Localization of intracellular compartments that exchange Na,K-ATPase molecules with the plasma membrane in a hormone-dependent manner

Changes in stresses sensitivity of ohmic heating-induced sublethally injured Staphylococcus aureus during repair: Potential mechanisms at the cellular and molecular levels

Ohmic heating (OH), an emerging food processing technology employed in the food processing industry, raises potential food safety concerns due to the recovery of sublethally injured pathogens such as Staphylococcus aureus (S. aureus). In the present study, sensitivity to various stress conditions and the changes in cellular-related factors of OH-injured S. aureus during repair were investigated. The results indicated that liquid media differences (nutrient broth (NB), phosphate-buffered saline (PBS), milk, and cucumber juice) affected the recovery process of injured cells. Nutrient enrichment determines the bacterial repair rate, and the rates of repair for these media were milk > NB > cucumber juice > PBS. The sensitivity of injured cells to various stressors, including different acids, temperature, nisin, simulated gastric fluid, and bile salt, increased during the injury phase and subsequently diminished upon repair. Additionally, the intracellular ATP content, enzyme activities (Na+/K+-ATPase, Ca2+/Mg2+-ATPase, and T-ATPase) and ion concentrations (Mg2+, K+, and Ca2+) gradually increased during repair. After 5 h of repair, the intracellular substances content of cell's was significantly higher than that of the injured bacteria without repair, while some indicators (e.g., Na+/K+-ATPase, K+, and Ca2+) were not restored to the untreated level. The results of this study indicated that OH-injured S. aureus exhibited strengthened resistance post-recovery, potentially due to the restoration of cellular structures. These findings have implications for optimizing food storage conditions and advancing OH processes in the food industry.

Evidence That Binding of Cyclic GMP to the Extracellular Domain of NKA (Sodium-Potassium ATPase) Mediates Natriuresis

BACKGROUND

Extracellular renal interstitial guanosine cyclic 3',5'-monophosphate (cGMP) inhibits renal proximal tubule (RPT) sodium (Na+) reabsorption via Src (Src family kinase) activation. Through which target extracellular cGMP acts to induce natriuresis is unknown. We hypothesized that cGMP binds to the extracellular α1-subunit of NKA (sodium-potassium ATPase) on RPT basolateral membranes to inhibit Na+ transport similar to ouabain-a cardiotonic steroid.

METHODS

Urine Na+ excretion was measured in uninephrectomized 12-week-old female Sprague-Dawley rats that received renal interstitial infusions of vehicle (5% dextrose in water), cGMP (18, 36, and 72 μg/kg per minute; 30 minutes each), or cGMP+rostafuroxin (12 ng/kg per minute) or were subjected to pressure-natriuresis±rostafuroxin infusion. Rostafuroxin is a digitoxigenin derivative that displaces ouabain from NKA.

RESULTS

Renal interstitial cGMP and raised renal perfusion pressure induced natriuresis and increased phosphorylated SrcTyr416 and Erk 1/2 (extracellular signal-regulated protein kinase 1/2)Thr202/Tyr204; these responses were abolished with rostafuroxin coinfusion. To assess cGMP binding to NKA, we performed competitive binding studies with isolated rat RPTs using bodipy-ouabain (2 μM)+cGMP (10 µM) or rostafuroxin (10 µM) and 8-biotin-11-cGMP (2 μM)+ouabain (10 μM) or rostafuroxin (10 µM). cGMP or rostafuroxin reduced bodipy-ouabain fluorescence intensity, and ouabain or rostafuroxin reduced 8-biotin-11-cGMP staining. We cross-linked isolated rat RPTs with 4-N3-PET-8-biotin-11-cGMP (2 μM); 8-N3-6-biotin-10-cAMP served as negative control. Precipitation with streptavidin beads followed by immunoblot analysis showed that RPTs after cross-linking with 4-N3-PET-8-biotin-11-cGMP exhibited a significantly stronger signal for NKA than non-cross-linked samples and cross-linked or non-cross-linked 8-N3-6-biotin-10-cAMP RPTs. Ouabain (10 μM) reduced NKA in cross-linked 4-N3-PET-8-biotin-11-cGMP RPTs confirming fluorescence staining. 4-N3-PET-8-biotin-11-cGMP cross-linked samples were separated by SDS gel electrophoresis and slices corresponding to NKA molecular weight excised and processed for mass spectrometry. NKA was the second most abundant protein with 50 unique NKA peptides covering 47% of amino acids in NKA. Molecular modeling demonstrated a potential cGMP docking site in the ouabain-binding pocket of NKA.

CONCLUSIONS

cGMP can bind to NKA and thereby mediate natriuresis.

Measuring enzyme activities in crude homogenates: Na+/K+-ATPase as a case study in optimizing assays

In this review of assays of Na⁺/K⁺-ATPase (NKA), we explore the choices made by researchers assaying the enzyme to investigate its role in physiological regulation. We survey NKA structure and function in the context of how it is typically assayed, and how technical choices influence what can be said about the enzyme. In comparing different methods for extraction and assay of NKA, we identified a series of common pitfalls that compromise the veracity of results. We include experimental work to directly demonstrate how choices in detergents, salts and substrates influence NKA activities measured in crude homogenates. Our review of assay approaches integrates what is known from enzymology, biomedical physiology, cell biology and evolutionary biology, offering a more robust method for assaying the enzyme in meaningful ways, identifying caveats and future directions to explore its structure and function. The goal is to provide the sort of background on the enzyme that should be considered in exploring the function of the enzyme in comparative physiology.

Sodium bicarbonate cotransporter NBCe2 gene variants increase sodium and bicarbonate transport in human renal proximal tubule cells

RATIONALE

Salt sensitivity of blood pressure affects >30% of the hypertensive and >15% of the normotensive population. Variants of the electrogenic sodium bicarbonate cotransporter NBCe2 gene, SLC4A5, are associated with increased blood pressure in several ethnic groups. SLC4A5 variants are also highly associated with salt sensitivity, independent of hypertension. However, little is known about how NBCe2 contributes to salt sensitivity, although NBCe2 regulates renal tubular sodium bicarbonate transport. We hypothesized that SLC4A5 rs10177833 and rs7571842 increase NBCe2 expression and human renal proximal tubule cell (hRPTC) sodium transport and may be a cause of salt sensitivity of blood pressure.

OBJECTIVE

To characterize the hRPTC ion transport of wild-type (WT) and homozygous variants (HV) of SLC4A5.

METHODS AND RESULTS

The expressions of NBCe2 mRNA and protein were not different between hRPTCs carrying WT or HV SLC4A5 before or after dopaminergic or angiotensin (II and III) stimulation. However, luminal to basolateral sodium transport, NHE3 protein, and Cl-/HCO3- exchanger activity in hRPTCs were higher in HV than WT SLC4A5. Increasing intracellular sodium enhanced the apical location of NBCe2 in HV hRPTCs (4.24±0.35% to 11.06±1.72% (P<0.05, N = 3, 2-way ANOVA, Holm-Sidak test)) as determined by Total Internal Reflection Fluorescence Microscopy (TIRFM). In hRPTCs isolated from kidney tissue, increasing intracellular sodium enhanced bicarbonate-dependent pH recovery rate and increased NBCe2 mRNA and protein expressions to a greater extent in HV than WT SLC4A5 (+38.00±6.23% vs HV normal salt (P<0.01, N = 4, 2-way ANOVA, Holm-Sidak test)). In hRPTCs isolated from freshly voided urine, bicarbonate-dependent pH recovery was also faster in those from salt-sensitive and carriers of HV SLC4A5 than from salt-resistant and carriers of WT SLC4A5. The faster NBCe2-specific bicarbonate-dependent pH recovery rate in HV SCL4A5 was normalized by SLC4A5- but not SLC4A4-shRNA. The binding of purified hepatocyte nuclear factor type 4A (HNF4A) to DNA was increased in hRPTCs carrying HV SLC4A5 rs7571842 but not rs10177833. The faster NBCe2-specific bicarbonate-dependent pH recovery rate in HV SCL4A5 was abolished by HNF4A antagonists.

CONCLUSION

NBCe2 activity is stimulated by an increase in intracellular sodium and is hyper-responsive in hRPTCs carrying HV SLC4A5 rs7571842 through an aberrant HNF4A-mediated mechanism.

Phosphorylation of rat kidney Na-K pump at Ser938 is required for rapid angiotensin II-dependent stimulation of activity and trafficking in proximal tubule cells

How angiotensin (ANG) II acutely stimulates the Na-K pump in proximal tubules is only partially understood, limiting insight into how ANG II increases blood pressure. First, we tested whether ANG II increases the number of pumps in plasma membranes of native rat proximal tubules under conditions of rapid activation. We found that exposure to 100 pM ANG II for 2 min, which was previously shown to increase affinity of the Na-K pump for Na and stimulate activity threefold, increased the amount of the Na-K pump in plasma membranes of native tubules by 33%. Second, we tested whether previously observed increases in phosphorylation of the Na-K pump at Ser(938) were part of the stimulatory mechanism. These experiments were carried out in opossum kidney cells, cultured proximal tubules stably coexpressing the ANG type 1 (AT1) receptor, and either wild-type or a S938A mutant of rat kidney Na-K pump under conditions found by others to stimulate activity. We found that 10 min of incubation in 10 pM ANG II stimulated activity of wild-type pumps from 2.3 to 3.5 nmol K · mg protein(-1) · min(-1) and increased the amount of the pump in the plasma membrane by 80% but had no effect on cells expressing the S938A mutant. We conclude that acute stimulation of Na-K pump activity in native rat proximal tubules includes increased trafficking to the plasma membrane and that phosphorylation at Ser(938) is part of the mechanism by which ANG II directly stimulates activity and trafficking of the rat kidney Na-K pump in opossum kidney cells.

In vivo effects of 17β-estradiol on cardiac Na(+)/K(+)-ATPase expression and activity in rat heart

In this study the in vivo effects of estradiol in regulating Na(+)/K(+)-ATPase function in rat heart was studied. Adult male Wistar rats were treated with estradiol (40μg/kg, i.p.) and after 24h the animals were sacrificed and the heart excised. Following estradiol administration, cardiac Na(+)/K(+)-ATPase activity, expression of the α1 subunit, and phosphorylation of the α1 subunit were significantly increased. These animals also had significantly decreased levels of digoxin-like immunoreactive factor(s). Na(+) levels were also significantly reduced but to a level that was still within the normal physiological range, highlighting the ability of the Na(+)/K(+)-ATPase to balance the ionic composition following treatment with estradiol. Estradiol treated rats also showed increased phosphorylation of protein kinase B (Akt), and extracellular-signal-regulated kinase 1/2 (ERK1/2). We therefore suggest a role for Akt and/or ERK1/2 in estradiol-mediated regulation of cardiac Na(+)/K(+)-ATPase expression and activity in rat heart.

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.

Changes in the expression and current of the Na+/K+ pump in the snail nervous system after exposure to a static magnetic field

Compelling evidence supports the use of a moderate static magnetic field (SMF) for therapeutic purposes. In order to provide insight into the mechanisms underlying SMF treatment, it is essential to examine the cellular responses elicited by therapeutically applied SMF, especially in the nervous system. The Na(+)/K(+) pump, by creating and maintaining the gradient of Na(+) and K(+) ions across the plasma membrane, regulates the physiological properties of neurons. In this study, we examined the expression of the Na(+)/K(+) pump in the isolated brain-subesophageal ganglion complex of the garden snail Helix pomatia, along with the immunoreactivity and current of the Na(+)/K(+) pump in isolated snail neurons after 15 min exposure to a moderate (10 mT) SMF. Western blot and immunofluorescence analysis revealed that 10 mT SMF did not significantly change the expression of the Na(+)/K(+) pump α-subunit in the snail brain and the neuronal cell body. However, our immunofluorescence data showed that SMF treatment induced a significant increase in the Na(+)/K(+) pump α-subunit expression in the neuronal plasma membrane area. This change in Na(+)/K(+) pump expression was reflected in pump activity as demonstrated by the pump current measurements. Whole-cell patch-clamp recordings from isolated snail neurons revealed that Na(+)/K(+) pump current density was significantly increased after the 10 mT SMF treatment. The SMF-induced increase was different in the two groups of control snail neurons, as defined by the pump current level. The results obtained could represent a physiologically important response of neurons to 10 mT SMF comparable in strength to therapeutic applications.

Activation of AMP-activated protein kinase stimulates Na+,K+-ATPase activity in skeletal muscle cells

Contraction stimulates Na(+),K(+)-ATPase and AMP-activated protein kinase (AMPK) activity in skeletal muscle. Whether AMPK activation affects Na(+),K(+)-ATPase activity in skeletal muscle remains to be determined. Short term stimulation of rat L6 myotubes with the AMPK activator 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR), activates AMPK and promotes translocation of the Na(+),K(+)-ATPase α(1)-subunit to the plasma membrane and increases Na(+),K(+)-ATPase activity as assessed by ouabain-sensitive (86)Rb(+) uptake. Cyanide-induced artificial anoxia, as well as a direct AMPK activator (A-769662) also increase AMPK phosphorylation and Na(+),K(+)-ATPase activity. Thus, different stimuli that target AMPK concomitantly increase Na(+),K(+)-ATPase activity. The effect of AICAR on Na(+),K(+)-ATPase in L6 myotubes was attenuated by Compound C, an AMPK inhibitor, as well as siRNA-mediated AMPK silencing. The effects of AICAR on Na(+),K(+)-ATPase were completely abolished in cultured primary mouse muscle cells lacking AMPK α-subunits. AMPK stimulation leads to Na(+),K(+)-ATPase α(1)-subunit dephosphorylation at Ser(18), which may prevent endocytosis of the sodium pump. AICAR stimulation leads to methylation and dephosphorylation of the catalytic subunit of the protein phosphatase (PP) 2A in L6 myotubes. Moreover, AICAR-triggered dephosphorylation of the Na(+),K(+)-ATPase was prevented in L6 myotubes deficient in PP2A-specific protein phosphatase methylesterase-1 (PME-1), indicating a role for the PP2A·PME-1 complex in AMPK-mediated regulation of Na(+),K(+)-ATPase. Thus contrary to the common paradigm, we report AMPK-dependent activation of an energy-consuming ion pumping process. This activation may be a potential mechanism by which exercise and metabolic stress activate the sodium pump in skeletal muscle.

N-myc downstream-regulated gene 2, a novel estrogen-targeted gene, is involved in the regulation of Na+/K+-ATPase

Na(+)/K(+)-ATPase, a plasma membrane protein abundantly expressed in epithelial tissues, has been identified and linked to numerous biological events, including ion transport and reabsorption. In Na(+)/K(+)-ATPase, the β-subunit plays a fundamental role in the structural integrity and functional maturation of holoenzyme. Estrogens are important circulating hormones that can regulate Na(+)/K(+)-ATPase abundance and activity; however, the specific molecules participating in this process are largely unknown. Here, we characterize that N-myc downstream-regulated gene 2 (NDRG2) is an estrogen up-regulated gene. 17β-Estradiol binds with estrogen receptor β but not estrogen receptor α to up-regulate NDRG2 expression via transcriptional activation. We also find that NDRG2 interacts with the β1-subunit of Na(+)/K(+)-ATPase and stabilizes the β1-subunit by inhibiting its ubiquitination and degradation. NDRG2-induced prolongation of the β1-subunit protein half-life is accompanied by a similar increase in Na(+)/K(+)-ATPase-mediated Na(+) transport and Na(+) current in epithelial cells. In addition, NDRG2 silencing largely attenuates the accumulation of β1-subunit regulated by 17β-estradiol. Our results demonstrate that estrogen/NDRG2/Na(+)/K(+)-ATPase β1 pathway is important in promoting Na(+)/K(+)-ATPase activity and suggest this novel pathway might have substantial roles in ion transport, fluid balance, and homeostasis.

Role of SRC family kinase in extracellular renal cyclic guanosine 3',5'-monophosphate- and pressure-induced natriuresis

cGMP functions as an extracellular (paracrine) messenger acting at the renal proximal tubule and is an important modulator of pressure-natriuresis (P-N). The signaling pathway activated by cGMP in the tubule cell basolateral membrane remains unknown. We hypothesized that renal interstitial microinfusion of cGMP (50 nmol/kg per minute) or P-N would be accompanied by increased renal protein levels of phospho-Src (Tyr 416) and that the natriuresis would be decreased by Src inhibition. Renal interstitial cGMP-induced natriuresis was blocked by Src inhibitor PP2 (2.0±0.4 versus 0.5±0.01 μEq/g per minute; P<0.001). The inactive analog of PP2, PP3, had no effect on cGMP-induced natriuresis. SU6656, another Src inhibitor, also inhibited cGMP-induced natriuresis (2.0±0.4 versus 1.02±0.01 μEq/g per minute; P<0.001). Renal interstitial cGMP infusion increased phospho-Src protein levels 5.6-fold at 15 minutes and 6.8-fold at 30 minutes compared with vehicle infusion but returned toward basal levels after 60 minutes. PP2 also blunted P-N (3.1±0.1 versus 1.1±0.3 μEq/g per minute; P<0.01) despite a similar increase in blood pressure. PP3 had no effect on P-N. Phospho-Src protein levels increased during P-N in vehicle- (1.8-fold) and PP3-treated (2.1-fold) groups compared with the sham-operated group. PP2 blocked the pressure-induced increase in renal phospho-Src protein levels. PP2 had no effect on renal hemodynamics but decreased both fractional excretion of Na(+) and lithium. Both extracellular cGMP and increased renal perfusion pressure increased renal phospho-Src protein levels and induced natriuresis in an Src-dependent manner, demonstrating that Src is an important downstream signaling molecule for extracellular cGMP-induced natriuresis.

AS160 associates with the Na+,K+-ATPase and mediates the adenosine monophosphate-stimulated protein kinase-dependent regulation of sodium pump surface expression

The sodium pump interacts with AS160, a protein that regulates the trafficking of the GLUT4 glucose transporter. This interaction drives the internalization of the sodium pump from the cell surface, and this process is in turn controlled by the energy-sensing kinase adenosine monophosphate-stimulated protein kinase.

The Na⁺,K⁺-ATPase is the major active transport protein found in the plasma membranes of most epithelial cell types. The regulation of Na⁺,K⁺-ATPase activity involves a variety of mechanisms, including regulated endocytosis and recycling. Our efforts to identify novel Na⁺,K⁺-ATPase binding partners revealed a direct association between the Na⁺,K⁺-ATPase and AS160, a Rab-GTPase-activating protein. In COS cells, coexpression of AS160 and Na⁺,K⁺-ATPase led to the intracellular retention of the sodium pump. We find that AS160 interacts with the large cytoplasmic NP domain of the α-subunit of the Na⁺,K⁺-ATPase. Inhibition of the activity of the adenosine monophosphate-stimulated protein kinase (AMPK) in Madin-Darby canine kidney cells through treatment with Compound C induces Na⁺,K⁺-ATPase endocytosis. This effect of Compound C is prevented through the short hairpin RNA-mediated knockdown of AS160, demonstrating that AMPK and AS160 participate in a common pathway to modulate the cell surface expression of the Na⁺,K⁺-ATPase.

Phosphorylation of the Na+,K+-ATPase and the H+,K+-ATPase

Phosphorylation is a widely used, reversible means of regulating enzymatic activity. Among the important phosphorylation targets are the Na(+),K(+)- and H(+),K(+)-ATPases that pump ions against their chemical gradients to uphold ionic concentration differences over the plasma membrane. The two pumps are very homologous, and at least one of the phosphorylation sites is conserved, namely a cAMP activated protein kinase (PKA) site, which is important for regulating pumping activity, either by changing the cellular distribution of the ATPases or by directly altering the kinetic properties as supported by electrophysiological results presented here. We further review the other proposed pump phosphorylations.

Caveolin-1 and dopamine-mediated internalization of NaKATPase in human renal proximal tubule cells

In moderate sodium-replete states, dopamine 1-like receptors (D1R/D5R) are responsible for regulating >50% of renal sodium excretion. This is partly mediated by internalization and inactivation of NaKATPase, when associated with adapter protein 2. We used dopaminergic stimulation via fenoldopam (D1-like receptor agonist) to study the interaction among D1-like receptors, caveolin-1 (CAV1), and the G protein-coupled receptor kinase type 4 in cultured human renal proximal tubule cells (RPTCs). We compared 2 groups of RPTCs, 1 of cell lines that were isolated from normal subjects (nRPTCs) and a second group of cell lines that have D1-like receptors that are uncoupled (uncoupled RPTCs) from adenylyl cyclase second messengers. In nRPTCs, fenoldopam increased the plasma membrane expression of D1R (10.0-fold) and CAV1 (1.3-fold) and markedly decreased G protein-coupled receptor kinase type 4 by 94+/-8%; no effects were seen in uncoupled RPTCs. Fenoldopam also increased the association of adapter protein 2 and NaKATPase by 53+/-9% in nRPTCs but not in uncoupled RPTCs. When CAV1 expression was reduced by 86.0+/-8.5% using small interfering RNA, restimulation of the D1-like receptors with fenoldopam in nRPTCs resulted in only a 7+/-9% increase in association between adapter protein 2 and NaKATPase. Basal CAV1 expression and association with G protein-coupled receptor kinase type 4 was decreased in uncoupled RPTCs (58+/-5% decrease in association) relative to nRPTCs. We conclude that the scaffolding protein CAV1 is necessary for the association of D1-like receptors with G protein-coupled receptor kinase type 4 and the adapter protein 2-associated reduction in plasma membrane NaKATPase.

Ubiquitination participates in the lysosomal degradation of Na,K-ATPase in steady-state conditions

The alveolar epithelial cell (AEC) Na,K-ATPase contributes to vectorial Na(+) transport and plays an important role in keeping the lungs free of edema. We determined, by cell surface labeling with biotin and immunofluorescence, that approximately 30% of total Na,K-ATPase is at the plasma membrane of AEC in steady-state conditions. The half-life of the plasma membrane Na,K-ATPase was about 4 hours, and the incorporation of new Na,K-ATPase to the plasma membrane was Brefeldin A sensitive. Both protein kinase C (PKC) inhibition with bisindolylmaleimide (10 microM) and infection with an adenovirus expressing dominant-negative PKCzeta prevented Na,K-ATPase degradation. In cells expressing the Na,K-ATPase alpha1-subunit lacking the PKC phosphorylation sites, the plasma membrane Na,K-ATPase had a moderate increase in half-life. We also found that the Na,K-ATPase was ubiquitinated in steady-state conditions and that proteasomal inhibitors prevented its degradation. Interestingly, mutation of the four lysines described to be necessary for ubiquitination and endocytosis of the Na,K-ATPase in injurious conditions did not have an effect on its half-life in steady-state conditions. Lysosomal inhibitors prevented Na,K-ATPase degradation, and co-localization of Na,K-ATPase and lysosomes was found after labeling and chasing the plasma membrane Na,K-ATPase for 4 hours. Accordingly, we provide evidence suggesting that phosphorylation and ubiquitination are necessary for the steady-state degradation of the plasma membrane Na,K-ATPase in the lysosomes in alveolar epithelial cells.

Na⁺,K⁺-ATPase as the Target Enzyme for Organic and Inorganic Compounds

This paper gives an overview of the literature data concerning specific and non specific inhibitors of Na⁺,K⁺-ATPase receptor. The immobilization approaches developed to improve the rather low time and temperature stability of Na⁺,K⁺-ATPase, as well to preserve the enzyme properties were overviewed. The functional immobilization of Na⁺,K⁺-ATPase receptor as the target, with preservation of the full functional protein activity and access of various substances to an optimum number of binding sites under controlled conditions in the combination with high sensitive technology for the detection of enzyme activity is the basis for application of this enzyme in medical, pharmaceutical and environmental research.

Trafficking of Na-K-ATPase and dopamine receptor molecules induced by changes in intracellular sodium concentration of renal epithelial cells

Most of the transepithelial transport of sodium in proximal tubules occurs through the coordinated action of the apical sodium/proton exchanger and the basolateral Na-K-ATPase. Hormones that regulate proximal tubule sodium excretion regulate the activities of these proteins. We have previously demonstrated that the level of intracellular sodium concentration modulates the regulation of Na-K-ATPase activity by angiotensin II and dopamine. An increase of a few millimolars in intracellular sodium concentration leads to increased Na-K-ATPase activity without a statistically significant increase in the number of plasma membrane Na-K-ATPase molecules, as determined by cell surface protein biotinylation. Using total internal reflection fluorescence, we detected an increased number of Na-K-ATPase molecules in cytosolic compartments adjacent to the plasma membrane, suggesting that the increased intracellular sodium concentration induces a movement of Na-K-ATPase molecules toward the plasma membrane. While intracellular compartments containing Na-K-ATPase molecules are very close to the plasma membrane, compartments containing type 1 dopamine receptors (D1Rs) are distributed in different parts of the cell cytosol. Fluorescence determinations indicate that an increased intracellular sodium concentration induces the increased colocalization of dopamine receptors with Na-K-ATPase molecules in the region of the plasma membrane. We propose that under in vivo conditions, in response to a sodium load in the lumen of proximal tubules, an increased level of intracellular sodium in epithelial cells is an early event that triggers the cellular response that leads to dopamine inhibition of proximal tubule sodium reabsorption.

G-protein-coupled receptor-mediated traffic of Na,K-ATPase to the plasma membrane requires the binding of adaptor protein 1 to a Tyr-255-based sequence in the alpha-subunit

Motion of integral membrane proteins to the plasma membrane in response to G-protein-coupled receptor signals requires selective cargo recognition motifs that bind adaptor protein 1 and clathrin. Angiotensin II, through the activation of AT1 receptors, promotes the recruitment to the plasma membrane of Na,K-ATPase molecules from intracellular compartments. We present evidence to demonstrate that a tyrosine-based sequence (IVVY-255) present within the Na,K-ATPase alpha1-subunit is involved in the binding of adaptor protein 1. Mutation of Tyr-255 to a phenylalanine residue in the Na,K-ATPase alpha1-subunit greatly reduces the angiotensin II-dependent activation of Na,K-ATPase, recruitment of Na,K-ATPase molecules to the plasma membrane, and association of adaptor protein 1 with Na,K-ATPase alpha1-subunit molecules. To determine protein-protein interaction, we used fluorescence resonance energy transfer between fluorophores attached to the Na,K-ATPase alpha1-subunit and adaptor protein 1. Although angiotensin II activation of AT1 receptors induces a significant increase in the level of fluorescence resonance energy transfer between the two molecules, this effect was blunted in cells expressing the Tyr-255 mutant. Thus, results from different methods and techniques suggest that the Tyr-255-based sequence within the NKA alpha1-subunit is the site of adaptor protein 1 binding in response to the G-protein-coupled receptor signals produced by angiotensin II binding to AT1 receptors.