Na⁺i,K⁺i-Dependent and -Independent Signaling Triggered by Cardiotonic Steroids: Facts and Artifacts

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

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

The Na/K-ATPase role as a signal transducer in lung inflammation

Acute respiratory distress syndrome (ARDS) is marked by damage to the capillary endothelium and alveolar epithelium following edema formation and cell infiltration. Currently, there are no effective treatments for severe ARDS. Pathologies such as sepsis, pneumonia, fat embolism, and severe trauma may cause ARDS with respiratory failure. The primary mechanism of edema clearance is the epithelial cells' Na/K-ATPase (NKA) activity. NKA is an enzyme that maintains the electrochemical gradient and cell homeostasis by transporting Na+ and K+ ions across the cell membrane. Direct injury on alveolar cells or changes in ion transport caused by infections decreases the NKA activity, loosening tight junctions in epithelial cells and causing edema formation. In addition, NKA acts as a receptor triggering signal transduction in response to the binding of cardiac glycosides. The ouabain (a cardiac glycoside) and oleic acid induce lung injury by targeting NKA. Besides enzymatic inhibition, the NKA triggers intracellular signal transduction, fostering proinflammatory cytokines production and contributing to lung injury. Herein, we reviewed and discussed the crucial role of NKA in edema clearance, lung injury, and intracellular signaling pathway activation leading to lung inflammation, thus putting the NKA as a protagonist in lung injury pathology.

Timed use of digoxin prevents heart ischemia-reperfusion injury through a REV-ERBα-UPS signaling pathway

Myocardial ischemia-reperfusion injury (MIRI) induces life-threatening damages to the cardiac tissue and pharmacological means to achieve cardioprotection are sorely needed. MIRI severity varies along the day-night cycle and is molecularly linked to components of the cellular clock including the nuclear receptor REV-ERBα, a transcriptional repressor. Here we show that digoxin administration in mice is cardioprotective when timed to trigger REV-ERBα protein degradation. In cardiomyocytes, digoxin increases REV-ERBα ubiquitinylation and proteasomal degradation, which depend on REV-ERBα ability to bind its natural ligand, heme. Inhibition of the membrane-bound Src tyrosine-kinase partially alleviated digoxin-induced REV-ERBα degradation. In untreated cardiomyocytes, REV-ERBα proteolysis is controlled by known (HUWE1, FBXW7, SIAH2) or novel (CBL, UBE4B) E3 ubiquitin ligases and the proteasome subunit PSMB5. Only SIAH2 and PSMB5 contributed to digoxin-induced degradation of REV-ERBα. Thus, controlling REV-ERBα proteostasis through the ubiquitin-proteasome system is an appealing cardioprotective strategy. Our data support the timed use of clinically-approved cardiotonic steroids in prophylactic cardioprotection.

Na,K-ATPase Acts as a Beta-Amyloid Receptor Triggering Src Kinase Activation

Beta-amyloid (Aβ) has a dual role, both as an important factor in the pathology of Alzheimer's disease and as a regulator in brain physiology. The inhibitory effect of Aβ₄₂ oligomers on Na,K-ATPase contributes to neuronal dysfunction in Alzheimer's disease. Still, the physiological role of the monomeric form of Aβ₄₂ interaction with Na,K-ATPase remains unclear. We report that Na,K-ATPase serves as a receptor for Aβ₄₂ monomer, triggering Src kinase activation. The co-localization of Aβ₄₂ with α1- and β1-subunits of Na,K-ATPase, and Na,K-ATPase with Src kinase in SH-SY5Y neuroblastoma cells, was observed. Treatment of cells with 100 nM Aβ₄₂ causes Src kinase activation, but does not alter Na,K-ATPase transport activity. The interaction of Aβ₄₂ with α1β1 Na,K-ATPase isozyme leads to activation of Src kinase associated with the enzyme. Notably, prevention of Na,K-ATPase:Src kinase interaction by a specific inhibitor pNaKtide disrupts the Aβ-induced Src kinase activation. Stimulatory effect of Aβ₄₂ on Src kinase was lost under hypoxic conditions, which was similar to the effect of specific Na,K-ATPase ligands, the cardiotonic steroids. Our findings identify Na,K-ATPase as a Aβ₄₂ receptor, thus opening a prospect on exploring the physiological and pathological Src kinase activation caused by Aβ₄₂ in the nervous system.

Sodium Ions as Regulators of Transcription in Mammalian Cells

The maintenance of an uneven distribution of Na+ and K+ ions between the cytoplasm and extracellular medium is the basis for the functioning of any animal cell. Changes in the intracellular ratio of these cations occur in response to numerous stimuli and are important for the cell activity regulation. Numerous experimental data have shown that gene transcription in mammalian cells can be regulated by changes in the intracellular [Na+]_i/[K+]_i ratio. Here, we discuss possible mechanisms of such regulation in various cell types, with special attention to the [Ca2+]-independent signaling pathways that suggest the presence of an intracellular sensor of monovalent cations. As such sensor, we propose the secondary structures of nucleic acids called G-quadruplexes. They are widely represented in mammalian genomes and are often found in the promoters of genes encoding transcription factors.

The Cardenolide Glycoside Acovenoside A Interferes with Epidermal Growth Factor Receptor Trafficking in Non-Small Cell Lung Cancer Cells

Cardenolide glycosides are natural compounds known to inhibit the ion pumping function of the Na+/K+-ATPase in cellular systems. Interestingly, various cancer cell types are highly susceptible to cardenolide glycosides. Herein, we explore the cardenolide glycoside Acovenoside A (AcoA) with respect to its influences on human A549 non-small cell lung cancer (NSCLC) cells. We found that exposure to AcoA, digoxin and ouabain increases intracellular sodium and ATP levels indicating that the ion pumping function of the transmembrane Na+/K+-ATPase is effectively inhibited. Like digoxin and ouabain, AcoA inhibits transcription factor NF-κB activation and induces apoptotic cell death in NSCLC cells. This was confirmed by a preclinical in vivo model in which AcoA treatment of NSCLC xenografts grown on chick chorioallantoic membranes inhibited the expression of proliferation antigen Ki-67 and induced apoptotic DNA strand breaks. We aimed to elucidate the underlying mechanisms. The Na+/K+-ATPase transmembrane complex contains Src kinase and epidermal growth factor receptor (EGFR). Indeed, we found that AcoA activates Src kinase in A549 cells, but not in a cell-free assay using recombinant Src kinase. Src kinase is a downstream target of EGFR, and correlation analysis using the NCI60 database pointed to a role of EGFR in cardenolide glycoside-induced cancer cell death. Accordingly, NSCLC cells expressing hyperphosphorylated EGFRmut exhibited resistance to AcoA. To investigate the interaction between cardenolide glycosides and EGFR in detail, we performed immunoblotting studies: Whereas ligand binding and EGFR phosphorylation were not significantly affected, ubiquitinated EGFR accumulated after prolonged incubation with AcoA. To visualize EGFR trafficking we used A549 cells transfected with a fluorescent biosensor which binds to activated EGFR. Pretreatment with AcoA and digoxin induced accumulation of EGFR in endosomal compartments thus inhibiting EGF-induced EGFR degradation comparable to the Na+ ionophore monensin, a known inducer of EGFR endosomal arrest. Intracellular Na+ concentrations regulate EGFR trafficking and signaling. Na+ homeostasis is maintained by the Na+/K+-ATPase, which might account for its close interaction with the EGFR. Cardenolide glycosides inhibit the ATP-dependent Na+/K+ exchange through the Na+/K+-ATPase resulting in higher intracellular Na+ levels. Our data provide first evidence that this impedes efficient EGFR trafficking at the endosomal compartment.

Nutraceutical, Dietary, and Lifestyle Options for Prevention and Treatment of Ventricular Hypertrophy and Heart Failure

Although well documented drug therapies are available for the management of ventricular hypertrophy (VH) and heart failure (HF), most patients nonetheless experience a downhill course, and further therapeutic measures are needed. Nutraceutical, dietary, and lifestyle measures may have particular merit in this regard, as they are currently available, relatively safe and inexpensive, and can lend themselves to primary prevention as well. A consideration of the pathogenic mechanisms underlying the VH/HF syndrome suggests that measures which control oxidative and endoplasmic reticulum (ER) stress, that support effective nitric oxide and hydrogen sulfide bioactivity, that prevent a reduction in cardiomyocyte pH, and that boost the production of protective hormones, such as fibroblast growth factor 21 (FGF21), while suppressing fibroblast growth factor 23 (FGF23) and marinobufagenin, may have utility for preventing and controlling this syndrome. Agents considered in this essay include phycocyanobilin, N-acetylcysteine, lipoic acid, ferulic acid, zinc, selenium, ubiquinol, astaxanthin, melatonin, tauroursodeoxycholic acid, berberine, citrulline, high-dose folate, cocoa flavanols, hawthorn extract, dietary nitrate, high-dose biotin, soy isoflavones, taurine, carnitine, magnesium orotate, EPA-rich fish oil, glycine, and copper. The potential advantages of whole-food plant-based diets, moderation in salt intake, avoidance of phosphate additives, and regular exercise training and sauna sessions are also discussed. There should be considerable scope for the development of functional foods and supplements which make it more convenient and affordable for patients to consume complementary combinations of the agents discussed here. Research Strategy: Key word searching of PubMed was employed to locate the research papers whose findings are cited in this essay.

Ouabain-Induced Gene Expression Changes in Human iPSC-Derived Neuron Culture Expressing Dopamine and cAMP-Regulated Phosphoprotein 32 and GABA Receptors

Cardiotonic steroids (CTS) are specific inhibitors and endogenous ligands of a key enzyme in the CNS-the Na⁺, K⁺-ATPase, which maintains and creates an ion gradient on the plasma membrane of neurons. CTS cause the activation of various signaling cascades and changes in gene expression in neurons and other cell types. It is known that intracerebroventricular injection of cardiotonic steroid ouabain causes mania-like behavior in rodents, in part due to activation of dopamine-related signaling cascades in the dopamine and cAMP-regulated phosphoprotein 32 (DARPP-32) expressing medium spiny neurons in the striatum. Dopaminergic projections in the striatum innervate these GABAergic medium spiny neurons. The objective of this study was to assess changes in the expression of all genes in human iPSC-derived expressing DARPP-32 and GABA receptors neurons under the influence of ouabain. We noted a large number of statistically significant upregulated and downregulated genes after a 16-h incubation with non-toxic concentration (30 nM) of ouabain. These changes in the transcriptional activity were accomplished with activation of MAP-kinase ERK1/2 and transcriptional factor cAMP response element-binding protein (CREB). Thus, it can be concluded that 30 nM ouabain incubated for 16 h with human iPSC-derived expressing DARPP-32 and GABA receptors neurons activates genes associated with neuronal maturation and synapse formation, by increasing the expression of genes associated with translation, vesicular transport, and increased electron transport chain function. At the same time, the expression of genes associated with proliferation, migration, and early development of neurons decreases. These data indicate that non-toxic concentrations of ouabain may induce neuronal maturation, neurite growth, and increased synaptogenesis in dopamine-receptive GABAergic neurons, suggesting formation of plasticity and the establishment of new neuronal junctions.

Ouabain Suppresses IL-6/STAT3 Signaling and Promotes Cytokine Secretion in Cultured Skeletal Muscle Cells

The cardiotonic steroids (CTS), such as ouabain and marinobufagenin, are thought to be adrenocortical hormones secreted during exercise and the stress response. The catalytic α-subunit of Na,K-ATPase (NKA) is a CTS receptor, whose largest pool is located in skeletal muscles, indicating that muscles are a major target for CTS. Skeletal muscles contribute to adaptations to exercise by secreting interleukin-6 (IL-6) and plethora of other cytokines, which exert paracrine and endocrine effects in muscles and non-muscle tissues. Here, we determined whether ouabain, a prototypical CTS, modulates IL-6 signaling and secretion in the cultured human skeletal muscle cells. Ouabain (2.5–50 nM) suppressed the abundance of STAT3, a key transcription factor downstream of the IL-6 receptor, as well as its basal and IL-6-stimulated phosphorylation. Conversely, ouabain (50 nM) increased the phosphorylation of ERK1/2, Akt, p70S6K, and S6 ribosomal protein, indicating activation of the ERK1/2 and the Akt-mTOR pathways. Proteasome inhibitor MG-132 blocked the ouabain-induced suppression of the total STAT3, but did not prevent the dephosphorylation of STAT3. Ouabain (50 nM) suppressed hypoxia-inducible factor-1α (HIF-1α), a modulator of STAT3 signaling, but gene silencing of HIF-1α and/or its partner protein HIF-1β did not mimic effects of ouabain on the phosphorylation of STAT3. Ouabain (50 nM) failed to suppress the phosphorylation of STAT3 and HIF-1α in rat L6 skeletal muscle cells, which express the ouabain-resistant α1-subunit of NKA. We also found that ouabain (100 nM) promoted the secretion of IL-6, IL-8, GM-CSF, and TNF-α from the skeletal muscle cells of healthy subjects, and the secretion of GM-CSF from cells of subjects with the type 2 diabetes. Marinobufagenin (10 nM), another important CTS, did not alter the secretion of these cytokines. In conclusion, our study shows that ouabain suppresses the IL-6 signaling via STAT3, but promotes the secretion of IL-6 and other cytokines, which might represent a negative feedback in the IL-6/STAT3 pathway. Collectively, our results implicate a role for CTS and NKA in regulation of the IL-6 signaling and secretion in skeletal muscle.

Retinoschisin and Cardiac Glycoside Crosstalk at the Retinal Na/K-ATPase

Purpose

Mutations in the RS1 gene, which encodes retinoschisin, cause X-linked juvenile retinoschisis, a retinal dystrophy in males. Retinoschisin specifically interacts with the retinal sodium–potassium adenosine triphosphatase (Na/K-ATPase), a transmembrane ion pump. Na/K-ATPases also bind cardiac glycosides, which control the activity of the pump and have been linked to disturbances in retinal homeostasis. In this study, we investigated the crosstalk between retinoschisin and cardiac glycosides at the retinal Na/K-ATPase and the consequences of this interplay on retinal integrity.

Methods

The effect of cardiac glycosides (ouabain and digoxin) on the binding of retinoschisin to the retinal Na/K-ATPase was investigated via western blot and immunocytochemistry. Also, the influence of retinoschisin on the binding of cardiac glycosides was analyzed via enzymatic assays, which quantified cardiac glycoside-sensitive Na/K-ATPase pump activity. Moreover, retinoschisin-dependent binding of tritium-labeled ouabain to the Na/K-ATPase was determined. Finally, a reciprocal effect of retinoschisin and cardiac glycosides on Na/K-ATPase localization and photoreceptor degeneration was addressed using immunohistochemistry in retinoschisin-deficient murine retinal explants.

Results

Cardiac glycosides displaced retinoschisin from the retinal Na/K-ATPase; however, retinoschisin did not affect cardiac glycoside binding. Notably, cardiac glycosides reduced the capacity of retinoschisin to regulate Na/K-ATPase localization and to protect against photoreceptor degeneration.

Conclusions

Our findings reveal opposing effects of retinoschisin and cardiac glycosides on retinal Na/K-ATPase binding and on retinal integrity, suggesting that a fine-tuned interplay between both components is required to maintain retinal homeostasis. This observation provides new insight into the mechanisms underlying the pathological effects of cardiac glycoside treatment on retinal integrity.

Ouabain-Induced Cell Death and Survival. Role of α1-Na,K-ATPase-Mediated Signaling and [Na+]i/[K+]i-Dependent Gene Expression

Ouabain is of cardiotonic steroids (CTS) family that is plant-derived compounds and is known for many years as therapeutic and cytotoxic agents. They are specific inhibitors of Na,K-ATPase, the enzyme, which pumps Na+ and K+ across plasma membrane of animal cells. Treatment of cells by CTS affects various cellular functions connected with the maintenance of the transmembrane gradient of Na+ and K+. Numerous studies demonstrated that binding of CTS to Na,K-ATPase not only suppresses its activity but also induces some signal pathways. This review is focused on different mechanisms of two ouabain effects: their ability (1) to protect rodent cells from apoptosis through the expression of [Na+]i-sensitive genes and (2) to trigger death of non-rodents cells (so-called «oncosis»), possessing combined markers of «classic» necrosis and «classic» apoptosis. Detailed study of oncosis demonstrated that the elevation of the [Na+]i/[K+]i ratio is not a sufficient for its triggering. Non-rodent cell death is determined by the characteristic property of "sensitive" to ouabain α1-subunit of Na,K-ATPase. In this case, ouabain binding leads to enzyme conformational changes triggering the activation of p38 mitogen-activated protein kinases (MAPK) signaling. The survival of rodent cells with ouabain-«resistant» α1-subunit is connected with another conformational transition induced by ouabain binding that results in the activation of ERK 1/2 signaling pathway.

Circulating Ouabain Modulates Expression of Claudins in Rat Intestine and Cerebral Blood Vessels

The ability of exogenous low ouabain concentrations to affect claudin expression and therefore epithelial barrier properties was demonstrated previously in cultured cell studies. We hypothesized that chronic elevation of circulating ouabain in vivo can affect the expression of claudins and tight junction permeability in different tissues. We tested this hypothesis in rats intraperitoneally injected with ouabain (1 μg/kg) for 4 days. Rat jejunum, colon and brain frontal lobes, which are variable in the expressed claudins and tight junction permeability, were examined. Moreover, the porcine jejunum cell line IPEC-J2 was studied. In IPEC-J2-cells, ouabain (10 nM, 19 days of incubation) stimulated epithelial barrier formation, increased transepithelial resistance and the level of cSrc-kinase activation by phosphorylation, accompanied with an increased expression of claudin-1, -5 and down-regulation of claudin-12; the expression of claudin-3, -4, -8 and tricellulin was not changed. In the jejunum, chronic ouabain increased the expression of claudin-1, -3 and -5 without an effect on claudin-2 and -4 expression. In the colon, only down-regulation of claudin-3 was observed. Chronic ouabain protected the intestine transepithelial resistance against functional injury induced by lipopolysaccharide treatment or by modeled acute microgravity; this regulation was most pronounced in the jejunum. Claudin-1 was also up-regulated in cerebral blood vessels. This was associated with reduction of claudin-3 expression while the expression of claudin-5 and occludin was not affected. Altogether, our results confirm that circulating ouabain can functionally and tissue-specifically affect barrier properties of epithelial and endothelial tissues via Na,K-ATPase-mediated modulation of claudins expression.

Ouabain and Marinobufagenin: Physiological Effects on Human Epithelial and Endothelial Cells

Long-term study on the identification of Na,K-ATPase endogenous inhibitors in mammalian tissues has resulted in the discovery of ouabain, marinobufagenin (MBG), and other cardiotonic steroids (CTS) in the blood plasma. Production of ouabain and MBG is increased in essential hypertension and other diseases associated with hypervolemia. Here, we compared the effects of ouabain and MBG on the Na,K-ATPase activity (measured as the transport of Na+, K+, and Rb⁺ ions) and proliferation and death of human renal epithelial cells (HRECs) and human umbilical vein endothelial cells (HUVEC) expressing α1-Na,K-ATPase. Ouabain concentration that provided the half-maximal inhibition of the Rb⁺ influx (IC₅₀) into HRECs and HUVECs was 0.07 μM. In both types of cells, the IC₅₀ values for MBG were 10 times higher than for ouabain. Incubation of HREC and HUVEC with 0.001-0.01 μM ouabain for 30 h resulted in 40% increase in the [3H]thymidine incorporation into DNA; further elevation of ouabain concentration to 0.1 μM completely suppressed DNA synthesis. MBG at the concentration of 0.1 μM activated DNA synthesis by 25% in HRECs, but not in HUVECs; 1 μM MBG completely inhibited DNA synthesis in HRECs and by 50% in HUVECs. In contrast to HRECs, incubation of HUVECs in the serum-free medium induced apoptosis, which was almost completely suppressed by ouabain and MBG at the concentrations of 0.1 and 3 μM, respectively. Based on these data, we can conclude that (i) the effect of MBG at the concentrations detected in the blood plasma (<0.01 μM) on HRECs and HUVECs was not due to the changes in the [Na+]_i/[K+]_i ratio; (ii) the effect of physiological concentrations of ouabain on these cells might be mediated by the activation of Na,K-ATPase, leading to cell proliferation.

Effects of Chronic Caffeine Administration on Behavioral and Molecular Adaptations to Sensory Contact Model Induced Stress in Adolescent Male Mice

Previous studies have shown that caffeine attenuates stress-induced mood dysfunction and memory deterioration through neuronal adenosine A2A receptors antagonism. However, whether caffeine exerts this effect through modulating other molecular targets, which interfere with the resilience to social defeat stress in adolescent male mice is unknown. This study was conducted to investigate the role of caffeine in the behavioral responses to social stress induced by the sensory contact model (SCM) and the possible alteration of the gene expression level of Na/K ATPase pump. Adolescent male mice were exposed to SCM for 12 days. Caffeine was administered intraperitoneal daily for 14 days after SCM. The time spent in interaction zone, social interaction ratio, preference index to novel objects, time spent in the open arms and immobility time in forced swimming test were used to measure the locomotor activity, social avoidance, short-term memory, anxiety and depression in mice. The results showed that chronic treatment with caffeine for 14 days improved locomotor activity, reversed the avoidance of social behavior, improved preference to novel objects, and reversed depression induced by social defeat stress in adolescent male mice, suggesting the enhancement of the resilience to social defeat stress induced by caffeine. Moreover, caffeine treatment did alter gene expression levels of Na/K ATPase isoforms in both prefrontal cortex and hippocampus. Altered gene expression was significant in most cases and correlates with the observed behavioral changes. Taken together, our findings provide new insight into the effects of chronic caffeine administration on locomotor activity, social avoidance, short-term memory and depression in adolescent male mice exposed to SCM.

Na,K-ATPase as a target for endogenous cardiotonic steroids: What's the evidence?

With an exception of few reports, the plasma concentration of ouabain and marinobufagenin, mostly studied cardiotonic steroids (CTS) assessed by immunoassay techniques, is less than 1 nM. During the last 3 decades, the implication of these endogenous CTS in the pathogenesis of hypertension and other volume-expanded disorders is widely disputed. The threshold for inhibition by CTS of human and rodent α1-Na,K-ATPase is ∼1 and 1000 nM, respectively, that rules out the functioning of endogenous CTS (ECTS) as natriuretic hormones and regulators of cell adhesion, cell-to-cell communication, gene transcription and translation, which are mediated by dissipation of the transmembrane gradients of monovalent cations. In several types of cells ouabain and marinobufagenin at concentrations corresponding to its plasma level activate Na,K-ATPase, decrease the [Na⁺]_i/[K⁺]_i-ratio and increase cell proliferation. Possible physiological significance and mechanism of non-canonical Na⁺_i/K⁺_i-dependent and Na⁺_i/K⁺_i-independent cell responses to CTS are discussed.

Bright and Early: Inhibiting Human Cytomegalovirus by Targeting Major Immediate-Early Gene Expression or Protein Function

The human cytomegalovirus (HCMV), one of eight human herpesviruses, establishes lifelong latent infections in most people worldwide. Primary or reactivated HCMV infections cause severe disease in immunosuppressed patients and congenital defects in children. There is no vaccine for HCMV, and the currently approved antivirals come with major limitations. Most approved HCMV antivirals target late molecular processes in the viral replication cycle including DNA replication and packaging. “Bright and early” events in HCMV infection have not been exploited for systemic prevention or treatment of disease. Initiation of HCMV replication depends on transcription from the viral major immediate-early (IE) gene. Alternative transcripts produced from this gene give rise to the IE1 and IE2 families of viral proteins, which localize to the host cell nucleus. The IE1 and IE2 proteins are believed to control all subsequent early and late events in HCMV replication, including reactivation from latency, in part by antagonizing intrinsic and innate immune responses. Here we provide an update on the regulation of major IE gene expression and the functions of IE1 and IE2 proteins. We will relate this insight to experimental approaches that target IE gene expression or protein function via molecular gene silencing and editing or small chemical inhibitors.

Low Ouabain Doses and AMP-Activated Protein Kinase as Factors Supporting Electrogenesis in Skeletal Muscle

Many motor disorders are associated with depolarization of the membrane of skeletal muscle fibers due to the impaired functioning of Na,K-ATPase. Here, we studied the role of ouabain (specific Na,K-ATPase ligand) and AMP-activated protein kinase (key regulator of muscle metabolism) in the maintenance of muscle electrogenesis; the levels of these endogenous factors are directly related to the motor activity. After 4-day intraperitoneal administration of ouabain (1 µg/kg daily), a hyperpolarization of sarcolemma was registered in isolated rat diaphragm muscles due to an increase in the electrogenic activity of Na,K-ATPase. In acute experiments, addition of nanomolar ouabain concentrations to the bathing solution resulted in the muscle membrane hyperpolarization within 15 min. The effect of ouabain reversed to membrane depolarization with the increase in the external potassium concentration. It is possible that Na,K-ATPase activation by ouabain may be regulated by such factors as specific subcellular location, interaction with molecular partners, and changes in the ionic balance. Preventive administration of the AMP-activated protein kinase activator AICAR (5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside; 400 mg/kg body weight daily for 7 days) in chronic experiments resulted in the stabilization of the endplate structure and abolishment of depolarization of the rat soleus muscle membrane caused by the motor activity cessation. The obtained data can be useful for creating approaches for correction of muscle dysfunction, especially at the early stages, prior to the development of muscle atrophy.

Transcriptomic changes triggered by ouabain in rat cerebellum granule cells: Role of α3- and α1-Na+,K+-ATPase-mediated signaling

It was shown previously that inhibition of the ubiquitous α1 isoform of Na⁺,K⁺-ATPase by ouabain sharply affects gene expression profile via elevation of intracellular [Na⁺]_i/[K⁺]_i ratio. Unlike other cells, neurons are abundant in the α3 isoform of Na⁺,K⁺-ATPase, whose affinity in rodents to ouabain is 10⁴-fold higher compared to the α1 isoform. With these sharp differences in mind, we compared transcriptomic changes in rat cerebellum granule cells triggered by inhibition of α1- and α3-Na⁺,K⁺-ATPase isoforms. Inhibition of α1- and α3-Na⁺,K⁺-ATPase isoforms by 1 mM ouabain resulted in dissipation of transmembrane Na⁺ and K⁺ gradients and differential expression of 994 transcripts, whereas selective inhibition of α3-Na⁺,K⁺-ATPase isoform by 100 nM ouabain affected expression of 144 transcripts without any impact on the [Na⁺]_i/[K⁺]_i ratio. The list of genes whose expression was affected by 1 mM ouabain by more than 2-fold was abundant in intermediates of intracellular signaling and transcription regulators, including augmented content of Npas4, Fos, Junb, Atf3, and Klf4 mRNAs, whose upregulated expression was demonstrated in neurons subjected to electrical and glutamatergic stimulation. The role [Na⁺]_i/[K⁺]_i-mediated signaling in transcriptomic changes involved in memory formation and storage should be examined further.

Transcriptomic changes triggered by ouabain in rat cerebellum granule cells: Role of α3- and α1-Na+,K+-ATPase-mediated signaling

It was shown previously that inhibition of the ubiquitous α1 isoform of Na+,K+-ATPase by ouabain sharply affects gene expression profile via elevation of intracellular [Na+]i/[K+]i ratio. Unlike other cells, neurons are abundant in the α3 isoform of Na+,K+-ATPase, whose affinity in rodents to ouabain is 104-fold higher compared to the α1 isoform. With these sharp differences in mind, we compared transcriptomic changes in rat cerebellum granule cells triggered by inhibition of α1- and α3-Na+,K+-ATPase isoforms. Inhibition of α1- and α3-Na+,K+-ATPase isoforms by 1 mM ouabain resulted in dissipation of transmembrane Na+ and K+ gradients and differential expression of 994 transcripts, whereas selective inhibition of α3-Na+,K+-ATPase isoform by 100 nM ouabain affected expression of 144 transcripts without any impact on the [Na+]i/[K+]i ratio. The list of genes whose expression was affected by 1 mM ouabain by more than 2-fold was abundant in intermediates of intracellular signaling and transcription regulators, including augmented content of Npas4, Fos, Junb, Atf3, and Klf4 mRNAs, whose upregulated expression was demonstrated in neurons subjected to electrical and glutamatergic stimulation. The role [Na+]i/[K+]i-mediated signaling in transcriptomic changes involved in memory formation and storage should be examined further.

Cardiotonic Steroids and the Sodium Trade Balance: New Insights into Trade-Off Mechanisms Mediated by the Na⁺/K⁺-ATPase

In 1972 Neal Bricker presented the "trade-off" hypothesis in which he detailed the role of physiological adaptation processes in mediating some of the pathophysiology associated with declines in renal function. In the late 1990's Xie and Askari published seminal studies indicating that the Na⁺/K⁺-ATPase (NKA) was not only an ion pump, but also a signal transducer that interacts with several signaling partners. Since this discovery, numerous studies from multiple laboratories have shown that the NKA is a central player in mediating some of these long-term "trade-offs" of the physiological adaptation processes which Bricker originally proposed in the 1970's. In fact, NKA ligands such as cardiotonic steroids (CTS), have been shown to signal through NKA, and consequently been implicated in mediating both adaptive and maladaptive responses to volume overload such as fibrosis and oxidative stress. In this review we will emphasize the role the NKA plays in this "trade-off" with respect to CTS signaling and its implication in inflammation and fibrosis in target organs including the heart, kidney, and vasculature. As inflammation and fibrosis exhibit key roles in the pathogenesis of a number of clinical disorders such as chronic kidney disease, heart failure, atherosclerosis, obesity, preeclampsia, and aging, this review will also highlight the role of newly discovered NKA signaling partners in mediating some of these conditions.

Glutathionylation of Na,K-ATPase Alpha-Subunit Alters Enzyme Conformation and Sensitivity to Trypsinolysis

We found earlier that Na,K-ATPase is purified from duck salt glands in partially glutathionylated state (up to 13 of the 23 cysteine residues of the Na,K-ATPase catalytic α-subunit can be S-glutathionylated). To determine the effect of glutathionylation on the enzyme conformation, we have analyzed the products of trypsinolysis of Na,K-ATPase α-subunit in different conformations with different extent of glutathionylation. Incubation of the protein in the E1 conformation with trypsin produced a large fragment with a molecular mass (MM) of 80 kDa with the following formation of smaller fragments with MM 40, 35.5, and 23 kDa. Tryptic digestion of Na,K-ATPase in the E2 conformation also resulted in the generation of the fragments with MM 40, 35.5, and 23 kDa. Deglutathionylation of Na,K-ATPase α-subunit increases the rate of proteolysis of the enzyme in both E1 and E2 conformations. The pattern of tryptic digestion of the α-subunit in E2 conformation additionally glutathionylated with oxidized glutathione is similar to that of partially deglutathionylated Na,K-ATPase. The pattern of tryptic digestion of the additionally glutathionylated α-subunit in E1 conformation is similar to that of the native enzyme. The highest rate of trypsinolysis was observed for the α-subunit in complex with ouabain (E2-OBN conformation). Additional glutathionylation increased the content of high-molecular-weight fragments among the digestion products, as compared to the native and deglutathionylated enzymes. The data obtained were confirmed using molecular modeling that revealed that number of sites accessible for trypsinolysis is higher in the E2P-OBN conformation than in the E1- and E2-conformations and that glutathionylation decreases the number of sites accessible for trypsin. Therefore, glutathionylation affects enzyme conformation and its sensitivity to trypsinolysis. The mechanisms responsible for the changes in the Na,K-ATPase sensitivity to trypsinolysis depending on the level of enzyme glutathionylation and increase in the enzyme sensitivity to proteolysis upon its binding to ouabain, as well as physiological role of these phenomena are discussed.