Interaction with the Na,K-ATPase and Tissue Distribution of FXYD5 (Related to Ion Channel)

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.

Generation of the novel anti-FXYD5 monoclonal antibody and its application to the diagnosis of pancreatic and lung cancer

Despite recent advances in cancer treatments, pancreatic cancer has a dismal prognosis globally. Early detection of cancer cells and effective treatments for recalcitrant tumors are required, but the innovative therapeutic tools remain in development. Cancer-specific antigens expressed only on cancer cells may help resolve these problems, and antibodies to such antigens have potential in basic research and clinical applications. To generate specific antibodies that bind to proteins expressed on the surface of pancreatic cancer cells, we immunized mice with human pancreatic cancer MIA PaCa-2 cells, and isolated a hybridoma that produces a monoclonal antibody (mAb), named 12-13.8. This antibody was applied to molecular biological experiments such as immunocytochemistry, immunoblotting, flow cytometry, and immunoprecipitation. In addition, we showed that mAb 12-13.8 could accumulate in tumors, through in vivo experiments using cancer-bearing mice. Immunohistochemical staining of pancreatic and lung tumor tissues indicated that the increase of the staining strength by mAb 12-13.8 positively and inversely correlated with the patients' cancer recurrence and survival rate, respectively. We identified the FXYD5 protein as the target protein of mAb 12-13.8, by a human protein array screening system. The FXYD5 protein is overexpressed in various types of cancer and is modified by O-linked glycosylation. We confirmed the binding of the FXYD5 protein to mAb 12-13.8 by using FXYD5-knockout MIA PaCa-2 cells, and detailed epitope mapping identified amino acid residues 45-52 as the minimal peptide sequence. Our results indicate that mAb 12-13.8 could be a valuable tool for FXYD5 studies, and useful in diagnostic and drug delivery applications for cancer patients.

Kidney-specific KO of the circadian clock protein PER1 alters renal Na+ handling, aldosterone levels, and kidney/adrenal gene expression

PERIOD 1 (PER1) is a circadian clock transcription factor that is regulated by aldosterone, a hormone that increases blood volume and Na+ retention to increase blood pressure. Male global Per1 knockout (KO) mice develop reduced night/day differences in Na+ excretion in response to a high-salt diet plus desoxycorticosterone pivalate treatment (HS + DOCP), a model of salt-sensitive hypertension. In addition, global Per1 KO mice exhibit higher aldosterone levels on a normal-salt diet. To determine the role of Per1 in the kidney, male kidney-specific Per1 KO (KS-Per1 KO) mice were generated using Ksp-cadherin Cre recombinase to remove exons 2-8 of Per1 in the distal nephron and collecting duct. Male KS-Per1 KO mice have increased Na+ retention but have normal diurnal differences in Na+ excretion in response to HS + DOCP. The increased Na+ retention is associated with altered expression of glucocorticoid and mineralocorticoid receptors, increased serum aldosterone, and increased medullary endothelin-1 compared with control mice. Adrenal gland gene expression analysis revealed that circadian clock and aldosterone synthesis genes have altered expression in KS-Per1 KO mice compared with control mice. These results emphasize the importance of the circadian clock not only in maintaining rhythms of physiological functions but also for adaptability in response to environmental cues, such as HS + DOCP, to maintain overall homeostasis. Given the prevalence of salt-sensitive hypertension in the general population, these findings have important implications for our understanding of how circadian clock proteins regulate homeostasis.NEW & NOTEWORTHY For the first time, we show that knockout of the circadian clock transcription factor PERIOD 1 using kidney-specific cadherin Cre results in increased renal Na+ reabsorption, increased aldosterone levels, and changes in gene expression in both the kidney and adrenal gland. Diurnal changes in renal Na+ excretion were not observed, demonstrating that the clock protein PER1 in the kidney is important for maintaining homeostasis and that this effect may be independent of time of day.

FXYD domain containing ion transport regulator 5 (FXYD5) silencing promotes cell viability and alleviates inflammatory response in cerulein-induced AR42J cells by blocking JAK2/STAT3 signaling pathway

Acute pancreatitis (AP), which causes severe morbidity and mortality, is a heavy burden for people clinically and financially. This study was designed to explore the mechanism of AP and try to find effective therapies against AP. The expression of FXYD5 was interfered by performing transfection. RT-qPCR and Western blot were utilized to measure FXYD5 expression. In addition, the viability, apoptosis and inflammatory response were evaluated using CCK-8, TUNEL and ELISA, respectively. Moreover, Western blot was employed to measure the expressions of apoptosis-, inflammation- and signaling pathway-related proteins. FXYD5 was found to be overexpressed in AP patients and AP cell model. The results showed that in cerulein-induced AR42J cells, cell viability was remarkably increased, and apoptosis was inhibited compared to the normal FXYD5-expressing group because FXYD5 was downregulated. Similarly, in such cells, interference with FXYD5 significantly suppressed the inflammatory response. In addition, Western blot analysis revealed that JAK2/STAT3 signaling was also strongly inhibited by FXYD5 interference. However, the effect of FXYD5 downregulation was reversed upon simultaneous activation of JAK2/STAT3 signaling. In conclusion, downregulation of FXYD5 could promote cell viability and alleviate inflammatory response in cerulein-induced AP via blocking JAK2/STAT3 signaling pathway.

Aldosterone-Regulated Sodium Transport and Blood Pressure

Aldosterone is a major mineralocorticoid steroid hormone secreted by glomerulosa cells in the adrenal cortex. It regulates a variety of physiological responses including those to oxidative stress, inflammation, fluid disruption, and abnormal blood pressure through its actions on various tissues including the kidney, heart, and the central nervous system. Aldosterone synthesis is primarily regulated by angiotensin II, K⁺ concentration, and adrenocorticotrophic hormone. Elevated serum aldosterone levels increase blood pressure largely by increasing Na⁺ re-absorption in the kidney through regulating transcription and activity of the epithelial sodium channel (ENaC). This review focuses on the signaling pathways involved in aldosterone synthesis and its effects on Na⁺ reabsorption through ENaC.

Discovery and Characterization of the Phospholemman/SIMP/Viroporin Superfamily

Using bioinformatic approaches, we present evidence of distant relatedness among the Ephemerovirus Viroporin family, the Rhabdoviridae Putative Viroporin U5 family, the Phospholemman family, and the Small Integral Membrane Protein family. Our approach is based on the transitivity property of homology complemented with five validation criteria: (1) significant sequence similarity and alignment coverage, (2) compatibility of topology of transmembrane segments, (3) overlap of hydropathy profiles, (4) conservation of protein domains, and (5) conservation of sequence motifs. Our results indicate that Pfam protein domains PF02038 and PF15831 can be found in or projected onto members of all four families. In addition, we identified a 26-residue motif conserved across the superfamily. This motif is characterized by hydrophobic residues that help anchor the protein to the membrane and charged residues that constitute phosphorylation sites. In addition, all members of the four families with annotated function are either responsible for or affect the transport of ions into and/or out of the cell. Taken together, these results justify the creation of the novel Phospholemman/SIMP/Viroporin superfamily. Given that transport proteins can be found not just in cells, but also in viruses, the ability to relate viroporin protein families with their eukaryotic and bacterial counterparts is an important development in this superfamily.

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 protein isoforms differentially modulate human Na/K pump function

Tight regulation of the Na/K pump is essential for cellular function because this heteromeric protein builds and maintains the electrochemical gradients for Na+ and K+ that energize electrical signaling and secondary active transport. We studied the regulation of the ubiquitous human α1β1 pump isoform by five human FXYD proteins normally located in muscle, kidney, and neurons. The function of Na/K pump α1β1 expressed in Xenopus oocytes with or without FXYD isoforms was evaluated using two-electrode voltage clamp and patch clamp. Through evaluation of the partial reactions in the absence of K+ but presence of Na+ in the external milieu, we demonstrate that each FXYD subunit alters the equilibrium between E1P(3Na) and E2P, the phosphorylated conformations with Na+ occluded and free from Na+, respectively, thereby altering the apparent affinity for Na+. This modification of Na+ interaction shapes the small effects of FXYD proteins on the apparent affinity for external K+ at physiological Na+. FXYD6 distinctively accelerated both the Na+-deocclusion and the pump-turnover rates. All FXYD isoforms altered the apparent affinity for intracellular Na+ in patches, an effect that was observed only in the presence of intracellular K+. Therefore, FXYD proteins alter the selectivity of the pump for intracellular ions, an effect that could be due to the altered equilibrium between E1 and E2, the two major pump conformations, and/or to small changes in ion affinities that are exacerbated when both ions are present. Lastly, we observed a drastic reduction of Na/K pump surface expression when it was coexpressed with FXYD1 or FXYD6, with the former being relieved by injection of PKA's catalytic subunit into the oocyte. Our results indicate that a prominent effect of FXYD1 and FXYD6, and plausibly other FXYDs, is the regulation of Na/K pump trafficking.

Clinical significance of P-class pumps in cancer

P-class pumps are specific ion transporters involved in maintaining intracellular/extracellular ion homeostasis, gene transcription, and cell proliferation and migration in all eukaryotic cells. The present review aimed to evaluate the role of P-type pumps [Na⁺/K⁺ ATPase (NKA), H⁺/K⁺ ATPase (HKA) and Ca2⁺-ATPase] in cancer cells across three fronts, namely structure, function and genetic expression. It has been shown that administration of specific P-class pumps inhibitors can have different effects by: i) Altering pump function; ii) inhibiting cell proliferation; iii) inducing apoptosis; iv) modifying metabolic pathways; and v) induce sensitivity to chemotherapy and lead to antitumor effects. For example, the NKA β2 subunit can be downregulated by gemcitabine, resulting in increased apoptosis of cancer cells. The sarcoendoplasmic reticulum calcium ATPase can be inhibited by thapsigargin resulting in decreased prostate tumor volume, whereas the HKA α subunit can be affected by proton pump inhibitors in gastric cancer cell lines, inducing apoptosis. In conclusion, the present review highlighted the central role of P-class pumps and their possible use and role as anticancer cellular targets for novel therapeutic chemical agents.

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.

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.

Prognostic and Clinicopathological Significance of EphB3 and Dysadherin Expression in Extrahepatic Cholangiocarcinoma

Aim

EphB3 and dysadherin are involved in tumorigenesis and progression of many neoplasms. However, the roles of EphB3 and dysadherin in extrahepatic cholangiocarcinoma (ECC) remain to be revealed. In this study, we aimed to evaluate the expression of EphB3 and dysadherin, and investigate their clinicopathological significance in ECC.

Methods

We examined EphB3 and dysadherin expression in 100 ECC, 30 peritumoral tissues, 10 adenoma and 15 normal biliary tract tissues using EnVision immunohistochemistry. The relationship between EphB3 or dysadherin expression and clinicopathological features was evaluated using the χ² test or Fisher’s exact test. The overall survival of ECC patients was analyzed using Kaplan-Meier univariate survival analysis and Log rank tests.

Results

We found that EphB3 expression was significantly down-regulated and dysadherin expression was significantly up-regulated in ECC tissues compared with normal tissues (P < 0.01). EphB3 expression was negatively correlated with dysadherin expression in ECC (P < 0.01). The positive rate of EphB3 expression and negative rate of dysadherin expression was significantly higher in patients with well-differentiated type, no lymph node metastasis, no surrounding tissues and organs invasion, early TNM stages (I + II) and radical resection (P < 0.01). The survival of ECC patients with positive EphB3 or negative dysadherin expression was significantly longer than patients with negative EphB3 or positive dysadherin expression (P < 0.01). Cox multivariate analysis demonstrated that negative EphB3 or positive dysadherin expression were independent poor prognostic factors in ECC patients. The ROC curves suggested that EphB3 and dysadherin combined diagnostic efficacy (AUC=0.688, 95%CI: 0.603-0.772) was significantly higher EphB3 diagnostic efficacy (AUC=0.654, 95%CI: 0.564-0.743) or dysadherin diagnostic efficacy (AUC=0.648, 95%CI: 0.558-0.737) alone.

Conclusion

EphB3 and dysadherin are involved in the carcinogenesis and progression of ECC, and ECC patients with negative EphB3 or positive dysadherin expression have a poor prognosis.

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.

Inactivity and exercise training differentially regulate abundance of Na+-K+-ATPase in human skeletal muscle

Physical inactivity is a global health risk that can be addressed through application of exercise training suitable for an individual's health and age. People's willingness to participate in physical activity is often limited by an initially poor physical capability and early onset of fatigue. One factor associated with muscle fatigue during intense contractions is an inexcitability of skeletal muscle cells, reflecting impaired transmembrane Na⁺/K⁺ exchange and membrane depolarization, which are regulated via the transmembranous protein Na⁺-K⁺-ATPase (NKA). This short review focuses on the plasticity of NKA in skeletal muscle in humans after periods of altered usage, exploring NKA upregulation with exercise training and downregulation with physical inactivity. In human skeletal muscle, the NKA content quantified by [³H]ouabain binding site content shows robust, yet tightly constrained, upregulation of 8-22% with physical training, across a broad range of exercise training types. Muscle NKA content in humans undergoes extensive downregulation with injury that involves substantial muscular inactivity. Surprisingly, however, no reduction in NKA content was found in the single study that investigated short-term disuse. Despite clear findings that exercise training and injury modulate NKA content, the adaptability of the individual NKA isoforms in muscle (α_1-3 and β_1-3) and of the accessory and regulatory protein FXYD1 are surprisingly inconsistent across studies, for exercise training as well as for injury/disuse. Potential reasons for this are explored. Finally, we provide suggestions for future studies to provide greater understanding of NKA regulation during exercise training and inactivity in humans.

A strategy for generating cancer-specific monoclonal antibodies to aberrant O-glycoproteins: identification of a novel dysadherin-Tn antibody

Successful application of potent antibody-based T-cell engaging immunotherapeutic strategies is currently limited mainly to hematological cancers. One major reason is the lack of well-characterized antigens on solid tumors with sufficient cancer specific expression. Aberrantly O-glycosylated proteins contain promising cancer-specific O-glycopeptide epitopes suitable for immunotherapeutic applications, but currently only few examples of such antibody epitopes have been identified. We previously showed that chimeric antigen receptor T-cells directed towards aberrantly O-glycosylated MUC1 can control malignant growth in a mouse model. Here, we present a discovery platform for the generation of cancer-specific monoclonal antibodies targeting aberrant O-glycoproteins. The strategy is based on cancer cell lines engineered to homogeneously express the truncated Tn O-glycoform, the so-called SimpleCells. We used SimpleCells of different cancer origin to elicit monoclonal antibodies with selectivity for aberrant O-glycoproteins. For validation we selected and characterized one monoclonal antibody (6C5) directed to a Tn-glycopeptide in dysadherin (FXYD5), known to be upregulated in cancer and promote metastasis. While dysadherin is widely expressed also in normal cells, we demonstrated that the 6C5 epitope is specifically expressed in 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.

A GWAS meta-analysis from 5 population-based cohorts implicates ion channel genes in the pathogenesis of irritable bowel syndrome

BACKGROUND

Irritable bowel syndrome (IBS) shows genetic predisposition, however, large-scale, powered gene mapping studies are lacking. We sought to exploit existing genetic (genotype) and epidemiological (questionnaire) data from a series of population-based cohorts for IBS genome-wide association studies (GWAS) and their meta-analysis.

METHODS

Based on questionnaire data compatible with Rome III Criteria, we identified a total of 1335 IBS cases and 9768 asymptomatic individuals from 5 independent European genotyped cohorts. Individual GWAS were carried out with sex-adjusted logistic regression under an additive model, followed by meta-analysis using the inverse variance method. Functional annotation of significant results was obtained via a computational pipeline exploiting ontology and interaction networks, and tissue-specific and gene set enrichment analyses.

KEY RESULTS

Suggestive GWAS signals (P ≤ 5.0 × 10^-6 ) were detected for 7 genomic regions, harboring 64 gene candidates to affect IBS risk via functional or expression changes. Functional annotation of this gene set convincingly (best FDR-corrected P = 3.1 × 10^-10 ) highlighted regulation of ion channel activity as the most plausible pathway affecting IBS risk.

CONCLUSION & INFERENCES

Our results confirm the feasibility of population-based studies for gene-discovery efforts in IBS, identify risk genes and loci to be prioritized in independent follow-ups, and pinpoint ion channels as important players and potential therapeutic targets warranting further investigation.

Na+ , K+ -ATPase activity in children with autism spectrum disorder: Searching for the reason(s) of its decrease in blood cells

Na⁺, K⁺‐ATPase (NKA) activity, which establishes the sodium and potassium gradient across the cell membrane and is instrumental in the propagation of the nerve impulses, is altered in a number of neurological and neuropsychiatric disorders, including autism spectrum disorders (ASD). In the present work, we examined a wide range of biochemical and cellular parameters in the attempt to understand the reason(s) for the severe decrease in NKA activity in erythrocytes of ASD children that we reported previously. NKA activity in leukocytes was found to be decreased independently from alteration in plasma membrane fluidity. The different subunits were evaluated for gene expression in leukocytes and for protein expression in erythrocytes: small differences in gene expression between ASD and typically developing children were not apparently paralleled by differences in protein expression. Moreover, no gross difference in erythrocyte plasma membrane oxidative modifications was detectable, although oxidative stress in blood samples from ASD children was confirmed by increased expression of NRF2 mRNA. Interestingly, gene expression of some NKA subunits correlated with clinical features. Excess inhibitory metals or ouabain‐like activities, which might account for NKA activity decrease, were ruled out. Plasma membrane cholesterol, but not phosphatidylcholine and phosphatidlserine, was slighty decreased in erythrocytes from ASD children. Although no compelling results were obtained, our data suggest that alteration in the erytrocyte lipid moiety or subtle oxidative modifications in NKA structure are likely candidates for the observed decrease in NKA activity. These findings are discussed in the light of the relevance of NKA in ASD. Autism Res2018, 11: 1388–1403. © 2018 International Society for Autism Research, Wiley Periodicals, Inc.

Lay Summary

The activity of the cell membrane enzyme NKA, which is instrumental in the propagation of the nerve impulses, is severely decreased in erythrocytes from ASD children and in other brain disorders, yet no explanation has been provided for this observation. We strived to find a biological/biochemical cause of such alteration, but most queries went unsolved because of the complexity of NKA regulation. As NKA activity is altered in many brain disorders, we stress the relevance of studies aimed at understanding its regulation in ASD.

Role of the Na+/K+-ATPase ion pump in male reproduction and embryo development

Na⁺/K⁺-ATPase was one of the first ion pumps studied because of its importance in maintaining osmotic and ionic balances between intracellular and extracellular environments, through the exchange of three Na⁺ ions out and two K⁺ ions into a cell. This enzyme, which comprises two main subunits (α and β), with or without an auxiliary polypeptide (γ), can have specific biochemical properties depending on the expression of associated isoforms (α1β1 and/or α2β1) in the cell. In addition to the importance of Na⁺/K⁺-ATPase in ensuring the function of many tissues (e.g. brain, heart and kidney), in the reproductive tract this protein is essential for embryo development because of its roles in blastocoel formation and embryo hatching. In the context of male reproduction, the discovery of a very specific subunit (α4), apparently restricted to male germ cells, only expressed after puberty and able to influence sperm function (e.g. motility and capacitation), opened a remarkable field for further investigations regarding sperm biology. Therefore, the present review focuses on the importance of Na⁺/K⁺-ATPase on male reproduction and embryo development.

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.

Greasing the wheels or a spanner in the works? Regulation of the cardiac sodium pump by palmitoylation

The ubiquitous sodium/potassium ATPase (Na pump) is the most abundant primary active transporter at the cell surface of multiple cell types, including ventricular myocytes in the heart. The activity of the Na pump establishes transmembrane ion gradients that control numerous events at the cell surface, positioning it as a key regulator of the contractile and metabolic state of the myocardium. Defects in Na pump activity and regulation elevate intracellular Na in cardiac muscle, playing a causal role in the development of cardiac hypertrophy, diastolic dysfunction, arrhythmias and heart failure. Palmitoylation is the reversible conjugation of the fatty acid palmitate to specific protein cysteine residues; all subunits of the cardiac Na pump are palmitoylated. Palmitoylation of the pump's accessory subunit phospholemman (PLM) by the cell surface palmitoyl acyl transferase DHHC5 leads to pump inhibition, possibly by altering the relationship between the pump catalytic α subunit and specifically bound membrane lipids. In this review, we discuss the functional impact of PLM palmitoylation on the cardiac Na pump and the molecular basis of recognition of PLM by its palmitoylating enzyme DHHC5, as well as effects of palmitoylation on Na pump cell surface abundance in the cardiac muscle. We also highlight the numerous unanswered questions regarding the cellular control of this fundamentally important regulatory process.

Impaired AQP2 trafficking in Fxyd1 knockout mice: A role for FXYD1 in regulated vesicular transport

The final adjustment of urine volume occurs in the inner medullary collecting duct (IMCD), chiefly mediated by the water channel aquaporin 2 (AQP2). With vasopressin stimulation, AQP2 accumulation in the apical plasma membrane of principal cells allows water reabsorption from the lumen. We report that FXYD1 (phospholemman), better known as a regulator of Na,K-ATPase, has a role in AQP2 trafficking. Daytime urine of Fxyd1 knockout mice was more dilute than WT despite similar serum vasopressin, but both genotypes could concentrate urine during water deprivation. FXYD1 was found in IMCD. In WT mice, phosphorylated FXYD1 was detected intracellularly, and vasopressin induced its dephosphorylation. We tested the hypothesis that the dilute urine in knockouts was caused by alteration of AQP2 trafficking. In WT mice at baseline, FXYD1 and AQP2 were not strongly co-localized, but elevation of vasopressin produced translocation of both FXYD1 and AQP2 to the apical plasma membrane. In kidney slices, baseline AQP2 distribution was more scattered in the Fxyd1 knockout than in WT. Apical recruitment of AQP2 occurred in vasopressin-treated Fxyd1 knockout slices, but upon vasopressin washout, there was more rapid reversal of apical AQP2 localization and more heterogeneous cytoplasmic distribution of AQP2. Notably, in sucrose gradients, AQP2 was present in a detergent-resistant membrane domain that had lower sedimentation density in the knockout than in WT, and vasopressin treatment normalized its density. We propose that FXYD1 plays a role in regulating AQP2 retention in apical membrane, and that this involves transfers between raft-like membrane domains in endosomes and plasma membranes.

FXYD8, a Novel Regulator of Renal Na+/K+-ATPase in the Euryhaline Teleost, Tetraodon nigroviridis

FXYD proteins are important regulators of Na+/K+-ATPase (NKA) activity in mammals. As an inhabitant of estuaries, the pufferfish (Tetraodon nigroviridis) responds to ambient salinity changes with efficient osmoregulation, including alterations in branchial, and renal NKA activities. Previous studies on teleostean FXYDs have mainly focused on the expression and potential functions of FXYD proteins in gills. The goal of the present study was to elucidate the potential role of FXYD8, a member of the fish FXYD protein family, in the modulation of NKA activity in the kidneys of this euryhaline pufferfish by using molecular, biochemical, and physiological approaches. The results demonstrate that T. nigroviridis FXYD8 (TnFXYD8) interacts with NKA in renal tubules. Meanwhile, the protein expression of renal TnFXYD8 was found to be significantly upregulated in hyperosmotic seawater-acclimated pufferfish. Moreover, overexpression of TnFXYD8 in Xenopus oocytes decreased NKA activity. Our results suggest the FXYD8 is able to modulate NKA activity through inhibitory effects upon salinity challenge. The present study further extends our understanding of the functions of FXYD proteins, the regulators of NKA, in vertebrates.

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.

Early vertebrate origin and diversification of small transmembrane regulators of cellular ion transport

KEY POINTS

Small transmembrane proteins such as FXYDs, which interact with Na⁺ ,K⁺ -ATPase, and the micropeptides that interact with sarco/endoplasmic reticulum Ca²⁺ -ATPase play fundamental roles in regulation of ion transport in vertebrates. Uncertain evolutionary origins and phylogenetic relationships among these regulators of ion transport have led to inconsistencies in their classification across vertebrate species, thus hampering comparative studies of their functions. We discovered the first FXYD homologue in sea lamprey, a basal jawless vertebrate, which suggests small transmembrane regulators of ion transport emerged early in the vertebrate lineage. We also identified 13 gene subfamilies of FXYDs and propose a revised, phylogeny-based FXYD classification that is consistent across vertebrate species. These findings provide an improved framework for investigating physiological and pathophysiological functions of small transmembrane regulators of ion transport.

ABSTRACT

Small transmembrane proteins are important for regulation of cellular ion transport. The most prominent among these are members of the FXYD family (FXYD1-12), which regulate Na⁺ ,K⁺ -ATPase, and phospholamban, sarcolipin, myoregulin and DWORF, which regulate the sarco/endoplasmic reticulum Ca²⁺ -ATPase (SERCA). FXYDs and regulators of SERCA are present in fishes, as well as terrestrial vertebrates; however, their evolutionary origins and phylogenetic relationships are obscure, thus hampering comparative physiological studies. Here we discovered that sea lamprey (Petromyzon marinus), a representative of extant jawless vertebrates (Cyclostomata), expresses an FXYD homologue, which strongly suggests that FXYDs predate the emergence of fishes and other jawed vertebrates (Gnathostomata). Using a combination of sequence-based phylogenetic analysis and conservation of local chromosome context, we determined that FXYDs markedly diversified in the lineages leading to cartilaginous fishes (Chondrichthyes) and bony vertebrates (Euteleostomi). Diversification of SERCA regulators was much less extensive, indicating they operate under different evolutionary constraints. Finally, we found that FXYDs in extant vertebrates can be classified into 13 gene subfamilies, which do not always correspond to the established FXYD classification. We therefore propose a revised classification that is based on evolutionary history of FXYDs and that is consistent across vertebrate species. Collectively, our findings provide an improved framework for investigating the function of ion transport in health and disease.

The X-linked juvenile retinoschisis protein retinoschisin is a novel regulator of mitogen-activated protein kinase signalling and apoptosis in the retina

X-linked juvenile retinoschisis (XLRS) is a hereditary retinal dystrophy in young males, caused by mutations in the RS1 gene. The function of the encoded protein, termed retinoschisin, and the molecular mechanisms underlying XLRS pathogenesis are still unresolved, although a direct interaction partner of the secreted retinoschisin, the retinal Na/K-ATPase, was recently identified. Earlier gene expression studies in retinoschisin-deficient (Rs1h^-/Y ) mice provided a first indication of pathological up-regulation of mitogen-activated protein (MAP) kinase signalling in disease pathogenesis. To further investigate the role for retinoschisin in MAP kinase regulation, we exposed Y-79 cells and murine Rs1h^-/Y retinae to recombinant retinoschisin and the XLRS-associated mutant RS1-C59S. Although normal retinoschisin stably bound to retinal cells, RS1-C59S exhibited a strongly reduced binding affinity. Simultaneously, exposure to normal retinoschisin significantly reduced phosphorylation of C-RAF and MAP kinases ERK1/2 in Y-79 cells and murine Rs1h^-/Y retinae. Expression of MAP kinase target genes C-FOS and EGR1 was also down-regulated in both model systems. Finally, retinoschisin treatment decreased pro-apoptotic BAX-2 transcript levels in Y-79 cells and Rs1h^-/Y retinae. Upon retinoschisin treatment, these cells showed increased resistance against apoptosis, reflected by decreased caspase-3 activity (in Y-79 cells) and increased photoreceptor survival (in Rs1h^-/Y retinal explants). RS1-C59S did not influence C-RAF or ERK1/2 activation, C-FOS or EGR1 expression, or apoptosis. Our data imply that retinoschisin is a novel regulator of MAP kinase signalling and exerts an anti-apoptotic effect on retinal cells. We therefore discuss that disturbances of MAP kinase signalling by retinoschisin deficiency could be an initial step in XLRS pathogenesis.

Autocrine MCP-1/CCR2 signaling stimulates proliferation and migration of renal carcinoma cells

The chemokine monocyte chemoattractant protein-1 [MCP-1; also known as chemokine (C-C motif) ligand 2] is an important mediator of monocyte recruitment during inflammatory processes. Pathologically high expression levels of MCP-1 by tumor cells have been observed in a variety of cancer types. In the majority of cases, high MCP-1 expression is associated with a poor prognosis, as infiltration of the tumor with inflammatory monocytes promotes tumor progression and metastasis. MCP-1 is also expressed in renal cell carcinoma (RCC). In the present study, the function and the regulation of MCP-1 was investigated in two RCC cell lines, CaKi-1 and 786-O. In both cell lines, expression of MCP-1 was significantly enhanced compared with non-cancerous control cells. As expected, secretion of MCP-1 into the medium facilitated the recruitment of peripheral blood monocytes via the chemokine (C-C motif) receptor type 2 (CCR2). As expression of CCR2 was also detected in 786-O and CaKi-1 cells, the effect of autocrine MCP-1/CCR2 signaling was evaluated in these cells. In proliferation assays, administration of an MCP-1 neutralizing antibody or of a CCR2 antagonist to CaKi-1 and 786-O cells significantly decreased cell growth; supplementation of the growth medium with recombinant human MCP-1 had no additional effect on proliferation. The migration ability of RCC cells was impaired by MCP-1 neutralization or pharmacological CCR2 inhibition, while it was stimulated by the addition of recombinant human MCP-1, compared with untreated control cells. Finally, substantial differences in the regulation of MCP-1 expression were observed between RCC cell lines. In CaKi-1 cells, expression of MCP-1 appears to be largely mediated by the transcription factor nuclear factor of activated T cells 5, while in 786-O cells, deletion of the tumor suppressor gene Von-Hippel-Lindau appeared to be responsible for MCP-1 upregulation, as suggested by previous studies. Taken together, the results of the current study indicate that expression of MCP-1 in RCC cells promotes tumor progression and metastasis not only by paracrine, but also by autocrine, MCP-1/CCR2 signaling events, enhancing cell proliferation and migration ability. Therefore, the present findings suggest the MCP-1/CCR2 axis is a potential target for future therapeutic strategies in the treatment of metastatic RCC.

Quantitative Targeted Absolute Proteomics of Transporters and Pharmacoproteomics-Based Reconstruction of P-Glycoprotein Function in Mouse Small Intestine

The purpose of this study was to investigate whether a pharmacokinetic model integrating in vitro mdr1a efflux activity (which we previously reported) with in vitro/in vivo differences in protein expression level can reconstruct intestinal mdr1a function. In situ intestinal permeability-surface area product ratio between wild-type and mdr1a/1b (-/-) mice is one of the parameters used to describe intestinal mdr1a function. The reconstructed ratios of six mdr1a substrates (dexamethasone, digoxin, loperamide, quinidine, verapamil, vinblastine) and one nonsubstrate (diazepam) were consistent with the observed values reported by Adachi et al. within 2.1-fold difference. Thus, intestinal mdr1a function can be reconstructed by our pharmacoproteomic modeling approach. Furthermore, we evaluated regional differences in protein expression levels of mouse intestinal transporters. Sixteen (mdr1a, mrp4, bcrp, abcg5, abcg8, glut1, 4f2hc, sglt1, lat2, pept1, mct1, slc22a18, ostβ, villin1, Na(+)/K(+)-ATPase, γ-gtp) out of 46 target molecules were detected by employing our established quantitative targeted absolute proteomics technique. The protein expression amounts of mdr1a and bcrp increased progressively from duodenum to ileum. Sglt1, lat2, and 4f2hc were highly expressed in jejunum and ileum. Mct1 and ostβ were highly expressed in ileum. The quantitative expression profiles established here should be helpful to understand and predict intestinal transporter functions.

Na,K-ATPase regulation in skeletal muscle

Skeletal muscle contains one of the largest and the most dynamic pools of Na,K-ATPase (NKA) in the body. Under resting conditions, NKA in skeletal muscle operates at only a fraction of maximal pumping capacity, but it can be markedly activated when demands for ion transport increase, such as during exercise or following food intake. Given the size, capacity, and dynamic range of the NKA pool in skeletal muscle, its tight regulation is essential to maintain whole body homeostasis as well as muscle function. To reconcile functional needs of systemic homeostasis with those of skeletal muscle, NKA is regulated in a coordinated manner by extrinsic stimuli, such as hormones and nerve-derived factors, as well as by local stimuli arising in skeletal muscle fibers, such as contractions and muscle energy status. These stimuli regulate NKA acutely by controlling its enzymatic activity and/or its distribution between the plasma membrane and the intracellular storage compartment. They also regulate NKA chronically by controlling NKA gene expression, thus determining total NKA content in skeletal muscle and its maximal pumping capacity. This review focuses on molecular mechanisms that underlie regulation of NKA in skeletal muscle by major extrinsic and local stimuli. Special emphasis is given to stimuli and mechanisms linking regulation of NKA and energy metabolism in skeletal muscle, such as insulin and the energy-sensing AMP-activated protein kinase. Finally, the recently uncovered roles for glutathionylation, nitric oxide, and extracellular K(+) in the regulation of NKA in skeletal muscle are highlighted.

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.

Different Modulatory Mechanisms of Renal FXYD12 for Na(+)-K(+)-ATPase between Two Closely Related Medakas upon Salinity Challenge

Upon salinity challenge, the Na⁺-K⁺-ATPase (NKA) of fish kidney plays a crucial role in maintaining ion and water balance. Moreover, the FXYD protein family was found to be a regulator of NKA. Our preliminary results revealed that fxyd12 was highly expressed in the kidneys of the two closely related euryhaline medaka species (Oryzias dancena and O. latipes) from different natural habitats (brackish water and fresh water). In this study, we investigated the expression and association of renal FXYD12 and NKA α-subunit as well as potential functions of FXYD12 in the two medakas. These findings illustrated and compared the regulatory roles of FXYD12 for NKA in kidneys of the two medakas in response to salinity changes. In this study, at the mRNA and/or protein level, the expression patterns were similar for renal FXYD12 and NKA in the two medakas. However, different patterns of NKA activities and different interaction levels between FXYD12 and NKA were found in the kidneys of these two medakas. The results revealed that different strategies were used in the kidneys of the two medaka species upon salinity challenge. On the other hand, gene knockdown experiments demonstrated that the function of O. dancena FXYD12 allowed maintenance of a high level of NKA activity. The results of the present study indicated that the kidneys of the examined euryhaline medakas originating from brackish water and fresh water exhibited different modulatory mechanisms through which renal FXYD12 enhanced NKA activity to maintain internal homeostasis. Our findings broadened the knowledge of expression and functions of FXYD proteins, the modulators of NKA, in vertebrates.

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.

Na+/Ca2+ exchange and Na+/K+-ATPase in the heart

This paper is the third in a series of reviews published in this issue resulting from the University of California Davis Cardiovascular Symposium 2014: Systems approach to understanding cardiac excitation–contraction coupling and arrhythmias: Na⁺ channel and Na⁺ transport. The goal of the symposium was to bring together experts in the field to discuss points of consensus and controversy on the topic of sodium in the heart. The present review focuses on cardiac Na⁺/Ca²⁺ exchange (NCX) and Na⁺/K⁺-ATPase (NKA). While the relevance of Ca²⁺ homeostasis in cardiac function has been extensively investigated, the role of Na⁺ regulation in shaping heart function is often overlooked. Small changes in the cytoplasmic Na⁺ content have multiple effects on the heart by influencing intracellular Ca²⁺ and pH levels thereby modulating heart contractility. Therefore it is essential for heart cells to maintain Na⁺ homeostasis. Among the proteins that accomplish this task are the Na⁺/Ca²⁺ exchanger (NCX) and the Na⁺/K⁺ pump (NKA). By transporting three Na⁺ ions into the cytoplasm in exchange for one Ca²⁺ moved out, NCX is one of the main Na⁺ influx mechanisms in cardiomyocytes. Acting in the opposite direction, NKA moves Na⁺ ions from the cytoplasm to the extracellular space against their gradient by utilizing the energy released from ATP hydrolysis. A fine balance between these two processes controls the net amount of intracellular Na⁺ and aberrations in either of these two systems can have a large impact on cardiac contractility. Due to the relevant role of these two proteins in Na⁺ homeostasis, the emphasis of this review is on recent developments regarding the cardiac Na⁺/Ca²⁺ exchanger (NCX1) and Na⁺/K⁺ pump and the controversies that still persist in the field.

The Circadian Clock in the Regulation of Renal Rhythms

Since the kidney is integral to maintenance of fluid and ion homeostasis, and therefore blood pressure regulation, its proper function is paramount. Circadian fluctuations in blood pressure, renal blood flow, glomerular filtration rate, and sodium and water excretion have been documented for decades, if not longer. Recent studies on the role of circadian clock proteins in the regulation of a variety of renal transport genes suggest that the molecular clock in the kidney controls circadian fluctuations in renal function. The circadian clock appears to be a critical regulator of renal function with important implications for the treatment of renal pathologies, which include chronic kidney disease and hypertension. The development, regulation, and mechanism of the kidney clock are reviewed here.

Clock genes in hypertension: novel insights from rodent models

The circadian clock plays an integral role in the regulation of physiological processes, including the regulation of blood pressure. However, deregulation of the clock can lead to pathophysiological states including hypertension. Recent work has implicated the circadian clock genes in the regulation of processes in the heart, kidney, vasculature, and the metabolic organs, which are all critical in the regulation of the blood pressure. The goal of this review is to provide an introduction and general overview into the role of circadian clock genes in the regulation of blood pressure with a focus on their deregulation in the etiology of hypertension. This review will focus on the core circadian clock genes CLOCK, BMAL1, Per, and Cry.

Paradoxical activation of the sodium chloride cotransporter (NCC) without hypertension in kidney deficient in a regulatory subunit of Na,K-ATPase, FXYD2

Na,K‐ATPase generates the driving force for sodium reabsorption in the kidney. Na,K‐ATPase functional properties are regulated by small proteins belonging to the FXYD family. In kidney FXYD2 is the most abundant: it is an inhibitory subunit expressed in almost every nephron segment. Its absence should increase sodium pump activity and promote Na⁺ retention, however, no obvious renal phenotype was detected in mice with global deletion of FXYD2 (Arystarkhova et al. 2013). Here, increased total cortical Na,K‐ATPase activity was documented in the Fxyd2^−/− mouse, without increased α1β1 subunit expression. We tested the hypothesis that adaptations occur in distal convoluted tubule (DCT), a major site of sodium adjustments. Na,K‐ATPase immunoreactivity in DCT was unchanged, and there was no DCT hypoplasia. There was a marked activation of thiazide‐sensitive sodium chloride cotransporter (NCC; Slc12a3) in DCT, predicted to increase Na⁺ reabsorption in this segment. Specifically, NCC total increased 30% and NCC phosphorylated at T53 and S71, associated with activation, increased 4‐6 fold. The phosphorylation of the closely related thick ascending limb (TAL) apical NKCC2 (Slc12a1) increased at least twofold. Abundance of the total and cleaved (activated) forms of ENaC α‐subunit was not different between genotypes. Nonetheless, no elevation of blood pressure was evident despite the fact that NCC and NKCC2 are in states permissive for Na⁺ retention. Activation of NCC and NKCC2 may reflect an intracellular linkage to elevated Na,K‐ATPase activity or a compensatory response to Na⁺ loss proximal to the TAL and DCT.

We discovered a substantial activation of renal NCC cotransporter in mice genetically depleted for the regulatory inhibitory subunit of Na,K‐ATPase, FXYD2. Surprisingly, no significant changes in urine output as well as elevation of blood pressure were detected suggesting compensatory adaptation elsewhere in nephron

Identification of fxyd genes from the spotted scat (Scatophagus argus): molecular cloning, tissue-specific expression, and response to acute hyposaline stress

By interacting with Na(+), K(+)-ATPase (NKA), the FXYD domain-containing ion transport regulator (FXYD) is involved in teleost osmoregulation, but knowledge of FXYD in marine fish is limited. In the present study, fxyd11 and fxyd12 were identified from the spotted scat (Scatophagus argus), and the two members of the FXYD protein family were expressed in a tissue-specific manner. Fxyd11 mRNA was predominantly expressed in gills, whereas fxyd12 mRNA was mainly distributed in kidneys and intestines. Acute hyposaline stress altered the activity of NKA and the expression of fxyd11 and fxyd12 in gills, kidneys, and intestines. Branchial fxyd11 mRNA expression remained at a low level during freshwater acclimation, whereas NKA activity increased, showing a negative correlation that differed from previous reports. Similarly, renal expression of fxyd11 and fxyd12 mRNA was negatively correlated with NKA activity. Unlike in gills and kidneys, intestinal NKA activity and mRNA expression of fxyd11 and fxyd12 were comparably suppressed. Taken together, the salinity-dependent expression of fxyd11 and fxyd12, and correlation with NKA activity suggested that both fxyd11 and fxyd12 were involved in the response to acute hyposaline challenge in the spotted scat.

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.

A role for the circadian clock protein Per1 in the regulation of the NaCl co-transporter (NCC) and the with-no-lysine kinase (WNK) cascade in mouse distal convoluted tubule cells

It has been well established that blood pressure and renal function undergo circadian fluctuations. We have demonstrated that the circadian protein Per1 regulates multiple genes involved in sodium transport in the collecting duct of the kidney. However, the role of Per1 in other parts of the nephron has not been investigated. The distal convoluted tubule (DCT) plays a critical role in renal sodium reabsorption. Sodium is reabsorbed in this segment through the actions of the NaCl co-transporter (NCC), which is regulated by the with-no-lysine kinases (WNKs). The goal of this study was to test if Per1 regulates sodium transport in the DCT through modulation of NCC and the WNK kinases, WNK1 and WNK4. Pharmacological blockade of nuclear Per1 entry resulted in decreased mRNA expression of NCC and WNK1 but increased expression of WNK4 in the renal cortex of mice. These findings were confirmed by using Per1 siRNA and pharmacological blockade of Per1 nuclear entry in mDCT15 cells, a model of the mouse distal convoluted tubule. Transcriptional regulation was demonstrated by changes in short lived heterogeneous nuclear RNA. Chromatin immunoprecipitation experiments demonstrated interaction of Per1 and CLOCK with the promoters of NCC, WNK1, and WNK4. This interaction was modulated by blockade of Per1 nuclear entry. Importantly, NCC protein expression and NCC activity, as measured by thiazide-sensitive, chloride-dependent (22)Na uptake, were decreased upon pharmacological inhibition of Per1 nuclear entry. Taken together, these data demonstrate a role for Per1 in the transcriptional regulation of NCC, WNK1, and WNK4.

A role for the circadian clock protein Per1 in the regulation of aldosterone levels and renal Na+ retention

The circadian clock plays an important role in the regulation of physiological processes, including renal function and blood pressure. We have previously shown that the circadian protein period (Per)1 regulates the expression of multiple Na(+) transport genes in the collecting duct, including the α-subunit of the renal epithelial Na(+) channel. Consistent with this finding, Per1 knockout mice exhibit dramatically lower blood pressure than wild-type mice. We have also recently demonstrated the potential opposing actions of cryptochrome (Cry)2 on Per1 target genes. Recent work by others has demonstrated that Cry1/2 regulates aldosterone production through increased expression of the adrenal gland-specific rate-limiting enzyme 3β-dehydrogenase isomerase (3β-HSD). Therefore, we tested the hypothesis that Per1 plays a role in the regulation of aldosterone levels and renal Na(+) retention. Using RNA silencing and pharmacological blockade of Per1 nuclear entry in the NCI-H295R human adrenal cell line, we showed that Per1 regulates 3β-HSD expression in vitro. These results were confirmed in vivo: mice with reduced levels of Per1 had decreased levels of plasma aldosterone and decreased mRNA expression of 3β-HSD. We postulated that mice with reduced Per1 would have a renal Na(+)-retaining defect. Indeed, metabolic cage experiments demonstrated that Per1 heterozygotes excreted more urinary Na(+) compared with wild-type mice. Taken together, these data support the hypothesis that Per1 regulates aldosterone levels and that Per1 plays an integral role in the regulation of Na(+) retention.

Role of Per1 and the mineralocorticoid receptor in the coordinate regulation of αENaC in renal cortical collecting duct cells

Renal function and blood pressure (BP) exhibit a circadian pattern of variation, but the molecular mechanism underlying this circadian regulation is not fully understood. We have previously shown that the circadian clock protein Per1 positively regulates the basal and aldosterone-mediated expression of the alpha subunit of the renal epithelial sodium channel (αENaC). The mechanism of this regulation has not been determined however. To further elucidate the mechanism of mineralocorticoid receptor (MR) and Per1 action, site-directed mutagenesis, DNA pull-down assays and chromatin immunoprecipitation (ChIP) methods were used to investigate the coordinate regulation of αENaC by Per1 and MR. Mutation of two circadian response E-boxes in the human αENaC promoter abolished both basal and aldosterone-mediated promoter activity. DNA pull down assays demonstrated the interaction of both MR and Per1 with the E-boxes from the αENaC promoter. These observations were corroborated by ChIP experiments showing increased occupancy of MR and Per1 on an E-box of the αENaC promoter in the presence of aldosterone. This is the first report of an aldosterone-mediated increase in Per1 on a target gene promoter. Taken together, these results demonstrate the novel finding that Per1 and MR mediate the aldosterone response of αENaC through DNA/protein interaction in renal collecting duct cells.

Opposing actions of Per1 and Cry2 in the regulation of Per1 target gene expression in the liver and kidney

Mounting evidence suggests that the circadian clock plays an integral role in the regulation of many physiological processes including blood pressure, renal function, and metabolism. The canonical molecular clock functions via activation of circadian target genes by Clock/Bmal1 and repression of Clock/Bmal1 activity by Per1-3 and Cry1/2. However, we have previously shown that Per1 activates genes important for renal sodium reabsorption, which contradicts the canonical role of Per1 as a repressor. Moreover, Per1 knockout (KO) mice exhibit a lowered blood pressure and heavier body weight phenotype similar to Clock KO mice, and opposite that of Cry1/2 KO mice. Recent work has highlighted the potential role of Per1 in repression of Cry2. Therefore, we postulated that Per1 potentially activates target genes through a Cry2-Clock/Bmal1-dependent mechanism, in which Per1 antagonizes Cry2, preventing its repression of Clock/Bmal1. This hypothesis was tested in vitro and in vivo. The Per1 target genes αENaC and Fxyd5 were identified as Clock targets in mpkCCDc14 cells, a model of the renal cortical collecting duct. We identified PPARα and DEC1 as novel Per1 targets in the mouse hepatocyte cell line, AML12, and in the liver in vivo. Per1 knockdown resulted in upregulation of Cry2 in vitro, and this result was confirmed in vivo in mice with reduced expression of Per1. Importantly, siRNA-mediated knockdown of Cry2 and Per1 demonstrated opposing actions for Cry2 and Per1 on Per1 target genes, supporting the potential Cry2-Clock/Bmal1-dependent mechanism underlying Per1 action in the liver and kidney.

Novel regulation of cardiac Na pump via phospholemman

As the only quantitatively significant Na efflux pathway from cardiac cells, the Na/K ATPase (Na pump) is the primary regulator of intracellular Na. The transmembrane Na gradient it establishes is essential for normal electrical excitability, numerous coupled-transport processes and, as the driving force for Na/Ca exchange, thus setting cardiac Ca load and contractility. As Na influx varies with electrical excitation, heart rate and pathology, the dynamic regulation of Na efflux is essential. It is now widely recognized that phospholemman, a 72 amino acid accessory protein which forms part of the Na pump complex, is the key nexus linking cellular signaling to pump regulation. Phospholemman is the target of a variety of post-translational modifications (including phosphorylation, palmitoylation and glutathionation) and these can dynamically alter the activity of the Na pump. This review summarizes our current understanding of the multiple regulatory mechanisms that converge on phospholemman and govern NA pump activity in the heart. The corrected Fig. 4 is reproduced below. The publisher would like to apologize for any inconvenience caused. [corrected].

Mechanism of the circadian clock in physiology

It has been well established that the circadian clock plays a crucial role in the regulation of almost every physiological process. It also plays a critical role in pathophysiological states including those of obesity and diabetes. Recent evidence has highlighted the potential for targeting the circadian clock as a potential drug target. New studies have also demonstrated the existence of "clock-independent effects" of the circadian proteins, leading to exciting new avenues of research in the circadian clock field in physiology. The goal of this review is to provide an introduction to and overview of the circadian clock in physiology, including mechanisms, targets, and role in disease states. The role of the circadian clocks in the regulation of the cardiovascular system, renal function, metabolism, the endocrine system, immune, and reproductive systems will be discussed.

ATP4A gene regulatory network for fine-tuning of proton pump and ion channels

The ATP4A encodes α subunit of H(+), K(+)-ATPase that contains catalytic sites of the enzyme forming pores through cell membrane which allows the ion transport. H(+), K(+)-ATPase is a membrane bound P-type ATPase enzyme which is found on the surface of parietal cells and uses the energy derived from each cycle of ATP hydrolysis that can help in exchanging ions (H(+), K(+) and Cl(-)) across the cell membrane secreting acid into the gastric lumen. The 3-D model of α-subunit of H(+), K(+)-ATPase was generated by homology modeling. It was evaluated and validated on the basis of free energies and amino acid residues. The inhibitor binding amino acid active pockets were identified in the 3-D model by molecular docking. The two drugs Omeprazole and Rabeprazole were found more potent interactions with generated model of α-subunit of H(+), K(+)-ATPase on the basis of their affinity between drug-protein interactions. We have generated ATP4A gene regulatory networks for interactions with other proteins which involved in regulation that can help in fine-tuning of proton pump and ion channels. These findings provide a new dimension for discovery and development of proton pump inhibitors and gene regulation of the ATPase. It can be helpful in better understanding of human physiology and also using synthetic biology strategy for reprogramming of parietal cells for control of gastric ulcers.

Identification of novel cell-adhesion molecules in peripheral nerves using a signal-sequence trap

The development and maintenance of myelinated nerves in the PNS requires constant and reciprocal communication between Schwann cells and their associated axons. However, little is known about the nature of the cell-surface molecules that mediate axon-glial interactions at the onset of myelination and during maintenance of the myelin sheath in the adult. Based on the rationale that such molecules contain a signal sequence in order to be presented on the cell surface, we have employed a eukaryotic-based, signal-sequence-trap approach to identify novel secreted and membrane-bound molecules that are expressed in myelinating and non-myelinating Schwann cells. Using cDNA libraries derived from dbcAMP-stimulated primary Schwann cells and 3-day-old rat sciatic nerve mRNAs, we generated an extensive list of novel molecules expressed in myelinating nerves in the PNS. Many of the identified proteins are cell-adhesion molecules (CAMs) and extracellular matrix (ECM) components, most of which have not been described previously in Schwann cells. In addition, we have identified several signaling receptors, growth and differentiation factors, ecto-enzymes and proteins that are associated with the endoplasmic reticulum and the Golgi network. We further examined the expression of several of the novel molecules in Schwann cells in culture and in rat sciatic nerve by primer-specific, real-time PCR and in situ hybridization. Our results indicate that myelinating Schwann cells express a battery of novel CAMs that might mediate their interactions with the underlying axons.