Differential phosphorylation of multiple sites in purified protein I by cyclic AMP-dependent and calcium-dependent protein kinases

The cAMP cascade in the nervous system: molecular sites of action and possible relevance to neuronal plasticity

Many intercellular messages regulate the activity of their target cells by altering the intracellular level of cAMP and, as a consequence, the phosphorylation state of proteins which serve as substrates for cAMP-dependent protein kinase. Such regulation plays a crucial role in neuronal development, neuronal function, and neuronal plasticity (e.g., elementary learning mechanisms). Ample information has been accumulated in recent years on the enzymes that regulate the level of cAMP or respond to it, on the regulation of cAMP synthesis by neurohormones, neurotransmitters, ions, and toxins, on neuronal-specific substrate proteins that are phosphorylated by the cAMP-dependent kinase, and on the interaction of the cAMP-cascade with other second-messenger systems within neurons. Such data, obtained by a combination of molecular-biological, biochemical, and cellular approaches, shed light on the detailed mechanisms by which modulation of a ubiquitous molecular cascade leads to a great variety of short-term as well as long-term specific neuronal responses and alterations.

Calmodulin-dependent multifunctional protein kinase. Evidence for isoenzyme forms in mammalian tissues

Calcium/calmodulin-dependent multifunctional protein kinases, extensively purified from rat brain (with apparent molecular mass 640 kDa), rabbit liver (300 kDa) and rabbit skeletal muscle (700 kDa), were analysed for their structural, immunological, and enzymatic properties. The immunological cross-reactivity with affinity-purified polyclonal antibodies to the 50-kDa catalytic subunit of the brain calmodulin-dependent protein kinase confirmed the presence of common antigenic determinants in all subunits of the protein kinases. One-dimensional phosphopeptide patterns, obtained by digestion of the autophosphorylated protein kinases with S. aureus V8 protease, and two-dimensional fingerprints of the 125I-labelled proteins digested with a combination of trypsin and chymotrypsin, revealed a close similarity between the two subunits (51 kDa and 53 kDa) of the liver enzyme. Similar identity was observed between the 56-kDa and/or 58-kDa polypeptides of the skeletal muscle calmodulin-dependent protein kinase. The data suggest that the subunits of the liver and muscle protein kinases may be derived by partial proteolysis or by autophosphorylation. The peptide patterns for the 50-kDa and 60-kDa subunits of the brain enzyme confirmed that the two catalytic subunits represented distinct protein products. The comparison of the phosphopeptide maps and the two-dimensional peptide fingerprints, indicated considerable structural homology among the 50-kDa and 60-kDa subunits of the brain calmodulin-dependent protein kinase and the liver and muscle polypeptides. However, a significant number of unique peptides in the liver 51-kDa subunit, skeletal muscle 56-kDa, and the brain 50-kDa and 60-kDa polypeptides were observed and suggest the existence of isoenzyme forms. All calmodulin-dependent protein kinases rapidly phosphorylated synapsin I with a stoichiometry of 3-5 mol phosphate/mol protein. The two-dimensional separation of phosphopeptides obtained by tryptic/chymotryptic digestion of 32P-labelled synapsin I indicated that the same peptides were phosphorylated by all the calmodulin-dependent protein kinases. Such data represent the first structural and immunological comparison of the liver calmodulin-dependent protein kinase with the enzymes isolated from brain and skeletal muscle. The findings indicate the presence of a family of highly conserved calmodulin-dependent multifunctional protein kinases, with similar structural, immunological and enzymatic properties. The individual catalytic subunits appear to represent the expression of distinct protein products or isoenzymes which are selectively expressed in mammalian tissues.

Altered calcium signal transduction after chronic ethanol consumption

Calcium and calcium-calmodulin dependent phosphorylation of several protein bands was found altered in synaptosomal membranes prepared from ethanol treated rats. The ethanol induced effect on Ca++ and Ca++-calmodulin phosphorylation presented regional differences. In particular 32P incorporation was lower in the striatum and cerebellum, higher in the hippocampus and unmodified in the cortex. Part of the phosphorylated bands had an apparent molecular weight similar to that of the phosphoproteins involved in neurotransmission. These results extend previous observations indicating that calcium movement control is modified during chronic ethanol consumption and suggest that ethanol may interfere at various steps in the calcium-promoted events.

Detection and characterization of some new basic proteins in chicken postsynaptic densities

Chicken brain postsynaptic density (PSD) polypeptides, obtained by treating synaptosomes with 0.5% Triton X-100 and then further purified on a sucrose gradient, are demonstrated to contain four basic proteins of 76K (pI greater than 9.2), 58K (pI 8.1-8.8, heterogeneous), 40K (pI 9.0), and 24K (pI 8.9). Nonequilibrium pH gradient-sodium dodecyl sulfate two-dimensional gels further reveal six more basic proteins with pI values higher than 9.2: 76K, 52K, 47K, 45K, 36K, and 34K. These basic proteins are a major part of the total chicken PSD polypeptides appearing on the gels. Some of these basic proteins (58K, 52K, 47K, 36K, 24K, and two at 76K) are distinguishable from those of brain mitochondria, the major contaminant. The 40K and 34K proteins may be common mitochondrial polypeptides. The 45K protein is probably a mitochondrial contaminant. A number of proteins including 76K (synapsin I-like protein) and 58K, along with some other minor ones, can be phosphorylated by endogenous protein kinase(s) in the presence of Ca2+, Mg2+, and [gamma-32P]ATP. No PSD basic proteins bind Ca2+.

Properties of cAMP-induced transmembrane current in mollusc neurons

The effects of different substances affecting some or other chains of cAMP metabolism on the properties of transmembrane current induced by cAMP injection have been studied in Helix pomatia neurons. It was found that preinjection of papaverine or adenosine triphosphate from a microelectrode into the neuron increases the cAMP-current amplitude. Injection of dibutyryl-cAMP does not induce transmembrane current by itself but results in a noticeable reversible inhibition of cAMP-current. Extracellular administration of trifluoperazine produces either an increase or a decrease of the cAMP-current amplitude in different cells but in both cases its removal restores the initial amplitude of cAMP-current. Furosemide has no appreciable effect on cAMP-current. An increase in intracellular Ca2+ concentration (by iontophoretic injection through a microelectrode, generation of a burst of action potentials, application of dinitrophenol, tetraphenylphosphonium or caffeine) considerably enhances the amplitude of cAMP-current. The amplitude of cAMP-current remains increased for many minutes after return of the intracellular Ca2+ level to its initial value. A possible physiological role of the observed effect is discussed.

Depolarization-dependent protein phosphorylation in rat cortical synaptosomes: characterization of active protein kinases by phosphopeptide analysis of substrates

Depolarization of synaptosomes is known to cause a calcium-dependent increase in the phosphorylation of a number of proteins. It was the aim of this study to determine which protein kinases are activated on depolarization by analyzing the incorporation of 32Pi into synaptosomal phosphoproteins and phosphopeptides. The following well-characterized phosphoproteins were chosen for study: phosphoprotein "87K," synapsin Ia and Ib, phosphoproteins IIIa and IIIb, the catalytic subunits of calmodulin kinase II, and the B-50 protein. Each was initially identified as a phosphoprotein in lysed synaptosomes after incubation with [gamma-32P]ATP. Mobility on two-dimensional polyacrylamide gels and phosphorylation by specific protein kinases were the primary criteria used for identification. A technique was developed that allowed simultaneous analysis of the phosphopeptides derived from all of these proteins. Phosphopeptides were characterized in lysed synaptosomes after activating cyclic AMP-, calmodulin-, and phospholipid-stimulated protein kinases in the presence of [gamma-32P]ATP. Phosphoproteins labelled in intact synaptosomes after incubation with 32Pi were then compared with those seen after ATP-labelling of lysed synaptosomes. As expected from previous work, phosphoprotein "87K," and synapsin Ia and Ib were labelled, but for the first time, phosphoproteins IIIa, IIIb, and the B-50 protein were identified as being labelled in intact synaptosomes; the calmodulin kinase II subunits were hardly phosphorylated. From a comparison of the phosphopeptide profiles it was found that cyclic AMP-, calmodulin-, and phospholipid-stimulated protein kinases are all active in intact synaptosomes and their activity is dependent on extrasynaptosomal calcium. The activation of cyclic AMP-stimulated protein kinases in intact synaptosomes was confirmed by the addition of dibutyryl cyclic AMP and theophylline which specifically increased the labelling of phosphopeptides in synapsin Ia and Ib and in phosphoproteins IIIa and IIIb. On depolarization of intact synaptosomes, a number of phosphopeptides showed increased labelling and the pattern suggested that cyclic AMP-, calmodulin-, and phospholipid-stimulated protein kinases were all activated. No new peptides were phosphorylated, suggesting that depolarization simply increased the activity of already active protein kinases and that there was no depolarization-specific increase in protein phosphorylation.

Phosphorylation of microtubule-associated protein 2 at distinct sites by calmodulin-dependent and cyclic-AMP-dependent kinases

Microtubule-associated protein 2 (MAP2) is an excellent substrate for both cyclic-AMP (cAMP)-dependent and Ca2+/calmodulin-dependent kinases. A recently purified cytosolic Ca2+/calmodulin-dependent kinase (now designated CaM kinase II) phosphorylates MAP2 as a major substrate. We now report that microtubule-associated cAMP-dependent and calmodulin-dependent protein kinases phosphorylate MAP2 on separate sites. Tryptic phosphopeptide digestion and two-dimensional phosphopeptide mapping revealed 11 major peptides phosphorylated by microtubule-associated cAMP-dependent kinase and five major peptide species phosphorylated by calmodulin-dependent kinase. All 11 of the cAMP-dependently phosphorylated peptides were phosphorylated on serine residues, whereas four of five major peptides phosphorylated by the calmodulin-dependent kinase were phosphorylated on threonine. Only one peptide spot phosphorylated by both kinases was indistinguishable by both migration and phosphoamino acid site. The results indicate that cAMP-dependent and calmodulin-dependent kinases may regulate microtubule and cytoskeletal dynamics by phosphorylation of MAP2 at distinct sites.

Phylogenetic survey of proteins related to synapsin I and biochemical analysis of four such proteins from fish brain

A phylogenetic survey of proteins immunologically related to Synapsin I, a major synaptic vesicle-associated phosphoprotein in mammals was carried out. Proteins antigenically related to Synapsin I were found by use of radioimmunoassay and other radioimmunochemical techniques in the nervous systems of several vertebrate and invertebrate species, which included birds, reptiles, amphibians, fish, echinoderms, arthropods, and mollusks. Four proteins present in fish brain, antigenically related to Synapsin I, were further studied and found to resemble mammalian Synapsin I in several respects. Like Synapsin I, the fish proteins were present in high amounts in nervous tissue, were enriched in synaptosomal fractions of brain where they were substrates for endogenous protein kinases, were acid extractable, and were sensitive to digestion by collagenase. In addition, two-dimensional peptide-mapping analysis revealed some homology between major phosphopeptide fragments of Synapsin I and the fish proteins. The results indicate that proteins related to Synapsin I are wide-spread in the animal kingdom.

Resolution of rat brain synaptic phosphoprotein B-50 into multiple forms by two-dimensional electrophoresis: evidence for multisite phosphorylation

Phosphoprotein B-50 was extracted from rat brain membranes by alkaline extraction and purified by ammonium sulphate precipitation and flat-bed isoelectric focusing. The purified protein shows microheterogeneity upon isoelectric focusing in a narrow pH gradient (pH 3.5-5.0). As visualized by two-dimensional gel electrophoresis, B-50 resolved into four clearly separated forms which differ slightly in isoelectric point. The forms are in part mutually convertible by exhaustive phosphorylation (using protein kinase C) and dephosphorylation (using Escherichia coli alkaline phosphatase). Proteolysis with Staphylococcus aureus protease yielded two radioactive peptides. Analysis of their molecular weights and the time course of their formation suggests that B-50 was cleaved at only one specific site. Our data indicate the presence of more than one phosphorylatable site. The possibility that the heterogeneity of B-50 was in part due to a glycoprotein nature of B-50 was studied extensively. However, none of the six different methods used revealed the presence of glyco-moieties in B-50.

A multifunctional calmodulin-stimulated phosphatase

This review summarizes current knowledge concerning structure-function, substrate specificity, localization, and regulatory properties of calcineurin. Calcineurin is composed of two nonidentical subunits, one of which is responsible for catalytic activity and calmodulin binding while the other subunit contains four high-affinity Ca2+-binding sites. The enzyme possesses calmodulin-stimulated and metal ion-dependent phosphatase activity toward several nonprotein and phosphoseryl-, phosphothreonyl- and phosphotyrosyl-containing protein substrates. These recent results suggest that the protein may play a multifunctional role in interactions between the Ca2+/CaM second messenger system and other second messenger systems.

The use of Synapsin I as a biochemical marker for neuronal damage by trimethyltin

The content of Synapsin I (Protein I) was examined in brain regions of adult rats exposed to trimethyltin (TMT), and in control animals. Long Evans hooded rats were intragastrically dosed with 4 mg TMT hydroxide/kg body weight for 4 days. No perturbations in Synapsin I levels were evident by 24 h following the fourth dose; however, by 36 h, a significant decrease of 28% in Synapsin I level was present in the hippocampus of TMT treated animals. This decrease was selective, no other brain region examined was affected. As determined by regional analysis of inorganic tin, this specificity was not due to a profound preferential accumulation of tin in the hippocampus. Despite the absence of an alteration in Synapsin I levels at 24 h, morphological examination revealed perturbation in the normal uniform arrangement of granule cell neurons, with dead neurons diffusely distributed throughout the facia dentata. At 36 h, these changes were only slightly more extensive. In contrast, examination of the terminal projection area of these cells, the mossy boutons, showed to be unaffected at 24 h after the 4th dose of TMT. However, by 36 h, many of the mossy boutons contained dense bodies and showed signs of degeneration. This result suggested that the loss of Synapsin I coincides with degeneration of the nerve terminal region. In order to better establish the temporal correlation, a less severe dosing regimen (only 3 days of exposure to 4 mg TMT/kg body wt) was utilized to attenuate the time course of necrosis. Again, necrotic changes were visible in the perikaryon by 1 day after termination of toxicant dosing.(ABSTRACT TRUNCATED AT 250 WORDS)

Synapsin I in nerve terminals: selective association with small synaptic vesicles

Immunocytochemistry revealed that synapsin I is preferentially (and possibly exclusively) associated with small (40- to 60-nanometer) synaptic vesicles and not with large (greater than 60-nanometer) dense-core vesicles in bovine hypothalamus. These observations may explain why synapsin I is found exclusively in neurons, since small synaptic vesicles are specific to neurons whereas large dense-core vesicles in neurons may be considered the equivalent of secretory organelles in endocrine cells.

Turnover of inositol phospholipids and signal transduction

Various extracellular informational signals such as those from a group of hormones and some neurotransmitters appear to be passed from the cell surface into the cell interior by two routes, protein kinase C activation and Ca2+ mobilization. Both routes usually become available as the result of an interaction of a single ligand and a receptor and act synergistically to evoke subsequent cellular responses such as release reactions. The signal-dependent breakdown of inositol phospholipids, particularly phosphatidylinositol bisphosphate, now appears to be a key event for initiating these processes.

On the disposition of a phosphorylated protein ("synapsin I") and its associated kinases in synaptosomes from rat brain

Endogenous phosphorylation of synapsin I (protein I), a phosphoprotein located on the surface of synaptic vesicles, was studied in vesicles prepared from synaptosomes lysed in the absence (control) or presence of 50 microM-cyclic AMP ("cAMP-treated"). Compared to synaptic plasma membrane (SPM) fractions prepared in parallel, and confirming previous work, the vesicle fractions were highly enriched on a unit protein basis in Ca2+-calmodulin-dependent kinase activity towards synapsin I. In contrast, with control vesicles the magnitude of the total phosphorylation of synapsin I in the presence of cyclic AMP was similar to that observed in SPM, but regulation by cyclic AMP was only partial. In "cAMP-treated" vesicles, however, synapsin I phosphorylation was highly enriched compared to SPM and the activity was virtually independent of cyclic AMP. The results show that while the free catalytic subunit of the cyclic AMP-dependent kinase remains associated with synapsin I during vesicle isolation the holoenzyme remains bound to membrane fragments, probably through its regulatory subunit.

cAMP, not Ca2+/calmodulin, regulates the phosphorylation of acetylcholine receptor in Torpedo californica electroplax

The regulation of the phosphorylation of the acetylcholine receptor in electroplax membranes from Torpedo californica and of purified acetylcholine receptor was investigated. The phosphorylation of the membrane-bound acetylcholine receptor was not stimulated by Ca2+/calmodulin, nor was it inhibited by EGTA, but it was stimulated by the catalytic subunit of cAMP-dependent protein kinase, and was blocked by the protein inhibitor of cAMP-dependent protein kinase. Purified acetylcholine receptor was not phosphorylated by Ca2+/calmodulin-dependent protein kinase activity in electroplax membranes, nor by partially purified Ca2+/calmodulin-dependent protein kinases from soluble or particulate fractions from the electroplax. Of the four acetylcholine receptor subunits, termed alpha, beta, gamma and delta, only the gamma- and delta-subunits were phosphorylated by the cAMP-dependent protein kinase (+ cAMP), or by its purified catalytic subunits.

Depolarization-dependent protein phosphorylation in rat cortical synaptosomes: the effects of calcium, strontium and barium

Depolarization of synaptosomes increases the phosphorylation of a number of proteins in a calcium-dependent manner. The concentration of calcium required for optimum stimulation was 0.1 mM, with higher concentrations up to 2.5 mM being progressively less effective. Calcium was significantly better than strontium at increasing depolarization-dependent protein phosphorylation, while barium had no stimulating effect at concentrations above 0.1 mM. The order of potency of these ions is consistent with a calmodulin-stimulated protein kinase being activated on entry of calcium into synaptosomes, but is not consistent with the known efficacy of these ions in stimulating neurotransmitter release. The data show for the first time that phosphorylation of proteins may not be a prerequisite for neurotransmitter release.

Protein phosphorylation in the brain

Protein phosphorylation represents an approach, sometimes the only approach available, to study the molecular basis for a wide variety of neurophysiological phenomena. The injection of protein kinases or protein kinase inhibitors into neurones has provided direct evidence that activation of protein kinases has an obligatory role in the mechanisms by which numerous extracellular signals produce specific physiological responses in neurones. A diversity of substrate proteins for the kinases have already been found. In several instances, the identity and functional role of these substrate proteins have been established.

Isolation of postsynaptic densities retaining their membrane attachment

A method is described for the preparation of a subcellular fraction, 30-50% pure, of intact postsynaptic units from rat cerebral cortex. The isolation procedure is based on chemical dissociation of the synaptic cleft as described by Crawford, Osborne & Potter followed by sonication of the extracted membranes and separation of the postsynaptic units on a discontinuous sucrose gradient. This preparation provides the first practical procedure for the isolation of postsynaptic densities, prominent organelles of unknown function, without the use of detergents, enabling retention of the postsynaptic membrane in association with the postsynaptic density. The preparation shows enhanced binding of spiroperidol, a dopamine agonist, which, in conjunction with morphological evidence, indicates that the preparation is sufficiently intact to enable study of the interaction of the postsynaptic membrane with the postsynaptic density. Actin, alpha- and beta-tubulin and postsynaptic density protein constitute the major proteins in the preparation; they are present in amounts of 41, 54, 57 and 74 micrograms per mg protein, respectively; as compared to 54, 59, 55 and 9 micrograms per mg protein of the synaptic junctional membrane used as starting material. The utility of the preparation for a number of localization studies, including ion translocating adenosine 5'-triphosphatases, protein kinases and their substrates is discussed.

Relationship between NGF-mediated volume increase and "priming effect" in fast and slow reacting clones of PC12 pheochromocytoma cells. Role of cAMP

Nerve Growth Factor (NGF)-mediated fiber outgrowth in pheochromocytoma PC12 cells is a slow process, developing over a period of several days. However, if these cells are pre-exposed to NGF for 7-10 days, renewed NGF treatment of the subcultured cells elicits fiber outgrowth within 24 h, comparable to the rate of response of physiological target cells to NGF. The present experiments demonstrated that this effect, previously termed "priming", was accompanied by a 60% increase in the volume of the PC12 cells, and that the dose-response curves for NGF-mediated induction of fiber outgrowth and for the increase in cell volume were very similar. Furthermore, the rates of NGF-mediated fiber outgrowth and of cell volume increase were both much slower in conventional PC12 cells (slow-reacting) compared to a newly-selected, fast-responding (FR)subclone of PC12 cells. These results suggested a possible causal relationship between the increase in cell volume and the induction of fiber outgrowth. However, when the cells were pre-exposed for 7 days to dibutyryl-cAMP (db-cAMP), the increase in cell volume was 3-fold higher than that effected by NGF. Nevertheless, db-cAMP had only a very limited ability to "prime" the cells for a subsequent response to NGF. Thus, the induction of cell volume increase and the increased availability of structural elements is not sufficient to explain the "priming" effect of NGF. The effects of db-cAMP are discussed in the context of a possible role of cAMP as a second messenger in the action of NGF.

Metabolite-controlled phosphorylation of hepatic phosphofructokinase proceeds by cAMP-dependent protein kinase

In hepatocytes 32P-incorporation into rat liver phosphofructokinase is stimulated by glucose as well as by glucagon, the effects of both stimuli being prevented by L-alanine [Eur. J. Biochem. (1982) 122, 175]. The phosphopeptides of the enzyme derived from limited proteolysis by subtilisin and from exhaustive tryptic digestion were analyzed either by one-dimensional mapping on sodium dodecyl sulphate-polyacrylamide slab gels and by fingerprint mapping, respectively. It is shown that in vivo stimulation of 32P-incorporation by glucose or by glucose plus glucagon results in identical phosphopeptide maps, and that these maps were identical with those obtained from phosphofructokinase phosphorylated in vitro with catalytic subunit of cAMP-dependent protein kinase. It is concluded that in the intact liver cell phosphofructokinase is phosphorylated by cAMP-dependent protein kinase but that the state of phosphorylation is modified by metabolite control.

Evidence that the synaptic phosphoprotein B-50 is localized exclusively in nerve tissue

The localization of the phosphoprotein B-50 (molecular weight 48,000 isoelectric point 4.5) in the rat has been studied. Inspection of endogenous phosphorylation patterns of the particulate as well as the cytosolic subcellular fractions from a variety of peripheral organs failed to demonstrate phosphorylation of a molecular weight 48,000 protein. Only in the particulate fractions from brain tissue was there endogenous phosphorylation of the B-50 protein. Two-dimensional analysis (isoelectric focusing and sodium dodecyl sulfate polyacrylamide gel electrophoresis) and in immunochemical detection method employing an anti B-50 antiserum revealed the presence of B-50 in particulate material from brain, but not in that of other tissues. Therefore the data were interpreted as pointing to the localization of B-50 in nervous tissue. In addition, the regional distribution of endogenous B-50 phosphorylation was studied using synaptosomal plasma membranes (SPM) obtained from individual rat brain regions. The highest value was found in SPM of septal origin, the lowest in SPM from the medulla spinalis. The relationship of the high value for B-50 phosphorylation in the septum to the sensitivity of that brain area to ACTH1-24 is discussed.

Presynaptic localization of phosphoprotein B-50

A major phosphoprotein of synaptic membranes, the phosphorylation of which is stimulated by Ca2+ and inhibited by ACTh, appears to be identical with protein B-50 described by Zwiers, Schotman and Gispen [40]. We have investigated its subsynaptic localization by means of a variety of subfractionation techniques and compared it with that of a number of other phosphoproteins found in synaptic membranes. It appears to be predominantly, if not exclusively, associated with presynaptic membranes of low bouyant density. This localization pattern is similar to, but somewhat more extreme than that exhibited by Protein I, as a brain specific phosphoprotein studied by Greengard and his collaborators [11].