Calmodulin plays a pivotal role in cellular regulation

Histofluorescence study and biochemical assay of catecholamines (dopamine and noradrenaline) during the course of arm-tip regeneration in the starfish, Asterina gibbosa (Echinodermata, Asteroidea)

In the starfish, Asterina gibbosa, the histofluorescence method gives evidence of an aminergic activity in the course of arm-tip regeneration. This activity can be detected as variations in the localization and intensity of a green fluorescence in the nerve structures of the arm, the radial nerve, and the circumoral nerve ring connected to it. Biochemical assays reveal that dopamine levels increase on the 2nd and 4th days of regeneration, when the blastema is forming and when differentiation commences. The level of noradrenaline also increases on the 2nd day after amputation of the arm.

Effect of calcium on superoxide production by phagocytic vesicles from rabbit alveolar macrophages

Phagocytic vesicles from rabbit lung macrophages produced superoxide in the presence of NADH or NADPH. At 37 degrees C, these vesicles generated 51+/-7.8 nmol O(2) (-)/min per mg protein in the presence of 0.5 mM NADPH. The apparent K(m) for NADPH and NADH (66 and 266 muM, respectively), the pH optimum for the reaction (6.9), and the cyanide insensitivity were similar to properties of plasma membrane-rich fractions of stimulated polymorphonuclear leukocytes studied by others. The activity of the phagocytic vesicles was trypsin sensitive. The specific superoxide-generating activity of macrophage phagocytic vesicles isolated from cells incubated up to 90 min with phagocytic particles remained constant. Calcium in micromolar concentrations inhibited the NADPH-dependent O(2) (-)-generating activity of phagocytic vesicles. In a physiological ionic medium (100 mM KCl, 2.5 mM MgCl(2), 30 mM imidazole-HCl, pH 6.9), a maximal inhibition of O(2) (-) generation by phagocytic vesicles of 80% was observed at 40 muM free Ca(2+). The half maximum inhibitory effect was at 0.7 muM Ca(2+). Variations of the calcium concentration resulted in rapid and reversible alterations in O(2) (-)-forming activity. Preincubation of phagocytic vesicles in the presence of EGTA rendered their O(2) (-) generation rate in the presence of NADPH insensitive to alterations in the free calcium concentration. This desensitization by low EGTA concentrations (</=100 muM) was reversible by the addition of excess calcium, but desensitization by high EGTA concentrations (>1 mM) was not reversible by the addition of calcium either in the presence or absence of purified rabbit lung macrophage or bovine brain calmodulins. Furthermore, trifluoperazine, a drug that inhibits calmodulin-stimulated reactions, did not alter the activity or the calcium sensitivity of the superoxide-generating system of sensitive phagocytic vesicles. Peripheral plasma membrane vesicles (podosomes) prepared by gentle sonication of macrophages possessed on O(2) (-)-generating system with similar properties to those of phagocytic vesicles. We conclude that the activated O(2) (-)-generating system of rabbit lung macrophages has its initial localization in the plasmalemma and undergoes subsequent internalization into phagocytic vesicles, where it can function for prolonged periods of time. Calcium at concentrations likely to exist in macrophage cytoplasm exerts a regulatory effect on the activated system.

Purification of calmodulin from Chlamydomonas: calmodulin occurs in cell bodies and flagella

Calmodulin has been purified from cell bodies of the green alga Chlamydomonas by Ca++-dependent affinity chromatography on fluphenazine-Sepharose 4B. Calmodulin from this primitive organism closely resembles that from bovine brain in a number of properties, including (a) binding to fluphenazine in a Ca++-dependent, reversible manner, (b) functioning as a heat-stable, Ca++-dependent activator of cyclic nucleotide phosphodiesterase, and (c) electrophoretic mobility in SDS-polyacrylamide gels in both the presence and absence of Ca++, which causes a shift in the relative mobility of calmodulin. Calmodulin has also been identified by the criteria of phosphodiesterase activation and electrophoretic mobility in both the detergent soluble "membrane plus matrix" and the axoneme fractions of Chlamydomonas flagella. Calmodulin is not associated with the partially purified 12S or 18S dynein ATPases of Chlamydomonas. The presence of calmodulin in the flagellum suggests that it is involved in one or more of the Ca++-dependent activities of this organelle.

Effect of denervation on the endogenous phosphorylating activity in the cytosol of rat skeletal muscle

An endogenous phosphorylating activity is demonstrated in the cytosol from soleus muscle of the rat which is markedly stimulated after severing the motor nerve fibers to this muscle. The [gamma-32P]AT[ phosphotransferase reaction is heat-labile, dependent upon Mg2+ but not Ca2+ or cyclic GMP, inhibited by a cyclic AMP dependent protein kinase inhibitor, and directly related to the amount of cytosolic protein which provides the endogenous source of both the protein kinase enzyme, ATP, cyclic AMP and phosphorylatable protein substrate. The time-course of the delayed transitory stimulation of the cytosolic phosphorylating activity of the denervated soleus may involve neurotropic factors.

Calcium dependent regulation of brain and cardiac muscle adenylate cyclase

The very close interdependence of Ca2+ and hormones in the overall metabolism of cyclic nucleotides has recently been emphasized by Cheung. Clearly the results presented here show that [Ca2+] in the physiological range (less than 10(-7) M to greater than 10(-6) M) has profound effects on the activity of adenylate cyclase from both brain and cardiac muscle. Whereas both brain and cardiac cyclase exhibit a Ca2+ dependent inhibition (perhaps mediated by calmodulin), only the brain cyclase is activated by Ca2+ via calmodulin. With both cyclases there is an inverse relationship between the inhibition of cyclase and the activation of calmodulin dependent (cAMP and cGMP) phosphodiesterase as a function of Ca2+ concentration. Because the IC50's for Ca2+ are the same in both heart and brain, the possibility exists that the Ca2+ inhibitory site of both cyclases is similar and perhaps identical. Considering the ability of Ca2+ to both stimulate and inhibit cyclase, one could imagine that in different species, tissues, or regions of the same tissue, there could exist multiple populations of cyclase, that is a cyclase which would only show Ca2+ dependent inhibition, Ca2+ dependent stimulation, or the biphasic response to Ca2+ (FIGURE 7). The fact that Ca2+ still regulates adenylate cyclase after various stimuli (histamine, NaF, etc.) suggests that Ca2+ may function to regulate the cyclase over shorter time periods (regardless of its state of stimulation) and that other affectors of cyclase (e.g., hormones) would serve to regulate the cyclase over longer time periods.

Calcium-dependent protein kinase: widespread occurrence in various tissues and phyla of the animal kingdom and comparison of effects of phospholipid, calmodulin, and trifluoperazine

A widespread occurrence of Ca2+-dependent protein kinase was shown in various tissues and phyla of the animal kingdom. Phosphatidylserine appeared to be more effective than calmodulin in supporting the Ca2+-dependent phosphotransferase activity. The phospholipid-sensitive Ca2+-dependent protein kinase activity, distributed in both the cytosolic and particulate fractions, was not inhibited by trifluoperazine, a specific inhibitor of calmodulin-sensitive, Ca2+-dependent reactions or processes. The enzyme activity levels, compared to those of cyclic AMP-dependent and cyclic GMP-dependent protein kinases, were exceedingly high in certain tissues (such as brain and spleen) and exhibited a much greater disparity among tissues. The Ka for Ca2+ was about 100 microM in the presence of phosphatidylserine; the value was as low as 2 microM in the presence of phosphatidylserine and diolein. It is suggested that phospholipid-sensitive Ca2+-dependent protein kinase may mediate certain actions of Ca2+ in tissues, acting independently or in a complementary manner with other protein phosphorylation systems stimulated by calmodulin-Ca2+, cyclic AMP, or cyclic GMP.

Activation of skeletal muscle myosin light chain kinase by calcium(2+) and calmodulin

Many biological processes are now known to be regulated by Ca2+ via calmodulin (CM). Although a general mechanistic model by which Ca2+ and calmodulin modulate many of these activities has been proposed, an accurate quantitative model is not available. A detailed analysis of skeletal muscle myosin light chain kinase activation was undertaken in order to determine the stoichiometries and equilibrium constants of Ca2+, calmodulin, and enzyme catalytic subunit in the activation process. The analysis indicates that activation is a sequential, fully reversible process requiring both Ca2+ and calmodulin. The first step of the activation process appears to require binding of Ca2+ to all four divalent metal binding sites on calmodulin for form the complex, Ca42+-calmodulin. This complex then interacts with the inactive catalytic subunit of the enzyme to form the active holoenzyme complex, Ca42+-calmodulin-enzyme. Formation of the holoenzyme follows simply hyperbolic kinetics, indicating 1:1 stoichiometry of Ca42+-calmodulin to catalytic subunit. The rate equation derived from the mechanistic model was used to determine the values of KCa2+ and KCM, the intrinsic activation constants for each step of the activation process. KCa2+ and KCM were found to have values of 10 microM and 0.86 nM, respectively, at 10 mM Mg2+. The rate equation using these equilibrium constants accurately predicts the extent of enzyme activation over a wide range of Ca2+ and calmodulin concentrations. The kinetic model and analytical techniques employed herein may be generally applicable to other enzymes with similar regulatory schemes.

Micromolar Ca2+ stimulates fusion of lipid vesicles with planar bilayers containing a calcium-binding protein

Fusion of phospholipid vesicles with a planar phospholipid bilayer membrane that contains a calcium-binding protein appears to mimic the essential aspects of cytoplasmic-vesicle fusion with plasma membranes (exocytosis) in that (i) there is a low basal rate of fusion in the absence of Ca2+, (ii) this basal rate is enormously increased by micromolar (approximately 10 microM) amounts of Ca2+, and (iii) this rate is not increased by millimolar Mg2+. Essential to this process is an osmotic gradient across the planar membrane, with the side containing the vesicles hyperosmotic to the opposite side. Similar osmotic gradients or their equivalent may be crucial for biological fusion events.

Calcium control of the intestinal microvillus cytoskeleton: its implications for the regulation of microfilament organizations

The microvillus core-filament bundle from intestinal epithelial cells is a highly ordered structure containing actin and four major associated proteins. Two of these, villin and calmodulin, bind calcium ions (Kd approximately 10(-6) M) in the physiologically important range. Because ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid is present throughout the purification and the isolated cores contain levels of calcium substoichiometric to calmodulin, the protein is bound in the structure without calcium saturation. 10-[3-(4-Methyl-1-piperazinyl)propyl]-2-trifluoromethylphenothiazine, a calmodulin-specific drug, removes the protein from the cores without visibly affecting their ultrastructure. Calmodulin-depleted cores rebind exogenously supplied brain calmodulin. Although the core filaments are stable when the calcium level is less than 10(-7) M, they dissassemble when it is greater than 10(-6) M. This appears to be due to the calcium-sensitive allosteric transition of villin from an F-actin bundling protein to an F-actin severing protein. The actions of the two calcium-binding proteins, villin and calmodulin, are discussed in terms of the calcium sensitivity of the filament bundle. We suggest that villin may act as a calcium-sensitive factor regulating microfilament assembly and disassembly and that calmodulin serves as a buffer modulating the free calcium concentration. This hypothesis may explain some aspects of the physiological process of calcium uptake in the intestine and of the effects of calcium fluxes on the submembranous organization of microfilaments in other cells and tissues.

A semi-automated method for the determination of multiple membrane ATPase activities

A simple semi-automated method for the determination of erythrocyte plasma membrane (Mg2+)-ATPase, (Na+ + K+ + Mg2+)-ATPase, and the (Ca2+ + Mg2+)-ATPase is described in detail. A nonautomated ATPase enzyme assay for multiple membrane ATPases and automated methods for determination of inorganic phosphate as well as membrane-bound protein are described. Together, the methods represent a system that has the flexibility and sensitivity for basic research applications and the capacity to process relatively large numbers of samples as is common in clinical laboratory screening.

Phosphorylase kinase from rabbit skeletal muscle: identification of the calmodulin-binding subunits

Phosphorylase kinase has the structure (alpha beta gamma delta)4 where the delta-subunit is identical to the calcium-binding protein termed calmodulin [Shenolikar et al. (1979) Eur. J. Biochem. 100, 329--337]. The delta-subunit was tightly bound to phosphorylase kinase in the absence of calcium ions, and its rate of exchange with [14C]calmodulin was only 15% per week. The delta-subunit remained associated with phophorylase kinase in the presence of 8 M urea provided that calcium ions were present and this property enabled electrophoretic techniques to be used which demonstrated that the delta-subunit was associated with the gamma-subunit. This finding was confirmed by cross-linking experiments with dimethylsuberimidate which resulted in the formation of a gamma delta complex. Phosphorylase kinase was shown to bind one additional molecule of calmodulin per alpha beta gamma delta unit, termed the delta'-subunit. Glycerol gradient centrifugation in the presence of [14C]calmodulin indicated that the interaction of the delta'-subunit with phosphorylase kinase only occurred in the presence of calcium ions, and that the Kd value was near 0.01 microM. This was similar to the concentration of delta'-subunit which produced half-maximal activation. The delta'-subunit did not remain associated with phosphorylase kinase in the presence of 8 M urea, either in the presence or absence of calcium ions. The very slow exchange between the delta-subunit and [14C]calmodulin, and the calcium-dependent binding of the delta'-subunit allowed cross-linking experiments to be used which demonstrated that the delta'-subunit was bound to both the alpha and beta subunits. This result was supported by the finding that selective proteolysis of either the alpha-subunit, or the alpha and beta subunits, decreased or abolished the ability of phosphorylase kinase to bind to calmodulin-Sepharose. The roles of the different subunits in the regulation of phosphorylase kinase activity are discussed.

Calcium-regulated phosphorylation in synaptosomal cytosol: dependence on calmodulin

Calcium stimulated the phosphorylation of several specific synaptosomal cytosolic proteins. The effects of calcium were both concentration and time dependent and were most apparent for proteins with molecular weights of 50,000, 55,000, and 60,000. Exogenous calcium (1.0-100 microM) enhanced the net incorporation of phosphate into protein by as much as 23-fold. In the absence of added calcium, the calcium chelator [ethylenebis(oxyethylenenitriolo)]tetraacetic acid did not lower the phosphorylation of any protein below control levels. The antipsychotic, fluphenazine (1.0-100 microM), caused a concentration-dependent decrease in calcium-stimulated protein phosphorylation. When the heat-stable calcium-binding protein, calmodulin, was removed from synaptosomal cytosol by affinity chromatography on fluphenazine-Sepharose, calcium-stimulated protein phosphorylation was abolished. Responsiveness to calcium could be restored by the addition of calmodulin to the phosphorylation assay. These results indicate that calcium-dependent protein kinases are of major importance in regulating the phosphorylation of specific cytosolic proteins in neuronal tissue. Furthermore, it would appear that one of the three substrates under investigation is specific to synaptosomal cytosol whereas the other two are present in both the cytosol and membrane fractions.

Overview article: biostructure of blood platelets

Advances in biochemistry, physiology, immunology, and cytochemistry, combined with a variety of new approaches for the evaluation of fine structure, have yielded new insights into the structural physiology and pathology of blood platelets. Subpopulations of platelet granules have been clearly defined; they include the catalase containing organelles referred to as peroxisomes; lysosomes enclosing hydrolytic enzymes; and the alpha-granules in which platelet factor 4, mitogenic factor, beta thromboglobulin, thrombin sensitive protein, fibrinogen, and coagulation factor V are localized. Features of platelet membrane systems have been particularly well-delineated, and recent evidence suggests that membrane complexes serve as the sarcoplasmic reticulum of platelets and the site of prostaglandin synthesis. Improved understanding of platelet biostructure resulting from these observations has made it possible to develop specific relationships between defects in structure and pathological behavior of the cells in vitro and in vivo.

Calcium-dependent hormonal regulation of amino acid transport and cyclic AMP accumulation in rat hepatocyte monolayer cultures

The effect of glucagon, epinephrine, norepinephrine, dexamethasone, insulin, and dexamethasone plus glucagon on the transport of 2-aminoisobutyric acid (AIB) and that of glucagon on the production of cyclic AMP were examined in rat hepatocyte monolayer cultures under three different culture conditions involving calcium. The hepatocytes were studied in calcium-contaning medium after treatment with or without 0.033% dimethyl sulfoxide, the solvent for the calcium ionophore A23187 (calcium controls); calcium-free medium after treatment with A23187 (calcium-depleted); and calcium-containing medium after treatment with ionophore (calcium-restored). The basal and hormonally regulated rates of AIB transport for hepatocytes in calcium control and calcium-depleted cultures were comparable. The restoration of calcium in calcium-restored cultures increased the basal and the hormonally stimulated transport of AIB when compared to the other conditions. Calcium markedly enhanced the stimulation of AIB transport in cultures treated with glucagon, catecholamines, and dexamethasone plus glucagon. The level of cyclic AMP production in response to glucagon in calcium control and calcium-depleted cultures was the same and it was conspicuously higher than the level in calcium-restored cultures. Varying the concentration of calcium in the medium used to maintain the hepatocytes in calcium control cultures did not affect the stimulation of AIB transport or cyclic AMP production by glucagon. However, in calcium-restored cultures, increasing the calcium concentration of the medium resulted in increased stimulation of AIB transport and decreased production of cyclic AMP by glucagon. In the calcium-restored cultures, calcium in the absence of glucagon enhanced AIB transport but had no effect on cyclic AMP production. Cultures maintained for 6 hr in calcium-free medium after the depletion of calcium showed a 6- to 7-fold increase in the production of cyclic AMP in response to glucagon, but no stimulation of AIB transport. We suggest that mobilization of cellular calcium by glucagon either directly or through cyclic AMP mediates its stimulation of amino acid transport.

Vasopressin rapidly stimulates Na entry and Na-K pump activity in quiescent cultures of mouse 3T3 cells

Addition of (Arg) vasopressin to quiescent cultures of Swiss 3T3 cells rapidly stimulates an ouabain-sensitive 86Rb uptake. In contrast the hormone has no significant effect on the rate of efflux of this cation from preloaded cells. The stimulation of 86Rb uptake is cycloheximide-insensitive, occurs within minutes of hormone addition and results from an increase in the Vmax of the uptake system. Vasopressin stimulates ion uptake in a concentration-dependent fashion (1--100 ng/ml); oxytocin also stimulated the Na-K pump but at significantly higher concentrations. The stimulation of the Na-K pump by vasopressin is apparently mediated by an increase in Na entry into the cells, since the hormone 1) strikingly shifts the concentration dependence on Na+ of the Na-K pump, 2) increases 22Na uptake, and 3) increases intracellular Na contents when the efflux of this ion is blocked by ouabain. Since vasopressin is a potent mitogen for Swiss 3T3 cells, the results provide further evidence in support of a possible role of monovalent ion fluxes in signalling the initiation of growth stimulation.