Okadaic acid-induced inhibition of B-50 dephosphorylation by presynaptic membrane-associated protein phosphatases

Okadaic acid and cyclosporin A modulate [(3)H]GABA release from rat brain synaptosomes

Rat brain synaptosomes were used to investigate the effect of okadaic acid, an inhibitor of protein phosphatase 1 and 2A, and cyclosporin A, an inhibitor of protein phosphatase 2B (calcineurin), on [(3)H]GABA release. Release of [(3)H]GABA was evoked by 4-aminopyridine in the presence of calcium and by alpha-latrotoxin in the presence and absence of calcium. Pretreatment of synaptosomes with 1 microM okadaic acid reduced [(3)H]GABA release evoked by 4-aminopyridine by about 40%. The effect of alpha-latrotoxin on [(3)H]GABA release was stimulated by okadaic acid. This stimulation was equal in both media. The stimulating effect of 4-aminopyridine and alpha-latrotoxin on [(3)H]GABA release was activated when synaptosomes were pretreated with cyclosporin A. Activation of 4-aminopyridine-evoked [(3)H]GABA release was observed at 1 microM cyclosporin A, but the toxin effect was enhanced only when concentration of cyclosporin A was increased to 10 microM. The level of cyclosporin A activation depended on alpha-latrotoxin concentrations used - a higher stimulating effect of cyclosporin A was observed with lower toxin concentration. These results suggest that in calcium medium 4-aminopyridine- and alpha-latrotoxin-evoked [(3)H]GABA release was realized by different mechanisms.

Muscarinic receptor- and phorbol ester-stimulated phosphorylation of protein kinase C substrates in adult and neonatal cortical slices

The neuron-specific protein B-50 (GAP-43) is a major presynaptic substrate for protein kinase C (PKC). Phosphorylation of B-50 by PKC at serine-41 is functionally related to signal transduction in association with process outgrowth and neurotransmitter release. Thus, it is important to characterize the factors which modulate phosphorylation of B-50 by PKC. Phosphoinositide (PI)-coupled muscarinic acetylcholine receptor (mAchR) activation would be expected to increase PKC activity through production of the second messenger, diacylglycerol. To test the hypothesis that activation of mAchR also increases phosphorylation of B-50, protein phosphorylation has been examined in cerebral cortical slices in response to the cholinergic agonist, carbachol (Cch) in comparison to the phorbol ester, 4beta-phorbol 12, 13-dibutyrate (PDB), a known activator of PKC. At short times of incubation with 1 mM Cch, a concentration which maximally activates PI metabolism, increased phosphorylation of a group of synaptosomal proteins, including B-50 and myristoylated, alanine-rich C kinase substrate (MARCKS), was observed. This increase was approximately half of that obtained in response to 1 microM PDB. Differing patterns of protein phosphorylation were observed in neonatal and adult slices: neonatal samples contained more MARCKS and a PKC substrate with a Mr of 46 kDa. Phosphorylation of B-50 and MARCKS was sensitive to Cch in both cases. Immunoblotting demonstrated less m1 acetylcholine receptor (the predominant mAchR subtype coupled to PI metabolism in the cortex) in neonatal, as compared to adult, synaptosomal fractions. These results are consistent with a coupling between mAchR-stimulated PI metabolism and PKC-mediated protein phosphorylation that is developmentally regulated.

Decreased phosphorylation of GAP-43/B-50 in striatal synaptic plasma membranes after circling motor activity

The effects of spontaneous circling motor activity on the in vitro phosphorylation of the protein kinase C substrate GAP-43/B-50 was studied on striatal membranes of developing rats (30 days of age). At this time of postnatal development, permanent plastic changes in cholinergic and dopaminergic systems are produced by physiological motor activity. Exercised animals showed a significant reduction of 31% in the level of GAP-43/B-50 endogenous phosphorylation in the contralateral striatum respect to the ipsilateral side (P < 0.01), while control animals did not show asymmetric differences. Compared to controls, the contralateral striatum of exercised animals showed a 33% reduction in the incorporation of 32P-phosphate into GAP-43/B-50 30 minutes post-exercise (P < 0.01). This change in GAP-43/B-50 phosphorylation was correlated with the running speed developed by the animals (r:0.8986, P = 0.015). GAP-43/B-50 immunoblots revealed no changes in the amount of this protein in any group. Moreover, a significant variation of 25% (P < 0.05) in the PKC activity was seen between both exercised striata. Interhemispheric differences were not found in control animals. We conclude that endogenous phosphorylation of this protein is also altered by motor activity in the same period that permanent changes in striatal neuroreceptors are triggered after motor training.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Regulation of in vitro phosphorylation of the casein kinase II sites in B-50 (GAP-43)

Casein kinase II (CKII) phosphorylates the rat neuronal growth-associated protein B-50 (GAP-43) at serines 191/192 and threonines 88, 89 and/or 95 both in vitro and in neuronal growth cones. Since little is known concerning regulation of the phosphorylation of these sites, these studies were undertaken to characterize the factors which determine the degree of B-50 phosphorylation by CKII in vitro. Phosphorylation of rat B-50 on serine and threonine residues by recombinant human CKII is stimulated by polylysine. Maximal stimulation occurs at 10 microg/ml of polylysine, a concentration which has no effect on protein kinase C (PKC)-mediated phosphorylation of B-50. Digestion with Staphylococcus aureus V8 protease demonstrates CKII-mediated phosphorylation of B-501-132 and the C-terminal fragment S3/S4. Phosphorylation of B-50 by either CKII or PKC is inhibited by the N-terminal monoclonal antibody NM2, while the C-terminal antibody NM6 has no effect on phosphorylation by either protein kinase. Protein phosphatase 2A dephosphorylates both the CKII and PKC sites, while protein phosphatases 2B and 1 are more selective for the PKC site. These results indicate that the phosphorylations of B-50 by CKII and PKC are determined by distinct regulatory signals in vivo.

Time course and involvement of protein kinase C-mediated phosphorylation of F1/GAP-43 in area CA3 after mossy fiber stimulation

1. Protein kinase C (PKC) activity and phosphorylation of F1/growth associated protein (GAP)-43, a PKC substrate, have been proposed to play key roles in the maintenance of long-term potentiation (LTP) at the synapses of Schaffer collateral/commissural on pyramidal neurons in CA1 (Akers et al., 1986). We have studied in the involvement of PKC and PKC-dependent protein phosphorylation of F1/GAP-3 in in vitro LTP observed at the synapses of mossy fiber (MF) on CA3 pyramidal neurons of rat hippocampus by post hoc in vitro phosphorylation. 2. After LTP was induced in CA3 in either the presence or absence of D-2-amino-5-phosphonovaleric acid (AP5), an NMDA receptor antagonist, the CA3 region was dissected for in vitro phosphorylation assay. In vivo phosphorylation of F1/GAP-43 was increased in membranes at 1 and 5 min after tetanic stimulation (TS) but not at 60 min after TS. 3. The degree of phosphorylation of F1/GAP-43 in the cytosol was inversely related to that in membranes at each time point after LTP. 4. The similar biochemical changes obtained from either control slices or AP5-treated slices indicate that LTP and the underlying biochemical changes are independent of the NMDA receptor. Immunoreactivity of the phosphorylated F1/GAP-43 in LTP slices was not significantly different from control, indicating that results from western blotting and post hoc in vitro phosphorylation are consistent. 5. Post hoc in vitro phosphorylation of F1/GAP-43 was PKC-mediated since phosphorylation of F1/GAP-43 was altered by the PKC activation cofactors, Ca2+, phosphatidylserine and phorbol ester. 6. Calmodulin (CaM) at > 5 microM inhibited phosphorylation, consistent with the presence of CaM-binding activity at the site on F1/GAP-43 acted upon by PKC. 7. We conclude that phosphorylation of F1/GAP-43 is associated with the induction but not the maintenance phase of MF-CA3 LTP.

Postnatal changes in serine/threonine protein phosphatases and their association with the microtubules

The activities and protein levels of three serine/threonine protein phosphatases were determined in homogenates and microtubule preparations from rat brain at various ages from postnatal day 1 (P1) through adulthood. The activities and levels of the calcium/calmodulin-dependent protein phosphatase, phosphatase 2B increased significantly from P1 to P21 in brain homogenates and remained elevated in the adult. The association of phosphatase 2B with microtubules was also found to be increased in the adult compared to the neonate (P3). In contrast, protein phosphatase 2A in brain homogenates decreased significantly from P1 to adult. However, the association of phosphatase 2A with the microtubules was found to increase with age. Finally, the activity and levels of phosphatase 1 in brain homogenates did not change with postnatal age, although the association of phosphatase 1 with microtubules was significantly decreased in the adult brain compared to P3. These studies clearly indicate that the activity, levels and association of these serine/threonine phosphatases with microtubules are independently regulated during postnatal development and suggest unique roles of phosphatase 1, phosphatase 2A and phosphatase 2B in modulating the phosphorylation state and function of microtubule-associated proteins at different postnatal ages.

Cloning and promoter analysis of the human B-50/GAP-43 gene

We here report isolation of exon 1 and analysis of the human B-50 promoter. A human genomic lambda EMBL3 library was screened with a homologous PCR probe. Two independent clones were analyzed and partially sequenced: They contained up to 5 kb sequence upstream of the translation start site and approx 13 kb of intron 1 sequence. There was a high degree of homology between the rat and the human gene with 100% homology from -504 to -427, with respect to the translation start codon. However, relatively long GT and GA repeats as seen in the rat gene were absent. Various promoter-reporter constructs, containing 5.0 to 0.12 kb of the upstream region, were transfected into undifferentiated and neuroectodermally differentiated P19-EC. Two promoter activities were found. The minimal fragment with promoter activity still responsive to differentiation was the 0.22 kb construct, similar to rat promoter P2. We conclude that the human B-50 gene is expressed in a similar way to the rat B-50 gene, based on the presence of two transcripts, the high degree of homology between the rat and the human sequence, and the two promoter activities found in P19-EC cells.

Mechanisms of muscarinic receptor-stimulated expression of c-fos in SH-SY5Y cells

In this study, the signal cascade transducing carbachol stimulation into c-fos expression in SH-SY5Y neuroblastoma cells was investigated. 1,2-Diacylglycerol formation and c-fos expression were mediated via stimulation of muscarinic M1 receptors and the first 5 min of receptor stimulation were critical for these events. Application of 1,2-dioctanoylglycerol induced c-fos expression and this, as well as carbachol-stimulated c-fos expression, was inhibited by protein kinase C inhibitors. Increasing the intracellular Ca2+ concentration had only small effects on c-fos expression. There was a dependency on extracellular Ca2+ for maximal c-fos expression and 1,2-diacylglycerol formation. The carbachol-stimulated c-fos expression was potentiated by application of the protein phosphatase inhibitor okadaic acid. These results demonstrate the importance of 1,2-diacylglycerol formation for muscarinic receptor-stimulated, protein kinase C-mediated c-fos expression in the SH-SY5Y cells and that this cascade is counteracted by an okadaic acid-sensitive protein phosphatase.

Differential activities of protein phosphatase types 1 and 2A in cytosolic and particulate fractions from rat forebrain

The activities and concentrations of protein phosphatase type 1 (PP1) and type 2A (PP2A) were compared in cytosol and particulate fractions of rat forebrain. Although the activity of PP2A was highest in the cytosol, immunoblot analysis with a PP2A-specific antibody showed that there were significant levels of the enzyme in the particulate fraction. There was no significant difference between the concentration of PP2A in the cytosol and particulate fractions such that the low activity of PP2A in the particulate fraction represents an inactivation of this form of the enzyme. Similar analysis in skeletal muscle, heart, and liver showed this finding was unique to the brain. Similarly, the majority of PP1 activity was recovered in the cytosol, but most PP1 enzyme was associated with the particulate fraction. Comparison with other tissues showed that the activities of PP1 in the particulate fractions were similar but that the forebrain contained significantly more enzyme than the other tissues. Thus, like PP2A it appears that the specific activity of PP1 in the particulate fraction of rat forebrain is much lower than that of the cytosol and of the particulate fractions of other tissues. Elution of PP1 and PP2A from membranes with 0.5 M NaCl plus 0.3% Triton X-100 resulted in severalfold activation of both enzymes. That the majority of PP1 and PP2A in rat forebrain are associated with membrane structures but in a low activity state suggests that novel regulatory mechanisms exist that have considerable and unique potential for activation of protein dephosphorylation.

Monoclonal antibody NM2 recognizes the protein kinase C phosphorylation site in B-50 (GAP-43) and in neurogranin (BICKS)

Mouse monoclonal B-50 antibodies (Mabs) were screened to select a Mab that may interfere with suggested functions of B-50 (GAP-43), such as involvement in neurotransmitter release. Because the Mab NM2 reacted with peptide fragments of rat B-50 containing the unique protein kinase C (PKC) phosphorylation site at serine-41, it was selected and characterized in comparison with another Mab NM6 unreactive with these fragments. NM2, but not NM6, recognized neurogranin (BICKS), another PKC substrate, containing a homologous sequence to rat B-50 (34-52). To narrow down the epitope domain synthetic B-50 peptides were tested in ELISAs. In contrast to NM6, NM2 immunoreacted with B-50 (39-51) peptide, but not with B-50 (43-51) peptide or a C-terminal B-50 peptide. Preabsorption by B-50 (39-51) peptide of NM2 inhibited the binding of NM2 to rat B-50 in contrast to NM6. NM2 selectively inhibited phosphorylation of B-50 during endogenous phosphorylation of synaptosomal plasma membrane proteins. Preabsorption of NM2 by B-50 (39-51) peptide abolished this inhibition. In conclusion, NM2 recognizes the QASFR peptide in B-50 and neurogranin. Therefore, NM2 may be a useful tool in physiological studies of the role of PKC-mediated phosphorylation and calmodulin binding of B-50 and neurogranin.

Okadaic acid induces early changes in microtubule-associated protein 2 and tau phosphorylation prior to neurodegeneration in cultured cortical neurons

Microtubules and their associated proteins play a prominent role in many physiological and morphological aspects of brain function. Abnormal deposition of the microtubule-associated proteins (MAPs), MAP2 and tau, is a prominent aspect of Alzheimer's disease. MAP2 and tau are heat-stable phosphoproteins subject to high rates of phosphorylation/dephosphorylation. The phosphorylation state of these proteins modulates their affinity for tubulin and thereby affects the structure of the neuronal cytoskeleton. The dinoflagellate toxin okadaic acid is a potent and specific inhibitor of protein phosphatases 1 and 2A. In cultured rat cortical neurons and a human neuroblastoma cell line (MSN), okadaic acid induces increased phosphorylation of MAP2 and tau concomitant with early changes in the neuronal cytoskeleton and ultimately leads to cell death. These results suggest that the diminished rate of MAP2 and tau dephosphorylation affects the stability of the neuronal cytoskeleton. The effect of okadaic acid was not restricted to neurons. Astrocytes stained with antibodies to glial fibrillary acidic protein (GFAP) showed increased GFAP staining and changes in astrocyte morphology from a flat shape to a stellate appearance with long processes.

Phorbol ester- and glutamate-sensitive phosphorylation of hippocampal membrane proteins from adult and neonatal rats

The phosphoinositide (PI) second messenger system in the neonatal rat brain is differentially stimulated as compared to that of the adult by agonists such as glutamate. Among the factors that might contribute to the neonatal pattern is the nature of phosphorylated membrane-bound proteins which could regulate this receptor-mediated response. This study was undertaken to compare membrane protein phosphorylation under conditions that affect PI hydrolysis in neonatal and adult rat hippocampus. Two-dimensional gel analysis revealed enhanced basal phosphorylation of two membrane proteins (M(r): 46,000 and 80,000; pI: 4.4 and 4.2, respectively) in the neonatal hippocampus when compared to the adult. The former phosphoprotein is present only in neonatal hippocampus. Phosphorylation of a 48,000 M(r) protein with a pI of 4.5 is prominent in hippocampal membranes from both neonatal and adult rats. After incubation of neonatal hippocampal slices with an active phorbol ester, 12-O-tetradecanoyl phorbol-13-acetate (TPA), all three proteins show decreased post-hoc phosphorylation. Slices from neonatal rats incubated with glutamate demonstrated no alteration in the phosphorylation of any of these proteins, while those from adult rats produced a marked change in phosphorylation of the 80,000 M(r) protein. The data suggest that phosphorylation of this protein from neonates is not yet as efficiently coupled to receptor stimulation as that from the adult. Immunoblot analysis revealed that the 48,000 M(r) protein is the growth-associated protein B-50/GAP-43 and that the 80,000 M(r) protein is a membrane-associated form of the MARCKS protein.(ABSTRACT TRUNCATED AT 250 WORDS)

An okadaic acid-sensitive protein phosphatase counteracts protein kinase C-induced phosphorylation in SH-SY5Y cells

Protein phosphorylation and subsequent dephosphorylation was studied in digitonin-permeabilized neuroblastoma SH-SY5Y cells by measuring the incorporation of [32P]phosphate into myelin basic protein (MBP). 1,2-Dioctanoyl-sn-glycerol (DOG) and calcium synergistically induced phosphorylation of MBP, which was inhibited by the protein kinase C (PKC) pseudosubstrate peptide (PKC19-36). The phosphorylation increased for 10 min when a net dephosphorylation started to appear. The dephosphorylation was inhibited by okadaic acid. Regardless of calcium concentration, the presence of DOG was necessary for significant effects of okadaic acid on MBP phosphorylation. H7 and staurosporine dose-dependently inhibited the phosphorylation of MBP, induced by DOG and calcium in the presence of okadaic acid. Different PKC pseudosubstrate peptides were applied and all showed an inhibitory effect on the phosphorylation of MBP under these conditions. These results demonstrate the presence, in SH-SY5Y cells, of a protein phosphatase, possibly protein phosphatase 2A, with a high basal activity that counteracts PKC-induced phosphorylation.

Studies on the role of B-50 (GAP-43) in the mechanism of Ca(2+)-induced noradrenaline release: lack of involvement of protein kinase C after the Ca2+ trigger

The involvement of B-50, protein kinase C (PKC), and PKC-mediated B-50 phosphorylation in the mechanism of Ca(2+)-induced noradrenaline (NA) release was studied in highly purified rat cerebrocortical synaptosomes permeated with streptolysin-O. Under optimal permeation conditions, 12% of the total NA content (8.9 pmol of NA/mg of synaptosomal protein) was released in a largely (> 60%) ATP-dependent manner as a result of an elevation of the free Ca2+ concentration from 10(-8) to 10(-5) M Ca2+. The Ca2+ sensitivity in the micromolar range is identical for [3H]NA and endogenous NA release, indicating that Ca(2+)-induced [3H]NA release originates from vesicular pools in noradrenergic synaptosomes. Ca(2+)-induced NA release was inhibited by either N- or C-terminal-directed anti-B-50 antibodies, confirming a role of B-50 in the process of exocytosis. In addition, both anti-B-50 antibodies inhibited PKC-mediated B-50 phosphorylation with a similar difference in inhibitory potency as observed for NA release. However, in a number of experiments, evidence was obtained challenging a direct role of PKC and PKC-mediated B-50 phosphorylation in Ca(2+)-induced NA release. PKC pseudosubstrate PKC19-36, which inhibited B-50 phosphorylation (IC50 value, 10(-5) M), failed to inhibit Ca(2+)-induced NA release, even when added before the Ca2+ trigger. Similar results were obtained with PKC inhibitor H-7, whereas polymyxin B inhibited B-50 phosphorylation as well as Ca(2+)-induced NA release. Concerning the Ca2+ sensitivity, we demonstrate that PKC-mediated B-50 phosphorylation is initiated at a slightly higher Ca2+ concentration than NA release. Moreover, phorbol ester-induced PKC down-regulation was not paralleled by a decrease in Ca(2+)-induced NA release from streptolysin-O-permeated synaptosomes. Finally, the Ca(2+)- and phorbol ester-induced NA release was found to be additive, suggesting that they stimulate release through different mechanisms. In summary, we show that B-50 is involved in Ca(2+)-induced NA release from streptolysin-O-permeated synaptosomes. Evidence is presented challenging a role of PKC-mediated B-50 phosphorylation in the mechanism of NA exocytosis after Ca2+ influx. An involvement of PKC or PKC-mediated B-50 phosphorylation before the Ca2+ trigger is not ruled out. We suggest that the degree of B-50 phosphorylation, rather than its phosphorylation after PKC activation itself, is important in the molecular cascade after the Ca2+ influx resulting in exocytosis of NA.

Protein phosphatases 1 and 2A dephosphorylate B-50 in presynaptic plasma membranes from rat brain

The protein B-50 is dephosphorylated in rat cortical synaptic plasma membranes (SPM) by protein phosphatase type 1 and 2A (PP-1 and PP-2A)-like activities. The present studies further demonstrate that B-50 is dephosphorylated not only by a spontaneously active PP-1-like enzyme, but also by a latent form after pretreatment of SPM with 0.2 mM cobalt/20 micrograms of trypsin/ml. The activity revealed by cobalt/trypsin was inhibited by inhibitor-2 and by high concentrations (microM) of okadaic acid, identifying it as a latent form of PP-1. In the presence of inhibitor-2 to block PP-1, histone H1 (16-64 micrograms/ml) and spermine (2 mM) increased B-50 dephosphorylation. This sensitivity to polycations and the reversal of their effects on B-50 dephosphorylation by 2 nM okadaic acid are indicative of PP-2A-like activity. PP-1- and PP-2A-like activities from SPM were further displayed by using exogenous phosphorylase alpha and histone H1 as substrates. Both PP-1 and PP-2A in rat SPM were immunologically identified with monospecific antibodies against the C-termini of catalytic subunits of rabbit skeletal muscle PP-1 and PP-2A. Okadaic acid-induced alteration of B-50 phosphorylation, consistent with inhibition of protein phosphatase activity, was demonstrated in rat cortical synaptosomes after immunoprecipitation with affinity-purified anti-B-50 immunoglobulin G. These results provide further evidence that SPM-bound PP-1 and PP-2A-like enzymes that share considerable similarities with their cytosolic counterparts may act as physiologically important phosphatases for B-50.