ABSTRACT
Introducción: La grasa de las articulaciones sinoviales puede servir para el mantenimiento de la estructura articular. Nuestro objetivo es analizar la evolución de la degeneración articular en rodillas con y sin paquete adiposo. Material y metodología: En 6 ovejas se efectuó la sección del ligamento cruzado anterior en ambas rodillas, para provocar una artrosis. En un grupo se preservó el paquete adiposo y en otro grupo se extirpó completamente. Realizamos un estudio histológico y de biología molecular analizando la expresión, en la membrana sinovial, el hueso subcondral, cartílago, grasa, menisco y líquido sinovial, de RUNX2, PTHrP, catepsina-K y MCP1. Resultados: No encontramos diferencias morfológicas. Encontramos aumento de la expresión de RUNX2 en membrana sinovial, PTHrP y Catepsina K en líquido sinovial en el grupo sin grasa y aumento de la expresión RUNX2 en el menisco y MCP1 en líquido sinovial en el grupo con grasa. Conclusión: La grasa infrapatelar participa en el proceso inflamatorio que acompaña en la artrosis, pues la resección de la grasa de Hoffa altera los marcadores proinflamatorios, mientras que el modelo con la grasa intacta incrementa el marcador proinflamatorio MCP1 en líquido sinovial.(AU)
Introduction: The fat of the synovial joints can be used to maintain the joint structure. Our objective is to analyze the evolution of joint degeneration in knees with and without adipose pack. Material and methodology: In six sheep, the anterior cruciate ligament was sectioned in both knees, to cause osteoarthritis. In one group the fat pack was preserved and in another group it was completely removed. We performed a histological and molecular biology study analyzing the expression, in the synovial membrane, subchondral bone, cartilage, fat, meniscus, and synovial fluid, of RUNX2, PTHrP, cathepsin-K, and MCP1. Results: We did not find morphological differences. We found increased expression of RUNX2 in synovial membrane, PTHrP and Cathepsin K in synovial fluid in the group without fat, and increased expression of RUNX2 in the meniscus and MCP1 in synovial fluid in the group with fat. Conclusion: Infrapatellar fat participates in the inflammatory process that accompanies osteoarthritis, since Hoffa fat pad resection alters pro-inflammatory markers, while the model with intact fat increases the pro-inflammatory marker MCP1 in synovial fluid.(AU)
Subject(s)
Animals , Stifle/injuries , Synovial Fluid , Cartilage , Osteoarthritis , SheepABSTRACT
Introducción: La grasa de las articulaciones sinoviales puede servir para el mantenimiento de la estructura articular. Nuestro objetivo es analizar la evolución de la degeneración articular en rodillas con y sin paquete adiposo. Material y metodología: En 6 ovejas se efectuó la sección del ligamento cruzado anterior en ambas rodillas, para provocar una artrosis. En un grupo se preservó el paquete adiposo y en otro grupo se extirpó completamente. Realizamos un estudio histológico y de biología molecular analizando la expresión, en la membrana sinovial, el hueso subcondral, cartílago, grasa, menisco y líquido sinovial, de RUNX2, PTHrP, catepsina-K y MCP1. Resultados: No encontramos diferencias morfológicas. Encontramos aumento de la expresión de RUNX2 en membrana sinovial, PTHrP y Catepsina K en líquido sinovial en el grupo sin grasa y aumento de la expresión RUNX2 en el menisco y MCP1 en líquido sinovial en el grupo con grasa. Conclusión: La grasa infrapatelar participa en el proceso inflamatorio que acompaña en la artrosis, pues la resección de la grasa de Hoffa altera los marcadores proinflamatorios, mientras que el modelo con la grasa intacta incrementa el marcador proinflamatorio MCP1 en líquido sinovial.(AU)
Introduction: The fat of the synovial joints can be used to maintain the joint structure. Our objective is to analyze the evolution of joint degeneration in knees with and without adipose pack. Material and methodology: In six sheep, the anterior cruciate ligament was sectioned in both knees, to cause osteoarthritis. In one group the fat pack was preserved and in another group it was completely removed. We performed a histological and molecular biology study analyzing the expression, in the synovial membrane, subchondral bone, cartilage, fat, meniscus, and synovial fluid, of RUNX2, PTHrP, cathepsin-K, and MCP1. Results: We did not find morphological differences. We found increased expression of RUNX2 in synovial membrane, PTHrP and Cathepsin K in synovial fluid in the group without fat, and increased expression of RUNX2 in the meniscus and MCP1 in synovial fluid in the group with fat. Conclusion: Infrapatellar fat participates in the inflammatory process that accompanies osteoarthritis, since Hoffa fat pad resection alters pro-inflammatory markers, while the model with intact fat increases the pro-inflammatory marker MCP1 in synovial fluid.(AU)
Subject(s)
Animals , Stifle/injuries , Synovial Fluid , Cartilage , Osteoarthritis , SheepABSTRACT
INTRODUCTION: The fat of the synovial joints can be used to maintain the joint structure. Our objective is to analyse the evolution of joint degeneration in knees with and without adipose pack. MATERIAL AND METHODOLOGY: In six sheep, the anterior cruciate ligament was sectioned in both knees, to cause osteoarthritis. In one group the fat pack was preserved and in another group it was completely removed. We performed a histological and molecular biology study analyzing the expression, in the synovial membrane, subchondral bone, cartilage, fat, meniscus, and synovial fluid, of RUNX2, PTHrP, cathepsin-K, and MCP1. RESULTS: We did not find morphological differences. We found increased expression of RUNX2 in synovial membrane, PTHrP and Cathepsin K in synovial fluid in the group without fat, and increased expression of RUNX2 in the meniscus and MCP1 in synovial fluid in the group with fat. CONCLUSION: Infrapatellar fat participates in the inflammatory process that accompanies osteoarthritis, since Hoffa fat pad resection alters pro-inflammatory markers, while the model with intact fat increases the pro-inflammatory marker MCP1 in synovial fluid.
ABSTRACT
INTRODUCTION: The fat of the synovial joints can be used to maintain the joint structure. Our objective is to analyze the evolution of joint degeneration in knees with and without adipose pack. MATERIAL AND METHODOLOGY: In six sheep, the anterior cruciate ligament was sectioned in both knees, to cause osteoarthritis. In one group the fat pack was preserved and in another group it was completely removed. We performed a histological and molecular biology study analyzing the expression, in the synovial membrane, subchondral bone, cartilage, fat, meniscus, and synovial fluid, of RUNX2, PTHrP, cathepsin-K, and MCP1. RESULTS: We did not find morphological differences. We found increased expression of RUNX2 in synovial membrane, PTHrP and Cathepsin K in synovial fluid in the group without fat, and increased expression of RUNX2 in the meniscus and MCP1 in synovial fluid in the group with fat. CONCLUSION: Infrapatellar fat participates in the inflammatory process that accompanies osteoarthritis, since Hoffa fat pad resection alters pro-inflammatory markers, while the model with intact fat increases the pro-inflammatory marker MCP1 in synovial fluid.
ABSTRACT
OBJECTIVE: To analyze the level of pro-inflammatory cytokines in osteoarthritis knee joint fat pad in relation to the subcutaneous fat of the thigh. MATERIAL AND METHODS: We performed a study of fat of the knee joint adipose affected of osteoarthritis and subcutaneous fat of the thigh of the same side to the greater distance of the joint in six patients with severe gonarthrosis, with a mean age of 68 years (range: 55-81 years). From the fat samples the progenitor mesenchymal cells were obtained. The supernatants of mesenchymal cells obtained to analyze inflammatory factors (IL-1b, IL6, IL9, IL1ra, IL12, IL13, IL15) and angiogenic (VEGF, PDGF bb) and immunomodulatory cytokines (IP-10 and INF-) means of two samples. RESULTS: Quantitative analysis revealed a significant (p 0.05) decrease in IL-1b, IL6, IL8, IL9, IL1ra, IL12, IL13 and increase of IL15 in Hoffa fat pad versus subcutaneous adipose tissue. Likewise, the analysis of angiogenic factors such as VEGF and PDGF, as well as factors IP-10 and INF- presented a significant decrease (p 0.05) in Hoffa fat pad versus subcutaneous adipose tissue. DISCUSSION: Mesenchymal cells from the adipose tissue of the severe osteoarthritic knee show a significant decrease in inflammatory cytokines even in the chronic state and a significant decrease in angiogenic factors and immunomodulatory cytokines (IP10 and INF).
OBJETIVO: Analizar el nivel de citoquinas proinflamatorias en la grasa articular del paquete adiposo en pacientes con gonartrosis, en relación con la grasa subcutánea del muslo. MATERIAL Y MÉTODOS: Efectuamos un estudio de grasa del paquete adiposo articular de la rodilla afectada de artrosis y de la grasa subcutánea del muslo del mismo lado, a la mayor distancia de la articulación en seis pacientes con gonartrosis grave, con una edad media de 68 años (rango: 55-81 años). De las muestras de grasa se obtuvieron las células mesenquimales progenitoras. Los sobrenadantes de células mesenquimales obtenidas se utilizaron para analizar factores inflamatorios (IL-1b, IL6, IL8, IL9, IL1ra, IL12, IL13, IL15) y angiogénicos (VEGF, PDGF bb), así como citoquinas inmunomoduladoras (IP-10 e INF-) y se compararon las medias de dos muestras. RESULTADOS: El análisis cuantitativo reveló una disminución significativa (p 0.05) de IL-1b, IL6, IL8, IL9, IL1ra, IL12, IL13 y un aumento de IL15 en la grasa de Hoffa frente al tejido adiposo subcutáneo. Del mismo modo, el análisis de factores angiógenicos como VEGF y PDGF bb, al igual que los factores IP-10 e INF- presentaron una disminución significativa en la grasa de Hoffa (p 0.05) frente al tejido adiposo subcutáneo. DISCUSIÓN: Las células mesenquimales del paquete adiposo articular de la rodilla artrósica grave muestran una disminución significativa de citoquinas inflamatorias, aun en el estado crónico, y una disminución significativa de factores angiogénicos y citoquinas inmunomoduladoras (IP10 e INF).
Subject(s)
Cytokines , Osteoarthritis, Knee , Subcutaneous Fat , Adipose Tissue , Aged , Aged, 80 and over , Cytokines/metabolism , Humans , Knee Joint , Middle Aged , Osteoarthritis, Knee/metabolism , Subcutaneous Fat/metabolism , ThighABSTRACT
There is empirical evidence that having some personality characteristics increases the risk of developing depression. This is the first study which analyses the role of personality dimensions, assessed by the Alternative Five Factor Model, in the development of depressive symptoms in adult burn survivors across time. Participants were 109 adult burn survivors admitted to a Burns Unit. Personality was assessed by the Zuckerman-Kuhlman Personality Questionnaire and depression symptoms by the Beck Depression Inventory. After adjusting by age, gender and burn size, results showed that high Neuroticism-Anxiety (N-Anx) and Aggression-Hostility (Agg-Host) were related to higher depression scores when compared with low N-Anx and Agg-Host groups along the six months follow-up. Moreover, Activity and Impulsive-Sensation Seeking factors were involved in statistically significant different depressive symptom development trajectories during the six months after burn. These findings suggest that personality factors could be used to identify the most vulnerable patients, who could develop severe mood symptoms at different points in their recovery.
Subject(s)
Burns/psychology , Depression/psychology , Personality , Adult , Aggression/psychology , Anxiety/psychology , Anxiety Disorders/psychology , Female , Follow-Up Studies , Hostility , Humans , Impulsive Behavior , Male , Middle Aged , Neuroticism , Personality Inventory , Severity of Illness IndexABSTRACT
P1B-type ATPases transport a variety of metals (Cd2+, Zn2+, Pb2+, Co2+, Cu2+, Ag+, Cu+) across biomembranes. Characteristic sequences CP[C/H/S] in transmembrane fragment H6 were observed in the putative transporting metal site of the founding members of this subfamily (initially named CPx-ATPases). In spite of their importance for metal homeostasis and biotolerance, their mechanisms of ion selectivity are not understood. Studies of better-characterized P(II)-type ATPases (Ca-ATPase and Na,K-ATPase) have identified three transmembrane segments that participate in ion binding and transport. Testing the hypothesis that metal specificity is determined by conserved amino acids located in the equivalent transmembrane segments of P1B-type ATPases (H6, H7, and H8), 234 P1B-ATPase protein sequences were analyzed. This showed that although H6 contains characteristic CPX or XPC sequences, conserved amino acids in H7 and H8 provide signature sequences that predict the metal selectivity in each of five P1B-ATPase subgroups identified. These invariant amino acids contain diverse side chains (thiol, hydroxyl, carbonyl, amide, imidazolium) that can participate in transient metal coordination during transport and consequently determine the particular metal selectivity of each enzyme. Each subgroup shares additional structural characteristics such as the presence (or absence) of particular amino-terminal metal-binding domains and the number of putative transmembrane segments. These differences suggest unique functional characteristics for each subgroup in addition to their particular metal specificity.
Subject(s)
Adenosine Triphosphatases/chemistry , Metals, Heavy/metabolism , Amino Acid Sequence , Biological Transport , Molecular Sequence Data , Protein Conformation , Sequence Homology, Amino AcidABSTRACT
To gain insight into the structure and conformational coupling in the Na,K-ATPase, this study characterized the reaction of the alpha1 subunit transmembrane cysteines with a small probe. Intact HeLa cells expressing heterologous Na,K-ATPase were treated with (microm) HgCl(2) after placing the enzyme predominantly in either of two conformations, phosphorylated E2P.Na/E2P or dephosphorylated ATP.E1. K/ATP.E1. Under both conditions the treatment led to enzyme inactivation following a double exponential kinetic as determined by ouabain-sensitive K(+) uptake measurements. However, the rate constant of the slow reacting component was ten times larger when the protein was probed in a medium that would favor enzyme phosphorylation. Enzymes carrying mutations of cysteines located in the alpha1 subunit transmembrane region were used to identify the reacting-SH groups. Replacement Cys104Ser reduced enzyme inactivation by removing the slow reacting component under both treatment conditions. Replacement of Cys964 reduced the inactivation rate constant of the fast reacting component (79%) and removed the slow reacting component when the dephosphorylated enzyme was treated with Hg(2+). Moreover, Cys964Ser substituted enzyme was insensitive to Hg(2+) when treated under phosphorylation conditions. These results indicate that Cys964 is involved in the fast inactivation by Hg(2+). Although the double mutant Cys964, 104Ser was still partially inactivated by treatment under nonphosphorylating conditions, an enzyme devoid of transmembrane cysteines was insensitive to Hg(2+) under all treatment conditions. Thus, this enzyme provides a background where accessibility of engineered transmembrane cysteines can be tested.
Subject(s)
Sodium-Potassium-Exchanging ATPase/chemistry , Cell Membrane/enzymology , Cysteine/chemistry , Enzyme Inhibitors/pharmacology , HeLa Cells , Humans , In Vitro Techniques , Kinetics , Mercury/pharmacology , Molecular Probes , Mutagenesis, Site-Directed , Phosphorylation , Protein Conformation , Protein Subunits , Sodium-Potassium-Exchanging ATPase/antagonists & inhibitors , Sodium-Potassium-Exchanging ATPase/geneticsABSTRACT
The roles of Ser775 and Glu779, two amino acids in the putative fifth transmembrane segment of the Na,K-ATPase alpha subunit, in determining the voltage and extracellular K+ (K+(o)) dependence of enzyme-mediated ion transport, were examined in this study. HeLa cells expressing the alpha1 subunit of sheep Na,K-ATPase were voltage clamped via patch electrodes containing solutions with 115 mM Na+ (37 degrees C). Na,K-pump current produced by the ouabain-resistant control enzyme (RD), containing amino acid substitutions Gln111Arg and Asn122Asp, displayed a membrane potential and K+(o) dependence similar to wild-type Na,K-ATPase during superfusion with 0 and 148 mM Na+-containing salt solutions. Additional substitution of alanine at Ser775 or Glu779 produced 155- and 15-fold increases, respectively, in the K+(o) concentration that half-maximally activated Na,K-pump current at 0 mV in extracellular Na+-free solutions. However, the voltage dependence of Na,K-pump current was unchanged in RD and alanine-substituted enzymes. Thus, large changes in apparent K+(o) affinity could be produced by mutations in the fifth transmembrane segment of the Na,K-ATPase with little effect on voltage-dependent properties of K+ transport. One interpretation of these results is that protein structures responsible for the kinetics of K+(o) binding and/or occlusion may be distinct, at least in part, from those that are responsible for the voltage dependence of K+(o) binding to the Na,K-ATPase.
Subject(s)
Glutamic Acid/metabolism , Point Mutation/physiology , Potassium/metabolism , Serine/metabolism , Sodium-Potassium-Exchanging ATPase/metabolism , Glutamic Acid/genetics , HeLa Cells , Humans , Membrane Potentials/physiology , Serine/genetics , Sodium-Potassium-Exchanging ATPase/geneticsABSTRACT
Na,K-ATPase containing the amino acid substitution glutamate to alanine at position 779 of the alpha subunit (Glu779Ala) supports a high level of Na-ATPase and electrogenic Na+-Na+ exchange activity in the absence of K+. In microsomal preparations of Glu779Ala enzyme, the Na+ concentration for half maximal activation of Na-ATPase activity was 161 +/- 14 mM (n = 3). Furthermore, enzyme activity with 800 mM Na+ was found to be similar in the presence and absence of 20 mM K+. These results showed that Na+, with low affinity, could stimulate enzyme turnover as effectively as K+. To gain further insight into the mechanism of this enzyme activity, HeLa cells expressing Glu779Ala enzyme were voltage clamped with patch electrodes containing 115 mM Na+ during superfusion in K+-free solutions. Electrogenic Na+-Na+ exchange was observed as an ouabain-inhibitable outward current whose amplitude was proportional to extracellular Na+ (Na+(o)) concentration. At all Na+(o) concentrations tested (3-148 mM), exchange current was maximal at negative membrane potentials (V(M)), but decreased as V(M) became more positive. Analyzing this current at each V(M) with a Hill equation showed that Na+-Na+ exchange had a high-affinity, low-capacity component with an apparent Na+(o) affinity at 0 mV (K0(0.5)) of 13.4 +/- 0.6 mM and a low-affinity, high-capacity component with a K0(0.5) of 120 +/- 13 mM (n = 17). Both high- and low-affinity exchange components were V(M) dependent, dissipating 30 +/- 3% and 82 +/- 6% (n = 17) of the membrane dielectric, respectively. The low-affinity, but not the high-affinity exchange component was inhibited with 2 mM free ADP in the patch electrode solution. These results suggest that the high-affinity component of electrogenic Na+-Na+ exchange could be explained by Na+(o) acting as a low-affinity K+ congener; however, the low-affinity component of electrogenic exchange appeared to be due to forward enzyme cycling activated by Na+(o) binding at a Na+-specific site deep in the membrane dielectric. A pseudo six-state model for the Na,K-ATPase was developed to simulate these data and the results of the accompanying paper (Peluffo, R.D., J.M. Argüello, and J.R. Berlin. 2000. J. Gen. Physiol. 116:47-59). This model showed that alterations in the kinetics of extracellular ion-dependent reactions alone could explain the effects of Glu779Ala substitution on the Na,K-ATPase.
Subject(s)
Alanine/metabolism , Amino Acid Substitution/physiology , Glutamic Acid/metabolism , Sodium-Potassium-Exchanging ATPase/metabolism , Adenosine Diphosphate/pharmacology , Alanine/genetics , Enzyme Inhibitors/pharmacology , Glutamic Acid/genetics , HeLa Cells , Humans , Membrane Potentials/drug effects , Ouabain/pharmacology , Sodium-Potassium-Exchanging ATPase/drug effects , Sodium-Potassium-Exchanging ATPase/geneticsABSTRACT
The structural-functional roles of 23 cysteines present in the sheep (Na,K)-ATPase alpha1 subunit were studied using site directed mutagenesis, expression, and kinetics analysis. Twenty of these cysteines were individually substituted by alanine or serine. Cys452, Cys455 and Cys456 were simultaneously replaced by serine. These substitutions were introduced into an ouabain resistant alpha1 sheep isoform and expressed in HeLa cells under ouabain selective pressure. HeLa cells transfected with a cDNA encoding for replacements of Cys242 did not survive ouabain selective pressure. Single substitutions of the remaining cysteines yielded functional enzymes, although some had reduced turnover rates. Only minor variations were observed in the enzyme Na(+) and K(+) dependence as a result of these replacements. Some substitutions apparently affect the E1<-->E2 equilibrium as suggested by changes in the K(m) of ATP acting at its low affinity binding site. These results indicate that individual cysteines, with the exception of Cys242, are not essential for enzyme function. Furthermore, this suggests that the presence of putative disulfide bridges is not required for alpha1 subunit folding and subsequent activity. A (Na,K)-ATPase lacking cysteine residues in the transmembrane region was constructed (Cys104, 138, 336, 802, 911, 930, 964, 983Xxx). No alteration in the K(1/2) of Na(+) or K(+) for (Na,K)-ATPase activation was observed in the resulting enzyme, although it showed a 50% reduction in turnover rate. ATP binding at the high affinity site was not affected. However, a displacement in the E1<-->E2 equilibrium toward the E1 form was indicated by a small decrease in the K(m) of ATP at the low affinity site accompanied by an increase in IC(50) for vanadate inhibition. Thus, the transmembrane cysteine-deficient (Na,K)-ATPase appears functional with no critical alteration in its interactions with physiological ligands.
Subject(s)
Cysteine/chemistry , Sodium-Potassium-Exchanging ATPase/chemistry , Adenosine Triphosphate/pharmacology , Animals , Enzyme Activation , Membrane Proteins/chemistry , Mutagenesis, Site-Directed , Phosphorylation , Potassium/pharmacology , Sheep , Sodium/pharmacology , Sodium-Potassium-Exchanging ATPase/genetics , Sulfhydryl ReagentsABSTRACT
Oxygen-containing amino acids in the transmembrane region of the Na, K-ATPase alpha subunit were studied to identify residues involved in Na+ and/or K+ coordination by the enzyme. Conserved residues located in the polar face of transmembrane helices were selected using helical wheel and topological models of the enzyme. Alanine substitution of these residues were introduced into an ouabain-resistant sheep alpha1 isoform and expressed in HeLa cells. The capacity to generate essential Na+ and K+ gradients and thus support cell growth was used as an initial indication of the functionality of heterologous enzymes. Enzymes carrying alanine substitution of Ser94, Thr136, Ser140, Gln143, Glu144, Glu282, Thr334, Thr338, Thr340, Ser814, Tyr817, Glu818, Glu821, Ser822, Gln854, and Tyr994 supported cell growth, while those carrying substitutions Gln923Ala, Thr955Ala, and Asp995Ala did not. To study the effects of these latter replacements on cation binding, they were introduced into the wild-type alpha1 sheep isoform and expressed in mouse NIH3T3 cells where [3H]ouabain binding was utilized to probe the heterologous proteins. These substitutions did not affect ouabain, K+, or Na+ binding. Expression levels of these enzymes were similar to that of control. However, the level of Gln923Ala-, Thr955Ala-, or Asp995Ala-substituted enzyme at the plasma membrane was significantly lower than that of the wild-type isoform. Thus, these substitutions appear to impair the maturation process or targeting of the enzyme to the plasma membrane, but not cation-enzyme interactions. These results complete previous studies which have identified Ser755, Asp804, and Asp808 as absolutely essential for Na+ and K+ transport by the enzyme. Thus, it is significant that most transmembrane conserved-oxygen-containing residues in the Na,K-ATPase can be replaced without substantially affecting cation-enzyme interactions to the extent of preventing enzyme function. Consequently, other chemical groups, aromatic rings or backbone carbonyls, should be considered in models of cation-binding sites.
Subject(s)
Alanine/chemistry , Amino Acids/chemistry , Ion Channels/chemistry , Oxygen/chemistry , Sodium-Potassium-Exchanging ATPase/metabolism , 3T3 Cells , Animals , Dose-Response Relationship, Drug , HeLa Cells , Humans , Ion Transport , Mice , Models, Biological , Mutagenesis, Site-Directed , Ouabain/pharmacology , Potassium/pharmacology , Protein Isoforms , Sodium/pharmacologyABSTRACT
The functional roles of Tyr771, Thr772, and Asn776 in the fifth transmembrane segment of the Na, K-ATPase alpha subunit were studied using site-directed mutagenesis, expression, and kinetics analysis. Nonconservative replacements Thr772Tyr and Asn776Ala led to reduced Na,K-ATPase turnover. Replacements at these positions (Asn776Ala, Thr772Leu, and Thr772Tyr) also led to high Na-ATPase activity (in the absence of K+). However, Thr772- and Asn776-substituted enzymes showed only small alterations in the apparent Na+ and K+ affinities (K1/2 for Na,K-ATPase activation). Thus, the high Na-ATPase activity does not appear related to cation-binding alterations. It is probably associated with conformational alterations which lead to an acceleration of enzyme dephosphorylation by Na+ acting at the extracellular space (Argüello et al. J. Biol. Chem. 271, 24610-24616, 1996). Nonconservative substitutions at position 771 (Tyr771Ala and Tyr771Ser) produced a significant decrease of enzyme turnover. Enzyme-Na+ interaction was greatly changed in these enzymes, while their activation by K+ did not appear affected. Although the Na+ K1/2 for Na,K-ATPase stimulation was unchanged (Tyr771Ala, Tyr771Ser), the activation by this cation showed no cooperativity (Tyr771Ala, nHill = 0.75; Tyr771Ser, nHill = 0.92; Control, nHill = 2.28). Substitution Tyr771Phe did not lead to a significant reduction in the cooperativity of the ATPase Na+ dependence (nHill = 1.91). All Tyr771-substituted enzymes showed low steady-state levels of phosphoenzyme during Na-activated phosphorylation by ATP. Phosphorylation levels were not increased by oligomycin, although the drug bound and inactivated Tyr771-substituted enzymes. No E1 left and right arrow E2 equilibrium alterations were detected using inhibition by vanadate as a probe. The data suggest that Tyr771 might play a central role in Na+ binding and occlusion without participating in K+-enzyme interactions.
Subject(s)
Amino Acid Substitution/physiology , Oxygen/physiology , Sodium-Potassium-Exchanging ATPase/physiology , Asparagine/physiology , HeLa Cells , Humans , Mutagenesis, Site-Directed , Protein Conformation , Threonine/physiology , Tyrosine/physiologyABSTRACT
Our studies have concentrated on two aspects of the Na,K-ATPase, the first relates to the identification of amino acids involved in binding Na+ and K+ during the catalytic cycle and the second involves defining how cardiac glycosides inhibit the enzyme. To date, three amino acids, Ser775, Asp804 and Asp808, all located in transmembrane regions five and six, have been shown to play a major role in K+ binding. These findings are based on site directed mutagenesis and expression studies. In order to understand how cardiac glycosides interact with the Na,K-ATPase, studies again involving mutagenesis coupled with expression have been used. More specifically, amino acid residues have been substituted in an ouabain sensitive alpha subunit using random mutagenesis, and the ability of the resulting enzyme to confer resistance to ouabain sensitive cells was determined. Interestingly, the amino acids of the alpha subunit which alter ouabain sensitivity cluster in two major regions, one comprised of the first and second transmembrane spanning domains and the extracellular loop joining them, and the second formed by the extracellular halves of transmembrane regions four, five, six and seven. As noted above, transmembrane regions five and six also contain the three amino acid residues Ser775, Asp804 and Asp808 which play a key role in cation transport, possibly binding K+. Thus, it is reasonable to propose that cardiac glycosides bind to two sites, the N- terminal region and the central region which contains the cation binding sites. Cardiac glycoside binding to the center region may lock the cation transport region into a configuration such that the enzyme cannot go through the conformational change required for ion transport.
Subject(s)
Sodium-Potassium-Exchanging ATPase/metabolism , Amino Acid Sequence , Animals , Binding Sites/physiology , Cardiac Glycosides/pharmacology , Enzyme Inhibitors/pharmacology , Ligands , Molecular Sequence Data , Sodium-Potassium-Exchanging ATPase/antagonists & inhibitors , Sodium-Potassium-Exchanging ATPase/geneticsABSTRACT
This paper summarizes experiments concerned with the functional consequences of mutations in cytoplasmic regions of the alpha 1 subunit of the Na,K-ATPase, in particular the amino terminus, the first cytoplasmic loop between transmembrane segments M2 and M3, and the major cytoplasmic loop between M4 and M5. In the first mutation (alpha 1M32), 32 residues were removed from the N-terminus. The second mutation (E233K) was in the putative beta strand of M2-M3 loop and the third, comprised the replacement of the amino terminal half of loop M4-M5 of the Na,K-ATPase with the homologous segment (residues 356-519) of the gastric H,K-ATPase. The first two mutations, either separately or in combination (alpha 1M32E233K), shift the equilibrium between the major conformational states of the enzyme, E1 and E2, in favor of E1 as manifested by increased apparent affinity for ATP, lower catalytic turnover, and decreased sensitivity to inhibition by vanadate. The striking changes observed with alpha 1M32E233K suggests interactions between the N-terminus, the beta-strand in the M2-M3 loop and the catalytic phosphorylation site. The behavior of these mutants contrasts with that of least one mutant involving substitution of a residue in the putative cation binding pocket, namely S775A in the fifth transmembrane segment (Arguello, J.M., & Lingrel, J. B. J. Biol. Chem. 270: 22764-22771, 1995). Although its K+/ATP antagonism resembles that of the foregoing cytoplasmic mutants, its vanadate sensitivity is unaltered suggesting that changes in apparent affinity for ATP are secondary to changes in K+ ligation. The question of cation selectivity, in particular that of Na+ versus protons, has been addressed in structure/function analysis of a cytoplasmic chimera involving the M4-M5 loop. Transport studies performed in the presence or absence of Na+ and at low versus high pH indicate a marked alteration in cation affinity and/or selectivity. This results suggests coupling of an alteration in the large M4-M5 cytoplasmic domain to cation binding in, presumably, the juxtapositioned transmembrane domain.
Subject(s)
Cytoplasm/metabolism , Isoenzymes/metabolism , Mutation/genetics , Sodium-Potassium-Exchanging ATPase/metabolism , Amino Acid Substitution/genetics , Protein ConformationABSTRACT
From the structural data obtained by systematically altering residues of the Na,K-ATPase, we are beginning to understand portions of how this active cation transporter couples hydrolysis of ATP with the vectorial movement of cations against their ionic gradients. In addition, the inhibitory action of cardiac glycosides and their interaction sites on the protein has focused our attentions on a catalytic core of the protein involving the H5-H6 transmembrane segment. In future investigations, both the ATP and the Na+ sites of the Na,K-ATPase must be uncovered to refine the structural picture of this complex transporter.
Subject(s)
Cardiac Glycosides/metabolism , Ouabain/metabolism , Potassium/metabolism , Protein Structure, Secondary , Sodium-Potassium-Exchanging ATPase/chemistry , Sodium-Potassium-Exchanging ATPase/metabolism , Sodium/metabolism , Amino Acid Sequence , Animals , Binding Sites , Cations, Monovalent/metabolism , Cell Membrane/enzymology , Kinetics , Molecular Sequence Data , Ouabain/pharmacology , SheepSubject(s)
Sodium-Potassium-Exchanging ATPase/chemistry , Sodium-Potassium-Exchanging ATPase/metabolism , Amino Acid Substitution , Animals , HeLa Cells , Humans , Kinetics , Point Mutation , Potassium/metabolism , Potassium/pharmacology , Recombinant Proteins/chemistry , Recombinant Proteins/metabolism , Sheep , TransfectionABSTRACT
Substitution of alanine for Ser775 in a ouabain-resistant alpha1 sheep isoform causes a 30-fold decrease in apparent affinity for K+ as an activator of the Na,K-ATPase, as well as an increase in apparent affinity for ATP (Arguello, J. M., and Lingrel, J. B (1995) J. Biol. Chem. 270, 22764-22771). This study was carried out to determine whether Ser775 is a direct cation-ligating residue or whether the change in apparent affinity for K+ is secondary to a conformational alteration as evidenced in the change in ATP affinity, with the following results. Kinetics of K+(Rb+) influx into intact cells show that the change is due to a change in K+ interaction at the extracellular surface. The K+ dependence of formation of K+-occluded enzyme (E2(K)) and of the rate of formation of deoccluded enzyme from E2(K) indicate that the Ser775 --> Ala mutation results in a marked increase (>/=30-fold) in rate of release of K+ from E2(K). The high affinity Na+-like competitive antagonist 1,3-dibromo2,4,6-tris-(methylisothiouronium)benzene (Br2TITU), which interacts with the E1 conformation and blocks cytoplasmic cation binding (Hoving, S., Bar-Shimon, M., Tijmes, J. J. , Tal, D. M., and Karlish, S. J. D. (1995) J. Biol. Chem. 270, 29788-29793), inhibits Na+-ATPase of the mutant less than the control enzyme. With intact cells, Br2TITU acts as a competitive inhibitor of extracellular K+ activation of both the mutant and control enzymes. In this case, the mutant was more sensitive to inhibition. With vanadate as a probe of conformation, a difference in conformational equilibrium between the mutant and control enzymes could not be detected under turnover conditions (Na+- ATPase) in the absence of K+. These results indicate that the increase in apparent affinity for ATP effected by the Ser775 --> Ala mutation is secondary to a change in intrinsic cation affinity/selectivity. The large change in affinity for extracellular K+ compared with cytoplasmic Na+ and to Br2TITU binding supports the conclusion that the serine hydroxyl is either part of the K+-gate structure or a direct cation-ligating residue that is shared by at least one Na+ ion, albeit with less consequence on rate constants for Na+ binding or release compared with K+.
Subject(s)
Serine , Sodium-Potassium-Exchanging ATPase/chemistry , Sodium-Potassium-Exchanging ATPase/metabolism , Adenosine Triphosphate/metabolism , Alanine , Animals , Binding Sites , Cations, Monovalent/metabolism , Enzyme Inhibitors/pharmacology , HeLa Cells , Humans , Isothiuronium/analogs & derivatives , Isothiuronium/pharmacology , Kinetics , Macromolecular Substances , Mutagenesis, Site-Directed , Ouabain/pharmacology , Point Mutation , Potassium/metabolism , Recombinant Proteins/chemistry , Recombinant Proteins/metabolism , Rubidium/metabolism , Sheep , Time Factors , TransfectionABSTRACT
The functional roles of Asp804 and Asp808, located in the sixth transmembrane segment of the Na,K-ATPase alpha subunit, were examined. Nonconservative replacement of these residues yielded enzymes unable to support cell viability. Only the conservative substitution, Ala808 --> Glu, was able to maintain the essential cation gradients (Van Huysse, J. W., Kuntzweiler, T. A., and Lingrel, J. B (1996) FEBS Lett. 389, 179-185). Asp804 and Asp808 were replaced by Ala, Asn, and Glu in the sheep alpha1 subunit and expressed in a mouse cell line where [3H]ouabain binding was utilized to probe the exogenous proteins. All of the heterologous proteins were targeted into the plasma membrane, bound ouabain and nucleotides, and adopted E1Na, E1ATP, and E2P conformations. K+ competition of ouabain binding to sheep alpha1 and Asp808 --> Glu enzymes displayed IC50 values of 4.11 mM (nHill = 1.4) and 23.8 mM (nHill = 1.6), respectively. All other substituted proteins lacked this K+-ouabain antagonism, e.g. 150 mM KCl did not inhibit ouabain binding. Na+ antagonized ouabain binding to all the expressed isoforms, however, the proteins carrying nonconservative substitutions displayed reduced Hill coefficients (nHill
Subject(s)
Aspartic Acid/chemistry , Membrane Proteins/metabolism , Sodium-Potassium-Exchanging ATPase/metabolism , 3T3 Cells , Animals , Aspartic Acid/metabolism , Cations , Membrane Proteins/chemistry , Membrane Proteins/genetics , Mice , Mutagenesis, Site-Directed , Sodium-Potassium-Exchanging ATPase/chemistry , Sodium-Potassium-Exchanging ATPase/geneticsABSTRACT
The effects of changing Glu-779, located in the fifth transmembrane segment of the Na,K-ATPase alpha subunit, on the phosphorylation characteristics and ion transport properties of the enzyme were investigated. HeLa cells were transfected with cDNA coding the E779A substitution in an ouabain-resistant sheep alpha1 subunit (RD). Steady state phosphorylation stimulated by Na+ concentrations less than 20 mM or by imidazole were similar for RD and E779A enzymes, an indication that phosphorylation and Na+ occlusion were not altered by this mutation. With E779A enzyme, higher Na+ concentrations reduced the level of phosphoenzyme and stimulated Na-ATPase activity in the absence of K+. These effects were a consequence of Na+ increasing the rate of protein dephosphorylation. In voltage-clamped HeLa cells expressing E779A enzyme, a prominent electrogenic Na+-Na+ exchange was observed in the absence of extracellular K+. Thus, increased Na-ATPase activity and Na+-dependent dephosphorylation result from Na+ acting as a K+ congener with low affinity at extracellular binding sites. These data suggest that E779A does not directly participate in ion binding but does affect the connection between extracellular ion binding and intracellular enzyme dephosphorylation. In cells expressing control RD enzyme, Na,K-pump current was dependent on membrane potential and extracellular K+ concentration. However, Na,K-pump current in cells expressing E779A enzyme was voltage independent at all extracellular K+ tested. These results indicate that Glu-779 may be part of the access channel determining the voltage dependence of ion transport by the Na, K-ATPase.
