the past decade, a correlation between enhanced expression of FXYD5 and tumor progression has been established for various tumor types. In this review, current knowledge on FXYD5 is discussed, including experimental data on the functional effects of FXYD5 on the Na⁺/K⁺-ATPase. FXYD5 modulates cellular junctions, influences chemokine production, and affects cell adhesion. The accumulated data may provide a basis for understanding the molecular mechanisms underlying FXYD5 mediated phenotypes.

AEG-1 expression is an independent prognostic factor in rectal cancer patients with preoperative radiotherapy: a study in a Swedish clinical trial

Background:

Preoperative radiotherapy (RT) is widely used to downstage rectal tumours, but the rate of recurrence varies significantly. Therefore, new biomarkers are needed for better treatment and prognosis. It has been shown that astrocyte elevated gene-1 (AEG-1) is a key mediator of migration, invasion, and treatment resistance. Our aim was to analyse the AEG-1 expression in relation to RT in rectal cancer patients and to test its radiosensitising properties.

Methods:

The AEG-1 expression was examined by immunohistochemistry in 158 patients from the Swedish clinical trial of RT. Furthermore, we inhibited the AEG-1 expression by siRNA in five colon cancer cell lines and measured the survival after irradiation by colony-forming assay.

Results:

The AEG-1 expression was increased in the primary tumours compared with the normal mucosa independently of the RT (P<0.01). High AEG-1 expression in the primary tumour of the patients treated with RT correlated independently with higher risk of distant recurrence (P=0.009) and worse disease-free survival (P=0.007). Downregulation of AEG-1 revealed a decreased survival after radiation in radioresistant colon cancer cell lines.

Conclusions:

The AEG-1 expression was independently related to distant recurrence and disease-free survival in rectal cancer patients with RT and could therefore be a marker to discriminate patients for distant relapse.

Overexpression of FXYD-3 is involved in the tumorigenesis and development of esophageal squamous cell carcinoma

Objective. To investigate the association of FXYD-3 expression with clinicopathological variables and PINCH in patients with ESCC. Patients and Methods. Expression of FXYD-3 protein was immunohistochemically examined in normal esophageal mucous (n = 20) and ESCC (n = 64). Results. Expression of FXYD-3 in the cytoplasm markedly increased from normal esophageal epithelial cells to primary ESCC (P = 0.001). The expression of FXYD-3 was correlated with TNM stages and depth of tumor invasion. Furthermore, the cases with lymph node metastasis tended to show a higher frequency of positive expression than those without metastasis (P = 0.086), and FXYD-3 expression tended to be positively related to the expression of PINCH (P = 0.063). Moreover, the cases positive for both proteins had the highest frequency of lymph node metastasis (P = 0.001). However, FXYD-3 expression was not correlated with patient's gender (P = 0.847), age (P = 0.876), tumor location (P = 0.279), size (P = 0.771), grade of differentiation (P = 0.279), and survival (P = 0.113). Conclusion. Overexpression of FXYD-3 in the cytoplasm may play an important role in the tumorigenesis and development in the human ESCC, particularly in combination with PINCH expression.

Down-regulation of FXYD3 is induced by transforming growth factor-β signaling via ZEB1/δEF1 in human mammary epithelial cells

FXYD3, a regulator of Na, K-ATPase, was identified as an mRNA overexpressed in murine breast cancers induced by neu oncogene, which had inactivated transforming growth factor (TGF)-β signaling due to the defect of TGF-β receptor I (TβRI) expression. To elucidate whether the expression of FXYD3 mRNA was regulated by TGF-β signaling, we used a normal human mammary epithelial cell line, MCF-10A which responds to TGF-β and tumor necrosis factor (TNF)-α, followed by induction of epithelial-to-mesenchymal transition (EMT). Here, we showed that FXYD3 at plasma membrane in epithelial state of MCF-10A cells was decreased by treatment of TGF-β and TNF-α. The repression of FXYD3 mRNA induced by TGF-β and TNF-α in MCF-10A cells was abolished by TβRI inhibitor or Smad3 inhibitor, but not by small interfering RNA (siRNA) for Smad2. In addition, expression level of FXYD3 mRNA was up-regulated by the silencing of ZEB1/δEF1 transcriptional repressor which was a down-stream target gene of TGF-β and an inducer of EMT. On the other hand, expression level and cellular localization of E-cadherin and N-cadherin were not changed by siRNA for FXYD3 in MCF-10A and human breast cancer MCF-7 cells. These results suggest that FXYD3 is a target gene of TGF-β signaling through ZEB1/δEF1, but is not directly involved in EMT.

Post-transcriptional control of Na,K-ATPase activity and cell growth by a splice variant of FXYD2 protein with modified mRNA

In kidney, FXYD proteins regulate Na,K-ATPase in a nephron segment-specific way. FXYD2 is the most abundant renal FXYD but is not expressed in most renal cell lines unless induced by hypertonicity. Expression by transfection of FXYD2a or FXYD2b splice variants in NRK-52E cells reduces the apparent Na(+) affinity of the Na,K-ATPase and slows the cell proliferation rate. Based on RT-PCR, mRNAs for both splice variants were expressed in wild type NRK-52E cells as low abundance species. DNA sequencing of the PCR products revealed a base alteration from C to T in FXYD2b but not FXYD2a from both untreated and hypertonicity-treated NRK-52E cells. The 172C→T sequence change exposed a cryptic KKXX endoplasmic reticulum retrieval signal via a premature stop codon. The truncation affected trafficking of FXYD2b and its association with Na,K-ATPase and blocked its effect on enzyme kinetics and cell growth. The data may be explained by altered splicing or selective RNA editing of FXYD2b, a supplementary process that would ensure that it was inactive even if transcribed and translated, in these cells that normally express only FXYD2a. 172C→T mutation was also identified after mutagenesis of FXYD2b by error-prone PCR coupled with a selection for cell proliferation. Furthermore, the error-prone PCR alone introduced the mutation with high frequency, implying a structural peculiarity. The data confirm truncation of FXYD2b as a potential mechanism to regulate the amount of FXYD2 at the cell surface to control activity of Na,K-ATPase and cell growth.

Translocation of Pseudomonas aeruginosa from the intestinal tract is mediated by the binding of ExoS to an Na,K-ATPase regulator, FXYD3

The intestinal tract is considered the most important reservoir of Pseudomonas aeruginosa in intensive care units (ICUs). Gut colonization by P. aeruginosa underlies the development of invasive infections such as gut-derived sepsis. Intestinal colonization by P. aeruginosa is associated with higher ICU mortality rates. The translocation of endogenous P. aeruginosa from the colonized intestinal tract is an important pathogenic phenomenon. Here we identify bacterial and host proteins associated with bacterial penetration through the intestinal epithelial barrier. We first show by comparative genomic hybridization analysis that the exoS gene, encoding the type III effector protein, ExoS, was specifically detected in a clinical isolate that showed higher virulence in silkworms following midgut injection. We further show using a silkworm oral infection model that exoS is required both for virulence and for bacterial translocation from the midgut to the hemolymph. Using a bacterial two-hybrid screen, we show that the mammalian factor FXYD3, which colocalizes with and regulates the function of Na,K-ATPase, directly binds ExoS. A pulldown assay revealed that ExoS binds to the transmembrane domain of FXYD3, which also interacts with Na,K-ATPase. Na,K-ATPase controls the structure and barrier function of tight junctions in epithelial cells. Collectively, our results suggest that ExoS facilitates P. aeruginosa penetration through the intestinal epithelial barrier by binding to FXYD3 and thereby impairing the defense function of tight junctions against bacterial penetration.

Identification of zebrafish Fxyd11a protein that is highly expressed in ion-transporting epithelium of the gill and skin and its possible role in ion homeostasis

FXYD proteins, small single-transmembrane proteins, have been proposed to be auxiliary regulatory subunits of Na⁺–K⁺-ATPase and have recently been implied in ion osmoregulation of teleost fish. In freshwater (FW) fish, numerous ions are actively taken up through mitochondrion-rich cells (MRCs) of the gill and skin epithelia, using the Na⁺ electrochemical gradient generated by Na⁺–K⁺-ATPase. In the present study, to understand the molecular mechanism for the regulation of Na⁺–K⁺-ATPase in MRCs of FW fish, we sought to identify FXYD proteins expressed in MRCs of zebrafish. Reverse-transcriptase PCR studies of adult zebrafish tissues revealed that, out of eight fxyd genes found in zebrafish database, only zebrafish fxyd11 (zfxyd11) mRNA exhibited a gill-specific expression. Double immunofluorescence staining showed that zFxyd11 is abundantly expressed in MRCs rich in Na⁺–K⁺-ATPase (NaK-MRCs) but not in those rich in vacuolar-type H⁺-transporting ATPase. An in situ proximity ligation assay demonstrated its close association with Na⁺–K⁺-ATPase in NaK-MRCs. The zfxyd11 mRNA expression was detectable at 1 day postfertilization, and its expression levels in the whole larvae and adult gills were regulated in response to changes in environmental ionic concentrations. Furthermore, knockdown of zFxyd11 resulted in a significant increase in the number of Na⁺–K⁺-ATPase–positive cells in the larval skin. These results suggest that zFxyd11 may regulate the transport ability of NaK-MRCs by modulating Na⁺–K⁺-ATPase activity, and may be involved in the regulation of body fluid and electrolyte homeostasis.

The physiological significance of the cardiotonic steroid/ouabain-binding site of the Na,K-ATPase

The Na,K-ATPase is the membrane "pump" that generates the Na(+) and K(+) gradients across the plasma membrane that drives many physiological processes. This enzyme is highly sensitive to inhibition by cardiotonic steroids, most notably the digitalis/ouabain class of compounds, which have been used for centuries to treat congestive heart failure and arrhythmias. The amino acids that constitute the ouabain-binding site are highly conserved across the evolutionary spectrum. This could be fortuitous or could result from this site being conserved because it has an important biological function. New physiological approaches using genetically engineered mice are being used to define the biological significance of the "receptor function" of the Na,K-ATPase and its regulation by potential endogenous cardiotonic steroid-like compounds. These studies extend the reach of earlier studies involving the biochemical purification of endogenous regulatory ligands.

Down-regulation of FXYD3 expression in human lung cancers: its mechanism and potential role in carcinogenesis

FXYD3 is a FXYD-containing Na,K-ATPase ion channel regulator first identified as a protein overexpressed in murine breast tumors initiated by oncogenic ras or neu. However, our preliminary study revealed that FXYD3 expression was down-regulated in oncogenic KRAS-transduced airway epithelial cells. This contradiction led us to investigate the role of FXYD3 in carcinogenesis of the lung. FXYD3 mRNA and protein levels were lower in most of the lung cancer cell lines than in either the noncancerous lung tissue or airway epithelial cells. Protein levels were also lower in a considerable proportion of primary lung cancers than in nontumoral airway epithelia; FXYD3 expression levels decreased in parallel with the dedifferentiation process. Also, a somatic point mutation, g55c (D19H), was found in one cell line. Forced expression of the wild-type FXYD3, but not the mutant, restored the well-demarcated distribution of cortical actin in cancer cells that had lost FXYD3 expression, suggesting FXYD3 plays a role in the maintenance of cytoskeletal integrity. However, no association between FXYD3 expression and its promoter's methylation status was observed. Therefore, inactivation of FXYD3 through a gene mutation or unknown mechanism could be one cause of the atypical shapes of cancer cells and play a potential role in the progression of lung cancer.

A link between FXYD3 (Mat-8)-mediated Na,K-ATPase regulation and differentiation of Caco-2 intestinal epithelial cells

FXYD3 (Mat-8) proteins are regulators of Na,K-ATPase. In normal tissue, FXYD3 is mainly expressed in stomach and colon, but it is also overexpressed in cancer cells, suggesting a role in tumorogenesis. We show that FXYD3 silencing has no effect on cell proliferation but promotes cell apoptosis and prevents cell differentiation of human colon adenocarcinoma cells (Caco-2), which is reflected by a reduction in alkaline phosphatase and villin expression, a change in several other differentiation markers, and a decrease in transepithelial resistance. Inhibition of cell differentiation in FXYD3-deficient cells is accompanied by an increase in the apparent Na+ and K+ affinities of Na,K-ATPase, reflecting the absence of Na,K-pump regulation by FXYD3. In addition, we observe a decrease in the maximal Na,K-ATPase activity due to a decrease in its turnover number, which correlates with a change in Na,K-ATPase isozyme expression that is characteristic of cancer cells. Overall, our results suggest an important role of FXYD3 in cell differentiation of Caco-2 cells. One possibility is that FXYD3 silencing prevents proper regulation of Na,K-ATPase, which leads to perturbation of cellular Na+ and K+ homeostasis and changes in the expression of Na,K-ATPase isozymes, whose functional properties are incompatible with Caco-2 cell differentiation.

Structural arrangement and conformational dynamics of the gamma subunit of the Na+/K+-ATPase

The Na+/K+-ATPase couples the chemical energy in ATP to transport Na+ and K+ across the plasma membrane against a concentration gradient. The ion pump is composed of two mandatory subunits: the alpha subunit, which is the major catalytic subunit, and the beta subunit, which is required for proper trafficking of the complex to the plasma membrane. In some tissues, the ion pump also contains an optional third subunit, gamma, which modulates the pump activity. To examine the conformational dynamics of the gamma subunit during ion transport and its position in relation to the alpha and the beta subunits, we have used fluorescence resonance energy transfer under voltage clamp conditions. From these experiments, evidence is provided that the gamma subunit is located adjacent to the M2-M6-M9 pocket of the alpha subunit at the transmembrane-extracellular interface. We have also used fluorescence resonance energy transfer to investigate the relative movement of the three subunits as the ion pump shuttles between the two main conformational states, E1 and E2, as described by the Albers-Post scheme. The results from this study suggest that there is no relative change in distance between the alpha and gamma subunits but there is a relative change in distance between the beta and gamma subunits during the E2 to E1 transition. It was also observed that labeling the gamma subunit at specific residues with fluorophores induces a decrease in K+-induced stationary current. This result could be due to a perturbation in the K+ branch of the reaction cycle of the pump, representing a new way to inhibit the pump.

Structural and functional interactions between FXYD5 and the Na+-K+-ATPase

FXYD5 is a member of a family of tissue-specific regulators of the Na(+)-K(+)-ATPase expressed in kidney tubules. Previously, we have shown that FXYD5 interacts with the alphabeta-subunits of the Na(+)-K(+)-ATPase and increases its V(max) (Lubarski I, Pihakaski-Maunsbach K, Karlish SJ, Maunsbach AB, Garty H. J Biol Chem 280: 37717-37724, 2005). The current study further characterizes structural interaction and structure-function relationships of FXYD5. FXYD5/FXYD4 chimeras expressed in Xenopus laevis oocytes have been used to demonstrate that both the high-affinity association with the pump and the increase in V(max) are mediated by the transmembrane domain of FXYD5. Several amino acids that participate in the high-affinity interaction between FXYD5 and the alpha-subunit of the Na(+)-K(+)-ATPase have been identified. The data suggest that different FXYD proteins interact similarly with the Na(+)-K(+)-ATPase and their transmembrane domains play a key role in both the structural interactions and functional effects. Other experiments have identified at least one splice variant of FXYD5 with 10 additional amino acids at the COOH terminus, suggesting the possibility of other functional effects not mediated by the transmembrane domain. FXYD5 could be specifically bound to wheat germ agglutinin beads, indicating that it is glycosylated. However, unlike previous findings in metastatic cells, such glycosylation does not evoke a large increase in the size of the protein expressed in native epithelia and X. laevis oocytes.

Dynamic expression of FXYD6 in the inner ear suggests a role of the protein in endolymph homeostasis and neuronal activity

A key protein in the production and in the maintenance of the endocochlear potential is the Na,K-ATPase. Previously, we have shown that FXYD6 is a modulator of the Na,K-ATPase expressed in the inner ear (Delprat et al. [2007] J Biol Chem 282:7450-7456). To investigate the potential role of FXYD6 in inner ear function, we studied the developmental expression of FXYD6. Reverse transcriptase-polymerase chain reaction analysis demonstrates that FXYD6 is present as two splice variants. Both variants coimmunoprecipitate with Na,K-ATPase after expression in Xenopus oocytes. Immunohistochemistry of the cochlea (from birth to postnatal day 30) shows that FXYD6 is expressed in several epithelial cells important for endolymph homeostasis. Marked similarities were found in the developmental expression patterns of FXYD6 and Na,K-ATPase, suggesting functional cooperation between the two proteins in the generation and maintenance of the endocochlear potential and ion composition of the endolymph.