Na+,K(+)-ATPase

Heterogeneity of signal transduction by Na-K-ATPase α-isoforms: role of Src interaction

Of the four Na-K-ATPase α-isoforms, the ubiquitous α1 Na-K-ATPase possesses both ion transport and Src-dependent signaling functions. Mechanistically, we have identified two putative pairs of domain interactions between α1 Na-K-ATPase and Src that are critical for α1 signaling function. Our subsequent report that α2 Na-K-ATPase lacks these putative Src-binding sites and fails to carry on Src-dependent signaling further supported our proposed model of direct interaction between α1 Na-K-ATPase and Src but fell short of providing evidence for a causative role. This hypothesis was specifically tested here by introducing key residues of the two putative Src-interacting domains present on α1 but not α2 sequence into the α2 polypeptide, generating stable cell lines expressing this mutant, and comparing its signaling properties to those of α2-expressing cells. The mutant α2 was fully functional as a Na-K-ATPase. In contrast to wild-type α2, the mutant gained α1-like signaling function, capable of Src interaction and regulation. Consistently, the expression of mutant α2 redistributed Src into caveolin-1-enriched fractions and allowed ouabain to activate Src-mediated signaling cascades, unlike wild-type α2 cells. Finally, mutant α2 cells exhibited a growth phenotype similar to that of the α1 cells and proliferated much faster than wild-type α2 cells. These findings reveal the structural requirements for the Na-K-ATPase to function as a Src-dependent receptor and provide strong evidence of isoform-specific Src interaction involving the identified key amino acids. The sequences surrounding the putative Src-binding sites in α2 are highly conserved across species, suggesting that the lack of Src binding may play a physiologically important and isoform-specific role.

Gene silencing reveals multiple functions of Na+/K+-ATPase in the salmon louse (Lepeophtheirus salmonis)

Na⁺/K⁺-ATPase has a key function in a variety of physiological processes including membrane excitability, osmoregulation, regulation of cell volume, and transport of nutrients. While knowledge about Na⁺/K⁺-ATPase function in osmoregulation in crustaceans is extensive, the role of this enzyme in other physiological and developmental processes is scarce. Here, we report characterization, transcriptional distribution and likely functions of the newly identified L. salmonis Na⁺/K⁺-ATPase (LsalNa⁺/K⁺-ATPase) α subunit in various developmental stages. The complete mRNA sequence was identified, with 3003 bp open reading frame encoding a putative protein of 1001 amino acids. Putative protein sequence of LsalNa⁺/K⁺-ATPase revealed all typical features of Na⁺/K⁺-ATPase and demonstrated high sequence identity to other invertebrate and vertebrate species. Quantitative RT-PCR analysis revealed higher LsalNa⁺/K⁺-ATPase transcript level in free-living stages in comparison to parasitic stages. In situ hybridization analysis of copepodids and adult lice revealed LsalNa⁺/K⁺-ATPase transcript localization in a wide variety of tissues such as nervous system, intestine, reproductive system, and subcuticular and glandular tissue. RNAi mediated knock-down of LsalNa⁺/K⁺-ATPase caused locomotion impairment, and affected reproduction and feeding. Morphological analysis of dsRNA treated animals revealed muscle degeneration in larval stages, severe changes in the oocyte formation and maturation in females and abnormalities in tegmental glands. Thus, the study represents an important foundation for further functional investigation and identification of physiological pathways in which Na⁺/K⁺-ATPase is directly or indirectly involved.

Snakes exhibit tissue-specific variation in cardiotonic steroid sensitivity of Na+/K+-ATPase

Toads are among several groups of organisms chemically defended with lethal concentrations of cardiotonic steroids. As a result, most predators that prey on amphibians avoid toads. However, several species of snakes have gained resistance-conferring mutations of Na⁺/K⁺-ATPase, the molecular target of cardiotonic steroids, and can feed on toads readily. Despite recent advances in our understanding of this adaptation at the genetic level, we have lacked functional evidence for how mutations of Na⁺/K⁺-ATPase account for cardiotonic steroid resistance in snake tissues. To address this issue, it is necessary to determine how the Na⁺/K⁺-ATPases of snakes react to the toxins. Some tissues might have Na⁺/K⁺-ATPases that are more susceptible than others and can thus provide clues about how the toxins influence organismal function. Here we provide a mechanistic link between observed Na⁺/K⁺-ATPase substitutions and observed resistance using actual snake Na⁺/K⁺-ATPases. We used an in vitro approach to determine the tissue-specific levels of sensitivity to cardiotonic steroids in select resistant and non-resistant snakes. We compared the sensitivities of select tissues within and between species. Our results suggest that resistant snakes contain highly resistant Na⁺/K⁺-ATPases in their heart and kidney, both of which rely heavily on the enzymes to function, whereas tissues that do not rely as heavily on Na⁺/K⁺-ATPases or might be protected from cardiotonic steroids by other means (liver, gut, and brain) contain non-resistant forms of the enzyme. This study reveals functional evidence that tissue-level target-site insensitivity to cardiotonic steroids varies not only among species but also across tissues within resistant taxa.

Changes in contractile and metabolic parameters of skeletal muscle as rats age from 3 to 12 months

Laboratory rats are considered mature at 3 months despite that musculoskeletal growth is still occurring. Changes in muscle physiological and biochemical characteristics during development from 3 months, however, are not well understood. Whole muscles and single skinned fibres from fast-twitch extensor digitorum longus (EDL) and predominantly slow-twitch soleus (SOL) muscles were examined from male Sprague-Dawley rats (3, 6, 9, 12 months). Ca²⁺ sensitivity of contractile apparatus decreased with age in both fast- (~ 0.04 pCa units) and slow-twitch (~ 0.07 pCa units) muscle fibres, and specific force increased (by ~ 50% and ~ 25%, respectively). Myosin heavy chain composition of EDL and SOL muscles altered to a small extent with age (decrease in MHCIIa proportion after 3 months). Glycogen content increased with age (~ 80% in EDL and 25% in SOL) and GLUT4 protein density decreased (~ 35 and 20%, respectively), whereas the glycogen-related enzymes were little changed. GAPDH protein content was relatively constant in both muscle types, but COXIV protein decreased ~ 40% in SOL muscle. Calsequestrin (CSQ) and SERCA densities remained relatively constant with age, whereas there was a progressive ~ 2-3 fold increase in CSQ-like proteins, though their role and importance remain unclear. There was also ~ 40% decrease in the density of the Na⁺, K⁺-ATPase (NKA) α1 subunit in EDL and the α2 subunit in SOL. These findings emphasise there are substantial changes in skeletal muscle function and the density of key proteins during early to mid-adulthood in rats, which need to be considered in the design and interpretation of experiments.

A possible mechanism for low affinity of silkworm Na+/K+-ATPase for K. J Bioenerg Biomembr 2017;49:463-472. [PMID: 29047027 DOI: 10.1007/s10863-017-9729-5]

The affinity for K⁺ of silkworm nerve Na⁺/K⁺-ATPase is markedly lower than that of mammalian Na⁺/K⁺-ATPase (Homareda 2010). In order to obtain clues on the molecular basis of the difference in K⁺ affinities, we cloned cDNAs of silkworm (Bombyx mori) nerve Na⁺/K⁺-ATPase α and β subunits, and analyzed the deduced amino acid sequences. The molecular masses of the α and β subunits were presumed to be 111.5 kDa with ten transmembrane segments and 37.7 kDa with a single transmembrane segment, respectively. The α subunit showed 75% identity and 93% homology with the pig Na⁺/K⁺-ATPase α1 subunit. On the other hand, the amino acid identity of the β subunit with mammalian counterparts was as low as 30%. Cloned α and β cDNAs were co-expressed in cultured silkworm ovary-derived cells, BM-N cells, which lack endogenous Na⁺/K⁺-ATPase. Na⁺/K⁺-ATPase expressed in the cultured cells showed a low affinity for K⁺ and a high affinity for Na⁺, characteristic of the silkworm nerve Na⁺/K⁺-ATPase. These results suggest that the β subunit is responsible for the affinity for K⁺ of Na⁺/K⁺-ATPase.

Expression and potential roles of sodium-potassium ATPase and E-cadherin in human gastric adenocarcinoma

Background

Gastric adenocarcinoma originates from an abnormal epithelium. The aim of this study was to investigate the expression of sodium-potassium ATPase (NKA), a transmembrane protein located in the epithelium for Na⁺ and K⁺ transportation, and E-cadherin, which are both crucial for the epithelium and adherens junction, as potential gastric cancer biomarkers.

Methods

45 patients diagnosed with gastric adenocarcinoma were recruited. Immunohistochemistry and immunofluorescence were conducted to for localization of NKA α1-, β1-isoform, and E-cadherin. NKA enzyme activity was determined by NADH-linked methods and immunoblotting of NKA α1-, β1-isoform, and E-cadherin were performed to evaluate protein expression.

Results

Immunostaining revealed that NKA was co-localized with E-cadherin in the glands of the gastric epithelium. Both NKA activity and α1-isoform protein expression were reduced in the study group (P < 0.05), indicating impaired NKA functions. In the adherens junctions, the NKA β1-isoform and E-cadherin were significantly reduced in the study groups (P < 0.05), indicating the adhesion force between cells may have been weakened.

Conclusions

A significant decrease in NKA function (protein and activity) and E-cadherin in tumor lesions appear promising biomarker for gastric adenocarcinoma. Therefore, developing screening methods for detecting NKA function may be beneficial for the early diagnosis of gastric cancer. In our knowledge, this study was the first to investigate the NKA and E-cadherin expression in the relation of gastric adenocarcinoma in human patients.

RH421 binds into the ATP-binding site on the Na+/K+-ATPase

The Na⁺/K⁺-ATPase plays a key role in ion transport across the plasma membrane of all animal cells. The voltage-sensitive styrylpyrimidium dye RH421 has been used in several laboratories for monitoring of Na⁺/K⁺-ATPase kinetics. It is known, that RH421 can interact with the enzyme and it can influence its activity at micromolar concentrations, but structural details of this interaction are only poorly understood. Experiments with isolated large cytoplasmic loop (C45) of Na⁺/K⁺-ATPase revealed that RH421 can interact with this part of the protein with dissociation constant 1μM. The Trp-to-RH421 FRET performed on six single-tryptophan mutants revealed that RH421 binds directly into the ATP-binding site. This conclusion was further supported by results from molecular docking, site-directed mutagenesis and by competitive experiments using ATP. Experiments with C45/DPPC mixture revealed that RH421 can bind to both C45 and lipids, but only the former interaction was influenced by the presence of ATP.

Anion-capped metallohost allows extremely slow guest uptake and on-demand acceleration of guest exchange

The switching of molecular recognition selectivity is important for tuning molecular functions based on host–guest binding. While the switching processes in artificial functional molecules are usually driven by changes of the thermodynamic stabilities, non-equilibrium phenomena also play an important role in biological systems. Thus, here we designed a host–guest system utilizing a non-equilibrium kinetically trapped state for on-demand and time-programmable control of molecular functions. We synthesized a bis(saloph) macrocyclic cobalt(III) metallohost 1(OTf)₂, which has anion caps at both sides of the cation-binding site. The anion caps effectively retard the guest uptake/release so that we can easily make a non-equilibrium kinetically trapped state. Indeed, we can obtain a long-lived kinetically trapped state {[1·K]³⁺+La³⁺} prior to the formation of the thermodynamically more stable state {[1·La]⁵⁺+K⁺}. The guest exchange to the more stable state from this kinetically trapped state is significantly accelerated by exchange of TfO⁻ anion caps by AcO⁻ in an on-demand manner.

Host—guest assemblies can exploit stimuli-responsive guest binding and release for molecular recognition, but are typically governed by thermodynamics alone. Here, the authors design macrocycles with removable and exchangeable anion caps, allowing for the kinetic trapping and on-demand exchange of guest ions.

Identification of Antifungal H+-ATPase Inhibitors with Effect on Plasma Membrane Potential

The plasma membrane H⁺-ATPase (Pma1) is an essential fungal protein and a proposed target for new antifungal medications. The compounds in a small-molecule library containing ∼191,000 commercially available compounds were screened for their ability to inhibit Saccharomyces cerevisiae plasma membranes containing Pma1. The overall hit rate was 0.2%, corresponding to 407 compounds. These hit compounds were further evaluated for ATPase selectivity and broad-spectrum antifungal activity. Following this work, one Pma1 inhibitor series based on compound 14 and analogs was selected for further evaluation. This compound series was able to depolarize the membrane and inhibit extracellular acidification in intact fungal cells concomitantly with a significant increase in intracellular ATP levels. Collectively, we suggest that these effects may be a common feature of Pma1 inhibitors. Additionally, the work uncovered a dual mechanism for the previously identified cationic peptide BM2, revealing fungal membrane disruption, in addition to Pma1 inhibition. The methods presented here provide a solid platform for the evaluation of Pma1-specific inhibitors in a drug development setting. The present inhibitors could serve as a starting point for the development of new antifungal agents with a novel mode of action.

Cardiotonic Steroids Stimulate Macrophage Inflammatory Responses Through a Pathway Involving CD36, TLR4, and Na/K-ATPase

OBJECTIVE

Circulating levels of cardiotonic steroids (CTS) are elevated in various chronic inflammatory conditions, but the role of CTS in inflammation remains largely unknown. We have previously shown that the CTS ouabain stimulates proinflammatory responses in murine macrophages. In this study, we aim to explore the mechanism how CTS induce proinflammatory responses in primary murine and human macrophages.

APPROACH AND RESULTS

Using both murine peritoneal macrophages and human monocyte-derived macrophages, we demonstrated that ouabain activated NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells), leading to proinflammatory cytokine (eg, MCP-1 [monocyte chemotactic protein 1], TNF-α [tumor necrosis factor-α], IL-1β [interleukin-1β], and IL-6) production. By applying siRNA techniques and murine peritoneal macrophages isolated from genetically modified mice, we showed that macrophages partially deficient in Na/K-ATPase, the receptor for CTS, or fully deficient in the scavenger receptor CD36 or TLR4 (Toll-like receptor) were resistant to ouabain-induced NF-κB activation, suggesting an indispensable role of these 3 receptors in this pathway. Mechanistically, this effect of ouabain was independent of the ion transport function of the Na/K-ATPase. Instead, ouabain stimulated a signaling complex, including Na/K-ATPase, CD36, and TLR4. Subsequently, TLR4 recruited MyD88 adaptor protein for NF-κB activation. Furthermore, intraperitoneal injection of ouabain into mice specifically recruited Ly6C⁺CCR2⁺ monocyte subtypes to the peritoneal cavities, indicating that the CTS ouabain triggers inflammation in vivo.

CONCLUSIONS

CTS activate NF-κB leading to proinflammatory cytokine production in primary macrophages through a signaling complex, including CD36, TLR4, and Na/K-ATPase. These findings warrant further studies on endogenous CTS in chronic inflammatory diseases, such as atherosclerosis.

Inhibition of Non Canonical HIV-1 Tat Secretion Through the Cellular Na+,K+-ATPase Blocks HIV-1 Infection

Besides its essential role in the activation of HIV-1 gene expression, the viral Tat protein has the unusual property of trafficking in and out of cells. In contrast to Tat internalization, the mechanism involved in extracellular Tat release has so far remained elusive. Here we show that Tat secretion occurs through a Golgi-independent pathway requiring binding of Tat with three short, non-consecutive intracytoplasmic loops at the C-terminus of the cellular Na⁺,K⁺-ATPase pump alpha subunit. Ouabain, a pump inhibitor, blocked this interaction and prevented Tat secretion; virions produced in the presence of this drug were less infectious, consistent the capacity of virion-associated Tat to increase HIV-1 infectivity. Treatment of CD4+ T-cells with short peptides corresponding to the Tat-binding regions of the pump alpha subunit impaired extracellular Tat release and blocked HIV-1 replication. Thus, non canonical, extracellular Tat secretion is essential for viral infectivity.

Subcellular localization of Na/K-ATPase isoforms in ventricular myocytes

The sodium/potassium ATPase (NKA) is essential for establishing the normal intracellular [Na⁺] and [K⁺] and transmembrane gradients that are essential for many cellular functions, including cardiac electrophysiology and contractility. Different NKA isoforms exhibit differential expression levels, cellular localization, and function in different tissues and species. Prior work has indicated that the NKA-α1 isoform is quantitatively predominant in cardiac myocytes, but that the α2 isoform is preferentially concentrated in the transverse tubules (TT), possibly at junctions with the sarcoplasmic reticulum (SR) where α2 may preferentially modulate cardiac contractility. Here we measured subcellular localization of NKA-α1 and α2 using super-resolution microscopy (STED and STORM) and isoform-selective antibodies in mouse ventricular myocytes. We confirm the preferential localization of NKA-α2 in TT vs. surface sarcolemma, but also show that α2 is relatively excluded from longitudinal TT elements. In contrast NKA-α1 is relatively uniformly expressed in all three sarcolemmal regions. We also tested the hypothesis that NKA-α2 (vs. α1) is preferentially concentrated at SR junctional sites near ryanodine receptors (RyR2). The results refute this hypothesis, in that NKA-α1 and α2 were equally close to RyR2 at the TT, with no preferential NKA isoform localization near RyR2. We conclude that in contrast to relatively uniform NKA-α1 distribution, NKA-α2 is preferentially concentrated in the truly transverse (and not longitudinal) TT elements. However, NKA-α2 does not preferentially cluster at RyR2 junctions, so the TT NKA-α2 concentration may suffice for preferential effects of NKA-α2 inhibition on cardiac contractility.

A quantitative shRNA screen identifies ATP1A1 as a gene that regulates cytotoxicity by aurilide B

Genome-wide RNA interference (RNAi) with pooled and barcoded short-hairpin RNA (shRNA) libraries provides a powerful tool for identifying cellular components that are relevant to the modes/mechanisms of action (MoA) of bioactive compounds. shRNAs that affect cellular sensitivity to a given compound can be identified by deep sequencing of shRNA-specific barcodes. We used multiplex barcode sequencing technology by adding sample-specific index tags to PCR primers during sequence library preparation, enabling parallel analysis of multiple samples. An shRNA library screen with this system revealed that downregulation of ATP1A1, an α-subunit of Na⁺/K⁺ ATPase, conferred significant sensitivity to aurilide B, a natural marine product that induces mitochondria-mediated apoptosis. Combined treatment with ouabain which inhibits Na⁺/K⁺ ATPase by targeting α-subunits potentiated sensitivity to aurilide B, suggesting that ATP1A1 regulates mitochondria-mediated apoptosis. Our results indicate that multiplex sequencing facilitates the use of pooled shRNA library screening for the identification of combination drug therapy targets.

Downregulation of ATP1A1 promotes cancer development in renal cell carcinoma

BACKGROUND

Aberrant expression of Na+/K+-ATPase α1 subunit (ATP1A1) is widely observed in multiple types of tumors, and its tissue-specific expression relates to cancer development. However, the functions and molecular mechanisms in renal cell carcinoma (RCC) are not fully understood.

METHODS

We investigated the ATP1A1 expression changes and possible roles in RCC through a quantitative proteomic approach and an integrative biochemical assessment. We detected ATP1A1 in RCC with LC-MS/MS, and further validated its expression with immunohistochemical analyses of 80 pairs of the RCC tumor and non-tumor tissues samples. The association of ATP1A1 expression with RCC pathology was statistically analyzed. Cell proliferation, migration and apoptosis were measured by CCK-8, boyden chamber assay and flow cytometry, respectively. The production of reactive oxygen species (ROS) was labeled with a single staining using a commercial kit, and was further detected with flow cytometry.

RESULTS

The ATP1A1 shows a significantly decreased expression in human RCC tissues than in the adjacent non-tumor tissues. The RCC patients with ATP1A1-positive expression exhibit longer overall survival time than the ATP1A1-negative patients. The exogenous overexpression of ATP1A1 inhibits RCC cell proliferation and cell migration by increasing the production of ROS. In addition, ATP1A1-mediated Raf/MEK/ERK signaling pathway is suppressed in RCC cells, indicating the possible occurrence of induced cell apoptosis.

CONCLUSIONS

Our in vitro and in vivo data of ATP1A1 inhibitory roles in RCC progression suggest that ATP1A1 is a potential novel suppressor protein for renal cancer.

On the Many Actions of Ouabain: Pro-Cystogenic Effects in Autosomal Dominant Polycystic Kidney Disease

Ouabain and other cardenolides are steroidal compounds originally discovered in plants. Cardenolides were first used as poisons, but after finding their beneficial cardiotonic effects, they were rapidly included in the medical pharmacopeia. The use of cardenolides to treat congestive heart failure remained empirical for centuries and only relatively recently, their mechanisms of action became better understood. A breakthrough came with the discovery that ouabain and other cardenolides exist as endogenous compounds that circulate in the bloodstream of mammals. This elevated these compounds to the category of hormones and opened new lines of investigation directed to further study their biological role. Another important discovery was the finding that the effect of ouabain was mediated not only by inhibition of the activity of the Na,K-ATPase (NKA), but by the unexpected role of NKA as a receptor and a signal transducer, which activates a complex cascade of intracellular second messengers in the cell. This broadened the interest for ouabain and showed that it exerts actions that go beyond its cardiotonic effect. It is now clear that ouabain regulates multiple cell functions, including cell proliferation and hypertrophy, apoptosis, cell adhesion, cell migration, and cell metabolism in a cell and tissue type specific manner. This review article focuses on the cardenolide ouabain and discusses its various in vitro and in vivo effects, its role as an endogenous compound, its mechanisms of action, and its potential use as a therapeutic agent; placing especial emphasis on our findings of ouabain as a pro-cystogenic agent in autosomal dominant polycystic kidney disease (ADPKD).

Seawater-drowning-induced acute lung injury: From molecular mechanisms to potential treatments

Drowning is a crucial public safety problem and is the third leading cause of accidental fatality, claiming ~372,000 lives annually, worldwide. In near-drowning patients, acute lung injury (ALI) or acute respiratory distress syndrome (ARDS) is one of the most common complications. Approximately 1/3 of near-drowning patients fulfill the criteria for ALI or ARDS. In the present article, the current literature of near-drowning, pathophysiologic changes and the molecular mechanisms of seawater-drowning-induced ALI and ARDS was reviewed. Seawater is three times more hyperosmolar than plasma, and following inhalation of seawater the hyperosmotic seawater may cause serious injury in the lung and alveoli. The perturbing effects of seawater may be primarily categorized into insufficiency of pulmonary surfactant, blood-air barrier disruption, formation of pulmonary edema, inflammation, oxidative stress, autophagy, apoptosis and various other hypertonic stimulation. Potential treatments for seawater-induced ALI/ARDS were also presented, in addition to suggestions for further studies. A total of nine therapeutic strategies had been tested and all had focused on modulating the over-activated immunoreactions. In conclusion, seawater drowning is a complex injury process and the exact mechanisms and potential treatments require further exploration.

Noradrenergic β-Adrenoceptor-Mediated Intracellular Molecular Mechanism of Na-K ATPase Subunit Expression in C6 Cells

Rapid eye movement sleep deprivation-associated elevated noradrenaline increases and decreases neuronal and glial Na-K ATPase activity, respectively. In this study, using C6 cell-line as a model, we investigated the possible intracellular molecular mechanism of noradrenaline-induced decreased glial Na-K ATPase activity. The cells were treated with noradrenaline in the presence or absence of adrenoceptor antagonists, modulators of extra- and intracellular Ca⁺⁺ and modulators of intracellular signalling pathways. We observed that noradrenaline acting on β-adrenoceptor decreased Na-K ATPase activity and mRNA expression of the catalytic α2-Na-K ATPase subunit in the C6 cells. Further, cAMP and protein kinase-A mediated release of intracellular Ca⁺⁺ played a critical role in such decreased α2-Na-K ATPase expression. In contrast, noradrenaline acting on β-adrenoceptor up-regulated the expression of regulatory β2-Na-K ATPase subunit, which although was cAMP and Ca⁺⁺ dependent, was independent of protein kinase-A and protein kinase-C. Combining these with previous findings (including ours) we have proposed a working model for noradrenaline-induced suppression of glial Na-K ATPase activity and alteration in its subunit expression. The findings help understanding noradrenaline-associated maintenance of brain excitability during health and altered states, particularly in relation to rapid eye movement sleep and its deprivation when the noradrenaline level is naturally altered.

Src-independent ERK signaling through the rat α3 isoform of Na/K-ATPase

The Na/K-ATPase α1 polypeptide supports both ion-pumping and signaling functions. The Na/K-ATPase α3 polypeptide differs from α1 in both its primary structure and its tissue distribution. The expression of α3 seems particularly important in neurons, and recent clinical evidence supports a unique role of this isoform in normal brain function. The nature of this specific role of α3 has remained elusive, because the ubiquitous presence of α1 has hindered efforts to characterize α3-specific functions in mammalian cell systems. Using Na/K-ATPase α1 knockdown pig kidney cells (PY-17), we generated the first stable mammalian cell line expressing a ouabain-resistant form of rat Na/K-ATPase α3 in the absence of endogenous pig α1 detectable by Western blotting. In these cells, Na/K-ATPase α3 formed a functional ion-pumping enzyme and rescued the expression of Na/K-ATPase β1 and caveolin-1 to levels comparable with those observed in PY-17 cells rescued with a rat Na/K-ATPase α1 (AAC-19). The α3-containing enzymes had lower Na+ affinity and lower ouabain-sensitive transport activity than their α1-containing counterparts under basal conditions, but showed a greater capacity to be activated when intracellular Na+ was increased. In contrast to Na/K-ATPase α1, α3 could not regulate Src. Upon exposure to ouabain, Src activation did not occur, yet ERK was activated through Src-independent pathways involving PI3K and PKC. Hence, α3 expression confers signaling and pumping properties that are clearly distinct from that of cells expressing Na/K-ATPase α1.

Protein Carbonylation of an Amino Acid Residue of the Na/K-ATPase α1 Subunit Determines Na/K-ATPase Signaling and Sodium Transport in Renal Proximal Tubular Cells

BACKGROUND

We have demonstrated that cardiotonic steroids, such as ouabain, signaling through the Na/K-ATPase, regulate sodium reabsorption in the renal proximal tubule. By direct carbonylation modification of the Pro222 residue in the actuator (A) domain of pig Na/K-ATPase α1 subunit, reactive oxygen species are required for ouabain-stimulated Na/K-ATPase/c-Src signaling and subsequent regulation of active transepithelial (22)Na(+) transport. In the present study we sought to determine the functional role of Pro222 carbonylation in Na/K-ATPase signaling and sodium handling.

METHODS AND RESULTS

Stable pig α1 knockdown LLC-PK1-originated PY-17 cells were rescued by expressing wild-type rat α1 and rat α1 with a single mutation of Pro224 (corresponding to pig Pro222) to Ala. This mutation does not affect ouabain-induced inhibition of Na/K-ATPase activity, but abolishes the effects of ouabain on Na/K-ATPase/c-Src signaling, protein carbonylation, Na/K-ATPase endocytosis, and active transepithelial (22)Na(+) transport.

CONCLUSIONS

Direct carbonylation modification of Pro224 in the rat α1 subunit determines ouabain-mediated Na/K-ATPase signal transduction and subsequent regulation of renal proximal tubule sodium transport.

Cardenolides from the Apocynaceae family and their anticancer activity

Cardenolides, as a group of natural products that can bind to Na(+)/K(+)-ATPase with an inhibiting activity, are traditionally used to treat congestive heart failure. Recent studies have demonstrated that the strong tumor cytotoxicities of cardenolides are mainly due to inducing the tumor cells apoptosis through different expression and cellular location of Na(+)/K(+)-ATPase α-subunits. The leaves, flesh, seeds and juices of numerous plants from the genera of Nerium, Thevetia, Cerbera, Apocynum and Strophanthus in Apocynaceae family, are the major sources of natural cardenolides. So far, 109 cardenolides have been isolated and identified from this family, and about a quarter of them are reported to exhibit the capability to regulate cancer cell survival and death through multiple signaling pathways. In this review, we compile the phytochemical characteristics and anticancer activity of the cardenolides from this family.

Inhibitory efficacy of bufadienolides on Na+,K+-pump activity versus cell proliferation

Bufadienolides are cytotoxic drugs that may form the basis for anticancer agents. Due to structural and functional similarity to cardiotonic glycosides, application is restricted. We, therefore, investigated correlation of their putative anticancer effects with inhibition of Na⁺,K⁺pumps. The natural bufalin and three derivatives were tested. The anticancer effects of the drugs were checked by observing their inhibitory effects on proliferation of rat liver cancer cells using MTT assay. Inhibition of Na⁺,K⁺-pump was determined by measuring pump-mediated current of rat α1/β1 and α2/β1 Na⁺,K⁺pumps expressed in Xenopus oocytes.

All tested bufadienolides inhibited cell proliferation and Na⁺,K⁺pump activity. An activity coefficient A=100xIC₅₀^{Na,K pump}/IC₅₀^{proliferation} was used to describe drug effectivity as anticancer drug. Natural bufalin exhibited lowest effectivity on cell proliferation, and also the A value for rat α1 isoform was the lowest (0.08), the α2 isoform was much less sensitive (A=1.00). The highest A values were obtained for the BF238 derivative with A=0.88 and 2.64 for the α1 and α2 isoforms, respectively. Therefore, we suggest that search for bufalin derivatives with high anticancer effect and low affinity for both Na⁺,K⁺pump isoforms may be a promising strategy for development of anticancer drugs.

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Effects of bufadienolides on Na pump are not correlated with their cytotoxicity.

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Bufadienolides with high anticancer effect but low side effect may exist.

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BF238 may form a basis for further anticancer drug research and development.

Allosteric Mechanisms of Molecular Machines at the Membrane: Transport by Sodium-Coupled Symporters

Solute transport across cell membranes is ubiquitous in biology as an essential physiological process. Secondary active transporters couple the unfavorable process of solute transport against its concentration gradient to the energetically favorable transport of one or several ions. The study of such transporters over several decades indicates that their function involves complex allosteric mechanisms that are progressively being revealed in atomistic detail. We focus on two well-characterized sodium-coupled symporters: the bacterial amino acid transporter LeuT, which is the prototype for the "gated pore" mechanism in the mammalian synaptic monoamine transporters, and the archaeal GltPh, which is the prototype for the "elevator" mechanism in the mammalian excitatory amino acid transporters. We present the evidence for the role of allostery in the context of a quantitative formalism that can reconcile biochemical and biophysical data and thereby connects directly to recent insights into the molecular structure and dynamics of these proteins. We demonstrate that, while the structures and mechanisms of these transporters are very different, the available data suggest a common role of specific models of allostery in their functions. We argue that such allosteric mechanisms appear essential not only for sodium-coupled symport in general but also for the function of other types of molecular machines in the membrane.

Structural analysis of the α subunit of Na(+)/K(+) ATPase genes in invertebrates

The Na(+)/K(+) ATPase is a ubiquitous pump coordinating the transport of Na(+) and K(+) across the membrane of cells and its role is fundamental to cellular functions. It is heteromer in eukaryotes including two or three subunits (α, β and γ which is specific to the vertebrates). The catalytic functions of the enzyme have been attributed to the α subunit. Several complete α protein sequences are available, but only few gene structures were characterized. We identified the genomic sequences coding the α-subunit of the Na(+)/K(+) ATPase, from the whole-genome shotgun contigs (WGS), NCBI Genomes (chromosome), Genomic Survey Sequences (GSS) and High Throughput Genomic Sequences (HTGS) databases across distinct phyla. One copy of the α subunit gene was found in Annelida, Arthropoda, Cnidaria, Echinodermata, Hemichordata, Mollusca, Placozoa, Porifera, Platyhelminthes, Urochordata, but the nematodes seem to possess 2 to 4 copies. The number of introns varied from 0 (Platyhelminthes) to 26 (Porifera); and their localization and length are also highly variable. Molecular phylogenies (Maximum Likelihood and Maximum Parsimony methods) showed some clusters constituted by (Chordata/(Echinodermata/Hemichordata)) or (Plathelminthes/(Annelida/Mollusca)) and a basal position for Porifera. These structural analyses increase our knowledge about the evolutionary events of the α subunit genes in the invertebrates.

Methylation-dependent and independent regulatory regions in the Na,K-ATPase alpha4 (Atp1a4) gene may impact its testis-specific expression

The α4 Na,K-ATPase is a sperm-specific protein essential for sperm motility and fertility yet little is known about the mechanisms that regulate its expression in germ cells. Here, the potential involvement of DNA methylation in regulating the expression of this sperm-specific protein is explored. A single, intragenic CpG island (Mα4-CGI) was identified in the gene encoding the mouse α4 Na,K-ATPase (Atp1a4), which displayed reduced methylation in mouse sperm (cells that contain α4) compared to mouse kidney (tissue that lacks α4 expression). Unlike the intragenic CGI, the putative promoter (the -700 to +200 region relative to the transcriptional start site) of Atp1a4 did not show differential methylation between kidney and sperm nevertheless it did drive methylation-dependent reporter gene expression in the male germ cell line GC-1spg. Furthermore, treatment of GC-1spg cells with 5-aza2-deoxycytidine led to upregulation of the α4 transcript and decreased methylation of both the Atp1a4 promoter and the Mα4-CGI. In addition, Atp1a4 expression in mouse embryonic stem cells deficient in DNA methyltransferases suggests that both maintenance and de novo methylation are involved in regulating its expression. In an attempt to define the regulatory function of the Mα4-CGI, possible roles of the Mα4-CGI in regulating Atp1a4 expression via methylation-dependent transcriptional elongation inhibition in somatic cells and via its ability to repress promoter activity in germ cells were uncovered. In all, our data suggests that both the promoter and the intragenic CGI could combine to provide multiple modes of regulation for optimizing the Atp1a4 expression level in a cell type-specific manner.

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.

Organ-Specific and Size-Dependent Ag Nanoparticle Toxicity in Gills and Intestines of Adult Zebrafish

We studied adult zebrafish to determine whether the size of 20 and 110 nm citrate-coated silver nanoparticles (AgC NPs) differentially impact the gills and intestines, known target organs for Ag toxicity in fish. Following exposure for 4 h, 4 days, or 4 days plus a 7 day depuration period, we obtained different toxicokinetic profiles for different particle sizes, as determined by Ag content of the tissues. Ionic AgNO3 served as a positive control. The gills showed a significantly higher Ag content for the 20 nm particles at 4 h and 4 days than the 110 nm particles, while the values were more similar in the intestines. Both particle types were retained in the intestines even after depuration. These toxicokinetics were accompanied by striking size-dependent differences in the ultrastructural features and histopathology in the target organs in response to the particulates. Ag staining of the gills and intestines confirmed prominent Ag deposition in the basolateral membranes for the 20 nm but not for the 110 nm particles. Furthermore, it was possible to link the site of tissue deposition to disruption of the Na(+)/K(+) ion channel, which is also localized to the basolateral membrane. This was confirmed by a reduction in ATPase activity and immunohistochemical detection of the α subunit of this channel in both target organs, with the 20 nm particles causing significantly higher inhibition and disruption than the larger size particles or AgNO3. These results demonstrate the importance of particle size in determining the hazardous impact of AgNPs in the gills and intestines of adult zebrafish.

Transcriptional regulators of Na,K-ATPase subunits

The Na,K-ATPase classically serves as an ion pump creating an electrochemical gradient across the plasma membrane that is essential for transepithelial transport, nutrient uptake and membrane potential. In addition, Na,K-ATPase also functions as a receptor, a signal transducer and a cell adhesion molecule. With such diverse roles, it is understandable that the Na,K-ATPase subunits, the catalytic α-subunit, the β-subunit and the FXYD proteins, are controlled extensively during development and to accommodate physiological needs. The spatial and temporal expression of Na,K-ATPase is partially regulated at the transcriptional level. Numerous transcription factors, hormones, growth factors, lipids, and extracellular stimuli modulate the transcription of the Na,K-ATPase subunits. Moreover, epigenetic mechanisms also contribute to the regulation of Na,K-ATPase expression. With the ever growing knowledge about diseases associated with the malfunction of Na,K-ATPase, this review aims at summarizing the best-characterized transcription regulators that modulate Na,K-ATPase subunit levels. As abnormal expression of Na,K-ATPase subunits has been observed in many carcinoma, we will also discuss transcription factors that are associated with epithelial-mesenchymal transition, a crucial step in the progression of many tumors to malignant disease.

Expression of rat Na-K-ATPase α2 enables ion pumping but not ouabain-induced signaling in α1-deficient porcine renal epithelial cells

Na-K-ATPase is a fundamental component of ion transport. Four α isoforms of the Na-K-ATPase catalytic α subunit are expressed in human cells. The ubiquitous Na-K-ATPase α1 was recently discovered to also mediate signal transduction through Src kinase. In contrast, α2 expression is limited to a few cell types including myocytes, where it is coupled to the Na(+)/Ca(2+) exchanger. To test whether rat Na-K-ATPase α2 is capable of cellular signaling like its α1 counterpart in a recipient mammalian system, we used an α1 knockdown pig renal epithelial cell (PY-17) to create an α2-expressing cell line with no detectable level of α1 expression. These cells exhibited normal ouabain-sensitive ATPase, but failed to effectively regulate Src. In contrast to α1-expressing cells, ouabain did not stimulate Src kinase or downstream effectors such as ERK and Akt in α2 cells, although their signaling apparatus was intact as evidenced by EGF-mediated signal transduction. Additionally, α2 cells were unable to rescue caveolin-1. Unlike the NaKtide sequence derived from Na-K-ATPase α1, which downregulates basal Src activity, the corresponding α2 NaKtide was unable to inhibit Src in vitro. Finally, coimmunoprecipitation of cellular Src was diminished in α2 cells. These findings indicate that Na-K-ATPase α2 does not regulate Src and, therefore, may not serve the same role in signal transduction as α1. This further implies that the signaling mechanism of Na-K-ATPase is isoform specific, thereby supporting a model where α1 and α2 isoforms play distinct roles in mediating contraction and signaling in myocytes.

Thermodynamics of information processing based on enzyme kinetics: An exactly solvable model of an information pump

Motivated by the recent proposed models of the information engine [Proc. Natl. Acad. Sci. USA 109, 11641 (2012)] and the information refrigerator [Phys. Rev. Lett. 111, 030602 (2013)], we propose a minimal model of the information pump and the information eraser based on enzyme kinetics. This device can either pump molecules against the chemical potential gradient by consuming the information to be encoded in the bit stream or (partially) erase the information initially encoded in the bit stream by consuming the Gibbs free energy. The dynamics of this model is solved exactly, and the "phase diagram" of the operation regimes is determined. The efficiency and the power of the information machine is analyzed. The validity of the second law of thermodynamics within our model is clarified. Our model offers a simple paradigm for the investigating of the thermodynamics of information processing involving the chemical potential in small systems.

ATPases, ion exchangers and human sperm motility

Human sperm has several mechanisms to control its ionic milieu, such as the Na,K-ATPase (NKA), the Ca-ATPase of the plasma membrane (PMCA), the Na+/Ca2+-exchanger (NCX) and the Na+/H+-exchanger (NHE). On the other hand, the dynein-ATPase is the intracellular motor for sperm motility. In this work, we evaluated NKA, PMCA, NHE, NCX and dynein-ATPase activities in human sperm and investigated their correlation with sperm motility. Sperm motility was measured by Computer Assisted Semen Analysis. It was found that the NKA activity is inhibited by ouabain with twoKi(7.9×10−9and 9.8×10−5 M), which is consistent with the presence of two isoforms of α subunit of the NKA in the sperm plasma membranes (α1 and α4), being α4 more sensitive to ouabain. The decrease in NKA activity is associated with a reduction in sperm motility. In addition, sperm motility was evaluated in the presence of known inhibitors of NHE, PMCA and NCX, such as amiloride, eosin, and KB-R7943, respectively, as well as in the presence of nigericin after incubation with ouabain. Amiloride, eosin and KB-R7943 significantly reduced sperm motility. Nigericin reversed the effect of ouabain and amiloride on sperm motility. Dynein-ATPase activity was inhibited by acidic pH and micromolar concentrations of Ca2+. We explain our results in terms of inhibition of the dynein-ATPase in the presence of higher cytosolic H+and Ca2+, and therefore inhibition of sperm motility.

Signaling function of Na,K-ATPase induced by ouabain against LPS as an inflammation model in hippocampus

BACKGROUND

Ouabain (OUA) is a newly recognized hormone that is synthesized in the adrenal cortex and hypothalamus. Low doses of OUA can activate a signaling pathway by interaction with Na,K-ATPase, which is protective against a number of insults. OUA has central and peripheral anti-inflammatory effects. Lipopolysaccharide (LPS), via toll-like receptor 4 activation, is a widely used model to induce systemic inflammation. This study used a low OUA dose to evaluate its effects on inflammation induced by LPS injection in rats.

METHODS

Adult male Wistar rats received acute intraperitoneal (ip) OUA (1.8 μg/kg) or saline 20 minutes before LPS (200 μg/kg, ip) or saline injection. Some of the animals had their femoral artery catheterized in order to assess arterial blood pressure values before and after OUA administration. Na,K-ATPase activity, cytokine mRNA levels, apoptosis-related proteins, NF-κB activation brain-derived neurotrophic factor BDNF, corticosterone and TNF-α levels were measured.

RESULTS

OUA pretreatment decreased mRNA levels of the pro-inflammatory cytokines, inducible nitric oxide synthase (iNOS) and IL-1β, which are activated by LPS in the hippocampus, but with no effect on serum measures of these factors. None of these OUA effects were linked to Na,K-ATPase activity. The involvement of the inflammatory transcription factor NF-κB in the OUA effect was indicated by its prevention of LPS-induced nuclear translocation of the NF-κB subunit, RELA (p65), as well as the decreased cytosol levels of the NF-κB inhibitor, IKB, in the hippocampus. OUA pretreatment reversed the LPS-induced glial fibrillary acidic protein (GFAP) activation and associated inflammation in the dentate gyrus. OUA also prevented LPS-induced increases in the hippocampal Bax/Bcl2 ratio suggesting an anti-apoptotic action in the brain.

CONCLUSION

Our results suggest that a low dose of OUA has an important anti-inflammatory effect in the rat hippocampus. This effect was associated with decreased GFAP induction by LPS in the dentate gyrus, a brain area linked to adult neurogenesis.

A direct role for ATP1A1 in unconventional secretion of fibroblast growth factor 2

Previous studies proposed a role for the Na/K-ATPase in unconventional secretion of fibroblast growth factor 2 (FGF2). This conclusion was based upon pharmacological inhibition of FGF2 secretion in the presence of ouabain. However, neither independent experimental evidence nor a potential mechanism was provided. Based upon an unbiased RNAi screen, we now report the identification of ATP1A1, the α1-chain of the Na/K-ATPase, as a factor required for efficient secretion of FGF2. As opposed to ATP1A1, down-regulation of the β1- and β3-chains (ATP1B1 and ATP1B3) of the Na/K-ATPase did not affect FGF2 secretion, suggesting that they are dispensable for this process. These findings indicate that it is not the membrane potential-generating function of the Na/K-ATPase complex but rather a so far unidentified role of potentially unassembled α1-chains that is critical for unconventional secretion of FGF2. Consistently, in the absence of β-chains, we found a direct interaction between the cytoplasmic domain of ATP1A1 and FGF2 with submicromolar affinity. Based upon these observations, we propose that ATP1A1 is a recruitment factor for FGF2 at the inner leaflet of plasma membranes that may control phosphatidylinositol 4,5-bisphosphate-dependent membrane translocation as part of the unconventional secretory pathway of FGF2.

Advances in understanding the role of cardiac glycosides in control of sodium transport in renal tubules

Cardiotonic steroids have been used for the past 200 years in the treatment of congestive heart failure. As specific inhibitors of membrane-bound Na(+)/K(+) ATPase, they enhance cardiac contractility through increasing myocardial cell calcium concentration in response to the resulting increase in intracellular Na concentration. The half-minimal concentrations of cardiotonic steroids required to inhibit Na(+)/K(+) ATPase range from nanomolar to micromolar concentrations. In contrast, the circulating levels of cardiotonic steroids under physiological conditions are in the low picomolar concentration range in healthy subjects, increasing to high picomolar levels under pathophysiological conditions including chronic kidney disease and heart failure. Little is known about the physiological function of low picomolar concentrations of cardiotonic steroids. Recent studies have indicated that physiological concentrations of cardiotonic steroids acutely stimulate the activity of Na(+)/K(+) ATPase and activate an intracellular signaling pathway that regulates a variety of intracellular functions including cell growth and hypertrophy. The effects of circulating cardiotonic steroids on renal salt handling and total body sodium homeostasis are unknown. This review will focus on the role of low picomolar concentrations of cardiotonic steroids in renal Na(+)/K(+) ATPase activity, cell signaling, and blood pressure regulation.

Molecular characterization and transcriptional regulation of the Na +/K+ ATPase α subunit isoforms during development and salinity challenge in a teleost fish, the Senegalese sole (Solea senegalensis)

In the present work, five genes encoding different Na(+),K(+) ATPase (NKA) α-isoforms in the teleost Solea senegalensis are described for the first time. Sequence analysis of predicted polypeptides revealed a high degree of conservation across teleosts and mammals. Phylogenetic analysis clustered the five genes into three main clades: α1 (designated atp1a1a and atp1a1b), α2 (designated atp1a2) and α3 (designated atp1a3a and atp1a3b) isoforms. Transcriptional analysis in larvae showed distinct expression profiles during development. In juvenile tissues, the atp1a1a gene was highly expressed in osmoregulatory organs, atp1a2 in skeletal muscle, atp1a1b in brain and heart and atp1a3a and atp1a3b mainly in brain. Quantification of mRNA abundance after a salinity challenge showed that atp1a1a transcript levels increased significantly in the gill of soles transferred to high salinity water (60 ppt). In contrast, atp1a3a transcripts increased at low salinity (5 ppt). In situ hybridization (ISH) analysis revealed that the number of ionocytes expressing atp1a1a transcripts in the primary gill filaments was higher at 35 and 60 ppt than at 5 ppt and remained undetectable or at very low levels in the lamellae at 5 and 35 ppt but increased at 60 ppt. Immunohistochemistry showed a higher number of positive cells in the lamellae. Whole-mount analysis of atp1a1a mRNA in young sole larvae revealed that it was localized in gut, pronephric tubule, gill, otic vesicle, yolk sac ionocytes and chordacentrum. Moreover, atp1a1a mRNAs increased at mouth opening (3 DPH) in larvae incubated at 36 ppt with a greater signal in gills.

Involvement of Na/K-ATPase in hydrogen peroxide-induced activation of the Src/ERK pathway in LLC-PK1 cells

We have shown that Na/K-ATPase interacts with Src. Here, we test the role of this interaction in H2O2-induced activation of Src and ERK. We found that exposure of LLC-PK1 cells to H2O2 generated by the addition of glucose oxidase into the culture medium activated Src and ERK1/2. It also caused a modest reduction in the number of surface Na/K-ATPases and in ouabain-sensitive Rb(+) uptake. These effects of H2O2 seem similar to those induced by ouabain, a specific ligand of Na/K-ATPase, in LLC-PK1 cells. In accordance, we found that the effects of H2O2 on Src and ERK1/2 were inhibited in α1 Na/K-ATPase-knockdown PY-17 cells. Whereas expression of wild-type α1 or the A420P mutant α1 defective in Src regulation rescued the pumping activity in PY-17 cells, only α1, and not the A420P mutant, was able to restore the H2O2-induced activation of protein kinases. Consistent with this, disrupting the formation of the Na/K-ATPase/Src complex with pNaKtide attenuated the effects of H2O2 on the kinases. Moreover, a direct effect of H2O2 on Na/K-ATPase-mediated regulation of Src was demonstrated. Finally, H2O2 reduced the expression of E-cadherin through the Na/K-ATPase/Src-mediated signaling pathway. Taken together, the data suggest that the Na/K-ATPase/Src complex may serve as one of the receptor mechanisms for H2O2 to regulate Src/ERK protein kinases and consequently the phenotype of renal epithelial cells.

β-Catenin regulates membrane potential in muscle cells by regulating the α2 subunit of Na,K-ATPase

Muscle β-catenin has been shown to play a role in the formation of the neuromuscular junction (NMJ). Our previous studies showed that muscle-specific conditional knockout of β-catenin (HSA-β-cat(-/-) ) results in early postnatal death in mice. To understand the underlying mechanisms, we investigated the electrophysiological properties of muscle cells from HSA-β-cat(-/-) and control mice, and found that, in the absence of muscle β-catenin, the resting membrane potential (RMP) depolarised in muscle cells from the diaphragm, gastrocnemius and extensor digitorum longus muscles. Furthermore, in a primary line of mouse myoblasts (C2C12 cells) transfected with small-interfering RNAs targeting β-catenin, the RMP was depolarised as well. Finally, the expression levels of the α2 subunit of sodium/potassium adenosine triphosphatase were reduced by β-catenin knockdown in vitro or deletion in vivo. These results suggest a possible mechanism underlying the depolarised RMP in the absence of muscle β-catenin, and provide additional evidence supporting a role for β-catenin in the development of NMJs.

Increased constrictor tone induced by ouabain treatment in rats

Ouabain (Oua)-induced hypertension in rodents provides a model to study cardiovascular changes associated with human hypertension. We examined vascular function in rats after a long-term treatment with Oua. Systolic blood pressure was measured by tail-cuff plethysmography in male Sprague-Dawley rats treated with Oua (≈ 25 µg/d) or placebo for 8 weeks. Blood pressure increased in Oua-treated animals, reaching 30% above baseline systolic blood pressure after 7 weeks. At the end of treatment, vascular responses were studied in mesenteric resistance arteries (MRAs) by wire myography. Contraction to potassium chloride in intact and denuded arteries showed greater sensitivity in Oua-treated animals. Contraction to phenylephrine and relaxation to acetylcholine were similar between groups with a lower response to sodium nitroprusside in Oua-treated arteries. Sensitivity to endothelin-1 was higher in Oua-treated arteries. Na⁺-K⁺ ATPase activity was decreased in MRAs from Oua-treated animals, whereas protein expression of the Na⁺-K⁺ ATPase α₂ isoform was increased in heart and unchanged in mesenteric artery. Preincubation with indomethacin (10⁻⁵ M) or Nω-nitro-L-arginine methyl ester (10⁻⁴ M) abolished the differences in potassium chloride response and Na⁺-K⁺ ATPase activity. Changes in MRAs are consistent with enhanced vascular smooth muscle cell reactivity, a contributor to the increased vascular tone observed in this model of hypertension.

Stoichiometric relationship between Na(+) ions transported and glucose consumed in human erythrocytes: Bayesian analysis of (23)Na and (13)C NMR time course data

We examined the response of Na(+),K(+)-ATPase (NKA) to monensin, a Na(+) ionophore, with and without ouabain, an NKA inhibitor, in suspensions of human erythrocytes (red blood cells). A combination of (13)C and (23)Na NMR methods allowed the recording of intra- and extracellular Na(+), and (13)C-labeled glucose time courses. The net influx of Na(+) and the consumption of glucose were measured with and without NKA inhibited by ouabain. A Bayesian analysis was used to determine probability distributions of the parameter values of a minimalist mathematical model of the kinetics involved, and then used to infer the rates of Na(+) transported and glucose consumed. It was estimated that the numerical relationship between the number of Na(+) ions transported by NKA per molecule of glucose consumed by a red blood cell was close to the ratio 6.0:1.0, agreeing with theoretical prediction.

Adaptive evolution of the osmoregulation-related genes in cetaceans during secondary aquatic adaptation

Background

Osmoregulation was a primary challenge for cetaceans during the evolutionary transition from a terrestrial to a mainly hyperosmotic environment. Several physiological mechanisms have been suggested to maintain the water and salt balance in cetaceans, but their genetic and evolutionary bases remain poorly explored. The current study investigated the genes involved in osmoregulation in cetaceans and compared them with their counterparts in terrestrial mammals to test whether adaptive evolution occurred during secondary aquatic adaptation.

Results

The present study analyzed the molecular evolution of 11 osmoregulation-related genes in 11 cetacean species, which represented all of the major cetacean clades. The results demonstrated positive selection acting on angiotensin converting enzyme (ACE), angiotensinogen (AGT), SLC14A2, and aquaporin 2 (AQP2). This evidence for the positive selection of AQP2 and SLC14A2 suggests that the adaptive evolution of these genes has helped to enhance the capacity for water and urea transport, thereby leading to the concentration of urine, which is an efficient mechanism for maintaining the water balance. By contrast, a series of positively selected amino acid residues identified in the ACE and AGT (two key members of the renin-angiotensin-aldosterone system, RAAS) proteins of cetaceans suggests that RAAS might have been adapted to maintain the water and salt balance in response to a hyperosmotic environment. Radical amino acid changes in positively selected sites were distributed among most internal and terminal branches of the cetacean phylogeny, which suggests the pervasively adaptive evolution of osmoregulation since the origin of cetaceans and their subsequent diversification.

Conclusions

This is the first comprehensive analysis of the molecular evolution of osmoregulation-related genes in cetaceans in response to selection pressure from a generally hyperosmotic environment. Four genes, i.e., AQP2, SLC14A2, ACE, and AGT were subject to positive selection in cetaceans, which suggests that cetaceans may have adapted to maintain their water and salt balance. This also suggests that cetaceans may have evolved an effective and complex mechanism for osmoregulation.

Effects of obesity and estradiol on Na+/K+-ATPase and their relevance to cardiovascular diseases

Obesity is associated with aberrant sodium/potassium-ATPase (Na(+)/K(+)-ATPase) activity, apparently linked to hyperglycemic hyperinsulinemia, which may repress or inactivate the enzyme. The reduction of Na(+)/K(+)-ATPase activity in cardiac tissue induces myocyte death and cardiac dysfunction, leading to the development of myocardial dilation in animal models; this has also been documented in patients with heart failure (HF). During several pathological situations (cardiac insufficiency and HF) and in experimental models (obesity), the heart becomes more sensitive to the effect of cardiac glycosides, due to a decrease in Na(+)/K(+)-ATPase levels. The primary female sex steroid estradiol has long been recognized to be important in a wide variety of physiological processes. Numerous studies, including ours, have shown that estradiol is one of the major factors controlling the activity and expression of Na(+)/K(+)-ATPase in the cardiovascular (CV) system. However, the effects of estradiol on Na(+)/K(+)-ATPase in both normal and pathological conditions, such as obesity, remain unclear. Increasing our understanding of the molecular mechanisms by which estradiol mediates its effects on Na(+)/K(+)-ATPase function may help to develop new strategies for the treatment of CV diseases. Herein, we discuss the latest data from animal and clinical studies that have examined how pathophysiological conditions such as obesity and the action of estradiol regulate Na(+)/K(+)-ATPase activity.

Hypoxia-induced lipid peroxidation in the brain during postnatal ontogenesis

Reactive oxygen species (ROS) are common products of the physiological metabolic reactions, which are associated with cell signaling and with the pathogenesis of various nervous disorders. The brain tissue has the high rate of oxidative metabolic activity, high concentration of polyunsaturated fatty acids in membrane lipids, presence of iron ions and low capacity of antioxidant enzymes, which makes the brain very susceptible to ROS action and lipid peroxidation formation. Membranes of brain cortex show a higher production of thiobarbituric acid-reactive substances (TBARS) in prooxidant system (ADP.Fe(3+)/NADPH) than membranes from the heart or kidney. Lipid peroxidation influences numerous cellular functions through membrane-bound receptors or enzymes. The rate of brain cortex Na(+),K(+)-ATPase inhibition correlates well with the increase of TBARS or conjugated dienes and with changes of membrane fluidity. The experimental model of short-term hypoxia (simulating an altitude of 9000 m for 30 min) shows remarkable increase in TBARS in four different parts of the rat brain (cortex, subcortical structures, cerebellum and medulla oblongata) during the postnatal development of Wistar rat of both sexes. Young rats and males are more sensitive to oxygen changes than adult rats and females, respectively. Under normoxia or hypobaric hypoxia both ontogenetic aspects and sex differences play a major role in establishing the activity of erythrocyte catalase, which is an important part of the antioxidant defense of the organism. Rats pretreated with L-carnitine (and its derivatives) have lower TBARS levels after the exposure to hypobaric hypoxia. The protective effect of L-carnitine is comparable with the effect of tocopherol, well-known reactive species scavenger. Moreover, the plasma lactate increases after a short-term hypobaric hypoxia and decreases in L-carnitine pretreated rats. Acute hypobaric hypoxia and/or L-carnitine-pretreatment modify serum but not brain lactate dehydrogenase activity. The obtained data seem to be important because the variations in oxygen tension represent specific signals of regulating the activity of many specific systems in the organism.

Aldosterone-induced ENaC and basal Na+/K+-ATPase trafficking via protein kinase D1-phosphatidylinositol 4-kinaseIIIβ trans Golgi signalling in M1 cortical collecting duct cells

Aldosterone regulates Na(+) transport in the distal nephron through multiple mechanisms that include the transcriptional control of epithelial sodium channel (ENaC) and Na(+)/K(+)-ATPase subunits. Aldosterone also induces the rapid phosphorylation of Protein Kinase D1 (PKD1). PKD isoforms regulate protein trafficking, by the control of vesicle fission from the trans Golgi network (TGN) through activation of phosphatidylinositol 4-kinaseIIIβ (PI4KIIIβ). We report rapid ENaCγ translocation to the plasma membrane after 30 min aldosterone treatment in polarized M1 cortical collecting duct cells, which was significantly impaired in PKD1 shRNA-mediated knockdown cells. In PKD1-deficient cells, the ouabain-sensitive current was significantly reduced and Na(+)/K(+)-ATPase α and β subunits showed aberrant localization. PKD1 and PI4KIIIβ localize to the TGN, and aldosterone induced an interaction between PKD1 and PI4KIIIβ following aldosterone treatment. This study reveals a novel mechanism for rapid regulation of ENaC and the Na(+)/K(+)-ATPase, via directed trafficking through PKD1-PI4KIIIβ signalling at the level of the TGN.