Structure-function relationships in sarcoplasmic or endoplasmic reticulum type Ca2+ pumps

Novel quercetin derivatives in treatment of peroxynitrite-oxidized SERCA1

Sarco/endoplasmic reticulum calcium ATP-ase (SERCA) is regulated by low concentrations of peroxynitrite and inhibited by high levels, as indicated in human diseases. We studied quercetin (Q) and its novel derivatives monochloropivaloylquercetin (MPQ) and chloronaphthoquinonequercetin (CHQ) as agents with expected preventive properties against peroxynitrite-induced SERCA impairment. Q and MPQ protected the SERCA1 against peroxynitrite induced activity decrease, while CHQ potentiated the inhibitory effect of peroxynitrite. Quercetin derivatives were found to be weaker antioxidants compared with Q, as indicated by their ability to scavenge peroxynitrite and prevent of SERCA1 carbonylation, both decreasing in the order (Q > MPQ > CHQ). Quantum-chemical values of theoretical parameter E HOMO also indicated lower antioxidant capacities for MPQ and CHQ. Prooxidant properties estimated by calculations of frontier molecular orbitals (E LUMO) correlated with experimentally determined SH-group decrease induced by the compounds studied. Both methods showed a decrease of prooxidant properties as follows: CHQ > MPQ > Q. In addition, experimentally measured half-wave potentials indicated stronger prooxidant properties of quercetin derivatives as compared to Q. More expressive alterations of conformation in the transmembrane region of SERCA1 induced by quercetin derivatives, as compared with Q, may at least partially correlate with their higher lipophilicities. The protective effects of Q and MPQ on different isoforms of SERCA activity may be useful in prevention and treatment of inflammation or muscle diseases. The inhibitory effect of CHQ on SERCA isoforms may be beneficial in therapeutic approaches aimed at anti-tumor treatment.

On marathons and Sprints: an integrated quantitative proteomics and transcriptomics analysis of differences between slow and fast muscle fibers

Skeletal muscle tissue contains slow as well as fast twitch muscle fibers that possess different metabolic and contractile properties. Although the distribution of individual proteins in fast and slow fibers has been investigated extensively, a comprehensive proteomic analysis, which is key for any systems biology approach to muscle tissues, is missing. Here, we compared the global protein levels and gene expression profiles of the predominantly slow soleus and fast extensor digitorum longus muscles using the principle of in vivo stable isotope labeling with amino acids based on a fully lysine-6 labeled SILAC-mouse. We identified 551 proteins with significant quantitative differences between slow soleus and fast extensor digitorum longus fibers out of >2000 quantified proteins, which greatly extends the repertoire of proteins differentially regulated between both muscle types. Most of the differentially regulated proteins mediate cellular contraction, ion homeostasis, glycolysis, and oxidation, which reflect the major functional differences between both muscle types. Comparison of proteomics and transcriptomics data uncovered the existence of fiber-type specific posttranscriptional regulatory mechanisms resulting in differential accumulation of Myosin-8 and α-protein kinase 3 proteins and mRNAs among others. Phosphoproteome analysis of soleus and extensor digitorum longus muscles identified 2573 phosphosites on 973 proteins including 1040 novel phosphosites. The in vivo stable isotope labeling with amino acids-mouse approach used in our study provides a comprehensive view into the protein networks that direct fiber-type specific functions and allows a detailed dissection of the molecular composition of slow and fast muscle tissues with unprecedented resolution.

Proteomic detection of nitroproteins as potential biomarkers for cardiovascular disease

Increased levels of reactive oxygen and nitrogen species are linked to many human diseases and can be formed as an indirect result of the disease process. The accumulation of specific nitroproteins which correlate with pathological processes suggests that nitration of protein tyrosine represents a dynamic and selective process, rather than a random event. Indeed, in numerous clinical disorders associated with an upregulation in oxidative stress, tyrosine nitration has been limited to certain cell types and to selective sites of injury. Additionally, proteomic studies show that only certain proteins are nitrated in selective tissue extracts. A growing list of nitrated proteins link the negative effects of protein nitration with their accumulation in a wide variety of diseases related to oxidation. Nitration of tyrosine has been demonstrated in diverse proteins such as cytochrome c, actin, histone, superoxide dismutase, α-synuclein, albumin, and angiotensin II. In vitro and in vivo aspects of redox-proteomics of specific nitroproteins that could be relevant to biomarker analysis and understanding of cardiovascular disease mechanism will be discussed within this review.

Evidence for a new allele at the SERCA1 locus affecting pork meat quality in part through the imbalance of Ca2+ homeostasis

Sarcoendoplasmic reticulum Ca2+-ATPase 1 (SERCA1) as a Ca2+ release channel plays a key role in the relaxation of skeletal muscle through pumping cytosolic Ca2+ into the SR (sarcoplasmic reticulum). In this study, a novel single nucleotide polymorphism (SNP) in exon 8 (C > T) was detected by tetra-primer ARMS-PCR and the tissue expression pattern of SERCA1 was analyzed in eleven tissues. A model of primary skeletal muscle cells in vitro exposed to dexamethasone (DEX, a synthetic corticosteroid) was also employed to determine whether stress hormones cause an increase in intracellular Ca2+ concentration that is associated with alteration in SERCA1 and in turn subsequently affect meat quality. The results showed that the CC genotype has lower content intramuscular fat and higher water than pig carrying the genotype CT and CC. In addition, the additive effects were both significantly (P < 0.05) and allele T seemed to be associate with increase in intramuscular fat, while decrease in water content. Accompanied with previous studies, the high abundance of porcine SERCA1 was found in skeletal muscle tissue. DEX markedly down-regulated the expression of SERCA1, leading to Ca2+ overload. Furthermore, the imbalance of Ca2+ homeostasis up-regulated the transcription level of Calpain1. Taken together, we demonstrated a novel mechanism that the changes in expression of SERCA1 potential disturb the normal Ca2+ channel as well as the balance of Ca2+ homeostasis and which in turn finally activated Ca2+-dependent proteases such as Calpain1 which could affect meat quality.

SERCA pump isoforms: Their role in calcium transport and disease

The sarcoendoplasmic reticulum (SR) calcium transport ATPase (SERCA) is a pump that transports calcium ions from the cytoplasm into the SR. It is present in both animal and plant cells, although knowledge of SERCA in the latter is scant. The pump shares the catalytic properties of ion-motive ATPases of the P-type family, but has distinctive regulation properties. The SERCA pump is encoded by a family of three genes, SERCA1, 2, and 3, that are highly conserved but localized on different chromosomes. The SERCA isoform diversity is dramatically enhanced by alternative splicing of the transcripts, occurring mainly at the COOH-terminal. At present, more than 10 different SERCA isoforms have been detected at the protein level. These isoforms exhibit both tissue and developmental specificity, suggesting that they contribute to unique physiological properties of the tissue in which they are expressed. The function of the SERCA pump is modulated by the endogenous molecules phospholamban (PLB) and sarcolipin (SLN), expressed in cardiac and skeletal muscles. The mechanism of action of PLB on SERCA is well characterized, whereas that of SLN is only beginning to be understood. Because the SERCA pump plays a major role in muscle contraction, a number of investigations have focused on understanding its role in cardiac and skeletal muscle disease. These studies document that SERCA pump expression and activity are decreased in aging and in a variety of pathophysiological conditions including heart failure. Recently, SERCA pump gene transfer was shown to be effective in restoring contractile function in failing heart muscle, thus emphasizing its importance in muscle physiology and its potential use as a therapeutic agent.

The Assembly of Calcium Release Units in Cardiac Muscle

Calcium release units (CRUs) are constituted of specialized junctional domains of the sarcoplasmic reticulum (jSR) that bear calcium release channels, also called ryanodine receptors (RyRs). In cardiac muscle, CRUs come in three subtypes that differ in geometry, but have common molecular components. Peripheral couplings are formed by a junction of the jSR with the plasmalemma; dyads occur where the jSR is associated with transverse (T)-tubules; corbular SR is a jSR domain that is located within the cells and bears RyRs but does not associate with either plasmalemma or T-tubules. Using transmission electron microscopy, this study followed the formation of CRUs and their accrual of four components: the L-type channel dihydropyridine receptors (DHPRs) of plasmalemma/T-tubules; the RyRs of jSR; triadin (Tr) and junctin (JnC), two homologous components of the jSR membrane; and calsequestrin (CSQ), the internal calcium binding proteins. During differentiation, peripheral couplings are formed first and the others follow. RyRs and DHPRs are targeted to subdomains of the CRUs that face each other and are acquired in a concerted manner. Overexpressions of either junction (JnC or Tr) and of CSQ, singly or in conjunction, shed light on the specific role of JnC in the structural development, organization, and maintenance of jSR cisternae and on the independent synthetic pathways and targeting of JnC and CSQ. In addition, the structural cues provided by the overexpression models allow us to define sequential steps in the synthetic pathway for JnC and CSQ and their targeting to the CRUs of differentiating myocardium.

Differentiation of answer of glioma C6 cells to SERCA pump inhibitors by actin disorganization

Capacitative calcium entry, usually evoked by receptor-ligand binding, may be also studied in the model system of calcium release after SERCA pump inhibition. We have previously found that disorganization of actin cytoskeleton has no effect on calcium influx into glioma C6 cells after thapsigargin administration [Biochem. Biophys. Res. Commun. 296 (2002) 484]. In the present work we show that the effect of other SERCA pump inhibitors depends on the endoplasmic reticulum distribution in a cell. Changing this distribution leads to changes in calcium release from ER stores. Intensity of calcium influx in the capacitative phase of cell answer does not depend on actin cytoskeleton state; however, administration of cytochalasin D significantly slows down signal build-up. While cyclopiazonic acid acts very similarly to thapsigargin, cytoskeleton disorganization leads to rise of calcium signal after administration of 2,5-di-(t-butyl)-1,4-benzohydroquinone. This effect may be caused by specific binding of this inhibitor to SERCA3 isoform of pump protein only.

Coordinate downregulation of CaM kinase II and phospholamban accompanies contractile phenotype transition in the hyperthyroid rabbit soleus

This study investigated the effects of l-thyroxine-induced hyperthyroidism on Ca2+/calmodulin (CaM)-dependent protein kinase (CaM kinase II)-mediated sarcoplasmic reticulum (SR) protein phosphorylation, SR Ca2+pump (Ca2+-ATPase) activity, and contraction duration in slow-twitch soleus muscle of the rabbit. Phosphorylation of Ca2+-ATPase and phospholamban (PLN) by endogenous CaM kinase II was found to be significantly lower (30–50%) in soleus of the hyperthyroid compared with euthyroid rabbit. Western blotting analysis revealed higher levels of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) 1 (∼150%) Ca2+pump isoform, unaltered levels of SERCA2 Ca2+pump isoform, and lower levels of PLN (∼50%) and δ-, β-, and γ-CaM kinase II (40 ∼ 70%) in soleus of the hyperthyroid rabbit. SR vesicles from hyperthyroid rabbit soleus displayed approximately twofold higher ATP-energized Ca2+uptake and Ca2+-stimulated ATPase activities compared with that from euthyroid control. The Vmaxof Ca2+uptake (in nmol Ca2+·mg SR protein−1·min−1: euthyroid, 818 ± 73; hyperthyroid, 1,649 ± 90) but not the apparent affinity of the Ca2+-ATPase for Ca2+(euthyroid, 0.97 ± 0.02 μM, hyperthyroid, 1.09 ± 0.04 μM) differed significantly between the two groups. CaM kinase II-mediated stimulation of Ca2+uptake by soleus muscle SR was ∼60% lower in the hyperthyroid compared with euthyroid. Isometric twitch force of soleus measured in situ was significantly greater (∼36%), and the time to peak force and relaxation time were significantly lower (∼30–40%), in the hyperthyroid. These results demonstrate that thyroid hormone-induced transition in contractile properties of the rabbit soleus is associated with coordinate downregulation of the expression and function of PLN and CaM kinase II and selective upregulation of the expression and function of SERCA1, but not SERCA2, isoform of the SR Ca2+pump.

Molecular physiology of the SERCA and SPCA pumps

Intracellular Ca(2+)-transport ATPases exert a pivotal role in the endoplasmic reticulum and in the compartments of the cellular secretory pathway by maintaining a sufficiently high lumenal Ca(2+) (and Mn(2+)) concentration in these compartments required for an impressive number of vastly different cell functions. At the same time this lumenal Ca(2+) represents a store of releasable activator Ca(2+) controlling an equally impressive number of cytosolic functions. This review mainly focuses on the different Ca(2+)-transport ATPases found in the intracellular compartments of mainly animal non-muscle cells: the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) pumps. Although it is not our intention to treat the ATPases of the specialized sarcoplasmic reticulum in depth, we can hardly ignore the SERCA1 pump of fast-twitch skeletal muscle since its structure and function is by far the best understood and it can serve as a guide to understand the other members of the family. In a second part of this review we describe the relatively novel family of secretory pathway Ca(2+)/Mn(2+) ATPases (SPCA), which in eukaryotic cells are primarily found in the Golgi compartment.

2,3-Dimercaptopropanol inhibits Ca2+ transport in microsomes from brain but not from fast-skeletal muscle

Ca2+ is involved in the regulation of a variety of physiological processes, but a persistent increase in free cytosolic Ca2+ concentrations may contribute to cell injury. Dimercaprol (BAL) is a compound used in the treatment of mercury intoxication, but presents low therapeutic efficacy. The molecular mechanism responsible for the BAL toxicity is poorly known. In the present study, the effect of BAL and inorganic and organic mercury on Ca2+ transport by Ca2+-ATPases located in the sarco/endoplasmic reticulum of fast-skeletal muscle and brain was examined. Ca2+ uptake by brain and fast-skeletal muscle microsomes was inhibited in a dose-dependent manner by Hg2+. The calculated IC50 for Ca2+ uptake inhibition by HgCl2 was 1.05+/-0.09 microM (n = 8) for brain and 0.72+/-0.06 microM (n = 9) for muscle. The difference was significant at p < 0.01 (data expressed as mean +/- SD). At a low concentration (1 microM), 2,3-dimer-captopropanol had no effect on Ca2+ uptake by brain or muscle vesicles and did not abolish the inhibition caused by Hg2+. A high concentration of BAL (1 mM) nearly abolished the inhibition caused by 1.75 microM HgCl2 or 6 microM CH3HgCl in skeletal muscle. Surprisingly, at intermediate concentrations (40-100 microM) BAL partially inhibited Ca2+ transport in brain but had no effect on muscle. Furthermore, ATP hydrolysis by brain or muscle microsomes was not inhibited by BAL. These results suggest that in brain microsomes BAL affects in a different way Ca2+ transport and ATP hydrolysis. The increase in BAL concentration observed after toxic administration of this compound to experimental animals may contribute to deregulate Ca2+ homoeostasis and, consequently, to the neurotoxicity of BAL.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Corticosteroids decrease mRNA levels of SERCA pumps, whereas they increase sarcolipin mRNA in the rat diaphragm

1. In order to explore the potential role of the sarcoplasmic-endoplasmic reticulum Ca2+-ATPase (SERCA)-type pumps and of their modulators phospholamban (PLB) and sarcolipin (SLN) in the functional alterations of the diaphragm induced by corticosteroid treatment, expression of SERCA, PLB and SLN was assessed by RT-PCR in the diaphragm of rats treated daily for 5 days either with triamcinolone (80 mg kg-1, n = 8) or with saline (control; 0.6 ml, n = 8). 2. Triamcinolone treatment reduced the normalised overall amount of all SERCA mRNA in diaphragm by 70 % compared to controls (P < 0.05). This reduction was accounted for by a relatively larger decrease in the SERCA1 mRNA (-69 %, P < 0.05) whilst the decrease in SERCA2 mRNA (-49 %, P = 0.09) did not reach statistical significance. As a result the relative proportion of SERCA2 mRNA was increased from 43 +/- 7 % in control diaphragm to 52 +/- 4 % after triamcinolone treatment (P < 0.05). 3. Only the adult isoform of SERCA1 (i.e. SERCA1a) mRNA was found in the diaphragm of the 15-week-old control rats. Furthermore, triamcinolone treatment resulted in reduced levels of SERCA2a (-40 %, P < 0.05) and increased levels of SLN mRNA (+100 %, P < 0.05), while the decrease in PLB mRNA (-31 %, P = 0.277) did not reach statistical significance. SERCA1b, SERCA2b and SERCA3 mRNA levels fell below the detection limit in the diaphragm of both control and triamcinolone-treated rats. 4. Compared to control diaphragm, control rat heart showed a relatively high PLB/(SERCA1 + SERCA2) mRNA ratio of 7.88 while this ratio amounted only to 0.16 in control extensor digitorum longus (EDL) muscle. Remarkably, the SLN/(SERCA1 + SERCA2) mRNA ratio in normal cardiac muscle (0.96) was nearly the same as in diaphragm, but in EDL it amounted to only 0.05 that in diaphragm. This indicates the very low expression of SLN in rat EDL. 5. These data reveal that considerable alterations in SERCA mRNA levels accompany the functional changes seen in diaphragm after corticosteroid treatment. The relatively larger decrease in SERCA1 mRNA is in agreement with the selective type II fibre atrophy previously observed in the diaphragm of triamcinolone-treated rats, but the magnitude of SERCA alterations is more pronounced than expected on the basis of the structural changes in the diaphragm. The increase in SLN mRNA levels may represent a compensatory mechanism.

Kinetics studies of the cardiac Ca-ATPase expressed in Sf21 cells: new insights on Ca-ATPase regulation by phospholamban

Kinetics studies of the cardiac Ca-ATPase expressed in Sf21 cells (Spodoptera frugiperda insect cells) have been carried out to test the hypotheses that phospholamban inhibits Ca-ATPase cycling by decreasing the rate of the E1.Ca to E1'.Ca transition and/or the rate of phosphoenzyme hydrolysis. Three sample types were studied: Ca-ATPase expressed alone, Ca-ATPase coexpressed with wild-type phospholamban (the natural pentameric inhibitor), and Ca-ATPase coexpressed with the L37A-phospholamban mutant (a more potent monomeric inhibitor, in which Leu(37) is replaced by Ala). Phospholamban coupling to the Ca-ATPase was controlled using a monoclonal antibody against phospholamban. Gel electrophoresis and immunoblotting confirmed an equivalent ratio of Ca-ATPase and phospholamban in each sample (1 mol Ca-ATPase to 1.5 mol phospholamban). Steady-state ATPase activity assays at 37 degrees C, using 5 mM MgATP, showed that the phospholamban-containing samples had nearly equivalent maximum activity ( approximately 0.75 micromol. nmol Ca-ATPase(-1).min(-1) at 15 microM Ca(2+)), but that wild-type phospholamban and L37A-phospholamban increased the Ca-ATPase K(Ca) values by 200 nM and 400 nM, respectively. When steady-state Ca-ATPase phosphoenzyme levels were measured at 0 degrees C, using 1 microM MgATP, the K(Ca) values also shifted by 200 nM and 400 nM, respectively, similar to the results obtained by measuring ATP hydrolysis at 37 degrees C. Measurements of the time course of phosphoenzyme formation at 0 degrees C, using 1 microM MgATP and 268 nM ionized [Ca(2+)], indicated that L37A-phospholamban decreased the steady-state phosphoenzyme level to a greater extent (45%) than did wild-type phospholamban (33%), but neither wild-type nor L37A-phospholamban had any effect on the apparent rate of phosphoenzyme formation relative to that of Ca-ATPase expressed alone. Measurements of inorganic phosphate (P(i)) release concomitant with the phosphoenzyme formation studies showed that L37A-phospholamban decreased the steady-state rate of P(i) release to a greater extent (45%) than did wild-type phospholamban (33%). However, independent measurements of Ca-ATPase dephosphorylation after the addition of 5 mM EGTA to the phosphorylated enzyme showed that neither wild-type phospholamban nor L37A-phospholamban had any effect on the rate of phosphoenzyme decay relative to Ca-ATPase expressed alone. Computer simulation of the kinetics data indicated that phospholamban and L37A-phospholamban decreased twofold and fourfold, respectively, the equilibrium binding of the first Ca(2+) ion to the Ca-ATPase E1 intermediate, rather than inhibiting rate of the E.Ca to E'.Ca transition or the rate of phosphoenzyme decay. Therefore, we conclude that phospholamban inhibits Ca-ATPase cycling by decreasing Ca-ATPase Ca(2+) binding to the E1 intermediate.

Residues of the fourth transmembrane segments of the Na,K-ATPase and the gastric H,K-ATPase contribute to cation selectivity

We have generated protein chimeras to investigate the role of the fourth transmembrane segments (TM4) of the Na,K- and gastric H, K-ATPases in determining the distinct cation selectivities of these two pumps. Based on a helical wheel analysis, three residues of TM4 of the Na,K-ATPase were changed to their H,K-counterparts. A construct carrying three mutations in TM4 (L319F, N326Y, and T340S) and two control constructs were heterologously expressed in Xenopus laevis oocytes and in the pig kidney epithelial cell line LLC-PK(1). Biochemical ATPase assays demonstrated a large sodium-independent ATPase activity at pH 6.0 for the pump carrying the TM4 substitutions, whereas the control constructs exhibited little or no activity in the absence of sodium. Furthermore, at pH 6.0 the K(1/2)(Na(+)) shifted to 1.5 mM for the TM4 construct compared with 9.4 and 5.9 mM for the controls. In contrast, at pH 7.5 all three constructs had characteristics similar to wild type Na,K-ATPase. Large increases in K(1/2)(K(+)) were observed for the TM4 construct compared with the control constructs both in two-electrode voltage clamp experiments in Xenopus oocytes and in ATPase assays. ATPase assays also revealed a 10-fold shift in vanadate sensitivity for the TM4 construct. Based on these findings, it appears that the three identified TM4 residues play an important role in determining both the specific cation selectivities and the E(1)/E(2) conformational equilibria of the Na,K- and H,K-ATPase.

Relaxation of diaphragm muscle

Relaxation is the process by which, after contraction, the muscle actively returns to its initial conditions of length and load. In rhythmically active muscles such as diaphragm, relaxation is of physiological importance because diaphragm must return to a relatively constant resting position at the end of each contraction-relaxation cycle. Rapid and complete relaxation of the diaphragm is likely to play an important role in adaptation to changes in respiratory load and breathing frequency. Regulation of diaphragm relaxation at the molecular and cellular levels involves Ca(2+) removal from the myofilaments, active Ca(2+) pumping by the sarcoplasmic reticulum (SR), and decrease in the number of working cross bridges. The relative contribution of these mechanisms mainly depends on sarcomere length, muscle tension, and the intrinsic contractile function. Increased capacity of SR to take up Ca(2+) can arise from increased density of active SR pumping sites or in slow-twitch fibers from phosphorylation of phospholamban, whereas impaired coupling between ATP hydrolysis and Ca(2+) transport into the SR or intracellular acidosis reduces SR Ca(2+) pump activity. In experimental conditions of decreased contractile performance, slowed, enhanced, or unchanged relaxation rates have been reported in vitro. In vivo, a slowing in the rate of decline of the respiratory pressure is generally considered an early reliable index of respiratory muscle fatigue. Impaired relaxation rate may, in turn, favor mismatch between blood flow and metabolic demand, especially at high breathing frequencies.

Identification and functional expression of the plasma membrane calcium ATPase gene family from Caenorhabditis elegans

Calcium-pumping ATPases are an essential component of the intracellular calcium homeostasis system and have been characterized in a large variety of species and cell types. In mammalian genomes, these proteins are encoded by gene families whose individual members feature complex tissue-specific expression and alternative splicing. In the search for a less complex system that is more amenable to genetic manipulation, we have identified a family of three genes (mca-1, mca-2, and mca-3) encoding putative calcium ATPases in the Caenorhabditis elegans Genome Project data and completed their transcript structure. In this work, we report the cloning and functional expression of the mca-1 gene, which encodes a calcium-stimulated ATPase whose features resemble those of the plasma membrane calcium adenosine triphosphatase family of mammalian cells and appears to be regulated by a multipartite promoter.

Expression of the sarco/endoplasmic reticulum Ca(2+)-transport ATPase protein isoforms during regeneration from notexin-induced necrosis of rat soleus muscle

Expression levels of fast-twitch (SERCA1), slow-twitch (SERCA2a) and "housekeeping" (SERCA2b) isoforms of the sarcoplasmic reticulum Ca(2+)-transport ATPase were monitored during regeneration of rat soleus muscles following necrosis induced by the toxin notexin at the tissue level by Western blot analysis and at the cellular level by immunocytochemical analysis. Due to necrosis, levels of muscle-specific SERCA1 and SERCA2a isoforms dropped to low levels on the third day after injection of the toxin. Subsequently, during regeneration both isoforms recovered but with a different time course. Expression of the fast type SERCA1 increased first. This type showed its most pronounced increase between day 3 and 10. Expression of the slow type SERCA2a was biphasic. After an increase to approximately one third of the control value on days 5-10, it showed its main increase up to the control level between day 10 and 21. Expression levels of the house-keeping SERCA2b isoform remained relatively constant throughout the 4 weeks of regeneration. Between day 10 and 28, when new innervation is established, SERCA2a expression spread gradually over almost all fibers whereas the number of SERCA1-expressing fibers decreased and only a limited number of fibers co-expressed SERCA1 and SERCA2a. At 4 weeks of regeneration, expression of the fast isoform was found only in 12% of the fibers, whereas the slow form was found in 98% of the fibers. In the contralateral untreated soleus muscles, 26% SERCA1-positive and 81% SERCA2a-positive fibers were observed. Immunocytochemical analysis showed that SERCA1 and SERCA2a were co-expressed with fast and slow myosin isoforms in fibers of normal muscles but in regenerated muscle only slow myosin and slow SERCA isoforms correlated. The results show that during regeneration levels of fast and slow SERCA proteins change in a similar way as their mRNAs do. However, in regenerated soleus, unlike in normal muscle, expression of slow SERCA is coregulated only with the slow myosin isoform. This finding is in agreement with the fact that the number of slow type fibers is increased in regenerated soleus.

Expression of sarcoplasmic/endoplasmic reticulum Ca2+ ATPases in the rat extensor digitorum longus (EDL) muscle regenerating from notexin-induced necrosis

The level of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) mRNAs and proteins have been assessed by RT-PCR, immunoblotting and immunocytochemistry in the rat extensor digitorum longus (EDL) muscles during regeneration from notexin-induced necrosis. As a result of the necrosis, SERCA1 and SERCA2 declined on days 1 and 3 after administration of the toxin. Thereupon the mRNA of the fast isoform SERCA1 rapidly increased between days 5 and 10 to the normal level. The mRNA level of the "housekeeping" SERCA2b isoform increased markedly during the actual necrosis at days 1 and 5, probably due to invading cells. Then the mRNA level of the neonatal SERCA1b splice variant increased first, and exceeded the level of the adult SERCA1a transcript on day 5. At later stages of regeneration the neonatal form was gradually replaced by the adult SERCA1a form, thus recapitulating similar changes known to occur during normal ontogenesis. Along with SERCA1, the levels of the slow isoform (SERCA2a) mRNA and protein increased on day 5, but the SERCA2a mRNA levels never rose above 10% of SERCA1 and after 10 days gradually declined again. In the normal and regenerated muscles, SERCA1 was expressed in 97% of the fibres which accounted for the population of fast-twitch fibres (type IIa, type IIb and probably type IIx/d). SERCA2a was present in 6% of the fibres of normal muscle (mostly in the slow-twitch type I fibres). At the end of regeneration the number of fibres expressing SERCA2a was twice as high and were found in small groups, unlike in normal EDL, but about 50% of these clustered fibres also expressed SERCA1.

ATP7B (WND) protein

Wilson's disease is a genetic disorder of copper metabolism characterized by the excessive accumulation of this metal in the liver. The gene for Wilson's disease, designated ATP7B, encodes a copper transporting P-type ATPase expressed predominantly in the liver. Over 60 disease specific mutations of ATP7B have now been reported in patients with Wilson's disease. The gene for ATP7B is approximately 80 kb and contains 21 exons that encode an approximately 7.5 kb transcript. Recent studies that focus on the structure and expression of the ATP7B protein support its role as a copper transporter involved in the intracellular trafficking of copper in hepatocytes. The introduction of functional ATP7B protein by recombinant adenovirus mediated gene delivery will be a potential approach for correcting Wilson's disease.

Signal transduction and regulation of lung endothelial cell permeability. Interaction between calcium and cAMP

Pulmonary endothelium forms a semiselective barrier that regulates fluid balance and leukocyte trafficking. During the course of lung inflammation, neurohumoral mediators and oxidants act on endothelial cells to induce intercellular gaps permissive for transudation of proteinaceous fluid from blood into the interstitium. Intracellular signals activated by neurohumoral mediators and oxidants that evoke intercellular gap formation are incompletely understood. Cytosolic Ca2+ concentration ([Ca2+]i) and cAMP are two signals that importantly dictate cell-cell apposition. Although increased [Ca2+]i promotes disruption of the macrovascular endothelial cell barrier, increased cAMP enhances endothelial barrier function. Furthermore, during the course of inflammation, elevated endothelial cell [Ca2+]i decreases cAMP to facilitate intercellular gap formation. Given the significance of both [Ca2+]i and cAMP in mediating cell-cell apposition, this review addresses potential sites of cross talk between these two intracellular signaling pathways. Emerging data also indicate that endothelial cells derived from different vascular sites within the pulmonary circulation exhibit distinct sensitivities to permeability-inducing stimuli; that is, elevated [Ca2+]i promotes macrovascular but not microvascular barrier disruption. Thus this review also considers the roles of [Ca2+]i and cAMP in mediating site-specific alterations in endothelial permeability.

Modification of the pH dependence of animal and plant transport ATPases by sulfated polysaccharides

The effects of heparin and dextran sulfate 8,000 on two isoforms of the sarco/endoplasmic reticulum Ca(2+)-ATPase of different animal tissues and on the corn root H(+)-ATPase were examined. In the absence of sulfated polysaccharides the pH profile's of the three transport ATPases were quite different, but after the addition of heparin or dextran sulfate 8,000 the pH profiles of the three enzymes became similar, all showed maximal activity at pH 7.0. Potassium and sodium antagonized the effects of sulfated polysaccharides on the three transport ATPases, but the antagonism was considerably reduced at acidic pH values.

Characterization of the intracellular and the plasma membrane Ca2+-ATPases in fractionated pig brain membranes using calcium pump inhibitors

The Ca2+-ATPase activity of isolated membranes and purified plasma membrane ATPase from pig brain was measured in the presence of specific inhibitors. The inhibition of the enzymatic activity by vanadate presents a lower affinity in microsomes than in the synaptic plasma membrane vesicles, showing K0.5 of 0.4 and 0.2 microM, respectively. The purified enzyme showed a higher sensitivity to vanadate with a K0.5 of 0.10 microM. Thapsigargin (Tg) and 2,5-di(tert-butyl)-1,4-benzohydroquinone (BHQ) were stronger inhibitors of the Ca2+-ATPase activity in microsomes than in the synaptic membrane vesicles. The activity of the purified enzyme was not affected by Tg and only partially by BHQ. Cyclopiazonic acid inhibited the enzymatic activity in all fractions, being more sensitive in microsomes. The microsome preparation incorporated 32P from [gamma-32P]ATP into two main proteins that appear at approx 110,000 and 140,000. According to the inhibition pattern, the lower phosphorylated band was identified as the sarco(endo)plasmic reticulum Ca2+-ATPase, being in a higher percentage than the upper band. Synaptic membrane vesicles also incorporated radioactive 32P into two protein bands. The 140,000 protein (upper band) shows the typical behavior of the purified plasma membrane Ca2+-ATPase, being more abundant in this preparation than the organellar Ca2+-pump (lower band). This study highlights the heterogeneous nature of the Ca2+-ATPase activity measured in brain membrane fractions.

Molecular cloning of YlPMR1, a S. cerevisiae PMR1 homologue encoding a novel P-type secretory pathway Ca2+ -ATPase, in the yeast Yarrowia lipolytica

A novel P-type ATPase gene, Saccharomyces cerevisiae PMR1 homologue (YlPMR1), has been cloned and sequenced in the yeast, Yarrowia lipolytica. The putative gene product has 928 amino acids with a calculated molecular mass of 100050 Da and a pI of 5.15. The deduced amino-acid sequence analysis demonstrated that the cloned gene product contains all 10 of the conserved regions in P-type ATPases and exhibits 55% amino-acid identity to the S. cerevisiae PMR1 gene product; however, it shows a relatively lower homology to PMCA (24%) and SERCA (33%), confirming the presence of a third class of Ca2+-ATPase (secretory pathway Ca2+-ATPase, SPCA). The YlPMR1-disrupted strain shows defective growth in low Ca2+ or EGTA-containing medium. In fact, a longer lag time (60 h) was observed in YlPMR1-defective mutant cells during cultivation in EGTA-containing YPD medium. These growth defects were overcome by adding Ca2+ and Mn2+ into the medium. Interestingly, whereas Mn2+ inhibits growth of the control strain, it significantly improves the growth of YlPMR1-disrupted cells. These results suggest an involvement of the YlPMR1 gene product in Ca2+ and Mn2+ ion homeostasis in Y. lipolytica.

Recovery of Ca2+ pools and growth in Ca2+ pool-depleted cells is mediated by specific epoxyeicosatrienoic acids derived from arachidonic acid

Depletion of Ca2+ pools using the irreversible Ca2+ pump blocker, thapsigargin, induces DDT1MF-2 smooth muscle cells to enter a stable nonproliferative state. Reversal of this state can be mediated by high (20%) serum treatment, which induces new Ca2+ pump protein, return of Ca2+ pools, and reentry of cells into the cell cycle; the effect of serum can be mimicked by the essential fatty acids (EFA), arachidonic, linoleic, and alpha-linolenic acids (Graber, M.N., Alfonso, A., and Gill, D.L., (1996) J. Biol. Chem. 271, 883-888). The possible requirement for EFA metabolism in inducing recovery of Ca2+ pool-depleted growth-arrested cells was investigated. Neither cyclooxygenase or lipoxygenase inhibitors had any effect on arachidonic acid-induced growth recovery of thapsigargin-treated cells. In contrast, the cytochrome P-450 epoxygenase inhibitors, SKF525A and metyrapone, substantially reduced arachidonic acid-induced recovery of growth while having minimal effects on control cell growth. Both epoxygenase inhibitors completely prevented the arachidonic acid-induced recovery of bradykinin-releasable Ca2+-pumping pools, whereas cyclooxygenase and lipoxygenase inhibitors had no effect. The effectiveness of the four cytochrome P-450 metabolites of arachidonic acid on recovery of Ca2+ pools were compared; 8,9- and 11,12-epoxyeicosatrienoic acid (EET) at 1.5 microM were completely effective in recovering agonist-sensitive Ca2+ pools, whereas the 5,6- and 14,15-EETs were without effect. SKF525A did not block the action of 8,9- or 11, 12-EET indicating further P-450 metabolism was not required. Hydration of the active EET molecules prevented Ca2+ pool recovery since the dihydroxy-derivatives of both 8,9- and 11,12-EET were ineffective. The specificity of effectiveness among EET molecules for subsequent resumption of growth of thapsigargin-treated cells was the same as for Ca2+ pool recovery. Significantly, the P-450 inhibitors, SKF525A and metyrapone, both prevented the action of 20% serum in inducing recovery of thapsigargin-treated cells, whereas cyclooxygenase and lipoxygenase inhibitors were ineffective, indicating that EFAs are the active component within serum that is responsible for recovery of Ca2+ pool-depleted cells. The specific action of EETs in mediating recovery of Ca2+ pools and growth of thapsigargin-treated cells represents not only a novel action of epoxygenase products from EFAs, but also a potentially significant new signaling pathway that may effect translational control and regulate transition from a stationary to proliferative growth state.

Overexpression of the rat sarcoplasmic reticulum Ca2+ ATPase gene in the heart of transgenic mice accelerates calcium transients and cardiac relaxation

The Ca2+ ATPase of the sarcoplasmic reticulum (SERCA2) plays a dominant role in lowering cytoplasmic calcium levels during cardiac relaxation and reduction of its activity has been linked to delayed diastolic relaxation in hypothyroid and failing hearts. To determine the contractile alterations resulting from increased SERCA2 expression, we generated transgenic mice overexpressing a rat SERCA2 transgene. Characterization of a heterozygous transgenic mouse line (CJ5) showed that the amount of SERCA2 mRNA and protein increased 2. 6-fold and 1.2-fold, respectively, relative to control mice. Determination of the relative synthesis rate of SERCA2 protein showed an 82% increase. The mRNA levels of some of the other genes involved in calcium handling, such as the ryanodine receptor and calsequestrin, remained unchanged, but the mRNA levels of phospholamban and Na+/Ca2+ exchanger increased 1.4-fold and 1.8-fold, respectively. The increase in phospholamban or Na+/Ca2+ exchanger mRNAs did not, however, result in changes in protein levels. Functional analysis of calcium handling and contractile parameters in isolated cardiac myocytes indicated that the intracellular calcium decline (t1/2) and myocyte relengthening (t1/2) were accelerated by 23 and 22%, respectively. In addition, the rate of myocyte shortening was also significantly faster. In isolated papillary muscle from SERCA2 transgenic mice, the time to half maximum postrest potentiation was significantly shorter than in negative littermates. Furthermore, cardiac function measured in vivo, demonstrated significantly accelerated contraction and relaxation in SERCA2 transgenic mice that were further augmented in both groups with isoproterenol administration. Similar results were obtained for the contractile performance of myocytes isolated from a separate line (CJ2) of homozygous SERCA2 transgenic mice. Our findings suggest, for the first time, that increased SERCA2 expression is feasible in vivo and results in enhanced calcium transients, myocardial contractility, and relaxation that may have further therapeutic implications.

PMR1, a Ca2+-ATPase in yeast Golgi, has properties distinct from sarco/endoplasmic reticulum and plasma membrane calcium pumps

PMR1, a P-type ATPase cloned from the yeast Saccharomyces cerevisiae, was previously localized to the Golgi, and shown to be required for normal secretory processes (Antebi, A., and Fink, G.R. (1992) Mol. Biol. Cell 3, 633-654). We provide biochemical evidence that PMR1 is a Ca2+-transporting ATPase in the Golgi, a hitherto unusual location for a Ca2+ pump. As a starting point for structure-function analysis using a mutagenic approach, we used the strong and inducible heat shock promoter to direct high level expression of PMR1 from a multicopy plasmid. Yeast lysates were separated on sucrose density gradients, and fractions assayed for organellar markers. PMR1 is found in fractions containing the Golgi marker guanosine diphosphatase, and is associated with an ATP-dependent, protonophore-insensitive 45Ca2+ uptake activity. This activity is virtually abolished in the absence of the expression plasmid. Furthermore, replacement of the active site aspartate within the phosphorylation domain had the expected effect of abolishing Ca2+ transport activity entirely. Interestingly, the mutant enzymes (Asp-371 --> Glu and Asp-371 --> Asn) demonstrated proper targeting to the Golgi, unlike analogous mutations in the related yeast H+-ATPase. Detailed characterization of calcium transport by PMR1 showed that sensitivity to inhibitors (vanadate, thapsigargin, and cyclopiazonic acid) and affinity for substrates (MgATP and Ca2+) were different from the previously characterized sarco/endoplasmic reticulum and plasma membrane Ca2+-ATPases. PMR1 therefore represents a new and distinct P-type Ca2+-ATPase. Because close homologs of PMR1 have been cloned from rat and other organisms, we suggest that Ca2+-ATPases in the Golgi will form a discrete subgroup that are important for functioning of the secretory pathway.

Store-operated Ca2+ entry and coupling to Ca2+ pool depletion in thapsigargin-resistant cells

The release of Ca2+ from intracellular Ca2+ pumping pools and the entry of extracellular Ca2+ are tightly coupled events. The potent and specific intracellular Ca2+ pump inhibitor, thapsigargin, blocks Ca2+ accumulation and allows Ca2+ release from pools within mammalian cells, inducing major changes in endoplasmic reticulum function and cell growth. Recent studies characterized the pools of Ca2+ within permeabilized DC-3F/TG2 cells (a thapsigargin-resistant variant form of the DC-3F Chinese hamster lung fibroblast line, able to grow in 2 microM thapsigargin), revealing highly thapsigargin-resistant intracellular Ca2+ pumping activity capable of accumulating Ca2+ within an inositol 1,4,5-trisphosphate-releasable Ca2+ pool (Waldron, R. T., Short, A. D., and Gill, D. L. (1995) J. Biol. Chem. 270, 11955-11961). Using intact fura-2-loaded thapsigargin-resistant DC-3F/TG2 cells, the present study investigated the role of this unusual Ca2+ pumping activity in maintaining cytosolic Ca2+, generating Ca2+ signals, and mediating Ca2+ entry. The thapsigargin-resistant Ca2+ pumping pool was capable of generating rapid cytosolic Ca2+ signals in response to the phospholipase C-coupled agonist, oleoyl lysophosphatidic acid. The resting level of cytosolic Ca2+ in DC-3F/TG2 cells was 2-fold elevated compared with control cells (the parent DC-3F line), and transient extracellular Ca2+ removal induced a large "overshoot" in cytosolic Ca2+. The overshoot response was blocked by the Ca2+ influx inhibitor, SKF96365, and was kinetically identical to that induced in parent DC-3F cells after thapsigargin-induced Ca2+ pool emptying, indicating that the thapsigargin-resistant DC-3F/TG2 cells had "constitutively" opened Ca2+ entry channels coupled to an emptied or partially emptied thapsigargin-sensitive Ca2+ pumping pool. Even though oleoyl lysophosphatidic acid-mediated Ca2+ release induced little Ca2+ entry, complete ionomycin-activated emptying of the thapsigargin-resistant Ca2+ pool in DC-3F/TG2 cells induced a large, sustained entry of Ca2+ that was also completely blocked by SKF96365. The results revealed that the thapsigargin-resistant Ca2+ pump does maintain physiological Ca2+ levels, is able to fill an agonist-responsive Ca2+ pool in DC-3F/TG2 cells, and is likely responsible for the ability of these cells to function and grow in the presence of thapsigargin. In addition, Ca2+ influx in the resistant DC-3F/TG2 cells reflects emptying of pools that accumulate Ca2+ by both thapsigargin-sensitive and -resistant Ca2+ pumps; since these pumps accumulate Ca2+ in distinct pools in parent DC-3F cells, it is possible that more than one pool is coupled to Ca2+ influx in the resistant DC-3F/TG2 cells.

Membrane integrity and phospholipid movement influence the base exchange reaction in rat liver microsomes

Properties of Ca(2+)-stimulated incorporation of amincalcohols, serine and ethanolamine, into phospholipids, and factors regulating the reaction were studied in endoplasmic reticulum membranes isolated from rat liver. In contrast to apparent K(m) values for either aminoalcohol, maximal velocities of the reaction were significantly affected by Ca2+ concentration. No competition between these two soluble substrates used at equimolar concentrations close to their K(m) values was observed, suggesting the existence of two distinct phospholipid base exchange activities. The enzyme utilizing the electrically neutral serine was not sensitive to changes of membrane potential evoked by valinomycin in the presence of KCl. On the other hand, when positively charged ethanolamine served as a substrate, the enzyme activity was inhibited by 140 mM KCl and this effect was reversed by valinomycin. The rates of inhibition of phospholipid base exchange reactions by various thiol group modifying reagents were also found to differ. Cd2+ and lipophylic p-chloromercuribenzoic acid at micromolar concentrations were most effective. It can be suggested that -SH groups located within the hydrophobic core of the enzymes molecules are essential for the recognition of membrane substrates. However, the influence of the -SH group modifying reagents on the protein-facilitated phospholipid motion across endoplasmic reticulum membranes can not be excluded, since an integral protein-mediated transverse movement of phospholipids within the membrane bilayer and Ca(2+)-mediated changes in configuration of the phospholipid polar head groups seem to be a regulatory step of the reaction. Indeed, when the membrane integrity was disordered by detergents or an organic solvent, the reaction was inhibited, although not due to the transport of its water-soluble substrates is affected, but due to modulation of physical state of the membrane bilayer and, in consequence, the accessibility of phospholipid molecules.

Sites of regulatory interaction between calcium ATPases and phospholamban

In an effort to define the amino acids that are involved in functional interactions between phospholamban (PLN) and the Ca2+ ATPase of cardiac sarcoplasmic reticulum (SERCA2), we have co-expressed wild type and mutant forms of phospholamban with wild type and mutant forms of SERCA2, isolated microsomal fractions and measured Ca2+ dependence of Ca2+ transport. We have found that both charged and hydrophobic residues in the cytoplasmic domains of both PLN and SERCA2 make up the cytoplasmic interaction site. In SERCA2, this site is the linear sequence Lys-Asp-Asp-Lys-Pro-Val402: In PLN, the site is more diffuse and complex. Function was retained if the net charge over the first 20 amino acids was +1 or +2, but function was lost if the net charge was -3, -2, 0 or +3. Function was also lost if the long alkyl side chains of Val4, Leu7 or Ile12 were replaced with the methyl group of Ala. We have also obtained evidence that a site of functional interaction is present in the transmembrane domains of PLN and SERCA2.

Changes in mRNA levels of the sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase isoforms in the rat soleus muscle regenerating from notexin-induced necrosis

The relative mRNA levels corresponding to the different sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase isoforms (SERCA1a, SERCA1b, SERCA2a, SERCA2b and SERCA3) were measured by reverse transcriptase-PCR in rat soleus muscles regenerating after notexin-induced necrosis. The succession of appearance of the different types of SERCA mRNA species in regenerating muscle largely recapitulates those observed during normal ontogenesis. The mRNA levels of the muscle-specific isoforms SERCA1a and SERCA2a became very low on the first and third days after injection of the snake venom. It was only on the fifth day of regeneration that the mRNA of the neonatal variant of the fast-twitch skeletal SERCA1b isoform began to rise, well before the other SERCA transcripts. At 7 and 10 days, i.e. at a time when the new myofibres normally become reinnervated, the mRNA level of SERCA1a and SERCA2a increased markedly, but the fast-twitch skeletal SERCA1a isoform was still the most prominent. On day 21, in the advanced stage of regeneration, a switch in the relative expression levels of SERCA1a and SERCA2a mRNA was observed and the ratio of both isoforms became similar to that found in the normal soleus muscles. This was followed by a decline in the level of all SERCA mRNA species, so that on day 28 the levels of the sarcoplasmic/endoplasmatic-reticulum Ca(2+)-pump RNAs was again lower but their ratio remained similar to that of the untreated control soleus.

Evidence for the presence of two (Ca(2+)-Mg2+) ATPases with different sensitivities to thapsigargin and cyclopiazonic acid in the human flatworm Schistosoma mansoni

The subcellular localization of the (Ca(2+)-Mg2+)ATPase activities present in heterogeneous (P1), nuclear (P2), mitochondrial (P3) and microsomal (P4) fractions obtained by differential centrifugation of Schistosoma mansoni homogenate was investigated. In the microsomal fraction (P4), the (Ca(2+)-Mg2+)ATPase activity was completely blocked by 3 microM thapsigargin, whereas in the more heterogeneous fraction (P1), about 20-30% of this activity was resistant to the drug. The same pattern of inhibition was observed using 20 microM cyclopiazonic acid. The distribution pattern of (Ca(2+)-Mg2+)ATPase activity among the four subcellular fractions (P1 > P4 > > P3 > P2) was completely different from that of [3H]-ouabain binding sites (P1 > or = P4 = P2 > or = P3). These results indicate that the (Ca(2+)-Mg2+)ATPase in S. mansoni is predominantly of the SERCA type (localized in the endoplasmic reticulum). However, there is another enzyme, present in lower proportion that could have a plasma membrane origin (PMCA type), because it is resistant to thapsigargin and cyclopiazonic acid and its inhibition by tamoxifen is antagonized by calmodulin.

Ca2+ pools and cell growth: arachidonic acid induces recovery of cells growth-arrested by Ca2+ pool depletion

The intracellular Ca2+ pump blocker, thapsigargin, induces emptying of Ca2+ pools and entry of DDT1MF-2 smooth muscle cells into a quiescent G(0)-like growth state. Although thapsigargin blocks pumps essentially irreversibly, high serum (20%) induces appearance of new pump protein, return of functional pools, and reentry of cells into the cell cycle (Waldron, R. T., Short, A. D., Meadows, J.J., Ghosh, T. K., and Gill, D. L. (1994) J. Biol. Chem. 269, 11927-11933). Through analysis of the effects of defined serum components and growth supplements, we reveal here that the factors in serum responsible for inducing recovery of Ca2+ pools and growth in thapsigargin-arrested DDT1MF-2 cells are exactly mimicked by the three essential fatty acids, arachidonic, linoleic, and alpha-linolenic acids. The EC50 values for arachidonic and linoleic acids on growth induction of thapsigargin-arrested cells were the same, approximately 5 microM. Nonessential fatty acids, including myristic, palmitic, stearic, oleic, and arachidic acids, were without any effect. Although not proven to be the active component of serum, levels of arachidonic and linoleic acids in serum were sufficient to explain serum-induced growth recovery. Significantly, arachidonic or linoleic acids induced complete recovery of bradykinin-sensitive Ca2+ pools within 6 h of treatment of thapsigargin-arrested cells. Protein synthesis inhibitors (cycloheximide or puromycin) completely blocked the appearance of serum-induced or arachidonic acid-induced agonist-sensitive pools. The sensitivity and fatty acid specificity of Ca2+ pool recovery in thapsigargin-arrested cells were almost identical to that for growth recovery. No pool or growth recovery was observed with 5,8,11,14-eicosatetraynoic acid, the nonmetabolizable analogue of arachidonic acid, suggesting that conversion to eicosanoids underlies the pool and growth recovery induced by essential fatty acids. The results provide not only further information on the link between Ca2+ pools and cell growth but also evidence for a potentially important signaling pathway involved in inducing transition from a stationary to a proliferative growth state.

Bidirectional activity of the endoplasmic reticulum Ca(2+)-ATPase of bovine adrenal cortex

It is generally accepted that the ryanodine receptor and the inositol 1,4,5-trisphosphate receptor play major roles in the complex mechanisms by which agonists increase intracellular Ca2+ concentration. In these mechanisms, the endoplasmic reticulum Ca(2+)-ATPase has been attributed an accessory role of refilling the intracellular Ca2+ store. In the present study, the activity of the microsomal Ca(2+)-ATPase of bovine adrenal cortex was investigated. We show that the Ca(2+)-pumping activity of the Ca(2+)-ATPase is related to the ADP/ATP ratio. Our results also show that a brisk increase of the ADP/ATP ratio upon addition of exogenous ADP triggered a rapid release of Ca2+ from preloaded microsomes. ADP released Ca2+ in a dose-dependent manner with an EC50 of 2.98 +/- 0.78 mM. ADP-induced Ca2+ release was not prevented by heparin, ruling out the participation of the inositol 1,4,5-trisphosphate receptor. ADP-induced Ca2+ release could not be attributed to the mere inhibition of the Ca(2+)-ATPase, since the rate of ADP-induced Ca2+ release was 20 times faster than the rate of Ca2+ release induced by a maximal concentration of thapsigargin (2 microM). ADP-induced Ca2+ release experiments performed in the presence of [32P]PO4 revealed a concomitant production of [32P]ATP. ADP-induced [32P]ATP production was dose-dependent, with an EC50 of 5.50 +/- 0.70 mM. ADP-induced [32P]ATP production was prevented by ionomycin (10 microM) and by high concentrations of extramicrosomal Ca2+. These results demonstrate that the microsomal Ca(2+)-ATPase of adrenal cortex possesses a bidirectional activity that depends on ADP concentrations, the Ca2+ gradient across the microsomal membrane, and probably also ATP concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium pump kinetics determined in single erythrocyte ghosts by microphotolysis and confocal imaging

The activity of the plasma membrane calcium pump was measured in single cells. Human red blood cell ghosts were loaded with a fluorescent calcium indicator and either caged calcium and ATP (protocol A) or caged ATP and calcium (protocol B). In a suitably modified laser scanning microscope either calcium or ATP were released by a short UV light pulse. The time-dependent fluorescence intensity of the calcium indicator was then followed in single ghosts by repetitive confocal imaging. The fluorescence intensity was converted into calcium concentration, which in turn was used to derive the kinetic parameters of the calcium pump, the Michaelis-Menten constant Km, and the maximal transport rate vmax. Km and vmax values derived in this manner were 24 +/- 14 microM and 1.0 +/- 0.6 microM/(ghost s) for protocol A, and 4 +/- 3 microM and 1.0 +/- 0.6 microM/(ghost s) for protocol B, respectively. The difference between A and B is presumably caused by calmodulin, which is inactive in the experiments with protocol A. The possibilities to extend the new method to living nucleus-containing cells transiently transfected with mutants of the plasma membrane calcium pump are discussed.

Thapsigargin-resistant intracellular calcium pumps. Role in calcium pool function and growth of thapsigargin-resistant cells

Exposure of cells to the intracellular Ca2+ pump blocker, thapsigargin (TG), results in emptying of Ca2+ pools and termination of cell proliferation (Short, A. D., Bian, J., Ghosh, T. K., Waldron, R. T., Rybak, S. L., and Gill, D. L. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 4986-4990). DC-3F Chinese hamster lung cells were made resistant to TG by long-term stepwise exposure to increasing TG concentrations in culture (Gutheil, J. C., Hart, S. R., Belani, C. P., Melera, P. W., and Hussain, A. (1994) J. Biol. Chem. 269, 7976-7981). Since these cells (DC-3F/TG2) grow in the presence of TG, it was important to ascertain what Ca2+ pool function they retain. TG-resistant DC-3F/TG2 cells cultured with 2 microM TG had a doubling time (24 h) not significantly different from the parent DC-3F cells without TG. Analysis of TG-induced inhibition of 45Ca2+ uptake into permeabilized parent DC-3F cells revealed two distinct Ca2+ pump activities with 20,000-fold different sensitivities to TG; the IC50 values for TG were 200 pM and 4 microM, representing 80% and 20% of total pumping activity, respectively. Total pump activity in parent DC-3F and resistant DC-3F/TG2 cells was similar (0.23 +/- 0.10 and 0.18 +/- 0.08 nmol of Ca2+/10(6) cells, respectively). In DC-3F/TG2 cells, up to 100 nM TG had no effect on Ca2+ pumping; however, almost all pumping was blocked at higher TG concentrations with an IC50 of 5 microM. In both cell types, each Ca2+ pump activity (regardless of TG sensitivity) had high Ca2+ affinity (Km values congruent to 0.1 microM) and similar ATP dependence and vanadate sensitivity. In DC-3F cells, the TG-sensitive Ca2+ pool was releasable with inositol 1,4,5-trisphosphate (InsP3) or GTP and was oxalate-permeable; the TG-insensitive pool in these cells was not InsP3-releasable. GTP-induced Ca2+ uptake in the presence of oxalate indicated Ca2+ transfer between distinct pools in the DC-3F cells. In resistant DC-3F/TG2 cells, almost 50% of total TG-insensitive Ca2+ accumulation was releasable with InsP3; unlike the parent cells, this pool was not oxalate-permeable, and GTP induced no Ca2+ transfer between pools in the presence of oxalate. Thus, whereas InsP3 releases Ca2+ only from the high TG sensitivity Ca2+ pumping pool in parent DC-3F cells, in resistant DC-3F/TG2 cells the TG-resistant Ca2+ pumping pool now contains functional InsP3 receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of HIV expression by intracellular calcium pump inhibition

We have studied the role of intracellular calcium sequestration on human immunodeficiency virus (HIV) production by latently infected T-lymphocytic cells. Inhibition of the sarco-endoplasmic reticulum-type calcium transport ATPases by thapsigargin or cyclopiazonic acid induced activation of HIV production in the CEM-derived ACH-2 cells. An approximately 50% depletion of the thapsigargin-sensitive calcium pools as measured fluorimetrically of Indo-loaded cells fully activated virus production. Viral activation was manifest by increases in soluble viral core p24 production, increases in cellular immunofluorescent staining for viral antigens, and increased viral transcription as measured by HIV long terminal repeat-directed expression of the chloramphenicol acetyltransferase reporter gene. Virus induction could be blocked in a dose-dependent manner by the calcium channel blocker econazole. Virus production by the Jurkat-derived HIV-1-inducible J1.1 cells was not significantly stimulated by thapsigargin. These data indicate that intracellular calcium pool function is involved in the control of the transcription of proviral HIV in a cell type-specific manner within the T-lymphoid lineage and that ACH-2 cells represent a useful model for the study of calcium dependent activation of the transcription of proviral HIV.

Localization and identification of Ca2+ATPases in highly purified human platelet plasma and intracellular membranes. Evidence that the monoclonal antibody PL/IM 430 recognizes the SERCA 3 Ca2+ATPase in human platelets

The Ca2+ATPase activities of highly purified human platelet membranes prepared by high-voltage free-flow electrophoresis have been analysed by using [gamma-32P]ATP hydrolysis, recognition by antibodies and phosphoenzyme-complex formation. The Ca2+ATPase activity present in mixed membranes was found to be predominantly associated with intracellular membranes after subfractionation, with only a low level of activity associated with plasma membranes. The intracellular-membrane Ca2+ATPase activity was inhibited totally with thapsigargin (Tg), whereas the plasma-membrane Ca2+ATPase was not significantly affected, suggesting that the latter does not belong to the SERCA (sarco-endoplasmic-reticulum Ca2+ATPase) class. A monoclonal antibody, 5F10, raised to the red-cell membrane Ca2+ATPase [Cheng, Magocsi, Cooper, Penniston and Borke (1993) Cell Physiol. Biochem. 4, 31-43] recognized two bands at 135 and 150 kDa in mixed membranes and plasma membranes, and the corresponding bands in red-blood-cell membranes, confirming the Ca2+ATPase to be of the PMCA (plasma-membrane Ca2+ATPase) type. No recognition of any band was detected in intracellular membranes. Identification of the intracellular-membrane Ca2+ATPase activity was carried out with polyclonal antibodies with known specificity towards SERCA 2b (S.2b) and SERCA 3 (N89), and a monoclonal antibody, PL/IM 430, raised against platelet intracellular membranes. All of these antibodies recognized the 100 kDa Ca2+ATPase in mixed membranes and intracellular membranes, with little or no recognition of the activity in the plasma membranes. In some membrane preparations the antibody PL/IM 430 and antiserum N89 recognized similar degradation products, of 74, 70 and 40 kDa, in the intracellular-membrane fraction. The Ca2+ATPase recognized by PL/IM 430 was immunoprecipitated, and the immunoprecipitated protein was specifically recognized by the antiserum N89, but not by S.2b. Analysis of the phosphoenzyme-complex formation revealed potent phosphorylation of the 100 and 74 kDa peptides, both recognized by PL/IM 430 and N89. These studies report the presence of a PMCA in a purified plasma-membrane fraction from human platelets, and that the antibody PL/IM 430 recognizes the SERCA 3 Ca2+ATPase in intracellular membranes.

Ischemia-induced changes in catecholamine release and their mechanisms: a study using cultured bovine adrenal chromaffin cells

Ischemia-induced changes in neurotransmitter release and their mechanisms were examined using cultured bovine adrenal chromaffin cells. When the cells were incubated in glucose-free media equilibrated with 0% O2/100% N2 (ischemia), ATP content decreased and reached the minimum level within 40 min. Control incubation was done in media equilibrated with 21% O2 in N2. After 10-min incubation under ischemic conditions, basal catecholamine (CA) release was elevated and the elevation persisted up to 90 min. High K(+)-evoked CA release was transiently enhanced at 10 min, but after that, it decreased to reach the minimum level at 60 min. At 10 min, cytosolic free Ca2+ concentration ([Ca2+]i) and 45Ca2+ uptake of the resting cells (basal values) and high K(+)-evoked increases in these two parameters were unchanged, but CA release from permeabilized cells in response to Ca2+ in media was augmented. After 60-min incubation under ischemic conditions, basal [Ca2+]i was elevated: the elevation was observed even in the absence of extracellular Ca2+. In contrast, high K(+)-evoked increases in [Ca2+]i and in 45Ca2+ uptake were suppressed, but basal 45Ca2+ uptake into intact cells and CA release from permeabilized cells were unchanged. These results suggest that in an early phase (10 min) of ischemia, both basal and stimulation-evoked CA release are augmented because of increased sensitivity of exocytotic machinery to Ca2+. In the late phase (60 min), basal CA release is augmented because of an increase in basal [Ca2+]i, which is due to accumulation of Ca2+ derived from intracellular Ca2+ pools: stimulation-evoked CA release is suppressed because of inhibition of stimulation-evoked increase in [Ca2+]i, which is due to functional disturbance of voltage-dependent Ca2+ channels.