Differentiation of the drug-binding sites of calmodulin

The Biodegradation of Soil Organic Matter in Soil-Dwelling Humivorous Fauna

Soil organic matter contains more carbon than global vegetation and the atmosphere combined. Gaining access to this source of organic carbon is challenging and requires at least partial removal of polyphenolic and/or soil mineral protections, followed by subsequent enzymatic or chemical cleavage of diverse plant polysaccharides. Soil-feeding animals make significant contributions to the recycling of terrestrial organic matter. Some humivorous earthworms, beetles, and termites, among others, have evolved the ability to mineralize recalcitrant soil organic matter, thereby leading to their tremendous ecological success in the (sub)tropical areas. This ability largely relies on their symbiotic associations with a diverse community of gut microbes. Recent integrative omics studies, including genomics, metagenomics, and proteomics, provide deeper insights into the functions of gut symbionts. In reviewing this literature, we emphasized that understanding how these soil-feeding fauna catabolize soil organic substrates not only reveals the key microbes in the intestinal processes but also uncovers the potential novel enzymes with considerable biotechnological interests.

High-throughput amenable fluorescence-assays to screen for calmodulin-inhibitors

The KRAS gene is highly mutated in human cancers and the focus of current Ras drug development efforts. Recently the interface between the C-terminus of K-Ras and calmodulin (CaM) was proposed as a target site to block K-Ras driven cancer cell stemness. We therefore aimed at developing a high-throughput amenable screening assay to identify novel CaM-inhibitors as potential K-Ras stemness-signaling disruptors. A modulated time-resolved Förster resonance energy transfer (mTR-FRET)-assay was developed and benchmarked against an identically designed fluorescence anisotropy (FA)-assay. In both assays, two CaM-binding peptides were labeled with Eu(III)-chelate or fluorescein and used as single-label reporter probes that were displaced from CaM upon competitor binding. Thus, peptidic and small molecule competitors with nanomolar to micromolar affinities to CaM could be detected, including a peptide that was derived from the C-terminus of K-Ras. In order to detect CaM-residue specific covalent inhibitors, a cell lysate-based Förster resonance energy transfer (FRET)-assay was furthermore established. This assay enabled us to measure the slow, residue-specific, covalent inhibition by ophiobolin A in the presence of other endogenous proteins. In conclusion, we have developed a panel of fluorescence-assays that allows identification of conventional and covalent CaM-inhibitors as potential disruptors of K-Ras driven cancer cell stemness.

Pharmacological Strategies for Manipulating Plant Ca2+ Signalling

Calcium is one of the most pleiotropic second messengers in all living organisms. However, signalling specificity is encoded via spatio-temporally regulated signatures that act with surgical precision to elicit highly specific cellular responses. How this is brought about remains a big challenge in the plant field, in part due to a lack of specific tools to manipulate/interrogate the plant Ca2+ toolkit. In many cases, researchers resort to tools that were optimized in animal cells. However, the obviously large evolutionary distance between plants and animals implies that there is a good chance observed effects may not be specific to the intended plant target. Here, we provide an overview of pharmacological strategies that are commonly used to activate or inhibit plant Ca2+ signalling. We focus on highlighting modes of action where possible, and warn for potential pitfalls. Together, this review aims at guiding plant researchers through the Ca2+ pharmacology swamp.

Direct visualization of interaction between calmodulin and connexin45

Calmodulin (CaM) is an intracellular Ca²⁺ transducer involved in numerous activities in a broad Ca²⁺ signaling network. Previous studies have suggested that the Ca²⁺/CaM complex may participate in gap junction regulation via interaction with putative CaM-binding motifs in connexins; however, evidence of direct interactions between CaM and connexins has remained elusive to date due to challenges related to the study of membrane proteins. Here, we report the first direct interaction of CaM with Cx45 (connexin45) of γ-family in living cells under physiological conditions by monitoring bioluminescence resonance energy transfer. The interaction between CaM and Cx45 in cells is strongly dependent on intracellular Ca²⁺ concentration and can be blocked by the CaM inhibitor, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W7). We further reveal a CaM-binding site at the cytosolic loop (residues 164-186) of Cx45 using a peptide model. The strong binding (K_d ∼ 5 nM) observed between CaM and Cx45 peptide, monitored by fluorescence-labeled CaM, is found to be Ca²⁺-dependent. Furthermore, high-resolution nuclear magnetic resonance spectroscopy reveals that CaM and Cx45 peptide binding leads to global chemical shift changes of ¹⁵N-labeled CaM, but does not alter the size of the structure. Observations involving both N- and C-domains of CaM to interact with the Cx45 peptide differ from the embraced interaction with Cx50 from another connexin family. Such interaction further increases Ca²⁺ sensitivity of CaM, especially at the N-terminal domain. Results of the present study suggest that both helicity and the interaction mode of the cytosolic loop are likely to contribute to CaM's modulation of connexins.

Short- and long-term inhibition of cardiac inward-rectifier potassium channel current by an antiarrhythmic drug bepridil

Bepridil is an effective antiarrhythmic drug on supraventricular and ventricular arrhythmias, and inhibitor of calmodulin. Recent investigations have been elucidating that bepridil exerts antiarrhythmic effects through its acute and chronic application for patients. The aim of this study was to identify the efficacy and the potential mechanism of bepridil on the inward-rectifier potassium channel in neonatal rat cardiomyocytes in acute- and long-term conditions. Bepridil inhibited inward-rectifier potassium current (I K1) as a short-term effect with IC50 of 17 μM. Bepridil also reduced I K1 of neonatal cardiomyocytes when applied for 24 h in the culture medium with IC50 of 2.7 μM. Both a calmodulin inhibitor (W-7) and an inhibitor of calmodulin-kinase II (KN93) reduced I K1 when applied for 24 h as a long-term effect in the same fashion, suggesting that the long-term application of bepridil inhibits I K1 more potently than that of the short-term application through the inhibition of calmodulin kinase II pathway in cardiomyocytes.

Predicting drug-target interactions using probabilistic matrix factorization

Quantitative analysis of known drug-target interactions emerged in recent years as a useful approach for drug repurposing and assessing side effects. In the present study, we present a method that uses probabilistic matrix factorization (PMF) for this purpose, which is particularly useful for analyzing large interaction networks. DrugBank drugs clustered based on PMF latent variables show phenotypic similarity even in the absence of 3D shape similarity. Benchmarking computations show that the method outperforms those recently introduced provided that the input data set of known interactions is sufficiently large--which is the case for enzymes and ion channels, but not for G-protein coupled receptors (GPCRs) and nuclear receptors. Runs performed on DrugBank after hiding 70% of known interactions show that, on average, 88 of the top 100 predictions hit the hidden interactions. De novo predictions permit us to identify new potential interactions. Drug-target pairs implicated in neurobiological disorders are overrepresented among de novo predictions.

Insulin effect on glucose transport in thymocytes and splenocytes from rats with metabolic syndrome

Metabolic syndrome (MS) may comprise several clinical conditions such as obesity, diabetes and inflammatory disorders, which are characterized by metabolic imbalances. The study of glucose transport and regulation by insulin in lymphocytes is important, since the way they increase inflammation and susceptibility to infections are common in MS. We studied glucose internalization in isolated thymocytes and splenocytes, its regulation by insulin, and the role of three glucose transporters (Gluts) in control and in MS rats. Control glucose internalization and insulin responses were lower in splenocytes than in thymocytes. Control and insulin-induced glucose internalization in thymocytes declined with age, while transport by splenocyte continued to respond to insulin. Control thymocyte glucose internalization was blocked by antibodies against Glut 1 and 4, while the insulin response also was blocked by an anti-Glut 3 antibody. On four month old control and insulin-induced response, splenocyte transport was only blocked by Glut 1 and 4 antibodies. At six months splenocyte glucose internalization depended on Glut 1 and was less sensitive to the effects of an anti-Glut 4 antibody. In MS splenocytes the capacity of anti-Glut 1 antibodies to inhibit control and insulin-dependent glucose transport was less significant, and we found that in MS rats, glucose internalization was dependent on Glut 3 and Glut 4. In summary, the altered metabolic state present in MS rats shows signs of modulation of glucose internalization by the Glut1, Glut 3 and Glut 4 transporters, compared with its own age control.

Inhibition of nitric oxide-activated guanylyl cyclase by calmodulin antagonists

BACKGROUND AND PURPOSE

Nitric oxide (NO) controls numerous physiological processes by activation of its receptor, guanylyl cyclase (sGC), leading to the accumulation of 3'-5' cyclic guanosine monophosphate (cGMP). Ca(2+)-calmodulin (CaM) regulates both NO synthesis by NO synthase and cGMP hydrolysis by phosphodiesterase-1. We report that, unexpectedly, the CaM antagonists, calmidazolium, phenoxybenzamine and trifluoperazine, also inhibited cGMP accumulation in cerebellar cells evoked by an exogenous NO donor, with IC(50) values of 11, 80 and 180 microM respectively. Here we sought to elucidate the underlying mechanism(s).

EXPERIMENTAL APPROACH

We used cerebellar cell suspensions to determine the influence of CaM antagonists on all steps of the NO-cGMP pathway. Homogenized tissue and purified enzyme were used to test effects of calmidazolium on sGC activity.

KEY RESULTS

Inhibition of cGMP accumulation in the cells did not depend on changes in intracellular Ca(2+) concentration. Degradation of cGMP and inactivation of NO were both inhibited by the CaM antagonists, ruling out increased loss of cGMP or NO as explanations. Instead, calmidazolium directly inhibited purified sGC (IC(50)= 10 microM). The inhibition was not in competition with NO, nor did it arise from displacement of the haem moiety from sGC. Calmidazolium decreased enzyme V(max) and K(m), indicating that it acts in an uncompetitive manner.

CONCLUSIONS AND IMPLICATIONS

The disruption of every stage of NO signal transduction by common CaM antagonists, unrelated to CaM antagonism, cautions against their utility as pharmacological tools. More positively, the compounds exemplify a novel class of sGC inhibitors that, with improved selectivity, may be therapeutically valuable.

Calcium fingerprints induced by calmodulin interactors in eukaryotic cells

Calcium (Ca2+) is a ubiquitous second messenger which promotes cell responses through transient changes in intracellular concentrations. The prominent role of Ca2+ in cell physiology is mediated by a whole set of proteins constituting a Ca2+-signalling toolkit involved in Ca2+-signal generation, deciphering and arrest. The different Ca2+-signalosomes deliver Ca2+-signals with spatial and temporal dynamics to control the function of specific cell types. Among the intracellular proteins involved in Ca2+-signal deciphering, calmodulin (CaM) plays a pivotal role in controlling Ca2+-homeostasis and downstream Ca2+-based signalling events. Due to its ubiquitous expression in eukaryotic cells and the variety of proteins it interacts with, CaM is central in Ca2+-signalling networks. For these reasons, it is expected that disrupting or modifying CaM interactions with its target proteins will affect Ca2+-homeostasis and cellular responses. The resulting calcium response will vary depending on which interactions between CaM and target proteins are altered by the molecules and on the specific Ca2+-toolkit expressed in a given cell, even in the resting state. In the present paper, the effect of six classical CaM interactors (W5, W7, W12, W13, bifonazole and calmidazolium) was studied on Ca2+-signalling in tumor initiating cells isolated from human glioblastoma (TG1) and tobacco cells (BY-2) using the fluorescent Ca2+-sensitive Indo-1 dye and aequorin, respectively. Various Ca2+-fingerprints were obtained depending both on the CaM interactor used and the cell type investigated. These data demonstrate that interaction between the antagonists and CaM results in a differential inhibition of CaM-dependent proteins involved in Ca2+-signal regulation. In addition, the distinct Ca2+-fingerprints in tobacco and human tumor initiating glioblastoma cells induced by a given CaM interactor highlight the specificity of the Ca2+-signalosome in eukaryotic cells.

Bepridil up-regulates cardiac Na+ channels as a long-term effect by blunting proteasome signals through inhibition of calmodulin activity

BACKGROUND AND PURPOSE

Bepridil is an anti-arrhythmic agent with anti-electrical remodelling effects that target many cardiac ion channels, including the voltage-gated Na+ channel. However, long-term effects of bepridil on the Na+ channel remain unclear. We explored the long-term effect of bepridil on the Na+ channel in isolated neonatal rat cardiomyocytes and in a heterologous expression system of human Na(v)1.5 channel.

EXPERIMENTAL APPROACH

Na+ currents were recorded by whole-cell voltage-clamp technique. Na+ channel message and protein were evaluated by real-time RT-PCR and Western blot analysis.

KEY RESULTS

Treatment of cardiomyocytes with 10 micromol.L(-1) bepridil for 24 h augmented Na+ channel current (I(Na)) in a dose- and time-dependent manner. This long-term effect of bepridil was mimicked or masked by application of W-7, a calmodulin inhibitor, but not KN93 [2-[N-(2-hydroxyethyl)-N-(4-methoxy benzenesulphonyl)]-amino-N-(4-chlorocinnamyl)-N-methylbenzylamine], a Ca2+/calmodulin-dependent kinase inhibitor. During inhibition of protein synthesis by cycloheximide, the I(Na) increase due to bepridil was larger than the increase without cycloheximide. Bepridil and W-7 significantly slowed the time course of Na(v)1.5 protein degradation in neonatal cardiomyocytes, although the mRNA levels of Na(v)1.5 were not modified. Bepridil and W-7 did not increase I(Na) any further in the presence of the proteasome inhibitor MG132 [N-[(phenylmethoxy)carbonyl]-L-leucyl-N-[(1S)-1-formyl-3-methylbutyl]-L-leucinamide]. Bepridil, W-7 and MG132 but not KN93 significantly decreased 20S proteasome activity in a concentration-dependent manner.

CONCLUSIONS AND IMPLICATIONS

We conclude that long-term exposure of cardiomyocytes to bepridil at therapeutic concentrations inhibits calmodulin action, which decreased degradation of the Na(v)1.5 alpha-subunit, which in turn increased Na+ current.

S100A1 Is a Novel Molecular Chaperone and a Member of the Hsp70/Hsp90 Multichaperone Complex

Although calmodulin is known to be a component of the Hsp70/Hsp90 multichaperone complex, the functional role of the protein remains uncertain. In this study, we have identified S100A1, but not calmodulin or other S100 proteins, as a potent molecular chaperone and a new member of the multichaperone complex. Glutathione S-transferase pull-down assays and co-immunoprecipitation experiments indicated the formation of stable complexes between S100A1 and Hsp90, Hsp70, FKBP52, and CyP40 both in vitro and in mammalian cells. S100A1 potently protected citrate synthase, aldolase, glyceraldehyde-3-phosphate dehydrogenase, and rhodanese from heat-induced aggregation and suppressed the aggregation of chemically denatured rhodanese and citrate synthase during the refolding pathway. In addition, S100A1 suppressed the heat-induced inactivation of citrate synthase activity, similar to that for Hsp90 and p23. The chaperone activity of S100A1 was antagonized by calmodulin antagonists, such as fluphenazine and prenylamine, that is, indeed an intrinsic function of the protein. The overexpression of S100A1 in COS-7 cells protected transiently expressed firefly luciferase and Escherichia coli beta-galactosidase from inactivation during heat shock. The results demonstrate a novel physiological function for S100A1 and bring us closer to a comprehensive understanding of the molecular mechanisms of the Hsp70/Hsp90 multichaperone complex.

Molecular cloning and expression of the Modulatory subunit of the cyclic nucleotide-gated cation channel

The cDNA of three variants of a cyclic nucleotide-gated (CNG) channel modulatory subunit (CNG4c-CNG4e) has been cloned. CNG4c, CNG4d, and CNG4e differ slightly from each other within an amino-terminal sequence that was originally reported as part of the bovine retinal glutamic acid-rich protein (GARP). The core region of CNG4 is homologous to the second subunit of the human rod photoreceptor channel (hRCNC2b), suggesting that both proteins are alternatively spliced products of the bovine and human homologue of the same gene. CNG4 transcripts are present in retina, testis, kidney, heart, and brain. Expression of CNG4 in HEK293 cells did not lead to detectable currents. Coexpression of CNG4 with the principal subunit of the bovine testis CNG channel (CNG3) resulted in currents which differed in several aspects from that induced by CNG3 alone. The heterooligomeric CNG3/CNG4 and the homooligomeric CNG3 channels were modified by Ca2+-calmodulin and some calmodulin antagonists. The results suggest that CNG4 forms functional heterooligomeric channels with CNG3 in vitro and probably also in intact tissues.

Inhibition of cell growth and intracellular Ca2+ mobilization in human brain tumor cells by Ca2+ channel antagonists

The effects of various Ca2+ channel agonists and antagonists on tumor cell growth were investigated using U-373 MG human astrocytoma and SK-N-MC human neuroblastoma cell lines. Classical Ca2+ channel antagonists, verapamil, nifedipine, and diltiazem, and inorganic Ca2+ channel antagonists, Ni2+ and Co2+, inhibited growth of these tumor cells in a dose-dependent manner. Except Ni2+, these Ca2+ channel antagonists did not induce a significant cytotoxicity, suggesting that the growth-inhibitory effects of these drugs may be the result of the influence on the proliferative signaling mechanisms of these tumor cells. In contrast, Bay K-8644, a Ca2+ channel agonist, neither enhanced the growth of tumor cells nor increased intracellular Ca2+ concentration, indicating that voltage-sensitive Ca2+ channels may not be involved in tumor cell proliferation. Moreover, growth-inhibitory concentrations of Ca2+ channel antagonists significantly blocked agonist (carbachol or serum)-induced intracellular Ca2+ mobilization, which was monitored using Fura-2 fluorescence technique. These results suggest that the inhibition of the growth of human brain tumor cells induced by Ca2+ channel antagonists may not be the result of interaction with Ca2+ channels, but may be the result of the interference with agonist-induced intracellular Ca2+ mobilization, which is an important proliferative signaling mechanism.

Calcium channel blockers inhibit the (Ca2+ + Mg2+)-ATPase activity and the 125I-calmodulin binding in brain membranes

Ca2+ channel blockers belonging to three distinct chemical groups (dihydropyridines, phenylalkylamines and diphenylalkylamines) differentially inhibit the (Ca2+ + Mg2+)-ATPase activity of synaptic plasma membranes (Santos et al., J. Neurochem. 52, S49D, 1989). We now report that (-)-desmethoxyverapamil and flunarizine are the most potent inhibitors of the Ca(2+)-activated ATPase activity of synaptic plasma membranes, decreasing the Vmax by 41% and 37%, respectively, with no significant effects on the Km for Ca2+ (162.7 +/- 14.9 nM free [Ca2+]), while nitrendipine did not affect these parameters. Trifluoperazine was the most potent inhibitor of the Ca(2+)-activated ATPase of synaptic plasma membranes with an IC50 of 8-10 microM. To clarify whether the inhibitory effects of Ca2+ channel blockers and of trifluoperazine on the (Ca2+ + Mg2+)-ATPase occur through the inhibition of the interaction of calmodulin with the enzyme, we studied their effects on the binding of 125I-calmodulin to the membrane proteins separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), followed by electrotransfer to nitrocellulose and autoradiography. The autoradiograms revealed Ca(2+)-dependent CaM binding proteins of about 140, 70 and 55 kDa. Trifluoperazine (30-40 microM) inhibited by 50-60% the binding of 125I-calmodulin to the 140 kDa band, which probably includes the (Ca2+ + Mg2+)-ATPase protein. Flunarizine and (-)-desmethoxyverapamil (100 microM) inhibited the 125I-calmodulin binding to the 140 kDa peptides by 100 and 90%, respectively, and they inhibited by 55 and 40%, respectively, the binding of 125I-calmodulin to the peptides in the 70-55 kDa range, whereas nitrendipine did not show any effect. The results suggest that the inhibitory effects of (-)-desmethoxyverapamil and flunarizine, as well as trifluoperazine, on the (Ca2+ + Mg2+)-ATPase activity of synaptic plasma membranes are mediated by inhibition of the calmodulin interaction with the enzyme.

Effects of Ca2+ channel antagonists and their isomers on glibenclamide-sensitive K+ currents in follicle-enclosed Xenopus oocytes

Several Ca2+ channel antagonists were shown to inhibit glibenclamide-sensitive K+ currents in follicle-enclosed Xenopus oocytes. We have investigated the stereoselectivity of the effects of Ca2+ channel antagonists on the glibenclamide-sensitive K+ currents induced by Y-26763 (a K+ channel opener) in follicle-enclosed Xenopus oocytes. (-)-Bepridil and (+)-bepridil similarly suppressed Y-26763-induced K+ currents with IC50 values of 7.8 microM and 7.4 microM, respectively. The Ca2+ channel antagonists, (-)- and (+/-)-verapamil, and inactive (+)-verapamil suppressed Y-26763-induced K+ currents to similar extents and their IC50 values were 63.1 microM and 55.0 microM, respectively. The Ca2+ channel antagonist, SD-3211 and its less potent (-)-isomer, SD-3212, suppressed Y-26763-induced K+ currents with similar IC50 values of 10.7 microM and 8.9 microM, respectively. Of all the Ca2+ channel antagonists tested, only diltiazem exhibited stereoselectivity. The rank order of potencies (IC50 in microM) of four isomers of diltiazem to block Y-26763-induced K+ currents was (+)-trans (4.2) > (-)-trans (13.3) > (-)-cis (35.8) > (+)-cis (75.9), which was, however, opposite to that of their potencies as Ca2+ channel antagonists. These results indicate that blockade by Ca2+ channel antagonists of glibenclamide-sensitive K+ currents in follicle-enclosed Xenopus oocytes is not mediated by Ca2+ channel antagonism.

Blockade by antiarrhythmic drugs of glibenclamide-sensitive K+ channels in Xenopus oocytes

1. The outward K+ current induced by KRN2391 (K+ channel opener) in Xenopus oocytes is blocked by glibenclamide. We have investigated the effects of various classes (I-IV) of antiarrhythmic drugs on this KRN2391-induced response. 2. All class I antiarrhythmic drugs (Na+ channel blockers) tested concentration-dependently suppressed KRN2391-induced responses with the rank order of potency (IC50 in microM), disopyramide (17.8) > aprindine (29.5) > propafenone (63.1) > ajmaline (145) > quinidine (151). Flecainide, SUN1165, lignocaine, mexiletine and procainamide were much less potent (IC50, 450- > 1000 microM) than quinidine. 3. The class II antiarrhythmic drugs (beta-blockers), timolol, (-)- and (+/-)- propranolol, and (+)- propranolol (a non-beta-blocker) inhibited KRN2391-induced K+ currents in a concentration-dependent manner with values for IC50 (microM) of 79, 131, 151 and 129, respectively, whilst butoxamine, oxprenolol, alprenolol, pindolol, nadolol, metoprolol and acebutolol were either weak (IC50, 300 microM-600 microM) or virtually inactive (IC50, > 1000 microM). 4. The class III antiarrhythmic drugs, amiodarone and (+)-sotalol scarcely affected KRN2391 responses. 5. All class IV drugs (Ca2+ antagonists) tested suppressed KRN2391-induced responses in a concentration-dependent manner with an IC50 of 6.3 microM for bepridil, 38 microM for prenylamine, 85 microM for verapamil and 135 microM for diltiazem. 6. In conclusion, antiarrhythmic drugs of classes I, II and IV potently blocked glibenclamide-sensitive K+ channels in Xenopus oocytes.

Putative therapeutic applications of calmodulin antagonists

Calmodulin is the most important intracellular receptor protein for the second messenger calcium. The calcium-calmodulin complex regulates a number of physiological processes. An increasing number of pharmaceutical products is reported to interfere with the calcium-calmodulin complex. Despite the fact that the precise mechanisms of action of these so-called calmodulin antagonists await further clarification, reports accumulate in the literature indicating a broadening spectrum of putative therapeutic applications of calmodulin antagonists. Some of these applications, such as in cell proliferation, hypertension, congestive heart failure, arrhythmia and gastro-intestinal disorders, are discussed in the present review.

Inactivation of glibenclamide-sensitive K+ channels in Xenopus oocytes by various calmodulin antagonists

In follicle-enclosed Xenopus oocytes, extracellular application of cromakalim (a K+ channel opener) or intracellular injection of cAMP induces the smooth outward K+ current which is inactivated by glibenclamide. We found that cromakalim- or cAMP-induced K+ currents in the oocytes were rapidly, reversibly and dose-dependently blocked by various drugs having a calmodulin antagonizing activity in common, namely, by a selective calmodulin antagonist (W-7), antipsychotics (trifluoperazine, chlorpromazine, haloperidol), an antidepressant (amitriptyline), a beta-adrenoceptor blocker (propranolol), a local anesthetic (lidocaine) and a calcium antagonist (prenylamine). W-7, trifluoperazine, chlorpromazine and prenylamine were relatively potent blockers. For example, IC50 values to block cromakalim (100 microM)-induced K+ currents were 12 microM for trifluoperazine and 16 microM for W-7, which were close to their IC50 values to inhibit Ca2+/calmodulin-dependent phosphodiesterase (an index of the potency of calmodulin antagonists). IC50 values to inhibit cAMP (20 pmol/oocyte)-induced K+ currents were 126 microM for prenylamine and 129 microM for chlorpromazine. The IC50 values of all drugs tested to block cromakalim or cAMP responses were significantly correlated with their calmodulin-antagonizing potencies. Isoproterenol-induced K+ currents in the oocytes were also dose-dependently inhibited by glibenclamide, W-7 and trifluoperazine. These results suggest the possibility that the activity of glibenclamide-sensitive K+ channels in follicle-enclosed oocytes are regulated by calmodulin or a calmodulin-dependent process.

Production of human interferon-beta by Sendai virus and poly(rI).poly(rC): inhibition by neomycin

Neomycin inhibits the production of interferon-beta (IFN-beta) in human fibroblast cells in response to Sendai virus or to poly(rI).poly(rC) in a concentration-dependent manner, and to the greatest extent effective when added prior to or up to 2 h after induction. This inhibitory effect is negated when the protein kinase C activator, SC-9, is present during IFN-beta production in response to poly(rI).poly(rC), but not in response to Sendai virus. These results suggest that in human cells both virus and poly(rI).poly(rC) utilize an early neomycin-sensitive signal transduction step for the production of IFN-beta; because neomycin binds specific phosphatidylinositol phosphates, both of these inducers very likely require hydrolysis of these phosphates.

Analysis of a myosin-like protein and the role of calcium in the E. coli cell cycle

For a number of years now, we have argued that current models for the control of initiation of DNA synthesis, chromosomal partitioning and septum formation in Escherichia coli are unsatisfactory. Indeed, we could argue that despite considerable efforts, with the possible exception of dnaA and ftsZ, no genes specifically implicated in these control processes have been identified. In the cases of DnaA and FtsZ, no evidence has appeared to indicate how such molecules might be regulated to act once per cycle. In 1988, we formulated a specific proposal that the timing of cell cycle events in E. coli might be determined by a Ca++ flux, mediated by calcium-binding proteins and protein kinases and culminating, in the case of chromosome segregation and division, in the action of force-generating proteins such as myosin (Norris et al., 1988). In formulating this proposal, we took the view that the fundamental elements of cell cycle regulation are likely to be highly conserved across all species including prokaryotes. In this presentation, we shall describe the approaches we have been taking in order to test this hypothesis and to summarize the data obtained, in particular in relation to new genes identified which may play a role in the E. coli cell cycle. We shall also briefly indicate recent data from other laboratories consistent with our general hypothesis.

Effect of calmodulin antagonists on the interferon system: induction and action of interferons

Synthesis of interferon (IFN) in response to Sendai virus and the development of the resulting antiviral state were studied in human (BG-9) and murine (L-929) fibroblast cell cultures in the presence of the calmodulin antagonists, trifluoperazine (TFP) and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7). Compared to control cultures, 16-fold and 8-fold more IFN was formed in human and murine cells, respectively, when 10 microM TFP was present in the medium for 24 h prior to IFN induction with Sendai virus. W-7 did not affect IFN production in human cells, but enhanced it in L-929 cells by 4- to 8-fold. TFP inhibited the antiviral state induced by homologous IFN in the two cell systems, and at 20 microM, there was a 3,000-fold increase in vesicular stomatitis virus (VSV) yield. It also reduced the maintenance of the antiviral state in human cells. In contrast, W-7 had no effect on the development of the antiviral state in either of the two cell systems. Thus, calcium-calmodulin dependent cellular processes are involved in both induction of IFN and its action. The several patterns response to TFP and W-7 may reflect different ligand-binding sites on calmodulin.

Calmodulin discriminates between the two enantiomers of the receptor-operated calcium channel blocker SK&F 96365: a study using 1H-NMR and chiral HPLC

1H nuclear magnetic resonance at 360 MHz shows that SK&F 96365 (1-(beta-[3-(p-methoxyphenyl)-propyloxy]-p-methoxyphenethyl)-1H- imidazole hydrochloride), an antagonist of mammalian receptor-operated calcium channels, interacts with the calcium-binding regulatory protein calmodulin (CaM). This may be inferred by a number of chemical shift changes in the spectrum of the calcium-saturated protein induced by addition of the compound. Moreover, two well-resolved singlets corresponding to the 2-proton of the SK&F 96365 imidazolium moiety are observed in the spectrum over a wide range of protein:compound ratios. Separation of rac SK&F 96365 into its two enantiomers by high-performance liquid chromatography on a cellulose tris (4-methylbenzoate) column enabled us to show that the doubling of this NMR signal in the presence of CaM is due to a propensity of the protein to distinguish between the two optical isomers of the compound.

Fluorescence spectroscopic analysis of calpain II interactions with calcium and calmodulin antagonists

1. The intrinsic fluorescence of epoxysuccinyl-inhibited calpain II undergoes a Ca2(+)-dependent decrease which contrasts with the increase observed for calmodulin. 2. Calpain II was inhibited by the calmodulin antagonist toluidinylnaphthalenesulfonate (TNS), and a Ca2(+)-dependent increase in TNS fluorescence intensity was observed for epoxysuccinyl-inhibited calpain II. 3. The calmodulin antagonists calmidazolium CDZ and felodipine both caused decreases in the intrinsic fluorescence of epoxysuccinyl-inhibited calpain II. 4. Increasing concentrations of Ca2+ caused an increase in the fluorescence intensity of the inhibited enzyme in the presence of (CDZ), and a decrease in the presence of felodipine. 5. It is concluded from these studies that Ca2+ and calmodulin antagonists induce conformational changes in calpain II, and that changes occur in regions other than the Ca2(+)-binding domains.

Interaction of amiodarone and its analogs with calmodulin

Benzofurans have important actions on the electrical properties of myocardium; the biochemical basis of those actions is not known. Crystallographic examination of these compounds has revealed that benzofurans share structural homologies with the traditional calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalene and trifluoperazine. In the present study, the ability of amiodarone, desethylamiodarone, and benziodarone to displace the fluorescent ligand 8-anilino-1-naphthalene sulfonic acid (ANS) from calmodulin, to modulate the fluorescence emission of dansylcalmodulin, and to inhibit the activation by calmodulin of bovine brain cyclic nucleotide phosphodiesterase and human erythrocyte membrane Ca2+-ATPase were investigated at concentrations ranging from 10(-8) to 10(-6) M. These benzofurans displaced ANS from calmodulin with nearly equal efficiency upon forming a 1:1 complex with that protein. Each of these compounds also produced a decreased fluorescence emission of dansylcalmodulin, but with relative efficiencies being desethylamiodarone greater than amiodarone greater than benziodarone. Amiodarone and desethylamiodarone inhibited calmodulin-stimulable phosphodiesterase activity with similar potencies. Amiodarone and benziodarone inhibited calmodulin-stimulable Ca2+-ATPase activity equally, but desethylamiodarone had no effect. The observed differential effects of the amiodarone analogs suggest that calmodulin may possess multiple benzofuran-binding sites that are recognized by specific targets and ligands of this Ca2+-binding protein and that the cellular action of amiodarone and its analogs may reflect calmodulin antagonism.

Calmodulin antagonist action in smooth-muscle myosin phosphorylation. Different mechanisms for trifluoperazine and calmidazolium inhibition

The mechanism of inhibition of myosin phosphorylation by calmodulin antagonists [trifluoperazine (TFP) and calmidazolium (CAL)] was investigated in two enzyme-substrate systems: (1) mixtures of isolated myosin phosphorylatable light-chain (L20), myosin light-chain kinase (MCLKase) and calmodulin (CM); (2) synthetic self-assembled myosin filaments containing tightly bound endogenous MLCKase and CM. Double-reciprocal plots obtained with the first system were non-linear, indicating that the antagonists did not act exclusively on CM to inhibit MLCKase. First-order phosphorylation progress curves obtained at different CM antagonist concentrations for the more native filamentous myosin system indicated that the antagonists could also inhibit phosphorylation by interaction with the myosin phosphorylation site. Further analysis of these data in accordance with Reiner [(1969) Behavior of Enzyme Systems, 2nd edn., pp. 185-201, Van Nostrand-Reinhold, New York] showed that over a range of concentrations required to inhibit phosphorylation TFP interacted with free CM as well as with the myosin phosphorylation site: accordingly inhibition was of an activator- and substrate-depletion type. CAL inhibition was more CM-specific and operated via an activator-combination mechanism, inhibiting free CM as well as the CM-MLCKase complex. Both CM and the isolated L20 light chain antagonized the inhibitory effects of CAL and TFP, consistent with the above analysis.

Amiodarone is a potent calmodulin antagonist

The possible interaction between amiodarone, a potent antiarrhythmic and antianginal agent, and calmodulin (CaM) was investigated by three avenues of approach: (a) Effect of amiodarone on cardiac and vascular Ca2+/calmodulin-activated cyclic nucleotide phosphodiesterase (CaM-PDE); (b) Effect on the CaM-activated (Ca2+ + Mg2+)-ATPase from human erythrocytes; (c) Direct interaction between amiodarone and calmodulin measured by the effect of the drug on the fluorescence of 9-anthroylcholine (9AC) bound to calmodulin. Results show that amiodarone did not interact with basal activities of CaM-PDE and other isolated CaM-insensitive PDE forms as well as with (Ca2+ + Mg2+)-ATPase. Amiodarone inhibited calmodulin-activation of aortic CaM-PDE (Ki = 650 nM, substrate cGMP) and calmodulin-activation of erythrocyte ghosts (Ca2+ + Mg2+)-ATPase (IC50 = 4.5 microM) in an apparently competitive manner. Amiodarone decreased the fluorescence of the hydrophobic probe 9AC bound to calmodulin (IC50 = 5 microM). It is concluded that amiodarone is a potent calmodulin antagonist.

Blocking of the receptor-stimulated calcium entry into human platelets by verapamil and nicardipine

Verapamil (ED50 = 3 x 10(-6) M) and nicardipine (ED50 = 10(-6) M) inhibited the platelet activating factor (PAF)-induced increase of free cytosolic calcium concentration [( Ca2+]i) in quin2-loaded human platelets. In a Ca-free medium containing 5 mM BaCl2, PAF stimulated the inflow of Ba2+ ions which is completely abolished by verapamil and nicardipine. Simultaneous determination of quin2 fluorescence and 45Ca absorption showed that the action of verapamil is accounted for by blocking of the Ca2+ entry. Nicardipine suppresses also Ca2+ mobilization from intracellular stores. The effects of verapamil and nicardipine are not competitive with respect to PAF. The blockers reduce the [Ca2+]i increase induced by ADP, vasopressin, and PGH2 analogue U46619.

Synthetic peptides based on the calmodulin-binding domain of myosin light chain kinase inhibit activation of other calmodulin-dependent enzymes

Nanomolar concentrations of synthetic peptides corresponding to the calmodulin-binding domain of skeletal muscle myosin light chain kinase were found to inhibit calmodulin activation of seven well-characterized calmodulin-dependent enzymes: brain 61 kDa cyclic nucleotide phosphodiesterase, brain adenylate cyclase, Bordetella pertussis adenylate cyclase, red blood cell membrane Ca++-pump ATPase, brain calmodulin-dependent protein phosphatase (calcineurin), skeletal muscle phosphorylase b kinase, and brain multifunctional Ca++ (calmodulin)-dependent protein kinase. Inhibition could be entirely overcome by the addition of excess calmodulin. Thus, the myosin light chain kinase peptides used in this study may be useful antagonists for studying calmodulin-dependent enzymes and processes.

Binding of a dihydropyridine felodipine-analogue to calmodulin and related calcium-binding proteins

A dihydropyridine-affinity column was prepared by coupling a physiologically active and vasoselective amino-derivative of felodipine to divinylsulfone-activated Trisacryl GF2000. Calmodulin (CaM) as well as the homologous calcium-binding proteins skeletal and cardiac Troponin C (sTnC and cTnC) and S100b bound to this resin in a calcium-dependent manner. In contrast, other homologous proteins such as parvalbumin and the intestinal calcium-binding protein did not bind. Competition studies showed that CaM had a higher affinity for the felodipine-column than sTnC or cTnC. Through studies with a series of proteolytic fragments of CaM and sTnC, it was found that the felodipine binding site is located in the amino-terminal domain of the protein. These results illustrate the utility of affinity-chromatography for the study of dihydropyridine-binding proteins.