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

Review Article: Safety Assessment of the Mycotoxin Cyclopiazonic Acid

Cyclopiazonic acid (CPA) is an indol-tetramic acid mycotoxin and is produced by the nearly ubiquitous molds, Aspergillus and Penicillium. CPA produced by these molds has been identified in a number of food sources (including, but not limited to, grain, legumes, meat, milk, and cheese) and from parasitic infections of man and other animals. Few incidents of CPA mycotoxicoses have been reported because of the benign nature of the intoxication, the small amounts present, and its effects may be disguised with concurrent aflatoxicosis (some toxicity data may have been generated using aflatoxin-contaminated CPA). CPA is absorbed in the gastrointestinal tract and following oral administration; it has a half-life of approximately 30 hours and is excreted largely unchanged in the urine and feces. Cyclopiazonic acid is not considered to be a potent acute toxin as its oral LD50 in rodents is in the range of 30 to 70 mg/kg. Multiple dose studies also show a range of effects in several species and among mammalian models, the pig appears to be the most sensitive with a no-observable-effect level (NOEL) in the range of 1.0 mg/kg/day. The preponderance of evidence from the rat and other test animals supports this dose as a defensible estimate of a no effect level. The target organs of CPA toxicity appear to be muscle, hepatic tissue, and spleen, with a localization in the former, although a more apparent toxic change in the latter two. The toxicity and symptoms of CPA poisoning can be attributed to its ability to alter normal intracellular calcium flux through its inhibition of the reticular form of the Ca2+-ATPase pump. CPA was not teratogenic in mice. CPA is not considered a carcinogen and the weight of evidence militates against its characterization as a mutagen. Despite CPA-induced pathological changes ascribed to the spleen or bursa of Fabricius, there does not appear to be an effect on the immune system. In vitro studies imply a potential immunomodulatory effect of CPA, but in all of those reports very high concentrations of CPA were required and none of these findings have been supported with in vivo studies. Therefore, based on a NOEL of 1 mg/kg/day and accounting for species variation, an appropriate acceptable daily intake (ADI) would be approximately 10 μg/kg/day or 700 μg/day. In the context of human exposure, if the uppermost limit of CPA found in cheese is 4 μg/g and the average individual consumes 50 g of cheese daily, this allows an intake of 200 μg, less than one third of a traditionally established ADI.

1,2-Dichlorobenzene affects the formation of the phosphoenzyme stage during the catalytic cycle of the Ca(2+)-ATPase from sarcoplasmic reticulum

BACKGROUND

1,2-Dichlorobenzene (1,2-DCB) is a benzene-derived molecule with two Cl atoms that is commonly utilized in the synthesis of pesticides. 1,2-DCB can be absorbed by living creatures and its effects on naturally-occurring enzymatic systems, including the effects on Ca(2+)-ATPases, have been poorly studied. Therefore, we aimed to study the effect of 1,2-DCB on the Ca(2+)-ATPase from sarcoplasmic reticulum (SERCA), a critical regulator of intracellular Ca(2+) concentration.

RESULTS

Concentrations of 0.05-0.2 mM of 1,2-DCB were able to stimulate the hydrolytic activity of SERCA in a medium-containing Ca(2+)-ionophore. At higher concentrations (0.25-0.75 mM), 1,2-DCB inhibited the ATP hydrolysis to ~80 %. Moreover, ATP hydrolysis and Ca(2+) uptake in a medium supported by K-oxalate showed that starting at 0.05 mM,1,2-DCB was able to uncouple the ratio of hydrolysis/Ca(2+) transported. The effect of this compound on the integrity of the SR membrane loaded with Ca(2+) remained unaffected. Finally, the analysis of phosphorylation of SERCA by [γ-(32)P]ATP, starting under different conditions at 0° or 25 °C showed a reduction in the phosphoenzyme levels by 1,2-DCB, mostly at 0 °C.

CONCLUSIONS

The temperature-dependent decreased levels of phosphoenzyme by 1,2-DCB could be due to the acceleration of the dephosphorylation mechanism - E2P · Ca2 state to E2 and Pi, which explains the uncoupling of the ATP hydrolysis from the Ca(2+) transport.

3-Acylated tetramic and tetronic acids as natural metal binders: myth or reality?

Covering: up to 20153-Acylated tetramic and tetronic acids are characterized by a low pKa and are likely to be deprotonated under physiological conditions. In addition, their structure makes them excellent chelators of metallic cations. We will discuss the significance of these chemical properties with regard to the biological properties and mechanisms of action of these compounds, highlighting the importance of considering them as salts or chelates for biological purposes, rather than acids.

Cytotoxicity and immunotoxicity of cyclopiazonic acid on human cells

In this study, in vitro cytotoxicity and immunotoxicity of the mycotoxin cyclopiazonic acid (CPA) was evaluated on human cells. To evaluate cytoxicity, several cellular targets were used (CD34+, monocytes, THP-1 and Caco-2). Monocytes were more sensitive to CPA than the THP-1 monocytic cell line after 48h of incubation in the tested conditions. Half maximal inhibitory concentration (IC50) were determined to be 8.5 × 10(-8) and 1.75 × 10(-7)M for monocytes and THP1, respectively, while IC50>1.25 × 10(-7)M was observed for Caco-2 and CD34+ cells. The CPA effect on macrophage differentiation was also examined at non-cytotoxic concentrations. The monocyte differentiation process was markedly disturbed in the presence of CPA. After 6 days of culture, CD71 expression was downregulated, while CD14 and CD11a expressions did not change. Moreover, activated macrophages showed a raised burst activity and TNF-α secretion. Overall, the results indicated that CPA exhibited toxicity on various human cellular models. Moreover, at non-cytotoxic concentrations, CPA disturbed human monocytes differentiation into macrophages. This work contributes to understanding the immunosuppressive properties of this food-related toxin.

Ion pumps as biological targets for decavanadate

The putative applications of poly-, oligo- and mono-oxometalates in biochemistry, biology, pharmacology and medicine are rapidly attracting interest. In particular, these compounds may act as potent ion pump inhibitors and have the potential to play a role in the treatment of e.g. ulcers, cancer and ischemic heart disease. However, the mechanism of action is not completely understood in most cases, and even remains largely unknown in other cases. In the present review we discuss the most recent insights into the interaction between mono- and polyoxometalate ions with ion pumps, with particular focus on the interaction of decavanadate with Ca(2+)-ATPase. We also compare the proposed mode of action with those of established ion pump inhibitors which are currently in therapeutic use. Of the 18 classes of compounds which are known to act as ion pump inhibitors, the complete mechanism of inhibition is only known for a handful. It has, however, been established that most ion pump inhibitors bind mainly to the E2 ion pump conformation within the membrane domain from the extracellular side and block the cation release. Polyoxometalates such as decavanadate, in contrast, interact with Ca(2+)-ATPase near the nucleotide binding site domain or at a pocket involving several cytoplasmic domains, and therefore need to cross through the membrane bilayer. In contrast to monomeric vanadate, which only binds to the E2 conformation, decavanadate binds to all protein conformations, i.e. E1, E1P, E2 and E2P. Moreover, the specific interaction of decavanadate with sarcoplasmic reticulum Ca(2+)-ATPase has been shown to be non-competitive with respect to ATP and induces protein cysteine oxidation with concomitant vanadium reduction which might explain the high inhibitory capacity of V10 (IC50 = 15 μM) which is quite similar to the majority of the established therapeutic drugs.

Probing determinants of cyclopiazonic acid sensitivity of bacterial Ca2+-ATPases

Cyclopiazonic acid (CPA) is a specific and potent inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase 1a (SERCA1a). Despite high sequence similarity to SERCA1a, Listeria monocytogenes Ca(2+)-ATPase 1 (LMCA1) is not inhibited by CPA. To test whether a CPA binding site could be created while maintaining the functionality of the ATPase we targeted four amino acid positions in LMCA1 for mutational studies based on a multiple sequence alignment of SERCA-like Ca(2+)-ATPases and structural analysis of the CPA site. The identification of CPA-sensitive gain-of-function mutants pinpointed key determinants of the CPA binding site. The importance of these determinants was further underscored by the characterization of the CPA sensitivity of two additional bacterial Ca(2+)-ATPases from Lactococcus lactis and Bacillus cereus. The CPA sensitivity was predicted from their sequence compared with the LMCA1 results, and this was experimentally confirmed. Interestingly, a cluster of Lactococcus bacteria applied in the production of fermented cheese display Ca(2+)-ATPases that are predictably CPA insensitive and may originate from their coexistence with CPA-producing Penicillum and Aspergillus fungi in the cheese. The differences between bacterial and mammalian binding pockets encompassing the CPA site suggest that CPA derivatives that are specific for bacteria or other pathogens can be developed.

Hippocampal Gαq/₁₁ but not Gαo-coupled receptors are altered in aging

Normal aging may limit the signaling efficacy of certain GPCRs by disturbing the function of specific Gα-subunits and leading to deficient modulation of intracellular functions that subserve synaptic plasticity, learning and memory. Evidence suggests that Gαq/₁₁ is more sensitive to the effects of aging relative to other Gα-subunits, including Gαo. To test this hypothesis, the functionality of Gαq/₁₁ and Gαo were compared in the hippocampus of young (6 months) and aged (24 months) F344 × BNF₁ hybrid rats assessed for spatial learning ability. Basal GTPγS-binding to Gαq/₁₁ was significantly elevated in aged rats relative to young and but not reliably associated with spatial learning. mAChR stimulation of Gαq/₁₁ with oxotremorine-M produced equivocal GTPγS-binding between age groups although values tended to be lower in the aged hippocampus and were inversely related to basal activity. Downstream Gαq/₁₁ function was measured in hippocampal subregion CA₁ by determining changes in [Ca(2+)]i after mAChR and mGluR (DHPG) stimulation. mAChR-stimulated peak change in [Ca(2+)]i was lower in aged CA₁ relative to young while mGluR-mediated integrated [Ca(2+)]i responses tended to be larger in aged. GPCR modulation of [Ca(2+)]i was observed to depend on intracellular stores to a greater degree in aged than young. In contrast, measures of Gαo-mediated GTPγS-binding were stable across age, including basal, mAChR-, GABABR (baclofen)-stimulated levels. Overall, the data indicate that aging selectively modulates the activity of Gαq/₁₁ within the hippocampus leading to deficient modulation of [Ca(2+)]i following stimulation of mAChRs but these changes are not related to spatial learning.

Discovery of novel SERCA inhibitors by virtual screening of a large compound library

Two screening protocols based on recursive partitioning and computational ligand docking methodologies, respectively, were employed for virtual screens of a compound library with 345,000 entries for novel inhibitors of the enzyme sarco/endoplasmic reticulum calcium ATPase (SERCA), a potential target for cancer chemotherapy. A total of 72 compounds that were predicted to be potential inhibitors of SERCA were tested in bioassays and 17 displayed inhibitory potencies at concentrations below 100 μM. The majority of these inhibitors were composed of two phenyl rings tethered to each other by a short link of one to three atoms. Putative interactions between SERCA and the inhibitors were identified by inspection of docking-predicted poses and some of the structural features required for effective SERCA inhibition were determined by analysis of the classification pattern employed by the recursive partitioning models.

Comparison of current docking tools for the simulation of inhibitor binding by the transmembrane domain of the sarco/endoplasmic reticulum calcium ATPase

Inhibitors of the transmembrane protein sarco/endoplasmic reticulum calcium ATPase (SERCA) are invaluable tools for the study of the enzyme's physiological functions and they have been recognized as a promising new class of anticancer agents. For the discovery of novel enzyme inhibitors, small molecule docking for virtual screens of large compound libraries has become increasingly important. Since the performance of various docking routines varies considerably, depending on the target and the chemical nature of the ligand, we critically evaluated the performance of four frequently used programs - GOLD, AutoDock, Surflex-Dock, and FRED - for the docking of SERCA inhibitors based on the structures of thapsigargin, di-tert-butylhydroquinone, and cyclopiazonic acid. Evaluation criteria were docking accuracy using crystal structures as references, docking reproducibility, and correlation between docking scores and known bioactivities. The best overall results were obtained by GOLD and FRED. Docking runs with conformationally flexible binding sites produced no significant improvement of the results.

P-type ATPases as drug targets: tools for medicine and science

P-type ATPases catalyze the selective active transport of ions like H+, Na+, K+, Ca2+, Zn2+, and Cu2+ across diverse biological membrane systems. Many members of the P-type ATPase protein family, such as the Na+,K+-, H+,K+-, Ca2+-, and H+-ATPases, are involved in the development of pathophysiological conditions or provide critical function to pathogens. Therefore, they seem to be promising targets for future drugs and novel antifungal agents and herbicides. Here, we review the current knowledge about P-type ATPase inhibitors and their present use as tools in science, medicine, and biotechnology. Recent structural information on a variety of P-type ATPase family members signifies that all P-type ATPases can be expected to share a similar basic structure and a similar basic machinery of ion transport. The ion transport pathway crossing the membrane lipid bilayer is constructed of two access channels leading from either side of the membrane to the ion binding sites at a central cavity. The selective opening and closure of the access channels allows vectorial access/release of ions from the binding sites. Recent structural information along with new homology modeling of diverse P-type ATPases in complex with known ligands demonstrate that the most proficient way for the development of efficient and selective drugs is to target their ion transport pathway.

Cyclopiazonic acid is complexed to a divalent metal ion when bound to the sarcoplasmic reticulum Ca2+-ATPase

We have determined the structure of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) in an E2.P(i)-like form stabilized as a complex with MgF(4)(2-), an ATP analog, adenosine 5'-(beta,gamma-methylene)triphosphate (AMPPCP), and cyclopiazonic acid (CPA). The structure determined at 2.5A resolution leads to a significantly revised model of CPA binding when compared with earlier reports. It shows that a divalent metal ion is required for CPA binding through coordination of the tetramic acid moiety at a characteristic kink of the M1 helix found in all P-type ATPase structures, which is expected to be part of the cytoplasmic cation access pathway. Our model is consistent with the biochemical data on CPA function and provides new measures in structure-based drug design targeting Ca(2+)-ATPases, e.g. from pathogens. We also present an extended structural basis of ATP modulation pinpointing key residues at or near the ATP binding site. A structural comparison to the Na(+),K(+)-ATPase reveals that the Phe(93) side chain occupies the equivalent binding pocket of the CPA site in SERCA, suggesting an important role of this residue in stabilization of the potassium-occluded E2 state of Na(+),K(+)-ATPase.

Structure-based virtual screening for novel inhibitors of the sarco/endoplasmic reticulum calcium ATPase and their experimental evaluation

A public compound library with 260,000 compounds was screened virtually by computational docking for novel inhibitors of the transmembrane enzyme sarco/endoplasmic reticulum calcium ATPase (SERCA). Docking was performed with the program GOLD in conjunction with a high resolution X-ray crystal structure of SERCA. Compounds that were predicted to be active were tested in bioassays. Nineteen novel compounds were discovered that were capable of inhibiting the ATP hydrolysis activity of SERCA at concentrations below 50 microM. Crucial enzyme/inhibitor interactions were identified by analyzing the docking-predicted binding poses of active compounds. Like other SERCA inhibitors, the newly discovered compounds are of considerable medicinal interest because of their potential for cancer chemotherapy.

Capsazepine, a synthetic vanilloid that converts the Na,K-ATPase to Na-ATPase

Capsazepine (CPZ), a synthetic capsaicin analogue, inhibits ATP hydrolysis by Na,K-ATPase in the presence but not in the absence of K(+). Studies with purified membranes revealed that CPZ reduced Na(+)-dependent phosphorylation by interference with Na(+) binding from the intracellular side of the membrane. Kinetic analyses showed that CPZ stabilized an enzyme species that constitutively occluded K(+). Low-affinity ATP interaction with the enzyme was strongly reduced after CPZ treatment; in contrast, indirectly measured interaction with ADP was much increased, which suggests that composite regulatory communication with nucleotides takes place during turnover. Studies with lipid vesicles revealed that CPZ reduced ATP-dependent digitoxigenin-sensitive (22)Na(+) influx into K(+)-loaded vesicles only at saturating ATP concentrations. The drug apparently abolishes the regulatory effect of ATP on the pump. Drawing on previous homology modeling studies of Na,K-ATPase to atomic models of sarcoplasmic reticulum Ca-ATPase and on kinetic data, we propose that CPZ uncouples an Na(+) cycle from an Na(+)/K(+) cycle in the pump. The Na(+) cycle possibly involves transport through the recently characterized Na(+)-specific site. A shift to such an uncoupled mode is believed to produce pumps mediating uncoupled Na(+) efflux by modifying the transport stoichiometry of single pump units.

Effects of high-affinity inhibitors on partial reactions, charge movements, and conformational States of the Ca2+ transport ATPase (sarco-endoplasmic reticulum Ca2+ ATPase)

The inhibitory effects of thapsigargin, cyclopiazonic acid, and 2,5-di(tert-butyl)hydroquinone, and 1,3-dibromo-2,4,6-tri(methylisothiouronium)benzene on the Ca(2+) ATPase were characterized by comparative measurements of sequential reactions of the catalytic and transport cycle, including biochemical measurements and detection of charge movements within a single cycle. In addition, patterns of ATPase proteolytic digestion with proteinase K were derived to follow conformational changes through the cycle or after inhibitor binding. We find that thapsigargin, cyclopiazonic acid, and 2,5-di(tert-butyl)hydroquinone inhibit Ca(2+) binding and catalytic activation as demonstrated with isotopic tracers and lack of charge movement upon addition of Ca(2+) in the absence of ATP. It has been shown previously that binding of these inhibitors requires the E2 conformational state of the ATPase, obtained in the absence of Ca(2+). We demonstrate here that E2 state conformational features are in fact induced by these inhibitors on the ATPase even in the presence of Ca(2+). The resulting dead-end complex interferes with progress of the catalytic and transport cycle. Inhibition by 1,3-dibromo-2,4,6-tri(methylisothiouronium)benzene, on the other hand, is related to interference with a conformational transition of the phosphorylated intermediate (E1 approximately P . 2Ca(2+) to E2-P . 2Ca(2+) transition), as demonstrated by increased phosphoenzyme levels and absence of bound Ca(2+) translocation upon addition of ATP. This transition includes large movements of ATPase headpiece domains and transmembrane segments, produced through utilization of ATP-free energy as the "conformational work" of the pump. We conclude that the mechanism of high-affinity Ca(2+) ATPase inhibitors is based on global effects on protein conformation that interfere with ATPase cycling.

Molecular determinants of sarco/endoplasmic reticulum calcium ATPase inhibition by hydroquinone‐based compounds

The ion transport activity of the sarco/endoplasmic reticulum calcium ATPase (SERCA) is specifically and potently inhibited by the small molecule 2,5-di-tert-butylhydroquinone (BHQ). In this study, we investigated the relative importance of the nature and position of BHQ's four substituents for enzyme inhibition by employing a combination of experimental and computational techniques. The inhibitory potencies of 21 commercially available or synthesized BHQ derivatives were determined in ATPase activity assays, and 11 compounds were found to be active. Maximum inhibitory potency was observed in compounds with two para hydroxyl groups, whereas BHQ analogues with only one hydroxyl group were still active, albeit with a reduced potency. The results also demonstrated that two alkyl groups were an absolute requirement for activity, with the most potent compounds having 2,5-substituents with four or five carbon atoms at each position. Using the program GOLD in conjunction with the ChemScore scoring function, the structures of the BHQ analogues were docked into the crystal structure of SERCA mimicking the enzyme's E(2) conformation. Analysis of the docking results indicated that inhibitor binding to SERCA was primarily mediated by a hydrogen bond between a hydroxyl group and Asp-59 and by hydrophobic interactions involving the bulky inhibitor alkyl groups. Attempts to dock BHQ into crystal structures corresponding to the E(1) conformation of the enzyme failed, because the conformational changes accompanying the E(2)/E(1) transition severely restricted the size of the binding site, suggesting that BHQ stabilizes the enzyme in its E(2) form. The potential role of Glu309 in enzyme inhibition is discussed in the context of the computational results. The docking scores correlated reasonably well with the measured inhibitory potencies and allowed the distinction between active and inactive compounds, which is a key requirement for future virtual screening of large compound databases for novel SERCA inhibitors.

Interdomain communication in calcium pump as revealed in the crystal structures with transmembrane inhibitors

Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum is an ATP-driven Ca(2+) pump consisting of three cytoplasmic domains and 10 transmembrane helices. In the absence of Ca(2+), the three cytoplasmic domains gather to form a compact headpiece, but the ATPase is unstable without an inhibitor. Here we describe the crystal structures of Ca(2+)-ATPase in the absence of Ca(2+) stabilized with cyclopiazonic acid alone and in combination with other inhibitors. Cyclopiazonic acid is located in the transmembrane region of the protein near the cytoplasmic surface. The binding site partially overlaps with that of 2,5-di-tert-butyl-1,4-dihydroxybenzene but is separate from that of thapsigargin. The overall structure is significantly different from that stabilized with thapsigargin: The cytoplasmic headpiece is more upright, and the transmembrane helices M1-M4 are rearranged. Cyclopiazonic acid primarily alters the position of the M1' helix and thereby M2 and M4 and then M5. Because M5 is integrated into the phosphorylation domain, the whole cytoplasmic headpiece moves. These structural changes show how an event in the transmembrane domain can be transmitted to the cytoplasmic domain despite flexible links between them. They also reveal that Ca(2+)-ATPase has considerable plasticity even when fixed by a transmembrane inhibitor, presumably to accommodate thermal fluctuations.

The molecular basis for cyclopiazonic acid inhibition of the sarcoplasmic reticulum calcium pump

The sarcoplasmic reticulum Ca(2+)-ATPase is essential for calcium reuptake in the muscle contraction-relaxation cycle. Here we present structures of a calcium-free state with bound cyclopiazonic acid (CPA) and magnesium fluoride at 2.65 A resolution and a calcium-free state with bound CPA and ADP at 3.4A resolution. In both structures, CPA occupies the calcium access channel delimited by transmembrane segments M1-M4. Inhibition of Ca(2+)-ATPase is stabilized by a polar pocket that surrounds the tetramic acid of CPA and a hydrophobic platform that cradles the inhibitor. The calcium pump residues involved include Gln(56), Leu(61), Val(62), and Asn(101). We conclude that CPA inhibits the calcium pump by blocking the calcium access channel and immobilizing a subset of transmembrane helices. In the E2(CPA) structure, ADP is bound in a distinct orientation within the nucleotide binding pocket. The adenine ring is sandwiched between Arg(489) of the nucleotide-binding domain and Arg(678) of the phosphorylation domain. This mode of binding conforms to an adenine recognition motif commonly found in ATP-dependent proteins.

Mechanisms of digoxin and digitoxin on the production of corticosterone in zona fasciculata-reticularis cells of ovariectomized rats

Previous studies have indicated that digoxin (DG) inhibits testosterone production by rat testicular interstitial cells through both in vivo and in vitro experiments. DG and digitoxin (DT), but not ouabain, inhibit the progesterone, pregnenolone, and corticosterone secretion by rat granulosa cells, luteal cells, and zona fasciculata-reticularis (ZFR) cells, respectively. However, the effect of DG and DT on the enzyme kinetics of cytochrome P450 side chain cleavage enzyme (P450scc), the protein expression of P450scc and steroidogenic acute regulatory protein (StAR), and mRNA expression of StAR are unclear. ZFR cells were prepared from adrenocortical tissues of ovariectomized rats, and then challenged with adrenocorticotropin (ACTH), 8-Br-cAMP, forskolin, A23187, cyclopiazonic acid (CPA), nicotinic acid adenine dinucleotide phosphate (NAADP), trilostane, 25-OH-Cholesterol, progesterone, or deoxycorticosterone in the presence of DG, DT, or ouabain for 1 h. Enzyme kinetics of P450scc, protein expression of acute regulatory protein (StAR) and P450scc, and mRNA expression of StAR were investigated. DG and DT but not ouabain suppressed basal and other evoked-corticosterone release significantly. DG and DT also inhibited pregnenolone production. The Vmax of the DG and DT group was the same as the control group, but the Km was higher in DG- and DT-treated group than in control group. DT and ouabain significant suppressed mRNA expression of StAR. DG and DT had no effect on the P450scc and StAR protein expression at basal state, but diminished ACTH-induced StAR protein expression to basal level. These results indicated that DG and DT have an inhibitory effect on corticosterone production via a Na+, K+-ATPase-independent mechanism by diminishing actions on cAMP-, Ca2+-pathway, competitive inhibition of P450scc enzyme and reduction of StAR mRNA expression.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

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.

Inhibition of sarcoplasmic reticulum Ca2+-ATPase by miconazole

The inhibition of sarcoplasmic reticulum Ca2+-ATPase activity by miconazole was dependent on the concentration of ATP and membrane protein. Half-maximal inhibition was observed at 12 microM miconazole when the ATP concentration was 50 microM and the membrane protein was 0.05 mg/ml. When ATP was 1 mM, a low micromolar concentration of miconazole activated the enzyme, whereas higher concentrations inhibited it. A qualitatively similar response was observed when Ca2+ transport was measured. Likewise, the half-maximal inhibition value was higher when the membrane concentration was raised. Phosphorylation studies carried out after sample preequilibration in different experimental settings shed light on key partial reactions such as Ca2+ binding and ATP phosphorylation. The miconazole effect on Ca2+-ATPase activity can be attributed to stabilization of the Ca2+-free enzyme conformation giving rise to a decrease in the rate of the Ca2+ binding transition. The phosphoryl transfer reaction was not affected by miconazole.

Endoplasmic reticulum Ca2+-ATPase pump is up-regulated in calcium-transporting dental enamel cells: a non-housekeeping role for SERCA2b

Dental enamel-forming cells face a major challenge to avoid the cytotoxic effects of excess calcium. We have characterized sarcoplasmic/endoplasmic reticulum calcium-ATPase pumps (SERCA) in rat enamel cells to address the proposal that non-mitochondrial calcium stores play a dominant role in transcellular calcium transport. A single major isoform, SERCA2b, was detected during the protein-secretory and calcium-transport stages of enamel formation using reverse-transcriptase PCR, cDNA cloning, Northern analysis and immunoblotting. Most importantly, SERCA2b exhibited a specific 3-fold up-regulation to high expression levels during calcium transport, as determined by quantitative immunoblotting and ATPase assays. Sensitivity of the calcium-dependent ATPase to thapsigargin and three other SERCA inhibitors was characterized. These findings indicate that enamel cells are well-equipped to sequester calcium in endoplasmic reticulum stores and so protect against calcium toxicity, associate SERCA with transcellular calcium transport for the first time, and establish SERCA2b as a molecular and pharmacological target for future investigations of calcium transcytosis. The observed physiological regulation in enamel cells contradicts the widespread perception that SERCA2b is restricted to general housekeeping duties.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

Sustained high O2 use for Ca2+ handling in rat ventricular slices under decreased free shortening after ryanodine

We hypothesized that O2 wasting of Ca2+ handling in the excitation-contraction coupling in ryanodine-treated failing hearts might derive from an increased external Ca2+ extrusion via Na+/Ca2+ exchanger and futile Ca2+ cycling via sarcoplasmic reticulum (SR) Ca2+-ATPase. We tested this hypothesis by mechanoenergetic studies using rat left ventricular slices. After the slices were treated with ryanodine (0.1 microM), 1-Hz free shortening significantly decreased by 78-85%, whereas the observed O2 consumption (VO2) required for total Ca2+ handling, increased from 0.79 to 1.13 ml O2 x min(-1) x 100 g x LV(-1) (155.6% of control). We reconfirmed that cyclopiazonic acid (10 microM), a blocker of SR Ca2+-ATPase, decreased VO2 by 75-80% in normal slices. However, 100 microM of cyclopiazonic acid was needed to inhibit the VO2 by 80% after ryanodine treatment. Blockade of a sarcolemmal Na+/Ca2+ exchanger by KB-R7943 (10 microM) significantly decreased VO2 by 45% after ryanodine treatment without significant effects on normal slices. Our results indicated that the VO2 increase following ryanodine treatment was derived from a net change of an increased external Ca2+ extrusion via Na+/Ca2+ exchanger and futile Ca2+ cycling via SR Ca2+-ATPase.

Insight into the uncoupling mechanism of sarcoplasmic reticulum ATPase using the phosphorylating substrate UTP

Ca(2+) transport and UTP hydrolysis catalyzed by sarcoplasmic reticulum Ca(2+)-ATPase from skeletal muscle was studied. A passive Ca(2+) load inside microsomal vesicles clearly decreased the net uptake rate and the final accumulation of Ca(2+) but not the UTP hydrolysis rate, causing energy uncoupling. In the absence of passive leak, the Ca(2+)/P(i) coupling ratio was 0.7-0.8. UTP hydrolysis did not maintain a rapid component of Ca(2+) exchange between the cytoplasmic and lumenal compartments as occurs with ATP. The uncoupling process in the presence of UTP is associated with: (i) the absence of a steady state accumulation of ADP-insensitive phosphoenzyme; (ii) the cytoplasmic dissociation of Ca(2+) bound to the ADP-sensitive phosphoenzyme; and (iii) the absence of enzyme inhibition by cyclopiazonic acid. All these characteristics confirm the lack of enzyme conformations with low Ca(2+) affinity and point to the existence of an uncoupling mechanism mediated by a phosphorylated form of the enzyme. Suboptimal coupling values can be explained in molecular terms by the proposed functional model.

Clomipramine and related structures as inhibitors of the skeletal sarcoplasmic reticulum Ca2+ pump

The Ca2+-pumping activity of skeletal sarcoplasmic reticulum vesicles is half-maximally inhibited by 120 microM clomipramine, 250 microM desipramine, and 500 microM imipramine or trimipramine. The inhibition is attributed to the dihydrodibenzazepine moiety, since 3-(dimethylamino)propionitrile, reproducing the aliphatic amine chain, has no inhibitory action. The inhibition is shown as a marked decrease of Ca2+ binding at equilibrium in the absence of ATP and as a reduction of phosphorylation of the Ca2+-free conformation by inorganic phosphate. Therefore, the drug effect is consistent with preferential interaction of tricyclic antidepressants with the Ca2+-free conformation of the nonphosphorylated enzyme. An additional decrease in the apparent rate constant of enzyme dephosphorylation, i.e., in the release of phosphate from ATP during enzyme cycling was also noticed.

Effects of three different Ca(2+) pump ATPase inhibitors on evoked contractions in rabbit aorta and activities of Ca(2+) pump ATPases in porcine aorta

Using vascular smooth muscle, we describe the actions of three pharmacological tools, cyclopiazonic acid (CPA), thapsigargin (TG) and 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBHQ), which are presumed to act as selective inhibitors of the sarco-endoplasmic reticulum Ca(2+)-ATPases (SERCAs). In porcine aortic smooth muscle microsomes two Ca(2+)-ATPase activities have been described, one vanadate-sensitive and one vanadate-resistant, representing the Ca(2+)-ATPase activities of the plasma membrane and SERCAs, respectively. In agreement, CPA, TG and tBHQ, in the concentration range 0.1 microM to 0.1 mM, dose-dependently inhibit the Ca(2+)-ATPase activity only in the vanadate-resistant microsomes. However, 0.1 mM tBHQ also significantly inhibited the Ca(2+)-ATPase activity of vanadate-sensitive microsomes. In rabbit aortic rings, all three SERCA inhibitors produced a dose-dependant inhibition of contractions evoked by 20 mM caffeine or 1 microM phenylephrine (PE) in a Ca(2+)-free physiological solution. However, in PE-contracted rings, tBHQ (> or =30 microM) also significantly inhibited the ability of cromakalim to induce relaxation. In conclusion, the data suggest that CPA, TG and tBHQ can all act as selective SERCA inhibitors in both porcine and rabbit aortic smooth muscle. However, in contrast to CPA and TG, high concentrations of tBHQ can exhibit some nonspecific effects, which include inhibition of the plasma membrane Ca(2+)-ATPase and possibly K(+) channels regulated by cromakalim.

Inactivation of P2X2 purinoceptors by divalent cations

1. P2X2 channels are activated by extracellular ATP. Despite being commonly described as non-desensitizing, P2X2 receptors do desensitize or inactivate. In the unspliced, 472 amino acid isoform of the P2X2 receptor, inactivation required membrane disruption and the presence of extracellular Ca2+. 2. The ability to inactivate whole-cell currents developed slowly after breaking in. In contrast, currents from excised patches exhibited rapid (approximately 100 ms) inactivation with a dependence on extracellular Ca2+, ATP and voltage. 3. The inactivation rate increased with the fourth power of [Ca2+] suggesting that the functional channel may be a tetramer. Ca2+ had both a higher affinity and a larger Hill coefficient for inactivation than Mg2+, Ba2+ or Mn2+. Trivalent cations at concentrations up to the solubility product of ATP had no effect. The change in apparent co-operativity with ionic species suggests the presence of experimentally unresolved ligand-insensitive kinetic steps. 4. Based on the weak voltage dependence of inactivation and the lack of effect of intracellular Ca2+ buffers, the Ca2+-binding sites are probably located near the extracellular surface of the membrane. 5. The recovery from inactivation was slow, with a time constant of approximately 7 min. 6. Ca2+-sensitive inactivation only appeared when the membrane was disrupted in some manner. Treatment with actin and microtubule reagents did not induce inactivation, suggesting that an intact cytoskeleton is not necessary. 7. Inactivation rates observed in different patch configurations suggest that the induction of Ca2+-dependent inactivation was due to the loss of a diffusible cofactor located in the membrane or the cytoplasm.

Overlapping effects of S3 stalk segment mutations on the affinity of Ca2+-ATPase (SERCA) for thapsigargin and cyclopiazonic acid

Chimeric exchanges and mutations were produced in the Ca(2+)-ATPase (SERCA) to match (in the majority of cases) corresponding sequences of the Na(+),K(+)-ATPase. The effects of these mutations on the concentration dependence of the specific Ca(2+)-ATPase inhibition by thapsigargin (TG) and cyclopiazonic acid (CPA) were then determined. Extensive chimeric mutations on the large cytosolic loop, on the S4 stalk segment, and on the M3 transmembrane segments produced little or no modification of the Ca(2+)-ATPase sensitivity to either inhibitor. On the other hand, the presence of a six amino acid Na(+), K(+)-ATPase sequence within the S3 stalk segment of the Ca(2+)-ATPase raised 60-fold the apparent K(i) for TG and 250-fold the apparent K(i) for CPA. More limited mutations within the same S3 segment, however, affected differently the concentration dependence of the Ca(2+)-ATPase inhibition by TG or CPA. Specifically, single mutation of Phe256 to Val increased 20-fold the apparent K(i) for TG, while having very little effect on the apparent K(i) for CPA. These findings indicate significant overlap of the TG and CPA binding domains within the S3 stalk segment of the Ca(2+)-ATPase, where the contribution of each protein residue is dependent on the structures of the two inhibitors. Saturating concentrations of either or both TG and CPA produce an identical reduction of the affinity of the ATPase for ATP, suggesting that only one inhibitor can bind at any time due to significant overlap of their binding domains. It is suggested that perturbations produced by binding of either inhibitor within the stalk segment interfere with the long-range functional linkage between ATP utilization in the ATPase cytosolic region and Ca(2+) binding in the membrane-bound region.