Rat brain adenylate cyclase. Further studies on its stimulation by a Ca2+-binding protein

Functional coupling of cerebral ?-aminobutyric acid (GABA)(B) receptor with adenylate cyclase system: effect of phaclofen

Using synaptic membrane from bovine cerebral cortex, effects of ?-aminobutyric acid (GABA), (?)-baclofen, and phaclofen on the cyclic AMP formation mediated by adenylate cyclase were studied. In addition, the binding affinity of phaclofen, a GABA(B) antagonist, to synaptic membrane was compared with those of GABA and (?)-baclofen. GABA and (?)-baclofen, GABA(B) receptor agonists, induced significant inhibitions on the basal and forskolin-stimulated adenylate cyclase activities. Treatment of synaptic membrane with the islet-activating protein, pertussis toxin, completely eliminated the inhibitory effects of GABA and (?)-baclofen on the forskolin-stimulated adenylate cyclase activity. In solubilized fraction of synaptic membrane, the forskolin-stimulated adenylate cyclase was no longer affected by the additions of GABA and (?)-baclofen. Phaclofen displaced 50% of the bound [(3)H](?)-baclofen from synaptic membrane at the concentration of 10(?3) M, and also completely abolished inhibitory effects of GABA and (?)-baclofen on the forskolin-stimulated adenylate cyclase activity. These results suggest that GABA(B) receptor in synaptic membrane of the bovine cerebral cortex may be functionally coupled with adenylate cyclase system via Ni and/or No proteins. The present results also suggest that phaclofen may have selective affinity to the same binding sites as those of GABA(B) receptor agonists such as (?)-baclofen, and induce a suppressive effect on GABA(B) receptor mediated inhibition of adenylate cyclase.

A quantitative study of the Ca2+/calmodulin sensitivity of adenylyl cyclase in Aplysia, Drosophila, and rat

Studies in Aplysia and Drosophila have suggested that Ca2+/calmodulin-sensitive adenylyl cyclase may act as a site of convergence for the cellular representations of the conditioned stimulus (Ca2+ influx) and unconditioned stimulus (facilitatory transmitter) during elementary associative learning. This hypothesis predicts that the rise in intracellular free Ca2+ concentration produced by spike activity during the conditioned stimulus will cause an increase in the activity of adenylyl cyclase. However, published values for the Ca2+ sensitivity of Ca2+/calmodulin-sensitive adenylyl cyclase in mammals and in Drosophila vary widely. The difficulty in evaluating whether adenylyl cyclase would be activated by physiological elevations in intracellular Ca2+ levels is in part a consequence of the use of Ca2+/EGTA buffers, which are prone to several types of errors. Using a procedure that minimizes these errors, we have quantified the Ca2+ sensitivity of adenylyl cyclase in membranes from Aplysia, Drosophila, and rat brain with purified species-specific calmodulins. In all three species, adenylyl cyclase was activated by an increase in free Ca2+ concentration in the range caused by spike activity. Ca2+ sensitivity was dependent on both calmodulin concentration and Mg2+ concentration. Mg2+ raised the threshold for adenylyl cyclase activation by Ca2+ but also acted synergistically with Ca2+ to activate maximally adenylyl cyclase.

The effects of Ca2+ and calmodulin on adenylyl cyclase activity in plasma membranes derived from neural and non-neural cells

The regulation of adenylyl cyclase activity by varying concentrations of Ca2+ was examined in plasma membrane preparations derived from a number of neural and non-neural cells. Enzyme activity in neural tissue (i.e. cerebellum) neural-derived pheochromocytoma PC12 cells and certain endocrine cells (i.e. pancreatic RINm5f and parathyroid cells) was stimulated by physiologic concentrations of Ca2+ by a calmodulin (CaM)-dependent mechanism. In contrast, adenylyl cyclase activity in non-neural cells (e.g. platelets and GH3 cells) was not stimulated by Ca2+. In these latter sources, enzyme activity was inhibited by increasing concentrations of Ca2+, independent of CaM. In liver membranes, Ca2+ and/or CaM did not alter adenylyl cyclase activity. These results demonstrate that the effects exerted by physiologic concentrations of Ca2+ on adenylyl cyclase activity range from CaM-dependent stimulation of activity to no effect, to CaM-independent inhibition of activity. The actions of Ca2+ on adenylyl cyclase may be major contributors to the various synergistic or antagonistic interactions that are seen between cAMP-generating and Ca(2+)-mobilizing systems.

Synergistic activation of brain adenylate cyclase by calmodulin, and either GTP or catecholamines including dopamine

The effects of calmodulin (CaM), guanosine triphosphate (GTP) and dopaminergic or beta-adrenergic agonists on the activities of adenylate cyclase were studied in EGTA-washed lysed synaptosomal membranes from rat striatum and cerebral cortex. Based on the free calcium ion concentration-dependence of the enzymic activity, it was found that the stimulatory effect of CaM and GTP on adenylate cyclase was synergistic with maximum activation at pCa 6.2 for both striatal and cortical membranes. This was not due to the effect of CaM-dependent phosphodiesterase because exogenous CaM did not affect particulate phosphodiesterase activity. Added CaM not only enhanced the adenylate cyclase activity but acted cooperatively with dopaminergic or beta-adrenergic agonists in the presence of GTP. Most marked was enhancement found for the striatal SKF 38393- (a specific D1 agonist) and the cortical isoproterenol-dependent activities. A synergism was also found for CaM and forskolin. These findings strongly suggest that CaM is involved in the striatal dopaminergic as well as in the cortical beta-adrenergic systems.

An automated assay for adenylate cyclase using reversed-phase high-performance liquid chromatography

An automated high-performance liquid chromatographic method for the assay of 3',5'-cyclic AMP was developed using octylsilica. Total analysis time was 10.1 min, with cAMP eluting at 3 min. As little as 10 pmol of cyclic AMP could be detected by absorption at 260 nm. Peak height and area were linearly related to cyclic AMP concentration over at least two orders of magnitude. The analytical procedure gave good results in the assay of crude microsomal preparations of adenylate cyclase from both bovine brain and sea urchin eggs. The method was used to demonstrate that sea urchin adenylate cyclase is a Ca2+-activated enzyme.

Pyrethroid inhibition of basal and calmodulin stimulated Ca2+ ATPase and adenylate cyclase in rat brain

Effects of two classes of pyrethroids, permethrin and resmethrin (type I), cypermethrin and deltamethrin (type II), on basal (calmodulin-deficient) and calmodulin stimulated activities of Ca2+ ATPase and adenylate cyclase from rat brain were studied in vitro. None of the pyrethroids inhibited synaptosomal basal Ca2+ ATPase, but permethrin and deltamethrin inhibited basal adenylate cyclase in the nuclear fraction of a brain homogenate. Both groups of pyrethroids decreased the calmodulin activated Ca2+ ATPase and adenylate cyclase from brain synaptosomes and nuclear fraction. The results indicate that calmodulin-stimulated Ca2+ ATPase is more sensitive to type II pyrethroids and pyrethroids are more effective on calmodulin stimulated enzymes than basal enzyme activities. Since calmodulin, adenylate cyclase and Ca2+ ATPase are known to participate in various brain processes, it is possible that pyrethroids alter neural transmission, however, additional in vivo work would be needed to confirm this possibility.

Multiple defects in the activity of adenylate cyclase from the Drosophila memory mutant rutabaga

Adenylate cyclase in homogenates of Drosophila melanogaster is heterogeneous with respect to its affinity toward MgATP and its subcellular distribution. Km values for MgATP range, under similar assay conditions, from approximately 10(-5) M to approximately 10(-3) M, depending on the body region and on the subcellular localization of the enzyme. The majority of the enzyme in whole-body preparations is particulate, but various body regions differ in the relative proportion of the soluble enzyme. The memory mutant rutabaga lacks up to 35% of the total particulate activity. Even ligands that stimulate directly the catalytic subunit are incapable of bringing the activity of the mutant's enzyme to normal levels. The defect is differentially pronounced in various body parts and is associated with an altered responsiveness of the enzyme to Mg2+, to Ca2+, and to forskolin. It is suggested that rutabaga is lesioned in a subpopulation, or a functional state, of adenylate cyclase, which may play a role in memory formation.

Identification of calmodulin-sensitive and calmodulin-insensitive adenylate cyclase in rat kidney

Adenylate cyclase from rat kidney membranes solubilized with Lubrol-PX, was resolved into calmodulin-insensitive and calmodulin-sensitive forms using DEAE-Sephacel and calmodulin-Sepharose affinity chromatography. The major fraction, 90% of the activity recovered, did not bind to the calmodulin-Sepharose in the presence of Ca2+, and was insensitive to activation by calmodulin. The calmodulin-sensitive enzyme, approximately 10% of the recovered activity, bound to the affinity column and was eluted with buffer containing 2 mM EGTA. In the presence of free Ca2+, calmodulin increased the specific activity of the calmodulin-sensitive adenylate cyclase from 15.2 to 60.4 pmol/mg protein-1 min-1. Maximum stimulation occurred at 0.035-0.076 mM Ca2+. The apparent Ka for calmodulin was 8 nM. The calmodulin-mediated increase in activity was inhibited by trifluoperazine, but not by its analog trifluoperazine-5-oxide. In contrast, trifluoperazine did not inhibit the calmodulin-insensitive activity. The GTP analog, guanyl-5'-yl imidodiphosphate, did not activate either fraction. Furthermore, activation by calmodulin did not require the presence of a guanyl nucleotide. The present finding of a calmodulin-sensitive form of adenylate cyclase in kidney raises the possibility that a calmodulin-mediated mechanism is involved in the formation of cAMP in this organ.

Adenylate cyclase in the Drosophila memory mutant rutabaga displays an altered Ca2+ sensitivity

Adenylate cyclase in washed, crude membrane fractions prepared from the Drosophila conditioning mutant, rutabaga, displays an altered responsiveness to Ca2+. The results are of interest since the modulation of adenylate cyclase activity by Ca2+ has recently been suggested to play a role in molecular events that underlie memory formation.

Loss of calcium/calmodulin responsiveness in adenylate cyclase of rutabaga, a Drosophila learning mutant

We have isolated and mapped an X-linked recessive mutation in Drosophila that blocks associative learning, and have partially characterized it biochemically. The mutation affects adenylate cyclase activity. Cyclase activity from mutant flies differed from the wild-type enzyme in that it was not stimulated by calcium or calmodulin. Mutant cyclase activity did respond to guanyl nucleotides, fluoride, and monoamines, which suggests that the defect is neither in the hormone receptor nor in either known GTP-binding regulatory protein. The mutation possibly affects the catalytic subunit directly. We postulate that there is at least one other type of adenylate cyclase activity that is unaffected by the mutation and insensitive to calcium/calmodulin.

Calmodulin activates bovine-cardiac myosin light-chain kinase by increasing the affinity for myosin light-chain 2

The basic mechanism by which calmodulin activates bovine-cardiac muscle myosin light-chain kinase was investigated using highly purified preparations of mixed bovine-cardiac myosin light chains or isolated myosin light chain 2. The apparent contamination of these substrate proteins by calmodulin, as detected by activation of calmodulin-sensitive phosphodiesterase, was less than 4 parts/million and was undetectable by antibodies against calmodulin. The apparent KA for calmodulin was 2 nM and 20 nM in the presence of isolated myosin light-chain 2 and mixed myosin light chains, respectively. Purified bovine cardiac troponin C activated myosin light-chain kinase by about 10% at a concentration of 2 microM. Mixed myosin light chains were phosphorylated in the absence and presence of calmodulin and in the presence of calcium with a V of 11.1 and 11.0 mumol phosphate transferred min-1 (mg enzyme)-1, respectively. The apparent Km values for mixed myosin light chains were 8.0 and 0.35 mg/ml in the absence and presence of calmodulin, respectively. Similarly calmodulin lowered the Km value for isolated myosin light-chain 2 over 20-fold and increased the V value only about 1.5-fold. Activity observed in the absence of calmodulin was dependent on the presence of calcium and was suppressed by chelating free calcium either before or during a phosphorylation reaction. The apparent KA for calcium was 1.2 microM and 0.4 microM in the absence and presence of calmodulin. Activity in the absence of calmodulin was inhibited at very high concentrations of the 'specific' calmodulin antagonists W-7, trifluoperazine and R24571 with apparent IC50 values of 0.3 mM, 0.2 mM and 0.02 mM. Antibiotics raised against calmodulin suppressed completely the kinase activity in the presence of calmodulin but had no effect on the activity measured in its absence. These results suggest that calmodulin stimulates the activity of bovine-cardiac myosin light-chain kinase by increasing over 20-fold the affinity for its substrate myosin light-chain 2.

Vanadate and brain adenylate cyclase. Effect of spreading depression

The effect of vanadate on the adenylate cyclase activity of rat cerebral cortex homogenates is described. In the absence of ethyleneglycol-bis-(beta-aminoethyl ether) N,N'-tetraacetic acid (EGTA), 10(-6)M vanadate inhibited enzyme activity by 23%, while 10 (-4) M and 10(-3) M stimulated the enzyme by 14 and 90%, respectively. In the presence of 0.2 mM EGTA, 10 (-6) M to 10(-3) M vanadate had only stimulating effects (18-450%). Additive effects of vanadate and noradrenaline on adenylate cyclase activity suggest different sites of action of these agents. Interaction of vanadate with both fluoride and guanyl-5'-yl imidodiphosphate had an apparently competitive character. Adenylate cyclase maximally stimulated by fluoride (10 mM) was inhibited by vanadate. This inhibitory effect was more pronounced in the absence of EGTA. Adenylate cyclase in the homogenates from the rat cerebral cortex in vivo invaded by spreading depression was slightly increased (up to 38%). This effect was abolished by low (10 (-7) M) vanadate. The results suggest that brain adenylate cyclase is stimulated by vanadate via the guanine nucleotide regulatory protein. The mechanism of vanadate's action, its modulation by calcium ions and the possible physiological role of these effects are discussed.

Crayfish abdominal muscle adenylate cyclase. Studies on the stimulation by a Ca2+-binding protein

A plasma-membrane preparation of crayfish muscle showed an adenylate cyclase activity which is inhibited to about 80% of its original activity by 100 microM-EGTA. Measurements of the enzyme activity in the presence of 100 microM-EGTA and various concentrations of Ca2+ revealed an increase in enzyme activity of about 400%, indicating an adenylate cyclase which is dependent on Ca2+ for activity. Fluphenazine (1 mM), a blocker of the Ca2+-binding protein calmodulin, decreased enzyme activity to zero. The enzyme can be re-activated by the addition of certain concentrations of calmodulin to the assay medium. This suggests that crayfish muscle adenylate cyclase is dependent on Ca2+ and calmodulin for activity.

Ontogeny of calmodulin and calmodulin-dependent adenylate cyclase in rat brain

The development of calmodulin, calmodulin-dependent adenylate cyclase and beta-adrenergic receptors was studied in the rat brain. Membrane-bound calmodulin detected was approximately 40-50% of the total calmodulin throughout the postnatal development of either in the cerebrum or cerebellum. No significant difference was found between the quantitative patterns of the membrane-bound and cytosolic calmodulin during the entire period of postnatal development in either of these tissues. Both the cytosolic and membrane-bound calmodulin were present in low concentrations in the immature brain after birth. Their contents rapidly increased during the second postnatal week. Subsequently, the cytosolic calmodulin content remained constant, but showed a considerable decrease in the particulate fraction after day 14. Basal adenylate cyclase activity in the rat cerebrum slowly increased up to the second postnatal week and decreased after day 14. The responsiveness to calmodulin of this enzyme remained unaltered during postnatal development, whereas fluoride and guanine nucleotide sensitivities increased in the same period. The maximum number of (-)-[3H]dihydroalprenolol binding site sharply increased during day 9-14 in the rat cerebrum, although the dissociation constant Kd of the binding site was not affected by age. The results in the latter study suggest that calmodulin-dependent adenylate cyclase may be already present in the earlier postnatal ages of the rat brain, while the beta-adrenergic receptor and guanine nucleotide regulatory unit, both of which are required for a hormone-sensitive adenylate cyclase, may sharply increase in the second postnatal week.

Activation of adenylate cyclase by forskolin in rat brain and testis

Detergent-dispersed adenylate cyclase from rat cerebrum was detected in two components, one sensitive to Ca2+ and calmodulin and another sensitive to fluoride or guanyl-5'-yl imidodiphosphate (Gpp(NH)p). The enzyme activity of both components was markedly augmented by forskolin assayed in the presence or absence of other enzyme activators (e.g., NaF, Gpp(NH)p, calmodulin). The catalytic subunit fraction in which G/F protein was totally lacking was also activated by forskolin. During 1-35 days of postnatal development, the basal adenylate cyclase activities in either cerebrum and cerebellum particulate preparations progressively increased. While the fluoride sensitivity of the cerebrum and cerebellum enzyme increased during postnatal development, the responsiveness to forskolin remained unaltered. There was no enhancement of soluble adenylate cyclase (from rat testis) by forskolin under the assay conditions in which there was a marked stimulatory action on the particulate enzyme. The results seen with the solubilized enzyme, with either Lubrol PX or cholate, indicate that the effects of forskolin on the cyclase do not require either G/F protein or calmodulin and the results of our study of brain enzymes support this view. Data on soluble testis cyclase (a poor or absent response to forskolin by this enzyme) imply that it lacks a protein (other than the catalytic unit) which could confer greater stimulation. The present results do not rule out an alternative explanation that forskolin stimulates adenylate cyclase by a direct interaction with the catalytic subunit, if the catalytic proteins do differ widely in various species of cells and their response to this diterpene.

The periodic change in adenosine 3',5'-monophosphate concentration in sea-urchin eggs

The level of adenosine 3',5'-monophosphate (cyclic AMP) in the eggs of the sea urchin, Anthocidaris crassispina, was found to change periodically after fertilization. The minimum and maximum levels of cyclic AMP were 1.0 X 10(-7)M and 1.5 X 10(-6)M, respectively. The activity of adenylate cyclase in a 105 000 X g precipitate reached a plateau at 20 min after fertilization and stayed constant for at least 2 h. It was also found that 1.0 mM CaCl2 increased the activity of adenylate cyclase in the same precipitate from unfertilized eggs. In contrast, phosphodiesterase activity changed periodically and correlated with cyclic AMP levels in the eggs. Up to a concentration of 1.5 X 10(-6)M cyclic AMP, phosphodiesterase activity was low, but it became activated when the level of cyclic AMP rose beyond this level. These results indicate that the change in the intracellular level of cyclic AMP is regulated mainly by the change in phosphodiesterase activity.

Adenylate cyclase of Torpedo synaptosomes is inhibited by calcium and not affected by muscarinic ligands

Cholinergic synaptosomes isolated from the electric organ of Torpedo contain membrane-bound adenylate cyclase activity (approximately 6 pmol/mg protein/min), which is dependent on the presence of guanine nucleotides. The activity is strongly dependent on temperature and only slightly affected by NaCl. The Torpedo adenylate cyclase is completely inhibited by low levels of free Ca2+ (KD approximately 0.5 microM). This effect is not altered by either trifluoperazine or addition of exogenous calmodulin. Ca2+ has no effect on the activation step of the adenylate cyclase by guanyl-5'-yl imidodiphosphate (GppNHp), and Mn2+ abolishes the Ca2+-dependent inhibition of cyclic AMP synthesis. These findings suggest that Ca2+ exerts its effect by direct interaction with a site located on the catalytic subunit. Torpedo synaptosomes contain presynaptic inhibitory muscarinic receptors. The binding of muscarinic agonists to the receptors is modulated (to lower affinity) by GTP. However, muscarinic ligands, examined under a variety of assay conditions, have no effect on adenylate cyclase activity. These results suggest that although both the muscarinic receptor and the adenylate cyclase are coupled to G proteins, they either interact with different G proteins or are situated in different regions of the presynaptic membrane.

Calmodulin stimulation of adenylate cyclase in rat brain membranes does not require GTP

Calcium stimulates adenylate cyclase activity in rat cerebral cortical membranes with either ATP or AppNHp as substrate. In contrast, isoproterenol stimulates the cerebral cortical enzyme with ATP as substrate but not with AppNHp as substrate unless exogenous GTP is added. In rat striatal membranes, calcium or dopamine stimulate adenylate cyclase activity with ATP as substrate, but not with AppNHp as substrate. GTP restores the dopamine but not the calcium response. The inhibitory guanine nucleotide GDP-beta S antagonizes dopamine and GppNHp stimulation of the brain adenylate cyclases, but not stimulation by calcium of either rat cerebral cortical or striatal enzymes. Results indicate that GTP is not requisite to calcium-calmodulin activation of adenylate cyclases in brain membranes. In addition, calcium-calmodulin cannot activate striatal adenylate cyclases with a nonphosphorylating nucleotide, AppNHp, as substrate.

Problems in measurement of erythrocyte calcium

As calcium has increasingly been the object of study in erythrocyte physiology, we reviewed the current methodologies for determination of calcium by atomic absorption spectrometry. The published normal values for erythrocyte calcium vary from 5 to 500 mumol/liter of packed cells. A method based on Harrison and Long's determination of calcium is presented and shows normal red cell calcium concentration to be 0.0149 +/- 0.0023 mumol/ml of packed red cells. The influence of temperature and type of crucible used in ashing red cells is assessed. The method of additions is employed to corroborate our results.

Properties of detergent-dispersed adenylate cyclase from cerebral cortex. Presence of an inhibitor protein

Adenylate cyclase was solubilized from washed particulate fraction of rabbit cerebral cortex with the nonionic detergent Lubrol 12A9 and subjected to either gel filtration on Ultrogel AcA 34 or chromatography on DEAE Bio-Gel A. By both procedures the enzyme was resolved into two components, one insensitive to guanyl 5'-yl imidodiphosphate [Gpp(NH)p] and NaF but stimulated by Ca2+ and calmodulin, and another that was sensitive to Gpp(NH)p and NaF but relatively insensitive to Ca2+ and calmodulin. The data support the possibility that two independent forms of adenylate cyclase exist in cerebral cortex, one regulated by guanine nucleotide regulatory protein and another by Ca2+-calmodulin. Fractions containing the guanylnucleotide-sensitive activity were found to contain a factor that inhibited basal and Ca2+-stimulated adenylate cyclase in the Ca2+-sensitive fraction. The inhibitor was inactivated by heating at 60 degrees C and by incubation with trypsin. Inhibition was not time-dependent, and it was not due to destruction of cAMP by phosphodiesterase or of ATP by ATPase. Inhibitory action was not reversed by calmodulin and therefore it does not appear to be a calmodulin binding protein. Sucrose density gradient sedimentation indicated a sedimentation coefficient of 4S for the inhibitor; by this technique it co-sedimented with the adenylate cyclase sensitive to Gpp(NH)p and NaF.

Function of calmodulin in postsynaptic densities. I. Presence of a calmodulin-activatable cyclic nucleotide phosphodiesterase activity

The postsynaptic density (PSD) fraction from canine cerebra cortex was found to contain an endogenous cyclic nucleotide-phosphodiesterase activity that was independent on Mn2+ and/or Mg2+ but not on Ca2+. Maximal activity was obtained at 1 micrometer Mn2+. This cyclic nucleotide phosphodiesterase activity was not decreased upon removal of the calmodulin from the PSD fraction, nor was it increased by the addition of calmodulin to a postsynaptic density fraction deficient in calmodulin. The enzymatic activity could be extracted by sonication, with the soluble enzyme having properties similar to those found in the native structure. Two peaks of cyclic nucleotide phosphodiesterase activities could be obtained after S-300 Sephacryl column chromatography of this soluble fraction: fraction I (excluded peak) and fraction II (215,000 mol wt). The fraction I activity preferred cyclic AMP over cyclic GMP and was not activated by calmodulin. The fraction II activity has an approximately fourfold lower Km for cyclic GMP over cyclic AMP. This fraction II activity was activatable by calmodulin, which increased the Vmax and decreased the Km in the case of both cyclic nucleotides. We conclude that two activities are present in the PSD, one activatable, and one not activatable, by calmodulin.

Calmodulin activates prokaryotic adenylate cyclase

The adenylate cyclase of Bordetella pertussis is stimulated 100- to 1000-fold in a dose-dependent manner by calf brain calmodulin. The system has the following properties. (i) The activation is prevented by ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and restored by Ca2+. (ii) Oxidation of the methionine residues of calmodulin abolishes the ability to activate the cyclase. (iii) Trifluoperazine inhibits calmodulin-activated cyclase. (iv) A troponin C preparation stimulates the B. pertussis cyclase with < 0.01 the potency of calmodulin. Although calmodulin has not been demonstrated in prokaryotes, this is an example of a (eukaryotic) calmodulin effect in a prokaryote.

High levels of a heat-labile calmodulin-binding protein (CaM-BP80) in bovine neostriatum

Bovine brain contains a heat-labile, 80,000-dalton calmodulin-binding protein (CaM-BP80) which inhibits the calmodulin-dependent activities of cyclic 3',5'-nucleotide phosphodiesterase, adenylate cyclase, and Ca2+-ATPase in vitro. CaM-BP80 is composed of two polypeptides (60,000 and 18,500 daltons) present in a 1:1 ratio. An antibody directed against CaM-BP80 was raised in rabbits, and a radioimmunoassay was developed, having a sensitivity of 60 fmol of CaM-BP80. Using the radioimmunoassay, we determined the levels of CaM-BP80 in various bovine tissues. The protein was found primarily in the brain, present in particularly high levels in the neostriatum. These results, together with immunohistochemical localization of CaM-BP80 at the postsynaptic densities and the microtubules of postsynaptic dendrites [Wood, J.G., Wallace, R., Whitaker, J., & Cheung, W.Y. (1980) J. Cell Biol. 84, 66-76], suggest that the protein may have a role in the cerebrum at the site of neurotransmitter action and at the level of microtubular function.

Preparative isoelectric focusing and some properties of solubilized adenylate cyclase from rat brain

A new procedure for the purification of rat brain adenylate cyclase (ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1) is presented. The enzyme solubilized in Lubrol PX was purified either by molecular sieving or by hydrophobic chromatography, followed by a preparative isoelectric focusing step. For this purpose, a new isoelectric focusing technique was developed which allows a good resolution of adenylate cyclase in a short period of time. When resolved by this procedure, the enzyme migrated as a single molecular species with a pI of 6.3. When isoelectric focusing was performed in the presence of EGTA, two distinct peaks of activity could be detected at pI 6.1 and 7.3. This suggests that adenylate cyclase consists of two subunits held together by divalent ions. It is shown that the purified adenylate cyclase has a smaller sedimentation coefficient and is less hydrophobic than the native one. We conclude that the adenylate cyclase containing complex was at least partially disaggregated by this procedure.

Effect of estradiol on neurotransmitter sensitive adenylate cyclase. Its possible role in 'sexual differentiation'

Adenylate cyclase activity is lower in the hypothalamus of 5-day-old male rats than in females. This may be due to the presence of estradiol (E2) in this area of the male but not in the female since castration of the newborn male leads to an enzyme activity in the hypothalamus that is indistinguishable from that of the female and androgenization of the newborn female causes an enzyme activity level comparable to that of the male. In the adult, enzyme activity is highest when the concentration of E2 is at its lowest level; adenylate cyclase activity in the hypothalamus-preoptic area was found to be higher in the metestrus female than in proestrus or in the adult male. In vitro E2 was shown to reduce enzyme activity and this reduction was found to be dependent on induction of protein biosynthesis. In addition, there appears to be a requirement for Ca2+ in the E2-induced reduction of cyclase activity. Although F- activates the enzyme in all of the cases studied, the reduction in enzyme activity brought about by E2 is not reversed by F-, suggesting that the effect of E2 is not on the receptor. As sexual differentiation is brought about by the action of E2 during the first week after birth, it seems plausible to suggest that this interaction between hormone and enzyme is an early step in the sexual differentiation of the brain.

Calmodulin plays a pivotal role in cellular regulation

The role of calcium ions (Ca2+) in cell function is beginning to be unraveled at the molecular level as a result of recent research on calcium-binding proteins and particularly on calmodulin. These proteins interact reversibly with Ca2+ to form a protein . Ca2+ complex, whose activity is regulated by the cellular flux of Ca2+. Many of the effects of Ca2+ appear to be exerted through calmodulin-regulated enzymes.

Calcium-dependent desensitization of adenylate cyclase in rat cerebral cortical slices

Incubation of slices of rat cerebral cortical grey matter in Krebs-Ringer bicarbonate-glucose buffer induced a rapid decline in the responsiveness of the adenylate cyclase in subsequently prepared membrane preparations to stimulation by various activators of the enzyme. The loss of responsiveness was time- and temperature-dependent, showed an absolute dependence on extracellular calcium ions, and was mimicked by the presence of serine proteases in the incubation medium. The resultant adenylate cyclase preparation was partially responsive to activation by fluoride and guanylylimidodiphosphate but had become virtually unresponsive to activation by ganglioside, trypsin, or beta-adrenergic agonists. The loss of responsiveness of adenylate cyclase was not altered if slices were incubated with depolarizing agents, putative neurotransmitters, receptor blockers, serine protease inhibitors, or adenosine deaminase. The nature of the calcium-dependent mechanism involved in the loss responsiveness of membranal adenylate cyclase is unknown. A suggested mechanism for the loss of sensitivity is the action of a membrane-bound, calcium-dependent protease.

The activation of brain adenylate cyclase and brain cyclic-nucleotide phosphodiesterase by seven calmodulin derivatives

1. A comparison has been made of the ability of seven calmodulin derivatives to displace 125I-labeled calmodulin and to activate adenylate cyclase in a brain particulate fraction. The activation of brain-soluble cyclic-nucleotide phosphodiesterase by the same calmodulin derivatives was examined in parallel. 2. In general, the dose for half-maximal inhibition of 125I-labeled calmodulin binding and the apparent Km of adenylate cyclase activation were comparable in brain membranes. These concentrations were 20--40-times higher than the corresponding apparent Km values of activation of cyclic-nucleotide phosphodiesterase. 3. Modifying the single histidine residue or both tyrosine residues exerted no influence on the biological properties of calmodulin. The carboxymethylation of two methionine residues or the amidation of several carboxyl groups reduced the activation properties of calmodulin on adenylate cyclase and cyclic-nucleotide phosphodiesterase. Altering seven lysine or four arginine residues resulted in two proteins whose activation properties on adenylate cyclase and phosphodiesterase had been modified in a way suggesting that lysine and arginine residues play distinct roles in the interaction of native calmodulin with each enzyme.

Immunocytochemical localization of calmodulin and a heat-labile calmodulin-binding protein (CaM-BP80) in basal ganglia of mouse brain

Antisera to calmodulin, a Ca2%-dependent modulator protein, and a heat-labile calmodulin-binding protein have been used to localize these proteins in mouse caudate-putamen. The two proteins appear to be located at identical sites in this brain area. At the light microscopic level, calmodulin and calmodulin-binding protein are found within the cytoplasm and processes of large cells. At the electron microscopic level the proteins are associated with neuronal elements only, primarily at postsynaptic sites within neuronal somata and dendrites. Within the dendrites the immunocytochemical label is associated predominantly with the postsynaptic density and dendritic microtubules. These results are in accord with recent biochemical and immunihistochemical studies of calmodulin in brain and in dividing cells. Thus, calmodulin and the heat-labile calmodulin-binding protein may play a role in the nervous system at the site of neurotransmitter action and at the level of microtubular function.