Rat brain glial cells in primary culture and subculture contain the delta, epsilon and zeta subspecies of protein kinase C as well as the conventional subspecies

Protein Kinase C Controls the Excitability of Cortical Pyramidal Neurons by Regulating Kv2.2 Channel Activity

The family of voltage-gated potassium Kv2 channels consists of the Kv2.1 and Kv2.2 subtypes. Kv2.1 is constitutively highly phosphorylated in neurons and its function relies on its phosphorylation state. Whether the function of Kv2.2 is also dependent on its phosphorylation state remains unknown. Here, we investigated whether Kv2.2 channels can be phosphorylated by protein kinase C (PKC) and examined the effects of PKC-induced phosphorylation on their activity and function. Activation of PKC inhibited Kv2.2 currents and altered their steady-state activation in HEK293 cells. Point mutations and specific antibodies against phosphorylated S481 or S488 demonstrated the importance of these residues for the PKC-dependent modulation of Kv2.2. In layer II pyramidal neurons in cortical slices, activation of PKC similarly regulated native Kv2.2 channels and simultaneously reduced the frequency of action potentials. In conclusion, this study provides the first evidence to our knowledge that PKC-induced phosphorylation of the Kv2.2 channel controls the excitability of cortical pyramidal neurons.

Amyloid β peptide (25–35) activates protein kinase C leading to cyclooxygenase-2 induction and prostaglandin E2 release in primary midbrain astrocytes

Prostaglandins (PGs) are generated by the enzymatic activity of cyclooxygenase-1 and -2 (COX-1/2) and modulate several functions in the CNS such as the generation of fever, the sleep/wake cycle, and the perception of pain. Moreover, the induction of COX-2 and the generation of PGs has been linked to neuroinflammatory aspects of Alzheimer's disease (AD). Non-steroidal anti-inflammatory drugs (NSAIDs) that block COX enzymatic activity have been shown to reduce the incidence of AD in various epidemiological studies. While several reports investigated the expression of COX-2 in neurons and microglia, expression of COX-2 in astroglial cells has not been investigated in detail. Here we show that amyloid beta peptide 25-35 (Abeta(25-35)) induces COX-2 mRNA and protein synthesis and a subsequent release of prostaglandin E(2) (PGE(2)) in primary midbrain astrocytes. We further demonstrate that protein kinase C (PKC) is involved in Abeta(25-35)-induced COX-2/PGE(2) synthesis. PKC-inhibitors prevent Abeta(25-35)-induced COX-2 and PGE(2) synthesis. Furthermore Abeta(25-35) rapidly induces the phosphorylation and enzymatic activation of PKC in primary rat midbrain glial cells and in primary human astrocytes from post mortem tissue. Our data suggest that the PKC isoforms alpha and/or beta are most probably involved in Abeta(25-35)-induced expression of COX-2 in midbrain astrocytes. The potential role of astroglial cells in the phagocytosis of amyloid and the involvement of PGs in this process suggests that a modulation of PGs synthesis may be a putative target in the prevention of amyloid deposition.

Specific PKC isoforms regulate blastocoel formation during mouse preimplantation development

During early mammalian development, blastocyst morphogenesis is achieved by epithelial differentiation of trophectoderm (TE) and its segregation from the inner cell mass (ICM). Two major interrelated features of TE differentiation required for blastocoel formation include intercellular junction biogenesis and a directed ion transport system, mediated by Na+/K+ ATPase. We have examined the relative contribution of intercellular signalling mediated by protein kinase C (PKC) and gap junctional communication in TE differentiation and blastocyst cavitation. The distribution pattern of four (delta, theta, iota/lambda, zeta) PKC isoforms and PKCmicro/PKD1 showed partial colocalisation with the tight junction marker ZO-1alpha+ in TE and all four PKCs (delta, theta, iota/lambda, zeta) showed distinct TE/ICM staining patterns (predominantly at the cell membrane within the TE and cytoplasmic within the ICM), indicating their potential contribution to TE differentiation and blastocyst morphogenesis. Specific inhibition of PKCdelta and zeta activity significantly delayed blastocyst formation. Although modulation of these PKC isoforms failed to influence the already established programme of epithelial junctional differentiation within the TE, Na+/K+ ATPase alpha1 subunit was internalised from membrane to cytoplasm. Inhibition of gap junctional communication, in contrast, had no influence on any of these processes. Our results demonstrate for the first time that distinct PKC isotypes contribute to the regulation of cavitation in preimplantation embryos via target proteins including Na+/K+ ATPase.

PKC signalling regulates tight junction membrane assembly in the pre-implantation mouse embryo

Epithelial differentiation including tight junction (TJ) formation occurs exclusively within the trophectoderm (TE) lineage of the mouse blastocyst. Here we examine mechanisms by which TJ protein membrane assembly might be regulated by protein kinase C (PKC) in the embryo. To overcome the inherent staging asynchrony of individual blastomeres within intact embryos, we have used isolated inner cell masses (ICMs) from early blastocysts to induce epithelial differentiation in their outer cells responding to their new cell contact pattern. Two TJ proteins examined retain their order of membrane assembly in isolated ICMs in culture as during normal development (early-assembling ZO-2 and late-assembling ZO-1α+), but this process is highly accelerated. Using six chemical modulators of PKC activity, we show here that PKC signalling is involved in the regulation of TJ membrane assembly. While indolactam-mediated PKC activation stimulates membrane assembly of both TJ proteins, TPA-mediated PKC activation stimulates only that of ZO-1α+. The PKC inhibitors Ro-31-8220, Ro-31-8425 and Gö 6983 suppress the stimulatory effect of both PKC activators on membrane assembly to varying extents according to inhibitor and TJ protein examined. Gö 6983 similarly inhibits ZO-2 and ZO-1α+membrane assembly. PKC inhibition by Gö 6976 appeared to stimulate TJ membrane assembly. Despite the broad PKC isotype specificity of the inhibitors used, these data suggest that the two TJ proteins are differently regulated by PKC isotypes or subfamilies. As Gö 6983 uniquely affects aPKC (particularly PKCζ) and we find that both PKCδ and ζ relocate upon activator treatment to colocalise partially with the TJ proteins in isolated ICMs, we suggest that at least PKCδ and ζ may play a central role in regulating TJ membrane assembly.

Translocation of protein kinase C-betaII in astrocytes requires organized actin cytoskeleton and is not accompanied by synchronous RACK1 relocation

Protein kinase C (PKC)-betaII is the most abundant PKC isoform in astrocytes. Upon activation, this isoform of PKC translocates from the cytosol to the plasma membrane (PM). In this study, we investigated in astrocytes the modality of PKC-betaII translocation as far as the participation of the receptor for activated C kinase-1 (RACK1) and the requirement for intact cytoskeleton in the process. In astrocytes, Western blots and immunocytochemistry coupled to confocal microscopic quantitative analysis showed that after 5 min of phorbol-12-myristate-13-acetate (PMA) exposure, native PKC-betaII, but not PKC-betaI, is relocated efficiently from the cytosol to the PM. Translocation of PKC-betaII was not associated with synchronous RACK1 relocation. Furthermore, the quantity of PM-associated PKC-betaII that co-immunoprecipitated with PM-bound RACK1 increased following PMA exposure, indicating a post activation binding of the two proteins in the PM. Because RACK1 and PKC-betaII relocation seemed not to be synchronous, we hypothesized that an intermediate interaction with the cytoskeleton was taking place. In fact, we were able to show that pharmacological disruption of actin-based cytoskeleton greatly deranged PKC-betaII translocation to the PM. The requirement for intact actin cytoskeleton was specific, because depolymerization of tubulin had no effect on the ability of the kinase to translocate to the PM. These results indicate that in astrocytes, RACK1 and PKC-betaII synchronous relocation is not essential for relocation of PKC-betaII to the PM. In addition, we show for the first time that the integrity of the actin cytoskeleton plays a specific role in PKC-betaII movements in these cells. We hypothesize that in glial cells, rapidly occurring changes of actin cytoskeleton arrangement may be involved in the fast reprogramming of PKC targeting to specific PM location to phosphorylate substrates in different cellular locations.

Rottlerin, an inhibitor of protein kinase Cdelta (PKCdelta), inhibits astrocytic glutamate transport activity and reduces GLAST immunoreactivity by a mechanism that appears to be PKCdelta-independent

Protein kinase C (PKC) regulates the activity and/or cell surface expression of several different neurotransmitter transporters, including subtypes of glutamate transporters. In the present study, the effects of pharmacological inhibitors of PKC were studied in primary astrocyte cultures that express the glutamate aspartate transporter (GLAST) subtype of glutamate transporter. We found that general inhibitors of PKC, bisindolylmaleimide I (Bis I), bisindolylmaleimide II (Bis II), staurosporine and an inhibitor of classical PKCs, Gö6976, had no effect on Na+-dependent glutamate transport activity. However, rottlerin, a putative specific inhibitor of PKCdelta, decreased transport activity with an IC50 value (less than 10 micro m) that is comparable to that reported for inhibition of PKCdelta. The effect of rottlerin was very rapid (maximal effect within 5 min) and was due to a decrease in the capacity (Vmax) for transport. Rottlerin also caused a drastic loss of GLAST immunoreactivity within 5 min, suggesting that rottlerin accelerates GLAST degradation/proteolysis. Rottlerin had no effect on cell surface or total expression of the transferrin receptor, providing evidence that the effect on GLAST cannot be attributed to a non-specific internalization/degradation of plasma membrane proteins. Down-regulation of PKCdelta with chronic phorbol ester treatment did not block rottlerin-mediated inhibition of transport activity. These results suggest a novel mechanism for regulation of the GLAST subtype of glutamate transporter and indicate that there is a rottlerin target that is capable of controlling the levels of GLAST by controlling the rate of degradation or limited proteolysis. It appears that the target for rottlerin may not be PKCdelta.

Induction of pituitary adenylate cyclase-activating polypeptide mRNA in the medial parvocellular part of the paraventricular nucleus of rats following kainic-acid-induced seizure

We examined the effects of kainic acid (KA)-induced seizure on the expression of the pituitary adenylate cyclase-activating polypeptide (PACAP) gene in the paraventricular nucleus (PVN) of rats using in situ hybridization histochemistry. Subcutaneous administration of KA (12 mg/kg) in adult male Sprague-Dawley rats caused a progressive development of seizure behavior. An induction of the PACAP gene expression in the medial parvocellular part of the PVN (mpPVN) was observed 3, 6, 12, 24 and 48 h after subcutaneous administration of KA. From a nearly undetectable level, PACAP gene expression increased in the mpPVN and reached maximum 12 h after subcutaneous administration of KA. PACAP gene expression returned to near basal level 48 h after stimulation with KA. Using a specific monoclonal PACAP antibody, PACAP immunoreactivity (-IR) gradually increased during the following 24 h after KA administration. In controls, PACAP-IR was located exclusively in nerve fibers of the mpPVN, whereas KA administration induced PACAP-IR in cell bodies of the mpPVN, and a dense accumulation of PACAP-IR nerve fibers in the external zone of the median eminence was observed. Induction of the PACAP gene expression following KA-induced seizure was significantly reduced by pretreatment with diazepam or MK-801 (nonselective N-methly-D-aspartate receptor antagonist). These results suggest that PACAP in the hypothalamo-adenohypophysial system may have a hypophysiotropic role during KA-induced seizure.

The alpha isoform of protein kinase C is involved in signaling the response of desmosomes to wounding in cultured epithelial cells

Initiation of reepithelialization upon wounding is still poorly understood. To enhance this understanding, we focus here on changes in the adhesive state of desmosomes of cultured Madin-Darby canine kidney cells in response to wounding of confluent cell sheets. Previous results show that desmosomal adhesion in Madin-Darby canine kidney cells changes from a calcium-dependent state to calcium independence in confluent cell sheets. We show that this change, which requires culture confluence to develop, is rapidly reversed upon wounding of confluent cell sheets. Moreover, the change to calcium dependence in wound edge cells is propagated to cells hundreds of micrometers away from the wound edge. Rapid transition from calcium independence to calcium dependence also occurs when cells are treated with phorbol esters that activate PKC. PKC inhibitors, including the conventional isoform inhibitor Gö6976, cause rapid transition from calcium dependence to calcium independence, even in subconfluent cells. The cellular location of the alpha isoform of PKC correlates with the calcium dependence of desmosomes. Upon monolayer wounding, PKCalpha translocates rapidly to the cell periphery, becomes Triton X-100 insoluble, and also becomes concentrated in lamellipodia. The PKCalpha translocation upon wounding precedes both the increase in PKC activity in the membrane fraction and the reversion of desmosomes to calcium dependence. Specific depletion of PKCalpha with an antisense oligonucleotide increases the number of cells with calcium-independent desmosomes. These results show that PKCalpha participates in a novel signaling pathway that modulates desmosomal adhesion in response to wounding.

Expression and translocation of protein kinase C isoforms in rat microglial and astroglial cultures

Cellular distribution and activation by phorbol myristate acetate (PMA) of classical (alpha, betaI, betaII,gamma), novel (delta, epsilon, theta, eta), and atypical (zeta, iota) protein kinase C (PKC) isoforms were studied in cultured rat neonatal microglial and astroglial cells by Western blot analysis. Among the classical isoforms, only betaII was expressed in microglia and astrocytes in the same abundance. The expression of betaI in microglia was less abundant, while PKCalpha was not detectable in this cell type. PKCgamma was absent in both cell populations. A different pattern of expression was also found for novel and atypical isoenzymes: Both cell types expressed delta, theta, eta, zeta, and iota isoforms, but PKCepsilon was absent in microglia and the expression of PKCzeta and PKCiota in these cells was low compared to astrocytes. The pattern of PKC distribution in cytosolic and particulate fractions as well as activation by short (10 min) and prolonged (4 hr) PMA treatment in both cell types were similar. On the whole, in comparison with astrocytes, PKC in microglial cells was less expressed, both in terms of number of isoforms and level of expression. The microglial profile of PKC isoforms differed from that of rat peritoneal macrophages, which did express PKCalpha. Preliminary evidence suggests that the ability of PMA to enhance cyclic AMP responses in astrocytes, but not in microglia, is related to the different pattern of expression of PKCalpha and PKCepsilon in the two cell types.

Mitogenic signaling by ATP/P2Y purinergic receptors in astrocytes: involvement of a calcium-independent protein kinase C, extracellular signal-regulated protein kinase pathway distinct from the phosphatidylinositol-specific phospholipase C/calcium pathway

Activation of ATP/P2Y purinergic receptors stimulates proliferation of astrocytes, but the mitogenic signaling pathway linked to these G-protein-coupled receptors is unknown. We have investigated the role of extracellular signal-regulated protein kinase (ERK) in P2Y receptor-stimulated mitogenic signaling as well as the pathway that couples P2Y receptors to ERK. Downregulation of protein kinase C (PKC) in primary cultures of rat cerebral cortical astrocytes greatly reduced the ability of extracellular ATP to stimulate ERK. Because occupancy of P2Y receptors also leads to inositol phosphate formation, calcium mobilization, and PKC activation, we explored the possibility that signaling from P2Y receptors to ERK is mediated by a phosphatidylinositol-specific phospholipase C (PI-PLC)/calcium pathway. However, neither inhibition of PI-PLC nor chelation of calcium significantly reduced ATP-stimulated ERK activity. Moreover, a preferential inhibitor of calcium-dependent PKC isoforms, Gö 6976, was significantly less effective in blocking ATP-stimulated ERK activity than GF102903X, an inhibitor of both calcium-dependent and -independent PKC isoforms. Furthermore, ATP stimulated a rapid translocation of PKCdelta, a calcium-independent PKC isoform, but not PKCgamma, a calcium-dependent PKC isoform. ATP also stimulated a rapid increase in choline, and inhibition of phosphatidylcholine hydrolysis blocked ATP-evoked ERK activation. These results indicate that P2Y receptors in astrocytes are coupled independently to PI-PLC/calcium and ERK pathways and suggest that signaling from P2Y receptors to ERK involves a calcium-independent PKC isoform and hydrolysis of phosphatidylcholine by phospholipase D. In addition, we found that inhibition of ERK activation blocked extracellular ATP-stimulated DNA synthesis, thereby indicating that the ERK pathway mediates mitogenic signaling by P2Y receptors.

Activation of protein kinase C modulates cell-cell and cell-substratum adhesion of a human colorectal carcinoma cell line and restores 'normal' epithelial morphology

Abnormal cell adhesion is an important contributing factor in invasion and metastasis. Here, we show that morphologically 'normal' cell-cell and cell-substratum adhesion can be restored to a poorly differentiated carcinoma cell line by activation of protein kinase C (PKC). This cell line, VACO 10MS, grows as multicellular aggregates loosely attached to the substratum. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA, 7.5 nM) induces rapid adhesive changes with 2 components. First, within 15 min of TPA the cells become closely apposed, an event resembling the 'compaction' seen in the mouse early embryo. Next, over 2 hr, the cells spread, forming a monolayer. We show that compaction depends on extracellular calcium, E-cadherin-mediated adhesion and F-actin but not on protein synthesis, microtubules or substratum adhesion. By contrast, cell spreading is independent of cadherin and extracellular Ca2+ but involves the formation of focal contacts containing alpkha(v) integrin. TPA treatment causes rapid translocation of PKC-alpha to the insoluble fraction. During compaction, actin- and PKC-alpha-containing lamellae form over the entire aggregate surface, those adjacent to the substratum appearing to initiate spreading. Compaction does not involve increased phosphorylation of the cadherin/catenin complex. We conclude that activation of PKC-alpha restores 'normal' morphology to these poorly differentiated cells. Our results are of general interest in relation to the regulation of cell adhesion and, through further investigation, may lead to identification of novel targets for therapeutic suppression of invasion and metastasis.

Expression of protein kinase C gene family members is temporally and spatially regulated during neural development in vitro

We used primary cultures of rat hippocampal neurons and PCC7-Mz1 cells to correlate the expression of the protein kinase C (PKC) gene family with specific events during neural differentiation. Multipotent PCC7-Mz1 embryonic carcinoma stem cells develop into a tissue-like pattern of neuronal, fibroblast-like and astroglial cells by all-trans retinoic acid (RA) treatment. Western blot analyses demonstrate that PKCalpha, betaI, gamma, theta, mu, lambda, and zeta were constitutively expressed but the expression of PKCbetaII, delta, epsilon, and eta was up-regulated three days after addition of RA when cells mature morphologically. While the protein levels of the PKC isoforms betaII, delta and eta decreased after d6, when the major phenotypical alterations of the developing neurons were completed, PKCepsilon expression remained at a high level. Immunofluorescence studies demonstrated that PKCalpha, lambda and zeta were constantly expressed in stem cells and the arising cell types. PKCdelta was detected in all differentiated cell types, whereby PKCbetaII, gamma, epsilon, and zeta were solely found in the neuronal derivatives with PKCgamma predominantly located in the nuclei. PKCeta was weakly expressed at the Golgi complex of stem cells but expanded throughout the entire somata of all developing neurons. In contrast, PKCbetaII was abundant only in the somata of a minor fraction of all neurons (approximately 2.5%). Also, PKCepsilon was exclusively synthesized by a subpopulation of neurons (40+/-5%), where it was localized in the somata and in the axons. PKCzeta was persistently expressed in two forms, the full-length PKCzeta and the constitutively active, proteolytic product PKMzeta, reasoning that permanent PKCzeta activity is important for PCC7-Mz1 physiology. Fractionation of extracts from undifferentiated and differentiating PCC7-Mz1 cells revealed that the conventional cPKCalpha was partly and the cPKCbetaI and the novel nPKCs delta and epsilon were mainly membrane bound, implying that they were also in an active state. However, when using the PKC substrate MARCKS (myristoylated alanine-rich C kinase substrate) to monitor cellular PKC activity, we observed that activation of PKC by phorbol ester was required for complete MARCKS phosphorylation and its translocation from the membrane to the cytoplasm. Our data show that the cell type-specific expression, subcellular localization and activation of PKCs are regulated in an isoform-specific manner during neurogenesis suggesting that they are involved in the control of neural development and in particular in neuronal differentiation.

Learning-induced alterations in hippocampal PKC-immunoreactivity: a review and hypothesis of its functional significance

1. To localize protein kinase C (PKC) in the hippocampus, PKC activity measures, mRNA in situ hybridization, and [3H]phorbol ester binding techniques were used until in the 1980s antibodies became available for in situ immunocytochemistry. In the late 1980s, PKC-isoform-specific antibodies were first used to map hippocampal PKC at the cellular and subcellular level. The mammalian hippocampus contains all four Ca(2+)-dependent PKC isoforms, but the (sub)cellular localization is both isoform- and species-specific. 2. Hippocampally-dependent spatial and associative learning in rat, mice and rabbit induce an increase in PKC immunoreactivity (ir) in hippocampal principal cells studied 24 hours after the animals had learned the task. Among the four Ca(2+)-dependent PKC subtypes, this increase is selective for the gamma-isoform. The presence of the gamma-isoform in dendritic spines (the most likely site for synaptic plasticity and information storage), in contrast to PKC alpha, beta 1, and beta 2, may underlie the isoform-selectivity. 3. Compared to fully trained animals, subjects halfway training showed intermediate levels of increased PKC gamma-ir. Poor learners that were not able to learn the task showed considerably less enhanced PKC gamma-ir as compared to good learners. 4. Associative learning induced a decrease in astroglial PKC beta 2 and gamma-ir in those regions where a simultaneous increase in neuronal PKC gamma-ir was observed. This decrease most likely reflects PKC down-regulation, enabling the astrocytes to maintain their K+ buffering capacity necessary to support neuronal activity such as accompanying learning and memory. 5. Western blot analyses revealed that the increase in PKC gamma-ir was not due to an increase in total amount of PKC gamma, translocation, or the proteolytic generation of the fragment PKM. The increase in PKC gamma-ir must therefore reflect a learning-induced conformational change in the PKC gamma molecule that results in the exposure of the antigenic site(s). 6. Although a large number of hippocampal pyramidal cells display learning-induced enhancement of PKC gamma-ir at the 24 hours post-training time point, this does not indicate, however, that all synapses in these neurons are used, or that the maximal PKC signal transduction capacity per call has been reached. 7. The enhanced PKC gamma-ir may reflect a form of activated PKC, since PKC stimulation by phorbol esters (both in hippocampal slices and mildly aldehyde fixed sections) mimicked the increase in PKC gamma-ir similar as seen after learning. 8. The most likely transmitter systems which may have induced the altered PKC gamma-ir are acetylcholine and glutamate. Their contribution and interaction at the cellular level are depicted in a schematic circuit terminating on a CA1 pyramidal cell (Fig. 4). 9. Several functional roles for PKC gamma in learning and memory are discussed, and a hypothetical model is proposed based on an endogeneous PKC inhibitor protein that may explain altered antibody-binding to PKC gamma after learning (Fig. 6). 10. The immunocytochemical approach can contribute significantly to the ongoing attempts to decipher part of the cellular and biochemical mechanism of learning and memory. The development of ever more specific and better characterized antibodies reactive with different sites of proteins like PKC gamma will offer the necessary tools for further immunocytochemical research to help unravel complex brain functions.

Specific induction of protein kinase C delta subspecies after transient middle cerebral artery occlusion in the rat brain: inhibition by MK-801

Protein kinase C (PKC) consists of a family of closely related Ca2+/phospholipid-dependent phosphotransferase isozymes, most of which are present in the brain and are differentially activated by second messengers. Calcium-dependent PKC activity may cause neuronal degeneration after ischemic insult. PKC is also involved in trophic-factor signaling, indicating that activity of some PKC subspecies may be beneficial to the injured brain. Therefore, we screened long-term changes in the expression of multiple PKC subspecies after focal brain ischemia. Middle cerebral artery occlusion was produced by using an intraluminal suture for 30 min of 90 min. In in situ hybridization experiments, mRNA levels of PKC alpha, -beta, -gamma, -delta, -epsilon, and -zeta were decreased in the infarct core 4 hr after ischemia and were lost completely 12 hr after ischemia. In areas surrounding the core, PKC delta mRNA was specifically induced 4, 12, and 24 hr after ischemia in the cortex. At 3 and 7 d, the core and a rim around it showed increased mRNA levels of PKC delta. No other subspecies were induced. At 2 d, immunoblotting demonstrated increased levels of PKC delta protein in the perifocal tissue, and immunocytochemistry revealed an increased number of PKC delta-positive neurons in the perifocal cortex. In the core, PKC delta-positive macrophages and endothelial cells were seen. Pretreatment with MK-801, an NMDA antagonist, inhibited cortical PKC delta mRNA induction. The data show that focal brain ischemia induces PKC delta mRNA and protein but not other PKC subspecies through the activation of NMDA receptors and that the upregulation lasts for several days in neurons of the perifocal zone.

Adrenergic activation of phospholipase D in primary rat astrocytes

Phospholipase D (PLD) activity was investigated in astrocytes prepared from newborn rat cerebral cortex using the transphosphatidylation assay. Basal PLD activity was measurable and was found to be enhanced by ATP, carbachol and noradrenaline. The activation by noradrenaline (EC50, 0.68 microM) was mimicked by methoxamine (EC50, 65 microM), an alpha 1-specific adrenergic agonist, and was inhibited by prazosine, an alpha 1-specific adrenergic antagonist. Clonidin, an alpha 2-adrenergic agonist, slightly lowered PLD activity whereas beta-adrenergic drugs were without effect. Experiments with mitogens indicate that PLD activation in astrocytes may be involved in the control of astrocytic cell proliferation.

Addition of phosphatidylcholine-phospholipase C induces cellular redistribution and phosphorylation of protein kinase C zeta in C 6 glial cells

Phosphatidylcholine breakdown has been shown to play a critical role in signal transduction involving generation of a number of second messengers [Exton, J.H., Biochim. Biophys. Acta, 1212 (1994) 26-42]. In the present report we demonstrate by immunofluorescence that short-treatment of C 6 glial cells with phosphatidylcholine-hydrolyzing phospholipase C (PC-PLC), changes the intracellular localization of protein kinase C (PKC) zeta from the cytoplasm to a perinuclear region. Western blot analysis also showed a redistribution of PKC zeta after incubation of cells with PC-PLC. To test whether these changes were accompanied by an activation of the enzyme, we measured the extent of phosphorylation of PKC zeta by immunoprecipitation from 32P-labelled cells. Short-treatment with PC-PLC resulted in enhanced phosphorylation of the higher Mr PKC zeta in C 6 glial cells.

Age-dependent increases in protein kinase C-alpha beta immunoreactivity and activity in the human brain: possible in vivo modulatory effects on guanine nucleotide regulatory G(i) proteins

In the postmortem human brain (20 specimens of frontal cortex, Brodmann area 9) the abundance of immunoreactive protein kinase C (PKC-alpha beta) and the activity of PKC (calcium, phosphatidylserine, and phorbol ester-dependent enzymes) were determined to study the effect of aging (range 1 month to 89 years) on this regulatory enzyme. Also, the abundance of immunoreactive G protein subunits (G alpha i1/2, G alpha i3, G alpha o, G alpha s and G beta) were assessed in parallel to investigate possible relationships with PKC-alpha beta. The abundance of PKC-alpha beta was positively correlated with aging (r = 0.62, n = 20, P < 0.005). Moreover, PKC activity also showed a significant positive correlation with the age of the subject at death (r = 0.55, n = 14, P < 0.05). Because of the known in vitro modulatory role of PKC-alpha beta on G(i) proteins, the existence of an in vivo effect of brain PKC-alpha beta on various G proteins was assessed through correlation analyses. In the brain of the same subjects, there were significant negative correlations between the immunoreactivity of PKC-alpha beta and the immunoreactivities of G alpha i1/2 (r = -0.78, n = 13, P < 0.005) and G alpha i3 (r = -0.58, n = 15, P < 0.005). In the same brains, similar negative, although non-significant, correlations were found between the levels of PKC-alpha beta and those of G alpha o, G alpha s and G beta. The results demonstrate an up-regulation of brain PKC-alpha beta with aging and suggest the existence of a relevant in vivo modulatory role of this regulatory enzyme on inhibitory Gi proteins in the human brain during the process of aging.

Ethanol enhances the in situ phosphorylation of MARCKS and protein kinase C activity in primary cultures of astrocytes

Protein kinase C (PKC) plays an important regulatory role in astrocyte function. Chronic exposure to ethanol for 4 days resulted in an increase in Ca2+-dependent PKC activity in the supernatant fraction of astrocyte homogenates. Only Ca2+-independent PKC activity could be observed in the membrane fraction and this activity was unaffected by ethanol exposure. Chronic ethanol exposure also increased the in situ phosphorylation of MARCKS in permeabilized astrocytes both in the absence or presence of the PKC activator, phorbol 12 -myristate 13 -acetate (PMA). These results suggest an increase in the expression of one or more astrocytic PKC isoforms after chronic ethanol exposure.

Phorbol ester stimulation of phosphatidylcholine synthesis in four cultured neural cell lines: correlations with expression of protein kinase C isoforms

Phosphatidylcholine (PtdCho) can provide lipid second messengers involved in signal transduction pathways. As a measure of phospholipid turnover in response to extracellular stimulation, we investigated differential enhancement of [3H]choline incorporation into PtdCho by phorbol esters. In C6 rat glioma and SK-N-SH human neuroblastoma cells, [3H]PtdCho synthesis was 2-4 fold stimulated by beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA) when [3H]choline was incubated simultaneously with, or 15 min prior to, beta-TPA treatment. By contrast, in N1E-115 mouse and SK-N-MC human neuroblastoma cells, phorbol esters had no appreciable effect on [3H]choline incorporation; however, in all cells, 200 microM oleic acid enhanced PtdCho synthesis, indicating a stimulable process. Alterations by thymeleatoxin (TMT), an activator of conventional PKC isoforms (alpha, beta and gamma), were similar to beta-TPA. We investigated whether expression of specific PKC isoforms might correlate with these effects of phorbol esters on PtdCho synthesis. All cell lines bound phorbol esters, had PKC activity that was translocated by phorbol esters and differentially expressed isoforms of PKC. Northern and western blot analyses, using specific cDNA and antibodies for PKC-alpha, -beta, -gamma, -delta, -epsilon, and -zeta, revealed that expression of alpha-isoform predominated in C6 and SK-N-SH cells. In contrast, TPA-responsive beta-isoform predominated in SK-N-MC cells. gamma-PKC was not detected in any cells and only in C6 cells was PKC-delta present and translocated by beta-TPA treatment. PKC-epsilon was not detected in SK-N-MC cell lines but translocated with TPA treatment in the other three cell lines. PKC-zeta was present in all cells but was unaltered by TPA treatment. Accordingly, stimulation of PtdCho turnover by phorbol esters correlated only with expression of PKC-alpha; presence of PKC-beta alone was insufficient for a TPA response.

Identification and cellular localization of protein kinase C isoforms in cultures of rat type-1 astrocytes

We have examined which isoforms of protein kinase C were present in rat brain astrocytes and found that: (1) the total of calcium-independent isoforms was greater than the total of calcium-dependent isoforms; (2) there were differences in the intracellular distribution of different isoforms; and (3) the abundance of total protein kinase C was greater in astrocytes from cortex than astrocytes from diencephalon.

Protein expression of the alpha, gamma, delta and epsilon subspecies of protein kinase C changes as C6 glioma cells become contact inhibited and quiescent in the presence of serum

Total protein kinase C (PKC) activity and protein expression of the alpha and delta subspecies of PKC increases markedly as C6 glioma cells grow from low cell density to the contact-inhibited quiescent state (also known as G(o)) in the presence of serum. At the same time protein expression of PKC subspecies gamma and epsilon decreases while the beta I, beta II, iota and zeta subspecies did not change. Serum deprivation of growing C6 glioma cells does not induce the same changes in PKC subspecies protein expression. The findings support the growing view that there are significant differences between the G(o) states brought about by contact inhibition or serum deprivation.

Preparation of a novel monoclonal antibody specific for myelin basic protein phosphorylated on Thr98

Phosphorylation is one of a number of post-translational modifications resulting in charge microheterogeneity of myelin basic protein (MBP). This phosphorylation is claimed to destabilise the compact myelin sheath by decreasing the interaction of membrane bilayers, thereby creating or maintaining pockets of cytoplasm. To further investigate and localise MBP phosphorylation to discrete regions of the myelin sheath we raised a monoclonal antibody with specificity for a known phosphorylation site in MBP. A synthetic peptide was made by Fmoc peptide chemistry and phosphorylation of Thr98 was achieved on the resin by the global phosphorylation methodology, utilising dibenzyl-N,N-diethylphosphoramidite phosphitylation and t-butylhydroperoxide oxidation. The peptide coupled to tuberculin was used to immunise mice for monoclonal antibody production. The selected hybridoma (Clone P12) secreted an IgG2a antibody which reacted strongly with the phosphorylated immunogen and with phosphorylated fractions of bovine MBP obtained by ion exchange chromatography. The antibody had minimal reactivity with the unphosphorylated peptide; the same peptide phosphorylated at another site Ser102; a preparation of unphosphorylated MBP obtained by ion exchange chromatography; and with an irrelevant phosphorylated protein (histone). Similar phosphorylation state-specific monoclonal antibodies could be made to recognise other specific phosphorylation sites in MBP or other proteins. It is planned to use these antibodies to quantify and locate the extent of MBP phosphorylation in normal and multiple sclerosis myelin.