Characterization of protein kinase C activities in postsynaptic density fractions prepared from cerebral cortex, hippocampus, and cerebellum

Regulation of phosphorylation at Ser(1303) of GluN2B receptor in the postsynaptic density

Neuronal N-methyl-D-aspartate subtype of ionotropic glutamate receptor (NMDAR) that plays essential roles in excitatory synaptic transmission is regulated by phosphorylation. However, the kinases and phosphatases involved in this regulation are not completely known. We show that the GluN2B subunit of NMDAR is phosphorylated at Ser(1303) by protein kinase C (PKC) and is dephosphorylated by protein phosphatase 1 (PP1), but not protein phosphatase 2A (PP2A) in isolated postsynaptic density (PSD). Although PSD is known to harbor PKC, PP1 and PP2A, their ability to regulate phosphorylation of GluN2B-Ser(1303) would depend on the accessibility of GluN2B-Ser(1303) to these proteins. Since PSD preparation is likely to maintain the organization of its component proteins as inside neurons, accessibility of kinases and phosphatases to GluN2B-Ser(1303)in vivo would be addressed by experiments using this system. Using an antibody specific for the phosphorylated state of GluN2B-Ser(1303) we demonstrate that PP1 is the major phosphatase in rat brain PSD that can dephosphorylate the GluN2B-Ser(1303) endogenous to PSD. We also show that PKC present in PSD can phosphorylate GluN2B-Ser(1303). The events reported here might be important in regulating GluN2B-Ser(1303) phosphorylation in vivo.

The postsynaptic density

Glutamatergic synapses in the central nervous system are characterized by an electron-dense web underneath the postsynaptic membrane; this web is called the postsynaptic density (PSD). PSDs are composed of a dense network of several hundred proteins, creating a macromolecular complex that serves a wide range of functions. Prominent PSD proteins such as members of the MaGuk or ProSAP/Shank family build up a dense scaffold that creates an interface between clustered membrane-bound receptors, cell adhesion molecules and the actin-based cytoskeleton. Moreover, kinases, phosphatases and several proteins of different signalling pathways are specifically localized within the spine/PSD compartment. Small GTPases and regulating proteins are also enriched in PSDs being the molecular basis for regulated structural changes of cytoskeletal components within the synapse in response to external or internal stimuli, e.g. synaptic activation. This synaptic rearrangement (structural plasticity) is a rapid process and is believed to underlie learning and memory formation. The characterization of synapse/PSD proteins is especially important in the light of recent data suggesting that several mental disorders have their molecular defect at the synapse/PSD level.

Rapid isolation of synaptoneurosomes and postsynaptic densities from adult mouse hippocampus

Previous postsynaptic density (PSD) isolation methodologies have utilized either whole brain or discrete brain regions of relatively large mammals such as dogs and rats. The present report details a simple and highly effective procedure for the rapid isolation of PSDs from small amounts of adult mouse hippocampus that has several advantages. First, by substituting synaptoneurosomes for synaptosomes as starting material, we have decreased the steps, time, and amount of tissue required to isolate PSDs. Second, by modifying critical steps in the synaptic isolation protocols we were able to isolate PSDs from less than 200 mg of mouse hippocampi in 3 h. Electron micrographs of isolated synaptoneurosomes showed presynaptic vesicles and densely stained membranes representing PSDs. Morphological examination of these PSDs by electron microscopy revealed a preparation that seems to be quite pure, with little or no membrane contamination. A comparison by Western blot analysis of synaptoneurosome and PSD fractions suggests that this technique yields a purified sample. Moreover, two different protocols using swing and fixed bucket rotors were used for this small-scale PSD isolation and both resulted in a very pure partition, supporting the idea that this procedure is reliable and consistent.

Identification of novel phosphorylation sites on postsynaptic density proteins

Phosphorylation of the components of the postsynaptic density (PSD), a protein complex lining the postsynaptic membrane, may regulate synaptic structure and function. We carried out mass spectrometric analyses to identify phosphorylation sites on PSD proteins. Phosphopeptides were isolated from the total tryptic digest of a PSD fraction by immobilized metal affinity chromatography and analyzed by liquid chromatography and tandem mass spectrometry. The phosphorylated residues detected following in vitro phosphorylation in the presence of Ca2+/calmodulin included S-1058 on SynGAP and S-1662 and S-1668 on Shank3. Other phosphorylated residues were identified in control samples, presumably reflecting phosphorylation in the intact cell. These included the homologous residues, S-295 on PSD-95 and S-365 on PSD-93, located between the PDZ2 and PDZ3 domains of these proteins; and S-367 located on the actin-binding domain of beta-CaMKII. The sequence RXXSPV emerged as a common phosphorylation motif of three specialized PSD scaffolding proteins, PSD-95, PSD-93, and Shank3. Phosphorylated serine residues in several of the identified phosphorylation sites were followed by prolines, suggesting prominent involvement of proline directed kinases in the regulation of PSD components.

Aniracetam improves contextual fear conditioning and increases hippocampal gamma-PKC activation in DBA/2J mice

DBA/2J (D2) mice display poor contextual learning and have less membrane-bound hippocampal protein kinase C (PKC) compared with C57BL/6 (B6) mice. Aniracetam and oxiracetam were previously shown to improve contextual learning in D2 mice and increase PKC activity. This study investigated a possible mechanism for learning enhancement by examining the effects of aniracetam on contextual fear conditioning and activation of the y isoform of PKC (gamma-PKC) in male D2 mice. In comparison to animals treated with vehicle only (10% 2-hydroxypropyl-beta-cyclodextrin), mice treated with aniracetam (100 mg/kg) 30 min prior to fear conditioning training demonstrated significantly improved contextual learning when tested 30 min and 24 h after training. This corresponded with a significant increase in activated, membrane-bound hippocampal gamma-PKC 30 min after training. No increase in learning or gamma-PKC was found 5 min after training. These results suggest an altered time course of activation of gamma-PKC in response to treatment with aniracetam, which improves learning in D2 mice.

Isoforms of protein kinase C in postsynaptic densities after cerebral ischemia

Relatively mild ischemic insult can lead to delayed neuronal cell death in vulnerable brain regions. We provide evidence that the protein composition of the postsynaptic densities (PSD) undergoes rapid modification after 15 min postdecapitative as well as 5 min transient global ischemia. We observed a significant increase in cPKC and nPKC protein content in the postischemic PSD. Of the calcium-regulated PKC isoforms, the alpha and beta subtypes increase in PSD over ten times above the control values whereas gamma PKC, an isoform most abundant in the native PSD structure, shows relatively smaller changes under ischemic conditions. For the first time, the PSD membrane translocation of Ca(2+)-independent isoforms delta and epsilon is shown. The yield of the PSD protein preparation from the postischemic cortex was two times higher compared with control. This correlated with an abundant increase in electron density and changes in ultrastructure of PSD isolated from postischemic cortex. Also sections from CA1 gerbils hippocampus after transient ischemia showed persistent enlargement of postsynaptic densities up to 24 h of reperfusion. This was accompanied by elevation of the PSD/cytoskeleton-associated alpha, beta PKC immunoreactivity and other changes in neuronal and glial cell morphology typical of the early postischemic degeneration. Sustained changes in PKC composition and organization of postsynaptic membranes during and after ischemia may cause persistent alteration in synaptic transmission and subsequently contribute to delayed neuronal injury.

Alterations in hippocampal GAP-43 phosphorylation and protein level following contextual fear conditioning

C57BL/6 (B6) mice display better contextual learning than the DBA/2 (D2) mice. The possibility that GAP-43, is differentially affected as a function of strain and learning was investigated in the present study. No basal difference between C57BL/6J (B6) and DBA/2J (D2) mice in the amount of hippocampal GAP-43 was observed, but naive D2 mice have slightly lower basal levels of GAP-43 phosphorylation than do B6 mice. Interestingly, alterations in hippocampal GAP-43 protein levels and phosphorylation state in response to training for contextual learning were observed only in B6 mice. Immediate-shocked mice, serving as nonlearning controls, showed no GAP-43 alterations, nor did D2 mice subjected to either training condition. These results suggest that modulation of hippocampal GAP-43 may be important for contextual learning and that strain-specific alterations in GAP-43 may be part of a disrupted pathway in D2 mice that is essential for learning.

Pathophysiological implications of the structural organization of the excitatory synapse

The glutamatergic synapse is the key structure in the development of activity-dependent synaptic plasticity in the central nervous system. The analysis of the complex biochemical mechanisms at the basis of the long-term changes in synaptic efficacy have received a tremendous impulse by the observation that the post-synaptic constituents of the synapse can be separated and purified through a simple procedure involving detergent treatment of synaptosomes and differential centrifugation. In this fraction, called post-synaptic density (PSD), the functional interactions of its constituents are preserved. The various subunits of ionotropic glutamate receptors are held in register with the presynaptic active zone through their interaction with linker proteins. N-methyl-D-aspartate (NMDA) subunits NR2A and NR2B, bind to the PSD protein called PSD-95, which in turn binds neuroligins, providing a handle for interacting with neurexin, located in the plasma membrane at the presynaptic active zone. Additional clustering of NMDA receptors is provided through the binding of NRI subunits to the cytoskeletal protein alpha-actinin-2. AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) and kainate receptors are other important constituents of PSDs and bind to different anchoring proteins. Phosphorylation processes have long been known to modulate NMDA receptor functional activity: the finding that several protein kinases, particularly Ca2+/Calmodulin-dependent protein kinase II and protein tyrosine kinases of the src family, are major constituents of PSDs has allowed to demonstrate that these enzymes are localized in a strategic position of the glutamatergic synapse, so that their activation provides a means for NMDA receptor function regulation upon its activation. The relevance of these mechanisms has been demonstrated in experimental models of pathologies involving deficits in synaptic plasticity, such as in streptozotocin-induced diabetes and in an animal model of prenatal induced ablation of hippocampal neurons. Both animal models display disturbances in long-term potentiation and cognitive deficits, thus providing in vivo models to study pathology related changes in both the structure and the function of the excitatory synapse.

Protein kinase C: a physiological mediator of enhanced transmitter output

Protein kinase C (PKC), activated by either diacylglycerol and/or arachidonic acid, through the activation of presynaptic receptors or nerve or nerve depolarization is involved is involved in the enhancement of transmitter release from many neural types. This facilities is most likely mediated by the phosphorylation of proteins involved in vesicle dynamics although a role for ion channels cannot be ruled out. PKC is not fundamental to the release process but rather has a modulatory role of PKC is to help maintain transmitter output during prolonged or elevated levels of activation and this seems to parallel suggestions that PKC is involved in the movement of reserve pools of vesicles into release-study sites. presynaptic facilitatory actions mediated by PKC are also involved in integrated modulatory functions such as long term potentiation, again where it elevates or maintains transmitter output. Although studies have tried to identify specific roles for various PKC isoforms, the actions of phorbol esters in elevators transmitter release do not fit with known potencies on individual isoforms and lit suggests that PKC may be located at an intraneuronal location which is difficult to access for lipophilic phorbol esters and further work is required in this area.

Brain spectrin binding to the NMDA receptor is regulated by phosphorylation, calcium and calmodulin

The N-methyl-D-aspartate receptor (NMDA-R) and brain spectrin, a protein that links membrane proteins to the actin cytoskeleton, are major components of post-synaptic densities (PSDs). Since the activity of the NMDA-R channel is dependent on the integrity of actin and leads to calpain-mediated spectrin breakdown, we have investigated whether the actin-binding spectrin may interact directly with NMDA-Rs. Spectrin is reported here to interact selectively in vitro with the C-terminal cytoplasmic domains of the NR1a, NR2A and NR2B subunits of the NMDA-R but not with that of the AMPA receptor GluR1. Spectrin binds at NR2B sites distinct from those of alpha-actinin-2 and members of the PSD95/SAP90 family. The spectrin-NR2B interactions are antagonized by Ca2+ and fyn-mediated NR2B phosphorylation, but not by Ca2+/calmodulin (CaM) or by Ca2+/CaM-dependent protein kinase II-mediated NR2B phosphorylation. The spectrin-NR1 interactions are unaffected by Ca2+ but inhibited by CaM and by protein kinase A- and C-mediated phosphorylations of NR1. Finally, in rat synaptosomes, both spectrin and NR2B are loosened from membranes upon addition of physiological concentrations of calcium ions. The highly regulated linkage of the NMDA-R to spectrin may underlie the morphological changes that occur in neuronal dendrites concurrently with synaptic activity and plasticity.

Changes in protein kinase C and adenylate cyclase in the temporal lobe from subjects with schizophrenia

Changes in G-protein linked neurotransmitter receptors have been reported in a number of regions of the brain of schizophrenic subjects. These changes, if functional, could cause a change in proteins such as protein kinase C (PKC) and adenylate cyclase (AC) which are important components of the G-protein linked second messenger cascades. We therefore used autoradiography to measure the distribution and density of [3H]phorbol ester binding to PKC and [3H]forskolin binding to AC in tissue obtained at autopsy from schizophrenic and non-schizophrenic subjects (Controls). There were significant decreases in the density of PKC in the parahippocampal gyrus (687 +/- 60 vs. 885 +/- 51 fmol/mg TE; mean +/- SEM; p < 0.01) and in AC in the dentate gyrus (75 +/- 4.9 vs. 92 +/- 6.5, p < 0.05) from the schizophrenic subjects. These data could indicate that changes in neurotransmitter receptors in the hippocampus from subjects with schizophrenia could have resulted in a change in their associated second messenger systems.

High resolution immunogold analysis reveals distinct subcellular compartmentation of protein kinase C gamma and delta in rat Purkinje cells

High resolution immunogold cytochemistry was used to investigate the subcellular distribution of protein kinase C gamma and delta in Purkinje cells of the rat cerebellum. Postembedding incubation with an antibody raised to a peptide sequence near the C-terminus of protein kinase C gamma resulted in strong labelling along the dendrosomatic plasma membrane. A quantitative analysis indicated that this labelling reflected the existence of two pools of protein kinase C gamma; one membrane associated pool and one cytoplasmic pool located within 50 nm of the plasma membrane. The labelling along the plasma membrane showed a pronounced and abrupt increase when moving from the cell body into the axon initial segment. Gold particles signalling protein kinase C gamma were also enriched in putative Purkinje axon terminals in the dentate nucleus. The only organelle showing a consistent immunolabelling for protein kinase C gamma was the Golgi apparatus where the gold particles were restricted to the trans face. Protein kinase C gamma immunoreactivity also occurred in the Purkinje cell spines, with an enrichment in or near the postsynaptic density. Antibodies to protein kinase C delta produced a very different labelling pattern in the Purkinje cells. Most of the gold particles were associated with rough endoplasmic reticulum, particularly with those cisternae that were located close to the nucleus or in the nuclear indentations. No significant protein kinase C delta immunolabelling was detected at the plasma membrane or in Purkinje cell spines. The present data point to a highly specific compartmentation of the two major protein kinase C isozymes in Purkinje cells and suggest that these isozymes act on different substrates and hence have different regulatory functions within these neurons.

Isolation and characterization of synaptoneurosomes from single rat hippocampal slices

A technique for recovering functional synaptoneurosomes containing vesicularized elements of the presynapse and postsynapse into an enriched fraction has been modified to allow for small amounts of starting brain tissue. Single 400 microm rat hippocampal slices were homogenized and sequentially filtered in a 1 cc tuberculin syringe to produce an enriched synaptoneurosome fraction. Data from Western immunoblots for specific synaptic proteins suggest that these fractions are neurochemically similar to synaptosome fractions generated by sucrose gradients. Electron micrographs show that the 'small scale' preparations contain an abundant population of fused presynaptic and postsynaptic vesicularized bodies as previously published for synaptoneurosome fractions prepared from relatively large amounts of starting tissue. The single slice synaptoneurosome preparation is a quick, easy and reliable method for use in the study of synaptic function.

Effect of glutamate receptor phosphorylation by endogenous protein kinases on electrical activity of isolated postsynaptic densities of rat cortex and hippocampus

Postsynaptic densities (PSDs) were isolated from rat brain cortex and hippocampus, purified and incorporated into giant (5-80 microns in diameter) liposomes. Gigaohm seals were obtained with a patch-clamp pipette, and a giant liposome PSD-containing membrane patch, was excised and recorded. The PSD was always oriented in an inside-out configuration. This allowed receptor agonists or antagonists to be added from the interior of the recording pipette, and also the addition of different substances, such as ATP, calcium, calmodulin and others to the 'intracellular' side of the PSD, i.e. to the bath. alpha-Amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA) receptor agonists such as quisqualate or AMPA induced in the PSD a complex pattern of electrical activity, that was blocked by 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), but not by 2-aminophosphonovalerate (APV). The currents generated by 0.5-1 microM quisqualate were increased by about 100% when the PSDs were phosphorylated. Similar findings were obtained when the agonist was 0.2-2 microM kainate. These currents were also blocked by a non-N-methyl-D-aspartate (NMDA) receptor antagonist but not by APV, and were increased by about 70% by phosphorylation of the PSDs. Addition of 5-10 microM NMDA plus 1 microM glycine to the 'extracellular' side of the PSD, led to a characteristic pattern of activity, with the opening of multiple receptor ion channels. This was entirely blocked by 10 microM APV. Addition of extracellular Mg2+ (1-2 mM) induced a voltage-dependent block of the currents. Phosphorylation of the PSD led to an increase of Mg(2+)-blocked current of about 80%. The effect of phosphorylation on ion channel activity showed a markedly different requirement for calcium and for calmodulin among the AMPA, kainate and NMDA types of glutamate receptors, thus suggesting that each receptor type is coupled at the synapse with a unique complement of protein phosphokinases.

gamma Isoform-selective changes in PKC immunoreactivity after trace eyeblink conditioning in the rabbit hippocampus

An immunocytochemical examination of the rabbit hippocampus was done to determine which of the Ca(2+)-dependent protein kinase C (PKC) isoforms (PKC alpha, -beta I, -beta II, or -gamma) are involved in associative learning. The hippocampally dependent trace eyeblink conditioning task was used for behavioral training, and pseudoconditioned and naive animals served as controls. Significant increases (P < 0.05) in staining intensity were found with antibodies reactive with the catalytic or the regulatory domain of PKC gamma in conditioned animals compared with naive and pseudoconditioned subjects at a 24-h post-conditioning time point. The increase was found in CA1 and CA3 pyramidal cell bodies, in apical dendrites and the proximal part of the basilar dendrites, and in cell bodies of dentate granule cells. In contrast, no conditioning-specific changes were found for PKC alpha, -beta I, or -beta II in in hippocampal neurons. The increase in PKC gamma immunoreactivity (ir) was significantly less (P < 0.05) in poor learners than in good learners. The correlation between the degree of PKC gamma-ir and the total number of conditioned responses across training sessions was both positive and significant. These results suggest that PKC gamma is the major Ca(2+)-dependent PKC isoform involved in hippocampal neurons during acquisition of associative memories. Immunoblots revealed no conditioning-induced increase in the total amount or translocation of PKC gamma at the 24-h time point, and no proteolytic PKC fragments were observed. In agreement with the Western blot data, PKC activity did not differ among naive, pseudoconditioned, and trace conditioned animals. The conditioning-induced increase in antibody binding to the gamma-isoform must therefore be due to an increased access to the antigenic site(s) as a result of alteration in the tertiary structure of PKC gamma or in quaternary interactions of PKC gamma in situ.

Messengers from the synapses to the nucleus (MSNs) that establish late phase of long-term potentiation (LTP) and long-term memory

The late stage of long-term potentiation (LTP) and long-term memory is believed to be largely governed by altered gene expression for its generation and maintenance, while the early stages of LTP and memory are controlled mainly by the phosphorylation-dephosphorylation of the synaptic proteins. For the altered gene expression, synaptic information must be transmitted from the synaptic sites to the nucleus. This article describes the presence of specific messenger molecules that transmit synaptic information to the nucleus; these molecules are referred to as MSNs (Messengers from Synapse to the Nucleus). In addition, recent studies have indicated that certain transcription factors localize at postsynaptic sites as well as the nucleus, and may function as MSNs.

ERK2-type mitogen-activated protein kinase (MAPK) and its substrates in postsynaptic density fractions from the rat brain

Mitogen-activated protein kinase (MAPK) and MAPK kinase (MAPKK) were detected by Western blotting in the synaptic fraction prepared from the rat brain. There were two bands immunoreactive to the anti-MAPK antiserum in the soluble, P2, synaptosome, and synaptic plasma membrane fractions. These immunoreactive bands possibly corresponded to extracellular signal-regulated kinase (ERK) 1 and 2 (Boulton et al., 1991b), respectively. Only ERK2 was detected in the postsynaptic density (PSD) fraction. We then surveyed MAPK substrates in the synaptic fractions using purified Xenopus MAPK (ERK2-type MAPK), and found a number of MAPK substrates unique to the PSD fraction. Thus, ERK2 is present in the synapse, especially at the postsynaptic site, and it may play a role(s) in synaptic function via the phosphorylation of synapse-specific substrates. Developmental changes in ERK2 also supported its role in the synapse.

Expression of protein kinase C isozymes in hippocampal neurones in culture

Several protein kinase C (PKC) isozymes were analyzed by immunoblot and immunocytochemistry in cultures of hippocampal neurones at several stages of differentiation. Our findings reveal the existence of two distinct patterns of expression. Firstly, conventional PKC isozymes alpha, beta and gamma, that are expressed at very low levels during the initial stages and then increase continuously with time of culture. Secondly, novel PKC isozymes delta, epsilon and zeta, whose contents increase very early to reach a maximum after three days of culture and then progressively decline. Specific proteolysis for PKC isozymes beta and gamma was observed throughout the period studied. The developmental profile obtained for the different PKC isozymes is discussed in relation to the differentiation of hippocampal neurones in culture.

Protein kinases involved in the expression of long-term potentiation

This review describes the protein kinases that are involved in long-term potentiation (LTP). The following items are described. 1. Ca2+/calmodulin-dependent protein kinase II (CaMKII) and protein kinase C (PKC) may play pivotal roles in the different phases of the expression of LTP. This involvement has been indicated mainly by using specific inhibitor of these kinases. The involvement of the CaMKII alpha-subunit was confirmed in mutant mice which are deficient in the gene for the subunit. 2. Involvement of persistently active protein kinases in the maintenance of LTP has been proposed and, since then, several studies have focused upon the persistent kinase. Both PKC and CaMKII are possible sources of the persistent kinase activities. 3. Protein kinases other than CaMKII or PKC (ex. protein kinase A, tyrosine kinases, mitogen-activated kinase) also play roles in the expression of LTP. 4. Finally, the importance of postsynaptic density as a device where complex chemical reactions related to neuronal signal transduction occur is discussed.