G protein-coupled receptors regulate Na+,K+-ATPase activity and endocytosis by modulating the recruitment of adaptor protein 2 and clathrin

Osmoregulatory strategies of estuarine fish Scatophagus argus in response to environmental salinity changes

BACKGROUND

Scatophagus argus, an estuarine inhabitant, can rapidly adapt to different salinity environments. However, the knowledge of the molecular mechanisms underlying its strong salinity tolerance remains unclear. The gill, as the main osmoregulatory organ, plays a vital role in the salinity adaptation of the fish, and thus relative studies are constructive to reveal unique osmoregulatory mechanisms in S. argus.

RESULTS

In the present study, iTRAQ coupled with nanoLC-MS/MS techniques were employed to explore branchial osmoregulatory mechanisms in S. argus acclimated to different salinities. Among 1,604 identified proteins, 796 differentially expressed proteins (DEPs) were detected. To further assess osmoregulatory strategies in the gills under different salinities, DEPs related to osmoregulatory (22), non-directional (18), hypo- (52), and hypersaline (40) stress responses were selected. Functional annotation analysis of these selected DEPs indicated that the cellular ion regulation (e.g. Na⁺-K⁺-ATPase [NKA] and Na⁺-K⁺-2Cl^- cotransporter 1 [NKCC1]) and ATP synthesis were deeply involved in the osmoregulatory process. As an osmoregulatory protein, NKCC1 expression was inhibited under hyposaline stress but showed the opposite trend in hypersaline conditions. The expression levels of NKA α1 and β1 were only increased under hypersaline challenge. However, hyposaline treatments could enhance branchial NKA activity, which was inhibited under hypersaline environments, and correspondingly, reduced ATP content was observed in gill tissues exposed to hyposaline conditions, while its contents were increased in hypersaline groups. In vitro experiments indicated that Na⁺, K⁺, and Cl^- ions were pumped out of branchial cells under hypoosmotic stress, whereas they were absorbed into cells under hyperosmotic conditions. Based on our results, we speculated that NKCC1-mediated Na⁺ influx was inhibited, and proper Na⁺ efflux was maintained by improving NKA activity under hyposaline stress, promoting the rapid adaptation of branchial cells to the hyposaline condition. Meanwhile, branchial cells prevented excessive loss of ions by increasing NKA internalization and reducing ATP synthesis. In contrast, excess ions in cells exposed to the hyperosmotic medium were excreted with sufficient energy supply, and reduced NKA activity and enhanced NKCC1-mediated Na⁺ influx were considered a compensatory regulation.

CONCLUSIONS

S. argus exhibited divergent osmoregulatory strategies in the gills when encountering hypoosmotic and hyperosmotic stresses, facilitating effective adaptabilities to a wide range of environmental salinity fluctuation.

The inositol pyrophosphate 5-InsP7 drives sodium-potassium pump degradation by relieving an autoinhibitory domain of PI3K p85α

Sodium/potassium-transporting adenosine triphosphatase (Na⁺/K⁺-ATPase) is one of the most abundant cell membrane proteins and is essential for eukaryotes. Endogenous negative regulators have long been postulated to play an important role in regulating the activity and stability of Na⁺/K⁺-ATPase, but characterization of these regulators has been elusive. Mechanisms of regulating Na⁺/K⁺-ATPase homeostatic turnover are unknown. Here, we report that 5-diphosphoinositol 1,2,3,4,6-pentakisphosphate (5-InsP₇), generated by inositol hexakisphosphate kinase 1 (IP6K1), promotes physiological endocytosis and downstream degradation of Na⁺/K⁺-ATPase-α1. Deletion of IP6K1 elicits a twofold enrichment of Na⁺/K⁺-ATPase-α1 in plasma membranes of multiple tissues and cell types. Using a suite of synthetic chemical biology tools, we found that 5-InsP₇ binds the RhoGAP domain of phosphatidylinositol 3-kinase (PI3K) p85α to disinhibit its interaction with Na⁺/K⁺-ATPase-α1. This recruits adaptor protein 2 (AP2) and triggers the clathrin-mediated endocytosis of Na⁺/K⁺-ATPase-α1. Our study identifies 5-InsP₇ as an endogenous negative regulator of Na⁺/K⁺-ATPase-α1.

Aminopeptidases in Cardiovascular and Renal Function. Role as Predictive Renal Injury Biomarkers

Aminopeptidases (APs) are metalloenzymes that hydrolyze peptides and polypeptides by scission of the N-terminus amino acid and that also participate in the intracellular final digestion of proteins. APs play an important role in protein maturation, signal transduction, and cell-cycle control, among other processes. These enzymes are especially relevant in the control of cardiovascular and renal functions. APs participate in the regulation of the systemic and local renin-angiotensin system and also modulate the activity of neuropeptides, kinins, immunomodulatory peptides, and cytokines, even contributing to cholesterol uptake and angiogenesis. This review focuses on the role of four key APs, aspartyl-, alanyl-, glutamyl-, and leucyl-cystinyl-aminopeptidases, in the control of blood pressure (BP) and renal function and on their association with different cardiovascular and renal diseases. In this context, the effects of AP inhibitors are analyzed as therapeutic tools for BP control and renal diseases. Their role as urinary biomarkers of renal injury is also explored. The enzymatic activities of urinary APs, which act as hydrolyzing peptides on the luminal surface of the renal tubule, have emerged as early predictive renal injury biomarkers in both acute and chronic renal nephropathies, including those induced by nephrotoxic agents, obesity, hypertension, or diabetes. Hence, the analysis of urinary AP appears to be a promising diagnostic and prognostic approach to renal disease in both research and clinical settings.

Control of cardiac contraction by sodium: Promises, reckonings, and new beginnings

Several generations of cardiac physiologists have verified that basal cardiac contractility depends strongly on the transsarcolemmal Na gradient, and the underlying molecular mechanisms that link cardiac excitation-contraction coupling (ECC) to the Na gradient have been elucidated in good detail for more than 30 years. In brief, small increases of cytoplasmic Na push cardiac (NCX1) Na/Ca exchangers to increase contractility by increasing the myocyte Ca load. Accordingly, basal cardiac contractility is expected to be physiologically regulated by pathways that modify the cardiac Na gradient and the function of Na transporters. Assuming that this expectation is correct, it remains to be elucidated how in detail signaling pathways affecting the cardiac Na gradient are controlled in response to changing cardiac output requirements. Some puzzle pieces that may facilitate progress are outlined in this short review. Key open issues include (1) whether the concept of local Na gradients is viable, (2) how in detail Na channels, Na transporters and Na/K pumps are regulated by lipids and metabolic processes, (3) the physiological roles of Na/K pump inactivation, and (4) the possibility that key diffusible signaling molecules remain to be discovered.

Myo-inositol Effects on the Developing Respiratory Neural Control System

Myo-inositol is a highly abundant stereoisomer of the inositol family of sugar alcohols and forms the structural basis for a variety of polyphosphate derivatives including second messengers and membrane phospholipids. These derivatives regulate numerous cell processes including gene transcription, membrane excitability, vesicular trafficking, intracellular calcium signaling, and neuronal growth and development. Myo-inositol can be formed endogenously from the breakdown of glucose, is found in a variety of foods including breastmilk and is commercially available as a nutritional supplement. Abnormal myo-inositol metabolism has been shown to underlie the pathophysiology of a variety of clinical conditions including Down Syndrome, traumatic brain injury, bronchopulmonary dysplasia (BPD), and respiratory distress syndrome (RDS). Several animal studies have shown that myo-inositol may play a critical role in development of both the central and peripheral respiratory neural control system; a notable example is the neonatal apnea and respiratory insufficiency that manifests in a mouse model of myo-inositol depletion, an effect that is also postnatally lethal. This review focuses on myo-inositol (and some of its derivatives) and how it may play a role in respiratory neural control; we also discuss clinical evidence demonstrating a link between serum myo-inositol levels and the incidence of intermittent hypoxemia (IH) events (a surrogate measure of apnea of prematurity (AOP)) in preterm infants. Further, there are both animal and human infant studies that have demonstrated respiratory benefits following supplementation with myo-inositol, which highlights the prospects that nutritional requirements are important for appropriate development and maturation of the respiratory system.

Renal proximal tubule Na,K-ATPase is controlled by CREB-regulated transcriptional coactivators as well as salt-inducible kinase 1

Sodium reabsorption by the kidney is regulated by locally produced natriuretic and anti-natriuretic factors, including dopamine and norepinephrine, respectively. Previous studies indicated that signaling events initiated by these natriuretic and anti-natriuretic factors achieve their effects by altering the phosphorylation of Na,K-ATPase in the renal proximal tubule, and that protein kinase A (PKA) and calcium-mediated signaling pathways are involved. The same signaling pathways also control the transcription of the Na,K-ATPase β subunit gene atp1b1 in renal proximal tubule cells. In this report, evidence is presented that (1) both the recently discovered cAMP-regulated transcriptional coactivators (CRTCs) and salt-inducible kinase 1 (SIK1) contribute to the transcriptional regulation of atp1b1 in renal proximal tubule (RPT) cells and (2) renal effectors, including norepinephrine, dopamine, prostaglandins, and sodium, play a role. Exogenously expressed CRTCs stimulate atp1b1 transcription. Evidence for a role of endogenous CRTCs includes the loss of transcriptional regulation of atp1b1 by a dominant-negative CRTC, as well as by a CREB mutant, with an altered CRTC binding site. In a number of experimental systems, SIK phosphorylates CRTCs, which are then sequestered in the cytoplasm, preventing their nuclear effects. Consistent with such a role of SIK in primary RPT cells, atp1b1 transcription increased in the presence of a dominant-negative SIK1, and in addition, regulation by dopamine, norepinephrine, and monensin was disrupted by a dominant-negative SIK1. These latter observations can be explained if SIK1 is phosphorylated and inactivated in the presence of these renal effectors. Our results support the hypothesis that Na,K-ATPase in the renal proximal tubule is regulated at the transcriptional level via SIK1 and CRTCs by renal effectors, in addition to the previously reported control of the phosphorylation of Na,K-ATPase.

High CO2 Leads to Na,K-ATPase Endocytosis via c-Jun Amino-Terminal Kinase-Induced LMO7b Phosphorylation

The c-Jun amino-terminal kinase (JNK) plays a role in inflammation, proliferation, apoptosis, and cell adhesion and cell migration by phosphorylating paxillin and β-catenin. JNK phosphorylation downstream of AMP-activated protein kinase (AMPK) activation is required for high CO2 (hypercapnia)-induced Na,K-ATPase endocytosis in alveolar epithelial cells. Here, we provide evidence that during hypercapnia, JNK promotes the phosphorylation of LMO7b, a scaffolding protein, in vitro and in intact cells. LMO7b phosphorylation was blocked by exposing the cells to the JNK inhibitor SP600125 and by infecting cells with dominant-negative JNK or AMPK adenovirus. The knockdown of the endogenous LMO7b or overexpression of mutated LMO7b with alanine substitutions of five potential JNK phosphorylation sites (LMO7b-5SA) or only Ser-1295 rescued both LMO7b phosphorylation and the hypercapnia-induced Na,K-ATPase endocytosis. Moreover, high CO2 promoted the colocalization and interaction of LMO7b and the Na,K-ATPase α1 subunit at the plasma membrane, which were prevented by SP600125 or by transfecting cells with LMO7b-5SA. Collectively, our data suggest that hypercapnia leads to JNK-induced LMO7b phosphorylation at Ser-1295, which facilitates the interaction of LMO7b with Na,K-ATPase at the plasma membrane promoting the endocytosis of Na,K-ATPase in alveolar epithelial cells.

Inhibition of epidermal growth factor signaling by the cardiac glycoside ouabain in medulloblastoma

Epidermal growth factor (EGF) signaling regulates cell growth, proliferation, and differentiation. Upon receptor binding, EGF triggers cascades of downstream signaling, including the MAPK and phosphoinositide-3-kinase (PI3K)/Akt signaling pathways. Aberrant expression/activation of EGFR is found in multiple human cancers, including medulloblastoma, the most prevalent pediatric brain cancer, and often has been associated with metastasis, poor prognosis, and resistance to chemotherapy. Na,K-ATPase is an ion pump well known for its role in intracellular ion homeostasis. Recent studies showed that Na,K-ATPase also functions as a signaling platform and revealed a role in EGFR, MAPK, and PI3K signaling. While both EGFR and Na,K-ATPase seem to modulate similar signaling pathways, cardiac glycosides that are steroid-like inhibitors of Na,K-ATPase, exhibit antiproliferative and proapoptotic properties in cancer cells. Thus, we sought to better understand the relationship between EGF and cardiac glycoside signaling. Here, we show that in medulloblastoma cells, both EGF and ouabain activate Erk1/2 and PI3K/Akt signaling. Nevertheless, in medulloblastoma cells ouabain did not transactivate EGFR as has been reported in various other cell lines. Indeed, ouabain inhibited EGF-induced Erk1/2 and Akt activation and, moreover, prevented EGF-induced formation of actin stress fibers and cell motility, probably by activating a stress signaling response. Na,K-ATPase has been proposed to act as a signaling scaffold and our studies suggest that in medulloblastoma cells Na,K-ATPase might act as a check point to integrate EGF-associated signaling pathways. Thus, Na,K-ATPase might serve as a valid target to develop novel therapeutic approaches in tumors with aberrant activation of the EGFR signaling cascades.

Crosstalk between dopamine receptors and the Na⁺/K⁺-ATPase (review)

Dopamine (DA) receptors, which belong to the G protein-coupled receptor family, are the target of ~50% of all modern medicinal drugs and constitute a large and diverse class of proteins whose primary function is to transduce extracellular stimuli into intracellular signals. Na+/K+-ATPase (NKA) is ubiquitous and crucial for the maintenance of intracellular ion homeostasis and excitability. Furthermore, it plays a critical role in diverse effects, including clinical cardiotonic and cardioprotective effects, ischemic preconditioning in the brain, natriuresis, lung edema clearance and other processes. NKA regulation is of physiological and pharmacological importance and has species- and tissue-specific variations. The activation of DA receptors regulates NKA expression/activity and trafficking in various tissues and cells, for example in the kidney, lung, intestine, brain, non-pigmented ciliary epithelium and the vascular bed. DA receptor-mediated regulation of NKA mediates a diverse range of cellular responses and includes endocytosis/exocytosis, phosphorylation/dephosphorylation of the α subunit of NKA and multiple signaling pathways, including phosphatidylinositol (PI)-phospholipase C/protein kinase (PK) C, cAMP/PKA, PI3K, adaptor protein 2, tyrosine phosphatase and mitogen-activated protein kinase/extracellular signal-regulated protein kinase. Furthermore, in brain and HEK293T cells, D1 and D2 receptors exist in a complex with NKA. Among D1 and D2 receptors and NKA, regulations are reciprocal, which leads to crosstalk between DA receptors and NKA. In the present study, the current understanding of signaling mechanisms responsible for the crosstalk between DA receptors and NKA, as well as with specific consequent functions, is reviewed.

Redox control of cardiac excitability

Reactive oxygen species (ROS) have been associated with various human diseases, and considerable attention has been paid to investigate their physiological effects. Various ROS are synthesized in the mitochondria and accumulate in the cytoplasm if the cellular antioxidant defense mechanism fails. The critical balance of this ROS synthesis and antioxidant defense systems is termed the redox system of the cell. Various cardiovascular diseases have also been affected by redox to different degrees. ROS have been indicated as both detrimental and protective, via different cellular pathways, for cardiac myocyte functions, electrophysiology, and pharmacology. Mostly, the ROS functions depend on the type and amount of ROS synthesized. While the literature clearly indicates ROS effects on cardiac contractility, their effects on cardiac excitability are relatively under appreciated. Cardiac excitability depends on the functions of various cardiac sarcolemal or mitochondrial ion channels carrying various depolarizing or repolarizing currents that also maintain cellular ionic homeostasis. ROS alter the functions of these ion channels to various degrees to determine excitability by affecting the cellular resting potential and the morphology of the cardiac action potential. Thus, redox balance regulates cardiac excitability, and under pathological regulation, may alter action potential propagation to cause arrhythmia. Understanding how redox affects cellular excitability may lead to potential prophylaxis or treatment for various arrhythmias. This review will focus on the studies of redox and cardiac excitation.

Inhibition of Activity of GABA Transporter GAT1 by δ-Opioid Receptor

Analgesia is a well-documented effect of acupuncture. A critical role in pain sensation plays the nervous system, including the GABAergic system and opioid receptor (OR) activation. Here we investigated regulation of GABA transporter GAT1 by δOR in rats and in Xenopus oocytes. Synaptosomes of brain from rats chronically exposed to opiates exhibited reduced GABA uptake, indicating that GABA transport might be regulated by opioid receptors. For further investigation we have expressed GAT1 of mouse brain together with mouse δOR and μOR in Xenopus oocytes. The function of GAT1 was analyzed in terms of Na⁺-dependent [³H]GABA uptake as well as GAT1-mediated currents. Coexpression of δOR led to reduced number of fully functional GAT1 transporters, reduced substrate translocation, and GAT1-mediated current. Activation of δOR further reduced the rate of GABA uptake as well as GAT1-mediated current. Coexpression of μOR, as well as μOR activation, affected neither the number of transporters, nor rate of GABA uptake, nor GAT1-mediated current. Inhibition of GAT1-mediated current by activation of δOR was confirmed in whole-cell patch-clamp experiments on rat brain slices of periaqueductal gray. We conclude that inhibition of GAT1 function will strengthen the inhibitory action of the GABAergic system and hence may contribute to acupuncture-induced analgesia.

The effect of oxidative stress upon the intestinal epithelial uptake of butyrate

Our aim was to investigate the effect of oxidative stress upon butyrate uptake at the intestinal epithelial level. For this, IEC-6 cells were treated with tert-butylhydroperoxide 3000μM (tBOOH), which increased levels of oxidative stress biomarkers, while maintaining cellular viability. The effect of tBOOH upon uptake of [(14)C]butyrate ([(14)C]BT) (10μM) can be summarized as follows: (a) it caused a reduction in the intracellular accumulation of [(14)C]BT over time, (b) it strongly reduced total [(14)C]BT uptake but did not affect Na(+)-independent uptake of [(14)C]BT, and (c) it did not affect the kinetics of [(14)C]BT uptake at 37°C, but increased uptake at 4°C. Moreover, tBOOH increased the efflux of [(14)C]BT not mediated by breast cancer resistance protein. We thus conclude that tBOOH strongly inhibits Na(+)-coupled monocarboxylate cotransporter 1 (SMCT1)-mediated, but not H(+)-coupled monocarboxylate transporter (MCT1)-mediated butyrate uptake; moreover, it increases uptake and efflux of butyrate by passive diffusion. tBOOH did not affect the mRNA expression levels of MCT1 and SMCT1 nor their cell membrane insertion. Rather, its effect was dependent on extracellular signal regulated kinase 1/2 and protein tyrosine kinase activation and on the generation of reactive oxygen species by NADPH and xanthine oxidases and was partially prevented by the polyphenols quercetin and resveratrol. In conclusion, tBOOH is an effective inhibitor of SMCT1-mediated butyrate transport in non-tumoral intestinal epithelial cells. Given the important role played by butyrate in the intestine, this mechanism may contribute to the procarcinogenic and proinflammatory effect of oxidative stress at this level.

The role of TonEBP in regulation of AAD expression and dopamine production in renal proximal tubule cells upon hypertonic challenge

Renal proximal tubule cells overexpress aromatic l-amino acid decarboxylase (AAD) to produce dopamine, which inhibits salt absorption in the hypertonic environment. We examined the effect of TonEBP on AAD expression in human proximal tubule epithelial cells, HK-2 cell line. Confocal microscopy showed that after 2h of exposure to the hypertonic medium, TonEBP accumulation in nuclei increased as compared to the isotonic control. The activated TonEBP enhanced the mRNA expression of the representative downstream genes (i.e., SMIT and TauT). Meanwhile, AAD protein abundance also increased with TonEBP activation. EMSA and luciferase reporter assay showed that TonEBP was involved in transcriptional regulation of AAD upon hypertonic stress. Inactivation of TonEBP by the p38 inhibitor SB203580, or TonEBP shRNA significantly reduced AAD expression, which was rescued by re-expressing Myc-tagged TonEBP. Up-regulation of AAD increased dopamine synthesis, and dopamine inhibited NKA activity in hypertonic condition. These results suggested that TonEBP played an important role in the epithelial cells of renal proximal tubule upon hypertonic stress by enhancing AAD expression, which could promote dopamine secretion to negative regulate NKA activity. The elucidation of a new mechanism described in this study combined with previous findings provides more insights into this issue.

The paradox of dopamine and angiotensin II-mediated Na(+), K(+)-ATPase regulation in renal proximal tubules

Accumulated studies reported that the natruretic dopamine (DA) and the anti-natruretic angiotensin II (Ang II) represent an important mechanism to regulate renal Na(+) and water excretion through intracellular secondary messengers to inhibit or activate renal proximal tubule (PT) Na(+), K(+)-ATPase (NKA). The antagonistic actions were mediated by the phosphorylation of different position of NKA α₁-subunit and different Pals-associated tight junction protein (PATJ) PDZ domains, the different protein kinase C (PKC) isoforms (PKC-β, PKC-ζ), the common adenylyl cyclase (AC) pathway, and the crosstalk and balance between DA and Ang II to NKA regulation. Besides, Ang II-mediated NKA modulation has bi-phasic effects.

Dopamine regulation of Na+-K+-ATPase requires the PDZ-2 domain of sodium hydrogen regulatory factor-1 (NHERF-1) in opossum kidney cells

Na(+)-K(+)-ATPase activity in renal proximal tubule is regulated by several hormones including parathyroid hormone (PTH) and dopamine. The current experiments explore the role of Na(+)/H(+) exchanger regulatory factor 1 (NHERF-1) in dopamine-mediated regulation of Na(+)-K(+)-ATPase. We measured dopamine regulation of ouabain-sensitive (86)Rb uptake and Na(+)-K(+)-ATPase α1 subunit phosphorylation in wild-type opossum kidney (OK) (OK-WT) cells, OKH cells (NHERF-1-deficient), and OKH cells stably transfected with full-length human NHERF-1 (NF) or NHERF-1 constructs with mutated PDZ-1 (Z1) or PDZ-2 (Z2) domains. Treatment with 1 μM dopamine decreased ouabain-sensitive (86)Rb uptake, increased phosphorylation of Na(+)-K(+)-ATPase α1-subunit, and enhanced association of NHERF-1 with D1 receptor in OK-WT cells but not in OKH cells. Transfection with wild-type, full-length, or PDZ-1 domain-mutated NHERF-1 into OKH cells restored dopamine-mediated regulation of Na(+)-K(+)-ATPase and D1-like receptor association with NHERF-1. Dopamine did not regulate Na(+)-K(+)-ATPase or increase D1-like receptor association with NHERF-1 in OKH cells transfected with mutated PDZ-2 domain. Dopamine stimulated association of PKC-ζ with NHERF-1 in OK-WT and OKH cells transfected with full-length or PDZ-1 domain-mutated NHERF-1 but not in PDZ-2 domain-mutated NHERF-1-transfected OKH cells. These results suggest that NHERF-1 mediates Na(+)-K(+)-ATPase regulation by dopamine through its PDZ-2 domain.

Targeting of renal proximal tubule Na,K-ATPase by salt-inducible kinase

The renal proximal tubule (RPT) is a central locale for Na+ reabsorption, and blood pressure regulation. Na+ reabsorption in the RPT depends upon the Na,K-ATPase, which is controlled by a complex regulatory network, including Salt-Inducible Protein Kinase (SIK). SIKs are recently discovered members of the AMP-activated Protein Kinase (AMPK) family, which regulate salt homeostasis and metabolism in a number of tissues. In the RPT, SIK interacts with the Na,K-ATPase in the basolateral membrane (BM), regulating both the activity and level of Na,K-ATPase in the BM. Thus, Na,K-ATPase activity can be rapidly adjusted in response to changes in Na+ balance. Long-term changes in Na+ intake affect the state of SIK phosphorylation, and as a consequence the phosphorylation of TORCs, Transducers of Regulated CREB (cAMP Regulatory Element Binding Protein). Once phosphorylated, TORCs enter the nucleus, and activate transcription of the ATP1B1 gene encoding for the Na,K-ATPase beta subunit.

SIK1/SOS2 networks: decoding sodium signals via calcium-responsive protein kinase pathways

Changes in cellular ion levels can modulate distinct signaling networks aimed at correcting major disruptions in ion balances that might otherwise threaten cell growth and development. Salt-inducible kinase 1 (SIK1) and salt overly sensitive 2 (SOS2) are key protein kinases within such networks in mammalian and plant cells, respectively. In animals, SIK1 expression and activity are regulated in response to the salt content of the diet, and in plants SOS2 activity is controlled by the salinity of the soil. The specific ionic stress (elevated intracellular sodium) is followed by changes in intracellular calcium; the calcium signals are sensed by calcium-binding proteins and lead to activation of SIK1 or SOS2. These kinases target major plasma membrane transporters such as the Na⁺,K⁺-ATPase in mammalian cells, and Na⁺/H⁺ exchangers in the plasma membrane and membranes of intracellular vacuoles of plant cells. Activation of these networks prevents abnormal increases in intracellular sodium concentration.

Na+/K+-ATPase-mediated signal transduction and Na+/K+-ATPase regulation

A number of studies suggest that Na(+)/K(+)-ATPase in caveolae interacts with neighboring membrane proteins and organizes cytosolic cascades of signaling proteins to send messages to intracellular organelles in different tissues, mostly in cardiac myocytes. Low concentration of ouabain binding to Na(+)/K(+)-ATPase activates Src/epidermal growth factor receptor complex to initiate multiple signal pathways, which include PLC/IP3/CICR, PI3K, reactive oxygen species (ROS), PLC/DG/PKC/Raf/MEK/ERK1/2, and Ras/Raf/MEK/ERK1/2 pathways. In cardiac myocytes, the resulting downstream events include the induction of some early response proto-oncogenes, activation of transcription factors, activator protein-1, and nuclear factor-kappaB, the regulation of a number of cardiac growth-related genes, and the stimulation of protein synthesis and myocyte hypertrophy and apoptosis. Conversely, several factors acting through signal pathways, such as protein kinases, Ca(2+), ROS, etc., can modulate the activity of the Na(+)/K(+)-ATPase.

Reciprocal modulation of function between the D1 and D2 dopamine receptors and the Na+,K+-ATPase

It is well documented that dopamine can increase or decrease the activity of the Na+,K+-ATPase (NKA, sodium pump) in an organ-specific fashion. This regulation can occur, at least partially, via receptor-mediated second messenger activation and can promote NKA insertion or removal from the plasma membrane. Using co-immunoprecipitation and mass spectrometry, we now show that, in both brain and HEK293T cells, D1 and D2 dopamine receptors (DARs) can exist in a complex with the sodium pump. To determine the impact of NKA on DAR function, biological assays were conducted with NKA and DARs co-expressed in HEK293T cells. In this system, expression of NKA dramatically decreased D1 and D2 DAR densities with a concomitant functional decrease in DAR-mediated regulation of cAMP levels. Interestingly, pharmacological inhibition of endogenous or overexpressed NKA enhanced DAR function without altering receptor number or localization. Similarly, DAR function was also augmented by small interfering RNA reduction of the endogenous NKA. These data suggest that, under basal conditions, NKA negatively regulates DAR function via protein-protein interactions. In reciprocal fashion, expression of DARs decreases endogenous NKA function in the absence of dopamine, implicating DAR proteins as regulators of NKA activity. Notably, dopamine stimulation or pertussis toxin inhibition of D2 receptor signaling did not alter NKA activity, indicating that the D2-mediated decrease in NKA function is dependent upon protein-protein interactions rather than signaling molecules. This evidence for reciprocal regulation between DARs and NKA provides a novel control mechanism for both DAR signaling and cellular ion balance.

T3 increases Na-K-ATPase activity via a MAPK/ERK1/2-dependent pathway in rat adult alveolar epithelial cells

Thyroid hormone (T3) increases Na-K-ATPase activity in rat adult alveolar type II cells via a PI3K-dependent pathway. In these cells, dopamine and beta-adrenergic agonists can stimulate Na-K-ATPase activity through either PI3K or MAPK pathways. We assessed the role of the MAPK pathway in the stimulation of Na-K-ATPase by T3. In the adult rat alveolar type II-like cell line MP48, T3 enhanced MAPK/ERK1/2 activity in a dose-dependent manner. Increased ERK1/2 phosphorylation was observed within 5 min, peaked at 20 min, and then decreased. Two MEK1/2 inhibitors, U0126 and PD-98059, each abolished the T3-induced increase in the quantity of Na-K-ATPase alpha(1)-subunit plasma membrane protein and Na-K-ATPase activity. T3 also increased the phosphorylation of MAPK/p38; however, SB-203580, a specific inhibitor of MAPK/p38 activity, did not prevent the T3-induced Na-K-ATPase activity. SP-600125, a specific inhibitor of the MAPK/JNK pathway, also did not block the T3-induced Na-K-ATPase activity. Phorbol 12-myristate 13-acetate (PMA) significantly increased ERK1/2 phosphorylation and Na-K-ATPase activity. The PMA-induced Na-K-ATPase activity was inhibited by U0126. These data indicate that activation of MAPK-ERK1/2 was required for the T3-induced increase in Na-K-ATPase activity in addition to the requirement for the PI3K pathway.

SIK1 is part of a cell sodium-sensing network that regulates active sodium transport through a calcium-dependent process

In mammalian cells, active sodium transport and its derived functions (e.g., plasma membrane potential) are dictated by the activity of the Na(+),K(+)-ATPase (NK), whose regulation is essential for maintaining cell volume and composition, as well as other vital cell functions. Here we report the existence of a salt-inducible kinase-1 (SIK1) that associates constitutively with the NK regulatory complex and is responsible for increases in its catalytic activity following small elevations in intracellular sodium concentrations. Increases in intracellular sodium are paralleled by elevations in intracellular calcium through the reversible Na(+)/Ca(2+) exchanger, leading to the activation of SIK1 (Thr-322 phosphorylation) by a calcium calmodulin-dependent kinase. Activation of SIK1 results in the dephosphorylation of the NK alpha-subunit and an increase in its catalytic activity. A protein phosphatase 2A/phosphatase methylesterase-1 (PME-1) complex, which constitutively associates with the NK alpha-subunit, is activated by SIK1 through phosphorylation of PME-1 and its dissociation from the complex. These observations illustrate the existence of a distinct intracellular signaling network, with SIK1 at its core, which is triggered by a monovalent cation (Na(+)) and links sodium permeability to its active transport.

Aminopeptidase N reduces basolateral Na+ -K+ -ATPase in proximal tubule cells

Aminopeptidase N/CD13 (Anpep) is a membrane-bound protein that catalyzes the formation of natriuretic hexapeptide angiotensin IV (ANG IV) from ANG III. We previously reported that Anpep is more highly expressed in the kidneys of Dahl salt-resistant (SR/Jr) than salt-sensitive (SS/Jr) rats, Anpep maps to a quantitative trait locus for hypertension, and that the Dahl SR/Jr rat contains a functional polymorphism of the gene. This suggests that renal Anpep may be linked to salt sensitivity; however, its effect on renal Na handling has not been determined. Here, we examined regulation of basolateral Na(+)-K(+)-ATPase, a preeminent basolateral Na(+) transporter in proximal tubule cells, by Anpep in LLC-PK1 cells. Treatment of the cells with Anpep siRNA increased total cellular Na(+)-K(+)-ATPase activity and basolateral Na(+)-K(+)-ATPase abundance by approximately twofold. Conversely, Anpep overexpression reduced Na(+)-K(+)-ATPase activity and basolateral abundance by approximately 50%. Similar effects were observed after treatment with ANG IV (10 nM, x30 min and 12 h). ANG IV receptor (AGTRIV) knockdown via specific siRNA relieved the decreases in basolateral Na(+)-K(+)-ATPase levels and activity induced by Anpep overexpression. In sum, these results demonstrate that Anpep reduces basolateral Na(+)-K(+)-ATPase levels via ANG IV/AGTRIV signaling. This novel pathway may be important in renal adaptation to high salt.

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

BACKGROUND AND PURPOSE

Dopamine is a major regulator of sodium reabsorption in proximal tubule epithelia. By binding to D1-receptors, dopamine induces endocytosis of plasma membrane Na,K-ATPase, resulting in a reduced capacity of the cells to transport sodium, thus contributing to natriuresis. We have previously demonstrated several aspects of the molecular mechanism by which dopamine induces Na,K-ATPase endocytosis; however, the location of intracellular compartments containing Na,K-ATPase molecules has not been identified.

EXPERIMENTAL APPROACH

In this study, we used different approaches to determine the localization of Na,K-ATPase-containing intracellular compartments. By expression of fluorescent-tagged Na,K-ATPase molecules in opossum kidney cells, a cell culture model of proximal tubule epithelia, we used fluorescence microscopy to determine cellular distribution of the fluorescent molecules and the effects of dopamine on this distribution. By labelling cell surface Na,K-ATPase molecules from the cell exterior with either biotin or an epitope-tagged antibody, we determined the localization of the tagged Na,K-ATPase molecules after endocytosis induced by dopamine.

KEY RESULTS

In cells expressing fluorescent-tagged Na,K-ATPase molecules, there were intracellular compartments containing Na,K-ATPase molecules. These compartments were in very close proximity to the plasma membrane. Upon treatment of the cells with dopamine, the fluorescence labelling of these compartments was increased. The labelling of these compartments was also observed when the endocytosis of biotin- or antibody-tagged plasma membrane Na,K-ATPase molecules was induced by dopamine.

CONCLUSIONS AND IMPLICATIONS

The intracellular compartments containing Na,K-ATPase molecules are located just underneath the plasma membrane.

Genetics of arterial hypertension and hypotension

Human hypertension affects affects more than 20% of the adult population in industrialized countries, and it is implicated in millions of deaths worldwide each year from stroke, heart failure and ischemic heart disease. Available evidence suggests a major genetic impact on blood pressure regulation. Studies in monogenic hypertension revealed that renal salt and volume regulation systems are predominantly involved in the genesis of these disorders. Mutations here affect the synthesis of mineralocorticoids, the function of the mineralocorticoid receptor, epithelial sodium channels and their regulation by a new class of kinases, termed WNK kinases. It has been learned from monogenic hypotension that almost all ion transporters involved in the renal uptake of Na(+) have a major impact on blood pressure regulation. For essential hypertension as a complex disease, many candidate genes have been analysed. These include components of the renin-angiotensin-aldosterone system, adducin, beta-adrenoceptors, G protein subunits, regulators of G protein signalling (RGS) proteins, Rho kinases and G protein receptor kinases. At present, the individual impact of common polymorphisms in these genes on the observed blood pressure variation, on risk for stroke and as predictors of antihypertensive responses remains small and clinically irrelevant. Nevertheless, these studies have greatly augmented our knowledge on the regulation of renal functions, cellular signal transduction and the integration of both. Together, this provides the basis for the identification of novel drug targets and, hopefully, innovative antihypertensive drugs.

Phosphorylation of adaptor protein-2 mu2 is essential for Na+,K+-ATPase endocytosis in response to either G protein-coupled receptor or reactive oxygen species

Activation of G protein-coupled receptor by dopamine and hypoxia-generated reactive oxygen species promote Na+,K+-ATPase endocytosis. This effect is clathrin dependent and involves the activation of protein kinase C (PKC)-zeta and phosphorylation of the Na+,K+-ATPase alpha-subunit. Because the incorporation of cargo into clathrin vesicles requires association with adaptor proteins, we studied whether phosphorylation of adaptor protein (AP)-2 plays a role in its binding to the Na+,K+-ATPase alpha-subunit and thereby in its endocytosis. Dopamine induces a time-dependent phosphorylation of the AP-2 mu2 subunit. Using specific inhibitors and dominant-negative mutants, we establish that this effect was mediated by activation of the adaptor associated kinase 1/PKC-zeta isoform. Expression of the AP-2 mu2 bearing a mutation in its phosphorylation site (T156A) prevented Na+,K+-ATPase endocytosis and changes in activity induced by dopamine. Similarly, in lung alveolar epithelial cells, hypoxia-induced endocytosis of Na+,K+-ATPase requires the binding of AP-2 to the tyrosine-based motif (Tyr-537) located in the Na+,K+-ATPase alpha-subunit, and this effect requires phosphorylation of the AP-2 mu2 subunit. We conclude that phosphorylation of AP-2 mu2 subunit is essential for Na+,K+-ATPase endocytosis in response to a variety of signals, such as dopamine or reactive oxygen species.

The dopamine paradox in lung and kidney epithelia: sharing the same target but operating different signaling networks

Stimulation of dopamine receptors in the lung or kidney epithelia has distinct and opposite effects on the function of Na,K-ATPase, which results in increased Na(+) absorption across the alveolar epithelium and increased sodium excretion via the kidney epithelium. In the lung, dopamine increases Na,K-ATPase by increasing cell basolateral surface expression of Na(+),K(+)-ATPase molecules, whereas in the kidney epithelia it decreases Na(+),K(+)-ATPase activity by removing active units from the plasma membrane by endocytosis. The opposite effects of dopamine over the same target (the Na(+),K(+)-ATPase) involve the activation of a distinct signaling network that it is target specific, and has a different spatial resolution. Understanding the specific signaling pathways involved in these actions of dopamine and their hierarchical organization may facilitate the drug discovery process that could lead to the design of new therapeutic approaches to clear lung edema in patients with acute lung injury and to decrease fluid overload during congestive heart failure and hypertension.

Salt loading induces redistribution of the plasmalemmal Na/K-ATPase in proximal tubule cells

BACKGROUND

We have reported that digitalis-like substances (cardiotonic steroids), including marinobufagenin (MBG), induce endocytosis of the plasmalemmal Na/K-ATPase in LLC-PK1 cells. The current report addresses the potential relevance of plasmalemmal Na/K-ATPase redistribution to in vivo salt handling.

METHODS

Male Sprague-Dawley rats were given 1 week of a high salt (4.0% NaCl) or normal salt (0.4% NaCl) diet. Urinary sodium excretion, as well as MBG excretion, was monitored, and proximal tubules were isolated using a Percoll gradient method. Tubular (86)Rb uptake, Na/K-ATPase enzymatic activity, and Na/K-ATPase alpha1 subunit density were determined.

RESULTS

The high salt diet increased urinary sodium (17.8 +/- 1.8 vs. 2.5 +/- 0.3 mEq/day, P < 0.01) and MBG excretion (104 +/- 12 vs. 26 +/- 4 pmol/day), and decreased proximal tubular (86)Rb uptake (0.44 +/- 0.07 vs. 1.00 +/- 0.10, P < 0.01) and Na/K-ATPase enzymatic activity (5.1 +/- 1.1 vs. 9.9 +/- 1.6 micromol/mg pr/hr, P < 0.01) relative to the normal diet. Proximal tubular Na/K-ATPase alpha1 protein density was decreased in the plasmalemma fraction but increased in both early and late endosomes following the high salt diet. In rats fed a high salt diet, anti-MBG antibody caused a 60% reduction in urinary sodium excretion, substantial increases in proximal tubule (86)Rb uptake, and Na/K-ATPase enzymatic activity, as well as significant decreases in the early and late endosomal Na/K-ATPase alpha1 protein content.

CONCLUSION

These data suggest that redistribution of the proximal tubule Na/K-ATPase in response to endogenous cardiotonic steroids plays an important role in renal adaptation to salt loading.

“Subcellular Proteomics” of Neuromelanin Granules Isolated from the Human Brain

"Subcellular proteomics" is currently the most effective approach to characterize subcellular compartments. Based on the powerful combination of subcellular fractionation and protein identification by LC-MS/MS we were able for the first time to 1) isolate intact neuromelanin granules from the human brain and 2) establish the first protein profile of these granules. This compartment containing neuromelanin (NM) is primarily located in the primate's substantia nigra, one of the main brain regions that severely degenerates in Parkinson disease. We used mechanic tissue disaggregation, discontinuous sucrose gradient centrifugation, cell disruption, and organelle separation to isolate NM granules from human substantia nigra. Using transmission electron microscopy we demonstrated that the morphological characteristics of the isolated NM granules are similar to those described in human brain tissue. Fundamentally we found numerous proteins definitely demonstrating a close relationship of NM-containing granules with lysosomes or lysosome-related organelles originating from the endosome-lysosome lineage. Intriguingly we further revealed the presence of endoplasmic reticulum-derived chaperones, especially the transmembrane protein calnexin, which recently has been located in lysosome-related melanosomes and has been suggested to be a melanogenic chaperone.

The 14-3-3 protein translates the NA+,K+-ATPase {alpha}1-subunit phosphorylation signal into binding and activation of phosphoinositide 3-kinase during endocytosis

Clathrin-dependent endocytosis of Na(+),K(+)-ATPase molecules in response to G protein-coupled receptor signals is triggered by phosphorylation of the alpha-subunit and the binding of phosphoinositide 3-kinase. In this study, we describe a molecular mechanism linking phosphorylation of Na(+),K(+)-ATPase alpha-subunit to binding and activation of phosphoinositide 3-kinase. Co-immunoprecipitation studies, as well as experiments using confocal microscopy, revealed that dopamine favored the association of 14-3-3 protein with the basolateral plasma membrane and its co-localization with the Na(+),K(+)-ATPase alpha-subunit. The functional relevance of this interaction was established in opossum kidney cells expressing a 14-3-3 dominant negative mutant, where dopamine failed to decrease Na(+),K(+)-ATPase activity and to promote its endocytosis. The phosphorylated Ser-18 residue within the alpha-subunit N terminus is critical for 14-3-3 binding. Activation of phosphoinositide 3-kinase by dopamine during Na(+),K(+)-ATPase endocytosis requires the binding of the kinase to a proline-rich domain within the alpha-subunit, and this effect was blocked by the presence of a 14-3-3 dominant negative mutant. Thus, the 14-3-3 protein represents a critical linking mechanism for recruiting phosphoinositide 3-kinase to the site of Na(+),K(+)-ATPase endocytosis.

Ouabain induces endocytosis of plasmalemmal Na/K-ATPase in LLC-PK1 cells by a clathrin-dependent mechanism

BACKGROUND

We have demonstrated that ouabain causes dose- and time-dependent decreases in (86)Rb uptake in porcine proximal tubular (LLC-PK1) cells. The present study addresses the molecular mechanisms involved in this process.

METHODS

Studies were performed with cultured LLC-PK1 and Src family kinase deficient (SYF) cells.

RESULTS

We found that 50 nmol/L ouabain applied to the basal, but not apical, aspect for 12 hours caused decreases in the plasmalemmal Na/K-ATPase. This loss of plasmalemmal Na/K-ATPase reverses completely within 12 to 24 hours after removal of ouabain. Ouabain also increased the Na/K-ATPase content in both early and late endosomes, activated phosphatidylinositol 3-kinase (PI(3)K), and also caused a translocation of some Na/K-ATPase to the nucleus. Immunofluorescence demonstrated that the Na/K-ATPase colocalized with clathrin both before and after exposure to ouabain, and immunoprecipitation experiments confirmed that ouabain stimulated interactions among the Na/K-ATPase, adaptor protein-2 (AP-2), and clathrin. Potassium (K) depletion, chlorpromazine, or PI(3)K inhibition all significantly attenuated this ouabain-induced endocytosis. Inhibition of the ouabain-activated signaling process through Src by 4-Amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) significantly attenuated ouabain-induced endocytosis. Moreover, experiments performed in SYF cells demonstrated that ouabain induced increases in the endocytosis of the Na/K-ATPase when Src was reconstituted (SYF+), but not in the Src-deficient (SYF-) cells.

CONCLUSION

These data demonstrate that ouabain stimulates a clathrin-dependent endocytosis pathway that translocates the Na/K-ATPase to intracellular compartments, thus suggesting a potential role of endocytosis in ouabain-induced signal transduction as well as proximal tubule sodium handling.

Hypertension-linked mutation in the adducin alpha-subunit leads to higher AP2-mu2 phosphorylation and impaired Na+,K+-ATPase trafficking in response to GPCR signals and intracellular sodium

Alpha-adducin polymorphism in humans is associated with abnormal renal sodium handling and high blood pressure. The mechanisms by which mutations in adducin affect the renal set point for sodium excretion are not known. Decreases in Na+,K+-ATPase activity attributable to endocytosis of active units in renal tubule cells by dopamine regulates sodium excretion during high-salt diet. Milan rats carrying the hypertensive adducin phenotype have a higher renal tubule Na+,K+-ATPase activity, and their Na+,K+-ATPase molecules do not undergo endocytosis in response to dopamine as do those of the normotensive strain. Dopamine fails to promote the interaction between adaptins and the Na+,K+-ATPase because of adaptin-mu2 subunit hyperphosphorylation. Expression of the hypertensive rat or human variant of adducin into normal renal epithelial cells recreates the hypertensive phenotype with higher Na+,K+-ATPase activity, mu2-subunit hyperphosphorylation, and impaired Na+,K+-ATPase endocytosis. Thus, increased renal Na+,K+-ATPase activity and altered sodium reabsorption in certain forms of hypertension could be attributed to a mutant form of adducin that impairs the dynamic regulation of renal Na+,K+-ATPase endocytosis in response to natriuretic signals.

3,3′,5-Triiodo-l-thyronine Up-regulation of Na,K-ATPase Activity and Cell Surface Expression in Alveolar Epithelial Cells Is Src Kinase- and Phosphoinositide 3-Kinase-dependent

We previously reported that thyroid hormone, 3,3',5-triiodo-l-thyronine (T3), increased Na,K-ATPase activity of adult rat alveolar epithelial cells in a transcription-independent manner via increased cell surface expression of the alpha(1) and beta(1) subunits of Na,K-ATPase. Now we sought to identify signaling molecules necessary for T3 stimulation of Na,K-ATPase activity in alveolar epithelial cells. Whereas protein kinase A inhibitor H-8 and protein kinase C inhibitor bisindolymaleimide did not block the T3-induced increase in Na,K-ATPase activity, two inhibitors of phosphoinositide 3-kinase (PI3K), wortmannin and Ly294002, and two Src kinase inhibitors, PP1 and PP2, blocked the T3-induced Na,K-ATPase activity. T3 stimulated the activity of PI3K as measured by phosphatidylinositol 3-phosphate. T3 also stimulated the serine 473 phosphorylation of the PI3K downstream molecule PKB/Akt in a dose-dependent manner. Transient expression of a constitutively active mutant of the PI3K catalytic subunit p110 augmented Na,K-ATPase activity and increased the amount of cell surface Na,K-ATPase alpha(1) subunit protein. T3 also stimulated Src family kinase activity. Transient expression of a constitutively active Src kinase increased Na,K-ATPase activity, PI3K activity, and phosphorylation of PKB/Akt at serine 473. PP1 or PP2 blocked T3-stimulated PKB/Akt phosphorylation at serine 473 and PI3K activity that was activated by an active mutant of Src; however, wortmannin did not inhibit the T3-stimulated Src kinase activity. Although PP1 and wortmannin abolished the increase in Na,K-ATPase activity induced by the active mutant of Src, PP1 did not inhibit the active mutant of PI3K-up-regulated Na,K-ATPase activity. In summary, T3 stimulates the PI3K/PKB pathway via the Src family of tyrosine kinases, and activation of both the Src family kinases and PI3K is required for the T3-induced stimulation of Na,K-ATPase activity and its cell surface expression in adult rat alveolar epithelial cells.

Clathrin-mediated Endocytosis of Na+,K+-ATPase in Response to Parathyroid Hormone Requires ERK-dependent Phosphorylation of Ser-11 within the α1-Subunit

Parathyroid hormone (PTH) inhibits Na(+),K(+)-ATPase activity through protein kinase C- (PKC) and extracellular signal-regulated kinase- (ERK) dependent pathways and increases serine phosphorylation of the alpha(1)-subunit. To determine whether specific serine phosphorylation sites within the Na(+),K(+)-ATPase alpha(1)-subunit are involved in the Na(+),K(+)-ATPase responses to PTH, we examined the effect of PTH in opossum kidney cells stably transfected with wild type rat Na(+),K(+)-ATPase alpha(1)-subunit (WT), serine 11 to alanine mutant alpha(1)-subunit (S11A), or serine 18 to alanine mutant alpha(1)-subunit (S18A). PTH increased phosphorylation and endocytosis of the Na(+),K(+)-ATPase alpha(1)-subunit into clathrin-coated vesicles in cells transfected with WT and S18A rat Na(+),K(+)-ATPase alpha(1)-subunits. PTH did not increase the level of phosphorylation or stimulate translocation of Na(+),K(+)-ATPase alpha(1)-subunits into clathrin-coated vesicles in cells transfected with the S11A mutant. PTH inhibited ouabain-sensitive (86)Rb uptake and Na(+),K(+)-ATPase activity (ouabain-sensitive ATP hydrolysis) in WT- and S18A-transfected opossum kidney cells but not in S11A-transfected cells. Pretreatment of the cells with the PKC inhibitors and ERK inhibitor blocked PTH inhibition of (86)Rb uptake, Na(+),K(+)-ATPase activity, alpha(1)-subunit phosphorylation, and endocytosis in WT and S18A cells. Consistent with the notion that ERK phosphorylates Na(+),K(+)-ATPase alpha(1)-subunit, ERK was shown to be capable of causing phosphorylation of Na(+),K(+)-ATPase alpha(1)-subunit immunoprecipitated from WT and S18A but not from S11A-transfected cells. These results suggest that PTH regulates Na(+),K(+)-ATPase by PKC and ERK-dependent alpha(1)-subunit phosphorylation and that the phosphorylation requires the expression of a serine at the 11 position of the Na(+),K(+)-ATPase alpha(1)-subunit.

Basic fibroblast growth factor (bFGF) regulation of the plasma membrane calcium ATPase (PMCA) as part of an anti-apoptotic mechanism of action

Basic fibroblast growth factor (bFGF) preserves the viability of at least 13 different cells, including epithelial, endothelial, smooth muscle and neuronal cells. In spite of this profound and rather universal effect on cell viability, detailed studies regarding the mechanism of bFGF's action have not been conducted. Rather, most studies have simply shown that bFGF inhibits cells from undergoing programmed cell death (i.e. apoptosis). The most mechanistic studies to date have been conducted on either neurons or ovarian (granulosa) cells. These studies have shown that bFGF prevents apoptosis through both genomic and acute actions. Basic FGF's acute actions involved the maintenance of normal levels of intracellular free calcium levels ([Ca(2+)](i)). In granulosa cells, bFGF maintained [Ca(2+)](i) through a protein kinase C(delta) (PKCdelta)-dependent mechanism. Further, bFGF-activated PKCdelta maintained [Ca(2+)](i) by stimulating calcium efflux. The ability of bFGF to stimulate calcium efflux involved the plasma membrane calcium ATPase (PMCA). Interestingly, bFGF-activated PKCdelta appeared to regulate PMCA activity in part by promoting its membrane localization.

Intracellular Na+ regulates dopamine and angiotensin II receptors availability at the plasma membrane and their cellular responses in renal epithelia

The balance and cross-talk between natruretic and antinatruretic hormone receptors plays a critical role in the regulation of renal Na+ homeostasis, which is a major determinant of blood pressure. Dopamine and angiotensin II have antagonistic effects on renal Na+ and water excretion, which involves regulation of the Na+,K+-ATPase activity. Herein we demonstrate that angiotensin II (Ang II) stimulation of AT1 receptors in proximal tubule cells induces the recruitment of Na+,K+-ATPase molecules to the plasmalemma, in a process mediated by protein kinase Cbeta and interaction of the Na+,K+-ATPase with adaptor protein 1. Ang II stimulation led to phosphorylation of the alpha subunit Ser-11 and Ser-18 residues, and substitution of these amino acids with alanine residues completely abolished the Ang II-induced stimulation of Na+,K+-ATPase-mediated Rb+ transport. Thus, for Ang II-dependent stimulation of Na+,K+-ATPase activity, phosphorylation of these serine residues is essential and may constitute a triggering signal for recruitment of Na+,K+-ATPase molecules to the plasma membrane. When cells were treated simultaneously with saturating concentrations of dopamine and Ang II, either activation or inhibition of the Na+,K+-ATPase activity was produced dependent on the intracellular Na+ concentration, which was varied in a very narrow physiological range (9-19 mm). A small increase in intracellular Na+ concentrations induces the recruitment of D1 receptors to the plasma membrane and a reduction in plasma membrane AT1 receptors. Thus, one or more proteins may act as an intracellular Na+ concentration sensor and play a major regulatory role on the effect of hormones that regulate proximal tubule Na+ reabsorption.

Loss of murine Na+/myo-inositol cotransporter leads to brain myo-inositol depletion and central apnea

myo-Inositol (Ins) and its polyphosphoinositide derivatives that are important in membrane signaling have long been held to play a special role in brain metabolism. As polyphosphoinositides turn over rapidly and are exceptionally abundant in nervous tissue, high Ins levels in the range of 2-15 mm that have been observed in brain may be necessary to maintain the rates of phosphoinositide synthesis in diverse membrane locations within neurons. Cellular concentration gradients of this magnitude indicate a dependence on active Ins transport, especially at the time of growth and differentiation. The Na(+)/myo-inositol cotransporter (SMIT1 or SLC5A3) gene is highly expressed prenatally in the central nervous system and placenta. To gain more insight into brain Ins metabolism, while ascertaining the importance of SMIT1 as a transporter, we generated mice with a homozygous targeted deletion of this gene. Newborn SMIT1(-/-) animals have no evidence of SMIT1 mRNA, a 92% reduction in the level of brain Ins, an 84% reduction in whole body Ins, and expire shortly after birth due to hypoventilation. Gross pathologic and light microscopic examinations of each organ, as well as the placenta, of embryonic day 18.5 fetuses at near term gestation were normal. Based on [(3)H]acetate incorporation into phospholipids of lung tissue explants, immunostaining of lung tissue for surfactant protein A, B, and C, and electron microscopic examination of alveolar cells, there was no evidence of abnormal pulmonary surfactant production by type 2 pneumocytes in lung. Although no histologic lesions were detected in the nervous system, electrophysiological studies of the brainstem pre-Bötzinger respiratory control center demonstrated an abnormal rhythm discharge with periods of central apnea. The cause of death can be explained by the regulatory defect in brainstem control of ventilation. This model demonstrates the critical importance of SMIT1 in the developing nervous system. The high affinity SMIT1 transporter is responsible for the Ins concentration gradient in the murine fetal-placental unit.

Isoform-specific regulation of Na+,K+-ATPase endocytosis and recruitment to the plasma membrane

The Na(+),K(+)-ATPase traffics between the plasma membrane and intracellular compartments in response to acute changes in membrane receptor activation. These effects are accomplished by a time-dependent interaction of the Na(+),K(+)-ATPase alpha-subunit with specific intracellular signaling molecules either at the plasma membrane (endocytosis) or at the endosome's membranes (recruitment). Most of these studies have been performed in rat renal epithelial cells in which only the alpha(1) isoenzyme is present. Studies in neurons from the neostriatum in which all three alpha-subunit isoforms are present indicate that neurotransmitter-dependent regulation of Na(+),K(+)-ATPase activity displays isoform specificity and also suggest a more complex organization of the intracellular signaling networks controlling Na(+),K(+)-ATPase traffic in mammalian cells.

Ion pump-interacting proteins: promising new partners

The sorting and regulation of the Na,K and H,K-ATPases requires that the pump proteins must associate, at least transiently, with kinases, phosphatases, scaffolding molecules, and components of the cellular trafficking machinery. The identities of these interacting proteins and the nature of their associations with the pump polypeptides have yet to be elucidated. We have begun a series of yeast two-hybrid screens employing structurally defined segments of pump polypeptides as baits in order to gain insight into the nature and function of these interacting proteins.

Phosphorylation of the Na+,K+-ATPase in skeletal muscle: potential mechanism for changes in pump cell-surface abundance and activity

In skeletal muscle, insulin stimulation leads to phosphorylation of Na(+),K(+)-ATPase alpha-subunits on both serine/threonine and tyrosine residues, translocation of Na(+),K(+)-ATPase molecules to the plasma membrane, and increased Na(+),K(+)-ATPase activity. The molecular nature of the tyrosine kinase that phosphorylates Na(+),K(+)-ATPase is not yet identified. In vitro phosphorylation experiments show that the alpha-subunit of Na(+),K(+)-ATPase from skeletal muscle is a substrate for the tyrosine-specific protein kinase c-src. Tyrosine phosphorylation of the alpha-subunits of Na(+),K(+)-ATPase may be an important mechanism for insulin-mediated regulation of Na(+),K(+)-ATPase translocation and activity.

Analysis of Na+,K+-ATPase motion and incorporation into the plasma membrane in response to G protein-coupled receptor signals in living cells

Dopamine (DA) increases Na(+),K(+)-ATPase activity in lung alveolar epithelial cells. This effect is associated with an increase in Na(+),K(+)-ATPase molecules within the plasma membrane (). Analysis of Na(+),K(+)-ATPase motion was performed in real-time in alveolar cells stably expressing Na(+),K(+)-ATPase molecules carrying a fluorescent tag (green fluorescent protein) in the alpha-subunit. The data demonstrate a distinct (random walk) pattern of basal movement of Na(+),K(+)-ATPase-containing vesicles in nontreated cells. DA increased the directional movement (by 3.5 fold) of the vesicles and an increase in their velocity (by 25%) that consequently promoted the incorporation of vesicles into the plasma membrane. The movement of Na(+),K(+)-ATPase-containing vesicles and incorporation into the plasma membrane were microtubule dependent, and disruption of this network perturbed vesicle motion toward the plasma membrane and prevented the increase in the Na(+),K(+)-ATPase activity induced by DA. Thus, recruitment of new Na(+),K(+)-ATPase molecules into the plasma membrane appears to be a major mechanism by which dopamine increases total cell Na(+),K(+)-ATPase activity.

Effects of cardiac glycosides on sodium pump expression and function in LLC-PK1 and MDCK cells

BACKGROUND

The decreases in proximal tubule sodium reabsorption seen with chronic renal failure and volume expansion have been ascribed to circulating digitalis-like substances (DLS). However, the circulating concentrations of DLS do not acutely inhibit the sodium pump to a degree consistent with the observed changes in proximal tubule sodium reabsorption.

METHODS

We examined how cell lines that simulated proximal (LLC-PK1) and distal tubule (MDCK) cells responded to acute (30 min) and long-term (up to 12 hours) Na+,K+-ATPase inhibition with DLS.

RESULTS

In LLC-PK1, but not MDCK cells, low concentrations of ouabain decreased 86Rb uptake profoundly in a time and dose dependent manner. In LLC-PK1 cells grown to confluence, transcellular 22Na flux was markedly reduced in concert with the decreases in 86Rb uptake. Similar findings were observed with marinobufagenin (MBG) and deproteinated extract of serum derived from patients with chronic renal failure. However, inhibition of the Na+,K+-ATPase with low extracellular potassium concentrations did not produce any of these effects. Western and Northern blots detected no change in alpha1 Na+,K+-ATPase protein and message RNA, respectively, in LLC-PK1 cells treated with ouabain for 12 hours. However, the decrease in enzymatic activity of Na+,K+-ATPase of these cells was comparable to observed decreases in 86Rb uptake. Differential centrifugation as well as biotinylation experiments demonstrated a shift of the Na+,K+-ATPase from the plasmalemma with prolonged ouabain treatment.

CONCLUSIONS

The results show that binding of cardiac glycosides by proximal (but not distal) tubular cells results in internalization of Na+,K+-ATPase with the net effect to amplify inhibition of the Na+,K+-ATPase. As the circulating concentrations of DLS increase with chronic renal failure and volume expansion, we suggest that this phenomenon explains some of the decreased sodium reabsorption by the proximal tubule seen in these conditions.

Relevance of dopamine signals anchoring dynamin-2 to the plasma membrane during Na+,K+-ATPase endocytosis

Clathrin-dependent endocytosis of Na(+),K(+)-ATPase in response to dopamine regulates its catalytic activity in intact cells. Because fission of clathrin-coated pits requires dynamin, we examined the mechanisms by which dopamine receptor signals promote dynamin-2 recruitment and assembly at the site of Na(+),K(+)-ATPase endocytosis. Western blotting revealed that dopamine increased the association of dynamin-2 with the plasma membrane and with phosphatidylinositol 3-kinase. Dopamine inhibited Na(+),K(+)-ATPase activity in OK cells and in those overexpressing wild type dynamin-2 but not in cells expressing a dominant-negative mutant. Dephosphorylation of dynamin is important for its assembly. Dopamine increased protein phosphatase 2A activity and dephosphorylated dynamin-2. In cells expressing a dominant-negative mutant of protein phosphatase 2A, dopamine failed to dephosphorylate dynamin-2 and to reduce Na(+),K(+)-ATPase activity. Dynamin-2 is phosphorylated at Ser(848), and expression of the S848A mutant significantly blocked the inhibitory effect of dopamine. These results demonstrate a distinct signaling network originating from the dopamine receptor that regulates the state of dynamin-2 phosphorylation and that promotes its location (by interaction with phosphatidylinositol 3-kinase) at the site of Na(+),K(+)-ATPase endocytosis.

Translocation of Na(+),K(+)-ATPase is induced by Rho small GTPase in renal epithelial cells

The distribution of transmembrane proteins is considered to be crucial for their activities because these proteins mediate the information coming from outside of cells. A small GTPase Rho participates in many cellular functions through its downstream effectors. In this study, we examined the effects of RhoA on the distribution of Na(+),K(+)-ATPase, one of the transmembrane proteins. In polarized renal epithelium, Na(+),K(+)-ATPase is known to be localized at the basolateral membrane. By microinjection of the constitutively active mutant of RhoA (RhoA(Val14)) into cultured renal epithelial cells, Na(+),K(+)-ATPase was translocated to the spike-like protrusions over the apical surfaces. Microinjection of the constitutively active mutant of other Rho family GTPases, Rac1 or Cdcd42, did not induce the translocation. The translocation induced by RhoA(Val14) was inhibited by treatment with Y-27632, a Rho-kinase specific inhibitor, or by coinjection of the dominant negative mutant of Rho-kinase. These results indicate that Rho and Rho-kinase are involved in the regulation of the localization of Na(+),K(+)-ATPase. We also found that Na(+),K(+)-ATPase seemed to be colocalized with ERM proteins phosphorylated at T567 (ezrin), T564 (radixin), and T558 (moesin) in cells microinjected with RhoA(Val14).

Stimulation of protein kinase C pathway mediates endocytosis of human nongastric H+-K+-ATPase, ATP1AL1

The human nongastric H+-K+-ATPase, ATP1AL1, shown to reabsorb K+ in exchange for H+ or Na+, is localized in the luminal plasma membrane of renal epithelial cells. It is presumed that renal H+-K+-ATPases can be regulated by endocytosis. However, little is known about the molecular mechanisms that control plasma membrane expression of renal H+-K+-ATPases. In our study, activation of protein kinase C (PKC) using phorbol esters (phorbol 12-myristate 13-acetate) leads to clathrin-dependent internalization and intracellular accumulation of the ion pump in stably transfected Madin-Darby canine kidney cells. Functional inactivation of the H+-K+-ATPase by PKC activation is shown by intracellular pH measurements. Proton extrusion capacity of ATP1AL1-transfected cells is drastically reduced after phorbol 12-myristate 13-acetate incubation and can be prevented with the PKC blocker bisindolylmaleimide. Ion pump internalization and inactivation are specifically mediated by the PKC pathway, whereas activation of the protein kinase A pathway has no influence. Our results show that the nongastric H+-K+-ATPase is a specific target for the PKC pathway. Therefore, PKC-mediated phosphorylation is a potential regulatory mechanism for apical nongastric H+-K+-ATPase plasma membrane expression.

Inositol hexakisphosphate-mediated regulation of glutamate receptors in rat brain sections

D-myo-inositol 1,2,3,4,5,6-hexakisphosphate (InsP6), one of the most abundant inositol phosphates within cells, has been proposed to play a key role in vesicle trafficking and receptor compartmentalization. In the present study, we used in vitro receptor autoradiography, subcellular fractionation, and immunoblotting to investigate its effects on alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors. Qualitative and quantitative analysis of 3H-AMPA binding indicated that incubation of frozen-thawed brain sections with InsP6 at 35 degrees C enhanced AMPA receptor binding in several brain regions, with maximal increases in the hippocampus and cerebellum. Moreover, saturation kinetics demonstrated that InsP6-induced augmentation of AMPA binding was due to an increment in the maximal number of AMPA binding sites. At the immunological level, Western blots performed on crude mitochondrial/synaptic (P2) fractions revealed that InsP6 (but not InsP5 and InsP3) treatment increased glutamate receptor (GluR)1 and GluR2 subunits of AMPA receptors, an effect that was associated with concomitant reductions in microsomal (P3) fractions. Interestingly, the InsP6-induced modulation of AMPA receptor binding was blocked at room temperature, and pretreatment with heparin also dampered its action on both AMPA receptor binding and GluR subunits. These effects of InsP6 appear to be specific to AMPA receptors, as neither 3H-glutamate binding to NMDA receptors nor levels of NR1 and NR2A subunits in P2 and P3 fractions were affected. Taken together, our data strongly suggest that InsP6 specifically regulates AMPA receptor distribution, possibly through a clathrin-dependent process.

Molecular aspects of acute inhibition of Na(+)-H(+) exchanger NHE3 by A(2)-adenosine receptor agonists

Adenosine regulates Na(+) homeostasis by its acute effects on renal Na(+) transport. We have shown in heterologously transfected A6/C1 cells (renal cell line from Xenopus laevis) that adenosine-induced natriuresis may be effected partly via A(2) adenosine receptor-mediated inactivation of the renal brush border membrane Na(+)-H(+) exchanger NHE3. In this study we utilized A6/C1 cells stably expressing wild-type as well as mutated forms of NHE3 to assess the molecular mechanism underlying A(2)-dependent control of NHE3 function. Cell surface biotinylation combined with immunoprecipitation revealed that NHE3 is targeted exclusively to the apical domain and that the endogenous Xenopus NHE is located entirely on the basolateral side of A6/C1 transfectants. Stimulation of A(2)-adenosine receptors located on the basolateral side for 15 min with CPA (N6-cyclopentyladenosine) acutely decreased NHE3 activity (microspectrofluorimety). This effect was mimicked by 8-bromo-cAMP and entirely blocked by pharmacological inhibition of PKA (with H89) or singular substitution of two PKA target sites (serine 552 and serine 605) on NHE3. Downregulation of NHE3 activity by CPA was attributable to a reduction of NHE3 intrinsic transport activity without change in surface NHE3 protein at 15 min. At 30 min, the decrease in transport activity was associated with a decrease in apical membrane NHE3 antigen. In conclusion, two highly conserved target serine sites on NHE3 determine NHE3 modulation upon A(2)-receptor activation and NHE3 inactivation by adenosine proceeds via two phases with distinct mechanisms.

Intracellular sodium modulates the state of protein kinase C phosphorylation of rat proximal tubule Na+,K+-ATPase

The natriuretic hormone dopamine and the antinatriuretic hormone noradrenaline, acting on alpha-adrenergic receptors, have been shown to bidirectionally modulate the activity of renal tubular Na+,K+-adenosine triphosphate (ATPase). Here we have examined whether intracellular sodium concentration influences the effects of these bidirectional forces on the state of phosphorylation of Na+,K+-ATPase. Proximal tubules dissected from rat kidney were incubated with dopamine or the alpha-adrenergic agonist, oxymetazoline, and transiently permeabilized in a medium where sodium concentration ranged between 5 and 70 mM. The variations of sodium concentration in the medium had a proportional effect on intracellular sodium. Dopamine and protein kinase C (PKC) phosphorylate the catalytic subunit of rat Na+,K+-ATPase on the Ser23 residue. The level of PKC induced Na+,K+-ATPase phosphorylation was determined using an antibody that only recognizes Na+,K+-ATPase, which is not phosphorylated on its PKC site. Under basal conditions Na+,K+-ATPase was predominantly in its phosphorylated state. When intracellular sodium was increased, Na+,K+-ATPase was predominantly in its dephosphorylated state. Phosphorylation of Na+,K+-ATPase by dopamine was most pronounced when intracellular sodium was high, and dephosphorylation by oxymetazoline was most pronounced when intracellular sodium was low. The oxymetazoline effect was mimicked by the calcium ionophore A23187. An inhibitor of the calcium-dependent protein phosphatase, calcineurin, increased the state of Na+,K+-ATPase phosphorylation. The results imply that phosphorylation of renal Na+,K+-ATPase activity is modulated by the level of intracellular sodium and that this effect involves PKC and calcium signalling pathways. The findings may have implication for the regulation of salt excretion and sodium homeostasis.

Dopamine-induced inhibition of Na+-K+-ATPase activity requires integrity of actin cytoskeleton in opossum kidney cells

The present study evaluated the importance of the association between Na+-K+-ATPase and the actin cytoskeleton on dopamine-induced inhibition of Na+-K+-ATPase activity. The approach used measures the transepithelial transport of Na+ in monolayers of opossum kidney (OK) cells, when the Na+ delivered to Na+-K+-ATPase was increased at the saturating level by amphotericin B. The maximal amphotericin B (1.0 microg mL-1) induced increase in short-circuit current (Isc) was prevented by ouabain (100 microM) or removal of apical Na+. Dopamine (1 microM) applied from the apical side significantly decreased (29 +/- 5% reduction) the amphotericin B-induced increase in Isc, this being prevented by the D1-like receptor antagonist SKF 83566 (1 microM) and the protein kinase C (PKC) inhibitor chelerythrine (1 microM). Exposure of OK cells to cytochalasin B (1 microM) or cytochalasin D (1 microM), inhibitors of actin polymerization, from both cell sides reduced by 31 +/- 4% and 36 +/- 3% the amphotericin B-induced increase in Isc and abolished the inhibitory effect of apical dopamine (1 microM), but not that of the PKC activator phorbol-12,13-dibutyrate (PDBu; 100 nM). Colchicine (1 microM) failed to alter the inhibitory effects of dopamine. The relationship between Na+-K+-ATPase and the concentration of extracellular Na+ showed a Michaelis-Menten constant (Km) of 44.1 +/- 13.7 mM and a Vmax of 49.6 +/- 4.8 microA cm-2 in control monolayers. In the presence of apical dopamine (1 microM) or cytochalasin B (1 microM) Vmax values were significantly (P < 0.05) reduced without changes in Km values. These results are the first, obtained in live cells, showing that the PKC-dependent inhibition of Na+-K+-ATPase activity by dopamine requires the integrity of the association between actin cytoskeleton and Na+-K+-ATPase.

Tyrosine 537 within the Na+,K+-ATPase alpha-subunit is essential for AP-2 binding and clathrin-dependent endocytosis

In renal epithelial cells endocytosis of Na(+),K(+)-ATPase molecules is initiated by phosphorylation of its alpha(1)-subunit, leading to activation of phosphoinositide 3-kinase and adaptor protein-2 (AP-2)/clathrin recruitment. The present study was performed to establish the identity of the AP-2 recognition domain(s) within the Na(+),K(+)-ATPase alpha(1)-subunit. We identified a conserved sequence (Y(537)LEL) within the alpha(1)-subunit that represents an AP-2 binding site. Binding of AP-2 to the Na(+),K(+)-ATPase alpha(1)-subunit in response to dopamine (DA) was increased in OK cells stably expressing the wild type rodent alpha-subunit (OK-WT), but not in cells expressing the Y537A mutant (OK-Y537A). DA treatment was associated with increased alpha(1)-subunit abundance in clathrin vesicles from OK-WT but not from OK-Y537A cells. In addition, this mutation also impaired the ability of DA to inhibit Na(+),K(+)-ATPase activity. Because phorbol esters increase Na(+),K(+)-ATPase activity in OK cells, and this effect was not affected by the Y537A mutation, the present results suggest that the identified motif is specifically required for DA-induced AP-2 binding and Na(+),K(+)-ATPase endocytosis.

Na,K-ATPase overexpression improves alveolar fluid clearance in a rat model of elevated left atrial pressure

BACKGROUND

Acute elevation of left atrial pressure (LAP) increases extravascular water and impairs active Na(+) transport in rat lungs. We have reported that overexpression of Na,K-ATPase subunit genes in the alveolar epithelium increases alveolar fluid clearance (AFC) in normal and injured rat lungs with normal LAP. We reasoned that adenovirus-mediated transfer of an Na,K-ATPase beta-subunit gene to the alveolar epithelium could improve AFC in rat lungs in the presence of acutely elevated LAP.

METHODS AND RESULTS

Normal rats were infected with 4x10(9) plaque-forming units of E1a(-)/E3(-) recombinant adenoviruses that contained a cytomegalovirus promoter coupled to a rat Na,K-ATPase beta(1)-subunit cDNA (adbeta(1)) or no cDNA (adNull) 7 days before study. Na,K-ATPase alpha(1)- and beta(1)-subunit abundance in basolateral cell membranes isolated from the peripheral lung was significantly increased in adbeta(1)-infected lungs compared with sham and adNull-infected controls. In all groups, elevation of LAP reduced membrane-bound Na,K-ATPase abundance; however, abundance in adbeta(1)-infected lungs remained greater than in controls. AFC, measured with a fluid-filled isolated lung preparation in the presence of elevated LAP (15 cmH(2)O), in Na,K-ATPase beta(1)-subunit-overexpressing lungs was up to 100% greater than in controls and was not different from rats studied at normal LAP (0 cmH(2)O).

CONCLUSIONS

These data suggest that alveolar overexpression of an Na,K-ATPase beta(1)-subunit can counteract downregulation of membrane-bound solute transporters owing to elevated pulmonary vascular pressures and can restore active Na(+) transport and AFC in this rat model of acute hydrostatic pulmonary edema.