Protein kinase A (PKA) phosphorylation of Na+/K+-ATPase opens intracellular C-terminal water pathway leading to third Na+-binding site in molecular dynamics simulations

ATP1A1-linked diseases require a malfunctioning protein product from one allele

Heterozygous germline variants in ATP1A1, the gene encoding the α1 subunit of the Na+/K+-ATPase (NKA), have been linked to diseases including primary hyperaldosteronism and the peripheral neuropathy Charcot-Marie-Tooth disease (CMT). ATP1A1 variants that cause CMT induce loss-of-function of NKA. This heterodimeric (αβ) enzyme hydrolyzes ATP to establish transmembrane electrochemical gradients of Na+ and K+ that are essential for electrical signaling and cell survival. Of the 4 catalytic subunit isoforms, α1 is ubiquitously expressed and is the predominant paralog in peripheral axons. Human population sequencing datasets indicate strong negative selection against both missense and protein-null ATP1A1 variants. To test whether haploinsufficiency generated by heterozygous protein-null alleles are sufficient to cause disease, we tested the neuromuscular characteristics of heterozygous Atp1a1+/- knockout mice and their wildtype littermates, while also evaluating if exercise increased CMT penetrance. We found that Atp1a1+/- mice were phenotypically normal up to 18 months of age. Consistent with the observations in mice, we report clinical phenotyping of a healthy adult human who lacks any clinical features of known ATP1A1-related diseases despite carrying a plasma-membrane protein-null early truncation variant, p.Y148*. Taken together, these results suggest that a malfunctioning gene product is required for disease induction by ATP1A1 variants and that if any pathology is associated with protein-null variants, they may display low penetrance or high age of onset.

Melatonin integrates multidimensional regulation of Na+/K+-ATPase in ionocytes and promotes stress and ease response in hypoxia-induced air-breathing fish: lessons from integrative approach

As circadian regulator, melatonin is involved in many physiological processes including ionosmotic regulation in fishes. Na⁺/K⁺-ATPase (NKA), an ubiquitous Na⁺/K⁺ transporter in ionocyte epithelia that drives electrochemical Na⁺ gradients and systemic osmotic integration, is a target of stress in fish. However, it is not certain how melatonin regulates NKA functions in ionocyte epithelia and how it modulates the adaptive response such as stress and ease response in fish particularly in hypoxia condition. We, thus, examined the short-term in vivo action of melatonin on the dynamics of NKA regulation in branchial, renal and intestinal ionocytes of hypoxia-induced air-breathing fish (Anabas testudineus Bloch). Interestingly, we found a rise in plasma melatonin in fish when kept for 30 min of forced submergence in water and that indicates a role for melatonin in hypoxia tolerance. A fall in blood [Na⁺ _, K⁺] occurred in these hypoxic fish which later showed a recovery after melatonin treatment. Similarly, melatonin favored the fall in NKA activity in branchial and renal epithelia of hypoxic fish, though it remarkably stimulated its activities in non-stressed fish. Likewise, melatonin that produced differential pattern of mRNA expression in nkaα1-subunit isoforms (nkaα1a, nkaα1b and nkaα1c) and melatonin receptor isoforms (mtnr1a, mtnr1bb, mtnr1bb _x1x2 ) in the tested ionocyte epithelia, showed reversed expression in hypoxic fish. In addition, the rise in NKAα-protein abundance in branchial and renal epithelia of melatonin-treated hypoxic fish indicated a recovery action of melatonin. A higher NKAα-immunoreactivity was found in the immunohistochemical and immunofluorescent images of branchial ionocytes and renal proximal and distal ionocytes of hypoxic fish treated with melatonin. Furthermore, an activation of PKA and PKG-dependent phosphorylation was found in branchial epithelia of hypoxic fish. The generated integrative parabola model showed that melatonin has a maximum targeted action on NKA function in the renal epithelia, suggesting its lead role in the integration of ionosmotic balance during the recovery or ease response. Over all, the data indicate a multidimensional and preferential action of melatonin on NKA regulation in fish ionocytes that integrate the recovery action against hypoxia, thus pointing to a major role for melatonin in stress and ease response in this fish.

Changes in porcine nutrient transport physiology in response to Ascaris suum infection

Background

The roundworm Ascaris suum is one of the parasites with the greatest economic impact on pig farming. In this context, lower weight gain is hypothesized to be due to decreased nutrient absorption. This study aims at characterizing the effects of A. suum infection on intestinal nutrient transport processes and potential molecular mechanisms.

Methods

Three groups of six piglets each were infected orally (10,000 embryonated A. suum eggs) in a single dose (“single infection”). Another three groups were infected orally (1000 embryonated eggs) for 10 consecutive days (“trickle infection”). Animals were necropsied 21, 35 and 49 days post-infection (dpi). Three groups served as respective controls. The Ussing chamber technique was applied for the functional characterization of small intestinal tissues [short-circuit currents (I_sc) as induced by glucose, alanine and peptides; ³H-glucose net flux rates; tissue conductance (G_t)]. Transcription and expression levels of relevant cytokines and nutrient transporters were evaluated (qPCR/western blot).

Results

Peptide- and alanine-induced changes in I_sc were significantly decreased in the jejunum and ileum of the trickle-infected group at 49 dpi and in the ileum of the single-infected group at 49 dpi. No significant differences regarding glucose transport were observed between the Ascaris-infected groups and the control group in Ussing chamber experiments. Transcription levels of the glucose and peptide transporters as well as of selected transcription factors (transcription of signal transducer and activator of transcription 6 [STAT6] and hypoxia-inducible factor 1-alpha [Hif-1α]) were significantly increased in response to both infection types after some periods. The transcription of interleukins 4 and 13 varied between decrease and increase regarding the respective time points, as did the protein expression of glucose transporters. The expression of the peptide transporter PepT1 was significantly decreased in the ileal single-infected group at 35 dpi. Hif-1α was significantly increased in the ileal tissue from the single-infected group at 21 dpi and in the trickle-infected group at 35 dpi. The expression levels of Na⁺/K⁺-ATPase and ASCT1 remained unaffected.

Conclusions

In contrast to the current hypothesis, these results indicate that the nutrient deprivation induced by A. suum cannot be explained by transcriptional or expression changes alone and requires further studies.

Graphical abstract

Facet-regulated adhesion of double-stranded DNA on palladium surfaces

A better understanding of interactions between metal-nanomaterial surfaces and biomolecules such as DNAs is critical for their biomedical applications. Here we investigated double-stranded DNA (dsDNA) adhering to palladium (Pd) nanosheets with two different exposed facets, {100} and {111}, using a combined computational and experimental approach. Different dsDNA binding modes on the two surfaces were observed, with a surprising "upright" conformation on Pd(100) and a "flat" conformation on Pd(111). Molecular dynamics simulations showed a stronger binding of the dsDNA on Pd(111) than Pd(100), which resulted in significant conformational changes and hydrogen bond breakage in the dsDNA on Pd(111). Further analyses revealed that the different binding strengths were caused by the number and arrangement of water molecules in the first solvation shell (FSS) of the two Pd surfaces. The water hydrogen bond network in the FSS of Pd(100) is compact and resists the embedding of dsDNA, while it is less compact on Pd(111), which allows penetration of dsDNA and its direct contact with Pd(111) surface atoms, thereby exhibiting stronger binding. Further free energy calculations with umbrella sampling supported these observations. Finally, these computational predictions on the adsorption capacity of dsDNA on Pd surfaces were confirmed by gel electrophoresis experiments.

Protein Kinase C Mediates the Corticosterone-induced Sensitization of Dorsal Root Ganglion Neurons Innervating the Rat Stomach

BACKGROUND/AIMS

Gastric hypersensitivity contributes to abdominal pain in patients with functional dyspepsia. Recent studies showed that hormones induced by stress are correlated with visceral hypersensitivity. However, the precise mechanisms underlying gastric hypersensitivity remain largely unknown. The aim of the present study was designed to investigate the roles of corticosterone (CORT) on excitability of dorsal root ganglion (DRG) neurons innervating the stomach.

METHODS

DRG neurons innervating the stomach were labeled by DiI injection into the stomach wall. Patch clamp recordings were employed to examine neural excitability and voltage-gated sodium channel currents. Electromyograph technique was used to determine the responses of neck muscles to gastric distension.

RESULTS

Incubation of acutely isolated DRG neurons with CORT significantly depolarized action potential threshold and enhanced the number of action potentials induced by current stimulation of the neuron. Under voltage-clamp mode, incubation of CORT enhanced voltage-gated sodium current density of the recorded neurons. Pre-incubation of GF109203X, an inhibitor of protein kinase C, blocked the CORT-induced hyperexcitability and potentiation of sodium currents. However, pre-incubation of H-89, an inhibitor of protein kinase A, did not alter the sodium current density. More importantly, intraperitoneal injection of CORT produced gastric hypersensitivity of healthy rats, which was blocked by pre-administration of GF109203X but not H-89.

CONCLUSIONS

Our data strongly suggest that CORT rapidly enhanced neuronal excitability and sodium channel functions, which is most likely mediated by protein kinase C but not protein kinase A signaling pathway in DRG neurons innervating the stomach, thus underlying the gastric hypersensitivity induced by CORT injection.

The many faces of compartmentalized PKA signalosomes

Cellular signal transmission requires the dynamic formation of spatiotemporally controlled molecular interactions. At the cell surface information is received by receptor complexes and relayed through intracellular signaling platforms which organize the actions of functionally interacting signaling enzymes and substrates. The list of hormone or neurotransmitter pathways that utilize the ubiquitous cAMP-sensing protein kinase A (PKA) system is expansive. This requires that the specificity, duration, and intensity of PKA responses are spatially and temporally restricted. Hereby, scaffolding proteins take the center stage for ensuring proper signal transmission. They unite second messenger sensors, activators, effectors, and kinase substrates within cellular micro-domains to precisely control and route signal propagation. A-kinase anchoring proteins (AKAPs) organize such subcellular signalosomes by tethering the PKA holoenzyme to distinct cell compartments. AKAPs differ in their modular organization showing pathway specific arrangements of interaction motifs or domains. This enables the cell- and compartment- guided assembly of signalosomes with unique enzyme composition and function. The AKAP-mediated clustering of cAMP and other second messenger sensing and interacting signaling components along with functional successive enzymes facilitates the rapid and precise dissemination of incoming signals. This review article delineates examples for different means of PKA regulation and for snapshots of compartmentalized PKA signalosomes.

Glutamate Water Gates in the Ion Binding Pocket of Na+ Bound Na+, K+-ATPase

The dynamically changing protonation states of the six acidic amino acid residues in the ion binding pocket of the Na+, K+ -ATPase (NKA) during the ion transport cycle are proposed to drive ion binding, release and possibly determine Na+ or K+ selectivity. We use molecular dynamics (MD) and density functional theory (DFT) simulations to determine the protonation scheme of the Na+ bound conformation of NKA. MD simulations of all possible protonation schemes show that the bound Na+ ions are most stably bound when three or four protons reside in the binding sites, and that Glu954 in site III is always protonated. Glutamic acid residues in the three binding sites act as water gates, and their deprotonation triggers water entry to the binding sites. From DFT calculations of Na+ binding energies, we conclude that three protons in the binding site are needed to effectively bind Na+ from water and four are needed to release them in the next step. Protonation of Asp926 in site III will induce Na+ release, and Glu327, Glu954 and Glu779 are all likely to be protonated in the Na+ bound occluded conformation. Our data provides key insights into the role of protons in the Na+ binding and release mechanism of NKA.

Importance of a Potential Protein Kinase A Phosphorylation Site of Na+,K+-ATPase and Its Interaction Network for Na+ Binding

The molecular mechanism underlying PKA-mediated regulation of Na(+),K(+)-ATPase was explored in mutagenesis studies of the potential PKA site at Ser-938 and surrounding charged residues. The phosphomimetic mutations S938D/E interfered with Na(+) binding from the intracellular side of the membrane, whereas Na(+) binding from the extracellular side was unaffected. The reduction of Na(+) affinity is within the range expected for physiological regulation of the intracellular Na(+) concentration, thus supporting the hypothesis that PKA-mediated phosphorylation of Ser-938 regulates Na(+),K(+)-ATPase activity in vivo Ser-938 is located in the intracellular loop between transmembrane segments M8 and M9. An extended bonding network connects this loop with M10, the C terminus, and the Na(+) binding region. Charged residues Asp-997, Glu-998, Arg-1000, and Lys-1001 in M10, participating in this bonding network, are crucial to Na(+) interaction. Replacement of Arg-1005, also located in the vicinity of Ser-938, with alanine, lysine, methionine, or serine resulted in wild type-like Na(+) and K(+) affinities and catalytic turnover rate. However, when combined with the phosphomimetic mutation S938E only lysine substitution of Arg-1005 was compatible with Na(+),K(+)-ATPase function, and the Na(+) affinity of this double mutant was reduced even more than in single mutant S938E. This result indicates that the positive side chain of Arg-1005 or the lysine substituent plays a mechanistic role as interaction partner of phosphorylated Ser-938, transducing the phosphorylation signal into a reduced affinity of Na(+) site III. Electrostatic interaction of Glu-998 is of minor importance for the reduction of Na(+) affinity by phosphomimetic S938E as revealed by combining S938E with E998A.

A novel molecular dynamics approach to evaluate the effect of phosphorylation on multimeric protein interface: the αB-Crystallin case study

BACKGROUND

Phosphorylation is one of the most important post-translational modifications (PTM) employed by cells to regulate several cellular processes. Studying the effects of phosphorylations on protein structures allows to investigate the modulation mechanisms of several proteins including chaperones, like the small HSPs, which display different multimeric structures according to the phosphorylation of a few serine residues. In this context, the proposed study is aimed at finding a method to correlate different PTM patterns (in particular phosphorylations at the monomers interface of multimeric complexes) with the dynamic behaviour of the complex, using physicochemical parameters derived from molecular dynamics simulations in the timescale of nanoseconds.

RESULTS

We have developed a methodology relying on computing nine physicochemical parameters, derived from the analysis of short MD simulations, and combined with N identifiers that characterize the PTMs of the analysed protein. The nine general parameters were validated on three proteins, with known post-translational modified conformation and unmodified conformation. Then, we applied this approach to the case study of αB-Crystallin, a chaperone which multimeric state (up to 40 units) is supposed to be controlled by phosphorylation of Ser45 and Ser59. Phosphorylation of serines at the dimer interface induces the release of hexamers, the active state of αB-Crystallin. 30 ns of MD simulation were obtained for each possible combination of dimer phosphorylation state and average values of structural, dynamic, energetic and functional features were calculated on the equilibrated portion of the trajectories. Principal Component Analysis was applied to the parameters and the first five Principal Components, which summed up to 84 % of the total variance, were finally considered.

CONCLUSIONS

The validation of this approach on multimeric proteins, which structures were known both modified and unmodified, allowed us to propose a new approach that can be used to predict the impact of PTM patterns in multi-modified proteins using data collected from short molecular dynamics simulations. Analysis on the αB-Crystallin case study clusters together all-P dimers with all-P hexamers and no-P dimer with no-P hexamer and results suggest a great influence of Ser59 phosphorylation on chain B.

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.

The plasma membrane transport systems and adaptation to salinity

Salt stress represents one of the environmental challenges that drastically affect plant growth and yield. Evidence suggests that glycophytes and halophytes have a salt tolerance mechanisms working at the cellular level, and the plasma membrane (PM) is believed to be one facet of the cellular mechanisms. The responses of the PM transport proteins to salinity in contrasting species/cultivars were discussed. The review provides a comprehensive overview of the recent advances describing the crucial roles that the PM transport systems have in plant adaptation to salt. Several lines of evidence were presented to demonstrate the correlation between the PM transport proteins and adaptation of plants to high salinity. How alterations in these transport systems of the PM allow plants to cope with the salt stress was also addressed. Although inconsistencies exist in some of the information related to the responses of the PM transport proteins to salinity in different species/cultivars, their key roles in adaptation of plants to high salinity is obvious and evident, and cannot be precluded. Despite the promising results, detailed investigations at the cellular/molecular level are needed in some issues of the PM transport systems in response to salinity to further evaluate their implication in salt tolerance.

Physicochemical mechanisms of protein regulation by phosphorylation

Phosphorylation offers a dynamic way to regulate protein activity and subcellular localization, which is achieved through its reversibility and fast kinetics. Adding or removing a dianionic phosphate group somewhere on a protein often changes the protein’s structural properties, its stability and dynamics. Moreover, the majority of signaling pathways involve an extensive set of protein–protein interactions, and phosphorylation can be used to regulate and modulate protein–protein binding. Losses of phosphorylation sites, as a result of disease mutations, might disrupt protein binding and deregulate signal transduction. In this paper we focus on the effects of phosphorylation on protein stability, dynamics, and binding. We describe several physico-chemical mechanisms of protein regulation through phosphorylation and pay particular attention to phosphorylation in protein complexes and phosphorylation in the context of disorder–order and order–disorder transitions. Finally we assess the role of multiple phosphorylation sites in a protein molecule, their possible cooperativity and function.

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

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

Microsecond molecular dynamics simulations of Mg²⁺- and K⁺-bound E1 intermediate states of the calcium pump

We have performed microsecond molecular dynamics (MD) simulations to characterize the structural dynamics of cation-bound E1 intermediate states of the calcium pump (sarcoendoplasmic reticulum Ca²⁺-ATPase, SERCA) in atomic detail, including a lipid bilayer with aqueous solution on both sides. X-ray crystallography with 40 mM Mg²⁺ in the absence of Ca²⁺ has shown that SERCA adopts an E1 structure with transmembrane Ca²⁺-binding sites I and II exposed to the cytosol, stabilized by a single Mg²⁺ bound to a hybrid binding site I'. This Mg²⁺-bound E1 intermediate state, designated E1•Mg²⁺, is proposed to constitute a functional SERCA intermediate that catalyzes the transition from E2 to E1•2Ca²⁺ by facilitating H⁺/Ca²⁺ exchange. To test this hypothesis, we performed two independent MD simulations based on the E1•Mg²⁺ crystal structure, starting in the presence or absence of initially-bound Mg²⁺. Both simulations were performed for 1 µs in a solution containing 100 mM K⁺ and 5 mM Mg²⁺ in the absence of Ca²⁺, mimicking muscle cytosol during relaxation. In the presence of initially-bound Mg²⁺, SERCA site I' maintained Mg²⁺ binding during the entire MD trajectory, and the cytosolic headpiece maintained a semi-open structure. In the absence of initially-bound Mg²⁺, two K⁺ ions rapidly bound to sites I and I' and stayed loosely bound during most of the simulation, while the cytosolic headpiece shifted gradually to a more open structure. Thus MD simulations predict that both E1•Mg²⁺ and E•2K+ intermediate states of SERCA are populated in solution in the absence of Ca²⁺, with the more open 2K+-bound state being more abundant at physiological ion concentrations. We propose that the E1•2K⁺ state acts as a functional intermediate that facilitates the E2 to E1•2Ca²⁺ transition through two mechanisms: by pre-organizing transport sites for Ca²⁺ binding, and by partially opening the cytosolic headpiece prior to Ca²⁺ activation of nucleotide binding.

Site-specific separation and detection of phosphopeptide isomers with pH-mediated stacking capillary electrophoresis-electrospray ionization-tandem mass spectrometry

This study reported a pH-mediated stacking CE coupled with ESI MS/MS method to determine the phosphorylation sites of three synthetic phosphopeptides containing structural isomers. These phosphopeptides mimic the phosphopeptides (amino acid residues 12-25) derived from the trypsin-digested products of human lamin A/C protein. The LODs were determined to be 118, 132 and 1240 fmol for SGAQASS(19)TpPL(22)SPTR, SGAQASS(19)TPL(22)SpPTR, and SGAQASS(19)TpPL(22)SpPTR, respectively. The established method was employed to analyze the phosphorylation sites of the trypsin-digested products of glutathione S-transferase-lamin A/C (1-57) fusion protein that had been phosphorylated in vitro by cyclin-dependent kinase 1. The results indicated that this method is feasible to specifically determine the phosphorylation site from phosphopeptide isomers in the trypsin-digested products of a kinase-catalyzed phosphoprotein, which should benefit the investigation of protein kinase-mediated cellular signal transduction.

Protein kinase-dependent oxidative regulation of the cardiac Na+-K+ pump: evidence from in vivo and in vitro modulation of cell signalling

The widely reported stimulation of the cardiac Na(+)-K(+) pump by protein kinase A (PKA) should oppose other effects of PKA to increase contractility of the normal heart. It should also reduce harmful raised myocyte Na(+) levels in heart failure, yet blockade of the β1 adrenergic receptor (AR), coupled to PKA signalling, is beneficial. We treated rabbits with the β1 AR antagonist metoprolol to modulate PKA activity and studied cardiac myocytes ex vivo. Metoprolol increased electrogenic pump current (Ip) in voltage clamped myocytes and reduced glutathionylation of the β1 pump subunit, an oxidative modification causally related to pump inhibition. Activation of adenylyl cyclase with forskolin to enhance cAMP synthesis or inclusion of the catalytic subunit of PKA in patch pipette solutions abolished the increase in Ip in voltage clamped myocytes induced by treatment with metoprolol, supporting cAMP/PKA-mediated pump inhibition. Metoprolol reduced myocardial PKA and protein kinase C (PKC) activities, reduced coimmunoprecipitation of cytosolic p47(phox) and membranous p22(phox) NADPH oxidase subunits and reduced myocardial O2(•-)-sensitive dihydroethidium fluorescence. Treatment also enhanced coimmunoprecipitation of the β1 pump subunit with glutaredoxin 1 that catalyses de-glutathionylation. Since angiotensin II induces PKC-dependent activation of NADPH oxidase, we examined the effects of angiotensin-converting enzyme inhibition with captopril. This treatment had no effect on PKA activity but reduced the activity of PKC, reduced β1 subunit glutathionylation and increased Ip. The PKA-induced Na(+)-K(+) pump inhibition we report should act with other mechanisms that enhance contractility of the normal heart but accentuate the harmful effects of raised cytosolic Na(+) in the failing heart. This scheme is consistent with the efficacy of β1 AR blockade in the treatment of heart failure.

Oxidative regulation of the Na(+)-K(+) pump in the cardiovascular system

The Na(+)-K(+) pump is an essential heterodimeric membrane protein, which maintains electrochemical gradients for Na(+) and K(+) across cell membranes in all tissues. We have identified glutathionylation, a reversible posttranslational redox modification, of the Na(+)-K(+) pump's β1 subunit as a regulatory mechanism of pump activity. Oxidative inhibition of the Na(+)-K(+) pump by angiotensin II- and β1-adrenergic receptor-coupled signaling via NADPH oxidase activation demonstrates the relevance of this regulatory mechanism in cardiovascular physiology and pathophysiology. This has implications for dysregulation of intracellular Na(+) and Ca(2+) as well as increased oxidative stress in heart failure, myocardial ischemia-reperfusion, and regulation of vascular tone under conditions of elevated oxidative stress. Treatment strategies that are able to reverse this oxidative inhibition of the Na(+)-K(+) pump have the potential for cardiovascular-protective effects.

A specific and essential role for Na,K-ATPase α3 in neurons co-expressing α1 and α3

Most neurons co-express two catalytic isoforms of Na,K-ATPase, the ubiquitous α1, and the more selectively expressed α3. Although neurological syndromes are associated with α3 mutations, the specific role of this isoform is not completely understood. Here, we used electrophysiological and Na(+) imaging techniques to study the role of α3 in central nervous system neurons expressing both isoforms. Under basal conditions, selective inhibition of α3 using a low concentration of the cardiac glycoside, ouabain, resulted in a modest increase in intracellular Na(+) concentration ([Na(+)](i)) accompanied by membrane potential depolarization. When neurons were challenged with a large rapid increase in [Na(+)](i), similar to what could be expected following suprathreshold neuronal activity, selective inhibition of α3 almost completely abolished the capacity to restore [Na(+)](i) in soma and dendrite. Recordings of Na,K-ATPase specific current supported the notion that when [Na(+)](i) is elevated in the neuron, α3 is the predominant isoform responsible for rapid extrusion of Na(+). Low concentrations of ouabain were also found to disrupt cortical network oscillations, providing further support for the importance of α3 function in the central nervous system. The α isoforms express a well conserved protein kinase A consensus site, which is structurally associated with an Na(+) binding site. Following activation of protein kinase A, both the α3-dependent current and restoration of dendritic [Na(+)](i) were significantly attenuated, indicating that α3 is a target for phosphorylation and may participate in short term regulation of neuronal function.