Mechanism of inhibition of the calcium pump of sarcoplasmic reticulum by thapsigargin

Novel identification and modulation of the mechanosensitive Piezo1 channel in human myometrium

Approximately 10% of US births deliver preterm before 37 weeks of completed gestation. Premature infants are at risk for life-long debilitating morbidities and death, and spontaneous preterm labour explains 50% of preterm births. In all cases existing treatments are ineffective, and none are FDA approved. The mechanisms that initiate preterm labour are not well understood but may result from dysfunctional regulation of quiescence mechanisms. Human pregnancy is accompanied by large increases in blood flow, and the uterus must enlarge by orders of magnitude to accommodate the growing fetus. This mechanical strain suggests that stretch-activated channels may constitute a mechanism to explain gestational quiescence. Here we identify for the first time that Piezo1, a mechanosensitive cation channel, is present in the uterine smooth muscle and microvascular endothelium of pregnant myometrium. Piezo is downregulated during preterm labour, and stimulation of myometrial Piezo1 in an organ bath with the agonist Yoda1 relaxes the tissue in a dose-dependent fashion. Further, stimulation of Piezo1 while inhibiting protein kinase A, AKT, or endothelial nitric oxide synthase mutes the negative inotropic effects of Piezo1 activation, intimating that actions on the myocyte and endothelial nitric oxide signalling contribute to Piezo1-mediated contractile dynamics. Taken together, these data highlight the importance of stretch-activated channels in pregnancy maintenance and parturition, and identify Piezo1 as a tocolytic target of interest. KEY POINTS: Spontaneous preterm labour is a serious obstetric dilemma without a known cause or effective treatments. Piezo1 is a stretch-activated channel important to muscle contractile dynamics. Piezo1 is present in the myometrium and is dysregulated in women who experience preterm labour. Activation of Piezo1 by the agonist Yoda1 relaxes the myometrium in a dose-dependent fashion, indicating that Piezo1 modulation may have therapeutic benefits to treat preterm labour.

A mouse model of human mitofusin-2-related lipodystrophy exhibits adipose-specific mitochondrial stress and reduced leptin secretion

Mitochondrial dysfunction has been reported in obesity and insulin resistance, but primary genetic mitochondrial dysfunction is generally not associated with these, arguing against a straightforward causal relationship. A rare exception, recently identified in humans, is a syndrome of lower body adipose loss, leptin-deficient severe upper body adipose overgrowth, and insulin resistance caused by the p.Arg707Trp mutation in MFN2, encoding mitofusin 2. How the resulting selective form of mitochondrial dysfunction leads to tissue- and adipose depot-specific growth abnormalities and systemic biochemical perturbation is unknown. To address this, Mfn2R707W/R707W knock-in mice were generated and phenotyped on chow and high fat diets. Electron microscopy revealed adipose-specific mitochondrial morphological abnormalities. Oxidative phosphorylation measured in isolated mitochondria was unperturbed, but the cellular integrated stress response was activated in adipose tissue. Fat mass and distribution, body weight, and systemic glucose and lipid metabolism were unchanged, however serum leptin and adiponectin concentrations, and their secretion from adipose explants were reduced. Pharmacological induction of the integrated stress response in wild-type adipocytes also reduced secretion of leptin and adiponectin, suggesting an explanation for the in vivo findings. These data suggest that the p.Arg707Trp MFN2 mutation selectively perturbs mitochondrial morphology and activates the integrated stress response in adipose tissue. In mice, this does not disrupt most adipocyte functions or systemic metabolism, whereas in humans it is associated with pathological adipose remodelling and metabolic disease. In both species, disproportionate effects on leptin secretion may relate to cell autonomous induction of the integrated stress response.

Excitation-contraction coupling in mammalian skeletal muscle: Blending old and last-decade research

The excitation–contraction coupling (ECC) in skeletal muscle refers to the Ca²⁺-mediated link between the membrane excitation and the mechanical contraction. The initiation and propagation of an action potential through the membranous system of the sarcolemma and the tubular network lead to the activation of the Ca²⁺-release units (CRU): tightly coupled dihydropyridine and ryanodine (RyR) receptors. The RyR gating allows a rapid, massive, and highly regulated release of Ca²⁺ from the sarcoplasmic reticulum (SR). The release from triadic places generates a sarcomeric gradient of Ca²⁺ concentrations ([Ca²⁺]) depending on the distance of a subcellular region from the CRU. Upon release, the diffusing Ca²⁺ has multiple fates: binds to troponin C thus activating the contractile machinery, binds to classical sarcoplasmic Ca²⁺ buffers such as parvalbumin, adenosine triphosphate and, experimentally, fluorescent dyes, enters the mitochondria and the SR, or is recycled through the Na⁺/Ca²⁺ exchanger and store-operated Ca²⁺ entry (SOCE) mechanisms. To commemorate the 7^th decade after being coined, we comprehensively and critically reviewed “old”, historical landmarks and well-established concepts, and blended them with recent advances to have a complete, quantitative-focused landscape of the ECC. We discuss the: 1) elucidation of the CRU structures at near-atomic resolution and its implications for functional coupling; 2) reliable quantification of peak sarcoplasmic [Ca²⁺] using fast, low affinity Ca²⁺ dyes and the relative contributions of the Ca²⁺-binding mechanisms to the whole concert of Ca²⁺ fluxes inside the fibre; 3) articulation of this novel quantitative information with the unveiled structural details of the molecular machinery involved in mitochondrial Ca²⁺ handing to understand how and how much Ca²⁺ enters the mitochondria; 4) presence of the SOCE machinery and its different modes of activation, which awaits understanding of its magnitude and relevance in situ; 5) pharmacology of the ECC, and 6) emerging topics such as the use and potential applications of super-resolution and induced pluripotent stem cells (iPSC) in ECC. Blending the old with the new works better!

Tetramethylpyrazine alleviates endoplasmic reticulum stress‑activated apoptosis and related inflammation in chondrocytes

Excessive apoptosis of chondrocytes and degradation of the extracellular matrix (ECM) contribute to the typical pathological characteristics of osteoarthritis (OA). Various studies have reported that tetramethylpyrazine (TMP) protects against multiple disorders by inhibiting inflammation and oxidative stress. The present study investigated the effects of TMP on chondrocytes and evaluated the associated mechanisms. To determine the effect of TMP on OA and the underlying mechanisms, chondrocytes were incubated with TMP and IL-1β or thapsigargin (TG) Western blotting assays were performed to examine the expression levels of endoplasmic reticulum (ER) stress proteins, and TUNEL staining, fluorescence immunostaining and reverse transcription-quantitative PCR were used to determine the apoptosis levels, and catabolic and inflammatory factors. It was found that TMP protected chondrocytes by suppressing IL-1β-induced expression of glucose-regulated protein 78 (GRP78) and CHOP (an apoptotic protein). TMP regulated the TG-mediated upregulated expression of GRP78 and CHOP in the chondrocytes of rats, as well as markedly suppressed levels of ER stress-triggered inflammatory cytokines (TNF-α and IL-6). Furthermore, TMP modulated TG-induced changes in ECM catabolic metabolism in rat chondrocytes. Collectively, TMP alleviated ER-stress-activated apoptosis and related inflammation in chondrocytes, indicating that it has therapeutic potential for the treatment of OA.

Sulforaphane protects against oxidative stress‑induced apoptosis via activating SIRT1 in mouse osteoarthritis

Osteoarthritis (OA), the most common form of human joint disease, is characterized by progressive degeneration of the articular cartilage, synovitis and subchondral osteoporosis. Chondrocyte apoptosis is the primary pathogenic mechanism of OA and is considered to be a potential therapeutic target. Sulforaphane (SFN), a dietary isothiocyanate obtained from cruciferous vegetables, has been reported to exert an anti‑apoptotic effect by activating sirtuin 1 (SIRT1). To the best of our knowledge, however, the effects of SFN on apoptotic responses in OA have not been reported. In the present study, SFN was shown to significantly inhibit chondrocyte apoptosis while enhancing expression levels of SIRT1 in a H2O2‑induced OA mouse model. The anti‑apoptotic effect of SFN was reversed by SIRT1 small interfering RNA, implying that SIRT1 exerted a protective role against the effect of SFN on chondrocytes. The expression levels of C/EBP homologous protein, 78‑kDa glucose regulated protein, Bax, Bcl‑2 and cleaved caspase 3 were found to be downregulated in SFN‑treated mice. Furthermore, SFN ameliorated cartilage degradation in the OA mouse model. These findings indicate that SFN exerted an anti‑apoptotic effect on chondrocytes and ameliorated OA in vivo by activating the SIRT1 signaling pathway.

Effect of Rosolic acid on endothelial dysfunction under ER stress in pancreatic microenvironment

Endothelial cell (EC) dysfunction is the underlying cause for the development of several pathologies, and the interdependency between the pancreatic β-cells and ECs has been established in the pathophysiology of diabetes. ECs release several factors that govern the expression of genes involved in the proliferation, physiology, and survival of the β-cells. Of the known factors that collapse this intricately balanced system, endothelial dysfunction is the crucial condition that manifests as the causative factor for micro and macrovascular diseases. Our earlier studies demonstrated that activation of nuclear factor erythroid-related factor (Nrf2) renders protection to the ECs experiencing ER stress. In this study, using a co-culture system, the crosstalk between pancreatic cells under ER stress and ECs and the effect of a novel Nrf2 activator Rosolic Acid (RA), on the crosstalk was investigated. ECs pre-treated with different concentrations RA and co-cultured with thapsigargin-induced ER stressed pancreatic β-cells showed increased levels of Nrf2 and its downstream targets such as heme oxygenase-1 (HO-1) and NADPH-quinone oxidoreductase-1 (NQO-1), and reduction of ER stress evinced by the decreased levels of glucose-regulated protein (GRP) 78 and C/ERB homologous protein (CHOP). The sensitization of ECs using RA, offered protection to pancreatic cells against ER stress as displayed by increased intracellular insulin and upregulated expression of cell survival and proliferative genes BCl2 and PDX-1. In addition, RA treatment resulted in elevated levels of various angiogenic factors, while inflammatory (TNF-α and IL-1β) and apoptotic markers (CXCL10 and CCL2) decreased. RA treatment normalized the levels of 115 proteins of the 277, which were differentially regulated as revealed by proteomic studies of ER stressed pancreatic β-cells in co-culture conditions. These findings clearly indicate the role of small molecule activators of Nrf2 not only in restoring the functioning of pancreatic cells but also in increasing the cell mass. Further, the study impinges on the strategies that can be developed to balance the pancreatic microenvironment, leading to the restoration of β-cell mass and their normophysiology in diabetic patients.

Age-related alterations in fertilization-induced Ca2+ oscillations depend on the genetic background of mouse oocytes†

Maternal aging affects various aspects of oocytes' physiology, including the functionality of their nuclear apparatus and mitochondria. In the present paper, we wished to investigate whether advanced reproductive age impacts oocytes' ability to generate proper Ca2+ oscillations in response to monospermic fertilization. We examined three different mouse strains/crosses: inbred C57BL/6Tar, outbred Tar:SWISS, and hybrid F1 (C57BL/6Tar × CBA/Tar). The females were either 2-4 months old (young) or 13-16 months old (aged). We observed that the Ca2+ oscillatory pattern is altered in a strain-dependent manner and changes were more profound in aged C57BL/6Tar and F1 than in aged Tar:SWISS oocytes. We also showed that maternal aging differently affects the size of Ca2+ store and expression of Itpr1, Atp2a2, Erp44, and Pdia3 genes involved in Ca2+ homeostasis in oocytes of C57BL/6Tar, Tar:SWISS, and F1 genetic background, which may explain partially the differences in the extent of age-dependent changes in the Ca2+ oscillations in those oocytes. Maternal aging did not have any visible impact on the distribution of the ER cisterns in oocytes of all three genetic types. Finally, we showed that maternal aging alters the timing of the first embryonic interphase onset and that this timing correlates in C57BL/6Tar and Tar:SWISS oocytes with the frequency of fertilization-induced Ca2+ oscillations. Our results indicate that extreme caution is required when conclusions about oocyte/embryo physiological response to aging are made and complement an increasing amount of evidence that mammalian (including human) susceptibility to aging differs greatly depending on the genetic background of the individual.

Bile canaliculi contract autonomously by releasing calcium into hepatocytes via mechanosensitive calcium channel

Drug-induced hepatocellular cholestasis leads to altered bile flow. Bile is propelled along the bile canaliculi (BC) by actomyosin contractility, triggered by increased intracellular calcium (Ca²⁺). However, the source of increased intracellular Ca²⁺ and its relationship to transporter activity remains elusive. We identify the source of the intracellular Ca²⁺ involved in triggering BC contractions, and we elucidate how biliary pressure regulates Ca²⁺ homeostasis and associated BC contractions. Primary rat hepatocytes were cultured in collagen sandwich. Intra-canalicular Ca²⁺ was measured with fluo-8; and intra-cellular Ca²⁺ was measured with GCaMP. Pharmacological modulators of canonical Ca²⁺-channels were used to study the Ca²⁺-mediated regulation of BC contraction. BC contraction correlates with cyclic transfer of Ca²⁺ from BC to adjacent hepatocytes, and not with endoplasmic reticulum Ca²⁺. A mechanosensitive Ca²⁺ channel (MCC), Piezo-1, is preferentially localized at BC membranes. The Piezo-1 inhibitor GsMTx-4 blocks the Ca²⁺ transfer, resulting in cholestatic generation of BC-derived vesicles whereas Piezo-1 hyper-activation by Yoda1 increases the frequency of Ca²⁺ transfer and BC contraction cycles. Yoda1 can recover normal BC contractility in drug-induced hepatocellular cholestasis, supporting that Piezo-1 regulates BC contraction cycles. Finally, we show that hyper-activating Piezo-1 can be exploited to normalize bile flow in drug-induced hepatocellular cholestasis.

Sterile activation of invariant natural killer T cells by ER-stressed antigen-presenting cells

Invariant NKT (iNKT) cells have the unique ability to shape immunity during antitumor immune responses and other forms of sterile and nonsterile inflammation. Recent studies have highlighted a variety of classes of endogenous and pathogen-derived lipid antigens that can trigger iNKT cell activation under sterile and nonsterile conditions. However, the context and mechanisms that drive the presentation of self-lipid antigens in sterile inflammation remain unclear. Here we report that endoplasmic reticulum (ER)-stressed myeloid cells, via signaling events modulated by the protein kinase RNA-like ER kinase (PERK) pathway, increase CD1d-mediated presentation of immunogenic endogenous lipid species, which results in enhanced iNKT cell activation both in vitro and in vivo. In addition, we demonstrate that actin cytoskeletal reorganization during ER stress results in an altered distribution of CD1d on the cell surface, which contributes to enhanced iNKT cell activation. These results define a previously unidentified mechanism that controls iNKT cell activation during sterile inflammation.

Vitexin alleviates ER-stress-activated apoptosis and the related inflammation in chondrocytes and inhibits the degeneration of cartilage in rats

Excessive extracellular matrix degradation and chondrocyte apoptosis are the pathological features of osteoarthritis (OA).

Panax quinquefolium saponin inhibits endoplasmic reticulum stress-induced apoptosis and the associated inflammatory response in chondrocytes and attenuates the progression of osteoarthritis in rat

Treatments for osteoarthritis (OA) seek to restore chondrocyte function and inhibit cell apoptosis. Panax quinquefolium saponin (PQS) is the major active ingredient of Radix panacis quinquefolii (American ginseng), and has been demonstrated to exert anti-inflammatory and anti-apoptotic effects in various diseases. However, any potential effect of PQS on the pathological process of OA remains unclear. This work aimed to explore the role of PQS in chondrocytes and to clarify its potential mechanisms. We showed that PQS treatment could protect chondrocytes against endoplasmic reticulum (ER) stress and associated apoptosis induced by interleukin (IL)-1β. Also, PQS further attenuated triglyceride (TG)-induced ER stress and associated apoptosis. Moreover, PQS may inhibit the ER stress-activated NF-κB pathway and associated inflammatory response in chondrocytes. Finally, PQS abolished rat cartilage degeneration in an in-vivo OA model of the knee joint. Our results indicate that PQS may be a potential novel treatment for OA.

Calcium mobilizing treatment acts as a co-signal for TLR-mediated induction of Interleukin-12 (IL-12p70) secretion by murine bone marrow-derived dendritic cells

We sought to determine whether pharmacological calcium-mobilizing agents could act in cooperation with Toll-like receptor (TLR) signals to induce high-level IL-12 production from murine bone marrow-derived dendritic cells. We found that calcium mobilization alone induced no IL-12, yet dramatically enhanced IL-12p70 secretion elicited by TLR ligands. Enhanced IL-12 production induced by calcium ionophore plus single TLR ligands, but not through dual TLR ligands, was inhibited by the calcineurin antagonist cyclosporine A, suggesting divergent mechanisms of IL-12 induction. Dendritic cells activated with calciumionophore plus the TLR9 ligand ODN1826 could induce Th1 polarization in naïve murine CD4^pos T cells at levels equal or superior to dendritic cells activated with the most efficient TLR ligand pairing; ODN1826 plus bacterial lipopolysaccharide. Parallel analysis of 38 inflammation-associated soluble products showed calciumionophore enhancement was restricted to a small set of factors. These data demonstrate previously undocumented activation co-signals for IL-12 production by dendritic cells.

The unfolded protein response genes in human osteoarthritic chondrocytes: PERK emerges as a potential therapeutic target

BACKGROUND

The unfolded protein response (UPR) is activated following an endoplasmic reticulum (ER) stress. The aim of this study was to investigate the global expression of UPR genes in human OA chondrocytes in induced (I)-UPR conditions, and to explore the regulation and role of the UPR genes in homeostatic (H)-UPR conditions in human normal and OA chondrocytes.

METHODS

Gene expression was determined by PCR array and qPCR. Protein production in cartilage was determined by immunohistochemistry, gene silencing by specific siRNAs, and gene regulation by treating chondrocytes with cytokines and growth factors associated with cartilage pathobiology.

RESULTS

Several UPR genes, among them ERN1, PERK, and CREB3L2 were downregulated in OA compared to normal chondrocytes at both the mRNA and protein levels, but the ER stress response triggered by thapsigargin or tunicamycin treatment was similar in normal and OA chondrocytes. The activation of ER stress sensors (phosphorylated PERK, cleavage of ATF6B, and the spliced mRNA forms of XBP1) was not significantly increased in OA chondrocytes/cartilage. PDGF-BB and IL-6 significantly downregulated the expression of ERN1, PERK, and CREB3L2, but not that of ATF6B. Silencing experiments done under conditions of no ER stress (physiological conditions) revealed that decreasing ERN1 expression led to decreased COL2a1, MMP-13, ADAMTS4 and ADAMTS5 expression, while decreasing CREB3L2 and ATF6B led to decreased ADAMTS5 and ADAMTS4 expression, respectively. Importantly, the downregulation of PERK expression increased COL1a1 and suppressed COL2a1 expression.

CONCLUSIONS

Although the level of ER stress is not significantly increased in OA chondrocytes, these cells respond strongly to an acute ER stress despite the decreased expression of ERN1, PERK, and CREB3L2. Emerging findings revealed for the first time that these genes play a role in cartilage biology in conditions where an acute ER stress response is not triggered and OA is not characterized by an overall basal activation of the ER stress response. Importantly, these findings identify PERK as a potential target for new OA treatment avenues.

When isolated at full receptivity, in vitro fertilized wheat (Triticum aestivum, L.) egg cells reveal [Ca2+]cyt oscillation of intracellular origin

During in vitro fertilization of wheat (Triticum aestivum, L.) in egg cells isolated at various developmental stages, changes in cytosolic free calcium ([Ca2+]cyt) were observed. The dynamics of [Ca2+]cyt elevation varied, reflecting the difference in the developmental stage of the eggs used. [Ca2+]cyt oscillation was exclusively observed in fertile, mature egg cells fused with the sperm cell. To determine how [Ca2+]cyt oscillation in mature egg cells is generated, egg cells were incubated in thapsigargin, which proved to be a specific inhibitor of the endoplasmic reticulum (ER) Ca2+-ATPase in wheat egg cells. In unfertilized egg cells, the addition of thapsigargin caused an abrupt transient increase in [Ca2+]cyt in the absence of extracellular Ca2+, suggesting that an influx pathway for Ca2+ is activated by thapsigargin. The [Ca2+]cyt oscillation seemed to require the filling of an intracellular calcium store for the onset of which, calcium influx through the plasma membrane appeared essential. This was demonstrated by omitting extracellular calcium from (or adding GdCl3 to) the fusion medium, which prevented [Ca2+]cyt oscillation in mature egg cells fused with the sperm. Combined, these data permit the hypothesis that the first sperm-induced transient increase in [Ca2+]cyt depletes an intracellular Ca2+ store, triggering an increase in plasma membrane Ca2+ permeability, and this enhanced Ca2+ influx results in [Ca2+]cyt oscillation.

Cell stress induces upregulation of osteopontin via the ERK pathway in type II alveolar epithelial cells

Osteopontin (OPN) is a multifunctional protein that plays important roles in cell growth, differentiation, migration and tissue fibrosis. In human idiopathic pulmonary fibrosis and murine bleomycin-induced lung fibrosis, OPN is upregulated in type II alveolar epithelial cells (AEC II). However, the mechanism of OPN induction in AEC II is not fully understood. In this study, we demonstrate the molecular mechanism of OPN induction in AEC II and elucidate the functions of OPN in AEC II and lung fibroblasts. Human lung adenocarcinoma cells (A549) and mouse alveolar epithelial cells (MLE12), used as type II alveolar epithelial cell lines for in vitro assays, and human pulmonary alveolar epithelial cells (HPAEpiC) were treated with either bleomycin, doxorubicin or tunicamycin. The mechanism of OPN induction in these cells and its function as a pro-fibrotic cytokine on A549 and lung fibroblasts were analyzed. The DNA damaging reagents bleomycin and doxorubicin were found to induce OPN expression in A549, MLE12 and HPAEpiC. OPN expression was induced via activation of the extracellular signal-regulated protein kinase (ERK)-dependent signaling pathway in A549 and MLE12. The endoplasmic reticulum (ER) stress-inducing reagent tunicamycin induced OPN mRNA expression in A549, MLE12 and HPAEpiC, and OPN mRNA expression was induced via activation of the ERK-dependent signaling pathway in A549 and MLE12. Another ER stress-inducing reagent thapsigargin induced the expression of OPN mRNA as well as the subsequent production of OPN in A549 and MLE12. Furthermore, OPN promoted the proliferation of A549 and the migration of normal human lung fibroblasts. Inhibition of OPN by small interference RNA or neutralizing antibody suppressed both of these responses. The results of this study suggest that cell stress induces the upregulation of OPN in AEC II by signaling through the ERK pathway, and that upregulated OPN may play a role in fibrogenesis of the lung.

Superinhibitory phospholamban mutants compete with Ca2+ for binding to SERCA2a by stabilizing a unique nucleotide-dependent conformational state

Three cross-linkable phospholamban (PLB) mutants of increasing inhibitory strength (N30C-PLB < N27A,N30C,L37A-PLB (PLB3) < N27A,N30C,L37A,V49G-PLB (PLB4)) were used to determine whether PLB decreases the Ca(2+) affinity of SERCA2a by competing for Ca(2+) binding. The functional effects of N30C-PLB, PLB3, and PLB4 on Ca(2+)-ATPase activity and E1 approximately P formation were correlated with their binding interactions with SERCA2a measured by chemical cross-linking. Successively higher Ca(2+) concentrations were required to both activate the enzyme co-expressed with N30C-PLB, PLB3, and PLB4 and to dissociate N30C-PLB, PLB3, and PLB4 from SERCA2a, suggesting competition between PLB and Ca(2+) for binding to SERCA2a. This was confirmed with the Ca(2+) pump mutant, D351A, which is catalytically inactive but retains strong Ca(2+) binding. Increasingly higher Ca(2+) concentrations were also required to dissociate N30C-PLB, PLB3, and PLB4 from D351A, demonstrating directly that PLB antagonizes Ca(2+) binding. Finally, the specific conformation of E2 (Ca(2+)-free state of SERCA2a) that binds PLB was investigated using the Ca(2+)-pump inhibitors thapsigargin and vanadate. Cross-linking assays conducted in the absence of Ca(2+) showed that PLB bound preferentially to E2 with bound nucleotide, forming a remarkably stable complex that is highly resistant to both thapsigargin and vanadate. In the presence of ATP, N30C-PLB had an affinity for SERCA2a approaching that of vanadate (micromolar), whereas PLB3 and PLB4 had much higher affinities, severalfold greater than even thapsigargin (nanomolar or higher). We conclude that PLB decreases Ca(2+) binding to SERCA2a by stabilizing a unique E2.ATP state that is unable to bind thapsigargin or vanadate.

Calcium signalling in human spermatozoa: a specialized 'toolkit' of channels, transporters and stores

Ca(2+) is a ubiquitous intracellular messenger which encodes information by temporal and spatial patterns of concentration. In spermatozoa, several key functions, including acrosome reaction and motility, are regulated by cytoplasmic Ca(2+) concentration. Despite the very small size and apparent structural simplicity of spermatozoa, evidence is accumulating that they possess sophisticated mechanisms for regulation of cytoplasmic Ca(2+) concentration and generation of complex Ca(2+) signals. In this review, we consider the various components of the Ca(2+)-signalling 'toolkit' that have been characterized in somatic cells and summarize the evidence for their presence and activity in spermatozoa. In particular, data accumulated over the last few years show that spermatozoa possess one (and probably two) Ca(2+) stores as well as a range of plasma membrane pumps and channels. Selective regulation of the various components of the 'toolkit' by agonists probably allows spermatozoa to generate localized Ca(2+) signals despite their very small cytoplasmic volume, permitting the discrete and selective activation of cell functions.

Effects of inhibitors on luminal opening of Ca2+ binding sites in an E2P-like complex of sarcoplasmic reticulum Ca22+-ATPase with Be22+-fluoride

We document here the intrinsic fluorescence and 45Ca2+ binding properties of putative "E2P-related" complexes of Ca2+-free ATPase with fluoride, formed in the presence of magnesium, aluminum, or beryllium. Intrinsic fluorescence measurements suggest that in the absence of inhibitors, the ATPase complex with beryllium fluoride (but not those with magnesium or aluminum fluoride) does constitute an appropriate analog of the "ADP-insensitive" phosphorylated form of Ca2+-ATPase, the so-called "E2P" state. 45Ca2+ binding measurements, performed in the presence of 100 mm KCl, 5 mm Mg2+, and 20% Me2SO at pH 8, demonstrate that this ATPase complex with beryllium fluoride (but again not those with magnesium or aluminum fluoride) has its Ca2+ binding sites accessible for rapid, low affinity (submillimolar) binding of Ca2+ from the luminal side of SR. In addition, we specifically demonstrate that in this E2P-like form of ATPase, the presence of thapsigargin, 2,5-di-tert-butyl-1,4-dihydroxybenzene, or cyclopiazonic acid prevents 45Ca2+ binding (i.e. presumably prevents opening of the 45Ca2+ binding sites on the SR luminal side). Since crystals of E2P-related forms of ATPase have up to now been described in the presence of thapsigargin only, these results suggest that crystallizing an inhibitor-free E2P-like form of ATPase (like its complex with beryllium fluoride) would be highly desirable, to unambiguously confirm previous predictions about the exit pathway from the ATPase transmembrane Ca2+ binding sites to the SR luminal medium.

Genetic manipulation of calcium-handling proteins in cardiac myocytes. I. Experimental studies

Two genetic experimental approaches, de novo expression of parvalbumin (Parv) and overexpression of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2a), have been shown to increase relaxation rates in myocardial tissue. However, the relative effect of Parv and SERCA2a on systolic function and on β-adrenergic responsiveness at varied pacing rates is unknown. We used gene transfer in isolated rat adult cardiac myocytes to gain a fuller understanding of Parv/SERCA2a function. As demonstrated previously, when Parv is expressed in elevated concentration (>0.1 mM), the transduced myocytes showed a reduction in sarcomere-shortening amplitude: 129 ± 17, 81 ± 8, and 149 ± 14 nm for control, Parv, and SERCA2a, respectively. At physiological temperature, shortening amplitude responses of Parv and SERCA2a myocytes to the β-adrenergic agonist isoproterenol (Iso) were not statistically different from that of control myocytes. However, in SERCA2a myocytes, in which baseline was slightly elevated and the Iso-stimulated value was slightly lower, the increase in shortening was slightly less than in Parv or control myocytes: 108 ± 14, 169 ± 39, and 34 ± 12% for control, Parv, and SERCA2a, respectively. In another test set, Parv myocytes had the strongest early postrest potentiation among all groups studied (rest time = 2–10 s), and SERCA2a myocytes were the least sensitive to variations in stimulation rhythm. To replicate the deficient Ca2+ removal observed in heart failure, we used 150 nM thapsigargin. Under these conditions, control myocytes exhibited slowed relaxation, whereas Parv myocytes retained their rapid kinetics, showing that Parv is still able to control relaxation, even when SERCA2a function is impaired.

Cyclopiazonic acid reduces the coupling factor of the Ca2+-ATPase acting on Ca2+ binding

The mycotoxin cyclopiazonic acid (CPA) is a potent inhibitor of the sarcoplasmic reticulum Ca2+-ATPase. The compound decreases the affinity of the Ca2+-ATPase for Ca2+ and reduces the maximum specific activity of the enzyme. Furthermore, CPA abolishes the cooperativity of Ca2+ transport, showing a Ca2+/ATP ratio approximately 1 at any extent of Ca2+ saturation. There is also an effect on the Ca2+-binding mechanism, where the addition of CPA results in binding of only half-maximal amount of Ca2+ observed in its absence. The experimental data suggest that in the presence of CPA, only a single Ca2+ ion binds to the Ca2+-ATPase.

Genetic manipulation of calcium-handling proteins in cardiac myocytes. II. Mathematical modeling studies

We developed a mathematical model specific to rat ventricular myocytes that includes electrophysiological representation, ionic homeostasis, force production, and sarcomere movement. We used this model to interpret, analyze, and compare two genetic manipulations that have been shown to increase myocyte relaxation rates, parvalbumin (Parv) de novo expression, and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a) overexpression. The model was used to seek mechanistic insights into 1) the relative contribution of two mechanisms by which SERCA2a overexpression modifies Ca2+ sequestration, i.e., more pumps and an increase in the SERCA2a-to-phospholamban ratio, 2) the mechanisms behind postrest potentiation and how Parv and SERCA2a influence this response, and 3) why Parv myocytes retain their fast kinetics when endogenous SERCA2a is partially impaired by thapsigargin (a condition used to mimic diastolic dysfunction). The model was also utilized to predict whether Parv metal-binding characteristics might be modified to improve diastolic and systolic functions and whether Parv or SERCA2a might affect diastolic Ca2+ levels and myocyte energetics. One outcome of the model was to demonstrate a higher peak and total ATP consumption in SERCA2a myocytes and more even distribution of ATP throughout the cardiac cycle in Parv myocytes. This may have implications for failing hearts that are energetically compromised.

Specific Structural Requirements for the Inhibitory Effect of Thapsigargin on the Ca2+ ATPase SERCA

Mutational analysis of amino acid residues lining the thapsigargin (TG) binding cavity at the interface of the membrane surface and cytosolic headpiece was performed in the Ca(2+) ATPase (SERCA-1). Specific mutations such as F256V, I765A, and Y837A reduce not only the apparent affinity of the ATPase for TG but also the maximal inhibitory effect. The effect of mutations is dependent on the type and size of the substitute side chain, indicating that hydrophobic partitioning of TG and complementary molecular shapes are involved not only in binding but also in the inhibitory mechanism. A major factor determining the inhibitory effect of bound TG is its interference with conformational changes that are required for the progress of the ATPase cycle. Most prominent and specific is the TG interference with a wide displacement of the Phe-256 side chain that is associated with the E2 to E1.2Ca(2+) transition. The specificity of the TG inhibitory mechanism is emphasized by the finding that the F256V mutation does not interfere at all with the effect of 2,5-di-(t-butyl)-hydroquinone, which is another SERCA inhibitor bound by hydrophobic partitioning. The specificity of the inhibitory mechanism is also emphasized by the observation that within the concentration range producing total inhibition of wild-type SERCA-1, TG produces a 4-fold stimulation of the P-glycoprotein (multidrug transporter) ATPase.

Effects of peroxynitrite on sarcoplasmic reticulum Ca2+ pump in pig coronary artery smooth muscle

Peroxynitrite generated in arteries from superoxide and NO may damage Ca(2+) pumps. Here, we report the effects of peroxynitrite on ATP-dependent azide-insensitive uptake of Ca(2+) into pig coronary artery vesicular membrane fractions F2 [enriched in plasma membrane (PM)] and F3 [enriched in sarcoplasmic reticulum (SR)]. Membranes were pretreated with peroxynitrite and then with DTT to quench this agent. This pretreatment inhibited Ca(2+) uptake in a peroxynitrite concentration-dependent manner, but the effect was more severe in F3 than in F2. The inhibition was thus not overcome by excess DTT used to quench peroxynitrite and was not affected if catalase, SOD, or mannitol was added along with peroxynitrite. Such damage to the pump protein would be difficult to repair if produced during ischemia-reperfusion. The acylphosphates formed with ATP in F3 corresponded mainly to the SR Ca(2+) pump (110 kDa), but in F2 both PM (140 kDa) and 110-kDa bands were observed. Peroxynitrite treatment of F2 inhibited only the 110-kDa band. Inhibition of Ca(2+) uptake and acylphosphate formation from ATP correlated well in peroxynitrite-treated F3 samples. However, inhibition of acylphosphates from orthophosphate (reverse reaction of the pump) was slightly poorer. Peroxynitrite treatment also covalently cross-linked the pump protein, yielding no dimers but only larger oligomers. In contrast, cross-linking of the SR Ca(2+) pump in skeletal and cardiac muscles gives dimers as the first oligomers. Therefore, we speculate that SERCA2 has a different quaternary structure in the coronary artery smooth muscle.

Arachidonic acid inhibition of muscarinic receptor-mediated nitric oxide production occurs at the level of calcium mobilization in Chinese hamster ovary cells

Strong evidence supports that nitric oxide (NO) alters cell signaling pathways involving arachidonic acid (AA). Little is known, however, about the reciprocal modulation of nitrergic pathways by AA. The effects of exogenous AA on signal transduction of M1 muscarinic acetylcholine receptors were investigated in a model system of stably transfected Chinese hamster ovary cells. AA concentration-dependently inhibited the effects of carbachol in producing NO (IC50 = 191 microM) but did not alter inositol phosphate production or M1 receptor binding. AA inhibited both carbachol-induced transient and sustained increase in intracellular calcium concentration ([Ca2+]i; IC50 = 11 and 12 microM, respectively). Furthermore, AA-induced increase in [Ca2+]i cross-desensitizes with thapsigargin, but AA does not inhibit Ca(2+)-ATPase activity. These data support the concept that AA concentration-dependently inhibits receptor-mediated NO production at the level of calcium mobilization.

Evidence for a global inhibitor-induced conformation change on the Ca(2+)-ATPase of sarcoplasmic reticulum from paired inhibitor studies

The Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum is inhibited by a variety of hydrophobic, hydroxy-containing molecules. A kinetic method has been used to study competition between binding of pairs of inhibitors to the ATPase. The presence of 2,5-di-tert-butyl-1,4-dihydroxybenzene (BHQ) decreases the affinity of the ATPase for 2,5-dipropyl-1,4-dihydroxybenzene (PHQ), suggesting that PHQ and BHQ bind to the same site on the ATPase. In contrast, the presence of BHQ increases the affinity of the ATPase for curcumin and vice versa. This suggests that BHQ and curcumin bind to separate sites on the ATPase and that binding of the first inhibitor to the ATPase results in a change to a conformation with higher affinity for the second inhibitor. This is consistent with previous experiments with BHQ and thapsigargin suggesting a conformation change on inhibitor binding, E2 + I <--> 2; E2I <--> 2; E2(A)I, with E2(A)I having a higher affinity for the second inhibitor than E2. The affinity for BHQ is also increased by binding of diethylstilbesterol, ellagic acid, or nonylphenol, and the affinity for curcumin is also increased by ellagic acid. These results showing that binding of a variety of inhibitors of very different structures all result in a general increase in inhibitor affinity point to a global conformational change on the Ca(2+)-ATPase caused by inhibitor binding, as well as any local, inhibitor-specific changes in conformation.

Curcumin, a molecule that inhibits the Ca2+-ATPase of sarcoplasmic reticulum but increases the rate of accumulation of Ca2+

Curcumin, an important inhibitor of carcinogenesis, is an inhibitor of the ATPase activity of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum (SR). Inhibition by curcumin is structurally specific, requiring the presence of a pair of -OH groups at the 4-position of the rings. Inhibition is not competitive with ATP. Unexpectedly, addition of curcumin to SR vesicles leads to an increase in the rate of accumulation of Ca(2+), unlike other inhibitors of the Ca(2+)-ATPase that result in a reduced rate of accumulation. An increase in the rate of accumulation of Ca(2+) is seen in the presence of phosphate ion, which lowers the concentration of free Ca(2+) within the lumen of the SR, showing that the effect is not passive leak across the SR membrane. Rather, simulations suggest that the effect is to reduce the rate of slippage on the ATPase, a process in which a Ca(2+)-bound, phosphorylated intermediate releases its bound Ca(2+) on the cytoplasmic rather than on the lumenal side of the membrane. The structural specificity of the effects of curcumin on ATPase activity and on Ca(2+) accumulation is the same, and the apparent dissociation constants for the two effects are similar, suggesting that the two effects of curcumin could follow from binding to a single site on the ATPase.

Thapsigargin and dimethyl sulfoxide activate medium P(i)<-->HOH oxygen exchange catalyzed by sarcoplasmic reticulum Ca2+-ATPase

Thapsigargin is a potent inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase. It binds the Ca(2+)-free E2 conformation in the picomolar range, supposedly resulting in a largely catalytically inactive species. We now find that thapsigargin has little effect on medium P(i) <--> HOH oxygen exchange and that this activity is greatly stimulated (up to 30-fold) in the presence of 30% (v/v) Me(2)SO. Assuming a simple two-step mechanism, we have evaluated the effect of thapsigargin and Me(2)SO on the four rate constants governing the reaction of P(i) with Ca(2+)-ATPase. The principal effect of thapsigargin alone is to stimulate EP hydrolysis (k(-2)), whereas that of Me(2)SO is to greatly retard P(i) dissociation (k(-1)), accounting for its well known effect on increasing the apparent affinity for P(i). These effects persist when the agents are used in combination and substantially account for the activated oxygen exchange (v(exchange) = k(-2)[EP]). Kinetic simulations show that the overall rate constant for the formation of EP is very fast (approximately 300 s(-1)) when the exchange is maximal. Thapsigargin greatly stabilizes Ca(2+)-ATPase against denaturation in detergent in the absence of Ca(2+), as revealed by glutaraldehyde cross-linking, suggesting that the membrane helices lock together. It seems that the reactions at the phosphorylation site, associated with the activated exchange reaction, are occurring without much movement of the transport site helices, and we suggest that they may be associated solely with an occluded H+ state.

Unravelling the interaction of thapsigargin with the conformational states of Ca(2+)-ATPase from skeletal sarcoplasmic reticulum

Preincubation of thapsigargin with sarcoplasmic reticulum vesicles in the presence of high Ca(2+) or the addition of high Ca(2+) to microsomal vesicles preincubated with thapsigargin in the absence of Ca(2+) allowed full enzyme phosphorylation by ATP. However, the enzyme activity was not protected by high Ca(2+) even when the samples were subjected to gel filtration before ATP addition. Our data indicate that: (i) the enzyme in the Ca(2+)-bound conformation can be stabilized in the presence of thapsigargin; (ii) the conformational transition from the Ca(2+)-free to the Ca(2+)-bound state can be elicited by Ca(2+) when thapsigargin is present; (iii) thapsigargin binding occurs whether or not the enzyme is in the presence of Ca(2+), and so a ternary complex enzyme-Ca(2+)-thapsigargin may be formed; (iv) thapsigargin can be dissociated from the enzyme with a slow kinetics after dilution under drastic conditions; (v) the kinetics of Ca(2+) binding is clearly slowed down by thapsigargin; and (vi) thapsigargin does not affect the hydrolysis rate of phosphorylating substrates when measured in the absence of Ca(2+), indicating that thapsigargin specifically inhibits the Ca(2+)-dependent activity.

Coupling of AMPA receptors with the Na(+)/Ca(2+) exchanger in cultured rat astrocytes

Astrocytes exhibit three transmembrane Ca(2+) influx pathways: voltage-gated Ca(2+) channels (VGCCs), the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) class of glutamate receptors, and Na(+)/Ca(2+) exchangers. Each of these pathways is thought to be capable of mediating a significant increase in Ca(2+) concentration ([Ca(2+)](i)); however, the relative importance of each and their interdependence in the regulation astrocyte [Ca(2+)](i) is not known. We demonstrate here that 100 microM AMPA in the presence of 100 microM cyclothiazide (CTZ) causes an increase in [Ca(2+)](i) in cultured cerebral astrocytes that requires transmembrane Ca(2+) influx. This increase of [Ca(2+)](i) is blocked by 100 microM benzamil or 0.5 microM U-73122, which inhibit reverse-mode operation of the Na(+)/Ca(2+) exchanger by independent mechanisms. This response does not require Ca(2+) influx through VGCCs, nor does it depend upon a significant Ca(2+) influx through AMPA receptors (AMPARs). Additionally, AMPA in the presence of CTZ causes a depletion of thapsigargin-sensitive intracellular Ca(2+) stores, although depletion of these Ca(2+) stores does not decrease the peak [Ca(2+)](i) response to AMPA. We propose that activation of AMPARs in astrocytes can cause [Ca(2+)](i) to increase through the reverse mode operation of the Na(+)/Ca(2+) exchanger with an associated release of Ca(2+) from intracellular stores. This proposed mechanism requires neither Ca(2+)-permeant AMPARs nor the activation of VGCCs to be effective.

Release of Ca(2+) from intracellular stores and entry of extracellular Ca(2+) are involved in sea squirt sperm activation

A rise in intracellular free Ca(2+) concentration ([Ca(2+)](i)) is required to activate sperm of all organisms studied. Such elevation of [Ca(2+)](i) can occur either by influx of extracellular Ca(2+) or by release of Ca(2+) from intracellular stores. We have examined these sources of Ca(2+) in sperm from the sea squirt Ascidia ceratodes using mitochondrial translocation to evaluate activation and the Ca(2+)-sensitive dye fura-2 to monitor [Ca(2+)](i) by bulk spectrofluorometry. Sperm activation artificially evoked by incubation in high-pH seawater was inhibited by reducing seawater [Ca(2+)], as well as by the presence of high [K(+)](o) or the Ca channel blockers pimozide, penfluridol, or Ni(2+), but not nifedipine or Co(2+). The accompanying rise in [Ca(2+)](i) was also blocked by pimozide or penfluridol. These results indicate that activation produced by alkaline incubation involves opening of plasmalemmal voltage-dependent Ca channels and Ca(2+) entry to initiate mitochondrial translocation. Incubation in thimerosal or thapsigargin, but not ryanodine (even if combined with caffeine pretreatment), evoked sperm activation. Activation by thimerosal was insensitive to reduced external calcium and to Ca channel blockers. Sperm [Ca(2+)](i) increased upon incubation in high-pH or thimerosal-containing seawater, but only the high-pH-dependent elevation in [Ca(2+)](i) could be inhibited by pimozide or penfluridol. Treatment with the protonophore CCCP indicated that only a small percentage of sperm could release enough Ca(2+) from mitochondria to cause activation. Inositol 1,4,5-trisphosphate (IP(3)) delivered by liposomes or by permeabilization increased sperm activation. Both of these effects were blocked by heparin. We conclude that high external pH induces intracellular alkalization that directly or indirectly activates plasma membrane voltage-dependent Ca channels allowing entry of external Ca(2+) and that thimerosal stimulates release of Ca(2+) from IP(3)-sensitive intracellular stores.

Mutation to the glutamate in the fourth membrane segment of Na+,K+-ATPase and Ca2+-ATPase affects cation binding from both sides of the membrane and destabilizes the occluded enzyme forms

The functional consequences of mutations Glu329 --> Gln in the Na+,K+-ATPase and Glu309 --> Asp in the sarco(endo)plasmic reticulum Ca2+-ATPase were analyzed and compared. Relative to the wild-type Na+,K+-ATPase, the Glu329 --> Gln mutant exhibited a 20-fold reduction in the apparent K+ affinity determined by titration of the rate of ATP hydrolysis at 50 microM ATP, and the rate of release of occluded K+ or Rb+ to the cytoplasmic side of the membrane was up to 30-fold enhanced by the mutation, as measured in kinetic studies of the phosphorylation by ATP of enzyme equilibrated with K+ or Rb+. The apparent affinity for extracellular K+ was 12-fold reduced by the Glu329 --> Gln mutation, as determined by K+ titration of the dephosphorylation. The maximum rate of phosphorylation by ATP of the Na+ form of the enzyme was reduced more than 2-fold by the mutation, but this effect could be counteracted by stabilizing Na+ occlusion with oligomycin. Similar studies on the Glu309 --> Asp mutant of the Ca2+-ATPase showed that the maximum rate of phosphorylation of the Ca2+ form was 8-9-fold reduced relative to that of the wild-type Ca2+-ATPase, and no Ca2+ occlusion could be detected in the mutant. Dephosphorylation of the phosphoenzyme intermediate formed with Pi was blocked in the Ca2+-ATPase mutant. The sensitivity to inhibition by thapsigargin, which binds selectively to the putative proton-occluded form of the Ca2+-ATPase, was reduced almost 300-fold in the mutant at neutral pH, but only 3-4-fold at pH 6.0. These data indicate that the mutations destabilize the occluded enzyme forms and interfere with cation binding from the extracytoplasmic side as well as with the gating process at the cytoplasmic entrance to the cation occlusion pocket.

Role of the S3 stalk segment in the thapsigargin concentration dependence of sarco-endoplasmic reticulum Ca2+ ATPase inhibition

The sarco-endoplasmic reticulum Ca2+ ATPase (SERCA) is specifically inhibited by thapsigargin (TG), whereas the Na+,K+-ATPase is not. Large chimeric exchanges between Ca2+ and Na+,K+-ATPases (Norregaard, A., Vilsen, B., and Andersen, J. P. (1994) J. Biol. Chem. 269, 26598-26601), as well as photolabeling with a TG azido derivative (Hua, S., and Inesi, G. (1997) Biochemistry 36, 11865-11872), suggest that the S3-M3 (stalk and membrane-bound) region of the Ca2+ ATPase is involved in TG binding. We produced small site-directed changes in the S3 stalk segment of the Ca2+ ATPase and found that mutation of five amino acids to the corresponding Na+,K+-ATPase residues increases by 3 orders of magnitude the TG concentration required for inhibition of Ca2+ ATPase and coupled Ca2+ transport. A single mutation in the S3 stalk segment (Gly257 --> Ile) is sufficient to increase by 1 order of magnitude the TG concentration required to produce 50% inhibition. By comparison, mutations yielding a nine-amino acid homology in the M3 transmembrane segment, or a 25-amino acid homology in the S4 stalk segment, do not affect the ATPase sensitivity to TG. We suggest that specific binding of TG to the S3 stalk segment, in addition to stacking of the TG ring structure at the membrane interface, determines the high affinity of the ATPase for the inhibitor.

Cyclopiazonic acid effect on Ca2+-dependent conformational states of the sarcoplasmic reticulum ATPase. Implication for the enzyme turnover

The affinity of sarcoplasmic reticulum Ca2+-ATPase for cyclopiazonic acid is dependent on the conformational state of the enzyme. It is high in the absence of Ca2+ but low in its presence. When Ca2+ was added to the enzyme in the presence of equimolar toxin, the apparent rate constant for Ca2+ binding was 0.6 min-1 when measured at 37 degrees C. The apparent equilibrium constant for Ca2+ dissociation increased from 0.2 to 0.6 microM at neutral pH, and from 5.9 to 37 microM at pH 6.0. The apparent equilibrium constant for Ca2+ dissociation increased progressively as the amount of toxin increased above an equimolar level. Cyclopiazonic acid decreased phosphorylation by ATP and Ca2+ when the enzyme in the absence of Ca2+ was incubated in the presence of toxin, although no effect was observed after a preliminary incubation with Ca2+ at 37 degrees C. Cyclopiazonic acid incubated with the enzyme in the presence of Ca2+ could be eliminated with a Sephadex column. However, the toxin could not be removed when it was incubated with the enzyme in the absence of Ca2+. In the latter case, cyclopiazonic acid was eliminated when the enzyme in the presence of toxin was incubated with Ca2+ at 37 degrees C. Under turnover conditions and in the presence of 10 microM ATP, the toxin-enzyme interaction can be characterized by an apparent Kd of 7 nM. With an ATP concentration of 1 mM, the enzyme was inhibited completely at a toxin/enzyme molar ratio of approximately 10. Furthermore, enzyme activity was observed to recover at a toxin/enzyme molar ratio of 1 when the Ca2+ concentration was raised, which is consistent with the competitive character of cyclopiazonic acid and Ca2+. It is concluded that ATP and Ca2+ can protect against cyclopiazonic acid inhibition.

Luminal Ca2+ protects against thapsigargin inhibition in neuronal endoplasmic reticulum

Thapsigargin is a specific and potent inhibitor of sarco/endoplasmic reticulum Ca2+-ATPases. However, in whole rat brain microsomes, 1 microM thapsigargin had no significant effect on the 10-min time course of ATP-dependent Ca2+ uptake in the absence of the luminal Ca2+ chelator oxalate. In contrast, 50 mM oxalate resolved a thapsigargin-sensitive Ca2+ uptake rate (IC50 approximately 1 nM thapsigargin) five times that of a thapsigargin-insensitive rate. This remaining approximately 20% of the total ATP-dependent accumulation was insensitive to thapsigargin (up to 10 microM), slightly less sensitive to vanadate inhibition, and unresponsive to 5 microM inositol 1,4,5-trisphosphate or 10 mM caffeine. Measuring both 12-min Ca2+ uptake and initial Ca2+ uptake rates, the apparent thapsigargin sensitivity increased as oxalate concentrations increased from 10 to 50 mM, corresponding to a range of luminal free Ca2+ concentrations of approximately 300 down to 60 nM. Addition of oxalate during steady-state 45Ca accumulation rapidly resolved the aforementioned thapsigargin sensitivity. These results strongly suggest that luminal Ca2+ may protect a large portion of neuronal endoplasmic reticulum Ca2+ pumps against thapsigargin inhibition. Although high [Ca2+] has been previously shown to protect against thapsigargin inhibition in several reticular membrane preparations, our results suggest that luminal Ca2+ alone is responsible for mediating this effect in neurons.

Differential effects of general anesthetics on the quaternary structure of the Ca-ATPases of cardiac and skeletal sarcoplasmic reticulum

The effects of the general anesthetics hexanol, halothane, and diethyl ether on Ca-ATPase activity and on the oligomeric state of the Ca-ATPase of sarcoplasmic reticulum (SR) from cardiac and skeletal muscle were investigated. The effects of these general anesthetics on Ca-ATPase activity were similar in cardiac and skeletal SR and were characterized by stimulation of Ca-ATPase activity at lower concentrations of anesthetics and inhibition at higher concentrations. The distribution of the Ca-ATPase among its oligomeric states was estimated from the time-resolved phosphorescence anisotropy (TPA) decay of SR in which Ca-ATPase was covalently labeled with erythrosin isothiocyanate (ERITC) or with erythrosin iodoacetamide (ERIA). In contrast to the similar responses of Ca-ATPase activity, there were marked differences in the responses to general anesthetics of the TPA decay between cardiac and skeletal SR. In cardiac SR hexanol, halothane, and diethyl ether caused pronounced increases in the limiting anisotropy at very long times (r infinity), which indicate increases in the fraction of oligomers too large to rotate on the millisecond time scale of the experiments. In skeletal SR, by contrast, there were no significant changes in r infinity in response to the three general anesthetics. This difference between cardiac and skeletal SR in response to general anesthetics is not due to the presence of phospholamban in cardiac SR, since SR from AT-1 cells, which have the SERCA2a isoform of Ca-ATPase, but only trace levels of phospholamban, have increases in r infinity in response to the general anesthetics that resemble those in cardiac SR. Experiments with cardiac SR labeled with ERIA give similar results, showing that the results with ERITC are not an artifact of the labeling procedure. Increasing the ionic strength with LiCl diminished the proportion of large immobile oligomers of cardiac Ca-ATPase under control conditions but enhanced the formation of large oligomers in response to hexanol.

Aging-related changes in calcium oscillations in fertilized mouse oocytes

Aging of oocytes, being not fertilized after ovulation for a prolonged time, considerably affects normal development of the fertilized oocyte. We examined effects of the aging on a series of highly repetitive Ca2+ transients commonly seen in fertilized mouse oocytes (Ca2+ oscillations). Frequency of Ca2+ oscillations in the aged oocyte [20 hrs after induction of superovulation by i.p. human chorionic gonadotropin (hCG)] was significantly higher (34.1 +/- 5.8 l/hr) than the fresh oocyte (14 hr post-hCG, 21.8 +/- 7.9 l/hr). Rates of rise and fall of individual Ca2+ transient in the aged oocyte were significantly slower than the fresh oocyte, whereas durations of individual Ca2+ transients were similar. When extracellular Ca2+ was raised from 2.04 mM to 5.00 mM, aged oocytes showed significant prolongation of the duration of individual Ca2+ transient, that resulted in a sustained elevation of intracellular Ca2+ ([Ca2+]i) in 33% of the aged oocyte. Transient increase in [Ca2+]i by photolysis of a caged Ca2+, Nitr-5, injected into cytoplasm was completely restored in the fresh oocyte [fluorescence intensity of [Ca2+]i indicator dye Fluo-3 (F480) returned to 97 +/- 2% of the control level, time constant = 37 +/- 9 sec]. In contrast, in the aged oocyte, restoration of F480 following Nitr-5 photolysis was incomplete (115 +/- 12% of the control) and slow (time constant = 64 +/- 23 sec). Because inhibition of the Ca2+ pump of the endoplasmic reticulum (ER) by 5 microM thapsigargin almost completely inhibited restoration of F480 following Nitr-5 photolysis in the fresh oocyte, we conclude that the aging-related changes in Ca2+ oscillations may be accounted for by dysfunction of intracellular Ca2+ regulation, presumably of the Ca2+ pump of the ER.

Effects of phosphatidylethanolamines on the activity of the Ca(2+)-ATPase of sarcoplasmic reticulum

ATPase activities for the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum reconstituted into dioleoylphosphatidylethanolamine [di(C18:1)PE] are, at temperatures higher than 20 degrees C, lower than in dioleoylphosphatidylcholine [di(C18:1)PC], whereas in egg yolk phosphatidylethanolamine the activities are the same as in di(C18:1)PC up to 25 degrees C, suggesting that low ATPase activities occur when the phosphatidylethanol-amine species is in the hexagonal H11 phase. ATPase activities measured in mixtures of di(C18:1)PC and di(C18:1)PE do not change with changing di(C18:1)PE content up to 80%. It is concluded that curvature frustration in bilayers containing di(C18:1)PE has no effect on ATPase activity. The rates of phosphorylation and of Ca2+ transport are identical for the native ATPase and for the ATPase in di(C18:1)PE. Dephosphorylation of the phosphorylated ATPase in di(C18:1)PE at 25 degrees C is, however, slower than for the native ATPase, explaining the lower steady-state rate of ATP hydrolysis; in egg yolk phosphatidylethanolamine at 25 degrees C the rate of dephosphorylation is equal to that for the unreconstituted ATPase. Phosphorylation of the ATPase by P1 in the absence of Ca2+ is unaffected by reconstitution in di(C18:1)RE. The stoichiometry of Ca2+ binding to the ATPase is also unaltered. Studies of the effect of di(C18:1)PE on the fluorescence intensity of the ATPase labelled with 7-chloro-4-nitro-2,1,3-benzoxadiazole are consistent with an increase in the E1/E2 equilibrium constant, where E1 is the conformation of the ATPase with two high-affinity binding sites for Ca2+ exposed to the cytoplasm, and E2 is a conformation unable to bind cytoplasmic Ca2+. A slight increase in affinity for Ca2+ can be attributed to the observed increase in the E1/E2 equilibrium constant.

An investigation of the mechanism of inhibition of the Ca(2+)-ATPase by phospholamban

The Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum has been reconstituted with peptides corresponding to the hydrophobic domain of phospholamban (PLB) with or without the three Cys residues replaced by Ala, and with PLB with the three Cys residues replaced by Ala [PLBcys-(1-52)]. Reconstitution with the hydrophobic domain of PLB[PLB(25-52)] was found to decrease the apparent affinity of the ATPase for Ca2+ with no effect on the maximal rate of ATP hydrolysis observed at saturating concentrations of Ca2+. Reconstitution with PLBCys-(1-52) decreased both the apparent affinity for Ca2+ and the maximal activity; the effect on maximal activity followed from a decrease in the rate of the Ca2+ transport step (E1PCa2-->E2P) as observed with the hydrophilic domain PLB(1-25). The concentration dependences of the effects of the hydrophobic domain and of the whole PLB molecule were very similar, suggesting that the hydrophilic domain made little contribution to the affinity of the ATPase for PLB. The effect of PLB on the ATPase was dependent on the molar ratio of phospholipid to ATPase, suggesting partition of the PLB between its binding site on the ATPase and the bulk lipid phase in the membrane. Neither PLB nor its hydrophobic domain affected the rates of phosphorylation or dephosphorylation of the ATPase. Despite their effects on the apparent affinity of the ATPase for Ca2+, neither PLB nor its hydrophobic domain had any effect on the true affinity of the ATPase for Ca2+, as measured from changes in the tryptophan fluorescence of the ATPase. The effects of PLB on the activity of the ATPase are the sum of the effects of its hydrophilic and hydrophobic domains.

Structure-function relationships in the Ca(2+)-ATPase of sarcoplasmic reticulum: facts, speculations and questions for the future

Structural data on the Ca(2+)-ATPase of sarcoplasmic reticulum are integrated with kinetic data on Ca2+ transport. The emphasis is upon ATPase-ATPase interactions, the requirement for phospholipids, and the mechanism of Ca2+ translocation. The possible role of cytoplasmic [Ca2+] in the regulation of the synthesis of Ca(2+)-ATPase is discussed.

The mechanism of inhibition of the Ca(2+)-ATPase of skeletal-muscle sarcoplasmic reticulum by the cross-linker o-phthalaldehyde

Labelling the Ca(2+)-ATPase of skeletal-muscle sarcoplasmic reticulum with o-phthalaldehyde (OPA) results in loss of ATPase activity at a 1:1 molar ration of label to ATPase. The affinity of the ATPase for CA2+ is unaffected, as is the E1/E2 equilibrium constant. The rate of dissociation of Ca2+ from the Ca(2+)-bound ATPase is also unaffected and Mg2+ increases the rate of dissociation, as for the unlabelled ATPase. Effects of Mg2+ on the fluorescence intensity of the ATPase labelled with 4-(bromo-methyl)-6,7-dimethoxycoumarin are also unaffected by labelling with OPA, consistent with the fluorescence change reporting on Mg2+ binding at the gating site on the ATPase. The affinity of the ATPase for ATP is reduced by labelling, as is the rate of phosphorylation. The rate of phosphorylation is independent of the concentration of ATP above 25 microM ATP, so that the slow step is the first-order rate constant for phosphorylation by bound ATP. The rate of the back reaction between phosphorylated ATPase and ADP is little affected, suggesting that the slow step in phosphorylation could be the slow conformation step before phosphoryl transfer. The rate of dephosphorylation of the phosphorylated ATPase is also decreased, suggesting that a similar conformation change could be involved in the dephosphorylation step. The rate of the Ca(2+)-transport step appears to be unaffected by labelling. The net result of these changes is that the labelled ATPase is present predominantly in a Ca(2+)-free, phosphorylated form at steady state in the presence of ATP.

Labelling the Ca(2+)-ATPase of skeletal-muscle sarcoplasmic reticulum with the cross-linker o-phthalaldehyde

The Ca(2+)-ATPase in the sarcoplasmic reticulum of skeletal muscle reacts with o-phthalaldehyde (OPA) to form a fluorescent isoindole product. The stoichiometry of labelling of the ATPase is 9 nmol of isoindole/mg of ATPase, corresponding to a 1:1 molar ratio of isoindole: ATPase. There is no evidence for any intermolecular cross-linking. Isoindole formation is faster in the presence of methylamine, but the stoichiometry of labelling is unchanged, whereas in the presence of 2-mercaptoethanol the level of labelling is much higher. It is concluded that OPA reacts with a single Cys residue (defining the specificity of the reaction) in a fast step, subsequent reaction with a Lys residue to form the isoindole being rate-controlling. Labelling the ATPase with OPA in the absence of methylamine leads to total loss of ATPase activity, whereas in the presence of methylamine, the decrease in ATPase activity on reaction is small. We conclude that the loss of ATPase activity probably follows from formation of the intramolecular cross-link rather than from the initial modification of the Cys residue. Reaction with OPA is not affected by the presence of ATP, ADP or Ca2+, so that the reactive Cys is not part of a ligand-binding site. The fluorescence emission spectrum of the labelled ATPase indicates a hydrophobic environment for the isoindole ring.

Lipid structure and Ca(2+)-ATPase function

Effects of lipid structure on the function of the Ca(2+)-ATPase of skeletal muscle of sarcoplasmic reticulum are reviewed. Binding of phospholipids to the ATPase shows little specificity. Phosphatidylcholines with short (C14) or long (C24) fatty acyl chains have marked effects on the activity of the ATPase, including a change in the stoichiometry of Ca binding. Low ATPase activity in gel phase lipid follows from low rate of phosphorylation. Phosphatidylinositol 4-phosphate increases ATPase activity by increasing the rate of dephosphorylation of the phosphorylated ATPase. Stimulation is not seen with other anionic phospholipids; phosphatidic acid decreases ATPase activity in a Mg(2-)-dependent manner.

The structure and interactions of Ca(2+)-ATPase

Electron crystallographic studies on membrane crystals of Ca(2+)-ATPase reveal different patterns of ATPase-ATPase interactions depending on enzyme conformation. Physiologically relevant changes in Ca2+ concentration and membrane potential affect these interactions. Ca2+ induced difference FTIR spectra of Ca(2+)-ATPase triggered by photolysis of caged Ca2+ are consistent with changes in secondary structure and carboxylate groups upon Ca2+ binding; the changes are reversed during ATP hydrolysis suggesting that a phosphorylated enzyme form of low Ca2+ affinity is the dominant intermediate during Ca2+ transport. A two-channel model of Ca2+ translocation is proposed involving the membrane-spanning helices M2-M5 and M4, M5, M6 and M8 respectively, with separate but interacting Ca2+ binding sites.