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

Association analysis of FXYD5 with prognosis and immunological characteristics across pan-cancer

Background

The FXYD domain-containing ion transport regulator 5 (FXYD5) gene is a cancer promoter. However, evidence for an association between FXYD5 and various types of cancer is still lacking. Using multi-omics bioinformatics, our study aimed to reveal the expression distribution, prognostic value, immune infiltration correlation, and molecular functions of FXYD5.

Methods

Using pan-cancer multi-omics data (including The Cancer Genome Atlas, PrognoScan, Gene Expression Profiling Interactive Analysis, cBioPortal, Gene Expression Omnibus, TIMER and scTIME Portal), we assessed the differences in the expression and prognostic value of FXYD5 in malignant tumors. Furthermore, at the single-cell level, we analyze the expression distribution of FXYD5 across different cell types within the tumor microenvironment, and its relationship with the immune microenvironment. Finally, focusing on ovarian cancer, we conducted preliminary validation of the above findings using cell and molecular biology techniques.

Results

Our results indicated that FXYD5 was up-regulated in various tumor types and was positively associated with tumor progression. We also revealed that FXYD5 was ubiquitously expressed in microenvironmental cells at the single-cell level, and its upregulation was associated with enhanced immune infiltration, cancer-associated fibroblast infiltration, and dysfunction of tumor-infiltrating cytotoxic T lymphocyte. Additionally, its expression was positively correlated with immune checkpoint genes, DNA mismatch repair genes, MSI (microsatellite instability) and TMB (tumor mutational burden) across various cancers. Its higher expression in cytotoxic T lymphocytes attenuated its ability to predict patient survival with PD-L1 (programmed death-ligand 1) blockade therapy, and FXYD5 was found to be a potential regulator of tumor immune escape and resistance to cancer immunotherapies. Based on GSEA (gene set enrichment analysis) and experimental verification, FXYD5 activated TGF-β/SMAD signaling and drove EMT (epithelial-mesenchymal transition) to promote ovarian cancer progression.

Conclusion

In summary, our study revealed that FXYD5-TGFβ axis may coregulate the interaction between tumors, CAFs (carcinoma-associated fibroblasts) and immune cells to reshape the tumor immune microenvironment and promote tumorigenesis and tumor progression. Thus, FXYD5 could be used as an immune-related biomarker for diagnosing and predicting the prognosis of multiple cancer types. Therefore, our findings suggest that targeting FXYD5 in TME (tumor microenvironment) may be a promising therapeutic strategy.

AMPK and glucose deprivation exert an isoform-specific effect on the expression of Na+,K+-ATPase subunits in cultured myotubes

In skeletal muscle, Na+,K+-ATPase (NKA), a heterodimeric (α/β) P-type ATPase, has an essential role in maintenance of Na+ and K+ homeostasis, excitability, and contractility. AMP-activated protein kinase (AMPK), an energy sensor, increases the membrane abundance and activity of NKA in L6 myotubes, but its potential role in regulation of NKA content in skeletal muscle, which determines maximum capacity for Na+ and K+ transport, has not been clearly delineated. We examined whether energy stress and/or AMPK affect expression of NKA subunits in rat L6 and primary human myotubes. Energy stress, induced by glucose deprivation, increased protein content of NKAα1 and NKAα2 in L6 myotubes, while decreasing the content of NKAα1 in human myotubes. Pharmacological AMPK activators (AICAR, A-769662, and diflunisal) modulated expression of NKA subunits, but their effects only partially mimicked those that occurred in response to glucose deprivation, indicating that AMPK does not mediate all effects of energy stress on NKA expression. Gene silencing of AMPKα1/α2 increased protein levels of NKAα1 in L6 myotubes and NKAα1 mRNA levels in human myotubes, while decreasing NKAα2 protein levels in L6 myotubes. Collectively, our results suggest a role for energy stress and AMPK in modulation of NKA expression in skeletal muscle. However, their modulatory effects were not conserved between L6 myotubes and primary human myotubes, which suggests that coupling between energy stress, AMPK, and regulation of NKA expression in vitro depends on skeletal muscle cell model.

High-Intensity Training Represses FXYD5 and Glycosylates Na,K-ATPase in Type II Muscle Fibres, Which Are Linked with Improved Muscle K+ Handling and Performance

Na+/K+ ATPase (NKA) comprises several subunits to provide isozyme heterogeneity in a tissue-specific manner. An abundance of NKA α, β, and FXYD1 subunits is well-described in human skeletal muscle, but not much is known about FXYD5 (dysadherin), a regulator of NKA and β1 subunit glycosylation, especially with regard to fibre-type specificity and influence of sex and exercise training. Here, we investigated muscle fibre-type specific adaptations in FXYD5 and glycosylated NKAβ1 to high-intensity interval training (HIIT), as well as sex differences in FXYD5 abundance. In nine young males (23.8 ± 2.5 years of age) (mean ± SD), 3 weekly sessions of HIIT for 6 weeks enhanced muscle endurance (220 ± 102 vs. 119 ± 99 s, p < 0.01) and lowered leg K+ release during intense knee-extensor exercise (0.5 ± 0.8 vs. 1.0 ± 0.8 mmol·min–1, p < 0.01) while also increasing cumulated leg K+ reuptake 0–3 min into recovery (2.1 ± 1.5 vs. 0.3 ± 0.9 mmol, p < 0.01). In type IIa muscle fibres, HIIT lowered FXYD5 abundance (p < 0.01) and increased the relative distribution of glycosylated NKAβ1 (p < 0.05). FXYD5 abundance in type IIa muscle fibres correlated inversely with the maximal oxygen consumption (r = –0.53, p < 0.05). NKAα2 and β1 subunit abundances did not change with HIIT. In muscle fibres from 30 trained males and females, we observed no sex (p = 0.87) or fibre type differences (p = 0.44) in FXYD5 abundance. Thus, HIIT downregulates FXYD5 and increases the distribution of glycosylated NKAβ1 in type IIa muscle fibres, which is likely independent of a change in the number of NKA complexes. These adaptations may contribute to counter exercise-related K+ shifts and enhance muscle performance during intense exercise.

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.

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.

Effect of differentiation, de novo innervation, and electrical pulse stimulation on mRNA and protein expression of Na+,K+-ATPase, FXYD1, and FXYD5 in cultured human skeletal muscle cells

Denervation reduces the abundance of Na⁺,K⁺-ATPase (NKA) in skeletal muscle, while reinnervation increases it. Primary human skeletal muscle cells, the most widely used model to study human skeletal muscle in vitro, are usually cultured as myoblasts or myotubes without neurons and typically do not contract spontaneously, which might affect their ability to express and regulate NKA. We determined how differentiation, de novo innervation, and electrical pulse stimulation affect expression of NKA (α and β) subunits and NKA regulators FXYD1 (phospholemman) and FXYD5 (dysadherin). Differentiation of myoblasts into myotubes under low serum conditions increased expression of myogenic markers CD56 (NCAM1), desmin, myosin heavy chains, dihydropyridine receptor subunit α_1S, and SERCA2 as well as NKAα2 and FXYD1, while it decreased expression of FXYD5 mRNA. Myotubes, which were innervated de novo by motor neurons in co-culture with the embryonic rat spinal cord explants, started to contract spontaneously within 7–10 days. A short-term co-culture (10–11 days) promoted mRNA expression of myokines, such as IL-6, IL-7, IL-8, and IL-15, but did not affect mRNA expression of NKA, FXYDs, or myokines, such as musclin, cathepsin B, meteorin-like protein, or SPARC. A long-term co-culture (21 days) increased the protein abundance of NKAα1, NKAα2, FXYD1, and phospho-FXYD1^Ser68 without attendant changes in mRNA levels. Suppression of neuromuscular transmission with α-bungarotoxin or tubocurarine for 24 h did not alter NKA or FXYD mRNA expression. Electrical pulse stimulation (48 h) of non-innervated myotubes promoted mRNA expression of NKAβ2, NKAβ3, FXYD1, and FXYD5. In conclusion, low serum concentration promotes NKAα2 and FXYD1 expression, while de novo innervation is not essential for upregulation of NKAα2 and FXYD1 mRNA in cultured myotubes. Finally, although innervation and EPS both stimulate contractions of myotubes, they exert distinct effects on the expression of NKA and FXYDs.

The role of AMPK in regulation of Na+,K+-ATPase in skeletal muscle: does the gauge always plug the sink?

AMP-activated protein kinase (AMPK) is a cellular energy gauge and a major regulator of cellular energy homeostasis. Once activated, AMPK stimulates nutrient uptake and the ATP-producing catabolic pathways, while it suppresses the ATP-consuming anabolic pathways, thus helping to maintain the cellular energy balance under energy-deprived conditions. As much as ~ 20-25% of the whole-body ATP consumption occurs due to a reaction catalysed by Na⁺,K⁺-ATPase (NKA). Being the single most important sink of energy, NKA might seem to be an essential target of the AMPK-mediated energy saving measures, yet NKA is vital for maintenance of transmembrane Na⁺ and K⁺ gradients, water homeostasis, cellular excitability, and the Na⁺-coupled transport of nutrients and ions. Consistent with the model that AMPK regulates ATP consumption by NKA, activation of AMPK in the lung alveolar cells stimulates endocytosis of NKA, thus suppressing the transepithelial ion transport and the absorption of the alveolar fluid. In skeletal muscles, contractions activate NKA, which opposes a rundown of transmembrane ion gradients, as well as AMPK, which plays an important role in adaptations to exercise. Inhibition of NKA in contracting skeletal muscle accentuates perturbations in ion concentrations and accelerates development of fatigue. However, different models suggest that AMPK does not inhibit or even stimulates NKA in skeletal muscle, which appears to contradict the idea that AMPK maintains the cellular energy balance by always suppressing ATP-consuming processes. In this short review, we examine the role of AMPK in regulation of NKA in skeletal muscle and discuss the apparent paradox of AMPK-stimulated ATP consumption.

A FXYD5/TGF‑β/SMAD positive feedback loop drives epithelial‑to‑mesenchymal transition and promotes tumor growth and metastasis in ovarian cancer

Epithelial ovarian cancer is aggressive and lacks effective prognostic indicators or therapeutic targets. In the present study, using immunohistochemistry and bioinformatics analysis on ovarian cancer tissue data from The Obstetrics and Gynecology Hospital of Fudan University and The Cancer Genome Atlas database, it was identified that FXYD domain‑containing ion transport regulator 5 (FXYD5) expression was upregulated in the SKOV3‑IP cell line compared with its parental cell line, SKOV3, and in ovarian cancer tissues compared with in normal tissues. In addition, FXYD5 upregulation was predictive of poor patient survival. Furthermore, through various in vitro (Transwell assay, clonogenic assay and western blot analysis) and in vivo (nude mouse model) experiments, it was demonstrated that FXYD5 promoted the metastasis of ovarian cancer cells. Mechanistically, RNA sequencing, western blot analysis, a luciferase reporter assay and chromatin immunoprecipitation were performed to reveal that FXYD5 dispersed the SMAD7‑SMAD specific E3 ubiquitin protein ligase 2‑TGF‑β receptor 1 (TβR1) complex, deubiquitinated and stabilized TβR1, and subsequently enhanced transforming growth factor‑β (TGF‑β) signaling and sustained TGF‑β‑driven epithelial‑mesenchymal transition (EMT). The TGF‑β‑activated SMAD3/SMAD4 complex was in turn directly recruited to the FXYD5 promoter region, interacted with specific SMAD‑binding elements, and then promoted FXYD5 transcription. In brief, FXYD5 positively regulated TGF‑β/SMADs signaling activities, which in turn induced FXYD5 expression, creating a positive feedback loop to drive EMT in the process of ovarian cancer progression. Collectively, the findings of the present study suggested a mechanism through which FXYD5 serves a critical role in the constitutive activation of the TGF‑β/SMADs signaling pathways in ovarian cancer, and provided a promising therapeutic target for human ovarian cancer.

Hormonal regulation of Na+-K+-ATPase from the evolutionary perspective

Na⁺-K⁺-ATPase, an α/β heterodimer, is an ancient enzyme that maintains Na⁺ and K⁺ gradients, thus preserving cellular ion homeostasis. In multicellular organisms, this basic housekeeping function is integrated to fulfill the needs of specialized organs and preserve whole-body homeostasis. In vertebrates, Na⁺-K⁺-ATPase is essential for many fundamental physiological processes, such as nerve conduction, muscle contraction, nutrient absorption, and urine excretion. During vertebrate evolution, three key developments contributed to diversification and integration of Na⁺-K⁺-ATPase functions. Generation of novel α- and β-subunits led to formation of multiple Na⁺-K⁺-ATPase isoenyzmes with distinct functional characteristics. Development of a complex endocrine system enabled efficient coordination of diverse Na⁺-K⁺-ATPase functions. Emergence of FXYDs, small transmembrane proteins that regulate Na⁺-K⁺-ATPase, opened new ways to modulate its function. FXYDs are a vertebrate innovation and an important site of hormonal action, suggesting they played an especially prominent role in evolving interaction between Na⁺-K⁺-ATPase and the endocrine system in vertebrates.

Dysadherin specific drug conjugates for the treatment of thyroid cancers with aggressive phenotypes

Background

EDC1 is a novel type of antibody-drug conjugate which binds and inhibits the Na,K-ATPase on the surface of cancer cells expressing dysadherin. The purpose of this study was to determine the expression of dysadherin in different types of thyroid carcinoma, and evaluate the therapeutic potential of EDC1 for thyroid carcinomas.

Methods

Thyroid tissues from 158 patients were examined for dysadherin expression and correlation with clinicopathological features. Thyroid cancer cell lines were examined for the expression of dysadherin and effective dose range of EDC1.

RESULTS

One in 53 benign thyroid tissues and 62% of thyroid cancers expressed dysadherin. All anaplastic and a majority of papillary thyroid cancers overexpressed dysadherin, while 25% of follicular thyroid cancers was found to be positive for dysadherin. Dysadherin expression significantly correlated with extrathyroidal extension and lymph node metastases in papillary thyroid cancer. Five of six human thyroid cancer cell lines analyzed expressed high levels of dysadherin. Of those cells lines sensitive to EDC1, half maximal effective concentrations (EC50) were observed to be between 0.125 nM and 1 nM.

Conclusions

EDC1 showed selective inhibition of growth in thyroid cancer cells with moderate to high expression of dysadherin, thus could be a specific and effective treatment.

FXYD5 (dysadherin) may mediate metastatic progression through regulation of the β-Na+-K+-ATPase subunit in the 4T1 mouse breast cancer model

FXYD5 is a Na+-K+-ATPase regulator, expressed in a variety of normal epithelia. In parallel, it has been found to be associated with several types of cancer and effect lethal outcome by promoting metastasis. However, the molecular mechanism underlying FXYD5 mediated invasion has not yet been identified. In this study, using in vivo 4T1 murine breast cancer model, we found that FXYD5-specific shRNA significantly inhibited lung cancer metastasis, without having a substantial effect on primary tumor growth. Our study reveals that FXYD5 participates in multiple stages of metastatic development and exhibits more than one mode of E-cadherin regulation. We provide the first evidence that FXYD5-related morphological changes are mediated through its interaction with Na+-K+-ATPase. Experiments in cultured 4T1 cells have indicated that FXYD5 expression may downregulate the β1 isoform of the pump. This behavior could have implications on both transcellular interactions and intracellular events. Further studies suggest that differential localization of the adaptor protein Annexin A2 in FXYD5-expressing cells may correlate with matrix metalloproteinase 9 secretion and adhesion changes in 4T1 wild-type cells.

FXYD5 Is an Essential Mediator of the Inflammatory Response during Lung Injury

The alveolar epithelium secretes cytokines and chemokines that recruit immune cells to the lungs, which is essential for fighting infections but in excess can promote lung injury. Overexpression of FXYD5, a tissue-specific regulator of the Na,K-ATPase, in mice, impairs the alveolo-epithelial barrier, and FXYD5 overexpression in renal cells increases C-C chemokine ligand-2 (CCL2) secretion in response to lipopolysaccharide (LPS). The aim of this study was to determine whether FXYD5 contributes to the lung inflammation and injury. Exposure of alveolar epithelial cells (AEC) to LPS increased FXYD5 levels at the plasma membrane, and FXYD5 silencing prevented both the activation of NF-κB and the secretion of cytokines in response to LPS. Intratracheal instillation of LPS into mice increased FXYD5 levels in the lung. FXYD5 overexpression increased the recruitment of interstitial macrophages and classical monocytes to the lung in response to LPS. FXYD5 silencing decreased CCL2 levels, number of cells, and protein concentration in bronchoalveolar lavage fluid (BALF) after LPS treatment, indicating that FXYD5 is required for the NF-κB-stimulated epithelial production of CCL2, the influx of immune cells, and the increase in alveolo-epithelial permeability in response to LPS. Silencing of FXYD5 also prevented the activation of NF-κB and cytokine secretion in response to interferon α and TNF-α, suggesting that pro-inflammatory effects of FXYD5 are not limited to the LPS-induced pathway. Furthermore, in the absence of other stimuli, FXYD5 overexpression in AEC activated NF-κB and increased cytokine production, while FXYD5 overexpression in mice increased cytokine levels in BALF, indicating that FXYD5 is sufficient to induce the NF-κB-stimulated cytokine secretion by the alveolar epithelium. The FXYD5 overexpression also increased cell counts in BALF, which was prevented by silencing the CCL2 receptor (CCR2), or by treating mice with a CCR2-blocking antibody, confirming that FXYD5-induced CCL2 production leads to the recruitment of monocytes to the lung. Taken together, the data demonstrate that FXYD5 is a key contributor to inflammatory lung injury.

The O-glycosylated ectodomain of FXYD5 impairs adhesion by disrupting cell-cell trans-dimerization of Na,K-ATPase β1 subunits

FXYD5 (also known as dysadherin), a regulatory subunit of the Na,K-ATPase, impairs intercellular adhesion by a poorly understood mechanism. Here, we determined whether FXYD5 disrupts the trans-dimerization of Na,K-ATPase molecules located in neighboring cells. Mutagenesis of the Na,K-ATPase β1 subunit identified four conserved residues, including Y199, that are crucial for the intercellular Na,K-ATPase trans-dimerization and adhesion. Modulation of expression of FXYD5 or of the β1 subunit with intact or mutated β1-β1 binding sites demonstrated that the anti-adhesive effect of FXYD5 depends on the presence of Y199 in the β1 subunit. Immunodetection of the plasma membrane FXYD5 was prevented by the presence of O-glycans. Partial FXYD5 deglycosylation enabled antibody binding and showed that the protein level and the degree of O-glycosylation were greater in cancer than in normal cells. FXYD5-induced impairment of adhesion was abolished by both genetic and pharmacological inhibition of FXYD5 O-glycosylation. Therefore, the extracellular O-glycosylated domain of FXYD5 impairs adhesion by interfering with intercellular β1-β1 interactions, suggesting that the ratio between FXYD5 and α1-β1 heterodimer determines whether the Na,K-ATPase acts as a positive or negative regulator of intercellular adhesion.

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.

FXYD5 Protein Has a Pro-inflammatory Role in Epithelial Cells

The FXYD proteins are a family of small membrane proteins that share an invariant four amino acid signature motif F-X-Y-D and act as tissue-specific regulatory subunits of the Na,K-ATPase. FXYD5 (also termed dysadherin or RIC) is a structurally and functionally unique member of the FXYD family. As other FXYD proteins, FXYD5 specifically interacts with the Na,K-ATPase and alters its kinetics by increasing Vmax However, unlike other family members FXYD5 appears to have additional functions, which cannot be readily explained by modulation of transport kinetics. Knockdown of FXYD5 in MDA-MB-231 breast cancer cells largely decreases expression and secretion of the chemokine CCL2 (MCP-1). A related effect has also been observed in renal cell carcinoma cells. The current study aims to further characterize the relationship between the expression of FXYD5 and CCL2 secretion. We demonstrate that transfection of M1 epithelial cell line with FXYD5 largely increases lipopolysaccharide (LPS) stimulated CCL2 mRNA and secretion of the translated protein. We have completed a detailed analysis of the molecular events leading to the above response. Our key findings indicate that FXYD5 generates a late response by increasing the surface expression of the TNFα receptor, without affecting its total protein level, or mRNA transcription. LPS administration to mice demonstrates induced secretion of CCL2 and TNFα in FXYD5-expressing lung peripheral tissue, which suggests a possible role for FXYD5 in normal epithelia during inflammation.

Expression and clinical significance of FXYD3 in endometrial cancer

FXYD3 expression is upregulated in numerous cancer cell types. The present study compared the FXDY3 expression in normal endometrium, premalignant lesion and endometrial cancer tissue samples, and investigated the correlation between FXDY3 expression and clinicopathological features. FXYD3 expression was analyzed by streptavidin-peroxidase immunohistochemistry in 21 normal endometrial tissue samples, 18 atypical endometrial hyperplasia samples and 50 tissues obtained from patients diagnosed with endometrial cancer. The percentage of FXYD3-positive cell expression in the normal endometrium, atypical hyperplasia and endometrial cancer tissues samples was 0, 22, and 26%, respectively. The differences between the atypical hyperplasia and endometrial cancer groups were statistically significant when compared with the normal group (P=0.007 and P=0.037, respectively). There was no significant difference between the atypical hyperplasia and endometrial cancer groups. The percentage of FXYD3-positive cells correlated with the fertility frequency (P<0.05). In conclusion, FXYD3 is a potential biomarker for endometrial cancer, and its upregulation may be an early event in endometrial carcinoma progression. In addition, FXYD3 expression in endometrial carcinoma correlates with fertility frequency.

Modulation of cell polarization by the Na+-K+-ATPase-associated protein FXYD5 (dysadherin)

FXYD5 (dysadherin or also called a related to ion channel, RIC) is a transmembrane auxiliary subunit of the Na(+)-K(+)-ATPase shown to increase its maximal velocity (Vmax). FXYD5 has also been identified as a cancer-associated protein whose expression in tumor-derived cell lines impairs cytoskeletal organization and increases cell motility. Previously, we have demonstrated that the expression of FXYD5 in M1 cells derived from mouse kidney collecting duct impairs the formation of tight and adherence junctions. The current study aimed to further explore effects of FXYD5 at a single cell level. It was found that in M1, as well as three other cell lines, FXYD5 inhibits transformation of adhered single cells from the initial radial shape to a flattened, elongated shape in the first stage of monolayer formation. This is also correlated to less ordered actin cables and fewer focal points. Structure-function analysis has demonstrated that the transmembrane domain of FXYD5, and not its unique extracellular segment, mediates the inhibition of change in cell shape. This domain has been shown before to be involved in the association of FXYD5 with the Na(+)-K(+)-ATPase, which leads to the increase in Vmax. Furthermore, specific transmembrane point mutations in FXYD5 that either increase or decrease its effect on cell elongation had a corresponding effect on the coimmunoprecipitation of FXYD5 with α Na(+)-K(+)-ATPase. These findings lend support to the possibility that FXYD5 affects cell polarization through its transmembrane domain interaction with the Na(+)-K(+)-ATPase. Yet interaction of FXYD5 with other proteins cannot be excluded.

2011 Homer Smith Award: To serve and protect: classic and novel roles for Na+, K+ -adenosine triphosphatase

The ability of cells to maintain sharp ion gradients across their membranes is the foundation for the molecular transport and electrical excitability. Across animal species and cell types, Na(+),K(+)-adenosine triphosphatase (ATPase) is arguably the most powerful contributor to this phenomenon. By producing a steep concentration difference of sodium and potassium between the intracellular and extracellular milieu, Na(+),K(+)-ATPase in the tubules provides the driving force for renal sodium reabsorption. Pump activity is downregulated by natriuretic hormones, such as dopamine, and is upregulated by antinatriuretic hormones, such as angiotensin. In the past decade, studies have revealed a novel and surprising role: that Na(+),K(+)-ATPase is a transducer of signals from extracellular to intracellular compartments. The signaling function of Na(+),K(+)-ATPase is activated by ouabain, a mammalian steroid hormone, at far lower concentrations than those that inhibit pump activity. By promoting growth and inhibiting apoptosis, activation of Na(+),K(+)-ATPase exerts tissue-protective effects. Ouabain-stimulated Na(+),K(+)-ATPase signaling has recently shown clinical promise by protecting the malnourished embryonic kidney from adverse developmental programming. A deeper understanding of the tissue-protective role of Na(+),K(+)-ATPase signaling and the regulation of Na(+),K(+)-ATPase pumping activity is of fundamental importance for the understanding and treatment of kidney diseases and kidney-related hypertension.

The Na-K-ATPase α₁β₁ heterodimer as a cell adhesion molecule in epithelia

The ion gradients generated by the Na-K-ATPase play a critical role in epithelia by driving transepithelial transport of various solutes. The efficiency of this Na-K-ATPase-driven vectorial transport depends on the integrity of epithelial junctions that maintain polar distribution of membrane transporters, including the basolateral sodium pump, and restrict paracellular diffusion of solutes. The review summarizes the data showing that, in addition to pumping ions, the Na-K-ATPase located at the sites of cell-cell junction acts as a cell adhesion molecule by interacting with the Na-K-ATPase of the adjacent cell in the intercellular space accompanied by anchoring to the cytoskeleton in the cytoplasm. The review also discusses the experimental evidence on the importance of a specific amino acid region in the extracellular domain of the Na-K-ATPase β(1) subunit for the Na-K-ATPase trans-dimerization and intercellular adhesion. Furthermore, a possible role of N-glycans linked to the Na-K-ATPase β(1) subunit in regulation of epithelial junctions by modulating β(1)-β(1) interactions is discussed.

Intracellular trafficking of FXYD1 (phospholemman) and FXYD7 proteins in Xenopus oocytes and mammalian cells

FXYD proteins are a group of short single-span transmembrane proteins that interact with the Na(+)/K(+) ATPase and modulate its kinetic properties. This study characterizes intracellular trafficking of two FXYD family members, FXYD1 (phospholemman (PLM)) and FXYD7. Surface expression of PLM in Xenopus oocytes requires coexpression with the Na(+)/K(+) ATPase. On the other hand, the Na(+)/Ca(2+) exchanger, another PLM-interacting protein could not drive it to the cell surface. The Na(+)/K(+) ATPase-dependent surface expression of PLM could be facilitated by either a phosphorylation-mimicking mutation at Thr-69 or a truncation of three terminal arginine residues. Unlike PLM, FXYD7 could translocate to the cell surface of Xenopus oocytes independently of the coexpression of α1β1 Na(+)/K(+) ATPase. The Na(+)/K(+) ATPase-independent membrane translocation of FXYD7 requires O-glycosylation of at least two of three conserved threonines in its ectodomain. Subsequent experiments in mammalian cells confirmed the role of conserved extracellular threonine residues and demonstrated that FXYD7 protein, in which these have been mutated to alanine, is trapped in the endoplasmic reticulum and Golgi apparatus.

FXYD5 (dysadherin) regulates the paracellular permeability in cultured kidney collecting duct cells

FXYD5 (dysadherin or RIC) is a member of the FXYD family of single-span transmembrane proteins associated with the Na(+)-K(+)-ATPase. Several studies have demonstrated enhanced expression of FXYD5 during metastasis and effects on cell adhesion and motility. The current study examines effects of FXYD5 on the paracellular permeability in the mouse kidney collecting duct cell line M1. Expressing FXYD5 in these cells leads to a large decrease in amiloride-insensitive transepithelial electrical resistance as well as increased permeability to 4-kDa dextran. Impairment of cell-cell contact was also demonstrated by staining cells for the tight and adherence junction markers zonula occludens-1 and β-catenin, respectively. This is further supported by large expansions of the interstitial spaces, visualized in electron microscope images. Expressing FXYD5 in M1 cells resulted in a decrease in N-glycosylation of β1 Na(+)-K(+)-ATPase, while silencing it in H1299 cells had an opposite effect. This may provide a mechanism for the above effects, since normal glycosylation of β1 plays an important role in cell-cell contact formation (Vagin O, Tokhtaeva E, Sachs G. J Biol Chem 281: 39573-39587, 2006).

Expression of dysadherin in the human male reproductive tract and in spermatozoa

OBJECTIVE

To study expression of dysadherin in human testis, epididymis, and spermatozoa.

DESIGN

Prospective study.

SETTING

Basic research laboratory.

PATIENT(S)

Testis, epididymis, and testicular spermatozoa from patients under treatment and semen from volunteer donors.

INTERVENTION(S)

Reverse transcription-polymerase chain reaction, immunohistochemistry, immunocytochemistry, and Western immunoblotting.

MAIN OUTCOME MEASURE(S)

Dysadherin messenger RNA (mRNA) analysis in testis, epididymis, and ejaculated spermatozoa, immunohistochemistry of both tissues, Western immunoblotting of tissue/cell extracts, and immunocytochemistry of spermatozoa.

RESULT(S)

Dysadherin mRNA was found in testis, epididymis, and ejaculated spermatozoa. Whereas testis and spermatozoa exhibited a distinctive 91-kDa protein form, the epididymis showed a 50-kDa moiety, also found in MDA-MB-231 breast cancer cells. Nucleotide sequence analysis revealed >99% homology between testicular and somatic cell mRNA, suggesting differential protein glycosylation. Dysadherin was immunodetected in round spermatids and testicular/ejaculated spermatozoa. It localizes to the acrosomal region and flagellum and colocalized with E-cadherin in the head and with the Na(+),K(+)-ATPase α4 subunit in the flagellum.

CONCLUSION(S)

This is the first report on expression of dysadherin in the male gonad and in spermatozoa. Its colocalization with E-cadherin and Na(+),K(+)-ATPase leads us to postulate a role for dysadherin as a modulator of sperm function.

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.

S163 is critical for FXYD5 modulation of wound healing in airway epithelial cells

The FXYD family, which contains seven members, are tissue specific regulators of the Na,K-ATPase. Increased expression of FXYD5, a cancer-cell-associated membrane glycoprotein, has been associated with increased cell motility and metastatic potential. To better understand how FXYD5 may modulate cell motility, we analyzed S163, a conserved residue in all FXYD family members located in the C-terminus. Ectopic expression of human FXYD5 S163 mutants in HEK 293 cells showed that negative charge at S163 (S163D) decreased membrane localization, assessed by immunofluorescence. Coimmunoprecipitation studies revealed decreased FXYD5/Na,K-ATPase interaction for S163D compared with wild-type or S163A mutants. Interestingly, FXYD5 overexpression induced expression of vimentin, a marker of epithelial-mesenchymal transition, in murine airway epithelial cells. Because Na,K-ATPase expression is decreased in some forms of cancer and is critical for establishing cell polarity and suppressing cell motility, we analyzed S163 mutants in an epithelial cell scratch-wound model as a measure of cell migration. Wild-type FXYD5 overexpression increased reepithelialization (p<0.0001), which was further increased in S163D mutants (p<0.005). However, S163A mutants inhibited epithelial cell migration compared with wild-type FXYD5 overexpression (p<0.0001). We conclude that negative charge at S163 regulates FXYD5/Na,K-ATPase interaction and that this interaction modulates cell migration across a wound in airway epithelial cells.

Functional roles of Na,K-ATPase subunits

PURPOSE OF REVIEW

Na,K-ATPase is an oligomeric protein composed of alpha subunits, beta subunits and FXYD proteins. The catalytic alpha subunit hydrolyzes ATP and transports the cations. Increasing experimental evidence suggest that beta subunits and FXYD proteins essentially contribute to the variable physiological needs of Na,K-ATPase function in different tissues.

RECENT FINDINGS

Beta subunits have a crucial role in the structural and functional maturation of Na,K-ATPase and modulate its transport properties. The chaperone function of the beta subunit is essential, for example, in the formation of tight junctions and cell polarity. Recent studies suggest that beta subunits also have inherent functions, which are independent of Na,K-ATPase activity and which may be involved in cell-cell adhesiveness and in suppression of cell motility. As for FXYD proteins, they modulate Na,K-ATPase activity in a tissue-specific way, in some cases in close cooperation with posttranslational modifications such as phosphorylation.

SUMMARY

A better understanding of the multiple functional roles of the accessory subunits of Na,K-ATPase is crucial to appraise their influence on physiological processes and their implication in pathophysiological states.

FXYD5 modulates Na+ absorption and is increased in cystic fibrosis airway epithelia

FXYD5, also known as dysadherin, belongs to a family of tissue-specific regulators of the Na(+)-K(+)-ATPase. We determined the kinetic effects of FXYD5 on Na(+)-K(+)-ATPase pump activity in stably transfected Madin-Darby canine kidney cells. FXYD5 significantly increased the apparent affinity for Na(+) twofold and decreased the apparent affinity for K(+) by 60% with a twofold increase in V(max) of K(+), a pattern that would increase activity and Na(+) removal from the cell. To test the effect of increased Na(+) uptake on FXYD5 expression, we analyzed Madin-Darby canine kidney cells stably transfected with an inducible vector expressing all three subunits of the epithelial Na(+) channel (ENaC). Na(+)-K(+)-ATPase activity increased sixfold after 48-h ENaC induction, but FXYD5 expression decreased 75%. FXYD5 expression was also decreased in lung epithelia from mice that overexpress ENaC, suggesting that chronic Na(+) absorption by itself downregulates epithelial FXYD5 expression. Patients with cystic fibrosis (CF) display ENaC-mediated hyperabsorption of Na(+) in the airways, accompanied by increased Na(+)-K(+)-ATPase activity. However, FXYD5 was significantly increased in the lungs and nasal epithelium of CF mice as assessed by RT-PCR, immunohistochemistry, and immunoblot analysis (P < 0.001). FXYD5 was also upregulated in nasal scrapings from human CF patients compared with controls (P < 0.02). Treatment of human tracheal epithelial cells with a CFTR inhibitor (I-172) confirmed that loss of CFTR function correlated with increased FXYD5 expression (P < 0.001), which was abrogated by an inhibitor of NF-kappaB. Thus FXYD5 is upregulated in CF epithelia, and this change may exacerbate the Na(+) hyperabsorption and surface liquid dehydration observed in CF airway epithelia.