The (Na+ + K+)-ATPase of chick sensory neurons. Studies on biosynthesis and intracellular transport

Helicobacter pylori-Induced Decrease in Membrane Expression of Na,K-ATPase Leads to Gastric Injury

Helicobacter pylori is a highly prevalent human gastric pathogen that causes gastritis, ulcer disease, and gastric cancer. It is not yet fully understood how H. pylori injures the gastric epithelium. The Na,K-ATPase, an essential transporter found in virtually all mammalian cells, has been shown to be important for maintaining the barrier function of lung and kidney epithelia. H. pylori decreases levels of Na,K-ATPase in the plasma membrane of gastric epithelial cells, and the aim of this study was to demonstrate that this reduction led to gastric injury by impairing the epithelial barrier. Similar to H. pylori infection, the inhibition of Na,K-ATPase with ouabain decreased transepithelial electrical resistance and increased paracellular permeability in cell monolayers of human gastric cultured cells, 2D human gastric organoids, and gastric epithelium isolated from gerbils. Similar effects were caused by a partial shRNA silencing of Na,K-ATPase in human gastric organoids. Both H. pylori infection and ouabain exposure disrupted organization of adherens junctions in human gastric epithelia as demonstrated by E-cadherin immunofluorescence. Functional and structural impairment of epithelial integrity with a decrease in Na,K-ATPase amount or activity provides evidence that the H. pylori-induced downregulation of Na,K-ATPase plays a role in the complex mechanism of gastric disease induced by the bacteria.

Quantification of endogenous and exogenous protein expressions of Na,K-ATPase with super-resolution PALM/STORM imaging

Transient transfection of fluorescent fusion proteins is a key enabling technology in fluorescent microscopy to spatio-temporally map cellular protein distributions. Transient transfection of proteins may however bypass normal regulation of expression, leading to overexpression artefacts like misallocations and excess amounts. In this study we investigate the use of STORM and PALM microscopy to quantitatively monitor endogenous and exogenous protein expression. Through incorporation of an N-terminal hemagglutinin epitope to a mMaple3 fused Na,K-ATPase (α₁ isoform), we analyze the spatial and quantitative changes of plasma membrane Na,K-ATPase localization during competitive transient expression. Quantification of plasma membrane protein density revealed a time dependent increase of Na,K-ATPase, but no increase in size of protein clusters. Results show that after 41h transfection, the total plasma membrane density of Na,K-ATPase increased by 63% while the endogenous contribution was reduced by 16%.

Isolated polycystic liver disease genes define effectors of polycystin-1 function

Dominantly inherited isolated polycystic liver disease (PCLD) consists of liver cysts that are radiologically and pathologically identical to those seen in autosomal dominant polycystic kidney disease, but without clinically relevant kidney cysts. The causative genes are known for fewer than 40% of PCLD index cases. Here, we have used whole exome sequencing in a discovery cohort of 102 unrelated patients who were excluded for mutations in the 2 most common PCLD genes, PRKCSH and SEC63, to identify heterozygous loss-of-function mutations in 3 additional genes, ALG8, GANAB, and SEC61B. Similarly to PRKCSH and SEC63, these genes encode proteins that are integral to the protein biogenesis pathway in the endoplasmic reticulum. We inactivated these candidate genes in cell line models to show that loss of function of each results in defective maturation and trafficking of polycystin-1, the central determinant of cyst pathogenesis. Despite acting in a common pathway, each PCLD gene product demonstrated distinct effects on polycystin-1 biogenesis. We also found enrichment on a genome-wide basis of heterozygous mutations in the autosomal recessive polycystic kidney disease gene PKHD1, indicating that adult PKHD1 carriers can present with clinical PCLD. These findings define genetic and biochemical modulators of polycystin-1 function and provide a more complete definition of the spectrum of dominant human polycystic diseases.

NHA2 is expressed in distal nephron and regulated by dietary sodium

Increased renal reabsorption of sodium is a significant risk factor in hypertension. An established clinical marker for essential hypertension is elevated sodium lithium countertransport (SLC) activity. NHA2 is a newly identified Na⁺(Li⁺)/H⁺ antiporter with potential genetic links to hypertension, which has been shown to mediate SLC activity and H⁺-coupled Na⁺(Li⁺) efflux in kidney-derived MDCK cells. To evaluate a putative role in sodium homeostasis, we determined the effect of dietary salt on NHA2. In murine kidney sections, NHA2 localized apically to distal convoluted (both DCT1 and 2) and connecting tubules, partially overlapping in distribution with V-ATPase, AQP2, and NCC1 transporters. Mice fed a diet high in sodium chloride showed elevated transcripts and expression of NHA2 protein. We propose a model in which NHA2 plays a dual role in salt reabsorption or secretion, depending on the coupling ion (sodium or protons). The identified novel regulation of Na⁺/H⁺ antiporter in the kidney suggests new roles in salt homeostasis and disease.

Dual pulse-chase microscopy reveals early divergence in the biosynthetic trafficking of the Na,K-ATPase and E-cadherin

The trafficking of newly synthesized Na,K-ATPase and E-cadherin is observed in polarized epithelial cells. E-cadherin’s exit from the Golgi complex is not susceptible to 19°C temperature block. Furthermore, these proteins exit the Golgi and are delivered to the basolateral cell surface in separate vascular carriers.

Recent evidence indicates that newly synthesized membrane proteins that share the same distributions in the plasma membranes of polarized epithelial cells can pursue a variety of distinct trafficking routes as they travel from the Golgi complex to their common destination at the cell surface. In most polarized epithelial cells, both the Na,K-ATPase and E-cadherin are localized to the basolateral domains of the plasma membrane. To examine the itineraries pursued by newly synthesized Na,K-ATPase and E-cadherin in polarized MDCK epithelial cells, we used the SNAP and CLIP labeling systems to fluorescently tag temporally defined cohorts of these proteins and observe their behaviors simultaneously as they traverse the secretory pathway. These experiments reveal that E-cadherin is delivered to the cell surface substantially faster than is the Na,K-ATPase. Furthermore, the surface delivery of newly synthesized E-cadherin to the plasma membrane was not prevented by the 19°C temperature block that inhibits the trafficking of most proteins, including the Na,K-ATPase, out of the trans-Golgi network. Consistent with these distinct behaviors, populations of newly synthesized E-cadherin and Na,K-ATPase become separated from one another within the trans-Golgi network, suggesting that they are sorted into different carrier vesicles that mediate their post-Golgi trafficking.

Akt Substrate of 160 kD Regulates Na+,K+-ATPase Trafficking in Response to Energy Depletion and Renal Ischemia

Renal ischemia and reperfusion injury causes loss of renal epithelial cell polarity and perturbations in tubular solute and fluid transport. Na(+),K(+)-ATPase, which is normally found at the basolateral plasma membrane of renal epithelial cells, is internalized and accumulates in intracellular compartments after renal ischemic injury. We previously reported that the subcellular distribution of Na(+),K(+)-ATPase is modulated by direct binding to Akt substrate of 160 kD (AS160), a Rab GTPase-activating protein that regulates the trafficking of glucose transporter 4 in response to insulin and muscle contraction. Here, we investigated the effect of AS160 on Na(+),K(+)-ATPase trafficking in response to energy depletion. We found that AS160 is required for the intracellular accumulation of Na(+),K(+)-ATPase that occurs in response to energy depletion in cultured epithelial cells. Energy depletion led to dephosphorylation of AS160 at S588, which was required for the energy depletion-induced accumulation of Na,K-ATPase in intracellular compartments. In AS160-knockout mice, the effects of renal ischemia on the distribution of Na(+),K(+)-ATPase were substantially reduced in the epithelial cells of distal segments of the renal tubules. These data demonstrate that AS160 has a direct role in linking the trafficking of Na(+),K(+)-ATPase to the energy state of renal epithelial cells.

Activation of the Ca²+-sensing receptor induces deposition of tight junction components to the epithelial cell plasma membrane

The Ca(2+)-sensing receptor (CaSR) belongs to the G-protein-coupled receptor superfamily and plays essential roles in divalent ion homeostasis and cell differentiation. Because extracellular Ca(2+) is essential for the development of stable epithelial tight junctions (TJs), we hypothesized that the CaSR participates in regulating TJ assembly. We first assessed the expression of the CaSR in Madin-Darby canine kidney (MDCK) cells at steady state and following manipulations that modulate TJ assembly. Next, we examined the effects of CaSR agonists and antagonists on TJ assembly. Immunofluorescence studies indicate that endogenous CaSR is located at the basolateral pole of MDCK cells. Stable transfection of human CaSR in MDCK cells further reveals that this protein co-distributes with β-catenin on the basolateral membrane. Switching MDCK cells from low-Ca(2+) medium to medium containing a normal Ca(2+) concentration significantly increases CaSR expression at both the mRNA and protein levels. Exposure of MDCK cells maintained in low-Ca(2+) conditions to the CaSR agonists neomycin, Gd(3+) or R-568 causes the transient relocation of the tight junction components ZO-1 and occludin to sites of cell-cell contact, while inducing no significant changes in the expression of mRNAs encoding junction-associated proteins. Stimulation of CaSR also increases the interaction between ZO-1 and the F-actin-binding protein I-afadin. This effect does not involve activation of the AMP-activated protein kinase. By contrast, CaSR inhibition by NPS-2143 significantly decreases interaction of ZO-1 with I-afadin and reduces deposition of ZO-1 at the cell surface following a Ca(2+) switch from 5 µM to 200 µM [Ca(2+)]e. Pre-exposure of MDCK cells to the cell-permeant Ca(2+) chelator BAPTA-AM, similarly prevents TJ assembly caused by CaSR activation. Finally, stable transfection of MDCK cells with a cDNA encoding a human disease-associated gain-of-function mutant form of the CaSR increases the transepithelial electrical resistance of these cells in comparison to expression of the wild-type human CaSR. These observations suggest that the CaSR participates in regulating TJ assembly.

The Na-K-ATPase α₁β₁ heterodimer as a cell adhesion molecule in epithelia

The ion gradients generated by the Na-K-ATPase play a critical role in epithelia by driving transepithelial transport of various solutes. The efficiency of this Na-K-ATPase-driven vectorial transport depends on the integrity of epithelial junctions that maintain polar distribution of membrane transporters, including the basolateral sodium pump, and restrict paracellular diffusion of solutes. The review summarizes the data showing that, in addition to pumping ions, the Na-K-ATPase located at the sites of cell-cell junction acts as a cell adhesion molecule by interacting with the Na-K-ATPase of the adjacent cell in the intercellular space accompanied by anchoring to the cytoskeleton in the cytoplasm. The review also discusses the experimental evidence on the importance of a specific amino acid region in the extracellular domain of the Na-K-ATPase β(1) subunit for the Na-K-ATPase trans-dimerization and intercellular adhesion. Furthermore, a possible role of N-glycans linked to the Na-K-ATPase β(1) subunit in regulation of epithelial junctions by modulating β(1)-β(1) interactions is discussed.

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

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

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

N-glycan-dependent quality control of the Na,K-ATPase beta(2) subunit

Bulky hydrophilic N-glycans stabilize the proper tertiary structure of glycoproteins. In addition, N-glycans comprise the binding sites for the endoplasmic reticulum (ER)-resident lectins that assist correct folding of newly synthesized glycoproteins. To reveal the role of N-glycans in maturation of the Na,K-ATPase beta(2) subunit in the ER, the effects of preventing or modifying the beta(2) subunit N-glycosylation on trafficking of the subunit and its binding to the ER lectin chaperone, calnexin, were studied in MDCK cells. Preventing N-glycosylation abolishes binding of the beta(2) subunit to calnexin and results in the ER retention of the subunit. Furthermore, the fully N-glycosylated beta(2) subunit is retained in the ER when glycan-calnexin interactions are prevented by castanospermine, showing that N-glycan-mediated calnexin binding is required for correct subunit folding. Calnexin binding persists for several hours after translation is stopped with cycloheximide, suggesting that the beta(2) subunit undergoes repeated post-translational calnexin-assisted folding attempts. Homology modeling of the beta(2) subunit using the crystal structure of the alpha(1)-beta(1) Na,K-ATPase shows the presence of a relatively hydrophobic amino acid cluster proximal to N-glycosylation sites 2 and 7. Combined, but not separate, removal of sites 2 and 7 dramatically impairs calnexin binding and prevents the export of the beta(2) subunit from the ER. Similarly, hydrophilic substitution of two hydrophobic amino acids in this cluster disrupts both beta(2)-calnexin binding and trafficking of the subunit to the Golgi. Therefore, the hydrophobic residues in the proximity of N-glycans 2 and 7 are required for post-translational calnexin binding to these N-glycans in incompletely folded conformers, which, in turn, is necessary for maturation of the Na,K-ATPase beta(2) subunit.

Membrane proteins follow multiple pathways to the basolateral cell surface in polarized epithelial cells

Newly synthesized apical and basolateral membrane proteins are sorted from one another in polarized epithelial cells. The trans-Golgi network participates in this sorting process, but some basolateral proteins travel from the Golgi to recycling endosomes (REs) before their surface delivery. Using a novel system for pulse–chase microscopy, we have visualized the postsynthetic route pursued by a newly synthesized cohort of Na,K-ATPase. We find that the basolateral delivery of newly synthesized Na,K-ATPase occurs via a pathway distinct from that pursued by the vesicular stomatitis virus G protein (VSV-G). Na,K-ATPase surface delivery occurs at a faster rate than that observed for VSV-G. The Na,K-ATPase does not pass through the RE compartment en route to the plasma membrane, and Na,K-ATPase trafficking is not regulated by the same small GTPases as other basolateral proteins. Finally, Na,K-ATPase and VSV-G travel in separate post-Golgi transport intermediates, demonstrating directly that multiple routes exist for transport from the Golgi to the basolateral membrane in polarized epithelial cells.

beta-Subunit overexpression alters the stoicheometry of assembled Na-K-ATPase subunits in MDCK cells

In eukaryotic cells, the apparent maintenance of 1:1 stoicheometry between the Na-K-ATPase alpha- and beta-subunits led us to question whether this was alterable and thus if some form of regulation was involved. We have examined the consequences of overexpressing Na-K-ATPase beta1-subunits using Madin-Darby canine kidney (MDCK) cells expressing flag-tagged beta1-subunits (beta1flag) or Myc-tagged beta1-subunits (beta1myc) under the control of a tetracycline-dependent promoter. The induction of beta1flag subunit synthesis in MDCK cells, which increases beta1-subunit expression at the plasma membrane by more than twofold, while maintaining stable alpha1 expression levels, revealed that all mature beta1-subunits associate with alpha1-subunits, and no evidence of "free" beta1-subunits was obtained. Consequently, the ratio of assembled beta1- to alpha1-subunits is significantly increased when "extra" beta-subunits are expressed. An increased beta1/alpha1 stoicheometry is also observed in cells treated with tunicamycin, suggesting that the protein-protein interactions involved in these complexes are not dependent on glycosylation. Confocal images of cocultured beta1myc-expressing and beta1flag-expressing MDCK cells show colocalization of beta1myc and beta1flag subunits at the lateral membranes of neighboring cells, suggesting the occurrence of intercellular interactions between the beta-subunits. Immunoprecipitation using MDCK cells constitutively expressing beta1myc and tetracycline-regulated beta1flag subunits confirmed beta-beta-subunit interactions. These results demonstrate that the equimolar ratio of assembled beta1/alpha1-subunits of the Na-K-ATPase in kidney cells is not fixed by the inherent properties of the interacting subunits. It is likely that cellular mechanisms are present that regulate the individual Na-K-ATPase subunit abundance.

Molecular identity of the late sodium current in adult dog cardiomyocytes identified by Nav1.5 antisense inhibition

Late Na(+) current (I(NaL)) is a major component of the action potential plateau in human and canine myocardium. Since I(NaL) is increased in heart failure and ischemia, it represents a novel potential target for cardioprotection. However, the molecular identity of I(NaL) remains unclear. We tested the hypothesis that the cardiac Na(+) channel isoform (Na(v)1.5) is a major contributor to I(NaL) in adult dog ventricular cardiomyocytes (VCs). Cultured VCs were exposed to an antisense morpholino-based oligonucleotide (Na(v)1.5 asOligo) targeting the region around the start codon of Na(v)1.5 mRNA or a control nonsense oligonucleotide (nsOligo). Densities of both transient Na(+) current (I(NaT)) and I(NaL) (both in pA/pF) were monitored by whole cell patch clamp. In HEK293 cells expressing Na(v)1.5 or Na(v)1.2, Na(v)1.5 asOligo specifically silenced functional expression of Na(v)1.5 (up to 60% of the initial I(NaT)) but not Na(v)1.2. In both nsOligo-treated controls and untreated VCs, I(NaT) and I(NaL) remained unchanged for up to 5 days. However, both I(NaT) and I(NaL) decreased exponentially with similar time courses (tau = 46 and 56 h, respectively) after VCs were treated with Na(v)1.5 asOligo without changes in 1) decay kinetics, 2) steady-state activation and inactivation, and 3) the ratio of I(NaL) to I(NaT). Four days after exposure to Na(v)1.5 asOligo, I(NaT) and I(NaL) amounted to 68 +/- 6% (mean +/- SE; n = 20, P < 0.01) and 60 +/- 7% (n = 11, P < 0.018) of those in VCs treated by nsOligo, respectively. We conclude that in adult dog heart Na(v)1.5 sodium channels have a "functional half-life" of approximately 35 h (0.69tau) and make a major contribution to I(NaL).

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.

Target-determined expression of ?3 isoform of the Na+,K+-ATPase in the somatic nervous system of rat

Factors that determine the differential expression of isoforms of Na(+),K(+)-ATPase in the nervous system of vertebrates are not understood. To address this question we studied the expression of alpha(3) Na(+),K(+)-ATPase in the L5 dorsal root ganglia (DRG) of developing rat, the normal adult rat, and the adult rat after peripheral axotomy. During development, the first alpha(3) Na(+),K(+)-ATPase-positive DRG neurons appear by embryonic day 21. At birth, the L5 DRG have a full complement (14 +/- 2%) of these neurons. By 15 days after sciatic nerve transection in adult rat, the number of alpha(3) Na(+),K(+)-ATPase-positive DRG neurons and small myelinated L5 ventral root axons decreases to about 35% of control counts. These results combined with data from the literature suggest that the expression of alpha(3) Na(+),K(+)-ATPase by rat somatic neurons is determined by target-muscle spindle-derived factors.

Regulation of the Na,K-ATPase in MDCK cells by prostaglandin E1: a role for calcium as well as cAMP

Prostaglandins (PGs) play a significant role in the regulation of sodium reabsorption by the kidney, in addition to accumulating during inflammation as well as in several solid tumors. Previously, we presented evidence indicating that prostaglandin E(1) (PGE(1)), a supplement in the serum-free medium for MDCK cells, increases the activity of the Na,K-ATPase in MDCK cells, in addition to its growth stimulatory effect [J. Cell. Physiol. 151 (1992) 337]. This report defines the molecular mechanisms, and signaling pathways responsible for the increased Na,K-ATPase activity. Our results indicate that the increased activity of the Na,K-ATPase in MDCK monolayers treated with either PGE(1) or 8Bromocyclic AMP (8Br-cAMP) can be attributed to an increase in the rate of biosynthesis of the Na,K-ATPase, and an increase in the levels of Na,K-ATPase alpha and beta subunit mRNAs. As beta subunit mRNA increased to a larger extent than alpha subunit mRNA, transient transfection studies were conducted using a human beta1 promoter/luciferase construct [Nucleic Acids Res. 21 (1993) 2619]. While an 8Br-cAMP stimulation was observed (suggesting the involvement of cAMP), our results also suggest that the observed PGE(1) stimulation could be explained by the involvement of Ca(2+) as well protein kinase C (PKC). Consistent with the involvement of Ca(2+), TMB-8 (which inhibits Ca(2+) efflux from intracellular stores) inhibited the PGE(1) stimulation. Moreover, PGE(1) was observed to stimulate the translocation of PKC beta1 from the soluble to the particulate fraction. The translocation of PKC, the PGE(1) stimulation of transcription, and the PGE(1)-mediated increase in the beta subunit mRNA level were all inhibited by the PKC inhibitor Gö6989. These results can be explained by the involvement of two classes of cell surface receptors in mediating the PGE(1) stimulation, including the EP1subtype (which activates phospholipase C), as well as the EP2 subtype (which activates adenylate cyclase).

The highly conserved extracellular peptide, DSYG(893-896), is a critical structure for sodium pump function

The peptide sequence DSYG(893-896) of the sheep sodium pump alpha 1 subunit is highly conserved among all K(+)-transporting P-type ATPases. To obtain information about its function, single mutations were introduced and the mutants were expressed in yeast and analysed for enzymatic activity, ion recognition, and alpha/beta subunit interactions. Mutants of Ser894 or Tyr895 were all active. Conservative phenylalanine and tryptophan mutants of Tyr895 displayed properties that were similar to the properties of the wild-type enzyme. Replacement of the same amino acid by cysteine, however, produced heat-sensitive enzymes, indicating that the aromatic group contributes to the stability of the enzyme. Mutants of the neighbouring Ser894 recognized K(+) with altered apparent affinities. Thus, the Ser894-->Asp mutant displayed a threefold higher apparent affinity for K(+) (EC(50) = 1.4 +/- 0.06 mm) than the wild-type enzyme (EC(50) = 3.8 +/- 0.33 mm). In contrast, the mutant Ser894-->Ile had an almost sixfold lower apparent affinity for K(+) (EC(50) = 21.95 +/- 1.41 mm). Mutation of Asp893 or Gly896 produced inactive proteins. When an anti-beta 1 subunit immunoglobulin was used to co-immunoprecipitate the alpha 1 subunit, neither the Gly896-->Arg nor the Gly896-->Ile mutant could be visualized by subsequent probing with an anti-alpha 1 subunit immunoglobulin. On the other hand, co-immunoprecipitation was obtained with the inactive Asp893-->Arg and Asp893-->Glu mutants. Thus, it might be that Asp893 is involved in enzyme conformational transitions required for ATP hydrolysis and/or ion translocation. The results obtained here demonstrate the importance of the highly conserved peptide DSYG(893-896) for the function of alpha/beta heterodimeric P-type ATPases.

Na,K-ATPase beta1-subunit increases the translation efficiency of the alpha1-subunit in MSV-MDCK cells

The Na,K-ATPase consists of an alpha- and beta-subunit. Moloney sarcoma virus-transformed MDCK cells (MSV-MDCK) express low levels of Na,K-ATPase beta(1)-subunit. Ectopic expression of Na,K-ATPase beta(1)-subunit in these cells increased the protein levels of the alpha(1)-subunit of Na,K-ATPase. This increase was not due to altered transcription of the alpha(1)-subunit gene or half-life of the alpha(1)-subunit protein because both alpha(1)-subunit mRNA levels and half-life of the alpha(1)-subunit protein were comparable in MSV-MDCK and beta(1)-subunit expressing MSV-MDCK cells. However, short pulse labeling revealed that the initial translation rate of the alpha(1)-subunit in beta(1)-subunit expressing MSV-MDCK cells was six- to sevenfold higher compared with MSV-MDCK cells. The increased translation was specific to alpha(1)-subunit because translation rates of occludin and beta-catenin, membrane and cytosolic proteins, respectively, were not altered. In vitro cotranslation/translocation experiments using rabbit reticulocyte lysate and rough microsomes revealed that the alpha(1)-subunit mRNA is more efficiently translated in the presence of beta(1)-subunit. Furthermore, sucrose density gradient analysis revealed significantly more alpha(1)-subunit transcript associated with the polysomal fraction in beta(1)-subunit expressing MSV-MDCK cells compared with MSV-MDCK cells, indicating that in mammalian cells the Na,K-ATPase beta(1)-subunit is involved in facilitating the translation of the alpha(1)-subunit mRNA in the endoplasmic reticulum.

Mutational analysis of alpha-beta subunit interactions in the delivery of Na,K-ATPase heterodimers to the plasma membrane

The beta-subunit of the Na,K-ATPase is required to deliver functional alpha beta-heterodimers to the plasma membrane (PM) of baculovirus-infected insect cells. We have investigated the molecular determinants in the beta-subunit for the assembly and delivery processes. Trafficking of both subunits was analyzed by Western blots of fractionated membranes enriched in endoplasmic reticulum (ER), Golgi, and PM. Heterodimer assembly was evaluated by co-immunoprecipitation, and enzymatic activity was measured by ATPase assay. Elimination of enzymatic activity by D369A point mutation of the alpha-subunit had no effect on the compartmental distribution of the Na,K-ATPase, demonstrating that enzymatic functioning is not a prerequisite for PM delivery. Replacement of all three N-glycosylation site asparagines with glutamines produced no effect on the delivery to the PM or the activity of the enzyme, but increased susceptibility to degradation was observed. Analysis of beta-subunits in which the disulfide bonds were removed through substitution reveals that the bridges are important for PM targeting but not for assembly of the heterodimer. Assembly is supported by beta-subunits with greatly truncated extracellular domains. The presence of the amino-terminal domain and transmembrane segment is sufficient for assembly and PM delivery. Intermediate length truncated beta-subunits and some disulfide bridge substitution mutants assemble with the alpha-subunit but are not able to exit the ER. We conclude that there are different and separable requirements for the assembly of Na,K-ATPase heterodimer complexes, exit of the dimer from the ER, delivery to the PM, and catalytic activity of the dimer.

Stimulation of Na,K-ATPase by low potassium requires reactive oxygen species

The signaling pathway that transduces the stimulatory effect of low K+ on the biosynthesis of Na,K-ATPase remains largely unknown. The present study was undertaken to examine whether reactive oxygen species (ROS) mediated the effect of low K+ in Madin-Darby canine kidney (MDCK) cells. Low K+ increased ROS activity in a time- and dose-dependent manner, and this effect was abrogated by catalase and N-acetylcysteine (NAC). To determine the role of ROS in low-K+-induced gene expression, the cells were first stably transfected with expression constructs in which the reporter gene chloramphenicol acetyl transferase (CAT) was under the control of the avian Na,K-ATPase alpha-subunit 1.9 kb and 900-bp 5'-flanking regions that have a negative regulatory element. Low K+ increased the CAT expression in both constructs. Catalase or NAC inhibited the effect of low K+. To determine whether the increased CAT activity was mediated through releasing the repressive effect or a direct stimulation of the promoter, the cells were transfected with a CAT expression construct directed by a 96-bp promoter fragment that has no negative regulatory element. Low K+ also augmented the CAT activity expressed by this construct. More importantly, both catalase and NAC abolished the effect of low K+. Moreover, catalase and NAC also inhibited low-K+-induced increases in the Na,K-ATPase alpha1- and beta1-subunit protein abundance and ouabain binding sites. The antioxidants had no significant effect on the basal levels of CAT activity, protein abundance, or ouabain binding sites. In conclusion, low K+ enhances the Na,K-ATPase gene expression by a direct stimulation of the promoter activity, and ROS mediate this stimulation and also low-K+-induced increases in the Na,K-ATPase protein contents and cell surface molecules.

Na(+)/K(+) pump expression in the L8 rat myogenic cell line: effects of heterologous alpha subunit transfection

We have characterized the physiological and biochemical properties of the Na(+)/K(+) pump and its molecular expression in L8 rat muscle cells. Pump properties were measured by [(3)H]ouabain binding and (86)Rb uptake. Scatchard plot analysis of specific ouabain binding indicated the presence of a single family of binding sites with a B(max) of approximately 135 fmol/ mg P and a K(D) of 3.3 x 10(-8). (86)Rb uptake due to specific pump activity was found to be 20% of the total in L8 cells. The results indicated lower affinity of L8 cells for ouabain and lower activity of the pump than that reported for chick or rat skeletal muscle in primary culture. Both the alpha(1) and beta(1) protein and mRNA isoforms were expressed in myoblasts and in myotubes, while the alpha(2), alpha(3), and beta(2) isoforms were not detectable. We attempted to overcome low physiological expression of the Na(+)/K(+) pump by employing a vector expressing an avian high affinity alpha subunit. This allowed identification of the transfected subunit separate from that endogenously expressed in L8 cells. Successful transfection into L8 myoblasts and myotubes was recognized by anti-avian alpha subunit monoclonal antibodies. Fusion index, Na(+)/K(+) pump activity, and the level of the transmembrane resting potential were all significantly greater in transfected L8 (tL8) cells than in non-tL8. The total amount of alpha subunit (avian and rat) in tL8 cells was greater than that (only rat) in non-tL8 cells. This relatively high abundance of the Na(+)/K(+) pump in transfected cells may indicate that avian and rat alpha subunits hybridize to form functional pump complexes.

Immunochemical demonstration of a novel beta-subunit isoform of X, K-ATPase in human skeletal muscle

Recently we have identified mRNA encoding a hitherto unknown mammalian X,K-ATPase beta-subunit expressed predominantly in muscle tissue (Pestov, N. B. et al. (1999) FEBS Lett. 456, 243-248). Here we demonstrate the existence of the predicted protein, designated as beta(m) (beta(muscle)), in human adult skeletal muscle membranes using immunoblotting with beta(m)-specific antibodies generated against recombinant polypeptide formed by extramembrane beta(m) domains. The electrophoretic mobility of beta(m) was shown to be abnormally low due to the presence of Glu-rich sequences. In contrast to mature forms of other known X,K-ATPase beta-subunits, carbohydrate moiety of beta(m) is sensitive to endoglycosidase H and appears to be composed of short high-mannose or hybrid N-glycans. This finding argues in favor of an intracellular location of beta(m) in human skeletal muscle.

The roles of carbohydrate chains of the beta-subunit on the functional expression of gastric H(+),K(+)-ATPase

Gastric H(+),K(+)-ATPase consists of alpha and beta-subunits. The alpha-subunit is the catalytic subunit, and the beta-subunit is a glycoprotein stabilizing the alpha/beta complex in the membrane as a functional enzyme. There are seven putative N-glycosylation sites on the beta-subunit. In this study, we examined the roles of the carbohydrate chains of the beta-subunit by expressing the alpha-subunit together with the beta-subunit in which one, several, or all of the asparagine residues in the N-glycosylation sites were replaced by glutamine. Removing any one of seven carbohydrate chains from the beta-subunit retained the H(+),K(+)-ATPase activity. The effects of a series of progressive removals of carbohydrate chains on the H(+),K(+)-ATPase activity were cumulative, and removal of all carbohydrate chains resulted in the complete loss of H(+), K(+)-ATPase activity. Removal of any single carbohydrate chain did not affect the alpha/beta assembly; however, little alpha/beta assembly was observed after removal of all the carbohydrate chains from the beta-subunit. In contrast, removal of three carbohydrate chains inhibited the surface delivery of the beta-subunit and the alpha-subunit assembled with the beta-subunit, indicating that the surface delivery mechanism is more dependent on the carbohydrate chains than the expression of the H(+),K(+)-ATPase activity and alpha/beta assembly.

Na+/Ca2+ exchange in neonatal rat heart cells: antisense inhibition and protein half-life

Cardiac Na+/Ca2+ exchanger (NCX) protein half-life (t1/2) and antisense knockdown were studied in primary cultured neonatal rat cardiomyocytes. Protein t1/2 was determined using [35S]methionine with a pulse-chase protocol. The 35S signal in NCX was identified by immunoprecipitation and Western blotting. The t1/2 of NCX protein was 33 h. Low concentrations (0.5 microM) of chimeric, phosphorothioated antisense oligodeoxynucleotides (AS-oligos) targeted to the region around the start codon of NCX1 transcript were used to knock down NCX protein and activity. Control myocytes (no oligos or scrambled oligos for at least 4 days) exhibited spontaneous Ca2+ transients (measured with fura 2). The sustained ("diastolic") Ca2+ concentration in the cytosol ([Ca2+]cyt) of control cells was unaffected by cyclopiazonic acid (CPA) plus caffeine (Caf), which promote depletion of sarcoplasmic reticular Ca2+ stores, but [Ca2+]cyt rose in control cells when external Na+ was removed. In contrast, approximately 60% of cells treated with AS-oligos for at least 4 days did not exhibit spontaneous Ca2+ transients or respond to Na+-free medium; however, CPA + Caf did induce a prolonged elevation in [Ca2+]cyt in these cells. In all cells, 50 mM K+ increased [Ca2+]cyt. NCX protein was reduced by approximately 50% in cells treated with AS-oligos for 7 days but was not reduced after only 2 days. These biochemical data are consistent with the physiological evidence of NCX knockdown in approximately 60% of cells.

Insect Na(+)/K(+)-ATPase

Na(+)/K(+)-ATPase (sodium/potassium pump) is a P-type ion-motive ATPase found in the plasma membranes of animal cels. In vertebrates, the functions of this enzyme in nerves, heart and kidney are well characterized and characteristics a defined by different isoforms. In contrast, despite different tissue distributions, insects possess a single isoform of the alpha-subunit. A comparison of insect and vertebrate Na(+)/K(+)-ATPases reveals that although the mode of action and structure are very highly conserved, the specific roles of the enzyme in most tissues varies. However, the enzyme is essential for the function of nerve cells, and in this respect Na(+)/K(+)-ATPase appears to be fundamental in metazoan evolution.

Assembly of the chimeric Na+/K+-ATPase and H+/K+-ATPase beta-subunit with the Na+/K+-ATPase alpha-subunit

Two sets of chimeric beta-subunits were constructed from subunits of Torpedo californica Na+/K+-ATPase and pig gastric H+/K+-ATPase. Five unique restriction sites (SnaBI, EcoRV, MunI, SphI and EcoT22I) were created at equivalent positions of the respective cDNAs and were used as joining points for the construction. One set of chimeras (HxN series) was made by exchanging the 5' portion of the Na+/K+-ATPase beta-subunit cDNA with the corresponding portion of the H+/K+-ATPase beta-subunit cDNA at the respective joining point. Complementary constructs were also prepared (NxH series). In the HxN series, the chimera joined at the SnaBI site formed a stable trypsin resistant complex with the Na+/K+-ATPase alpha-subunit, which was functional with respect to ATP hydrolysis and pump current generation, although the activities were less than those of the complex with the Na+/K+-ATPase beta-subunit. Trypsin resistance decreased for the complex of the chimera joined at the EcoRV site. In the NxH series, the chimeras joined at the SnaBI site and the EcoRV site formed rather trypsin-resistant complexes, but the expressions of the alpha-subunits were below 50% of the control. The chimeras joined at the MunI, SphI and EcoT22I site formed complexes susceptible to tryptic digestion. None of the chimeras in the NxH series were functional. These results suggest that at least two regions of the Na+/K+-ATPase beta-subunit [SnaBI site(Tyr40) to EcoRV site(Ile89) and EcoT22I site(Cys176) to C-terminus)] are involved in stable assembly with the Na+/K+-ATPase alpha-subunit and that the cytoplasmic domain [N-terminus to SnaBI site(Tyr40)] is functionally replaceable with the corresponding domain of the H+/K+-ATPase beta-subunit.

N-Glycosylation is not a prerequisite for glutamate receptor function but Is essential for lectin modulation

All ionotropic glutamate receptor (iGluR) subunits analyzed so far are heavily N-glycosylated at multiple sites on their amino-terminal extracellular domains. Although the exact functional significance of this glycosylation remains to be determined, it has been suggested that N-glycosylation may be a precondition for the formation of functional ion channels. In particular, it has been argued that N-glycosylation is required for the formation of functional ligand binding sites. We analyzed heterologously expressed recombinant glutamate receptors (GluRs) of all three pharmacological subclasses of glutamate receptors, N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and kainate receptors. By expressing the GluR subunits in tunicamycin-treated, nonglycosylating Xenopus laevis oocytes, we determined that in neither case is N-glycosylation required for ion channel function, although for NMDA receptors, functional expression in the absence of N-glycosylation is very low. Furthermore, we analyzed and compared the interaction of the desensitization-inhibiting lectin concanavalin A (ConA) with all functional GluR subunits. We show that although ConA has its most pronounced effects on kainate receptors, it potentiates currents at most other receptor subtypes as well, including certain NMDA receptor subunits, although to a much lesser extent. One notable exception is the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor GluR2, which is not affected by ConA. Furthermore, we show that ConA acts directly via binding to the carbohydrate side chains of the receptor protein.

Biochemical characterization of the subunits of the Na+/K+ ATPase expressed in insect cells

The Na+/K+ ATPase is composed of two subunits called alpha and beta chains. In insect cells, independently expressed alpha and beta chains are localized to intracellular membranes. Sucrose density gradient sedimentation, crosslinking analysis, and immunoprecipitation of radio-labeled proteins show that the alpha chains expressed alone are in large aggregates of different molecular weights with less than 4% being monomeric. Analysis by non-reducing SDS-PAGE and immunoblotting show that the beta chains expressed alone are in Triton X-100 insoluble, disulfide-linked aggregates. Co-expression of both subunits in insect cells results in only a small fraction (less than 15%) of the alpha chains being assembled as the active recombinant enzyme, with at least 22% of the active recombinant enzyme localized to the plasma membrane as determined by a biochemical assay. The small amount of beta chain at the plasma membrane in cells that express both subunits is beyond the limit of detection by the biochemical assay. Immunoprecipitation of Triton X-100 soluble alpha chains from radio-labeled cells expressing both subunits shows that the alpha chains are mostly in large aggregates containing beta chains. These results suggest that, in insect cells, the availability of correctly folded beta chains is the rate limiting step in the assembly of active Na+/K+ ATPase.

Dexamethasone upregulates the Na-K-ATPase in rat alveolar epithelial cells

Previous studies in kidney, heart, and liver cells have demonstrated that dexamethasone regulates the expression of Na-K-ATPase. In the lungs, Na-K-ATPase has been reported in alveolar epithelial type II (ATII) cells and is thought to participate in active Na+ transport and lung edema clearance. The aim of this study was to determine whether Na-K-ATPase would be regulated by dexamethasone in cultured rat ATII cells. Regulation of the Na-K-ATPase by dexamethasone could lead to a greater understanding of its role in active Na+ transport and lung edema clearance. Rat ATII cells were isolated, plated for 24 h, and exposed to 10(-7) and 10(-8) M dexamethasone. These cells were harvested at 0, 3, 6, 12, and 24 h after dexamethasone exposure for determination of steady-state Na-K-ATPase mRNA transcript levels, protein expression, and function. The steady-state Na-K-ATPase beta1-mRNA transcript levels increased in ATII cells 6, 12, and 24 h after dexamethasone exposure (P < 0.05). However, the steady-state alpha1-mRNA transcript levels were unchanged. The protein expression for the alpha1- and beta1-subunits increased in ATII cells exposed to dexamethasone compared with controls in association with a temporal increase in Na-K-ATPase function after dexamethasone exposure. These results suggest that dexamethasone regulates Na-K-ATPase in ATII cells possibly by transcriptional, translational, and posttranslational mechanisms.

1,25-Dihydroxyvitamin D3 selectively induces increased expression of the Na,K-ATPase beta 1 subunit in avian myelomonocytic cells without a concomitant change in Na,K-ATPase activity

Treatment of avian myelomonocytic cells with 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) results in an approximately two fold increase in levels of Na,K-ATPase beta 1 subunit mRNA and protein (both total and plasma membrane-associated). The changes in beta 1 subunit expression occur in the absence of a detectable increase in expression of any of the three alpha subunit isoforms or in Na,K-ATPase activity. The selective induction of the expression of the beta subunit in avian myelomonocytic cells by 1,25(OH)2D3 reveals a previously unobserved feature of the regulation of Na,K-ATPase expression, while the targeting of beta subunit polypeptides to the plasma membrane in the absence of a corresponding increase in active Na,K-ATPase suggests that, in these cells, transport of the beta subunit to the plasma membrane may be independent of its binding to the alpha subunit.

Subunit interactions in the Na,K-ATPase explored with the yeast two-hybrid system

Subunit interactions of the alpha1- and beta1-subunits of the chicken Na,K-ATPase were explored with the yeast two-hybrid system. Gal4-fusion proteins containing domains of the alpha1- and beta1-subunits were designed for examining both intersubunit and intrasubunit protein-protein interactions. Regions of the alpha- and beta-subunits known to be involved in alpha-beta-subunit assembly were positive in two-hybrid assay, supporting the validity of the assays. A library of beta-subunit ectodomains with C-terminal truncations was screened to find the maximal truncation retaining an interaction with the alpha-subunit extracellular H7H8 loop (where H7 refers to the seventh membrane span, and so on). The maximal truncation removed all the cysteines involved in disulfide bridges, leaving only 63 amino acids of the beta-subunit ectodomain. Scanning alanine mutagenesis led to identification of an evolutionarily conserved sequence of four amino acids (SYGQ) in the extracellular H7H8 loop of the alpha-subunit that is crucial to alpha-beta-intersubunit interactions. Oligomerization studies with single domains failed to detect self-association of either of the two large cytosolic loops (H2H3 and H4H5) within the alpha-subunit. However, evidence was found for an interaction between these two cytoplasmic loops.

Role of glycosylation and disulfide bond formation in the beta subunit in the folding and functional expression of Na,K-ATPase

Initial folding is a prerequisite for subunit assembly in oligomeric proteins. In this study, we have compared the role of co-translational modifications in the acquisition of an assembly-competent conformation of the beta subunit, the assembly of which is required for the structural and functional maturation of the catalytic Na,K-ATPase alpha subunit. Cysteine or asparagine residues implicated in disulfide bond formation or N-glycosylation, respectively, in the Xenopus beta1 subunit were eliminated by site-directed mutagenesis, and the assembly efficiency of the mutants and the functional expression of Na+,K+ pumps were studied after expression in Xenopus oocytes. Our results show that lack of each one of the two most C-terminal disulfide bonds indeed permits short term but completely abolishes long term assembly of the beta subunit. On the other hand, lack of the most N-terminal disulfide bonds allows the expression of a small number of functional Na+,K+ pumps at the cell surface. Elimination of all three but not of one or two glycosylation sites produces beta subunits that remain stably expressed in the endoplasmic reticulum, in association with binding protein but not as irreversible aggregates. The assembly efficiency of nonglycosylated beta subunits is decreased but a reduced number of functional Na+,K+ pumps is expressed at the cell surface. The lack of sugars does not influence the apparent K+ or ouabain affinity of the Na+,K+ pumps. Thus, these data show that disulfide bond formation and N-glycosylation may play important but qualitatively distinct roles in the initial folding of oligomeric protein subunits. Moreover, the results suggest that an endoplasmic reticulum degradation pathway exists, which is glycosylation-dependent.

Na-K-ATPase in lacrimal gland acinar cell endosomal system: correcting a case of mistaken identity

Na-K-ATPase is associated with a variety of membrane populations in lacrimal acinar cells. Acinus-like structures formed by rabbit acinar cells in primary culture were incubated with horseradish peroxidase (HRP) to label basolateral and endosomal membranes and then analyzed by electron microscopy cytochemistry with the 3-3'-diaminobenzidine reaction or by fractionation and measurement of marker catalytic activities or immunoreactivities. HRP adsorbed to basolateral membranes at 4 degrees C. Fractionation showed it associated with low-density membranes enriched in acid phosphatase and TGN38 but containing only minor amounts of Na-K-ATPase. Cells internalized HRP to cytoplasmic vesicles, Golgi structures, and lysosomes at 37 degrees C. The major endosomal compartment revealed by fractionation coincided with major peaks of Na-K-ATPase and Rab6 and secondary peaks of galactosyltransferase and gamma-adaptin. Carbachol (10 microM) increased lysosomal and Golgi labeling. Thus most of the Na-K-ATPase is located in the basolateral membrane-oriented endosomal system, concentrated in a compartment possibly related to the trans-Golgi network. Constitutive and stimulation-accelerated traffic to and from this compartment may serve several exocrine cell functions.

The AMOG/beta 2 subunit of Na,K-ATPase is not necessary for long-term survival of telencephalic grafts

Adhesion molecule on glia (AMOG) represents the beta 2-subunit of murine Na,K-ATPase. Mice carrying a targeted deletion of the AMOG/beta 2 gene exhibit tremor and limb paralysis at postnatal day (P) 15 and die 2 days after the onset of symptoms. The brains of these mice show edema and swelling of astrocytic end feet. However, the cause of death has remained unclear. To identify long-term consequences of AMOG/beta 2 deficiency, we have grafted parts of the embryonic telencephalic anlage of AMOG/beta 2-deficient mice into the caudoputamen of wild-type mice and analyzed the grafts up to 500 days after transplantation. Histological, immunocytochemical, and in situ hybridization techniques were applied to examine histoarchitecture, proliferation, differentiation, and long-term survival of grafts. AMOG/beta 2-deficient telencephalic grafts develop normally and form solid neural tissue that cannot be distinguished from control grafts by morphological features or with immunocytochemical stains for neuronal and glial markers. No signs of degeneration can be found. Expression analysis, however, revealed that no AMOG/beta 2 protein of possible host origin can be detected in AMOG/beta 2-deficient grafts. Graft-borne astrocytes express neither the AMOG/beta 1 nor the AMOG/beta 2 subunit of Na,K-ATPase as examined with immunocytochemistry and in situ hybridization. These findings indicate that AMOG/beta 2 is not necessary for long-term survival of telencephalic graft tissue.

Coupled glucose transport and metabolism in cultured neuronal cells: determination of the rate-limiting step

In brain and nerves the phosphorylation of glucose, rather than its transport, is generally considered the major rate-limiting step in metabolism. Since little is known regarding the kinetic coupling between these processes in neuronal tissues, we investigated the transport and phosphorylation of [2-3H]glucose in two neuronal cell models: a stable neuroblastoma cell line (NCB20), and a primary culture of isolated rat dorsal root ganglia cells. When transport and phosphorylation were measured in series, phosphorylation was the limiting step, because intracellular glucose concentrations were the same as those outside of cells, and because the apparent Km for glucose utilization was lower than expected for the transport step. However, the apparent Km was still severalfold higher than the Km of hexokinase I. When [2-3H]glucose efflux and phosphorylation were measured from the same intracellular glucose pool in a parallel assay, rates of glucose efflux were three- to-fivefold greater than rates of phosphorylation. With the parallel assay, we observed that activation of glucose utilization by the sodium channel blocker veratridine caused a selective increase in glucose phosphorylation and was without effect on glucose transport. In contrast to results with glucose, both cell types accumulated 2-deoxy-D-[14C]glucose to concentrations severalfold greater than extracellular concentrations. We conclude from these studies that glucose utilization in neuronal cells is phosphorylation-limited, and that the coupling between transport and phosphorylation depends on the type of hexose used.

Modulation of an inactivating human cardiac K+ channel by protein kinase C

The transient outward current (ITO) is an important repolarizing component of the cardiac action potential. In native cardiac myocytes, ITO is modulated after activation of protein kinase C, although the molecular nature of this effect is not well understood. A channel recently cloned from human ventricular myocardium (Kv1.4, HK1) produces a rapidly inactivating K+ current, which has phenotypic similarities to the 4-aminopyridine-sensitive component of ITO. Therefore, we examined whether this recombinant channel was also modulated by protein kinase C activation by investigating the effects of the diacylglycerol analogue phorbol 12-myristate 13-acetate (PMA) on Kv1.4 K+ current expressed in Xenopus oocytes. At a concentration of 10 nmol/L, PMA caused a biphasic response with an initial increase (14 +/- 4%, mean +/- SEM) in current, which peaked in 14 minutes. This was followed by a significant reduction (40 +/- 11%) in the current within 30 minutes. There was no significant change in cell membrane electrical capacitance with 10 nmol/L PMA (1 +/- 1% decline in 30 minutes), demonstrating that loss of cell membrane surface area did not explain the reduction in K+ current, although cell capacitance did decrease when using a higher concentration of PMA (81 nmol/L). The inactive stereoisomer, 4 alpha-PMA, had no effect on Kv1.4 current, whereas preincubation with the protein kinase inhibitor staurosporine or protein kinase C-selective chelerythrine prevented the effects of PMA. When purified from a stably transfected mammalian cell line by using immunoprecipitation, the channel protein was readily phosphorylated in vitro by purified protein kinase C.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of antigenic sites on the Na+/K(+)-ATPase beta-subunit: their sequences and the effects of thiol reduction upon their structure

In contrast to the catalytic (alpha) subunit of the Na+/K(+)-ATPase holoenzyme, the glycoprotein (beta) subunit has proven to be a poor antigen for monoclonal antibody (Mab) production. However, in this work six Mabs directed against the beta-subunit of the lamb kidney holoenzyme have been isolated. These Mabs all recognize the holoenzyme, but their 'in solution' binding affinities for deglycosylated enzyme or isolated beta are generally at least 10-fold higher. Species specificity mapping, antibody patterns of binding to beta-fragments and competition binding studies indicated that there were only three distinct epitopes, with two antibodies binding in the NH2-terminal half (epitopes I and II) and 4 Mabs binding at the same or overlapping site (III) in the -COOH terminal half of beta. DNA sequence analysis of isolated collections of bacteriophage M13 that contain a 15 amino-acid 'epitope library' insert in the pIII protein, which enables them to bind to the antibodies, revealed the residues KYRDS (amino acids 111-115) and LETYP (amino acids 197-201) to be the deduced sequences for the epitopes of Mabs M19-P7-E5 (II) and M17-P5-F11 (III), respectively. The epitope I site was not, however, identified. Further studies showed that antibody binding to these three determinant sites had no affect on the Na+/K(+)-ATPase and K(+)-stimulated p-nitrophenylphosphatase (pNPPase) activities of either holoenzyme or deglycosylated enzyme, nor any affect on the cation- (Na+, K+ or Mg2+) and ouabain-induced conformational changes monitored with FITC-labeled deglycosylated enzyme. Interestingly, anti-beta Mab access to the three epitopes was increased following beta-mercaptoethanol inactivation of the holoenzyme, but this thiol reduction abolished the binding of two conformation-sensitive anti-alpha Mabs to the enzyme. These results are consistent with the previous suggestion of Kirley ((1990) J. Biol. Chem. 265, 4227-4232) that the beta-disulfide linkages not only maintain beta-structure but they are critical for maintaining alpha-conformation and holoenzyme activity.

Primary structure of avian H+/K(+)-ATPase beta-subunit

A cDNA encoding a beta-subunit of the avian H+/K(+)-ATPase was cloned from a chicken stomach cDNA library, and its nucleotide sequence determined. A comparison between all the available sequence data for the beta-subunits of P-type ATPases reveals several evolutionarily conserved regions. Overall identity was 66% when compared with mammalian H+/K(+)-ATPase beta-subunits, 34% identity when compared with the Na+/K(+)-ATPase beta 2-subunits, and 33% identity when compared with the Na+/K(+)-ATPase beta 1-subunits.

Localization of Na+/K(+)-ATPase in the bovine corneal endothelium

A mouse monoclonal antibody has been used to localize Na+/K(+)-ATPase in the bovine corneal endothelium. The specificity of the antibody was demonstrated by reaction with a single protein of molecular mass 100 kDa on Western blots and immunoprecipitation of a complex consisting of 100 kDa and 50 kDa subunits. Treatment of the immunoprecipitated antigen with Peptide N-Glycanase F produced no change in the molecular mass of the 100 kDa protein, but resulted in a progressive decrease in the molecular mass of the 50 kDa subunit, to yield a core protein of molecular mass about 33 kDa. The pattern of deglycosylation suggested the presence of three N-linked glycans attached to the 33 kDa protein core. These results were consistent with the antibody being specific for the alpha subunit of the Na+/K(+)-ATPase. Immunocytochemical studies at the light and electron microscopic level demonstrated antibody binding to both the basal and lateral membranes of bovine corneal endothelial cells. This suggested a baso-lateral distribution of Na+/K(+)-ATPase in these cells, rather than the previously proposed lateral membrane-only distribution.

Subcellular distribution of tissue kallikrein and Na,K-ATPase alpha-subunit in rat parotid striated duct cells

Intracellular protein distribution and sorting were examined in rat parotid striated duct cells, in which tissue kallikrein is apical, and Na,K-ATPase is basolateral. Electron-microscopic immunogold cytochemistry, with both polyclonal and monoclonal antibodies, demonstrated these enzymes at opposite poles of the cells and in distinct intracellular sites. Kallikrein was found within apical secretory granules, whereas Na,K-ATPase was present on basolateral cell membranes. In addition, kallikrein was localized throughout cisternae of all Golgi profiles, whereas Na,K-ATPase (alpha-subunit) was found only in small peripheral vesicles and/or lateral cisternal extensions of a basal subset of Golgi profiles. These differences in the subcellular distribution of the two marker antigens were most clearly seen with double immunogold labelling. Our results suggest that kallikrein, an apical, regulated secretory protein, and Na,K-ATPase, a basolateral, constitutively transported membrane protein, are segregated at (or prior to) the level of the Golgi apparatus rather than in the trans-Golgi network (TGN), as was expected.

Glutamate up-regulates alpha 1 and alpha 2 subunits of the sodium pump in astrocytes of mixed telencephalic cultures but not in pure astrocyte cultures

Prior work employing an in vitro model of the cerebral cortex has shown that sodium pump activity is a critical determinant for neuronal survival of glutamate stimulation. We have hypothesized that up-regulation of total brain sodium pump activity will protect against potential excitotoxins. Increased sodium pump activity could theoretically occur by changes in the reaction rate (short-term) and/or by increased levels of sodium pump protein (long-term) and is potentially complex since the three catalytic (a) subunit isoforms of the sodium pump are distributed in a highly variable, cell-specific pattern in the brain. Short-term regulation (seconds to minutes) has been well studied: brain sodium pump exhibits a large dynamic range. In contrast, the possibility of long-term modulation of sodium pump activity has not been extensively explored. We used isoform specific antibodies and [3H]ouabain binding to determine whether prolonged stimulation of sodium pump activity in rodent telencephalic cultures increased total sodium pump enzyme. Exposure of mixed neuronal-glial cultures to high levels of glutamate (10 mM) for 18 h, which is highly toxic to neurons, was associated with an approximately 80% increase in alpha 1 and alpha 2 subunit expression by glia. Induction of alpha 2 subunit immunoreactivity was also associated with comparable changes in [3H]ouabain binding, suggesting that the up-regulation corresponded to functional alpha 2 protein. Shorter (30 min) glutamate treatments, which also killed neurons, did not produce similar changes in sodium pump expression. In contrast to mixed cultures, pure astrocyte cultures had undetectable alpha 2 and alpha 3 and moderate levels of alpha 1 protein, as confirmed by low levels of [3H]ouabain binding. Glutamate treatment using this protocol was associated with a decrease in alpha 1 sodium pump expression. We conclude that long-term regulation of the sodium pump can be demonstrated in glia which have developed in the presence of neurons. Both alpha 1 and alpha 2 isoforms of the sodium pump are involved in this response to glutamate.