Hydrophobic regions function in calmodulin-enzyme(s) interactions

Stage-dependent sequential organization of nascent smooth muscle cells and its implications for the gut coiling morphogenesis in Xenopus larva

In vertebrates, gut coiling proceeds left-right asymmetrically during expansion of the gastrointestinal tract with highly organized muscular structures facilitating peristalsis. In this report, we explored the mechanisms of larval gut coiling morphogenesis relevant to its nascent smooth muscle cells using highly transparent Xenopus early larvae. First, to visualize the dynamics of intestinal smooth muscle cells, whole-mount specimens were immunostained with anti-smooth muscle-specific actin (SM-actin) antibody. We found that the nascent gut of Xenopus early larvae gradually expands the SM-actin-positive region in a stage-dependent manner. Transverse orientation of smooth muscle cells was first established, and next, the cellular longitudinal orientation along the gut axis was followed to make a meshwork of the contractile cells. Finally, anisotropic torsion by the smooth muscle cells was generated in the center of gut coiling, suggesting that twisting force might be involved in the late phase of coiling morphogenesis of the gut. Administration of S-(-)-Blebbistatin to attenuate the actomyosin contraction in vivo resulted in cancellation of coiling of the gut. Development of decapitation embryos, trunk 'torso' explants, and gut-only explants revealed that initial coiling of the gut proceeds without interactions with the other parts of the body including the central nervous system. We newly developed an in vitro model to assess the gut coiling morphogenesis, indicating that coiling pattern of the nascent Xenopus gut is partially gut-autonomous. Using this gut explant culture technique, inhibition of actomyosin contraction was performed by administrating either actin polymerization inhibitor, myosin light chain kinase inhibitor, or calmodulin antagonist. All of these reagents decreased the extent of gut coiling morphogenesis in vitro. Taken together, these results suggest that the contraction force generated by actomyosin-rich intestinal smooth muscle cells during larval stages is essential for the normal coiling morphogenesis of this muscular tubular organ.

Exploring Conformational Space with Thermal Fluctuations Obtained by Normal-Mode Analysis

Proteins in their native states can be represented as ensembles of conformers in dynamical equilibrium. Thermal fluctuations are responsible for transitions between these conformers. Normal-modes analysis (NMA) using elastic network models (ENMs) provides an efficient procedure to explore global dynamics of proteins commonly associated with conformational transitions. In the present work, we present an iterative approach to explore protein conformational spaces by introducing structural distortions according to their equilibrium dynamics at room temperature. The approach can be used either to perform unbiased explorations of conformational space or to explore guided pathways connecting two different conformations, e.g., apo and holo forms. In order to test its performance, four proteins with different magnitudes of structural distortions upon ligand binding have been tested. In all cases, the conformational selection model has been confirmed and the conformational space between apo and holo forms has been encompassed. Different strategies have been tested that impact on the efficiency either to achieve a desired conformational change or to achieve a balanced exploration of the protein conformational multiplicity.

Lanmodulin: A Highly Selective Lanthanide-Binding Protein from a Lanthanide-Utilizing Bacterium

Lanthanides (Lns) have been shown recently to be essential cofactors in certain enzymes in methylotrophic bacteria. Here we identify in the model methylotroph, Methylobacterium extorquens, a highly selective Ln^III-binding protein, which we name lanmodulin (LanM). LanM possesses four metal-binding EF hand motifs, commonly associated with Ca^II-binding proteins. In contrast to other EF hand-containing proteins, however, LanM undergoes a large conformational change from a largely disordered state to a compact, ordered state in response to picomolar concentrations of all Ln^III (Ln = La-Lu, Y), whereas it only responds to Ca^II at near-millimolar concentrations. Mutagenesis of conserved proline residues present in LanM's EF hands, not encountered in Ca^II-binding EF hands, to alanine pushes Ca^II responsiveness into the micromolar concentration range while retaining picomolar Ln^III affinity, suggesting that these unique proline residues play a key role in ensuring metal selectivity in vivo. Identification and characterization of LanM provides insights into how biology selectively recognizes low-abundance Ln^III over higher-abundance Ca^II, pointing toward biotechnologies for detecting, sequestering, and separating these technologically important elements.

A high-throughput fluorimetric microarray with enhanced fluorescence and suppressed "coffee-ring" effects for the detection of calcium ions in blood

A rapid, ultrasensitive, and high-throughput fluorimetric microarray method has been developed using hydrophobic pattern as the microarray substrate and 3-aminopropyltriethoxysilane-coupled carboxylic acid calcium (APS-CCA) as the fluorescent probes for sensing Ca2+ ions in blood. The hydrophobic pattern of the developed Ca2+ analysis microarray could largely suppress the "coffee-ring" effects to facilitate the better distribution density of testing microspots toward the high-throughput detections, and especially prevent the cross-contamination of the multiple samples between adjacent microspots. Moreover, the use of APS matrix could endow the CCA probe the enhanced environmental stability and fluorescence intensity, which is about 2.3-fold higher than that of free CCA. The interactions between APS-CCA and Ca2+ ions were systematically characterized by UV-vis and fluorescence measurements including microscopy imaging. It was demonstrated that the fluorimetric microarray could display the strong capacity of specifically sensing Ca2+ ions with the minimal interferences from blood backgrounds. Such an APS-CCA-based fluorimetric microarray can allow for the analysis of Ca2+ ions down to 0.0050 mM in blood, promising a highly sensitive and selective detection candidate for Ca2+ ions to be applied in the clinical laboratory.

Modulation of Calmodulin and Protein Kinase C Activities by Pencillium Mycotoxins

Calmodulin (CaM), a calcium-binding protein, is found in high concentrations in mammalian brain where it plays a pivotal role in a large number of cellular functions. Protein kinase C (PKC), a multifunctional cytosolic enzyme, in the presence of both Ca2+ and phospholipids, transduce extracellular signals into intracellu-lar events. Both CaM and PKC are partially involved in maintaining Ca2+ homeostasis in the cell. Any fluctuations in the intracel-lular Ca2+ can modulate cellular functions and may contribute to neuronal dysfunction. Hence, the present investigation was initiated to study the effects of some selected penicillium (naturally occurring tremorgenic) mycotoxins like secalonic acid, citreoviridin, and verruculogen on CaM activity, active conformation of CaM and PKC activity. Stimulation of CaM-deflcient bovine brain 3′-5′ phosphodieste rase (PDE) indicated CaM activity. The modification of CaM active conformation was studied by the binding of fluorescent probe N-phenyl-1-napthylamine (NPN) to CaM. Alterations in the fluorescence of dansyl-CaM was used to study the effect of these compounds on complex formation between CaM and PDE. Rat brain cytosolic PKC was studied using 32P-ATP as a measure of altered protein phosphorylation. The concentrations of mycotoxins used were in the range of 10 to 50 μM. All three mycotoxins inhibited CaM-stimulated PDE activity in a concentration-dependent manner. Citreoviridin and secalonic acid inhibited NPN fluorescence and Ca2+-dependent complex formation of dansyl-CaM and PDE. The IC50 values for NPN fluorescence of citreoviridin and secalonic acid were 13 μM and 19 μM respectively. However, verruculogen showed little effect on NPN fluorescence and the Ca2+-dependent complex formation of dansyl-CaM and PDE. These mycotoxins also inhibited PKC activity in a concentration-dependent manner with IC50 values of 19.8, 25.7, and 38.4 μM for secalonic acid, citreoviridin, and verruculogen, respectively. The results of our study suggest that these mycotoxins at very low concentrations are interacting with CaM and PKC. Such an effect could lead to impairment of neurotransmission and result in neurotoxicity.

Evaluation of the calmodulin-SOX9 interaction by "magnetic fishing" coupled to mass spectrometry

Disruption of calmodulin (CaM)-based protein interactions has been touted as a potential means for modulating several disease pathways. Among these is SOX9, which is a DNA binding protein that is involved in chrondrocyte differentiation and regulation of the hormones that control sexual development. In this work, we employed a "magnetic fishing"/mass spectrometry assay in conjunction with intrinsic fluorescence to examine the interaction of CaM with the CaM-binding domain of SOX9 (SOX-CAL), and to assess the modulation of this interaction by known anti-CaM compounds. Our data show that there is a high affinity interaction between CaM and SOX-CAL (27±9 nM), and that SOX-CAL bound to the same location as the well-known CaM antagonist melittin; unexpectedly, we also found that addition of CaM-binding small molecules initially produced increased SOX-CAL binding, indicative of binding to both the well-known high-affinity CaM binding site and a second, lower-affinity binding site.

Mapping the sevoflurane-binding sites of calmodulin

General anesthetics, with sevoflurane (SF) being the first choice inhalational anesthetic agent, provide reversible, broad depressor effects on the nervous system yet have a narrow margin of safety. As characterization of low-affinity binding interactions of volatile substances is exceptionally challenging with the existing methods, none of the numerous cellular targets proposed as chief protagonists in anesthesia could yet be confirmed. The recognition that most critical functions modulated by volatile anesthetics are under the control of intracellular Ca²⁺ concentration, which in turn is primarily regulated by calmodulin (CaM), motivated us for characterization of the SF–CaM interaction. Solution NMR (Nuclear Magnetic Resonance) spectroscopy was used to identify SF-binding sites using chemical shift displacement, NOESY and heteronuclear Overhauser enhancement spectroscopy (HOESY) experiments. Binding affinities were measured using ITC (isothermal titration calorimetry). SF binds to both lobes of (Ca²⁺)₄-CaM with low mmol/L affinity whereas no interaction was observed in the absence of Ca²⁺. SF does not affect the calcium binding of CaM. The structurally closely related SF and isoflurane are shown to bind to the same clefts. The SF-binding clefts overlap with the binding sites of physiologically relevant ion channels and bioactive small molecules, but the binding affinity suggests it could only interfere with very weak CaM targets.

Calcium negatively regulates an intramolecular interaction between the N-terminal recoverin homology and EF-hand motif domains and the C-terminal C1 and catalytic domains of diacylglycerol kinase α

The type I diacylglycerol kinase (DGK) isozymes (α, β and γ) contain a shared recoverin homology (RVH) domain, a tandem repeat of Ca2+-binding EF-hand motifs, two cysteine-rich C1 domains, and the catalytic domain. We previously reported that a DGKα mutant lacking the RVH domain and EF-hands was constitutively active, implying that the N-terminal region (NTR) of DGKα, consisting of the RVH domain and EF-hand motifs, intramolecularly interacts with and masks the activity of the C-terminal region (CTR), containing the C1 and catalytic domains. In this study, we demonstrate that a glutathione S-transferase (GST)-fused DGKα-NTR construct physically binds to a green fluorescent protein (GFP)-fused DGKα-CTR construct. Moreover, co-precipitation of GFP-DGKα-CTR with GST-DGKα-NTR was clearly attenuated by the addition of 1 μM Ca2+. This result indicates that Ca2+ induces dissociation of the physical interaction between DGKα-NTR and DGKα-CTR. In addition to previously reported calcium-dependent changes in the hydrophobicity and net surface charge, Ca2+ also appeared to induce a decrease in the α-helical content of DGKα-NTR. These results suggest that Ca2+-induced conformational changes in the NTR release the intramolecular association between the NTR and the CTR of DGKα.

Differentiation of prostate cancer cells using flexible fluorescent polymers

Using water-soluble, fluorescent, flexible polymers, we have devised a novel methodology for identification and differentiation of prostate cancer cells. Using a stepwise linear discriminant analysis, we demonstrate that the differential modulations of the polymer emission intensities in the presence of conditioned cell culture media can be used to distinguish between prostate cancer subtypes and between cancerous and noncancer cells. The differences in the compositions of the conditioned cell culture media are likely contributing to different fluorescence spectral patterns of the polymers. This in vitro approach may provide a novel platform for the development of an alternative prostate cancer diagnostic and subtyping technique.

Structure and function of ALG-2, a penta-EF-hand calcium-dependent adaptor protein

ALG-2 (a gene product of PDCD6) is a 22-kD protein containing five serially repetitive EF-hand structures and belongs to the penta-EF-hand (PEF) family, including the subunits of typical calpains. ALG-2 is the most conserved protein among the PEF family members and its homologs are widely found in eukaryotes. X-ray crystal structures of various PEF proteins including ALG-2 have common features: presence of eight α-helices and dimer formation via paired EF5s that are positioned in anti-parallel orientation. ALG-2 forms a homodimer and a heterodimer with its closest paralog peflin. Like calmodulin, a well-known four-EF-hand protein, ALG-2 interacts with various proteins in a Ca(2+)-dependent fashion, but the binding motifs are completely different. With some exceptions, ALG-2-interacting proteins commonly contain Pro-rich regions, and ALG-2 recognizes at least two distinct Pro-containing motifs: PPYP(X)nYP (X, variable; n=4 in ALIX and PLSCR3) and PXPGF (represented by Sec31A). A shorter alternatively spliced isoform, lacking two residues and designated ALG-2(ΔGF122), does not bind ALIX but maintains binding capacity to Sec31A. X-ray crystal structural analyses have revealed that binding of calcium ions induces the configuration of the side chain of R125 so that it opens Pocket 1, which accepts PPYP, but Pocket 1 remains closed in the case of ALG-2(ΔGF122). ALG-2 dimer has two ligand-binding sites, each in a monomer molecule, and appears to function as a Ca(2+)-dependent adaptor protein to either stabilize a preformed complex or to bridge two proteins on scaffolds in systems of the endosomal sorting complex required for transport (ESCRT) and ER-to-Golgi transport.

Binding of calcium, magnesium, and target peptides to Cdc31, the centrin of yeast Saccharomyces cerevisiae

Cdc31, the Saccharomyces cerevisiae centrin, is an EF-hand calcium-binding protein essential for the cell division and mRNA nuclear export. We used biophysical techniques to investigate its calcium, magnesium, and protein target binding properties as well as their conformations in solution. We show here that Cdc31 displays one Ca(2+)/Mg(2+) mixed site in the N-terminal domain and two low-affinity Ca(2+) sites in the C-terminal domain. The affinity of Cdc31 for different natural target peptides (from Kar1, Sfi1, Sac3) that we obtained by isothermal titration calorimetry shows weakly Ca(2+), but also Mg(2+) dependence. The characteristics of target surface binding were shown to be similar; we highlight that the 1-4 hydrophobic amino acid motif, in a stable amphipathic α-helix, is critical for binding. Ca(2+) and Mg(2+) binding increase the α-helix content and stabilize the structure. Analysis of small-angle X-ray scattering experiments revealed that N- and C-terminal domains are not individualized in apo-Cdc31; in contrast, they are separated in the Mg(2+) state, creating a groove in the middle of the molecule that is occupied by the target peptide in the liganded form. Consequently, Mg(2+) seems to have consequences on Cdc31's function and could be important to stimulate interactions in resting cells.

A normal mode-based geometric simulation approach for exploring biologically relevant conformational transitions in proteins

A three-step approach for multiscale modeling of protein conformational changes is presented that incorporates information about preferred directions of protein motions into a geometric simulation algorithm. The first two steps are based on a rigid cluster normal-mode analysis (RCNMA). Low-frequency normal modes are used in the third step (NMSim) to extend the recently introduced idea of constrained geometric simulations of diffusive motions in proteins by biasing backbone motions of the protein, whereas side-chain motions are biased toward favorable rotamer states. The generated structures are iteratively corrected regarding steric clashes and stereochemical constraint violations. The approach allows performing three simulation types: unbiased exploration of conformational space; pathway generation by a targeted simulation; and radius of gyration-guided simulation. When applied to a data set of proteins with experimentally observed conformational changes, conformational variabilities are reproduced very well for 4 out of 5 proteins that show domain motions, with correlation coefficients r > 0.70 and as high as r = 0.92 in the case of adenylate kinase. In 7 out of 8 cases, NMSim simulations starting from unbound structures are able to sample conformations that are similar (root-mean-square deviation = 1.0-3.1 Å) to ligand bound conformations. An NMSim generated pathway of conformational change of adenylate kinase correctly describes the sequence of domain closing. The NMSim approach is a computationally efficient alternative to molecular dynamics simulations for conformational sampling of proteins. The generated conformations and pathways of conformational transitions can serve as input to docking approaches or as starting points for more sophisticated sampling techniques.

Fast photochemical oxidation of proteins for comparing structures of protein-ligand complexes: the calmodulin-peptide model system

Fast photochemical oxidation of proteins (FPOP) is a mass spectrometry-based protein footprinting method that modifies proteins on the microsecond time scale. Highly reactive (•)OH, produced by laser photolysis of hydrogen peroxide, oxidatively modifies the side chains of approximately one-half the common amino acids on this time scale. Because of the short labeling exposure, only solvent-accessible residues are sampled. Quantification of the modification extent for the apo and holo states of a protein-ligand complex provides structurally sensitive information at the amino-acid level to compare the structures of unknown protein complexes with known ones. We report here the use of FPOP to monitor the structural changes of calmodulin in its established binding to M13 of the skeletal muscle myosin light chain kinase. We use the outcome to establish the unknown structures resulting from binding with melittin and mastoparan. The structural comparison follows a comprehensive examination of the extent of FPOP modifications as measured by proteolysis and LC-MS/MS for each protein-ligand equilibrium. The results not only show that the three calmodulin-peptide complexes have similar structures but also reveal those regions of the protein that became more or less solvent-accessible upon binding. This approach has the potential for relatively high throughput, information-dense characterization of a series of protein-ligand complexes in biochemistry and drug discovery when the structure of one reference complex is known, as is the case for calmodulin and M13 of the skeletal muscle myosin light chain kinase, and the structures of related complexes are not.

The adrenergic receptor subtypes present in frog (Rana esculenta) skin

Frog skin transports ions and water under hormonal control. In spite of the fundamental role played by adrenergic stimulation in maintaining the water balance of the organism, the receptor subtype(s) present in the skin have not been identified yet. We measured the increase in short-circuit current (ISC, an estimate of ion transport) induced by cirazoline, clonidine, xamoterol, formoterol, or BRL 37344, in order to verify the presence of alpha1, alpha2, beta1, beta2, or beta3 receptor subtypes, respectively. Only after treatment with formoterol, BRL 37344 and, to a lesser extent, cirazoline was measured a significant increase in ISC (57%, 33.2%, and 4.7%, respectively). The formoterol and BRL 37344 concentrations producing half-maximal effect (EC50) were 1.12 and 70.1 nM, respectively. Moreover, the formoterol effect was inhibited by treatment with ICI 118551 (antagonist of beta2 receptors) while SR 59230A (antagonist of beta3 receptors) had no effect; opposite findings were obtained when the BRL 37344 stimulation was investigated. Finally, by measuring the transepithelial fluxes of 22Na+ and 36Cl-, we demonstrated that Na+ absorption is increased by activation of beta2 and beta3 and is cAMP-sensitive, whereas the Cl- secretion is only increased by activation of beta2 receptors and is cAMP- and calmodulin-sensitive.

Interaction of antagonists with calmodulin: insights from molecular dynamics simulations

We report results of 12 ns, all-atom molecular dynamics simulation (MDS) and Poisson-Boltzmann free energy calculations (PBFE) on calmodulin (CaM) bound to two molecules of trifluoperazine (TFP) and of N-(3,3, diphenylpropyl)- N'-[1- R-(3,4-bis-butoxyphenyl)-ethyl]-propylenediamine (DPD). X-ray data show very similar structures for the two complexes, yet the antagonists significantly differ with respect to their CaM binding affinities, the neutral DPD is much more potent. The goal of the study was to unravel the reason why TFP is less potent although its positive charge should facilitate binding. The electrostatic energy terms in CHARMM and binding free energy terms of the PBFE approach showed TFP a better antagonist, while inspection of hydrophobic contacts supports DPD binding. Detailed inspection of the amino acid contributions of PBFE calculations unravel that steric reasons oppose the favorable binding of TFP. Structural conditions are given for a successful drug design strategy, which may benefit also from charge-charge interactions.

Requirement of the TRPC1 Cation Channel in the Generation of Transient Ca2+ Oscillations by the Calcium-sensing Receptor

The calcium-sensing receptor (CaR) is an allosteric protein that responds to extracellular Ca(2+) ([Ca(2+)](o)) and aromatic amino acids with the production of different patterns of oscillations in intracellular Ca(2+) concentration ([Ca(2+)](i)). An increase in [Ca(2+)](o) stimulates phospholipase C-mediated production of inositol 1,4,5-trisphosphate and causes sinusoidal oscillations in [Ca(2+)](i). Conversely, aromatic amino acid-induced CaR activation does not stimulate phospholipase C but engages an unidentified signaling mechanism that promotes transient oscillations in [Ca(2+)](i). We show here that the [Ca(2+)](i) oscillations stimulated by aromatic amino acids were selectively abolished by TRPC1 down-regulation using either a pool of small inhibitory RNAs (siRNAs) or two different individual siRNAs that targeted different coding regions of TRPC1. Furthermore, [Ca(2+)](i) oscillations stimulated by aromatic amino acids were also abolished by inhibition of TRPC1 function with an antibody that binds the pore region of the channel. We also show that aromatic amino acid-stimulated [Ca(2+)](i) oscillations can be prevented by protein kinase C (PKC) inhibitors or siRNA-mediated PKCalpha down-regulation and impaired by either calmodulin antagonists or by the expression of a dominant-negative calmodulin mutant. We propose a model for the generation of CaR-mediated transient [Ca(2+)](i) oscillations that integrates its stimulation by aromatic amino acids with TRPC1 regulation by PKC and calmodulin.

Significance of the extra C-terminal tail of CaLP, a novel calmodulin-like protein involved in oyster calcium metabolism

Oyster (Pinctada fucata) calmodulin-like protein (CaLP), containing a C-terminally extra hydrophilic tail (150D-161K), is a novel protein involved in the regulation of oyster calcium metabolism. To investigate the importance of the extra fragment to the Ca(2+)/Mg(2+)-dependent conformational changes in the intact CaLP molecule and the interactions between CaLP and its target proteins, a truncated CaLP mutant (M-CaLP) devoid of the extended C-terminus was constructed and overexpressed in Escherichia coli. The conformational characteristics of M-CaLP were studied by CD and fluorescence spectroscopy and compared with those of the oyster CaM and CaLP. The far-UV CD results reveal that the extra tail has a strong effect on the Ca(2+)-induced, but a relatively weak effect on the Mg(2+)-induced conformational changes in CaLP. However, upon Ca2+ or Mg2+ binding, only slight changes for intrinsic phenylalanine and tyrosine fluorescence spectra between M-CaLP and CaLP are observed. Our results also indicate that the extra tail can significantly decrease the exposure of the hydrophobic patches in CaLP. Additionally, affinity chromatography demonstrates that the target binding of CaLP is greatly influenced by its additional tail. All our results implicate that the extra tail may play some important roles in the interactions between CaLP and its targets in vivo.

Changes in cyclic nucleotide phosphodiesterase activity and calmodulin concentration in heart muscle of cardiomyopathic hamsters

Cyclic nucleotides (cAMP and cGMP) phosphodiesterase (PDE) activities and expression are altered in the cardiac muscle of cardiomyopathic heart failure, and PDE inhibitors improve the abnormal muscle condition through changing the cyclic nucleotide concentration. These observations prompted us to investigate the role of calmodulin (CaM) in the regulation of cyclic nucleotide PDE activities, and moreover to study the modulation of the PDE isozymes in heart failure, using cardiac muscles of cardiomyopathic hamster. The CaM concentrations in the heart muscle of the normal control and cardiomyopathic hamsters (each of three to four hamsters) varied with cell fraction and with the age of the animal. The CaM concentrations in the soluble fraction obtained from cardiomyopathic hamster tissue were significantly increased at 25 and 32 weeks of age (2.02 +/- 0.62 microg/mg protein (mean +/- S.E.), and 3.21 +/- 0.95) compared with that obtained from the control (0.60 +/- 0.04) or cardiomyopathic (0.95 +/- 0.12) hamsters at 8 weeks of age. The solubilized PDE isolated from the hamster heart muscle (three or four hamsters in each age) by column chromatography on diethylaminoethyl (DEAE)-cellulose revealed three peaks of activity, which may correspond to the isozymes of PDE classified recently, namely PDE I, II, and III. These three peaks of activity, particularly peak III, seen in the soluble fraction of cardiomyopathic hamster heart declined in proportion to the age of the animal compared with that of the control hamster heart. In the cGMP-PDE assay system, the concentration of CaM inhibitor W-7 required for 50% inhibition (IC(50)) of PDE I, II, and III peak activities was 140, 29, and 46 microM, respectively, suggesting that PDE II is more sensitive to W-7. These results suggest that alteration in these isozyme activities accompanied with changes of CaM concentration may influence the cardiac muscle contractility in cardiomyopathic hamster via changes of cyclic nucleotide concentration.

The Structure of the Complex of Calmodulin with KAR-2

3'-(beta-Chloroethyl)-2',4'-dioxo-3,5'-spiro-oxazolidino-4-deacetoxyvinblastine (KAR-2) is a potent anti-microtubular agent that arrests mitosis in cancer cells without significant toxic side effects. In this study we demonstrate that in addition to targeting microtubules, KAR-2 also binds calmodulin, thereby countering the antagonistic effects of trifluoperazine. To determine the basis of both properties of KAR-2, the three-dimensional structure of its complex with Ca(2+)-calmodulin has been characterized both in solution using NMR and when crystallized using x-ray diffraction. Heterocorrelation ((1)H-(15)N heteronuclear single quantum coherence) spectra of (15)N-labeled calmodulin indicate a global conformation change (closure) of the protein upon its binding to KAR-2. The crystal structure at 2.12-A resolution reveals a more complete picture; KAR-2 binds to a novel structure created by amino acid residues of both the N- and C-terminal domains of calmodulin. Although first detected by x-ray diffraction of the crystallized ternary complex, this conformational change is consistent with its solution structure as characterized by NMR spectroscopy. It is noteworthy that a similar tertiary complex forms when calmodulin binds KAR-2 as when it binds trifluoperazine, even though the two ligands contact (for the most part) different amino acid residues. These observations explain the specificity of KAR-2 as an anti-microtubular agent; the drug interacts with a novel drug binding domain on calmodulin. Consequently, KAR-2 does not prevent calmodulin from binding most of its physiological targets.

Immunosuppressive activity of a molecule isolated from Artemisia annua on DTH responses compared with cyclosporin A

Artemisia belongs to the family of Compositea; there are different Artemisias in Iran, of which Artemisia annua L. is grown in the north of Iran. In this study, Artemisinin was extracted and purified from the whole plants. The purification of Artemisinin was performed using column chromatography in different polarities of solvents and the results were evaluated by Thin Layer Chromatography (TLC). (1)H-NMR (NMR-500) spectroscopy was used to characterize the purified Artemisinin. The immunosuppressive activity of Artemisinin was investigated on Balb/c mice by DTH response in comparison to cyclosporin A (CsA). The data indicated that Artemisinin could suppress the delayed type hypersensitivity (DTH) against sheep blood capsule in Balb/c mice. Also its inhibitory effect on calmodulin (CaM) structure was determined by fluorescence spectroscopy. The data indicated an inhibitory effect of that on the activity of calmodulin by increasing the fluorescence emission of calmodulin. Both in vivo (DTH response) and in vitro (spectrofluorometry) studies indicated the activity of Artemisinin as an immunosuppressive agent and that the fluorescence emission of calmodulin is more than cyclosporin A.

Mechanism linking NMDA receptor activation to modulation of voltage-gated sodium current in distal retina

In this study, we investigated the mechanism that links activation of N-methyl-D-aspartate (NMDA) receptors to inhibition of voltage-gated sodium channels in isolated catfish cone horizontal cells. NMDA channels were activated in voltage-clamped cells incubated in low-calcium saline or dialyzed with the calcium chelator BAPTA to determine that calcium influx through NMDA channels is required for sodium channel modulation. To determine whether calcium influx through NMDA channels triggers calcium-induced calcium release (CICR), cells were loaded with the calcium-sensitive dye calcium green 2 and changes in relative fluorescence were measured in response to NMDA. Responses were compared with measurements obtained when caffeine depleted stores. Voltage-clamp studies demonstrated that CICR modulated sodium channels in a manner similar to that of NMDA. Blocking NMDA receptors with AP-7, blocking CICR with ruthenium red, depleting stores with caffeine, or dialyzing cells with calmodulin antagonists W-5 or peptide 290-309 all prevented sodium channel modulation. These results support the hypothesis that NMDA modulation of voltage-gated sodium channels in horizontal cells requires CICR and activation of a calmodulin-dependent signaling pathway.

Two-phase response of acid extrusion triggered by purinoceptor in Chinese hamster ovary cells

The functional characteristics of purinoceptors in Chinese hamster ovary (CHO) cells were investigated using a microphysiometer which detects small metabolic changes to living cells in real-time as variations of pH in the extracellular microenvironment. Uridine 5'-triphosphate (UTP) increased the extracellular acidification rate biphasically, namely a transient and a steady response were observed. The transient phase reached a peak (four- to fivefold the basal extracellular acidification rate in amplitude) within 20 s and was followed by the steady phase which was sustained for more than 1 min at an amplitude less than twofold the basal extracellular acidification rate. Both phases showed a concentration-dependent increase in response to UTP. However, there was a significant difference in the pEC(50) value for UTP between the transient (4.8) and steady phases (6.1). Like UTP, ATP increased the extracellular acidification rate, but alpha,beta-methyleneATP (alpha,beta-MeATP), 2-methylthioATP (2-MeSATP), ADP, UDP and adenosine did not. This result suggests that the acid is extruded through a P2Y(2) or P2Y(2)-like purinoceptor. 5-(N-ethyl-N-isopropyl) amiloride (EIPA) and 4-isopropyl-3-methylsulphonylbenzoyl-guanidine methanesulphonate (HOE642) suppressed both phases of the UTP-stimulated extracellular acidification rate response with high affinity (pIC(50): approximately 7.0). This result suggests that the Na(+)/H(+) exchanger 1 (NHE-1) predominantly mediates the UTP-induced acid extrusion response in CHO cells. Elimination of extracellular Ca(2+) or treatment with thapsigargin diminished both phases of the UTP-stimulated extracellular acidification rate. In addition, N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide hydrochloride (W-7) also abrogated the two phases. These results are consistent with the involvement of NHE-1 which is activated via Ca(2+)/calmodulin. Persistent exposure to UTP reduced both extracellular acidification rate phases, causing desensitization of the P2Y purinoceptor. This desensitization did not affect the acid extrusion response mediated by the alpha(1)-adrenoceptor.

Calcium, calmodulin, and calcium-calmodulin kinase II: heartbeat to heartbeat and beyond

Calcium (Ca) is the key regulator of cardiac contraction during excitation-contraction (E-C) coupling. However, differences exist between the amount of Ca being transported into the myocytes upon electrical stimulation as compared to Ca released from the sarcoplasmic reticulum (SR). Moreover, alterations in E-C coupling occur in cardiac hypertrophy and heart failure. In addition to the direct effects of Ca on the myofilaments, Ca plays a pivotal role in activation of a number of Ca-dependent proteins or second messengers, which can modulate E-C coupling. Of these proteins, calmodulin (CaM) and Ca-CaM-dependent kinase II (CaMKII) are of special interest in the heart because of their role of modulating Ca influx, SR Ca release, and SR Ca uptake during E-C coupling. Indeed, CaM and CaMKII may be associated with some ion channels and Ca transporters and both can modulate acute cellular Ca handling. In addition to the changes in Ca, CaM and CaMKII signals from beat-to-beat, changes may occur on a longer time scale. These may occur over seconds to minutes involving phosphorylation/dephosphorylation reactions, and even a longer time frame in altering gene transcription (excitation-transcription (E-T) coupling) in hypertrophic signaling and heart failure. Here we review the classical role of Ca in E-C coupling and extend this view to the role of the Ca-dependent proteins CaM and CaMKII in modulating E-C coupling and their contribution to E-T coupling.

Modulation of a voltage-gated calcium channel linked to activation of glutamate receptors and calcium-induced calcium release in the catfish retina

1. Catfish (Ictalurus punctatus) retinal cone horizontal cells contain an L-type calcium current that has been proposed to be involved in visual processing. Here we report on the modulation of this current by activation of glutamate receptors and calcium-induced calcium release (CICR) from intracellular calcium stores. 2. Fluorescence data obtained from isolated horizontal cells loaded with indo-1 provided evidence of calcium release from an intracellular calcium store sensitive to caffeine, calcium and ryanodine. In the presence of caffeine, ryanodine-sensitive stores released calcium in a transient manner. Release of calcium was blocked when cells were preincubated in BAPTA, in the presence of ruthenium red, or in low concentrations of ryanodine. 3. The release of calcium from ryanodine-sensitive stores directly corresponded with a decrease of the voltage-gated L-type calcium current amplitude. Caffeine-induced modulation of the calcium current was reduced in the presence of ruthenium red. 4. Activation of ionotropic kainate receptors on catfish cone horizontal cells triggered CICR from ryanodine-sensitive stores and mimicked inhibition of the voltage-gated calcium current. Kainate-induced inhibition of the calcium current was diminished when intracellular calcium stores were inhibited with ruthenium red or depleted with ryanodine, or when calmodulin antagonists or CaM kinase II inhibitors were present. 5. These results provide evidence that activation of an ionotropic glutamate receptor on catfish cone horizontal cells is linked to calcium release from ryanodine-sensitive intracellular calcium stores and modulation of the L-type calcium current activity. Inhibition of this calcium current directly or indirectly involves calmodulin and CaM kinase II and represents a possible mechanism used by horizontal cells to affect response properties of these cells.

Calmodulin Signals Capacitation and Triggers the Agonist-Induced Acrosome Reaction in Mouse Spermatozoa

Capacitated acrosome-intact spermatozoa interact with specific sugar residues on neoglycoproteins (ngps) or solubilized zona pellucida (ZP), the egg's extracellular glycocalyx, prior to the initiation of a signal transduction cascade that results in the fenestration and fusion of the sperm plasma membrane and the outer acrosomal membrane at multiple sites and exocytosis of acrosomal contents (i.e., induction of the acrosome reaction (AR)). The AR releases acrosomal contents at the site of sperm-zona binding and is thought to be a prerequisite event that allows spermatozoa to penetrate the ZP and fertilize the egg. Since Ca(2+)/calmodulin (CaM) plays a significant role in several cell signaling pathways and membrane fusion events, we have used a pharmacological approach to examine the role of CaM, a calcium-binding protein, in sperm capacitation and agonist-induced AR. Inclusion of CaM antagonists (calmodulin binding domain, calmidazolium, compound 48/80, ophiobolin A, W5, W7, and W13), either in in vitro capacitation medium or after sperm capacitation blocked the npg-/ZP-induced AR. Purified CaM largely reversed the AR blocking effects of antagonists during capacitation. Our results demonstrate that CaM plays an important role in priming (i.e., capacitation) of mouse spermatozoa as well as in the agonist-induced AR. These data allow us to propose that CaM regulates these events by modulating sperm membrane component(s).

Substrate recognition by the collagen-binding domain of Clostridium histolyticum class I collagenase

Clostridium histolyticum type I collagenase (ColG) has a segmental structure, S1+S2+S3a+S3b. S3a and S3b bound to insoluble collagen, but S2 did not, thus indicating that S3 forms a collagen-binding domain (CBD). Because S3a+S3b showed the most efficient binding to substrate, cooperative binding by both domains was suggested for the enzyme. Monomeric (S3b) and tandem (S3a+S3b) CBDs bound to atelocollagen, which contains only the collagenous region. However, they did not bind to telopeptides immobilized on Sepharose beads. These results suggested that the binding site(s) for the CBD is(are) present in the collagenous region. The CBD bound to immobilized collagenous peptides, (Pro-Hyp-Gly)(n) and (Pro-Pro-Gly)(n), only when n is large enough to allow the peptides to have a triple-helical conformation. They did not bind to various peptides with similar amino acid sequences or to gelatin, which lacks a triple-helical conformation. The CBD did not bind to immobilized Glc-Gal disaccharide, which is attached to the side chains of hydroxylysine residues in the collagenous region. These observations suggested that the CBD specifically recognizes the triple-helical conformation made by three polypeptide chains in the collagenous region.

Cation- and peptide-binding properties of human centrin 2

Centrin and calmodulin (CaM) are closely related four-EF-hand Ca(2+)-binding proteins. While CaM is monomeric, centrin 2 is dimeric and binds only two Ca(2+) per dimer, likely to site IV in each monomer. Ca(2+) binding to centrin 2 displays pronounced negative cooperativity and a [Ca(2+)](0.5) of 30 microM. As in CaM, Ca(2+) binding leads to the exposure of a hydrophobic probe-accessible patch on the surface of centrin 2. Provided Ca(2+) is present, centrin 2 forms a 1:1 peptide:monomer complex with melittin with an affinity of 100 nM. The complex binds four instead of two Ca(2+). Our data point to surprising differences in the mode of activation of these homologous proteins.

A new potent calmodulin antagonist with arylalkylamine structure: crystallographic, spectroscopic and functional studies

An arylalkylamine-type calmodulin antagonist, N-(3, 3-diphenylpropyl)-N'-[1-R-(3, 4-bis-butoxyphenyl)ethyl]-propylene-diamine (AAA) is presented and its complexes with calmodulin are characterized in solution and in the crystal. Near-UV circular dichroism spectra show that AAA binds to calmodulin with 2:1 stoichiometry in a Ca(2+)-dependent manner. The crystal structure with 2:1 stoichiometry is determined to 2.64 A resolution. The binding of AAA causes domain closure of calmodulin similar to that obtained with trifluoperazine. Solution and crystal data indicate that each of the two AAA molecules anchors in the hydrophobic pockets of calmodulin, overlapping with two trifluoperazine sites, i.e. at a hydrophobic pocket and an interdomain site. The two AAA molecules also interact with each other by hydrophobic forces. A competition enzymatic assay has revealed that AAA inhibits calmodulin-activated phosphodiesterase activity at two orders of magnitude lower concentration than trifluoperazine. The apparent dissociation constant of AAA to calmodulin is 18 nM, which is commensurable with that of target peptides. On the basis of the crystal structure, we propose that the high-affinity binding is mainly due to a favorable entropy term, as the AAA molecule makes multiple contacts in its complex with calmodulin.

Nerve growth factor activation of the extracellular signal-regulated kinase pathway is modulated by Ca(2+) and calmodulin

Nerve growth factor is a member of the neurotrophin family of trophic factors that have been reported to be essential for the survival and development of sympathetic neurons and a subset of sensory neurons. Nerve growth factor exerts its effects mainly by interaction with the specific receptor TrkA, which leads to the activation of several intracellular signaling pathways. Once activated, TrkA also allows for a rapid and moderate increase in intracellular calcium levels, which would contribute to the effects triggered by nerve growth factor in neurons. In this report, we analyzed the relationship of calcium to the activation of the Ras/extracellular signal-regulated kinase pathway in PC12 cells. We observed that calcium and calmodulin are both necessary for the acute activation of extracellular signal-regulated kinases after TrkA stimulation. We analyzed the elements of the pathway that lead to this activation, and we observed that calmodulin antagonists completely block the initial Raf-1 activation without affecting the function of upstream elements, such as Ras, Grb2, Shc, and Trk. We have broadened our study to other stimuli that activate extracellular signal-regulated kinases through tyrosine kinase receptors, and we have observed that calmodulin also modulates the activation of such kinases after epidermal growth factor receptor stimulation in PC12 cells and after TrkB stimulation in cultured chicken embryo motoneurons. Calmodulin seems to regulate the full activation of Raf-1 after Ras activation, since functional Ras is necessary for Raf-1 activation after nerve growth factor stimulation and calmodulin-Sepharose is able to precipitate Raf-1 in a calcium-dependent manner.

Ca(2+)/calmodulin-independent activation of calcineurin from Dictyostelium by unsaturated long chain fatty acids

This study describes a novel mode of activation for the Ca(2+)/calmodulin-dependent protein phosphatase calcineurin. Using purified calcineurin from Dictyostelium discoideum we found a reversible, Ca(2+)/calmodulin-independent activation by the long chain unsaturated fatty acids arachidonic acid, linoleic acid, and oleic acid, which was of the same magnitude as activation by Ca(2+)/calmodulin. Half-maximal stimulation of calcineurin occurred at fatty acid concentrations of approximately 10 microM with either p-nitrophenyl phosphate or RII phosphopeptide as substrates. The methyl ester of arachidonic acid and the saturated fatty acids palmitic acid and arachidic acid did not activate calcineurin. The activation was shown to be independent of the regulatory subunit, calcineurin B. Activation by Ca(2+)/calmodulin and fatty acids was not additive. In binding assays with immobilized calmodulin, arachidonic acid inhibited binding of calcineurin to calmodulin. Therefore fatty acids appear to mimic Ca(2+)/calmodulin action by binding to the calmodulin-binding site.

Calcium influx activates extracellular-regulated kinase/mitogen-activated protein kinase pathway through a calmodulin-sensitive mechanism in PC12 cells

Evidence suggests that membrane depolarization is able to promote neuronal survival through a sustained, although moderate, increase in the intracellular calcium. We have used the PC12 cell line to study the possible intracellular pathways that can be activated by calcium influx. Previously, we observed that membrane depolarization-induced calcium influx was able to activate the extracellular-regulated kinase (ERK)/mitogen-activated protein kinase pathway and most of this activation was calmodulin-dependent. We demonstrated that a part of the ERK activation is due to the phosphorylation of the epidermal growth factor receptor. Here, we show that both the epidermal growth factor receptor phosphorylation and the Shc-Grb2-Ras activation are not calmodulin-modulated. Moreover, dominant negative mutant Ha-ras (Asn-17) prevents the activation on ERKs by membrane depolarization, suggesting that Ras and calmodulin are both necessaries to activate ERKs by membrane depolarization. We failed to observe any significant induction and/or modulation of the A-Raf, B-Raf or c-Raf-1 kinase activities, thus suggesting the existence of a MEK kinase different from the classical Raf kinases that directly or indirectly can be modulated by Ca2+/calmodulin.

Structure and dynamics of calmodulin in solution

To characterize the dynamic behavior of calmodulin in solution, we have carried out molecular dynamics (MD) simulations of the Ca2+-loaded structure. The crystal structure of calmodulin was placed in a solvent sphere of radius 44 A, and 6 Cl- and 22 Na+ ions were included to neutralize the system and to model a 150 mM salt concentration. The total number of atoms was 32,867. During the 3-ns simulation, the structure exhibits large conformational changes on the nanosecond time scale. The central alpha-helix, which has been shown to unwind locally upon binding of calmodulin to target proteins, bends and unwinds near residue Arg74. We interpret this result as a preparative step in the more extensive structural transition observed in the "flexible linker" region 74-82 of the central helix upon complex formation. The major structural change is a reorientation of the two Ca2+-binding domains with respect to each other and a rearrangement of alpha-helices in the N-terminus domain that makes the hydrophobic target peptide binding site more accessible. This structural rearrangement brings the domains to a more favorable position for target binding, poised to achieve the orientation observed in the complex of calmodulin with myosin light-chain kinase. Analysis of solvent structure reveals an inhomogeneity in the mobility of water in the vicinity of the protein, which is attributable to the hydrophobic effect exerted by calmodulin's binding sites for target peptides.

Comparison of the Ca2+-binding properties of human recombinant calretinin-22k and calretinin

Calretinin-22k (CR-22k) is a splice product of calretinin (CR) found specifically in cancer cells, and possesses four EF-hands and a differently processed C-terminal end. The Ca2+-binding properties of recombinant human calretinin CR-22k were investigated by flow dialysis and spectroscopic methods and compared with those of CR. CR possesses four Ca2+-binding sites with positive cooperativity (nH = 1.3) and a [Ca2+]0.5 of 1.5 microM, plus one low affinity site with an intrinsic dissociation constant (K'D) of 0.5 mM. CR-22k contains three Ca2+-binding sites with nH of 1.3 and [Ca2+]0.5 of 1.2 microM, plus a low affinity site with K'D of 1 mM. All the sites seem to be of the Ca2+-specific type. Limited proteolysis and thiol reactivity suggest that that the C terminus of full-length CR, but not of CR-22k, is in close proximity of site I leading to mutual shielding. Circular dichroism (CD) spectra predict that the content of alpha-helix in CR and CR-22k is similar and that Ca2+ binding leads to very small changes in the CD spectra of both proteins. The optical properties are very similar for CR-22k and CR, even though CR-22k possesses one additional Trp at the C-terminal end, and revealed that the Trp residues are organized into a hydrophobic core in the metal-free proteins and become even better shielded from the aqueous environment upon binding of Ca2+. The fluorescence of the hydrophobic probe 2-p-toluidinylnaphtalene-6-sulfonate is markedly enhanced by the two proteins already in the absence of Ca2+ and is further increased by binding of Ca2+. The trypsinolysis patterns of CR and CR-22k are markedly dependent on the presence or absence of Ca2+. Together, our data suggest the presence of an allosteric conformational unit encompassing sites I-III for CR-22k and I-IV for CR, with a very similar conformation and conformational changes for both proteins. In the allosteric unit of CR, site IV is fully active, whereas in CR-22k this site has a 80-fold decreased affinity, due to the decreased amphiphilic properties of the C-terminal helix of this site. Some very specific Ca2+-dependent conformational changes suggest that both CR and CR-22k belong to the "sensor"-type family of Ca2+-binding proteins.

The role of cysteine residues in S100B dimerization and regulation of target protein activity

Previous studies have demonstrated that the two cysteine residues in the calcium-binding protein S100B are required for its extracellular functions. In the present study, a recombinant S100B protein and mutant S100Bs containing one or no cysteine residue(s) have been used to determine the contribution of cysteine residues to S100B dimerization and interaction with the intracellular target proteins aldolase, phosphoglucomutase, and the microtubule associated tau protein. Mutation of C68 to a valine or C84 to a serine, C68 to valine and C84 to serine, or C68 to valine and C84 to alanine did not significantly alter S100B activation of aldolase. However, mutation of C84 to serine resulted in calcium-independent S100B activation of phosphoglucomutase and a loss of S100B inhibition of tau phosphorylation by Ca2+/calmodulin-dependent protein kinase II. The altered functionality of the C84S mutant with phosphoglucomutase and tau was not due to altered physical properties or dimerization state. All of the mutants exhibited heat stability and calcium dependent conformational changes which were identical to recombinant S100B. In addition, S100B proteins containing two, one or no cysteine residues behaved as dimers in size exclusion chromatography experiments in the presence or absence of calcium as well as in the presence or absence of reducing agent. Dynamic light scattering and analytical ultracentrifugation experiments confirmed that dimerization was not affected by calcium or reducing agent. Altogether these results demonstrate that S100B dimerization is not calcium- or sulfhydryl-dependent. In summary, cysteine residues are not necessary for the noncovalent dimerization of S100B, but are important in certain S100B target protein-interactions.

The structure of a calmodulin mutant with a deletion in the central helix: implications for molecular recognition and protein binding

BACKGROUND

Calmodulin (CaM) is the major calcium-dependent regulator of a large variety of important intracellular processes in eukaryotes. The structure of CaM consists of two globular calcium-binding domains joined by a central 28-residue alpha helix. This linker helix has been hypothesized to act as a flexible tether and is crucial for the binding and activation of numerous target proteins. Although the way in which alterations of the central helix modulate the molecular recognition mechanism is not known exactly, the relative orientation of the globular domains seems to be of great importance. The structural analysis of central helix mutants may contribute to a better understanding of how changes in the conformation of CaM effect its function.

RESULTS

We have determined the crystal structure of a calcium-saturated mutant of chicken CaM (mut-2) that lacks two residues in the central helix, Thr79 and Asp80, at 1.8 A resolution. The mutated shorter central helix is straight, relative to that of the wild-type structure. The loss of a partial turn of the central alpha helix causes the C-terminal domain to rotate 220 degrees around the helix axis, with respect to the N-terminal domain. This rotation places the two domains on the same side of the central helix, in a cis orientation, rather than in the trans orientation found in wild-type structures.

CONCLUSIONS

The deletion of two residues in the central helix of CaM does not distort or cause a bending of the linker alpha helix. The main consequence of the mutation is a change in the relative orientation of the two globular calcium-binding domains, causing the hydrophobic patches in these domains to be closer and much less accessible to interact with the target enzymes. This may explain why this mutant of CaM shows a marked decrease in its ability to activate some enzymes while the mutation has little or no effect on its ability to activate others.

Calcium in suramin-induced rat sensory neuron toxicity in vitro

Suramin is an experimental chemotherapeutic agent and a neurotoxin which causes a dose-dependent peripheral neuropathy in vivo and inhibits dorsal root ganglion (DRG) neurite outgrowth in vitro. The mechanism of suramin-induced cyto- and neurotoxicity remains unclear. Calcium is a key signal transducer in cellular responses to a variety of physiological and pathogenic stimuli. In the present study, we have determined the role of calcium in suramin-induced neurotoxicity in dorsal root ganglion neurons in vitro. Suramin-induced inhibition of neurite outgrowth and induction of neuronal cell death were dose-related phenomena. A low level of extracellular calcium significantly reduced suramin-induced inhibition of neurite outgrowth and delayed neuronal cell death in vitro. Nimodipine (100 microM), an L-type voltage-sensitive calcium channel (VSCC) inhibitor, mimicked low calcium medium and protected neurite outgrowth in regular calcium medium supplemented with 300 microM suramin. TMB-8 (100 microM), an inhibitor of intracellular calcium release, failed to protect neurite outgrowth against the toxin. Calmidazolium (10 microM), a potent calmodulin inhibitor, and calpain inhibitor peptide (CIP, 10 microM) protected neurite outgrowth against suramin. The results support the hypothesis that the calcium signaling system is important in suramin-induced neurotoxicity. Influx of extracellular calcium is more important than release of intracellular calcium in causing cell injury in vitro.

HMN-709, a chlorobenzenesulfonamide derivative, is a new membrane-permeable calmodulin antagonist

Our objective is to describe the basic chemical and biological properties of the new calmodulin antagonist HMN-709 (2-[N-(2-aminoethyl)-N-(4-chlorobenzenesulfonyl)]amino-N-(4-flu orocinnamyl)-N-methylbenzylamine). This newly synthesized compound was found to inhibit the Ca2+/calmodulin-dependent activation of calmodulin kinase I, smooth muscle myosin light chain kinase and Ca2+-phosphodiesterase with IC50 values of 1.57+/-0.21, 2.29+/-0.09 and 0.30+/-0.08 microM (mean+/-S.E.), respectively. This compound showed little or no effect on the Ca2+/calmodulin-independent activation of protein kinase A, protein kinase C and basal phosphodiesterase. In addition, HMN-709 inhibited calmodulin kinase I competitively with respect to calmodulin (Ki=0.88 microM) and non-competitively with respect to ATP. Affinity chromatography, with HMN-709-coupled Sepharose HP, showed that the compound bound to calmodulin in a Ca(2+)-dependent manner and did not bind to calmodulin kinase I. These results suggest that HMN-709 antagonizes calmodulin by binding to Ca2+/calmodulin. HMN-709 inhibited collagen-induced platelet aggregation with an IC50 value of 11.80+/-0.86 microM (mean+/-S.E.) without inhibiting phorbol 12,13-dibutyrate-induced aggregation at doses up to 12 microM. HMN-709 appears to be a new, membrane-permeable calmodulin antagonist that may be used for studying the involvement of calmodulin in cellular processes.

Study of the activity of head ganglion cells of larvae of the Drosophila ts-mutant with altered capacity for learning and increased activational properties of calmodulin

The mitotic activity of cells of the head neural ganglion of Drosophila larvae of two genetic lines, the agts 3-mutant line, which possesses increased calmodulin activational properties and altered capacity for learning, and the wild type CS line, serving as a control, was studied. The value of the mitotic index, as a ratio of the number of dividing cells to their total number, was assessed. The mitotic index was calculated following the exposure of the larvae to a temperature of 37 degrees C for 30 min, and without exposure, at a temperature which was standard for the maintenance of Drosophila ts-mutants, 22 degrees C. A higher mitotic index was observed at 22 degrees C in the agts 3 line as compared with the CS line. Exposure to a temperature of 37 degrees C led to a sharp decrease in mitotic activity in both the lines investigated. The increase in mitotic index at 22 degrees C in the agts 3 line was presumptively related to an increase in the activational properties of calmodulin, which is characteristic for this line. Following preliminary treatment of the neural ganglia by the calmodulin inhibitor, trifluoperazine, at a concentration of 10(-3) M for 30 min, the difference between the mitotic index of the mutant and the control line disappeared due to its approximately three-fold decrease in the agts 3 line; this confirmed the hypothesis advanced and suggested an important role of calmodulin in the regulation of the mitotic activity of cells of the general ganglion of Drosophila larvae.

The calcium-dependent association and functional coupling of calmodulin with myocardial phospholipase A2. Implications for cardiac cycle-dependent alterations in phospholipolysis

Herein we demonstrate the calcium-dependent regulation of myocardial phospholipase A2 activity, which is mediated by a cytosolic protein constituent that can be chromatographically resolved from, and subsequently reconstituted with, purified myocardial phospholipase A2. Purification of this protein by sequential column chromatographies revealed an 18-kDa doublet, which was identified as calmodulin by Western blotting, calcium-dependent precipitation with W-7 agarose beads, and reconstitution of calcium-mediated phospholipase A2 inhibition with authentic homogeneous calmodulin. Calcium-induced calmodulin-mediated inhibition of myocardial phospholipase A2 was titrated by physiologic increments of calcium ion (Kd approximately 200 nM). Moreover, ternary complex affinity chromatography with calmodulin-Sepharose demonstrated that inhibition of myocardial phospholipase A2 activity by calmodulin resulted from the direct interaction of calmodulin with the myocardial phospholipase A2 catalytic complex. Exposure of cultured A-10 muscle cells to three structurally disparate calmodulin antagonists (W-7, trifluoperazine, and calmidazolium) resulted in the robust release of arachidonic acid, which was entirely ablated by pretreatment of cells with (E)-6-(bromomethylene)-3-(1-naphthalenyl)-2-H-tetrahydropyran-2-one. Collectively, this study identifies a novel mechanism whereby latent phospholipase A2 activity can be released from tonic inhibition by alterations in the interactions between the phospholipase A2 catalytic complex, calcium ion, and the intracellular calcium transducer, calmodulin.

Effect of calmodulin antagonists on calmodulin-induced biphasic modulation of Ca(2+)-induced Ca2+ release

1. Calmodulin (CaM) has a biphasic effect on Ca(2+)-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR): potentiation and inhibition at low (pCa > 6.0) and high (pCa 5) Ca2+ concentrations, respectively. To characterize the mode of action of CaM, we studied the effect of CaM antagonists on the CICR in skinned muscle fibres of the rabbit. Ca2+ release was measured by microfluorometry with Fura-2. 2. A CaM antagonist, trifluoperazine (TFP), potentiated the CICR in a dose-dependent manner (10-300 microM) at pCa 6, where a simple reversal of the CaM effect would be inhibition of the CICR. Furthermore, 100 microM TFP sensitized the CICR to Ca2+. A similar effect was produced by other CaM antagonists that were tested: chlorpromazine, W-7, mastoparan, and peptide fragment of CaM-binding residues of CaM-dependent protein kinase II. 3. The biphasic effect of CaM on the CICR was observed even in the presence of high concentrations of CaM antagonists or CaM-bindings peptides. 4. From these results we suggest that CaM has a unique mode of action on the CICR which is quite different from the effect of CaM on known enzymes.

ML-7 and W-7 facilitate thromboxane A2-mediated Ca2+ mobilization in rabbit platelets

The effects of 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-7), a myosin light chain kinase inhibitor, and (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), a calmodulin antagonist, on thromboxane A2 receptor-mediated signal transduction were examined in rabbit washed platelets. ML-7 and W-7 at 10-30 microM slightly potentiated the aggregation induced by a thromboxane A2 receptor agonist, 9,11-dideoxy-9 alpha,11 alpha- epoxymethanoprostaglandin F2 alpha (U46619), in spite of their known inhibitory actions. ML-7 and W-7 concentration-dependently enhanced U46619-induced phosphoinositide hydrolysis and the increase in internal free Ca2+ concentration in the presence or absence of external Ca2+. While ML-7 and W-7 inhibited basal GTPase activity, they augmented U46619-induced activation of GTPase in a concentration-dependent manner. The present results suggest that ML-7 and W-7 enhance thromboxane A2 receptor-mediated signal transduction at the receptor/G protein coupling, leading to the enhancement of phosphoinositide hydrolysis and Ca2+ mobilization, independently of the inhibition of myosin light chain kinase or calmodulin.

Partial characterization of binding sites of VA-045, a novel apovincaminic acid derivative, in rat brain membranes

1. We characterized the binding sites of VA-045 [(+)-eburunamenine-14- carboxylic acid (2-nitroxyethyl)ester] in the rat brain. 2. VA-045 showed no affinity for various types of well-known neurotransmitter-related receptors or channels. However, radiolabeled VA-045 ([3H]VA-045) bound to rat brain membranes in a saturable and reversible manner. The Kd and Bmax values of [3H]VA-045 binding were 58.2 nM and 2685 fmol/mg of protein, respectively. 3. The largest specific binding of [3H]VA-045 was observed in the cerebellum, among seven brain regions, and in subcellular synaptosomes. 4. Specific binding of [3H]VA-045 was inhibited by VA-045 (Ki = 0.06 microM), a levorotatory enantiomer of VA-045 (VA-213) and its structural analog, vinpocentine. Moreover, compounds with calmodulin antagonistic activity inhibited the [3H]VA-045 binding. 5. These results suggest that VA-045 binds to specific sites, which may resemble calmodulin, on synaptic membranes in the brain.

Effects of myosin light chain kinase and peptides on Ca2+ exchange with the N- and C-terminal Ca2+ binding sites of calmodulin

Myosin light chain kinase and peptides from the calmodulin (CaM) binding domains of myosin light chain kinase (RS-20, M-13), CaM kinase II, and the myristoylated alanine-rich protein kinase C substrate protein slowed Ca2+ dissociation from CaM's N-terminal sites from 405 +/- 75/s to 1.8-2.9/s and from CaM's C-terminal sites from 2.4 +/- 0.2/s to 0.1-0.4/s at 10 degrees C. Since Ca2+ dissociates 5-29 times faster from the N-terminal in these CaM.peptide complexes and both lobes are required for activation, Ca2+ dissociation from the N-terminal would control target protein inactivation. Ca2+ binds 70 times faster to the N-terminal (1.6 x 10(8) M-1 s-1) than the C-terminal sites (2.3 x 10(6) M-1 s-1). In a 0.6-ms half-width Ca2+ transient, Ca2+ occupied > 70% of the N-terminal but only 20% of the C-terminal sites. RS-20 produced a 9-fold and CaM kinase II a 6.3-fold increase in C-terminal Ca2+ affinity, suggesting that some target proteins may be bound to the C-terminal at resting [Ca2+]. When this is the case, Ca2+ exchange with the faster N-terminal sites may regulate CaM's activation and inactivation of these target proteins during a Ca2+ transient.

Complex assembly of calgranulins A and B, two S100-like calcium-binding proteins from pig granulocytes

Calgranulin A (CAGA) and calgranulin B (CAGB) are two S100-like calcium-binding proteins that in human, bovine and mouse granulocytes are associated into a heterocomplex. We have previously identified in pig granulocytes the porcine homologue of CAGA and a novel S100-like protein which was named calgranulin C (CAGC). As pig CAGA is not associated with CAGC, we herein investigate its possible association with other proteins. CAGA was purified from pig granulocytes by gel filtration followed by Mono Q chromatography. The purified fractions were analysed by SDS-polyacrylamide gel electrophoresis, isoelectric focusing, mass spectrometry, chemical cross-linking and hydrophobic interaction chromatography. The CAGA-associated protein was further characterized by amino acid sequencing. Two CAGA-containing fractions were isolated. One of them was identified as a CAGA homodimer. The other fraction consists of a heterocomplex containing CAGA and a pI 7.0 calcium-binding protein; this protein has a molecular mass of 15,877.9 +/- 3.8 Da (mean +/- SD) whereas it migrates on 10 and 16% polyacrylamide gels as a 24- and 20-kDa protein, respectively. The pI 7.0 protein was identified by internal amino acid sequencing as the porcine homologue of CAGB. The stoichiometry of the heterocomplex was estimated to be 1:1. Both the CAGA homodimer and CAGA/CAGB were found to be non-covalently associated. Unlike the homodimer, CAGA/CAGB was bound to a Phenyl Superose column in a calcium-dependent manner. Our results suggest that pig granulocytes contain, in addition to CAGC, a CAGA homodimer and a CAGA/CAGB heterodimer. It is proposed that CAGB/CAGB and the CAGA homodimer may play different roles in vivo.

Release of arachidonic acid via Ca2+ increase stimulated by pyrophosphonucleotides and bradykinin in mammary tumour cells

The relationship between the increase of intracellular Ca2+ and the release of arachidonic acid by bradykinin and pyrophosphonucleotides was studied in cultured mammary tumour cells, MMT060562. Bradykinin, ATP, UTP and UDP induced an increase of intracellular Ca2+ and the release of arachidonic acid from phospholipids into the extracellular fluid. Release of arachidonic acid was also induced by the application of the Ca2+ ionophore, A23187. Liberation of arachidonic acid by bradykinin and ATP was reduced by mepacrine, a blocker of phospholipase A2 and W-7, a calmodulin antagonist. It is suggested that the increase in cytosolic Ca(2+)-induced release of arachidonic acid occurs through activation of calmodulin-dependent phospholipase A2.

Anti-calmodulin potency of indol alkaloids in in vitro systems

We have demonstrated that bis-indol Vinca alkaloids of anti-mitotic activities (vinblastine, vincristine, and navelbine) bind to calmodulin in a Ca(2+)-dependent manner. We designed direct binding tests (fluorescence energy transfer and circular dichroism measurements) to quantify the interactions of bis-indol derivatives with calmodulin. The dissociation constants of calmodulin-navelbine and calmodulin-vinblastine complexes with 1:1 stoichiometry are 0.5 microM and 3 microM, respectively. These values indicate that the binding affinities of these Vinca alkaloids to calmodulin and tubulin are comparable. Immunological, enzyme kinetic and fluorescence anisotropy measurements showed that bis-indol alkaloids inhibit the interactions of calmodulin with target proteins. The results of indirect enzyme-linked immunosorbent assay showed that bis-indol alkaloids effectively antagonize with anti-calmodulin antibody for calmodulin binding (IC50 = 90 microM, 400 microM, and 430 microM for navelbine, vincristine and vinblastine, respectively). According to the fluorescence anisotropy and enzyme kinetic measurements, vinblastine, vincristine and vinblastine, similarly to trifluoperazine, the classic calmodulin antagonist, compete with target enzyme [phosphofructokinase (ATP: D-fructose 6-phosphate 1-phosphotransferase, EC 2.7.1.11)] for an inhibitory effect either on immunocomplex formation or on calmodulin-enzyme interaction. Navelbine appeared in our tests as the most potent drug in inhibiting the association of calmodulin to target proteins in comparison to other bis-indol derivatives. Since navelbine and vinblastine possess identical vindoline moiety, although they differ in the catharantine part, the difference in anti-calmodulin potencies is suggested to reside predominantly on this portion of the molecules. These findings might establish the pharmacological importance of these activities in the specificity and toxicity of the drugs.