Function of a calmodulin in postsynaptic densities. II. Presence of a calmodulin-activatable protein kinase activity

Simple Method for the Preparation of Postsynaptic Density Fraction from Mouse Brain

Postsynaptic density (PSD) is a morphologically and functionally specialized postsynaptic membrane structure of excitatory synapses. It contains hundreds of proteins such as neurotransmitter receptors, adhesion molecules, cytoskeletal proteins, and signaling enzymes. The study of the molecular architecture of the PSD is one of the most intriguing issues in neuroscience research. The isolation of the PSD from the brain of an animal is necessary for subsequent biochemical and morphological analyses. Many laboratories have developed methods to isolate PSD from the animal brain. In this chapter, we present a simple method to isolate PSD from the mouse brain using sucrose density gradient-based purification of synaptosomes followed by detergent extraction.

Activation State-Dependent Substrate Gating in Ca2+/Calmodulin-Dependent Protein Kinase II

Calcium/calmodulin-dependent protein kinase II (CaMKII) is highly concentrated in the brain where its activation by the Ca²⁺ sensor CaM, multivalent structure, and complex autoregulatory features make it an ideal translator of Ca²⁺ signals created by different patterns of neuronal activity. We provide direct evidence that graded levels of kinase activity and extent of T²⁸⁷ (T²⁸⁶ α isoform) autophosphorylation drive changes in catalytic output and substrate selectivity. The catalytic domains of CaMKII phosphorylate purified PSDs much more effectively when tethered together in the holoenzyme versus individual subunits. Using multisubstrate SPOT arrays, high-affinity substrates are preferentially phosphorylated with limited subunit activity per holoenzyme, whereas multiple subunits or maximal subunit activation is required for intermediate- and low-affinity, weak substrates, respectively. Using a monomeric form of CaMKII to control T²⁸⁷ autophosphorylation, we demonstrate that increased Ca²⁺/CaM-dependent activity for all substrates tested, with the extent of weak, low-affinity substrate phosphorylation governed by the extent of T²⁸⁷ autophosphorylation. Our data suggest T²⁸⁷ autophosphorylation regulates substrate gating, an intrinsic property of the catalytic domain, which is amplified within the multivalent architecture of the CaMKII holoenzyme.

Calcium-Stimulated Protein Phosphorylation in Synaptic Membranes

Synaptic membranes from rat brain contain several calcium-requiring protein kinase (PK) activities with different substrate specificities: (a) an activity (CaH-PK) effective at high concentrations of Ca2+ ion in the absence of Mg2+ (active on class F substrates); (b) a (Ca + Mg)-PK activity that is mediated by Ca2+ ion in the presence of Mg2+ (active on class B substrates); (c) (Ca-CaM)-PK activities that exhibit simultaneous requirements for both Ca2+ ion and CaM (for class C and D substrates). Also described are three activities (d-f) that do not require Ca2+ ion: (d) a Mg-PK activity in which the presence of Ca2+ causes the inhibition of phosphorylation (active on class A substrates); (e) an activity affecting a diverse group of substrates (class E substrates), the phosphorylation of which occurs in the presence of Mg2+ ion alone (Mg-PK activity) and is unaffected by the addition of Ca2+ ion and CaM, the substrates of which show different responses to several types of inhibitors; and, finally, (f) the previously well characterized cAMP-dependent PK activities. Several of the substrates of these kinases have been identified in a fairly unambiguous manner: among them are P43 (class A), as the alpha subunit of pyruvate dehydrogenase; P54 (class B), as the presynaptic protein B50; and the doublet P75-P80, as proteins IA and IB of Ueda and Greengard. The most interesting activity is that requiring both Ca2+ and CaM. The half-maximal stimulation (K0.5) for Ca2+ in the presence of CaM was found to be 1.0 microM Ca2+F in untreated membranes. There is little change in this value on prior EGTA extraction of the membranes, which removes the bulk of its Ca2+ and reduces its residual CaM by greater than or equal to 50%. The apparent K0.5 for CaM in the presence of excess Ca2+ ion was found to equal 0.4 microgram per reaction mixture (8 micrograms/ml) or 1.35 micrograms per reaction mixture (27 micrograms/ml), for the untreated and EGTA-treated membranes, respectively.

Ca2+/calmodulin protein kinase II and memory: learning-related changes in a localized region of the domestic chick brain

The role of calcium/calmodulin-dependent protein kinase II (CaMKII) in the recognition memory of visual imprinting was investigated. Domestic chicks were exposed to a training stimulus and learning strength measured. Trained chicks, together with untrained chicks, were killed either 1 h or 24 h after training. The intermediate and medial hyperstriatum ventrale/mesopallium (IMHV/IMM), a forebrain memory storage site, was removed together with a control brain region, the posterior pole of the neostriatum/nidopallium (PPN). Amounts of membrane total alphaCaMKII (tCaMKII) and Thr286-autophosphorylated alphaCaMKII (apCAMKII) were measured. For the IMHV/IMM 1 h group, apCaMKII amount and apCAMKII/tCaMKII increased as chicks learned. The magnitude of the molecular changes were positively correlated with learning strength. No learning-related effects were observed in PPN, or in either region at 24 h. These results suggest that CaMKII is involved in the formation of memory but not in its maintenance.

Synaptic metaplasticity and the local charge effect in postsynaptic densities

Synaptic plasticity might be one of the elementary processes that underlies higher brain functions, such as learning and memory. Intriguingly, the capacity of a synapse for plastic changes itself displays marked variation or plasticity. This higher-order plasticity, or metaplasticity, appears to depend on the same macromolecules as plasticity, most notably the NMDA receptor and Ca2+/calmodulin kinase II; yet we do not understand metaplasticity in molecular terms. Metaplasticity has a feedback-inhibition character that confers stability to synaptic patterns, whereas in plasticity, the molecular events implicated tend to have an opposite effect. As a resolution to this difference, we suggest that metaplasticity be considered in a biophysical context. It has been shown that autophosphorylation of Ca2+/calmodulin kinase II in postsynaptic densities generates changes in the local electrostatic potential sufficient to affect the direction of synaptic plasticity. We propose that this finding could help explain both the puzzling abundance of Ca2+/calmodulin kinase II in the postsynaptic density and the metaplasticity of synaptic transmission.

Retraction of spinule-type neurites from carp retinal horizontal cell dendrites during dark adaptation involves the activation of Ca2+/calmodulin-dependent protein kinase II

The formation of spinules at the terminal dendrites of retinal horizontal cells with the onset of light and their subsequent retraction during darkness is a remarkable example of synaptic plasticity where sensory experience modifies reversibly, and on a time scale of minutes the ultrastructure of synaptic connectivity. The signals and the subsequent intracellular cascades underlying the prominent morphological alterations are only partially understood. We show here that lowering the external calcium concentration did prevent dark- and AMPA-induced retraction of spinules in a eyecup preparation. Furthermore, spinule retraction was prevented in vivo by the injection of calmidazolium, an inhibitor of calmodulin, into the eyeball, and also by the injection of KN-62, an inhibitor of Ca2+/calmodulin-dependent protein kinase (CaMkII). We conclude that local Ca2+ influx through AMPA-gated channels followed by activation of CaMkII is an important step for spinule retraction during dark adaptation. The phosphorylation patterns of phosphoproteins derived from purified horizontal cells was affected by the inhibitors of calmodulin and CaMkII respectively. Some of the affected phosphoproteins appeared to be cytoskeleton-associated proteins, including GAP-43. Based on these observations, a putative scenario for the retraction of spinules is proposed.

An increase in phosphorylation of microtubule-associated protein 2 accompanies dendrite extension during the differentiation of cultured hippocampal neurones

Hippocampal neurones, from embryonic rats, were cultured for different times and the extension of dendrite-like processes was analysed morphologically and by immunofluorescence, using microtubule-associated protein 2 (MAP2) as a marker. Simultaneously, the changes in phosphorylation in MAP2 were analyzed and a correlation between dendrite sprouting and an increase in MAP2 phosphorylation was found. Phospho-MAP2 was cleaved by Staphylococcus aureus V8 protease limited proteolysis and its phosphopeptide pattern was compared to that obtained with two protein kinases (calcium/calmodulin-dependent kinase and protein kinase C) in vitro. An involvement of calcium/calmodulin-dependent protein kinase in the phosphorylation of MAP2, occurring simultaneously with dendrite extension during neuronal differentiation in vitro, is suggested.

Ca(2+)-binding proteins in rat synaptic fractions surveyed by the 45Ca2+ overlay method

Ca(2+)-binding proteins in the synaptic and subsynaptic fractions (P2, synaptosome, synaptic plasma membrane, and postsynaptic density [PSD]-enriched fractions) and soluble fraction of rat brain were surveyed by a 45Ca2+ overlay method. The PSD-enriched fraction from cerebral cortex contained two major Ca(2+)-binding proteins (55,000 M(r) and 19,000 M(r)) and a distinct group (in 140,000 M(r) region), and two minor ones (66,000 M(r) and 16,000 M(r)); and the fraction from cerebellum contained two (55,000 M(r) and 19,000 M(r)). The proteins with 55,000 M(r) and 19,000 M(r) were identified as tubulin and calmodulin, respectively, and present in all the fractions investigated. The Ca(2+)-binding proteins of 140,000 M(r) region were found only in the PSD-enriched fraction isolated from cerebral cortex: neither the PSD-enriched fraction isolated from cerebellum nor other subcellular fractions prepared from cerebral cortex and cerebellum contained the proteins. The 140,000 M(r) Ca(2+)-binding proteins were the substrates for the Ca2+/calmodulin-dependent protein kinase II associated with PSD, and no change in the Ca(2+)-binding was detected by the 45Ca2+ overlay method after phosphorylation of the proteins by the protein kinase. The 16,000 M(r) Ca(2+)-binding protein might be the beta-subunit of calcineurin. Calretinin and calbindin-D28k were also detected as Ca(2+)-binding proteins in the soluble fractions of both cerebral cortex and cerebellum.

Light and electron microscopic localization of alpha subunits of GTP-binding proteins, G(o) and Gi, in the cerebral cortex and hippocampus of rat brain

Antibodies that recognize alpha subunits of G(o), Gi2 and Gi3 were used to evaluate their association with synaptic junctions. G(o), but not Gi, was concentrated within perikaryal and dendritic cytoplasm of a small population of bipolar neurons. All three G-proteins were associated with the intracellular surface of dendritic, axonal and astrocytic plasma membranes and postsynaptic densities (PSDs). However, association with PSDs was more prevalent for the two Gi's than for G(o) while the association with terminals forming putatively excitatory synapses was more prevalent for G(o) and Gi3 than for Gi2. Thus, neuromodulators may modulate the release of excitatory transmitters via activation of presynaptic Gi3 and G(o) and also regulate the opening of Ca2+ and/or K+ channels via activation of Gi's and G(o) at PSDs.

Localization of a 82 kDa protein in postsynaptic density and its association with cytoskeletons

A fraction of synaptic junctional complex (SJC) was prepared from rat synaptosomes and served as antigen material to produce monoclonal antibodies (Mab) for examining the component proteins of the SJC. An antibody, Mab SJ-8, was obtained, which recognized a protein with a molecular weight of 82,000 Da in the SJC preparation by immunoblot analysis. The immunohistochemical localization of the 82 kDa protein was studied with the rat cerebellum. Mab SJ-8 labeled the peripheral areas of the Purkinje and granule cells. Small punctate areas were also stained in the molecular layer with SJ-8. Intracellular localization of the protein was examined with rat brain synaptosomes. Immunoelectron microscopy demonstrated that Mab SJ-8 strongly labeled the postsynaptic density (PSD) and also a fibrous network spreading out of it. However, the antibody did not label the pre- or post-synaptic membrane or the cleft material.

P400 protein is one of the major substrates for Ca2+/calmodulin-dependent protein kinase II in the postsynaptic density-enriched fraction isolated from rat cerebral cortex, hippocampus and cerebellum

Concanavalin A-binding glycoprotein with 250 K M(r) found in the postsynaptic density (PSD)-enriched preparation (or synaptic cytoskeleton) from rat cerebellum was identified with P400 protein from the physicochemical properties and enrichment in the cerebellum. Proteins homologous to the cerebellar 250 K M(r) protein occurred in the PSD-enriched preparations from rat cerebral cortex and from hippocampus, although the contents in the preparations were very low. The 250 K M(r) proteins in the PSD-enriched preparations from cerebellum and from cerebrum were highly phosphorylated by Ca2+/calmodulin (CaM)-dependent protein kinase II. The protein of synaptic plasma membrane (SPM) and PSD-enriched fractions prepared from cerebral cortex were not phosphorylated by the cAMP-dependent protein kinase endogenous to the fractions, whereas the protein from cerebellum was done in SPM and PSD-enriched fractions. The facts suggest that P400 or P400-like protein is closely associated with Ca2+/CaM-dependent protein kinase II in the PSD-enriched preparations, especially in the preparation from cerebral cortex. Phosphorylation of the protein by Ca2+/CaM-dependent protein kinase II may play an important role in the postsynaptic function in both cerebellum and at least in some areas of cerebrum.

The regulatory role of EF-hand motifs of pig 80K diacylglycerol kinase as assessed using truncation and deletion mutants

To elucidate the regulatory function of EF-hand motifs of pig 80K diacylglycerol (DG) kinase, we constructed and expressed several truncation and deletion mutants of the enzyme in E. coli or COS-7 cells. The bacterially expressed EF-hand region could bind Ca2+ and was suggested to undergo conformational change like calmodulin. A mutant enzyme lacking EF-hands lost Ca(2+)-binding activity, but could be fully activated by phosphatidylserine (PS) or deoxycholate in the absence of Ca2+. The full activation of the wild-type enzyme by PS, on the other hand, was totally dependent on Ca2+. Further, the wild-type enzyme expressed in COS-7 cells was exclusively soluble, whereas the EF-hand-deleted mutant was considerably associated with the membranes. The results suggest that under Ca(2+)-free condition, the EF-hand masks the PS-binding site of the DG kinase, and that the Ca(2+)-binding results in the exposure of the PS-binding site through the conformational change of the EF-hand region.

Possible role for calmodulin and the Ca2+/calmodulin-dependent protein kinase II in postsynaptic neurotransmission

The theory presented here is based on results from in vitro experiments and deals with three proteins in the postsynaptic density/membrane-namely, calmodulin, the Ca2+/calmodulin-dependent protein kinase, and the voltage-dependent Ca2+ channel. It is visualized that, in vivo in the polarized state of the membrane, calmodulin is bound to the kinase; upon depolarization of the membrane and the intrusion of Ca2+, Ca2(+)-bound calmodulin activates the autophosphorylation of the kinase. Calmodulin is visualized as having less affinity for the phosphorylated form of the kinase and is translocated to the voltage-dependent Ca2+ channel. There, with its bound Ca2+, it acts as a Ca2+ sensor, to close off the Ca2+ channel of the depolarized membrane. At the same time, it is thought that the configuration of the kinase is altered by its phosphorylated states; by interacting with Na+ and K+ channels, it alters the electrical properties of the membrane to regain the polarized state. Calmodulin is moved to the unphosphorylated kinase to complete the cycle, allowing the voltage-dependent Ca2+ channel to be receptive to Ca2+ flux upon the next cycle of depolarization. Thus, the theory tries to explain (i) why calmodulin and the kinase reside at the postsynaptic density/membrane site, and (ii) what function autophosphorylation of the kinase may play.

Calmodulin-dependent protein kinase II. Multifunctional roles in neuronal differentiation and synaptic plasticity

One of the most important mechanisms for regulating neuronal functions is through second messenger cascades that control protein kinases and the subsequent phosphorylation of substrate proteins. Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II) is the most abundant protein kinase in mammalian brain tissues, and the alpha-subunit of this kinase is the major protein and enzymatic molecule of synaptic junctions in many brain regions. CaM-kinase II regulates itself through a complex autophosphorylation mechanism whereby it becomes calcium-independent following its initial activation. This property has implicated CaM-kinase II as a potential molecular switch at central nervous system (CNS) synapses. Recent studies have suggested that CaM-kinase II is involved in many diverse phenomena such as epilepsy, sensory deprivation, ischemia, synapse formation, synaptic transmission, long-term potentiation, learning, and memory. During brain development, the expression of CaM-kinase II at both protein and mRNA levels coincides with the active periods of synapse formation and, therefore, factors regulating the genes encoding kinase subunits may play a role in the cell-to-cell recognition events that underlie neuronal differentiation and the establishment of mature synaptic functions. Recent findings have demonstrated that the mRNA encoding the alpha-subunit of CaM-kinase II is localized in neuronal dendrites. Current speculation suggests that the localized translation of dendritic mRNAs encoding specific synaptic proteins may be responsible for producing synapse-specific changes associated with the processing, storage, and retrieval of information in neural networks.

Autophosphorylation of neuronal calcium/calmodulin-stimulated protein kinase II

A unique feature of neuronal calcium/calmodulin-stimulated protein kinase II (CaM-PK II) is its autophosphorylation. A number of sites are involved and, depending on the in vitro conditions used, three serine and six threonine residues have been tentatively identified as autophosphorylation sites in the alpha subunit. These sites fall into three categories. Primary sites are phosphorylated in the presence of calcium and calmodulin, but under limiting conditions of temperature, ATP, Mg2+, or time. Secondary sites are phosphorylated in the presence of calcium and calmodulin under nonlimiting conditions. Autonomous sites are phosphorylated in the absence of calcium and calmodulin after initial phosphorylation of Thr-286. Mechanisms that lead to a decrease in CaM-PK II autophosphorylation include the thermolability of the enzyme and the activity of protein phosphatases. A range of in vitro inhibitors of CaM-PK II autophosphorylation have recently been identified. Autophosphorylation of CaM-PK II leads to a number of consequences in vitro, including generation of autonomous activity and subcellular redistribution, as well as alterations in conformation, activity, calmodulin binding, substrate specificity, and susceptibility to proteolysis. It is established that CaM-PK II is autophos-phorylated in neuronal cells under basal conditions. Depolarization and/or activation of receptors that lead to an increase in intracellular calcium induces a marked rise in the autophosphorylation of CaM-PK II in situ. The incorporation of phosphate is mainly found on Thr-286, but other sites are also phosphorylated at a slower rate. One consequence of the increase in CaM-PK II autophosphorylation in situ is an increase in the level of autonomous kinase activity. It is proposed that the formation of an autonomous enzyme is only one of the consequences of CaM-PK II autophosphorylation in situ and that some of the other consequences observed in vitro will also be seen. CaM-PK II is involved in the control of neuronal plasticity, including neurotransmitter release and long-term modulation of postreceptor events. In order to understand the function of CaM-PK II, it will be essential to ascertain more fully the mechanisms of its autophosphorylation in situ, including especially the sites involved, the consequences of this autophosphorylation for the kinase activity, and the relationships between the state of CaM-PK II autophosphorylation and the physiological events within neurons.

Contributions of dendritic spines and perforated synapses to synaptic plasticity

The dynamic nature of synaptic connections has presented morphologists with considerable problems which, from a structural perspective, have frustrated the development of ideas on synaptic plasticity. Gradually, however, progress has been made on concepts such as the structural remodelling and turnover of synapses. This has been considerably helped by the recent elaboration of unbiased stereological procedures. The major emphasis of this review is on naturally occurring synaptic plasticity, which is regarded as an ongoing process in the postdevelopmental CNS. The focus of attention are PSs, with their characteristically discontinuous synaptic active zone, since there is mounting evidence that this synaptic type is indicative of synaptic remodelling and turnover in the mature CNS. Since the majority of CNS synapses can only be considered in terms of their relationship to dendritic spines, the contribution of these spines to synaptic plasticity is discussed initially. Changes in the configuration of these spines appears to be crucial for the plasticity, and these can be viewed in terms of the significance of the cytoskeleton, of various dendritic organelles, and also of the biophysical properties of spines. Of the synaptic characteristics that may play a role in synaptic plasticity, the PSD, synaptic curvature, the spinule, coated vesicles, polyribosomes, and the spine apparatus have all been implicated. Each of these is assessed. Special emphasis is placed on PSs because of their ever-increasing significance in discussions of synaptic plasticity. The possibility of their being artefacts is dismissed on a number of grounds, including consideration of the results of serial section studies. Various roles, other than one in synaptic plasticity have been put forward in discussing PSs. Although relevant to synaptic plasticity, these include a role in increasing synaptic efficacy, as a more permanent type of synaptic connection, or as a route for the intercellular exchange of metabolites or membrane components. The consideration of many estimates of synaptic density, and of PS frequency, have proved misleading, since studies have reported diverse and sometimes low figures. A recent reassessment of PS frequency, using unbiased stereological procedures, has provided evidence that in some brain regions PSs may account for up to 40% of all synapses. All ideas that have been put forward to date regarding the role of PSs are examined, with particular attention being devoted to the major models of Nieto-Sampedro and co-workers.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of septal kindling on glutamate binding and calcium/calmodulin-dependent phosphorylation in a postsynaptic density fraction isolated from rat cerebral cortex

Postsynaptic density (PSD) fractions were isolated from the cerebral cortices of control and kindled rats and assayed for glutamate and gamma-aminobutyric acid-binding capacities and for the Ca2+/calmodulin-dependent protein kinase. Glutamate binding was found to be increased by approximately 50% in the PSDs isolated from kindled rats as compared to controls; this increase was almost completely from an increase in Bmax; Kd decreased only slightly. Studies with inhibitors indicate that the receptors involved were of the N-methyl-D-aspartate and quisqualate types. PSDs isolated from control and kindled rats did not differ in gamma-aminobutyric acid or flunitrazepam binding. The in vitro autophosphorylation of the Ca2+/calmodulin-dependent protein kinase was depressed by 45-76% in PSDs isolated from kindled rats as compared to controls, with little change in amount of the kinase. Therefore, we infer that (i) the kindled state is associated with an increase in glutamate activation of postsynaptic sites, allowing Ca2+ to enter dendritic spines, (ii) a change has occurred in activity of the protein kinase, which is the major cerebral cortex PSD protein, and (iii) perhaps major alterations in the PSD are a concomitant to the long-lasting nature of the kindled state.

Ca2+/calmodulin-dependent protein kinase II immunoreactivity in the rat hippocampus after forebrain ischemia

The influence of transient forebrain ischemia on the temporal alteration of Ca2+/calmodulin-dependent kinase II (CaM kinase II) in the rat hippocampus was analysed by the immunohistochemical method using antigen-affinity purified polyclonal antibodies against CaM kinase II of rat brain. Six to twenty-four hours after ischemia, CA1 and CA3 pyramidal cells, and dentate granule cells lost CaM kinase II immunoreactivity in neuronal perikarya, although immunoreactivity in the dendritic fields was preserved. The recovery of immunoreactivity of the CA3 pyramidal cells and dentate granule cells was noted 3 days after recirculation. Seven days after ischemia, immunoreactivity in the CA1 subfield was greatly reduced. These results suggest that CaM kinase II molecules in the CA1 subfield are preferentially located on the CA1 pyramidal cells and that CaM kinase II plays a critical role in the reconstruction of neuronal cytoskeleton and neuronal networks damaged by ischemic insult.

Properties of a protein kinase C activity in synaptic plasma membrane and postsynaptic density fractions isolated from canine cerebral cortex

Protein kinase C (PKC) activity (phosphorylation increased by addition of Ca2+/phosphatidylserine or Ca2+/phosphatidylserine/phorbol ester) was found in both a synaptic plasma membrane (SPM) and a postsynaptic density (PSD) fraction. The SPM fraction had as endogenous substrates 87K-, 60K-, 50K-, and 20K-Mr proteins, whereas the PSD fraction had only the 20K-Mr protein. The PKC activity was also detected using histone III-S as a substrate, in SPM but much less in PSD. Phosphorylations of histone and the endogenous substrates of PKC, assayed in the absence of Ca2+, were enhanced in the SPM prepared after treatment of brain homogenate with phorbol 12-myristate 13-acetate (TPA), but very little enhancement was found in PSD after such treatment. The SPM PKC activity (both for endogenous substrate proteins and for histone), which was enhanced by TPA treatment of brain homogenate, was inhibited by calcium (IC50, 3 x 10(-7) M). The phosphorylations of the 20K-Mr protein in PSD, and in SPM prepared with and without TPA treatment, were all inhibited by H-7. The 20K-Mr protein in the PSD fraction is also phosphorylated by a PSD Ca2+/calmodulin-dependent protein kinase II. The evidence indicates that both SPM and PSD fractions contain a PKC activity. Detergent treatment of SPM, to produce a purified PSD fraction, results in a PSD fraction that has lost most of the endogenous substrates, lost the TPA-induced enhanced activity assayed in the absence of Ca2+, and lost the inhibitory effect of low Ca2+ concentration.

Expression of calmodulin-dependent phosphodiesterase, calmodulin-dependent protein phosphatase, and other calmodulin-binding proteins in human SMS-KCNR neuroblastoma cells

Calmodulin (CaM)-dependent enzymes, such as CaM-dependent phosphodiesterase (CaM-PDE), CaM-dependent protein phosphatase (CN), and CaM-dependent protein kinase II (CaM kinase II), are found in high concentrations in differentiated mammalian neurons. In order to determine whether neuroblastoma cells express these CaM-dependent enzymes as a consequence of cellular differentiation, a series of experiments was performed on human SMS-KCNR neuroblastoma cells; these cells morphologically differentiate in response to retinoic acid and phorbol esters [12-O-tetradecanoylphorbol 13-acetate (TPA)]. Using biotinylated CaM overlay procedures, immunoblotting, and protein phosphorylation assays, we found that SMS-KCNR cells expressed CN and CaM-PDE, but did not appear to have other neuronal CaM-binding proteins. Exposure to retinoic acid, TPA, or conditioned media from human HTB-14 glioma cells did not markedly alter the expression of CaM-binding proteins; 21-day treatment with retinoic acid, however, did induce expression of novel CaM-binding proteins of 74 and 76 kilodaltons. Using affinity-purified polyclonal antibodies, CaM-PDE immunoreactivity was detected as a 75-kilodalton peptide in undifferentiated cells, but as a 61-kilodalton peptide in differentiated cells. CaM kinase II activity and subunit autophosphorylation was not evident in either undifferentiated or neurite-bearing cells; however, CaM-dependent phosphatase activity was seen. Immunoblot analysis with affinity-purified antibodies against CN indicated that this enzyme was present in SMS-KCNR cells regardless of their state of differentiation. Although SMS-KCNR cells did not show a complete pattern of neuronal CaM-binding proteins, particularly because CaM kinase II activity was lacking, they may be useful models for examination of CaM-PDE and CN expression. It is possible that CaM-dependent enzymes can be used as sensitive markers for terminal neuronal differentiation.

Calcium/calmodulin-dependent protein kinase activity in primary astrocyte cultures

Calcium, calmodulin-dependent protein kinase (Ca/CaM kinase) is an important component of calcium signalling mechanisms in the brain, but little is known about the properties of this protein phosphorylation system in astrocytes. Addition of calcium and calmodulin to supernatant or membrane fractions obtained from rat astrocytes in primary culture increased phosphate incorporation into an exogenously added substrate, casein, and into endogenous protein substrates; this increase was greater than that observed with either calcium alone or calmodulin alone. The calcium, calmodulin-stimulated increase was inhibited by trifluoperazine, and this inhibition could be overcome by the addition of excess calmodulin. The major substrates for Ca/CaM kinase activity were proteins with molecular weights of 59 and 53 kDa, which were similar, but not identical, to the subunits of Ca/CaM kinase type II from brain. The specific activity of Ca/CaM kinase and the phosphorylation of 59 kDa were increased in astrocyte cultures treated and maintained in dibutyryl cyclic adenosine monophosphate (dBcAMP). These results indicate that astrocytes contain Ca/CaM kinase activity and suggest an interaction between the cAMP and calcium/calmodulin messenger systems in these cells.

Presence of calcium/calmodulin-dependent protein kinase II in nerve terminals of rat brain

Calcium/calmodulin-dependent protein kinase type II, a multimeric 550-650 kilodalton enzyme composed of major alpha (50 kilodalton) and beta/beta' (60/58 kilodalton) subunits, is present in high concentrations in mammalian brain. Previous immunocytochemical studies indicated that the enzyme is enriched in cell bodies and dendrites, but did not show a clear-cut localization in nerve terminals. The present study presents evidence, using lesion-induced degenerations of pre- and postsynaptic neuronal populations in the neostriatum and substantia nigra, that calcium/calmodulin-dependent protein kinase II, as measured both by autophosphorylation of enzyme subunits and by synapsin I kinase activity, is present in high concentrations in several populations of presynaptic terminals. Lesions of the corticostriatal tract decreased the amount of enzyme by 30-40% in the neostriatum, a decrease similar to that seen in the same region of synapsin I, a general nerve terminal marker. Lesions of the striatonigral tract induced an even more pronounced decrease of the enzyme in the substantia nigra; this decrease was larger than the lesion-induced change of synapsin I seen in the same region. Our data therefore indicate that certain nerve terminal populations in the rat brain contain high levels of calcium/calmodulin-dependent protein kinase II.

Cell-specific localization of the alpha-subunit of calcium/calmodulin-dependent protein kinase II in Purkinje cells in rodent cerebellum

Brain calcium/calmodulin-dependent protein kinase type II, a multimeric 600-650 kDa enzyme composed of alpha- (50 kDa) and beta/beta' (60 and 58 kDa) subunits, may be formed by alpha- and beta-subunits combining in variable proportions in different types of neurons. This study presents evidence, using cerebella from mutant mice, that the alpha-subunit displays a restricted localization in the rodent cerebellum, being detectable only in Purkinje cells. Immunocytochemical analysis of normal rat cerebellum with an antibody selective for the alpha-subunit confirmed that this subunit was detectable only in Purkinje cells. In contrast, the beta/beta'-subunits appeared to be present in all types of cerebellar mutants examined. These results indicate that different cells of the cerebellum express distinct isozymic forms of the multifunctional calcium/calmodulin-dependent protein kinase type II. It appears that Purkinje cells primarily contain an isoenzyme formed by both alpha- and beta/beta'-subunits, and that non-Purkinje cells contain an isoenzyme formed primarily by beta/beta'-subunits.

Immunohistochemical localization of Ca2+/calmodulin-dependent protein kinase II in rat brain and various tissues

Polyclonal antibodies against Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) of rat brain were prepared by immunizing rabbits and then purified by antigen-affinity column. The antibodies which recognized both subunits of the enzyme with Mrs 49K and 60K were used for the study on the distribution of CaM kinase II in formalin-fixed, paraffin-embedded tissues. In the brain, a light-microscopic study demonstrated strong immunoreactivity in neuronal somata and dendrites and weak immunoreactivity in nuclei. The densely stained regions included cerebral cortex, hippocampal formation, striatum, substantia nigra, and cerebellar cortex. In substantia nigra, neurites were stained, but not neuronal somata. Electron microscopy revealed that the immunoreactive product was highly concentrated at the postsynaptic densities. In addition to neurons, weak immunoreactivity was also demonstrated in glial cells, such as astrocytes and ependymal cells of ventricles and epithelial cells of choroid plexus. In other tissues, strong immunoreactivity was observed in the islet of pancreas and moderate immunoreactivity in skeletal muscle and kidney tubules. Immunoreactivity was demonstrated in all of the tissues tested. The results suggest that CaM kinase II is widely distributed in the tissues.

Developmental changes in protein phosphorylation in chicken forebrain. II. Calmodulin stimulated phosphorylation

The development of calmodulin stimulated protein phosphorylation, with particular reference to calmodulin-stimulated protein kinase II (CMK II), was investigated in 3 subcellular fractions of chicken forebrain: cytosol (S3), crude synaptic plasma membranes (P2-M) and occluded cytosol (P2-S). Changes in the level of calmodulin-stimulated phosphorylation of endogenous proteins occurred over a protracted time course and were not complete until after day 52 post-hatching. By day 15 post-hatching, calmodulin-stimulated phosphoproteins characteristic of embryonic fractions had all disappeared and those characteristic of adult tissue were present but not necessarily at their mature levels. The levels of CMK II were estimated from the autophosphorylation of the alpha-subunit which was the only phosphoprotein present at 53,000 Da in the 3 fractions. Overall, calmodulin-stimulated phosphorylation and CMK II levels were low in embryonic brain and high in adult brain but two specific changes in CMK II were observed during development: (1) although CMK II concentrations increased in both membrane and cytosolic fractions until day 23 the kinase was predominantly cytoplasmic (approximately 75%) until day 23, after which it became increasingly membrane bound so that by day 52 post-hatching the majority of CMK II was present in the synaptic membrane fraction, and (2) the relative concentrations of the alpha- and beta-subunits changed from an alpha:beta-value of approximately 1:1 in the 19 day embryo to approximately 1:2 by 15 days post-hatch after which no further change was seen. The occurrence of major changes in the calmodulin stimulated protein phosphorylation system for up to 6-8 weeks after synapse formation is completed in the forebrain, provides further support for the existence of a synapse maturation phase of neuronal differentiation which is distinct from synapse formation. This phase involves only a specific subset of the developmental changes occurring in the calmodulin-stimulated phosphorylation system.

Neurochemical characteristics of a postsynaptic density fraction isolated from adult canine hippocampus

Postsynaptic density and synaptic membrane fractions isolated from hippocampal tissue have been compared to those previously isolated from cerebellum and cerebral cortex. In all respects examined, the isolated hippocampal preparations are similar to the cerebral cortex fractions. The morphology of the postsynaptic density (PSD) preparation is the same and the protein composition is similar, but with higher concentrations of the 51-kDa major protein and of calmodulin, and lower concentrations of actin, in the hippocampal PSD fraction. The binding characteristics for glutamate and GABA are also similar between the two fractions, but with higher Bmax and KD glutamate values and lower Bmax and higher KD GABA values for the hippocampal PSD preparation. Both preparations contain GABAA and GABAB receptors. The PSD fraction contains, as does the cerebral cortex fraction, a calmodulin-dependent binding of the Ca2+ channel antagonist, nitrendipine, as well as a cAMP-dependent and a Ca2+/calmodulin-dependent protein kinase, with the same respective substrates. The value of the hippocampal fractions for studies on long-term potentiation and on kindling in the hippocampus is discussed.

Active site-directed inhibition of Ca2+/calmodulin-dependent protein kinase type II by a bifunctional calmodulin-binding peptide

The activation of Ca2+/calmodulin (CaM)-dependent protein kinase II (CaM-KII) by Ca2+/CaM results in autophosphorylation and the generation of Ca2+/CaM-independent enzyme activity. We postulated that CaM binding and subsequent autophosphorylation alters the conformation of CaM-KII and exposes its substrate-binding and catalytic site(s). Previous peptide mapping studies on CaM-KII demonstrated the close proximity of CaM-binding and autophosphorylation domains. Analyses of the deduced amino acid sequences encoding CaM-KII have allowed the identification of its CaM-binding domain and have revealed two consensus phosphorylation sites that flank this regulatory domain. We report herein the distinct properties of two synthetic peptides modeled after the CaM-binding domain of CaM-KII. The first peptide binds CaM in a Ca2+-dependent manner and is an antagonist of CaM-KII activation (IC50 approximately equal to 75 nM). It does not, however, inhibit CaM-KII activity. A second peptide containing the same CaM-binding domain plus a putative autophosphorylation sequence at its N terminus displayed bifunctional regulatory properties. In addition to being a CaM antagonist, the latter was a potent inhibitor of Ca2+/CaM-independent kinase activity (IC50 approximately equal to 2 microM). We suggest that this bifunctional peptide represents an active site-directed inhibitory element of CaM-KII. The separation of CaM antagonist and active site-directed inhibitory properties of this peptide distinguishes CaM-KII from other CaM-dependent enzymes in which bifunctional regulatory properties appear to reside in the same peptide domain. These results indicate that the definition of site-directed inhibitory peptides should, in some cases, be expanded to include bona fide phosphorylation sites.

Distribution of calmodulin- and cyclic AMP-stimulated protein kinases in synaptosomes

The subcellular location of calmodulin- and cyclic AMP stimulated protein kinases was assessed in synaptosomes which were prepared on Percoll density gradients. The distribution of the protein kinases between the outside and the inside and between the soluble and membrane fractions was determined by incubating intact and lysed synaptosomes, as well as supernatant and pellet fractions obtained from lysed synaptosomes, in the presence of [gamma-32P]ATP. Protein kinase activity was assessed by the labelling of endogenous proteins, or exogenous peptide substrates, under conditions optimized for either calmodulin- or cyclic AMP-stimulated protein phosphorylation. When assessed by calmodulin-stimulated autophosphorylation of the alpha subunit of calmodulin kinase II, 44% of this enzyme was on the outside of synaptosomes, and 41% was in the 100,000 g supernatant. Using an exogenous peptide substrate, the distribution of total calmodulin-stimulated kinase activity was 27% on the outside and 34% in the supernatant. The high proportion of calmodulin kinase II on the outside of synaptosomes is consistent with its known localization at postsynaptic densities. The proportion of calmodulin kinase II which was soluble depended on the ionic strength conditions used to prepare the supernatant, but the results suggest that a major proportion of this enzyme which is inside synaptosomes is soluble. When assessed by cyclic AMP-stimulated phosphorylation of endogenous substrates, no cyclic AMP-stimulated kinase activity was observed on the outside of synaptosomes, whereas 21% was found with an exogenous peptide substrate. This suggests that if endogenous substrates are present on the outside of synaptosomes, then the enzyme does not have access to them. The cyclic AMP-stimulated protein kinase present inside synaptosomes was largely bound to membranes and/or the cytoskeleton, with only 10% found in the supernatant when assessed by endogenous protein phosphorylation and 25% with an exogenous substrate. The markedly different distribution of the calmodulin- and cyclic AMP-stimulated protein kinases presumably reflects differences in the functions of these enzymes at synapses.

The binding of a monoclonal antibody reactive with pp60v-src to the rat CNS both in vitro and in vivo: evidence that the epitope is present intracellularly as well as being associated with a number of antigenically related polypeptides located externally in the plasma membrane only in the synaptic region

A monoclonal antibody, F4, has been produced which reacts with an epitope possessing an unusual subcellular distribution. It binds to the external surface of the neuronal plasma membrane only in the region of the synapse. This is evidenced by binding of F4 to presynaptic terminals in unfixed cultures of rat cerebellum and to preparations of unfixed synaptosomes. In addition, much larger amounts of the epitope are present intracellularly. In fixed nervous tissue, the epitope is found in many neurons, and is associated mainly with presynaptic plasma membranes, synaptic vesicles, postsynaptic densities (cerebral cortex and hippocampus, but not cerebellum), rough endoplasmic reticulum, and the Golgi apparatus. The epitope is especially abundant in large neurons (e.g. pyramidal cells). Similar amounts of epitope are present in the chromaffin cells of the adrenal medulla. It is also expressed in ependymal cells in the brain, and in epithelial cells present in ducts of the medulla, but not cortex, of the kidney. However, the epitope is not found in glial cells in the brain, or in either liver, spleen, skeletal muscle, or testes. F4 is not species specific, as it binds to postmortem adult human cerebral cortex and neonatal cerebellum in a manner as described for the rat. It also binds to homogenates of brains of fish, chicken and mouse. The appearance of the epitope during development of the cerebellum in vivo and in vitro occurs in parallel with the differentiation of neurons and formation of synapses, though small amounts are also present in neuronal precursor cells. The F4 antibody can detect nanogram amounts of pp60v-src on immunodots. The strength of this reaction is high enough that F4 can be used to demonstrate pp60v-src-like immunoreactivity in Rous Sarcoma virus-transformed chick embryo fibroblasts. However, present evidence suggests that it may be premature to assign the immunocytochemical reactivity of F4 in the brain exclusively to pp60c-src. This conclusion is based on the fact that F4 reacts with several polypeptides from synaptic plasma membranes on Western blots of renaturing, two-dimensional gels that are dissimilar in size to pp60c-src, and from the fact that it can cross-react, albeit weakly, with several other serine protein kinases in an immunodot assay. Appreciation of this cross-reactivity, and of the evolutionary conservation of the epitope, as well as its sensitivity to denaturation, has led to our working hypothesis that F4 binds to a conformational epitope present on several polypeptides that may be most perfectly represented by some aspect of the catalytic domain of tyrosine protein kinases.

Phosphorylation of synaptic proteins in chick forebrain: changes with development and passive avoidance training

We have used synaptic plasma membranes (SPMs) and postsynaptic densities (PSDs) to study protein phosphorylation at the synapse in the developing chick forebrain and in 1-day-old chick forebrain following training on a passive avoidance task. Endogenous phosphorylation patterns in SPMs and PSDs prepared by extraction with n-octylglucoside isolated from chick forebrain were investigated by labelling with [32P]ATP. The phosphoprotein components of the SPM and PSD fractions were separated using sodium dodecyl sulphate gradient polyacrylamide gel electrophoresis. Autoradiography and densitometry of the Coomassie Blue protein staining pattern revealed phosphate incorporation into several SPM components including those of molecular mass 52, 37, and 29 kilodaltons (kDa). Bands of similar molecular mass were not phosphorylated in PSD fractions. This difference in phosphorylation between SPMs and PSDs was not due to the detergent n-octylglucoside. In a developmental study in which SPM and PSD fractions were prepared from 1-day-old, 14-day-old, and 21-day-old chickens, the phosphorylation patterns of SPMs were similar throughout, but striking differences occurred in PSDs, both in the level of phosphorylation and in the components phosphorylated. A time-course study was carried out in which phosphorylation of SPMs and PSDs from 1-day-old chicks trained on a passive avoidance task was compared with patterns from control chicks trained on a water-coated bead and untrained chicks. In SPMs prepared from forebrains removed 10 mins following training, a consistent but nonsignificant decrease (-21%) in phosphorylation of a 52 kDa band occurred in chicks with passive avoidance training compared with water-trained and untrained chicks.(ABSTRACT TRUNCATED AT 250 WORDS)

Calmodulin-dependent multifunctional protein kinase in Aspergillus nidulans

A Ca2+/calmodulin (CaM)-dependent multifunctional protein kinase has been isolated from Aspergillus nidulans and purified to homogeneity. Unlike any CaM-dependent multifunctional protein kinase described previously, the native enzyme from Aspergillus behaves as a monomer. The calculated molecular weight is 41,200. NaDodSO4/PAGE reveals a single protein band with an apparent Mr of 51,000. Two-dimensional isoelectric focusing/NaDodSO4/PAGE of the purified enzyme showed one major and one minor more acidic Coomassie blue-stained spot, both of which bind 125I-labeled calmodulin in a calcium-dependent manner. The kinase is autophosphorylated in a calcium- and CaM-dependent manner, yielding an increase in the amount and number of more acidic forms of the enzyme. The Aspergillus kinase catalyzes the Ca2+/CaM-dependent phosphorylation of known substrates of type II Ca2+/CaM-dependent protein kinases, including glycogen synthase, microtubule-associated protein 2, synapsin, tubulin, gizzard myosin light chain, and casein. Cross-reactivity between antiserum raised against native rat brain protein kinase II and 125I-labeled Aspergillus kinase has been detected. Two forms of CaM have been isolated from Aspergillus nidulans, both of which activate the Aspergillus kinase at lower concentrations than that required for activation by bovine brain CaM.

Classical conditioning induces long-term translocation of protein kinase C in rabbit hippocampal CA1 cells

The role of the Ca2+/phospholipid-dependent, diacylglycerol-activated enzyme protein kinase C (PKC) in rabbit eyelid conditioning was examined. PKC was partially purified from the CA1 region of hippocampal slices from naive, pseudoconditioned, and conditioned rabbits 24 hr after the rabbits were well conditioned. Crude membrane and cytosol fractions were prepared. In conditioned rabbits, significantly more PKC activity (63.3%) was associated with the membrane fraction (and significantly less with the cytosol fraction) compared to naive (42.0%) and pseudoconditioned (44.7%) animals. These differences in distribution of enzyme activity were paralleled by differences in stimulation of enzyme activity by Ca2+, phospholipid, and diacylglycerol. There were no between-group differences in basal protein kinase activity. These results suggest that there is a long-term translocation of PKC from cytosol to membrane as a result of conditioning. Autoradiographic binding of radioactive phorbol 12,13-dibutyrate to PKC demonstrated that almost all specific binding was in the stratum radiatum, a region containing the proximal apical dendrites of CA1 pyramidal neurons. Therefore, this may be the site of the conditioning-specific PKC translocation, a locus well-suited to underlie the biophysical effects of conditioning.

Two developmentally regulated isoenzymes of calmodulin-stimulated protein kinase II in rat forebrain

Soluble calmodulin-stimulated protein kinase II has been purified from adult and 10-day-old rat forebrain. By autoradiography, the alpha/beta subunit ratios of the 10-day and adult enzymes were 0.67 +/- 0.03 and 2.20 +/- 0.15, respectively. By silver staining, the alpha/beta subunit ratios were 1.02 +/- 0.06 and 2.36 +/- 0.10, respectively. The apparent holoenzyme molecular masses of the purified 10-day and adult enzymes were 500,000 daltons and 700,000 daltons. However, varying the purification conditions revealed higher and lower molecular mass forms at both ages and suggested that the form of the kinase that is usually purified is merely that which has the highest affinity for calmodulin-Sepharose and may not be the form of the kinase that exists in vivo. The subunits of the adult and 10-day enzymes were indistinguishable by one- and two-dimensional electrophoresis and one-dimensional proteolytic peptide maps. These results are consistent with the suggestion that at least two developmentally regulated isoenzymes of this kinase exist in rat forebrain.

Characterization of a soluble Mr-30,000 catalytic fragment of the neuronal calmodulin-dependent protein kinase II

Chymotryptic digestion of postsynaptic densities releases a soluble, catalytically active fragment of the alpha (Mr 50,000) subunit of the neuronal cytoskeletal calmodulin-dependent protein kinase II. The purified soluble form of the kinase likewise yields the fragment. Denaturation of the enzyme results in more extensive proteolytic degradation. 125I-Iodopeptide maps of the isolated catalytic portions of both forms of the enzyme are similar and are contained within the map of the isolated alpha subunit. Catalytic fragments of both forms of the enzyme comigrate on two-dimensional SDS-PAGE/isoelectric focusing with pI 6.7-7.2. The fragment phosphorylates microtubule-associated protein (MAP-2) but is not activated by Ca+2/calmodulin nor is it inhibited by trifluoperazine. Km values for MAP-2 and ATP are indistinguishable from those of the holoenzyme, while the Vmax is similar to that of the holoenzyme activated with Ca+2/calmodulin. Overlays of Western blots of fragment with 125I-calmodulin shows a loss of calmodulin binding. Both the number of phosphorylation sites and the ability to autophosphorylate are markedly reduced in the catalytic fragment. Evaluation of the hydrodynamic parameters of the purified fragment yielded Mr value of 25,600 with a frictional ratio (f/f0) of 1.12; the Mr value determined by SDS-PAGE was 30,000. Thus, the catalytic fragment appears to represent an activated form of the kinase with a monomeric, globular structure unlike the native enzyme which exhibits oligomerization and cytoskeletal association. These results are consistent with a tertiary structure for the calmodulin-dependent protein kinase that contains distinct domains responsible for catalytic activity, regulation by calmodulin, cytoskeletal association and the multimeric organization of enzyme subunits.

Nature and immunochemical characteristics of a Ca2+/calmodulin kinase activity endowed in a highly insoluble protein purified from adult rat brain

Using sequential extraction procedure of proteins from adult rat forebrain, a protein of Mr 52,000, insoluble in neutral detergents, capable of binding calmodulin in the presence of Ca2+, was isolated. Antibodies to this antigen had the capacity to inhibit the Ca2+/calmodulin-dependent kinase activity associated with this protein. This protein (52K) (in many respects identical to the major protein of postsynaptic densities) shares by itself the Ca2+/calmodulin-dependent kinase activity, thus differing from soluble Ca2+/calmodulin-dependent kinases isolated by others. Despite its insolubility in most detergents, the 52K protein is not particularly rich in hydrophobic amino acids. Its richness in cysteine and proline residues suggests that the active conformation of the enzyme is sustained by numerous disulfide bridges.

Independent protein kinases associated with the rat cerebral synaptic junction: comparison with cyclic AMP-dependent and Ca2+/calmodulin-dependent protein kinases in the synaptic junction

Independent protein kinases in the synaptic junction (SJ) isolated from rat cerebrum were characterized. SJ showed a protein kinase activity, phosphorylating intrinsic proteins, even in the absence of cyclic AMP or Ca2+ plus calmodulin (CaM) exogenously added. The activity was affected neither by Ca2+ concentrations in the physiological fluctuation range nor by the addition of specific ligands such as glutamate, aspartate, acetylcholine, and concanavalin A. The activity was not due to cyclic AMP-dependent protein kinase in SJ, since the activity was not inhibited by an inhibitor protein for cyclic AMP-dependent protein kinase, and since synapsin I was not specifically phosphorylated whereas cyclic AMP-dependent kinase appeared to phosphorylate selectively the protein in SJ. Phosphorylation of SJ proteins by the independent kinases was about one-third of that of the Ca2+/CaM-dependent protein kinase intrinsic to SJ. The apparent Km for ATP was estimated to be 700 microM. Proteins of 16K Mr and 117K Mr were specifically phosphorylated under the basic condition (in the absence of the substances known to activate specifically protein kinases), as well as six other proteins both under the basic conditions and in the presence of Ca2+ and CaM. The phosphorylation of 150K Mr, 60K Mr, 51K Mr, and 16K Mr SJ proteins was enhanced after prephosphorylation of SJ proteins by intrinsic kinase in the presence of Ca2+ and CaM.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of digestion with phospholipase A2 on endogenous protein phosphorylation in particulate fractions from rat brain synaptosomes

The endogenous phosphorylation of synapsin 1 in cyclic AMP-containing media was greatly decreased by digestion of synaptic vesicles and synaptosomal membranes with phospholipase A2, suggesting that the system is functionally dependent on the membrane structure. Treatment of the synaptic vesicle fraction with phospholipase A2 also caused a small but significant inhibition of the Ca2+/calmodulin-dependent phosphorylation of the same protein. The Ca2+/calmodulin-dependent phosphorylation of other major acceptors, and the basal phosphorylation of a 52-kD acceptor enriched in the vesicle fraction, remained unchanged after cleavage of the membrane phospholipids with phospholipase A2. The significance of the selective effect of phospholipase A2 treatment on endogenous membrane phosphorylation is discussed.

Chronic lithium treatment increases the phosphorylation of a 64-kDa protein in rat brains

Despite the wide clinical use of lithium in the treatment of manic depressive illness there is no adequate explanation for its mechanism of action. In the light of lithium's suggestive effects on the second messenger system in the brain, we studied the effects of chronic dietary lithium treatment (achieving blood levels in the therapeutic range) on protein phosphorylation in different areas of rat brain. An increase in the phosphorylation of a 64-kDa membrane-associated protein was evident in the lithium-treated rats compared to controls. This increase was observed only under basal phosphorylating conditions and was abolished when the phosphorylation was performed in the presence of Ca2+ or Ca2+ and calmodulin. The possibility that this 64-kDa protein affected by lithium is the beta-subunit of the calmodulin-dependent protein kinase or a different protein which co-migrates with it is discussed.

Demonstration by phase-partitioning in Triton X-114 solutions that phosphoprotein B-50 (F-1) from rat brain is an integral membrane protein

The Triton X-114 phase separation technique was employed to fractionate phosphoproteins present in membrane fragments from rat brain. Membranes were labelled with [gamma-32P]ATP in media containing Ca2+, Ca2+ plus calmodulin or cyclic AMP, and then treated with Triton X-114. Phosphoproteins recovered in the detergent-insoluble fraction, aqueous and detergent phases were detected by SDS-polyacrylamide gel electrophoresis and autoradiography. Of the proteins solubilised by the detergent, a known substrate of protein kinase C, the B-50 phosphoprotein (45 kD; also known as F-1), partitioned quantitatively into the detergent-rich phase, making it very probable that this phosphoprotein is an integral membrane protein. The detergent-rich phase also contained an 80 kD phosphoprotein, which probably corresponds to the widespread acidic 87 kD substrate of protein kinase C.

Molecular mechanisms of neuronal excitability: possible involvement of CaM kinase II in seizure activity

A type II calmodulin-dependent protein kinase (CaM kinase II) has been characterized in the synaptic region and may mediate some of the effects of Ca2+ on neuronal excitability. The activity of CaM kinase II is inhibited by anticonvulsant compounds and may be the molecular basis of their neuro-modulatory effects. The direct injection of purified CaM kinase II into invertebrate neurons has demonstrated that this kinase can directly alter specific ion conductances and neuronal activity. A long-lasting decrease in CaM kinase II activity is associated with septal kindling, an experimental model of epilepsy and long-term memory. In summary, CaM kinase II appears to be a central mediator of the effects of Ca2+ on neuronal function. Further investigation of this enzyme and its effects on neuronal activity may provide a molecular insight into an endogenous mechanism for modulating some of the effects of Ca2+ on neuronal excitability and may increase our understanding of the complex regulatory mechanisms that underlie the pathogenesis of seizure discharge and its regulation by anticonvulsant compounds.

Characteristics of a Ca2+/calmodulin-dependent binding of the Ca2+ channel antagonist, nitrendipine, to a postsynaptic density fraction isolated from canine cerebral cortex

Synaptic membrane (SM) and postsynaptic density (PSD) fractions isolated from the cerebral cortex (CTX) and cerebellum (CL) of the canine brain were found to contain one class of specific nitrendipine binding sites. The specific binding constants were: CTX-SM, Kd = 110 pM (Bmax = 126 fmol/mg protein); CTX-PSD, Kd = 207 pM (Bmax = 196 fmol/mg); CL-SM, Kd = 100 pM (Bmax = 65 fmol/mg); CL-PSD, Kd = 189 pM (Bmax = 80 fmol/mg). Treatment of the CTX-SM and CTX-PSD fractions with 0.5% deoxycholate and 1.0% N-lauroyl sarcosinate removed 88-91% and 42-51% of the nitrendipine binding, respectively, indicating that the major nitrendipine binding present in the SM fractions are of non-synaptic origin. Moreover, the percentages of total protein and specific nitrendipine binding removed from PSDs by these detergents were similar, indicating no preferential dissociation of the latter, and suggesting that the receptor protein is firmly bound and is probably an intrinsic component of the PSD fraction. Both Ca2+ and calmodulin were found to be important for the binding of nitrendipine to the CTX-SM and CTX-PSD fractions since: R24571, a calmodulin antagonist, was found to inhibit nitrendipine binding to the CTX-SM and CTX-PSD fractions with IC50 values of 1.1 microM and 0.9 microM, respectively; removal of Ca2+ from the CTX-SM and CTX-PSD fractions with 0.2 mM EGTA resulted in losses of specific nitrendipine binding of 80 and 90%, respectively; Ca2+ alone restored nitrendipine binding to EGTA-pretreated CTX-SM fractions and not to CTX-PSD fractions, with the latter needing both Ca2+ and calmodulin to restore nitrendipine binding; EGTA treatment removed 14-16% and 89-91% of nitrendipine bound to the CTX-SM and CTX-PSD fractions, respectively, suggesting that calmodulin (but not Ca2+) is needed to maintain the nitrendipine-nitrendipine receptor-calmodulin complex; Ca2+-reconstituted EGTA-pretreated CTX-SM fractions and the Ca2+ plus calmodulin-reconstituted EGTA-pretreated CTX-SM and CTX-PSD fractions were found to have similar binding constants to those for the corresponding native, untreated fractions; and the Ca2+/calmodulin dependency on nitrendipine binding was similar to the well-known Ca2+/calmodulin dependency on phosphorylation in EGTA-pretreated PSD fractions. It needed much less Ca2+ to saturate Ca2+/calmodulin-dependent phosphorylation of the pretreated CTX-PSD fractions than the nitrendipine binding. Yet, less calmodulin was needed to saturate nitrendipine binding than the phosphorylation.(ABSTRACT TRUNCATED AT 400 WORDS)

Subcellular distribution of a calmodulin-dependent protein kinase activity in rat cerebral cortex during development

The postnatal development of calmodulin-stimulated phosphorylation of endogenous proteins, in particular the autophosphorylated subunits of the calmodulin-stimulated protein kinase II, were investigated in subcellular fractions of rat cerebral cortex. The major subunit had a mol. wt. of 53,000 Da (designated 50 kDa) and the minor one a mol. wt. of 63,000 Da (designated 60 kDa). The 50-kDa subunit was found to be the only significant phosphoprotein in each fraction and throughout development at its molecular weight. However, the 60-kDa subunit was found to comigrate with other phosphoproteins that accounted for up to 15% of the radioactivity at this molecular weight and which differed between the fractions. 50-kDa autophosphorylation was found to be 3-fold greater in cytoplasmic fractions at day 10 and by adults was evenly distributed between membrane and cytoplasmic fractions. A similar pattern was also found for the total calmodulin-stimulated phosphorylation. Changes in autophosphorylation activity of the 50-kDa subunit were found to represent changes in kinase activity rather than alterations in phosphatase activity. In the membrane, this change was shown to be due to changes in the amount of enzyme. Although in the adult autophosphorylation activity is evenly distributed between membrane and soluble fractions, when differences in phosphatase activity and lack of autophosphorylation activity of the majority of post-synaptic density-associated kinase is taken into account, it is clear that the vast majority of the enzyme is membrane-bound. Phosphorylation of endogenous substrates paralleled the development of 50-kDa subunit autophosphorylation, most of which occurred between day 14 and day 30, a period which follows the most rapid phase of synaptogenesis. This pattern was different from that of the phosphorylation of myelin basic protein and two substrates of the calcium-phospholipid-dependent protein kinase. There was also a change in the ratio of autophosphorylation activity of the 50-kDa and 60-kDa subunits during development which appears to be due to a change in the amount of the subunits themselves. This ratio was the same in all fractions at any one age. We suggest that this change is due to the existence of at least two developmentally regulated isoenzymes in the cortex.

Characterization of Ca2+/calmodulin-dependent protein kinase associated with rat cerebral synaptic junction: substrate specificity and effect of autophosphorylation

The Ca2+/calmodulin (CaM)-dependent protein kinase associated with rat cerebral synaptic junction (SJ) was characterized, using the SJ fraction as the enzyme preparation, to clarify the functional significance of the enzyme in situ. The protein kinase was greatly activated in the presence of micromolar concentrations of both Ca2+ and calmodulin (EC50 for Ca2+, 1.0 microM; that for CaM, 100 nM). The Km for ATP was 150 microM. SJ proteins were phosphorylated without a lag time, and the phosphorylation reached its maximum within 2-10 min at 25 degrees C. The endogenous substrates consisted of four major (160K, 120K, 60K, and 51K Mr) and 10 minor proteins. Compared with the endogenous substrate phosphorylation, the phosphorylation of exogenously added proteins (myosin light chains from chicken muscle, casein, arginine-rich histone, microtubule-associated protein-2, tau-protein, and tubulin) was weak, although they are expected to be good substrates for the soluble form of the Ca2+/CaM-dependent protein kinase. Autophosphorylation of the enzyme in SJ inhibited its activity and did not alter the subcellular distribution of the enzyme.

Involvement of fodrin-binding proteins in the structure of the neuronal postsynaptic density and regulation by phosphorylation

Novel polypeptides with Mr values about 140,000 bind fodrin and spectrin and are enriched in the postsynaptic density (PSD) compared to other tissues or subcellular fractions. 125I-fodrin binding to these polypeptides is competed for by unlabeled spectrin. These polypeptides are distinct from ankyrin and its proteolytic fragments and from band 4.1 which also bind fodrin. Phosphorylation of PSDs by the endogenous calmodulin-dependent protein kinase markedly reduces 125I-fodrin binding to the transblotted preparation. Such an event may play a regulatory role in governing protein-protein interactions among elements of the PSD.