An Ibotenate-Selective Metabotropic Glutamate Receptor: Mediates Protein Phosphorylation in Cultured Hippocampal Pyramidal Neurons

Abstract: Previous results showed that within 30 s after glutamate stimulation of cultured rat hippocampal pyramidal neurons there occurred an elevation of Ca²⁺ and diacylglycerol, and the phosphorylation of three acidic protein kinase C substrates, i.e., an 87-kDa protein known as myristoylated alanine-rich C kinase substrate and a 120-and a 48-kDa protein. In addition, it was suggested that a metabotropic-type glutamate receptor might be responsible for the phosphorylation observed. This work examines the ability of metabotropic and ionotropic glutamate receptor agonists to quickly activate phospholipases in 1.26 mM versus 50 nM extracellular Ca²⁺ by measuring the generation of inositol phosphates. NMDA, quisqualate, and trans-(±)-1-amino-1,3-cyclopentanedicarboxylic acid did not stimulate the generation of inositol phosphates in the presence of normal or low extracellular Ca²⁺ in pyramidal neurons. Kainate stimulated the production of inositol phosphates in the presence of 1.26 mM extracellular Ca²⁺ but not in 50 nM extracellular Ca²⁺. Other than glutamate, only ibotenate was able to stimulate the generation of inositol phosphates in both normal and low extracellular Ca²⁺. The maximal response to ibotenate was approximately equal to that of glutamate, when pyramidal neurons were stimulated in 50 nM extracellular Ca²⁺. The generation of inositol phosphates by glutamate and ibotenate could be partially blocked (50–60% reduction) by pretreatment of neurons with pertussis toxin (250 ng/ml),-suggesting that a GTP-binding protein might be involved. In addition, ibotenate stimulated the immediate phosphorylation of the same three protein kinase C substrates as glutamate. The NMDA receptor blocker MK-801 had no effect on this phosphorylation. These results suggest that the stimulation of phosphorylation in pyramidal neurons by glutamate occurs predominantly through the activation of an ibotenate-selective metabotropic glutamate receptor.     

Time-Dependent Increase in Protein Phosphorylation Following One-Trial Enhancement in Hermissenda

Abstract: One-trial conditioning of the nudibranch mollusk Hermissenda produces short- and long-term changes in excitability (enhancement) of identified sensory neurons. To investigate the biochemical mechanisms underlying this example of plasticity, we have examined changes in protein phosphorylation at different times following the in vitro conditioning trial. Changes in the incorporation of ³²PO₄ into proteins were determined using two-dimensional polyacrylamide gel electrophoresis, autoradiography, and densitometry. Conditioning resulted in increases in levels of several phosphoproteins, five of which, ranging in apparent molecular mass from 22 to 55 kDa, were chosen for analysis. The increased phosphorylation of the 46- and 55-kDa phosphoproteins detected 2 h postconditioning was significantly greater than the level of phosphorylation detected in an unpaired control group, indicating that long-term enhancement is pairing specific. Statistically significant increases in phosphorylation as compared with the control group that received only light were detected immediately after conditioning (5 min) for the 55-, 46-, and 22-kDa phosphoproteins, at 1 h for the 55- and 46-kDa phosphoproteins, and at 2 h for the 55-, 46-, and 22-kDa phosphoproteins. The 46- and 55-kDa phosphoproteins are putative structural proteins, and the 22-kDa phosphoprotein is proposed to be a protein kinase C substrate previously identified in Hermissenda following multitrial classical conditioning. Time-dependent increases in protein phosphorylation may contribute to the induction and maintenance of different memory stages expressed in sensory neurons after one-trial conditioning.     

Syndapin I, a Synaptic Dynamin-binding Protein that Associates with the Neural Wiskott-Aldrich Syndrome Protein

The GTPase dynamin has been clearly implicated in clathrin-mediated endocytosis of synaptic vesicle membranes at the presynaptic nerve terminal. Here we describe a novel 52-kDa protein in rat brain that binds the proline-rich C terminus of dynamin. Syndapin I (synaptic, dynamin-associated protein I) is highly enriched in brain where it exists in a high molecular weight complex. Syndapin I can be involved in multiple protein–protein interactions via a src homology 3 (SH3) domain at the C terminus and two predicted coiled-coil stretches. Coprecipitation studies and blot overlay analyses revealed that syndapin I binds the brain-specific proteins dynamin I, synaptojanin, and synapsin I via an SH3 domain-specific interaction. Coimmunoprecipitation of dynamin I with antibodies recognizing syndapin I and colocalization of syndapin I with dynamin I at vesicular structures in primary neurons indicate that syndapin I associates with dynamin I in vivo and may play a role in synaptic vesicle endocytosis. Furthermore, syndapin I associates with the neural Wiskott-Aldrich syndrome protein, an actin-depolymerizing protein that regulates cytoskeletal rearrangement. These characteristics of syndapin I suggest a molecular link between cytoskeletal dynamics and synaptic vesicle recycling in the nerve terminal.     

Depolarization and Neurotransmitters Increase Neuronal Protein Tyrosine Phosphorylation

Abstract: In rat hippocampal slices and in neurons in primary culture, K⁺-induced depolarization increased markedly and rapidly tyrosine phosphorylation of a 110-kDa protein (pp110) and, to a lesser degree, of a 120-kDa protein (pp120), in a calcium-dependent fashion. Qlutamate, 1-aminocyclopentane- trans -1,3-dicarboxylic acid (an agonist of metabotropic glutamate receptors), and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (an agonist of ionotropic glutamate receptors) stimulated also tyrosine phosphorylation of pp110 and pp120. These effects were not observed in astrocytes in primary culture. In hippocampal slices tyrosine phosphorylation of pp110 and pp120 was stimulated by Ca²⁺-ionophores and by phorbol esters and antagonized by a chelator of intracellular Ca²⁺and by drugs that inhibit protein kinase C. Stimulation of muscarinic and α₁,-adrenergic receptors increased also tyrosine phosphorylation of pp110 and pp120. These results demonstrate that membrane depolarization and stimulation of neurotransmitter receptors activate a tyrosine phosphorylation pathway in neurons. This pathway involves an increase in intracellular Ca²⁺ concentrations and the activation of protein kinase C. It may provide a biochemical basis for some neurotrophic effects of electrical activity and neurotransmitters and may contribute to the role of tyrosine phosphorylation in long-term potentiation.     

Neurite Outgrowth and Protein Phosphorylation in Chick Embryonic Sensory Ganglia Induced by a Brief Exposure to 12-O-Tetradecanoylphorbol 13-Acetate

Abstract: An exposure to 12- O -tetradecanoylphorbol 13-acetate (TPA) at 20 n M for as short as 30 min was sufficient to elicit neurite outgrowth from explanted chick embryonic sensory ganglia. Attachment of the ganglia to the collagencoated substratum during exposure to TPA was essential for subsequent neurite outgrowth. Pulse-labeling with [³⁵S]-methionine indicated no significant difference in protein synthesis between control and TPA-treated ganglia. In vitro phosphorylation assay revealed a prominent protein kinase C substrate with an apparent molecular mass of 66,000 dalton (66 kDa) in chick embryo ganglia extracts. Treatment of intact ganglia with TPA for 30 min also specifically stimulated the phosphorylation of the same protein. When staurosporine, a potent inhibitor of protein kinase C, was present during TPA treatment, both neurite outgrowth and the phosphorylation of the 66-kDa protein were blocked. Biochemical analysis of the phosphorylated 66-kDa protein indicated that (1) phosphorylation was only in serine residue, (2) the pI value was 4.5, (3) after V8 protease digestion, two phosphorylated peptide fragments, 6.0 and 7.5 kDa in size, were produced, and (4) it cross-reacted with an antiserum raised against a 66-kDa neurofilament subunit from rat spinal cord. These results suggest that early activation of protein kinase C and the phosphorylation of the 66-kDa protein may be involved in neuritogenesis.     

Synapsin Ia, Synapsin Ib, Protein IIIa, and Protein IIIb, Four Related Synaptic Vesicle-Associated Phosphoproteins, Share Regional and Cellular Localization in Rat Brain

The regional and cellular distribution of four synaptic vesicle-associated proteins, synapsins Ia and Ib (Mr 86,000 and 80,000, collectively referred to as synapsin I) and proteins IIIa and IIIb (Mr 74,000 and 55,000, collectively referred to as protein III), has been compared in selected rat brain regions, using both radioimmunoassays and back-phosphorylation assays. Lesions of several neuronal populations in the basal ganglia (corticostriatal fibers, intrinsic striatal neurons, striatonigral fibers, nigrostriatal fibers) induced decreases in the levels of these various proteins that were highly correlated (r = 0.96-0.97). Moreover, the synaptic vesicle-associated phosphoproteins displayed a similar and widespread distribution throughout the CNS. This evidence for colocalization indicates that the majority of, and possibly all, CNS neurons and nerve terminals may contain both forms of synapsin I and both forms of protein III.     

Rat Brain Protein Phosphatase 2A: An Enzyme that May Regulate Autophosphorylated Protein Kinases

Abstract: Protein phosphatase 2A (PP2A) isolated from whole rat brain homogenate supernatants has been compared with that extracted from rat synaptosomal membranes. Both purified enzymes are comprised of the three known PP2A polypeptide chains of 65 (A subunit), 55 (B/B' subunit), and 38 (C subunit) kDa and have okadaic acid inhibition curves ( K _i = 0.05 n M ) nearly identical to that reported for skeletal muscle PP2A. The isolated 38-kDa subunit of rat brain PP2A appears to contain phosphotyrosine based on cross-reactivity with a specific monoclonal antibody (PY-20). Amino acid compositions and sequences of peptides isolated from the 65- and 38-kDa species correspond to regions of the cDNA-deduced sequences of the regulatory and catalytic subunits of protein phosphatase 2A from several sources. Studies reported here also demonstrate that autophosphorylated protein kinases, particularly Ca²⁺/calmodulin-dependent protein kinase II (CaM kinase II), are excellent substrates for brain PP2A. Furthermore, Ca²⁺-dependent K⁺-depolarization of hippocampal synaptosomes was accompanied by a sequential increase, then decrease, in CaM kinase II phosphorylation level over a 45-s time course. The decrease was blocked by 1 n M okadaic acid. These data demonstrate that the type 2A protein phosphatase is present at the synapses of CNS neurons where its localization could alter the functions of phosphoproteins involved in synaptic plasticity.     

Expression of Tyrosine Hydroxylase Gene in Cultured Hypothalamic Cells: Roles of Protein Kinase A and C

Abstract: In hypothalamic cells cultured in serum-free medium, the quantity of tyrosine hydroxylase mRNA increases after treatment with an activator of the protein kinase A pathway (8-bromoadenosine cyclic AMP, 3-isobutyl-1-methylxanthine, or forskolin) or an activator of protein kinase C (12- O -tetradecanoylphorbol 13-acetate or sn -1,2-diacylglycerol). The tyrosine hydroxylase mRNA level decreases in the cells after inhibition of protein kinase C with calphostin C or after depletion of protein kinase C by extended phorbol ester treatment. These data suggest that both protein kinase pathways regulate tyrosine hydroxylase gene expression in hypothalamic cells. As simultaneous activation of both pathways has less than an additive effect on the tyrosine hydroxylase mRNA level, they appear to be interrelated. Compared with the rapid and dramatic increase of the tyrosine hydroxylase mRNA level in pheochromocytoma cells, activation of the protein kinase A or protein kinase C pathway in the cultured hypothalamic cells induces slow changes of a small magnitude in the amount of tyrosine hydroxylase mRNA. The slow regulation of tyrosine hydroxylase gene expression in hypothalamic dopaminergic neurons corresponds to the relatively high stability of tyrosine hydroxylase mRNA (half-life = 14 ± 1 h) in these cells.     

A Protein Modulator Stimulates C Kinase-Dependent Phosphorylation of a 90K Substrate in Synaptic Membranes

We have identified and partially purified an acidic, heat-stable, noncalmodulin protein from bovine brain cytosol that stimulates Ca2+-dependent phosphorylation of an Mr 90K substrate in crude rat brain synaptic membranes. We show that this modulator of phosphorylation (MOP) enhances Ca2+- and phospholipid-dependent protein kinase (C kinase) phosphorylation of this 90K substrate. The 90K substrate is a higher Mr form of an 87K substrate that is a major C kinase substrate in rat brain. The Ca2+-dependent phosphorylation of both substrates is inhibited by the Ca2+-binding proteins S-100 and calmodulin. Both substrates yield phosphopeptide fragments of Mr 9K and 13K after limited proteolysis with V8 protease. Two-dimensional polyacrylamide gel electrophoresis reveals that they have similar acidic isoelectric points (pI 5.0). MOP enhances Ca2+-dependent phosphorylation of the 90K substrate whereas the phosphorylation of 87K is diminished. This reciprocal relationship suggests that the mobility of the 87K substrate in sodium dodecyl sulfate-polyacrylamide gels is decreased to 90K with increasing phosphorylation. MOP may be a novel protein modulator of C kinase-mediated phosphorylation in the nervous system.     

Protein Kinase C Modulates Calcium Channels in Isolated Presynaptic Nerve Terminals of Rat Hippocampus

Abstract: Nerve terminals (“synaptosomes”) isolated from rat brain hippocampus were loaded with the fluorescent Ca²⁺ indicator fura-2 and were subjected to depolarization with an elevated K⁺ concentration in a stopped-flow spectrophotometer to measure the activity of voltage-gated Ca²⁺ channels in the presynaptic membrane. Three components of Ca²⁺ influx were seen, which were tentatively identified as two classes of voltage-dependent Ca²⁺ channels with different inactivation kinetics (τ of ～60 ms and 1 s, respectively) and Na⁺/Ca²⁺ exchange working in the “reverse” mode. The activity of both classes of voltage-dependent Ca²⁺ channels was slightly augmented by the phorbol ester phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), but the effect of PMA was markedly enhanced by the protein phosphatase inhibitor okadaic acid (OKA). The PKC inhibitors calphostin C and dihydrosphingosine (DHS) caused a prompt decrease in voltage-dependent Ca²⁺ channel activity, but the effect of DHS could be showed by coaddition of OKA. These results suggest that the activity of presynaptic voltage-dependent Ca²⁺ channels in the hippocampus is under a dynamic balance between PKC phosphorylation (leading to activation) and protein phosphatase dephosphorylation (leading to inactivation) and that both of these metabolic pathways are tonically active in the nerve terminals.     

Characterization of a Nerve Growth Factor-Stimulated Protein Kinase in PC12 Cells Which Phosphorylates Microtubule-Associated Protein 2 and pp250

Treatment of PC12 cells with nerve growth factor (NGF) resulted in the rapid, but transient, activation of a protein kinase which specifically phosphorylated an endogenous 250-kDa cytoskeletal protein (pp250). We report that the microtubule-associated protein, MAP2, is an alternative substrate for the NGF-activated kinase. NGF treatment maximally activated the kinase within 5 min; however, the activity declined with longer exposure to NGF. The enzyme was localized predominantly in microsomal and soluble fractions and phosphorylated MAP2 on serine and threonine residues. The soluble enzyme was fractionated by DEAE chromatography and gel filtration and had an apparent Mr of 45,000. The enzyme was purified to near homogeneity by chromatofocussing and had a pI of 4.9. Kinetic analysis revealed that NGF treatment caused a sevenfold increase in Vmax for MAP2. The Km with respect to the MAP2 substrate was approximately 50 nM and was not altered by NGF treatment. A novel feature of the NGF-stimulated enzyme was its sharp dependence on Mn2+ concentration. The active enzyme is likely to be phosphorylated, because inclusion of phosphatase inhibitors was required for recovery of optimal activity and the activity was lost on treatment of the enzyme with alkaline phosphatase. Histones, tubulin, casein, bovine serum albumin, and the ribosomal subunit protein S-6 were not phosphorylated by this enzyme. The NGF-stimulated kinase was distinct from A kinase, C kinase, or other NGF-stimulated kinases. The rapid and transient activation of the protein kinase upon NGF treatment suggests that the enzyme may play a role in signal transduction in PC12 cells.     

Generation and Regulation of β-Amyloid Peptide Variants by Neurons

Abstract: Studies of processing of the Alzheimer β-amyloid precursor protein (βAPP) have been performed to date mostly in continuous cell lines and indicate the existence of two principal metabolic pathways: the "β-secretase" pathway, which generates β-amyloid (Aβ_1–40/42; ∼4 kDa), and the "α-secretase" pathway, which generates a smaller fragment, the "p3" peptide (Aβ_17–40/42; ∼3 kDa). To determine whether similar processing events underlie βAPP metabolism in neurons, media were examined following conditioning by primary neuronal cultures derived from embryonic day 17 rats. Immunoprecipitates of conditioned media derived from [³⁵S]methionine pulse-labeled primary neuronal cultures contained 4- and 3-kDa Aβ-related species. Radiosequencing analysis revealed that the 4-kDa band corresponded to conventional Aβ beginning at position Aβ(Asp¹), whereas both radio-sequencing and immunoprecipitation-mass spectrometry analyses indicated that the 3-kDa species in these conditioned media began with Aβ(Glu¹¹) at the N terminus, rather than Aβ(Leu¹⁷) as does the conventional p3 peptide. Either activation of protein kinase C or inhibition of protein phosphatase 1/2A increased soluble βAPP_α release and decreased generation of both the 4-kDa Aβ and the 3-kDa N-truncated Aβ. Unlike results obtained with continuously cultured cells, protein phosphatase 1/2A inhibitors were more potent at reducing Aβ secretion by neurons than were protein kinase C activators. These data indicate that rodent neurons generate abundant Aβ variant peptides and emphasize the role of protein phosphatases in modulating neuronal Aβ generation.     

Co-activation of PKA and PKC in cerebrocortical nerve terminals synergistically facilitates glutamate release

Protein kinase A and protein kinase C are involved in processes that enhance glutamate release at glutamatergic nerve terminals. However, it is not known whether these two kinases co-exist within the same nerve terminal, nor is it clear what impact their simultaneous activation may have on neurotransmitter release. In cerebrocortical nerve terminals, co-application of forskolin, which increases cAMP levels and activates protein kinase A, and 4beta-phorbol dibutyrate, a direct activator of protein kinase C, synergistically enhanced the spontaneous release of glutamate. This enhancement exhibited both tetrodotoxin-sensitive and tetrodotoxin-resistant components. Interestingly, the tetrodotoxin-resistant component of release was not observed when cyclic AMP-dependent protein kinase (PKA) and calcium- and phospholipid-dependent protein kinase (PKC) were activated separately, but developed slowly after the co-activation of the two kinases, accounting for 50% of the facilitated release. This release component was dependent on voltage-dependent Ca2+ channels that opened spontaneously after PKA and PKC activation and occurred in the absence of Na+ channel firing. These data provide functional evidence for the co-existence of PKA- and PKC-signalling pathways in a subpopulation of glutamatergic nerve terminals.     

Neurotrophin-3 and Brain-Derived Neurotrophic Factor Activate Multiple Signal Transduction Events but Are Not Survival Factors for Hippocampal Pyramidal Neurons

Abstract: Expression of the neurotrophin-3 (NT-3) receptor (TrkC) and the effects of NT-3 on signal transduction were investigated in highly enriched populations of embryonic rat hippocampal pyramidal neurons grown in bilaminar cultures. PCR analysis revealed that the predominant trkC isoform is K1, which lacks an insert in the kinase domain. Polyclonal TrkC-specific antibodies stained >90% of the neurons and revealed a single ～145-kDa protein in immunoblots of extracts from adult hippocampus and pyramidal neuron cultures. Addition of NT-3 (50 mg/ml) to these cultures induced the tyrosine phosphorylation of TrkC but not TrkB, as determined by anti-phosphotyrosine staining of immunoprecipitates; thus, all the effects of NT-3 are mediated through TrkC. NT-3 also increased the tyrosine phosphorylation of 42-, 44-, 49-, 55-, 95-, and 145-kDa proteins; the pattern induced by brain-derived neurotrophic factor (BDNF) was similar but not identical to that induced by NT-3, suggesting that subtle differences may exist in signaling by TrkB and TrkC receptors. Immunoprecipitation of p21^ras from ³²P-prelabeled cells showed that NT-3 increased the level of the GTP-bound form of the protein threefold over the control within 5 min. Mitogen-activated protein (MAP) kinase activity was maximally elevated by NT-3 within 2 min and then returned slowly toward baseline over the next 60 min. Tyrosine phosphorylation of phospholipase C-γ increased rapidly after NT-3, suggesting that this enzyme becomes activated. Consistent with this, the neurotrophin rapidly increased protein kinase C activity as well as intracellular Ca²⁺ levels. The effects of both NT-3 and BDNF on Ca²⁺ levels were attenuated in Ca²⁺-free medium, suggesting that both neurotrophins increase Ca²⁺ flux across the plasma membrane as well as release from internal stores. NT-3 also increased c-Fos expression in >80% of the cells; the effect peaked at 30 minand declined to baseline by 120 min. Despite the activation of ras-MAP kinase and phosphoinositide signaling pathways, neither NT-3 nor BDNF alone or in combination could sustain hippocampal pyramidal neurons deprived of glial support. We conclude that in this system NT-3 and BDNF do not appear to be acting as classical “neurotrophic” factors and that activation of the MAP kinase pathway is insufficient for the promotion of neuronal survival.     

Survival of Chick Embryonic Sensory Neurons in Culture Is Supported by Phorbol Esters

Sensory neurons of the chick embryo are supported in culture by several neurotrophic factors, including the phorbol esters. Because phorbol esters are known to activate one of the second messengers, namely, protein kinase C, it was of interest to see if the neurotrophic action of phorbol 12,13-dibutyrate (PDB) was related to the activation of protein kinase C in sensory neurons. Sensory neurons were obtained from dorsal root ganglia of 10-day-old chick embryos and maintained in a serum-free medium for several days to quantify survival and analyze protein kinase C activity. PDB (30 nM) supported the survival of approximately 50% of the total number of neurons plated. This value was comparable to that supported by nerve growth factor (NGF; 40 ng/ml). If PDB and NGF were added together, there was no additive effect on the survival. The protein kinase C activity of the particulate and cytosolic fractions of sensory neurons supported by NGF for 3 days was 1.26 +/- 0.1 and 2.9 +/- 0.32 pmol/min/mg of protein, respectively. In contrast, neurons supported by PDB showed an approximately 500% increase in enzyme activity in their particulate fraction. The enzyme activity of the cytosolic fraction was decreased by approximately 40%. If NGF-supported neurons were treated with PDB (30 nM) for 15 min, protein kinase C activity increased greater than 400% in the particulate fraction, whereas an approximately 50% decrease was observed in the cytosolic fraction. The protein kinase C value, expressed as a ratio of the activities in the particulate to cytosol fractions, showed large increases after phorbol treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Protein Kinase C of Sympathetic Neuronal Membrane Is Activated by Phorbol Ester-Correlation Between Transmitter Release, 45Ca2+ Uptake, and the Enzyme Activity

The effects of phorbol esters [phorbol 12,13-dibutyrate (PDB), 12-O-tetradecanoylphorbol 13-acetate (TPA), and phorbol 13-acetate] were investigated on the release of [3H]norepinephrine, 45Ca2+ accumulation, and protein kinase C activity in cultured sympathetic neurons of the chick embryo. Sympathetic neurons derived from 10-day-old chick embryo were cultured in serum-free medium supplemented with insulin, transferrin, and nerve growth factor. After 3 days, neurons were loaded with [3H]-norepinephrine and the release of [3H]norepinephrine was determined before and after electrical stimulation. Stimulation at 1 Hz for 15 s increased the release of [3H]-norepinephrine over the nonstimulation period. Stimulation-evoked release gradually declined with time during subsequent stimulation periods. Incubation of neurons in Ca2+-free Krebs solution containing 1 mM EGTA completely blocked stimulation-evoked release of [3H]-norepinephrine. Stimulation-evoked release of [3H]-norepinephrine was markedly facilitated by 3 and 10 nM PDB or TPA. The spontaneous release was also enhanced by PDB and TPA. The net accumulation of 45Ca2+ during stimulation of sympathetic neurons was increased by two- to fourfold in the presence of PDB or TPA. PDB at 1-100 nM produced a concentration-dependent increase in the activation of protein kinase C. PDB at 30 nM increased the activity of protein kinase C of the particulate fraction from 0.09 to 0.58 pmol/min/mg protein. There was no significant change in protein kinase C activity of the cytosolic fraction (0.14 pmol/min/mg versus 0.13 pmol/min/mg protein). The ratio of the particulate to cytosolic protein kinase C increased from a control value of 0.62 to 4.39 after treatment with 30 nM PDB. TPA (10 and 30 nM) also increased protein kinase C activity of the particulate fraction by six- to eightfold. Phorbol 13-acetate had no effect on protein kinase C activity, [3H]norepinephrine release, and 45Ca2+ accumulation. These results provide direct evidence that activation of protein kinase C enhances Ca2+ accumulation, which in turn leads to the facilitation of transmitter release in sympathetic neurons.     

蛋白激酶C对大鼠缺血海马突触体谷氨酸摄取的调控作用

采用大鼠海马脑片体外缺血模型,观察海马突触体内蛋白激酶Ｃ（ＰＫＣ）活性的变化,以及这种变化对突触体谷氨酸（ＧＬＵ）摄取的影响。结果显示：海马脑片体外“缺血”１０ｍｉｎ,其突触体内ＰＫＣ活性基本不变,而缺血３０ｍｉｎ,突触体内ＰＫＣ活性显著上升（Ｐ＜０．０１,ｎ＝６）;非Ｎ－甲基－Ｄ－天门冬氨酸（ＮＭＤＡ）受体拮抗剂ＤＮＱＸ有效地抑制ＰＫＣ活性的同时,可降低胞外ＧＬＵ的堆积,而ＮＭＤＡ受体阻断剂ＡＰ＿５无作用。进一步实验证明,ＰＫＣ激动剂ＰＤＢ浓度依赖性地抑制突触体对３Ｈ－ＧＬＵ的摄取（ＩＣ５０＝１３１±１０μｍｏｌ／Ｌ）,此抑制作用可由ＰＫＣ抑制剂Ｈ－７（１００μｍｏｌ／Ｌ）抵消。提示脑缺血诱发ＧＬＵ堆积的作用机理可能是：脑缺血引发钙内流导致ＧＬＵ过量释放,ＧＬＵ又通过突触前非ＮＭＤＡ受体激活ＰＫＣ,抑制其自身摄取,正反馈性加重胞外ＧＬＵ的堆积。     

Eicosanoid activation of protein kinase C epsilon: involvement in growth cone repellent signaling

Exposure of growing neurons to thrombin or semaphorin 3A stimulates a receptor-mediated signaling cascade that results in collapse of their growth cones. This collapse response necessitates eicosanoid production, as we have shown earlier. The present report investigates whether and which protein kinase C (PKC) isoforms may be activated by such eicosanoids. To examine these questions, we isolated growth cones from fetal rat brain and tested whether thrombin or the eicosanoid, 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE), could activate endogenous growth cone PKC. We show that both thrombin and 12(S)-HETE stimulate the phosphorylation of the myristoylated alanine-rich protein kinase C substrate, an 87-kDa adhesion site protein. Furthermore, we show both with immunoprecipitated and with recombinant PKC that 12(S)-HETE activation is selective for the epsilon isoform and does not require accessory proteins. Last, we demonstrate that PKC activation is necessary for thrombin-induced growth cone collapse. These data indicate that eicosanoid-mediated repellent effects result from the direct and selective activation of PKCepsilon and suggest the involvement of myristoylated alanine-rich protein kinase C substrate phosphorylation in growth cone collapse.     

Interaction of the C-terminal region of the rat serotonin transporter with MacMARCKS modulates 5-HT uptake regulation by protein kinase C

The serotonin transporter (SERT) mediates the re-uptake of released serotonin into presynaptic nerve terminals. Its activity is regulated by different mechanisms including protein kinase C (PKC) triggered internalization. Here, we used yeast 2-hybrid screening and cotransfection into 293 cells to identify a homologue of the myristoylated alanine-rich C kinase substrate (MARCKS), MacMARCKS, as a C-terminally interacting protein of SERT. Upon cotransfection with SERT, MacMARCKS caused a reduction in the maximal rate of [(3)H]serotonin uptake and reduced its down-regulation elicited by activation of PKC. Our data are consistent with MARCKS proteins regulating the plasma membrane dynamics of neurotransmitter transporters.