Multiple-sited interaction of caldesmon with Ca(2+)-calmodulin

The binding of Ca(2+)- and Ba(2+)-calmodulin to caldesmon and its functional consequence was investigated with three different calmodulin mutants. Two calmodulin mutants have pairs of cysteine residues substituted and oxidized to a disulphide bond in either the N- or C-terminal lobe (C41/75 and C85/112). The third mutant has phenylalanine-92 replaced by alanine (F92A). Binding measurements in the presence of Ca2+ by separation on native gels and by carbodiimide-induced cross-linking showed a lower affinity for caldesmon in all the mutants. When Ca2+ was replaced by Ba2+ the affinity of calmodulin for caldesmon was further reduced. The ability of Ca(2+)-calmodulin to release caldesmon's inhibition of the actin-tropomyosin-activated myosin ATPase was virtually abolished by mutation of phenylalanine-92 to alanine or by replacing Ba2+ for Ca2+ in native calmodulin. Both cysteine mutants retained their functional ability, but the increased concentration needed for 50% release of caldesmon inhibition reflected their decreased affinity. Ca2+ -calmodulin produced a broadening in the signals of the NMR spectrum of the 10 kDa Ca(2+)-calmodulin-binding C-terminal fragment of caldesmon arising from tryptophans -749 and -779 and caused an enhancement of maximum tryptophan fluorescence of 49% and a 16 nm blue shift of the maximum. Ca(2+)-calmodulin F92A produced a change in wavelength of 4 nm but no change in maximum, whereas Ca(2+)-calmodulin C41/75 binding produced a decrease in fluorescence with no shift of the maximum. We conclude that functional binding of Ca(2+)-calmodulin to caldesmon requires multiple interaction sites on both molecules. However, some structural modification in calmodulin does not abolish the caldesmon-related functionality. This suggests that various EF hand proteins can substitute for the calmodulin molecule.

Effect of cyclosporin A and analogues on cytosolic calcium and vasoconstriction: possible lack of relationship to immunosuppressive activity

1. The full therapeutic potential of the main immunosuppressive drug, cyclosporin A (CsA), is limited because of its side effects, namely nephrotoxicity and hypertension. Several lines of evidence suggest that the origin of both side effects could be CsA-induced vasoconstriction. However, the underlying molecular mechanisms are not well understood. 2. Diameter measurements of rat isolated mesenteric arteries showed an increase in noradrenaline- and [Arg]8vasopressin-induced vasoconstriction when arteries were pretreated with CsA. 3. Measurements in cultured vascular smooth muscle cells (VSMC) of either cytosolic calcium concentration or of 45Ca2+ efflux showed that CsA potentiated the calcium influx to several vasoconstrictor hormones: [Arg]8vasopressin, angiotensin II, endothelin-1 and 5-hydroxytryptamine. On the other hand, 45Ca2+ efflux in response to thapsigargin, which depletes calcium from intracellular pools, was not potentiated by CsA. 45Ca2+ uptake was not altered by CsA or by any of the analogues tested. 4. Time-course studies in cultured VSMC showed that maximal CsA-induced Ca2+ potentiation occurred after ca. 20 h and this effect was reversed over approximately the next 20 h. 5. To investigate the possible role played by the known intracellular targets of CsA, namely cyclophilin and calcineurin, CsA derivatives with variable potencies with respect to their immunosuppressive activity, were tested on the calcium influx to [Arg]8vasopressin. Derivatives devoid of immunosuppressive activity (cyclosporin H, PSC-833) potentiated calcium signalling, while the potent immunosuppressant, FK520, a close derivative of FK506, and MeVal4CsA, an antagonist of the immunosuppressive effect of CsA did not. The latter compound was unable to reverse the calcium potentiating effect of CsA. 6. Our results show that CsA increases the calcium influx to vasoconstrictor hormones in smooth muscle cells, which presumably increases vasoconstriction. Loading of the intracellular calcium pools appears not to be involved. Experiments with derivatives of CsA and FK520 suggest that interactions with cyclophilins and calcineurin are not the mechanism involved. This indicates, for the first time, that the immunosuppressive activity can be dissociated from the calcium potentiating effect of CsA in vascular smooth muscle.

The effects of caldesmon extraction on mechanical properties of skinned smooth muscle fibre preparations

The role of caldesmon in the regulation of smooth muscle contraction was investigated in chemically skinned smooth muscle fibres from the guinea-pig taenia coli. A 19-kDa C-terminal fragment of caldesmon gave a minor (<5%) reduction of force in fully thiophosphorylated fibres, but reduced force by about 50% at intermediate activation levels without affecting the level of light chain phosphorylation. An extraction procedure was developed using incubation in solutions containing high Mg2+ concentrations. Protein analysis revealed a selective decrease in the amount of caldesmon in the fibres. Maximal active force per cross-sectional area was unaffected. The Ca2+ dependence of active force was shifted towards lower Ca2+ concentrations and became less steep. The effects of extraction of caldesmon could in part be reversed by incubation in a solution containing purified caldesmon. The results are consistent with the hypothesis that caldesmon in smooth muscle thin filaments inhibits force generation and plays a role in regulating cooperative attachment of cross-bridges at sub-maximal levels of activation in smooth muscle.

Modulation of gelsolin-induced actin-filament severing by caldesmon and tropomyosin and the effect of these proteins on the actin activation of myosin Mg(2+)-ATPase activity

We have investigated the cumulative effects of three smooth-muscle actin-binding proteins, gelsolin, caldesmon and tropomyosin, on actin activation of myosin Mg(2+)-ATPase activity under low-ionic-strength conditions. A combination of tropomyosin (at a stoicheiometric ratio to actin) and gelsolin (at a molar ratio to actin of up to 1:100) showed essentially additive stimulatory effects that were counteracted by caldesmon. Suppression of the gelsolin-induced activation of the ATPase by caldesmon was higher in the presence of tropomyosin although it was not complete even at stoicheiometric amounts of both proteins to actin. Since activation of actin-activated ATPase activity of myosin by gelsolin is related to its severing action, it is concluded that caldesmon and tropomyosin cannot fully protect actin filaments against the severing activity of gelsolin. Direct analysis of the actin-severing activity of gelsolin by a fluorimetric assay using pyrene-labelled actin confirmed this conclusion. Tropomyosin and caldesmon in saturating amounts relative to actin inhibited the activity of gelsolin by between 21 and 40% and 25 and 48% respectively, depending on the molar ratio of gelsolin to actin. The inhibitory effect was increased with a combination of both (up to 67%) although it was evident that even under these conditions the actin filaments were not fully protected from being severed by gelsolin. These findings were corroborated by electron-microscopic investigation of actin filaments with or without tropomyosin and caldesmon after the addition of gelsolin.

Beta-adrenergic regulation of synaptic NMDA receptors by cAMP-dependent protein kinase

To identify the protein kinases regulating synaptic NMDA receptors, as well as the conditions favoring enhancement of NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) by phosphorylation, we studied the effects of kinase activation and inhibition in hippocampal neurons. Inhibition of cAMP-dependent protein kinase (PKA) prevented recovery of NMDA receptors from calcineurin-mediated dephosphorylation induced by synaptic activity, suggesting that tonically active PKA phosphorylates receptors during quiescent periods. Conversely, elevation of PKA activity by forskolin, cAMP analogs, or the beta-adrenergic receptor agonists norepinephrine and isoproterenol overcame the ability of calcineurin to depress the amplitude of NMDA EPSCs. Thus, stimulation of beta-adrenergic receptors during excitatory synaptic transmission can increase charge transfer and Ca2+ influx through NMDA receptors.

Effects of calponin on isometric force and shortening velocity in permeabilized taenia coli smooth muscle

Calponin, a thin filament-associated protein, inhibits actomyosin adenosinetriphosphatase in solution and has been suggested to modulate smooth muscle contractility. We used permeabilized guinea pig taenia coli smooth muscle to investigate whether calponin can modulate actin-myosin interaction in a more organized contractile system. Fibers were permeabilized with Triton X-100 and glycerol, which permit access of large macromolecules to the contractile apparatus. For contractures elicited by Ca2+ (6.6 microM + 0.1 microM calmodulin), the recombinant alpha-isoform of chicken gizzard calponin (CaP) decreased isometric force (Fo) and unloaded shortening velocity (Vus) in a dose-dependent manner; 1 microM CaP had minimal effects on force (< 10%) but reduced Vus by approximately 50% and 10 microM CaP reduced Fo to 27% of control and Vus to near zero levels. To eliminate any effects of the binding of calmodulin by CaP and consequent inhibition of myosin light chain kinase activity, we also studied fibers activated by thiophosphorylation of the myosin regulatory light chain. Fo was only moderately inhibited, remaining at approximately 75% of control in the presence of CaP (10 microM), whereas Vus was reduced to 32% of control. A similar inhibition was obtained with a mutant (CaPcys175) that retains the ability to bind to actin. CaP phosphorylated by protein kinase C and CaPcys175 mutant labeled with 1,5-IAEDANS, which bind actin poorly, were not effective inhibitors. Our results indicate that 1) CaP more strongly inhibits Vus (approximately cross-bridge cycle rate) than Fo (approximately number of activated cross bridges) and 2) the effects of CaP are related to its binding to actin. Thus the function of CaP in regulation of smooth muscle contractility may be more strongly related to its function as a modulator of velocity, as related to the "latch state," than as an "on-off" switch.

[The effect of a caldesmon fragment with a mol. weight of 38 kDa and calponin on the capacity of actin to form a "strong" form of binding with myosin heads]

Effect of calponin and 38 kD actin-binding proteolytic fragment of caldesmon on actin structure alterations, initiated by decoration of thin filaments by N-ethylmaleimide-modified skeletal myosin subfragment-1 (NEM-S1) and by phosphorylated smooth heavy meromyosin (pHMM), has been studied by polarized fluorimetry. F-actin of myosin-free ghost fiber was labeled with fluorescent probe fluoroscein-5-maleimide. Both the actin-binding regulatory proteins have been demonstrated to inhibit conformational changes of actin typical for the "strong" binding of myosin head to actin. Tropomyosin weakens the inhibitory effect of calponin and markedly increases the effect of the 38 kD fragment of caldesmon. The results indicate similarity of molecular mechanisms of the regulation of muscle contraction by calponin and the actin-binding fragment of caldesmon. It is proposed that the regulation of smooth muscle contraction by calponin and caldesmon is carried out via the inhibition of the formation of the stage AM in ATP hydrolysis cycle.

[The effect of calponin on the rate of actin filament movement]

The effect of calponin on the velocity of actin filaments sliding over skeletal and phosphorylated smooth myosins was studied by in vitro mobility assay. It was found that calponin, being part of an actin filament, inhibits the average velocity of thin filaments movement. The analysis of histograms of the velocities showed that in the presence of calponin, actin filaments are capable to slow down the sliding, stop moving and move with high velocity, characteristic of calponin-free filaments. Tropomyosin weakens the inhibiting effect of calponin. It is supposed that calponin inhibits the sliding of thin filaments in more "all or none" fashion.

Inhibition of smooth muscle actomyosin ATPase by caldesmon is associated with caldesmon-induced conformational changes in tropomyosin bound to actin

The smooth muscle tropomyosin isoforms beta and gamma were isolated in pure form and labeled with N-(1-pyrenyl)iodoacetamide (PIA) on the cysteine residues at either the N- or the C-terminal region (Cys-36 and Cys-190 of beta- and gamma-isoforms, respectively). The effect of caldesmon (CaD) on local conformational changes in different regions of the tropomyosin molecule was determined on the basis of changes in the excimer fluorescence (excited dimer of pyrene) formed in homodimers of tropomyosin isoforms. In the absence of actin, excimer fluorescence from the pyrene at Cys-190 of gamma-tropomyosin homodimer decreased in a simple manner on the addition of CaD, whereas the excimer from the Cys-36 of beta-tropomyosin homodimer exhibited a biphasic change, suggesting that additional weak binding sites exist near Cys-36. In the presence of actin, CaD-induced changes in the excimer fluorescence of pyrene-tropomyosin were observed only with Cys-36, and this change was associated with an inhibition of actin-activated myosin ATPase. A competition study with unlabeled tropomyosin isoforms indicated that the different excimer changes exhibited by beta- and gamma-tropomyosin in the presence of CaD were due to conformational changes in different regions of the tropomyosin molecule and not to differences in their affinities for CaD. Experiments with recombinant CaD mutants derived using the baculovirus expression system showed that the inhibition of tropomyosin potentiation of actomyosin ATPase by CaD requires the regions between residues 728-756 and 718-727 on the CaD molecule, although the latter region was sufficient for direct interaction with tropomyosin.

Calmodulin-binding peptides isolated from alpha-casein peptone

Peptides that inhibit calmodulin-dependent cyclic nucleotide phosphodiesterase were isolated from a pepsin digest of alpha-casein. Analysis of these peptides showed that they corresponded to the alpha S2-casein sequences 164-179 (Leu-Lys-Lys-Ile-Ser-Gln-Arg-Tyr-Gln-Lys-Phe-Ala-Leu-Pro-Gln-Tyr). 183-206 (Val-Tyr-Gln-His-Gln-Lys-Ala-Met-Lys-Pro-Trp-Ile-Gln-Pro-Lys-Thr-Lys-Val -Ile- Pro-Tyr-Val-Arg-Tyr) and 183-207 (C-terminus, Val-Tyr-Gln-His-Gln-Lys-Ala-Met-Lys-Pro-Trp-Ile- Gln-Pro-Lys-Thr-Lys-Val-Ile-Pro-Tyr-Val-Arg-Tyr-Leu). These peptides inhibited calmodulin-induced cyclic nucleotide phosphodiesterase activity over the range 1-50 microM without affecting the basal enzyme activity. These results demonstrated that the affinities of these peptides for calmodulin are comparable to the affinities of certain endogenous neurohormones and proteins that interact with calmodulin.

In vitro motility analysis of smooth muscle caldesmon control of actin-tropomyosin filament movement

We have used the in vitro motility assay to investigate the effect of caldesmon on the movement of actin-tropomyosin filaments over thiophosphorylated smooth muscle myosin and skeletal muscle heavy meromyosin. Using either motor, incorporation of up to 8 nM caldesmon inhibited filament movement by decreasing the proportion of filaments motile from > 85% to < 30%. There was a minimal effect on filament attachment and a modest decrease in motile filament velocity in this concentration range. The reduction in the proportion of filaments motile could be completely reversed by incorporation of an excess of calmodulin at pCa 4.5. The expressed C-terminal fragment, 606C, which retains caldesmon's inhibitory capacity but does not bind to myosin, decreased the proportion of filaments motile but had no effect on velocity. We conclude that the velocity reduction by whole caldesmon is due to actin-myosin cross-linking. A significant decrease in filament attachment was observed when caldesmon was added to an excess over actin (> 10 nM). In the absence of tropomyosin, addition of an excess of caldesmon caused a similar decrease in the filament density, but there was no effect on the proportion of filaments that were motile. Our results demonstrate that caldesmon can switch actin-tropomyosin from motile to non-motile states without controlling velocity of movement or weak binding affinity and show the inhibitory action of caldesmon in the motility assay to be functionally indistinguishable from that reported for troponin.

Influence of caltropin on the caldesmon induced polymerization of G-actin

The effect of caltropin (CaT) on the caldesmon (CaD)-G-actin interaction was monitored by viscosity measurements, bioassays measuring the release of inorganic phosphate (Pi) following G-actin polymerization and fluorescence studies using acrylodan labelled G-actin. CaD induced polymerization of G-actin into filaments in the absence of salt was accompanied by an increase in relative viscosity. This effect of CaD was essentially abolished by CaT in the presence of Ca2+. In bioassays the rate of Pi release was reduced significantly in the presence of Ca2+/CaT. Acrylodan labelled G-actin when excited at 375 nm exhibited an emission maximum at 478 nm. Polymerization of G-actin resulted in shifting the emission maximum to 465 nm. When CaD was added to G-actin containing Ca2+/CaT, the rate of G-actin polymerization was reduced considerably, suggesting that CaT interferes in the CaD-G-actin interaction.

Postsynaptic injection of CA2+/CaM induces synaptic potentiation requiring CaMKII and PKC activity

CA2+-regulated protein kinases play critical roles in long-term potentiation (LTP). To understand the role of Ca2+/calmodulin (CaM) signaling pathways in synaptic transmission better, Ca2+/CaM was injected into hippocampal CA1 neurons. Ca2+/CaM induced significant potentiation of excitatory synaptic responses, which was blocked by coinjection of a CaM-binding peptide and was not induced by injections of Ca2+ or CaM alone. Reciprocal experiments demonstrated that Ca2+/CaM-induced synaptic potentiation and tetanus-induced LTP occluded one another. Pseudosubstrate inhibitors or high-affinity substrates of CaMKII or PKC blocked Ca2/CaM-induced potentiation, indicating the requirement of CaMKII and PKC activities in synaptic potentiation. We suggest that postsynaptic levels of free Ca2+/CaM is a rate limiting factor and that functional cross-talk between Ca2+/CaM and PKC pathways occurs during the induction of LTP.

Multiple signals are required for function of the human granulocyte-macrophage colony-stimulating factor gene promoter in T cells

The human granulocyte-macrophage CSF (GM-CSF) gene is expressed in T cells in response to TCR activation that can be mimicked by treatment of the cells with PMA and Ca2+ ionophore. The gene contains a proximal functional promoter region (-620 to +34), as well as a powerful enhancer located 3 kb upstream, both of which are involved in the response of the gene to TCR activation. The proximal promoter contains a region termed CLEO (-54 to -31) that consists of a purine-rich element abutting an activator protein-1 (AP-1)-like site, as well as an upstream nuclear factor-kappa B (NF-kappa B) site (-85 to -76) and a CK-1 element (-101 to -92). We show in this work that mutations in either the purine-rich region of the CLEO element or the NF-kappa B site result in reduced PMA/Ca2+ activation of a 620-bp human GM-CSF promoter-luciferase reporter construct in Jurkat T cells by 65% and 50%, respectively. The major inducible protein complex that binds to the human CLEO (hCLEO) element is an AP-1-like complex that is inducible by PMA alone, but shows increased binding in response to PMA together with Ca2+ ionophore. Although the binding of this complex is not cyclosporin-sensitive, promoter induction is inhibited by cyclosporin treatment. A second weak inducible complex resembling nuclear factor of activated T cells (NF-AT) was also observed binding to the hCLEO region. By using recombinant proteins, we confirmed that AP-1, NF-ATp, and a higher order NF-ATp/AP-1 complex could all form with the hCLEO element, and we have also defined the sequence requirements for binding of each of these complexes. We found that expression of a constitutively active form of calcineurin could substitute for Ca2+ ionophore and synergize with PMA to activate the GM-CSF promoter, and conversely that mutant-activated Ras could substitute for PMA and cooperate with Ca2+ ionophore. Co-expression of Ras and calcineurin, however, did not activate the GM-CSF promoter, but required the additional expression of NF-kappa B p65. These results imply that at least three signals are required to activate the GM-CSF proximal promoter, and that the signals impinge on distinct transcription factors that bind to the hCLEO and NF-kappa B regions of the promoter.

Parallel inhibition of active force and relaxed fiber stiffness by caldesmon fragments at physiological ionic strength and temperature conditions: additional evidence that weak cross-bridge binding to actin is an essential intermediate for force generation

Previously we showed that stiffness of relaxed fibers and active force generated in single skinned fibers of rabbit psoas muscle are inhibited in parallel by actin-binding fragments of caldesmon, an actin-associated protein of smooth muscle, under conditions in which a large fraction of cross-bridges is weakly attached to actin (ionic strength of 50 mM and temperature of 5 degrees C). These results suggested that weak cross-bridge attachment to actin is essential for force generation. The present study provides evidence that this is also true for physiological ionic strength (170 mM) at temperatures up to 30 degrees C, suggesting that weak cross-bridge binding to actin is generally required for force generation. In addition, we show that the inhibition of active force is not a result of changes in cross-bridge cycling kinetics but apparently results from selective inhibition of weak cross-bridge binding to actin. Together with our previous biochemical, mechanical, and structural studies, these findings support the proposal that weak cross-bridge attachment to actin is an essential intermediate on the path to force generation and are consistent with the concept that isometric force mainly results from an increase in strain of the attached cross-bridge as a result of a structural change associated with the transition from a weakly bound to a strongly bound actomyosin complex. This mechanism is different from the processes responsible for quick tension recovery that were proposed by Huxley and Simmons (Proposed mechanism of force generation in striated muscle. Nature. 233:533-538.) to represent the elementary mechanism of force generation.

Are actin filaments moving under unloaded conditions in the in vitro motility assay?

With sliding actin-filament motility assays, filament velocity should be independent of changes in the level of actomyosin activation under unloaded conditions. Using a simple modification of the motility assay to measure relative changes in isometric force (activation), we determined that isometric force increased 200-fold with thiophosphorylation of the myosin regulatory light chain, and that with thiophosphorylated myosin, isometric force was further increased by the addition of saturating smooth-muscle tropomyosin (100%) or tropomyosin plus calponin (500%), and decreased with the addition of saturating caldesmon (-100%). Under "reducing conditions," filament velocity (2.0 microns/s) was constant for mixtures of dephosphorylated and thiophosphorylated myosin containing > 5% thiophosphorylated myosin, and was unaffected by the addition of saturating concentrations of tropomyosin or caldesmon. In contrast, "standard assay conditions" were found to be nonreducing. With fully thiophosphorylated smooth-muscle myosin, saturating smooth-muscle tropomyosin increased velocity to 150% of control, and caldesmon halted all filament motion; with fully dephosphorylated myosin (< 0.002 mol/mol) filaments only moved when tropomyosin or tropomyosin plus calponin was added. Taken together, these observations suggest that under "standard conditions" a mechanical load is present that is eliminated by "reducing conditions." Filament velocity was insensitive to changes in cross-bridge density, under all conditions, suggesting that noncycling cross-bridges, generated by photochemical oxidation of myosin, is a likely source of mechanical loading.

Tumor necrosis factor-alpha up-regulates Bcl-2 expression and decreases calcium-dependent apoptosis in human B cell lines

Group I and Epstein-Barr virus-negative Burkitt's lymphoma cell lines and the B104 lymphoma cell line which expresses a phenotype of immature B cells undergo apoptosis after cross-linking of their surface Ig receptors or after exposure to a calcium ionophore. We show here that tumor necrosis factor (TNF)-alpha protects these B cell lines against Ca(2+)-dependent apoptosis. Protection was associated with up-regulation of bcl-2 mRNA and protein expression. The increase of Bcl-2 expression induced by TNF-alpha was inhibited by chelerythrine, a specific inhibitor of protein kinase C (PKC), suggesting that Bcl-2 expression was dependent on PKC activation. Furthermore, we show that phorbol esters and cyclosporin A (CsA), which prevent Ca(2+)-dependent apoptosis, up-regulated Bcl-2 expression. The effect of CsA on Bcl-2 expression is controlled by calcineurin since we have shown that FK506 but not rapamycin had the same effect on Bcl-2 expression, whereas okadaic acid, an inhibitor of phosphatases 1, 2A and 2C, was ineffective. These data provide direct evidence that TNF-alpha prevents Ca(2+)-dependent apoptosis by a Bcl-2-dependent mechanism mediated by PKC.

Distinct molecular processes associated with isometric force generation and rapid tension recovery after quick release

It was proposed by Huxley and Simmons (Nature 1971, 233:533-538) that force-generating cross-bridges are attached to actin in several stable positions. In this concept, isometric force is generated by the same mechanism as the quick tension recovery after an abrupt release of length; i.e., when crossbridges proceed from the first postulated stable position to the second and/or subsequent positions, resulting in straining of the elastic elements within the cross-bridges. Therefore, isometric force is generated by cross-bridges in the second or even subsequent stable positions. However, through mechanical measurements of skinned rabbit psoas muscle fibers, we found that during isometric contraction only the first stable state is significantly occupied; i.e., isometric force is generated by cross-bridges in the first of the stable states. Thus, isometric force and the quick tension recovery appear to result from two distinctly different molecular processes. We propose that isometric force results from a structural change in the actomyosin complex associated with the transition from a weakly bound configuration to a strongly bound configuration before the reaction steps in the Huxley-Simmons model, whereas a major component of quick tension recovery originates from transitions among the subsequent strongly bound states. Mechanical, biochemical, and structural evidence for the two distinct processes is summarized and reviewed.

Synaptic desensitization of NMDA receptors by calcineurin

Desensitization is a phenomenon that is common to many ligand-gated ion channels but has been demonstrated only rarely with physiological stimulation. Numerous studies describe desensitization of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor by exogenous agonists, but whether synaptic stimulation causes desensitization has been unknown. Synaptic stimulation of NMDA receptors on rat hippocampal neurons resulted in desensitization that was prevented by intracellular 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), adenosine-5'-O-(3-thiotriphosphate) (ATP-gamma-S), or inhibitors of phosphatase 2B (calcineurin), but not by inhibitors of phosphatases 1 and 2A or of tyrosine phosphatases. Synaptic NMDA receptors may fluctuate between phosphorylated and dephosphorylated forms, depending on the rate of synaptic stimulation and the magnitude of the associated influx of calcium through NMDA receptors.

Effect of caldesmon on the assembly of smooth muscle myosin

Smooth muscle myosin filaments are much less stable than the skeletal muscle counterpart. Smooth myosin requires higher concentration of Mg2+ than skeletal myosin to form thick filaments and addition of ATP disassembles the dephosphorylated smooth muscle myosin filaments into monomers but not phosphorylated ones. We found that the addition of caldesmon to dephosphorylated myosin induced the formation of the filaments under the conditions where myosin by itself is soluble or disassembled. Although the induced filaments were short at 1 mM Mg2+, they became medium sized and seemed like side polar filaments with prominent 14 nm periodicity at higher Mg2+ conditions (8 mM). In the presence of F-actin, myosin filaments induced by caldesmon were associated along actin filaments to form large structures. The association of actin and myosin filaments was observed only in the presence of caldesmon, suggesting that caldesmon cross-linked actin and myosin filaments. This cross-linking was disrupted by the addition of calmodulin. Caldesmon-induced filament formation of dephosphorylated myosin in the presence of Mg(2+)-ATP may explain the existence of myosin filaments in relaxed smooth muscle fibers. A similar effect of telokin on myosin filament assembly was also examined and is discussed.

Caldesmon and low Mr isoform of tropomyosin are localized in neuronal growth cones

Neuronal growth cones move actively, accompanying changes in intracellular Ca2+ concentration. The movement of growth cones may partly depend on the actomyosin system, considering the presence of actin and myosin II. Yet, Ca(2+)-sensitive regulatory proteins for the actomyosin system have not been identified in growth cones. In the present study, caldesmon, an inhibitory protein on actin-myosin interaction, was detected in the growth cone fraction isolated from embryonic rat brain, using immunoblotting with the antibody to chicken gizzard caldesmon. Morphological evidence of caldesmon in growth cones of cultured rat neurons was obtained using the indirect immunofluorescence method. Since inhibition of caldesmon on actin-myosin interaction can be overcome by calmodulin and Ca2+, caldesmon may be involved in the Ca(2+)-dependent regulation in growth cone motility. Tropomyosin is another member of the actomyosin system whose function may be regulated by caldesmon in smooth and nonmuscle cells. A low Mr isoform of tropomyosin was distributed in the growth cone fraction. Using specific antibodies against tropomyosin isoforms, we further clarified morphologically that the low Mr isoform was localized in growth cones, but not the high Mr isoform. High Mr isoforms of tropomyosin were present in nonneuronal cells. Actin filaments in growth cones may be unstable, since low Mr tropomyosin binds to actin filaments with a lower affinity than high Mr isoforms. The instability of actin filaments may be suitable for the rapid movement and shape changes of growth cones.

An inhibitor for calcineurin, FK506, blocks induction of long-term depression in rat visual cortex

Long-term depression (LTD) of synaptic transmission, often used as an essential component in synaptic models for learning, memory and forgetting, can be produced in layer II/III of the visual cortex by a prolonged, low-frequency stimulation (LFS) of layer IV. The activation of Ca2+/calmodulin-dependent protein phosphatase, calcineurin, has been postulated to play a role in the induction of LTD. The recent introduction of a specific inhibitor for calcineurin, FK506, prompted the investigation of the involvement of this phosphatase in the induction of LTD in visual cortex. Thus, we administered FK506 at 1 microM to visual cortical slices of young rats, and found that it did not significantly affect field responses of layer II/III evoked by test stimulation of layer IV at 0.1 Hz, but prevented LTD of the responses from being induced by LFS (1 Hz for 15 min) in all the 10 slices tested. Without FK506, significant LTD was induced by the same parameters of LFS in 8 of the 12 slices. These results suggest the critical involvement of calcineurin in producing LTD in visual cortex.

Effect of unphosphorylated smooth muscle myosin on caldesmon-mediated regulation of actin filament velocity

The effect of smooth muscle myosin at different levels of light chain phosphorylation on caldesmon-mediated movement of actin filaments was investigated using an in vitro motility assay. Myosin at different levels of phosphorylation was obtained by mixing different proportions of fully phosphorylated and unphosphorylated myosin in monomeric form, while keeping the total myosin concentration constant. The average velocity of actin filaments containing tropomyosin was 1.20 +/- 0.046 microns s-1 at 30 degrees C with fully phosphorylated myosin. This velocity was not altered when the percentage of unphosphorylated myosin coated on the nitrocellulose surface was increased to 80%; further increases lowered the velocity. When the actin filaments with caldesmon bound at stoichiometric levels were used, filament velocity was unaffected until 50% of the myosin was unphosphorylated, but further increases in the percentage of unphosphorylated myosin induced a decrease in the velocity, and at 95% unphosphorylated myosin, filament movement had ceased. The decreased filament velocity in the presence of caldesmon was also observed when phosphorylated myosin was mixed with myosin rod instead of unphosphorylated myosin, but was not observed when the 38 kDa caldesmon C-terminal fragment, which lacks the myosin-binding domain, was used instead of intact caldesmon. These data indicate that the decreased filament velocity in the presence of caldesmon reflects the mechanical load produced by the tethering of actin to myosin through the interaction of the caldesmon N-terminal domain and the myosin S-2 region. The tethering effect mediated by caldesmon may play a role in smooth muscle contraction when a large number of myosin heads are dephosphorylated, as in force maintenance.

Suppression of GABAA receptor responses by NMDA application in hippocampal neurones acutely isolated from the adult guinea-pig

1. In acutely isolated hippocampal cells, NMDA and glutamate application suppressed GABAA receptor-mediated responses. We studied the cellular events underlying the interaction between the two classes of receptors by using a whole-cell voltage-clamp approach. 2. Following an NMDA application, an outward current mediated by GABAA receptor activation (GABA response) was suppressed for up to 12 s. The suppression of the GABA response was reduced when Ca2+ in the extracellular solution was replaced by Ba2+ or when intracellular BAPTA (1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) was increased from 1 to 10 mM. 3. Replacing ATP in the intracellular solution by adenosine-5'-O-3-thiotriphosphate reduced the suppressive effect of NMDA application on the GABA response. Okadaic acid, a phosphatase inhibitor, also prevented the NMDA-induced suppression of the GABA response. In addition, when the intracellular perfusing solution contained the calcineurin autoinhibitory fragment (50 microM), suppression of the GABA response by the NMDA current was also reduced. 4. Intracellular perfusion of an activated form of the Ca(2+)-dependent phosphatase, calcineurin, suppressed GABA responses. 5. The results show that NMDA responses elicited in hippocampal neurones transiently suppressed GABA responses. The data suggest that the functional linkage of the NMDA response with the GABA response was established via a Ca(2+)-dependent dephosphorylation process.

Response of guinea pig smooth and striated urethral sphincter to cromakalim, prazosin, nifedipine, nitroprusside, and electrical stimulation

Prazosin (an alpha-1-adrenergic blocker) and cromakalim (potassium channel opener), given alone, induced significant fatigue of the urethral sphincter at a concentration of 10(-4) M; both drugs combined achieved a significant sphincteric fatigue at a concentration of 10(-5) M each. To 10(-4) M hexamethonium (ganglionic smooth muscle blocker) and 10(-4) M decamethonium (nicotinic blocker of striated muscle) the striated urethral sphincter responded like striated muscle with no detectable function of its smooth muscle component. Therefore, the striated component seems to play a dominant role in sphincteric function. With calcium depletion or in the presence of a calcium channel blocker (10(-4) M nifedipine) the urethral sphincter showed a relative enhancement of response to electrical field stimulation when compared with smooth and skeletal muscle, whose responses were both significantly reduced. This phenomenon could not be explained with calcium-dependent, inhibitory, nitric oxide-releasing nerves, as the NO-synthase blocker N-nitro-L-arginine (10(-5) M to 5 x 10(-5) M) failed to induce the enhancement of sphincter contraction during electrostimulation found with calcium depletion. Still, NO-releasing nerves might play a role in sphincteric relaxation because sodium nitroprusside (10(-5) M) induced a significant relaxation of the urethral sphincter precontracted with 80 mM potassium. The potential to weaken sphincteric closure with drugs, exemplified by the results obtained in response to prazosin and cromakalim, would represent a therapeutic advance in the patient with neurogenic bladder dysfunction.