The inhibition of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase by macrocyclic lactones and cyclosporin A

Altered functional connectivity and network excitability in a model of cortical dysplasia

Focal cortical dysplasias (FCDs) are malformations of cortical development that often result in medically refractory epilepsy, with a greater incidence in the pediatric population. The relationship between the disturbed cortical morphology and epileptogenic activity of FCDs remains unclear. We used the BCNU (carmustine 1-3-bis-chloroethyl-nitrosourea) animal model of cortical dysplasia to evaluate neuronal and laminar alterations and how these result in altered activity of intracortical networks in early life. We corroborated the previously reported morphological anomalies characteristic of the BCNU model, comprising slightly larger and rounder neurons and abnormal cortical lamination. Next, the neuronal activity of live cortical slices was evaluated through large field-of-view calcium imaging as well as the neuronal response to a stimulus that leads to cortical hyperexcitability (pilocarpine). Examination of the joint activity of neuronal calcium time series allowed us to identify intracortical communication patterns and their response to pilocarpine. The baseline power density distribution of neurons in the cortex of BCNU-treated animals was different from that of control animals, with the former showing no modulation after stimulus. Moreover, the intracortical communication pattern differed between the two groups, with cortexes from BCNU-treated animals displaying decreased inter-layer connectivity as compared to control animals. Our results indicate that the altered anatomical organization of the cortex of BCNU-treated rats translates into altered functional networks that respond abnormally to a hyperexcitable stimulus and highlight the role of network dysfunction in the pathophysiology of cortical dysplasia.

Inhibition of SERCA and PMCA Ca2+-ATPase activities by polyoxotungstates

Plasma membrane calcium ATPases (PMCA) and sarco(endo) reticulum calcium ATPases (SERCA) are key proteins in the maintenance of calcium homeostasis. Herein, we compare for the first time the inhibition of SERCA and PMCA calcium pumps by several polyoxotungstates (POTs), namely by Wells-Dawson phosphotungstate anions [P₂W₁₈O₆₂]^6- (intact, {P₂W₁₈}), [P₂W₁₇O₆₁]^10- (monolacunary, {P₂W₁₇}), [P₂W₁₅O₅₆]^12- (trilacunary, {P₂W₁₅}), [H₂P₂W₁₂O₄₈]^12- (hexalacunary, {P₂W₁₂}), [H₃P₂W₁₅V₃O₆₂]^6- (trivanadium-substituted, {P₂W₁₅V₃}) and by Preyssler-type anion [NaP₅W₃₀O₁₁₀]^14- ({P₅W₃₀}). The speciation in the solutions of tested POTs was investigated by ³¹P and ⁵¹V NMR spectroscopy. The tested POTs inhibited SERCA Ca²⁺-ATPase activity, whereby the Preyssler POT showed the strongest effect, with an IC₅₀ value of 0.37 μM. For {P₂W₁₇} and {P₂W₁₅V₃} higher IC₅₀ values were determined: 0.72 and 0.95 μM, respectively. The studied POTs showed to be more potent inhibitors of PMCA Ca²⁺-ATPase activity, with lower IC₅₀ values for {P₂W₁₇}, {P₅W₃₀} and {P₂W₁₅V₃}.

Ivermectin represses Wnt/β-catenin signaling by binding to TELO2, a regulator of phosphatidylinositol 3-kinase-related kinases

Ivermectin (IVM), an avermectin-derivative anthelmintic, specifically binds to glutamate-gated chloride ion channels (GluCls), causing paralysis in invertebrates. IVM also exhibits other biological activities such as Wnt/β-catenin pathway inhibition in vertebrates that do not possess GluCls. This study showed that affinity purification using immobilized IVM B1a isolated TELO2, a cofactor of phosphatidylinositol 3-kinase-related kinases (PIKKs), as a specific IVM B1a-binding protein. TELO2 knockdown reduced cytoplasmic β-catenin and the transcriptional activation of β-catenin/TCF. IVM B1a bound to TELO2 through the C-terminal α-helix, in which mutations conferred IVM resistance. IVM reduced the TELO2 and PIKK protein levels and the AKT and S6 kinase phosphorylation levels. The inhibition of mTOR kinase reduced the cytoplasmic β-catenin level. Therefore, IVM binds to TELO2, inhibiting PIKKs and reducing the cytoplasmic β-catenin level. In conclusion, our data indicate TELO2 as a druggable target for human diseases involving abnormalities of the Wnt/β-catenin pathway and PIKKs, including mTOR.

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Ivermectin is a chemical suppressor of the eyeless phenotype in zebrafish embryos

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Ivermectin physically interacts with TELO2

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TELO2 mediates Wnt/β-catenin signaling inhibition by ivermectin

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Ivermectin reduces the PIKK protein levels and downstream signaling

Gold Compounds Inhibit the Ca2+-ATPase Activity of Brain PMCA and Human Neuroblastoma SH-SY5Y Cells and Decrease Cell Viability

Plasma membrane calcium ATPases (PMCA) are key proteins in the maintenance of calcium (Ca2+) homeostasis. Dysregulation of PMCA function is associated with several human pathologies, including neurodegenerative diseases, and, therefore, these proteins are potential drug targets to counteract those diseases. Gold compounds, namely of Au(I), are well-known for their therapeutic use in rheumatoid arthritis and other diseases for centuries. Herein, we report the ability of dichloro(2-pyridinecarboxylate)gold(III) (1), chlorotrimethylphosphinegold(I) (2), 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidenegold(I) chloride (3), and chlorotriphenylphosphinegold(I) (4) compounds to interfere with the Ca2+-ATPase activity of pig brain purified PMCA and with membranes from SH-SY5Y neuroblastoma cell cultures. The Au(III) compound (1) inhibits PMCA activity with the IC50 value of 4.9 µM, while Au(I) compounds (2, 3, and 4) inhibit the protein activity with IC50 values of 2.8, 21, and 0.9 µM, respectively. Regarding the native substrate MgATP, gold compounds 1 and 4 showed a non-competitive type of inhibition, whereas compounds 2 and 3 showed a mixed type of inhibition. All gold complexes showed cytotoxic effects on human neuroblastoma SH-SY5Y cells, although compounds 1 and 3 were more cytotoxic than compounds 2 and 4. In summary, this work shows that both Au (I and III) compounds are high-affinity inhibitors of the Ca2+-ATPase activity in purified PMCA fractions and in membranes from SH-SY5Y human neuroblastoma cells. Additionally, they exert strong cytotoxic effects.

Altered SERCA Expression in Breast Cancer

Background and Objectives: Calcium (Ca²⁺) signaling is critical for the normal functioning of various cellular activities. However, abnormal changes in cellular Ca²⁺ can contribute to pathological conditions, including various types of cancer. The maintenance of intracellular Ca²⁺ levels is achieved through tightly regulated processes that help maintain Ca²⁺ homeostasis. Several types of regulatory proteins are involved in controlling intracellular Ca²⁺ levels, including the sarco/endoplasmic reticulum (SR/ER) Ca²⁺ ATPase pump (SERCA), which maintains Ca²⁺ levels released from the SR/ER. In total, three ATPase SR/ER Ca²⁺-transporting (ATP2A) 1-3 genes exist, which encode for several isoforms whose expression profiles are tissue-specific. Recently, it has become clear that abnormal SERCA expression and activity are associated with various types of cancer, including breast cancer. Breast carcinomas represent 40% of all cancer types that affect women, with a wide variety of pathological and clinical conditions. Materials and methods: Using cBioPortal breast cancer patient data, Kaplan–Meier plots demonstrated that high ATP2A1 and ATP2A3 expression was associated with reduced patient survival. Results: The present study found significantly different SERCA specific-type expressions in a series of breast cancer cell lines. Moreover, bioinformatics analysis indicated that ATP2A1 and ATP2A3 expression was highly altered in patients with breast cancer. Conclusion: Overall, the present data suggest that SERCA gene-specific expressioncan possibly be considered as a crucial target for the control of breast cancer development and progression.

The Ca2+-ATPase Inhibition Potential of Gold(I, III) Compounds

The therapeutic applications of gold are well-known for many centuries. The most used gold compounds contain Au(I). Herein, we report, for the first time, the ability of four Au(I) and Au(III) complexes, namely dichloro (2-pyridinecarboxylate) Au(III) (abbreviated as 1), chlorotrimethylphosphine Au(I) (2), 1,3-bis(2,6-diisopropylphenyl) imidazole-2-ylidene Au(I) chloride (3), and chlorotriphenylphosphine Au(I) (4), to affect the sarcoplasmic reticulum (SR) Ca2+-ATPase activity. The tested gold compounds strongly inhibit the Ca2+-ATPase activity with different effects, being Au(I) compounds 2 and 4 the strongest, with half maximal inhibitory concentration (IC50) values of 0.8 and 0.9 µM, respectively. For Au(III) compound 1 and Au(I) compound 3, higher IC50 values are found (4.5 µM and 16.3 µM, respectively). The type of enzymatic inhibition is also different, with gold compounds 1 and 2 showing a non-competitive inhibition regarding the native substrate MgATP, whereas for Au compounds 3 and 4, a mixed type of inhibition is observed. Our data reveal, for the first time, Au(I) compounds with powerful inhibitory capacity towards SR Ca2+ATPase function. These results also show, unprecedently, that Au (III) and Au(I) compounds can act as P-type ATPase inhibitors, unveiling a potential application of these complexes.

Sarco/Endoplasmic Reticulum Calcium ATPase Inhibitors: Beyond Anticancer Perspective

The sarco/endoplasmic reticulum calcium ATPase (SERCA), which plays a key role in the maintenance of Ca²⁺ ion homeostasis, is an extensively studied enzyme, the inhibition of which has a considerable impact on cell life and death decision. To date, several SERCA inhibitors have been thoroughly studied and the most notable one, a derivative of the sesquiterpene lactone thapsigargin, is gradually approaching a clinical application. Meanwhile, new compounds with SERCA-inhibiting properties of natural, synthetic, or semisynthetic origin are being discovered and/or developed; some of these might also be suitable for the development of new drugs with improved performance. This review brings an up-to-date comprehensive overview of recently discovered compounds with the potential of SERCA inhibition, discusses their mechanism of action, and highlights their potential clinical applications, such as cancer treatment.

Targeting protein-protein interactions for therapeutic discovery via FRET-based high-throughput screening in living cells

We have developed a structure-based high-throughput screening (HTS) method, using time-resolved fluorescence resonance energy transfer (TR-FRET) that is sensitive to protein-protein interactions in living cells. The membrane protein complex between the cardiac sarcoplasmic reticulum Ca-ATPase (SERCA2a) and phospholamban (PLB), its Ca-dependent regulator, is a validated therapeutic target for reversing cardiac contractile dysfunction caused by aberrant calcium handling. However, efforts to develop compounds with SERCA2a-PLB specificity have yet to yield an effective drug. We co-expressed GFP-SERCA2a (donor) in the endoplasmic reticulum membrane of HEK293 cells with RFP-PLB (acceptor), and measured FRET using a fluorescence lifetime microplate reader. We screened a small-molecule library and identified 21 compounds (Hits) that changed FRET by >3SD. 10 of these Hits reproducibly alter SERCA2a-PLB structure and function. One compound increases SERCA2a calcium affinity in cardiac membranes but not in skeletal, suggesting that the compound is acting specifically on the SERCA2a-PLB complex, as needed for a drug to mitigate deficient calcium transport in heart failure. The excellent assay quality and correlation between structural and functional assays validate this method for large-scale HTS campaigns. This approach offers a powerful pathway to drug discovery for a wide range of protein-protein interaction targets that were previously considered “undruggable”.

A systematic assessment of chemical, genetic, and epigenetic factors influencing the activity of anticancer drug KP1019 (FFC14A)

KP1019 ([trans-RuCl₄(1H-indazole)₂]; FFC14A) is one of the promising ruthenium-based anticancer drugs undergoing clinical trials. Despite the pre-clinical and clinical success of KP1019, the mode of action and various factors capable of modulating its effects are largely unknown. Here, we used transcriptomics and genetic screening approaches in budding yeast model and deciphered various genetic targets and plethora of cellular pathways including cellular signaling, metal homeostasis, vacuolar transport, and lipid homeostasis that are primarily targeted by KP1019. We also demonstrated that KP1019 modulates the effects of TOR (target of rapamycin) signaling pathway and induces accumulation of neutral lipids (lipid droplets) in both yeast and HeLa cells. Interestingly, KP1019-mediated effects were found augmented with metal ions (Al³⁺/Ca²⁺/Cd²⁺/Cu²⁺/Mn²⁺/Na⁺/Zn²⁺), and neutralized by Fe²⁺, antioxidants, osmotic stabilizer, and ethanolamine. Additionally, our comprehensive screening of yeast histone H3/H4 mutant library revealed several histone residues that could significantly modulate the KP1019-induced toxicity. Altogether, our findings in both the yeast and HeLa cells provide molecular insights into mechanisms of action of KP1019 and various factors (chemical/genetic/epigenetic) that can alter the therapeutic efficiency of this clinically important anticancer drug.

Proteomic analysis of imatinib-resistant CML-T1 cells reveals calcium homeostasis as a potential therapeutic target

Chronic myeloid leukemia (CML) therapy has markedly improved patient prognosis after introduction of imatinib mesylate for clinical use. However, a subset of patients develops resistance to imatinib and other tyrosine kinase inhibitors (TKIs), mainly due to point mutations in the region encoding the kinase domain of the fused BCR-ABL oncogene. To identify potential therapeutic targets in imatinib-resistant CML cells, we derived imatinib-resistant CML-T1 human cell line clone (CML-T1/IR) by prolonged exposure to imatinib in growth media. Mutational analysis revealed that the Y235H mutation in BCR-ABL is probably the main cause of CML-T1/IR resistance to imatinib. To identify alternative therapeutic targets for selective elimination of imatinib-resistant cells, we compared the proteome profiles of CML-T1 and CML-T1/IR cells using 2-DE-MS. We identified eight differentially expressed proteins, with strongly upregulated Na⁺/H⁺ exchanger regulatory factor 1 (NHERF1) in the resistant cells, suggesting that this protein may influence cytosolic pH, Ca²⁺ concentration or signaling pathways such as Wnt in CML-T1/IR cells. We tested several compounds including drugs in clinical use that interfere with the aforementioned processes and tested their relative toxicity to CML-T1 and CML-T1/IR cells. Calcium channel blockers, calcium signaling antagonists and modulators of calcium homeostasis, namely thapsigargin, ionomycin, verapamil, carboxyamidotriazole and immunosuppressive drugs cyclosporine A and tacrolimus (FK-506) were selectively toxic to CML-T1/IR cells. The putative cellular targets of these compounds in CML-T1/IR cells are postulated in this study. We propose that Ca²⁺ homeostasis can be a potential therapeutic target in CML cells resistant to TKIs. We demonstrate that a proteomic approach may be used to characterize a TKI-resistant population of CML cells enabling future individualized treatment options for patients.

Kinetic characterization of Ca²⁺-ATPase (SERCA1) inhibition by tri-n-butyltin(IV) chloride. A docking conformation proposal

Organotin compounds, such as tri-n-butyltin(IV) chloride (TBT), are widespread toxicants which disrupt different functions in living organisms. TBT interacts with lipid membranes and membrane proteins. The inhibition of the calcium ATPase from sarcoplasmic reticulum membranes by TBT was studied. It was found that the ATPase inhibition could not be reverted in a large time scale; moreover, an excess of TBT over enzyme did not fully inhibit the ATPase activity; therefore, it was concluded that TBT irreversibly inhibits the enzyme, and this inhibition is accompanied by a decrease in the effective TBT concentration. The residual ATP hydrolysis activity was measured at different TBT concentrations with time, and the protective effect of different calcium concentrations on the TBT inhibition was also determined. The simplest kinetic mechanism to successfully explain all the observations and the kinetic behavior was found to be a single irreversible step of the inhibitor binding to the enzyme accompanied with a first-order inhibitor inactivation. A fluorescence study of fluorescein-5-isothiocyanate-labeled enzyme revealed that TBT binding to the enzyme entails a conformational change related to the high- to low-affinity calcium-binding state transition (E1 to E2 transition), resembling the conformational change induced by vanadate linked to the formation of E2 V complex from E1 state. A docking study allowed us to propose a binding pocket for TBT in the membrane region of E1 close to the high-affinity calcium-binding sites, as well as to define the interactions with amino acid residues interfering with calcium sites occupancy.

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.

Activation of rat transient receptor potential cation channel subfamily V member 1 channels by 2-aminoethoxydiphenyl borate

BACKGROUND

The transient receptor potential cation channel subfamily V member 1 (TRPV1) channel has been proved to be a molecular integrator of inflammatory pain sensation. 2-Aminoethoxydiphenyl borate (2-APB) and its analogs have been noticed as attractive candidates for the development of a selective TRPV1 agonist and/or antagonist. However, selectivity and effectiveness, species dependence, and the binding site(s) of 2-APB on TRPV1 channel protein remain controversial.

METHODS

The present study aimed to characterize acting sites of 2-APB on heterologously expressed rat TRPV1 (rTRPV1) channels in HEK 293 cells. Rat TRPV1 currents were recorded by cell-free, excised patch clamp techniques.

RESULTS

In inside-out and outside-out patch modes, 2-APB applied either side of the membrane dose-dependently activated rTRPV1 channels. 2-APB dose-dependently potentiated rTRPV1 currents, that activated by capsaicin, protons, or noxious heat. 2-APB potentiated the capsaicin-activated rTRPV1 current from both side of the patch membrane. A structural analogue of 2-APB, diphenylboronic anhydride, showed the same potentiation effect on the capsaicin-activated rTRPV1 current.

CONCLUSION

It is suggested that 2-APB directly opens rTRPV1 channels from both sides of the membrane and potentiates the opening of channels by inflammatory stimuli.

Long-term mTOR inhibitors administration evokes altered calcium homeostasis and platelet dysfunction in kidney transplant patients

The use of the mammal target of rapamycin (mTOR) inhibitors has been consolidated as the therapy of election for preventing graft rejection in kidney transplant patients, despite their immunosuppressive activity is less strong than anti-calcineurin agents like tacrolimus and cyclosporine A. Furthermore, as mTOR is widely expressed, rapamycin (a macrolide antibiotic produced by Streptomyces hygroscopicus) is recommended in patients presenting neoplasia due to its antiproliferative actions. Hence, we have investigated whether rapamycin presents side effects in the physiology of other cell types different from leucocytes, such as platelets. Blood samples were drawn from healthy volunteers and kidney transplant patients long-term medicated with rapamycin: sirolimus and everolimus. Platelets were either loaded with fura-2 or directly stimulated, and immunoassayed or fixed with Laemmli's buffer to perform the subsequent analysis of platelet physiology. Our results indicate that rapamycin evokes a biphasic time-dependent alteration in calcium homeostasis and function in platelets from kidney transplant patients under rapamycin regime, as demonstrated by the reduction in granule secretion observed and subsequent impairment of platelet aggregation in these patients compared with healthy volunteers. Platelet count was also reduced in these patients, thus 41% of patients presented thrombocytopenia. All together our results show that long-term administration of rapamycin to kidney transplant patients evokes alteration in platelet function.

Endoplasmic reticulum membrane reorganization is regulated by ionic homeostasis

Recently we described a new, evolutionarily conserved cellular stress response characterized by a reversible reorganization of endoplasmic reticulum (ER) membranes that is distinct from canonical ER stress and the unfolded protein response (UPR). Apogossypol, a putative broad spectrum BCL-2 family antagonist, was the prototype compound used to induce this ER membrane reorganization. Following microarray analysis of cells treated with apogossypol, we used connectivity mapping to identify a wide range of structurally diverse chemicals from different pharmacological classes and established their ability to induce ER membrane reorganization. Such structural diversity suggests that the mechanisms initiating ER membrane reorganization are also diverse and a major objective of the present study was to identify potentially common features of these mechanisms. In order to explore this, we used hierarchical clustering of transcription profiles for a number of chemicals that induce membrane reorganization and discovered two distinct clusters. One cluster contained chemicals with known effects on Ca²⁺ homeostasis. Support for this was provided by the findings that ER membrane reorganization was induced by agents that either deplete ER Ca²⁺ (thapsigargin) or cause an alteration in cellular Ca²⁺ handling (calmodulin antagonists). Furthermore, overexpression of the ER luminal Ca²⁺ sensor, STIM1, also evoked ER membrane reorganization. Although perturbation of Ca²⁺ homeostasis was clearly one mechanism by which some agents induced ER membrane reorganization, influx of extracellular Na⁺ but not Ca²⁺ was required for ER membrane reorganization induced by apogossypol and the related BCL-2 family antagonist, TW37, in both human and yeast cells. Not only is this novel, non-canonical ER stress response evolutionary conserved but so also are aspects of the mechanism of formation of ER membrane aggregates. Thus perturbation of ionic homeostasis is important in the regulation of ER membrane reorganization.

FKBP52 is involved in the regulation of SOCE channels in the human platelets and MEG 01 cells

Immunophilins are FK506-binding proteins that have been involved in the regulation of calcium homeostasis, either by modulating Ca(2+) channels located in the plasma membrane or in the rough endoplasmic reticulum (RE). We have investigated whether immunophilins would participate in the regulation of stored-operated Ca(2+) entry (SOCE) in human platelets and MEG 01. Both cell types were loaded with fura-2 for determining cytosolic calcium concentration changes ([Ca(2+)](c)), or stimulated and fixed to evaluate the protein interaction profile by performing immunoprecipitation and western blotting. We have found that incubation of platelets with FK506 increases Ca(2+) mobilization. Thapsigargin (TG)-evoked, Thr-evoked SOCE and TG-evoked Mn(2+) entry resulted in significant reduction by treatment of platelets with immunophilin antagonists. We confirmed by immunoprecipitation that immunophilins interact with transient receptor potential channel 1 (TRPC1) and Orai1 in human platelets. FK506 and rapamycin reduced the association between TRPC1 and Orai1 with FK506 binding protein (52) (FKBP52) in human platelets, and between TRPC1 and the type II IP(3)R, which association is known to be crucial for the maintenance of SOCE in human platelets. FKBP52 role in SOCE activation was confirmed by silencing FKBP52 using SiRNA FKBP52 in MEG 01 as demonstrated by single cell configuration imaging technique. TRPC1 silencing and depletion of cell of TRPC1 and FKBP52 simultaneously, impair activation of SOCE evoked by TG in MEG 01. Finally, in MEG 01 incubated with FK506 we observed a reduction in TRPC1/FKBP52 coupling, and similarly, FKBP52 silencing reduced the association between IP3R type II and TRPC1 during SOCE. All together, these results demonstrate that immunophilins participate in the regulation of SOCE in human platelets.

Intracellular cyclophilin A is an important Ca(2+) regulator in platelets and critically involved in arterial thrombus formation

Platelet adhesion and aggregation play a critical role in primary hemostasis. Uncontrolled platelet activation leads to pathologic thrombus formation and organ failure. The decisive central step for different processes of platelet activation is the increase in cytosolic Ca(2+) activity ([Ca(2+)](i)). Activation-dependent depletion of intracellular Ca(2+) stores triggers Ca(2+) entry from the extracellular space. Stromal interaction molecule 1 (STIM1) has been identified as a Ca(2+) sensor that regulates store-operated Ca(2+) entry through activation of the pore-forming subunit Orai1, the major store-operated Ca(2+) entry channel in platelets. In the present study, we show for the first time that the chaperone protein cyclophilin A (CyPA) acts as a Ca(2+) modulator in platelets. CyPA deficiency strongly blunted activation-induced Ca(2+) mobilization from intracellular stores and Ca(2+) influx from the extracellular compartment and thus impaired platelet activation substantially. Furthermore, the phosphorylation of the Ca(2+) sensor STIM1 was abrogated upon CyPA deficiency, as shown by immunoprecipitation studies. In a mouse model of arterial thrombosis, CyPA-deficient mice were protected against arterial thrombosis, whereas bleeding time was not affected. The results of the present study identified CyPA as an important Ca(2+) regulator in platelets, a critical mechanism for arterial thrombosis.

A diversity of SERCA Ca2+ pump inhibitors

The SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase) is probably the most extensively studied membrane protein transporter. There is a vast array of diverse inhibitors for the Ca2+ pump, and many have proved significant in helping to elucidate both the mechanism of transport and gaining conformational structures. Some SERCA inhibitors such as thapsigargin have been used extensively as pharmacological tools to probe the roles of Ca2+ stores in Ca2+ signalling processes. Furthermore, some inhibitors have been implicated in the cause of diseases associated with endocrine disruption by environmental pollutants, whereas others are being developed as potential anticancer agents. The present review therefore aims to highlight some of the wide range of chemically diverse inhibitors that are known, their mechanisms of action and their binding location on the Ca2+ ATPase. Additionally, some ideas for the future development of more useful isoform-specific inhibitors and anticancer drugs are presented.

Rapamycin (sirolimus) protects against hypoxic damage in primary heart cultures via Na+/Ca2+ exchanger activation

AIMS

Rapamycin (sirolimus) is an antibiotic that inhibits protein synthesis through mammalian targeting of rapamycin (mTOR) signaling, and is used as an immunosuppressant in the treatment of organ rejection in transplant recipients. Rapamycin confers preconditioning-like protection against ischemic-reperfusion injury in isolated mouse heart cultures. Our aim was to further define the role of rapamycin in intracellular Ca(2+) homeostasis and to investigate the mechanism by which rapamycin protects cardiomyocytes from hypoxic damage.

MAIN METHODS

We demonstrate here that rapamycin protects rat heart cultures from hypoxic-reoxygenation (H/R) damage, as revealed by assays of lactate dehydrogenase (LDH) and creatine kinase (CK) leakage to the medium, by MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) measurements, and desmin immunostaining. As a result of hypoxia, intracellular calcium levels ([Ca(2+)](i)) were elevated. However, treatment of heart cultures with rapamycin during hypoxia attenuated the increase of [Ca(2+)](i). Rapamycin also attenuated (45)Ca(2+) uptake into the sarcoplasmic reticulum (SR) of skinned heart cultures in a dose- and time-dependent manner. KB-R7943, which inhibits the "reverse" mode of Na(+)/Ca(2+) exchanger (NCX), protected heart cultures from H/R damage with or without the addition of rapamycin. Rapamycin decreased [Ca(2+)](i) following its elevation by extracellular Ca(2+) ([Ca(2+)](o)) influx, thapsigargin treatment, or depolarization with KCl.

KEY FINDINGS

We suggest that rapamycin induces cardioprotection against hypoxic/reoxygenation damage in primary heart cultures by stimulating NCX to extrude Ca(2+) outside the cardiomyocytes.

SIGNIFICANCE

According to our findings, rapamycin preserves Ca(2+) homeostasis and prevents Ca(2+) overload via extrusion of Ca(2+) surplus outside the sarcolemma, thereby protecting the cells from hypoxic stress.

Δ²,³-ivermectin ethyl secoester, a conjugated ivermectin derivative with leishmanicidal activity but without inhibitory effect on mammalian P-type ATPases

Looking at a new putative target for the large spectrum antiparasitic drug ivermectin, we recently showed that avermectin-derived drugs are active against promastigote and amastigote forms of Leishmania amazonensis at low micromolar concentrations. However, we then reported that at this concentration range ivermectin is also able to inhibit three important mammalian P-type ATPases so that unacceptable adverse effects could occur if this drug were used at such high doses therapeutically. The present work aimed to test the activity of ten ivermectin analogs on these rat ATPases in search of a compound with similar leishmanicidal activity but with no effect on the mammalian (host) ATPases at effective concentrations. We synthesized three new ivermectin analogs for testing on rat SERCA (1a and 1b), Na+, K+-ATPase (α₁ and α₂/α₃ isoforms) and H+/K+-ATPase activity, along with seven analogs already characterized for their leishmanicidal activity. Our main finding is that one of the prepared derivatives, Δ²,³-ivermectin ethyl secoester 8, is equipotent to ivermectin 1 for the in vitro leishmanicidal effects but is nearly without effect on the rat ATPases, indicating that it could have a better therapeutic index in vivo and could serve as a candidate for hit-to-lead progression. This conclusion is further supported by the fact that compound 8 produced only 6% (vs 77% for ivermectin) inhibition of the human kidney enzyme at 5 μM, a concentration corresponding to the IC₅₀ for the activity against L. amazonensis amastigotes.

SERCA2b activity is regulated by cyclophilins in human platelets

OBJECTIVE

The role of cyclophilins (chaperones that are widely expressed in different cell types, including human platelets) was explored in sarcoendoplasmic calcium (Ca(2+)) adenosine triphosphatase (SERCA) activity.

METHODS AND RESULTS

Cyclophilin inhibition by cyclosporin A (CsA) evoked a time- and concentration-dependent reduction of Ca(2+) uptake by SERCA2b. However, other Ca(2+)-adenosine triphosphatases expressed in platelets, such as SERCA3 and plasma membrane Ca(2+) adenosine triphophatase, remained unaltered after CsA treatment. Cypermethrin, a non-CsA-related calcineurin inhibitor, did not alter SERCA2b activity. Furthermore, SERCA2b was affected by other CsA analogues, which do not interfere with calcineurin, such as PKF-211-811-NX5 (NIM811) and sanglifehrin A. Inhibition of the immunophilin family members using FK506 (tacrolimus) did not alter SERCA2b ability to sequester Ca(2+) into the dense tubular system. Coimmunoprecipitation experiments confirmed that cyclophilin A associates with SERCA2b and stromal interaction molecule-1 in resting platelets. This interaction is attenuated by the physiological agonist thrombin but enhanced by treatment with CsA or sanglifehrin A.

CONCLUSIONS

Cyclophilin A is a regulator of SERCA2b in human platelets.

Ivermectin is a nonselective inhibitor of mammalian P-type ATPases

Ivermectin is a large spectrum antiparasitic drug that is very safe at the doses actually used. However, as it is being studied for new applications that would require higher doses, we should pay attention to its effects at high concentrations. As micromolar concentrations of ivermectin have been reported to inhibit the sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA), we decided to investigate its putative inhibitory effect on other two important P-type ATPases, namely the Na(+) , K(+)-ATPase and H(+)/K(+)-ATPase. We first extended the data on SERCA, using preparations from rat enriched in SERCA1a (extensor digitorum longus) and 1b (heart) isoforms. Secondly, we tested the effect of ivermectin in two preparations of rat Na(+), K(+)-ATPase in order to appreciate its putative selectivity towards the alpha(1) isoform (kidney) and the alpha(2)/alpha(3) isoforms (brain), and in an H(+)/K(+)-ATPase preparation from rat stomach. Ivermectin inhibited all these ATPases with similar IC(50) values (6-17 microM). With respect to the inhibition of the Na(+), K(+)-ATPase, ivermectin acts by a mechanism different from the classical cardiac glycosides, based on selectivity towards the isoforms, sensibility to the antagonistic effect of K(+) and to ionic conditions favoring different conformations of the enzyme. We conclude that ivermectin is a nonselective inhibitor of three important mammalian P-type ATPases, which is indicative of putative important adverse effects if this drug were used at high doses. As a consequence, we propose that novel analogs of ivermectin should be developed and tested both for their parasitic activity and in vitro effects on P-type ATPases.

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.

Functional groups of ryanodine receptors in rat ventricular cells

Ryanodine receptors (RyR2s) are ion channels in the sarcoplasmic reticulum (SR) that are responsible for Ca2+ release in rat ventricular myocytes. Localization of RyR2s is therefore crucial for our understanding of contraction and other Ca2+-dependent intracellular processes. Recent results (e.g. circular waves and Ca2+ sparks in perinuclear area) raised questions about the classical views of RyR2 distribution and organization within ventricular cells. A Ca2+ spark is a fluorescent signal reflecting the activation of a small group of RyR2s. Frequency and spatio-temporal characteristics of Ca2+ sparks depend on the state of cytoplasmic and intraluminal macromolecular complexes regulating cardiac RyR2 function. We employed electron microscopy, confocal imaging of spontaneous Ca2+ sparks and immunofluorescence to visualize the distribution of RyR2s in ventricular myocytes and to evaluate the local involvement of the macromolecular complexes in regulation of functional activity of the RyR2 group. An electron microscopy study revealed that the axial tubules of the transverse-axial tubular system probably do not have junctions with the network SR (nSR). The nSR was found to be wrapped around intermyofibrillar mitochondria and contained structures similar to feet of the junctional cleft. Treatment of ventricular myocytes with antibodies against RyR2 showed that in addition to the junctional SR, a small number of RyR2s can be localized at the middle of the sarcomere and in the zone of perinuclear mitochondria. Recordings of spontaneous Ca2+ sparks showed the existence of functional groups of RyR2s in these intracellular compartments. We found that within the sarcomere about 20% of Ca2+ sparks were not colocalized with the zone of the junctional or corbular SR (Z-line zone). The spatio-temporal characteristics of sparks found in the Z-line and A-band zones were very similar, whereas sparks from the zone of the perinuclear mitochondria were about 25% longer. Analysis of the initiation sites of Ca2+ sparks within the same junctional SR cluster suggested that 18-25 RyR2s are in the functional group producing a spark. Because of the similarity of the spatio-temporal characteristics of sarcomeric sparks and ultrastructural characteristics of nSR, we suggest that the functional groups of RyR2s in the middle of the sarcomere are macromolecular complexes of approximately 20 RyR2s with regulatory proteins. Our data allowed us to conclude that a significant number of functional RyR2s is located in the middle of the sarcomere and in the zone of perinuclear mitochondria. These RyR2s could contribute to excitation-contraction coupling, mitochondrial and nuclear signalling, and Ca2+-dependent gene regulation, but their existence raises many additional questions.

The immunophilin ligand GPI1046 protects neurons from the lethal effects of the HIV-1 proteins gp120 and Tat by modulating endoplasmic reticulum calcium load

The dysfunction and death of neuronal cells is thought to underlie the cognitive manifestations of human immunodeficiency virus (HIV)-associated neurological disorders. Although HIV-infected patients are living longer owing to the effectiveness of anti-retroviral therapies, the number of patients developing neurological disorders is on the rise. Thus, there is an escalating need for effective therapies to preserve cognitive function in HIV-infected patients. Using HIV-protein-induced neurotoxicity as a model system, we tested the effectiveness of a non-immunosuppressive immunophilin ligand to attenuate gp120 and Tat-induced modification of neuronal function. The immunophilin ligand GPI1046 attenuated endoplasmic reticulum (ER) calcium release induced by gp120 and Tat and protected neurons from the lethal effect of these neurotoxic HIV proteins. Both inositol 1,4,5 trisphosphate (IP(3)) and ryanodine-sensitive ER calcium release was attenuated by pre-incubation with GPI1046. Using the sarco/endoplasmic reticulum calcium pump inhibitor thapsigargin to release ER calcium, we determined that GPI1046 reduced the total ER calcium load. These findings suggest that non-immunosuppressive immunophilin ligands may be useful neuroprotective drugs in HIV dementia.

Diastolic dysfunction in human cardiac allografts is related with reduced SERCA2a gene expression

Previous studies demonstrated that impaired left ventricular (LV) relaxation in cardiac allografts limits exercise tolerance post-transplant despite preserved systolic ejection fraction (EF). This study tested in human cardiac allografts whether the isovolumic relaxation time (IVRT), which provides the basis for most of diastolic LV filling, relates with gene expression of regulatory proteins of calcium homeostasis or cardiac matrix proteins. Gene expression was studied in 31 heart transplant recipients (25 male, 6 female) 13-83 months post-transplant with LVEF >50%, LV end-diastolic pressure <20 mmHg, normal LV mass index and without allograft rejection or significant cardiac pathology. IVRT related with the other diastolic parameters e-wave velocity (r = -0.46; p = 0.01), e/a-wave ratio (r = -0.5; p < 0.01) but not with heart frequency (r = -0.16; p = 0.4). No relation of IVRT was observed for immunosuppression, mean rejection grade or other medication. IVRT was not related with gene expression of desmin, collagen I, phospholamban, the Na+-Ca2+ exchanger, the ryanodine receptor or interstitial fibrosis but correlated inversely with SERCA2a (r = -0.48; p = 0.02). Prolonged IVRT is associated with decreased SERCA2a expression in cardiac allografts without significant other pathology. Similar observations in non-transplanted patients with diastolic failure suggest that decreased SERCA2a expression is an important common pathomechanism.

In smooth muscle, FK506-binding protein modulates IP3 receptor-evoked Ca2+ release by mTOR and calcineurin

Ca2+ release from the sarcoplasmic reticulum (SR) by the IP3 receptors (IP3Rs) crucially regulates diverse cell signalling processes from reproduction to apoptosis. Release from the IP3R may be modulated by endogenous proteins associated with the receptor, such as the 12 kDa FK506-binding protein (FKBP12), either directly or indirectly by inhibition of the phosphatase calcineurin. Here, we report that, in addition to calcineurin, FKPBs modulate release through the mammalian target of rapamycin (mTOR), a kinase that potentiates Ca2+ release from the IP3R in smooth muscle. The presence of FKBP12 was confirmed in colonic myocytes and co-immunoprecipitated with the IP3R. In aortic smooth muscle, however, although present, FKBP12 did not co-immunoprecipitate with IP3R. In voltage-clamped single colonic myocytes rapamycin, which together with FKBP12 inhibits mTOR (but not calcineurin), decreased the rise in cytosolic Ca2+ concentration ([Ca2+]c) evoked by IP3R activation (by photolysis of caged IP3), without decreasing the SR luminal Ca2+ concentration ([Ca2+]l) as did the mTOR inhibitors RAD001 and LY294002. However, FK506, which with FKBP12 inhibits calcineurin (but not mTOR), potentiated the IP3-evoked [Ca2+]c increase. This potentiation was due to the inhibition of calcineurin; it was mimicked by the phosphatase inhibitors cypermethrin and okadaic acid. The latter two inhibitors also prevented the FK506-evoked increase as did a calcineurin inhibitory peptide (CiP). In aortic smooth muscle, where FKBP12 was not associated with IP3R, the IP3-mediated Ca2+ release was unaffected by FK506 or rapamycin. Together, these results suggest that FKBP12 has little direct effect on IP3-mediated Ca2+ release, even though it is associated with IP3R in colonic myocytes. However, FKBP12 might indirectly modulate Ca2+ release through two effector proteins: (1) mTOR, which potentiates and (2) calcineurin, which inhibits Ca2+ release from IP3R in smooth muscle.

Isolated Rafts from Adriamycin-Resistant P388 Cells Contain Functional ATPases and Provide an Easy Test System for P-glycoprotein?Related Activities

PURPOSE

P-glycoprotein (P-gp), a membrane ATPase expelling many structurally unrelated compounds out of cells, is one of the major contributors to multidrug resistance. It is enriched in cold TritonX-100 insoluble membrane domains (i.e., rafts). The purpose of this work was to characterize the ATPase activities of raft preparations from P388 cells overexpressing P-gp (P388/ADR) or devoid of P-gp (P388) and to establish a P-gp-enriched screening system for P-gp-interfering compounds.

METHODS

Rafts were extracted with cold TritonX-100. The ATPase activity was characterized in 96-well plates using a fluorescence assay.

RESULTS

The ATPase activity per mg protein was about five times higher in P388/ADR rafts than in crude membranes. The anti-P-gp antibody C219 inhibited 20% of the activity in P388/ADR rafts but only about 10% of the activity in P388/ADR crude membranes and had no effect on the activity of P388 rafts. The known P-gp-activating compounds verapamil, progesterone, and valinomycin revealed the typical bell-shaped activity/concentration profiles in P388/ADR rafts, indicative for activation at low compound concentrations and inhibition at concentrations >10 to 100 microM. The inhibitory effect was also observed in P388 rafts.

CONCLUSIONS

Extracted rafts are rich in functional ATPases. Rafts from P-gp-overexpressing cells display P-gp-typical ATPase activity and provide an easy, P-gp-enriched screening system.

Histologic features in primary cutaneous squamous cell carcinomas in immunocompromised patients focusing on organ transplant patients

BACKGROUND

In organ transplant recipients (OTR) there is an increased incidence of cutaneous squamous cell carcinomas (CSCC), and they may be aggressive.

OBJECTIVE

We wanted to determine whether any histologic features were characteristic or more common in CSCC of OTR.

METHODS

A total of 518 immunocompetent individuals with 601 primary CSCC were examined. Seventy-nine OTR with 231 primary CSCC, 53 renal OTR, 25 heart/heart lung OTR, and 1 liver OTR were examined.

RESULTS

In all patient populations, CSCC occurred dominantly in chronically sun-exposed skin. In OTR, however, a greater percentage occurred on the extremity, particularly the upper extremity, and they occurred at an earlier age. Features that were significantly more common in OTR were acantholytic changes, early dermal invasion, an infiltrative growth pattern with or without desmoplasia, and Bowen's disease with carcinoma. In OTR primary CSCC were also significantly deeper at time of diagnosis.

CONCLUSIONS

Our findings confirmed the association of ultraviolet radiation exposure with development of CSCC in OTR. The increased depth of the primary CSCC in OTR is surprising because these patients are followed closely for skin cancer compared with immunocompetent patients. The other morphologic features that were significantly more common in OTR may theoretically reflect not only the type of iatrogenic immunosuppression in these patients, but also other procarcinogenic effects of their medications.

The 12 kDa FK506-binding protein, FKBP12, modulates the Ca(2+)-flux properties of the type-3 ryanodine receptor

We have characterised the functional regulation of the type-3 ryanodine receptor by the 12 kDa FK506-binding protein. Wild-type type-3 ryanodine receptor and mutant type-3 ryanodine receptor in which the critical valine at position 2322 in the central 12 kDa FK506-binding protein binding site was substituted by aspartate, were stably expressed in human embryonic kidney cells. In contrast to the wild-type receptor, the mutant receptor was strongly impaired in binding to immobilised glutathione S-transferase 12 kDa FK506-binding protein. Caffeine-induced 45Ca(2+)-efflux was markedly increased in cells expressing mutant type-3 ryanodine receptor whereas the maximal-releasable Ca2+ was not affected. Confocal Ca2+ imaging provided clear evidence for a much higher sensitivity of the mutant receptor, which showed global Ca2+ release at about 20-fold lower caffeine concentrations than the wild-type receptor. Spontaneous Ca2+ sparks were observed in both wild-type- and mutant-expressing cells but the number of sparking cells was about 1.5-fold higher in the mutant group, suggesting that the degree of FK506 binding controls the stability of the closed state of ryanodine receptor channels. Furthermore, overexpression of 12 kDa FK506-binding protein decreased the number of sparking cells in the wild-type-expressing cells whereas it did not affect the number of sparking cells in cells expressing the mutant receptor. Concerning spark properties, the amplitude and duration of Ca2+ sparks mediated by mutant channels were significantly reduced in comparison to wild-type channels. This suggests that functional coupling between different mutant type-3 ryanodine receptor channels in a cluster is impaired. Our findings show for the first time that the central binding site for the 12 kDa FK506-binding protein of type-3 ryanodine receptor, encompassing the critical valine proline motif, plays a crucial role in the modulation of the Ca2+ release properties of the type-3 ryanodine receptor channel, including the regulation of both global Ca2+ responses and spontaneous Ca2+ sparks.

Calcineurin and intracellular Ca2+-release channels: regulation or association?

The Ca(2+)- and calmodulin-dependent phosphatase calcineurin was reported to interact with the inositol 1,4,5-trisphosphate receptor (IP(3)R) and the ryanodine receptor (RyR) and to modulate their phosphorylation status and activity. However, controversial data on the molecular mechanisms involved and on the functional relevance of calcineurin for these channel-complexes have been described. Hence, we will focus on the functional importance of calcineurin for IP(3)R and RyR function and on the different mechanisms by which Ca(2+)-dependent dephosphorylation can affect the gating of those intracellular Ca(2+)-release channels. Since many studies made use of immunosuppressive drugs that are inhibiting calcineurin activity, we will also have to take the different side effects of these drugs into account for the proper interpretation of the effects of calcineurin on intracellular Ca(2+)-release channels. In addition, it became recently known that various other phosphatases and kinases can associate with these channels, thereby forming macromolecular complexes. The relevance of these enzymes for IP(3)R and RyR functioning will be reviewed since in some cases they could interfere with the effects ascribed to calcineurin. Finally, we will discuss the downstream effects of calcineurin on the regulation of the expression levels of intracellular Ca(2+)-release channels as well as the relation between IP(3)R- and RyR-mediated Ca(2+) release and calcineurin-dependent gene expression.

Impaired muscle Ca2+ and K+ regulation contribute to poor exercise performance post-lung transplantation

Lung transplant recipients (LTx) exhibit marked peripheral limitations to exercise. We investigated whether skeletal muscle Ca2+ and K+ regulation might be abnormal in eight LTx and eight healthy controls. Peak oxygen consumption and arterialized venous plasma [K+] (where brackets denote concentration) were measured during incremental exercise. Vastus lateralis muscle was biopsied at rest and analyzed for sarcoplasmic reticulum Ca2+ release, Ca2+ uptake, and Ca2+-ATPase activity rates; fiber composition; Na+-K+-ATPase (K+-stimulated 3-O-methylfluorescein phosphatase) activity and content ([3H]ouabain binding sites); as well as for [H+] and H+-buffering capacity. Peak oxygen consumption was 47% less in LTx (P < 0.05). LTx had lower Ca2+ release (34%), Ca2+ uptake (31%), and Ca2+-ATPase activity (25%) than controls (P < 0.05), despite their higher type II fiber proportion (LTx, 75.0 +/- 5.8%; controls, 43.5 +/- 2.1%). Muscle [H+] was elevated in LTx (P < 0.01), but buffering capacity was similar to controls. Muscle 3-O-methylfluorescein phosphatase activity was 31% higher in LTx (P < 0.05), but [3H]ouabain binding content did not differ significantly. However, during exercise, the rise in plasma [K+]-to-work ratio was 2.6-fold greater in LTx (P < 0.05), indicating impaired K+ regulation. Thus grossly subnormal muscle calcium regulation, with impaired potassium regulation, may contribute to poor muscular performance in LTx.