Glutamate and gamma-aminobutyric acid content and release of synaptosomes from temporal lobe epilepsy patients

During surgical intervention in medically refractory temporal lobe epilepsy (TLE) patients, diagnosed with either mesial temporal lobe sclerosis (MTS)- or tumor (T)-associated TLE, biopsies were taken from the anterior temporal neocortex and the hippocampal region. Synaptosomes, isolated from these biopsies were used to study intrasynaptosomal Ca(2+) levels ([Ca(2+)](i)), and glutamate and gamma-aminobutyric acid (GABA) contents and release. All synaptosomal preparations demonstrated a basal [Ca(2+)](i) of about 200 nM, except neocortical synaptosomes from MTS-associated TLE patients (420 nM). K(+)-induced depolarization resulted in a robust increase of the basal [Ca(2+)](i) in all preparations. Neocortical synaptosomes from TLE patients contained 22.9 +/- 3.0 nmol glutamate and 4.6 +/- 0.5 nmol GABA per milligram synaptosomal protein, whereas rat cortical synaptosomes contained twice as much glutamate and four times as much GABA. Hippocampal synaptosomes from MTS-associated TLE patients, unlike those from T-associated TLE patients, contained about 70% less glutamate and 55% less GABA than neocortical synaptosomes. Expressed as percentage of total synaptosomal content, synaptosomes from MTS-associated TLE patients exhibited an increased basal and a reduced K(+)-induced glutamate and GABA release compared to rat cortical synaptosomes. In MTS-associated TLE patients, only GABA release from neocortical synaptosomes was partially Ca(2+)-dependent. Control experiments in rat synaptosomes demonstrated that at least part of the reduction in K(+)-induced release can be ascribed to resection-induced hypoxia in biopsies. Thus, synaptosomes from MTS-associated TLE patients exhibit a significant K(+)-induced increase in [Ca(2+)](i), but the consequent release of glutamate and GABA is severely impaired. Our data show that at least part of the differences in glutamate and GABA content and release between human biopsy material and fresh rat tissue is due to the resection time.

Anti-B-50 (GAP-43) antibodies decrease exocytosis of glutamate in permeated synaptosomes

The involvement of the protein kinase C substrate, B-50 (GAP-43), in the release of glutamate from small clear-cored vesicles in streptolysin-O-permeated synaptosomes was studied by using anti-B-50 antibodies. Glutamate release was induced from endogenous as well as 3H-labelled pools in a [Ca(2+)]-dependent manner. This Ca(2+)-induced release was partially ATP dependent and blocked by the light-chain fragment of tetanus toxin, demonstrating its vesicular nature. Comparison of the effects of anti-B-50 antibodies on glutamate and noradrenaline release from permeated synaptosomes revealed two major differences. Firstly, Ca(2+)-induced glutamate release was decreased only partially by anti-B-50 antibodies, whereas Ca(2+)-induced noradrenaline release was inhibited almost completely. Secondly, anti-B-50 antibodies significantly reduced basal glutamate release, but did not affect basal noradrenaline release. In view of the differences in exocytotic mechanisms of small clear-cored vesicles and large dense-cored vesicles, these data indicate that B-50 is important in the regulation of exocytosis of both types of neurotransmitters, probably at stages of vesicle recycling and/or vesicle recruitment, rather than in the Ca(2+)-induced fusion step.

Role of calcineurin in Ca2+-induced release of catecholamines and neuropeptides

Neurotransmission requires rapid docking, fusion, and recycling of neurotransmitter vesicles. Several of the proteins involved in this complex Ca2+-regulated mechanism have been identified as substrates for protein kinases and phosphatases, e.g., the synapsins, synaptotagmin, rabphilin3A, synaptobrevin, munc18, MARCKS, dynamin I, and B-50/GAP-43. So far most attention has focused on the role of kinases in the release processes, but recent evidence indicates that phosphatases may be as important. Therefore, we investigated the role of the Ca2+/calmodulin-dependent protein phosphatase calcineurin in exocytosis and subsequent vesicle recycling. Calcineurin-neutralizing antibodies, which blocked dynamin I dephosphorylation by endogenous synaptosomal calcineurin activity, but had no effect on the activity of protein phosphatases 1 or 2A, were introduced into rat permeabilized nerve terminals and inhibited Ca2+-induced release of [3H]noradrenaline and neuropeptide cholecystokinin-8 in a specific and concentration-dependent manner. Our data show that the Ca2+/calmodulin-dependent phosphatase calcineurin plays an essential role in exocytosis and/or vesicle recycling of noradrenaline and cholecystokinin-8, transmitters stored in large dense-cored vesicles.

Protein kinase C in synaptic plasticity: changes in the in situ phosphorylation state of identified pre- and postsynaptic substrates

1. Long-term potentiation and its counterpart long-term depression are two forms of activity dependent synaptic plasticity, in which protein kinases and protein phosphatases are essential. 2. B-50/GAP-43 and RC3/neurogranin are two defined neuronal PKC substrates with different synaptic localization. B-50/GAP-43 is a presynaptic protein and RC3/neurogranin is only found at the postsynaptic site. Measuring their phosphorylation state in hippocampal slices, allows us to simultaneously monitor changes in pre- and postsynaptic PKC mediated phosphorylation. 3. Induction of LTP in the CA1 field of the hippocampus is accompanied with an increase in the in situ phosphorylation of both B-50/GAP-43 and RC3/neurogranin, during narrow, partially overlapping, time windows. 4. Pharmacological data show that mGluR stimulation results in an increase in the in situ phosphorylation of B-50/GAP-43 and RC3/neurogranin.

Evidence for a role of calmodulin in calcium-induced noradrenaline release from permeated synaptosomes: effects of calmodulin antibodies and antagonists

The nervous tissue-specific protein B-50 (GAP-43), which has been implicated in the regulation of neurotransmitter release, is a member of a family of atypical calmodulin-binding proteins. To investigate to what extent calmodulin and the interaction between B-50 and calmodulin are involved in the mechanism of Ca(2+)-induced noradrenaline release, we introduced polyclonal anti-calmodulin antibodies, calmodulin, and the calmodulin antagonists trifluoperazine, W-7, calmidazolium, and polymyxin B into streptolysin-O-permeated synaptosomes prepared from rat cerebral cortex. Anti-calmodulin antibodies, which inhibited Ca2+/calmodulin-dependent protein kinase II autophosphorylation and calcineurin phosphatase activity, decreased Ca(2+)-induced nor-adrenaline release from permeated synaptosomes. Exogenous calmodulin failed to modulate release, indicating that if calmodulin is required for vesicle fusion it is still present in sufficient amounts in permeated synaptosomes. Although trifluoperazine, W-7, and calmidazolium inhibited Ca(2+)-induced release, they also strongly increased basal release. Polymyxin B potently inhibited Ca(2+)-induced noradrenaline release without affecting basal release. It is interesting that polymyxin B was also the only antagonist affecting the interaction between B-50 and calmodulin, thus lending further support to the hypothesis that B-50 serves as a local Ca(2+)-sensitive calmodulin store underneath the plasma membrane in the mechanism of neurotransmitter release. We conclude that calmodulin plays an important role in vesicular noradrenaline release, probably by activating Ca2+/calmodulin-dependent enzymes involved in the regulation of one or more steps in the release mechanism.

Ba2+ replaces Ca2+/calmodulin in the activation of protein phosphatases and in exocytosis of all major transmitters

Exocytosis from nerve terminals is triggered by depolarization-evoked Ca2+ entry, which also activates calmodulin and stimulates protein phosphorylation. Ba2+ is believed to replace Ca2+ in triggering exocytosis without activation of calmodulin and can therefore be used to unravel aspects of presynaptic function. We have analysed the cellular actions of Ba2+ in relation to its effect on transmitter release from isolated nerve terminals. Barium evoked specific release of amino acid transmitters, catecholamines and neuropeptides (EC50 0.2-0.5 mM), similar to K-/Ca(2+)-evoked release both in extent and kinetics. Ba(2+)-and Ca(2+)-evoked release were not additive. In contrast to Ca2+, Ba2+ triggered release which was insensitive to trifluoperizine and hardly stimulated protein phosphorylation. These observations are in accordance with the ability of Ba2+ to replace Ca2+ in exocytosis without activating calmodulin. Nevertheless, calmodulin appears to be essential for regular (Ca(2+)-triggered) exocytosis, given its sensitivity to trifluoperizine. Both Ba(2+)-and Ca(2+)-evoked release were blocked by okadaic acid. Furthermore, anti-calcineurin antibodies decreased Ba(2+)-evoked release. In conclusion, Ba2+ replaces Ca2+/calmodulin in the release of the same transmitter pool. Calmodulin-dependent phosphorylation appears not to be essential for transmitter release. Instead, our data implicate both Ca(2+)-dependent and -independent dephosphorylation in the events prior to neurotransmitter exocytosis.

N-terminal-specific anti-B-50 (GAP-43) antibodies inhibit Ca(2+)-induced noradrenaline release, B-50 phosphorylation and dephosphorylation, and calmodulin binding

B-50 (GAP-43) is a presynaptic protein kinase C (PKC) substrate implicated in the molecular mechanism of noradrenaline release. To evaluate the importance of the PKC phosphorylation site and calmodulin-binding domain of B-50 in the regulation of neurotransmitter release, we introduced two monoclonal antibodies to B-50 into streptolysin O-permeated synaptosomes isolated from rat cerebral cortex. NM2 antibodies directed to the N-terminal residues 39-43 of rat B-50 dose-dependently inhibited Ca(2+)-induced radiolabeled and endogenous noradrenaline release from permeated synaptosomes. NM6 C-terminal-directed (residues 132-213) anti-B-50 antibodies were without effect in the same dose range. NM2 inhibited PKC-mediated B-50 phosphorylation at Ser41 in synaptosomal plasma membranes and permeated synaptosomes, inhibited 32P-B-50 dephosphorylation by endogenous synaptosomal phosphatases, and inhibited the binding of calmodulin to synaptosomal B-50 in the absence of Ca2+. Similar concentrations of NM6 did not affect B-50 phosphorylation or dephosphorylation or B-50/calmodulin binding. We conclude that the N-terminal residues 39-43 of the rat B-50 protein play an important role in the process of Ca(2+)-induced noradrenaline release, presumably by serving as a local calmodulin store that is regulated in a Ca(2+)- and phosphorylation-dependent fashion.

Long-term potentiation and synaptic protein phosphorylation

Long-term potentiation (LTP) is a well known experimental model for studying the activity-dependent enhancement of synaptic plasticity, and because of its long duration and its associative properties, it has been proposed as a system to investigate the molecular mechanisms of memory formation. At present, there are several lines of evidence that indicate that pre- and postsynaptic kinases and their specific substrates are involved in molecular mechanisms underlying LTP. Many studies focus on the involvement of protein kinase C (PKC). One way to investigate the role of PKC in long-term potentiation is to determine the degree of phosphorylation of its substrates after in situ phosphorylation in hippocampal slices. Two possible targets are the presynaptic membrane-associated protein B-50 (a.k.a. GAP 43, neuromodulin and F1), which has been implicated in different forms of synaptical plasticity in the brain such as neurite outgrowth, hippocampal LTP and neurotransmitter release, and the postsynaptic protein neurogranin (a.k.a. RC3, BICKS and p17) which function remains to be determined. This review will focus on the protein kinase C activity in pre- and postsynaptic compartment during the early phase of LTP and the possible involvement of its substrates B-50 and neurogranin.

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.

B-50/GAP-43 binds to actin filaments without affecting actin polymerization and filament organization

To investigate a possible function of the nervous tissue-specific protein kinase C substrate B-50/GAP-43 in regulation of the dynamics of the submembranous cytoskeleton, we studied the interaction between purified B-50 and actin. Both the phosphorylated and dephosphorylated forms of B-50 cosedimented with filamentous actin (F-actin) in a Ca(2+)-independent manner. Neither B-50 nor phospho-B-50 had any effect on the kinetics of actin polymerization and on the critical concentration at steady state, as measured using pyrenylated actin. Light scattering of F-actin samples was not increased in the presence of B-50, suggesting that B-50 does not bundle actin filaments. The number of actin filaments, determined by [3H]cytochalasin B binding, was not affected by either phospho- or dephospho-B-50, indicating that B-50 has neither a severing nor a capping effect. These observations were confirmed by electron microscopic evaluation of negatively stained F-actin samples, which did not reveal any structural changes in the actin meshwork on addition of B-50. We conclude that B-50 is an actin-binding protein that does not directly affect actin dynamics.

Evidence for a role of protein kinase C substrate B-50 (GAP-43) in Ca(2+)-induced neuropeptide cholecystokinin-8 release from permeated synaptosomes

To study the involvement of the protein kinase C (PKC) substrate B-50 [also known as growth-associated protein-43 (GAP-43), neuromodulin, and F1] in presynaptic cholecystokinin-8 (CCK-8) release, highly purified synaptosomes from rat cerebral cortex were permeated with the bacterial toxin streptolysin O (SL-O). CCK-8 release from permeated synaptosomes, determined quantitatively by radioimmunoassay, could be induced by Ca2+ in a concentration-dependent manner (EC50 of approximately 10(-5) M). Ca(2+)-induced CCK-8 release was maximal at 10(-4) M Ca2+, amounting to approximately 10% of the initial 6,000 +/- 550 fmol of CCK-8 content/mg of synaptosomal protein. Only 30% of the Ca(2+)-induced CCK-8 release was dependent on the presence of exogenously added ATP. Two different monoclonal anti-B-50 antibodies were introduced into permeated synaptosomes to study their effect on Ca(2+)-induced CCK-8 release. The N-terminally directed antibodies (NM2), which inhibited PKC-mediated B-50 phosphorylation, inhibited Ca(2+)-induced CCK-8 release in a dose-dependent manner, whereas the C-terminally directed antibodies (NM6) affected neither B-50 phosphorylation nor CCK-8 release. The PKC inhibitors PKC19-36 and 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), which inhibited B-50 phosphorylation in permeated synaptosomes, had no effect on Ca(2+)-induced CCK-8 release. Our data strongly indicate that B-50 is involved in the mechanism of presynaptic CCK-8 release, at a step downstream of the Ca2+ trigger. As CCK-8 is stored in large dense-cored vesicles, we conclude that B-50 is an essential factor in the exocytosis from this type of neuropeptide-containing vesicle.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of ACTH-(1-24) on dopamine and noradrenaline release, B-50 phosphorylation and calmodulin binding to B-50 in vitro

ACTH-(1-24), 1 microM, enhanced the Ca(2+)-dependent release of [3H]dopamine ([3H]DA) from intact septal synaptosomes by approximately 30%, but had no effect on the release of [3H]noradrenaline ([3H]NA) from intact cortical synaptosomes. Since a strong correlation has been reported between B-50 (phosphorylation) and [3H]NA release from intact or streptolysin-O- (SL-O-) permeated cortical synaptosomes, we investigated whether the effects of ACTH-(1-24) on the release of radiolabelled transmitters are mediated by B-50. We observed that the increment in the release of [3H]DA from SL-O-permeated septal synaptosomes as a result of exposure to a high Ca2+ concentration was much less pronounced than that of the release of [3H]NA from SL-O permeated septal and cortical synaptosomes. ACTH-(1-24) concentration-dependently inhibited [3H]NA release from SL-O-permeated cortical synaptosomes (IC50 value of approximately 10 microM) when ACTH-(1-24) was added 150 s prior to the Ca2+ trigger. Simultaneous addition of ACTH-(1-24), SL-O and Ca(2+)-buffers to cortical synaptosomes did not lead to a change in [3H]NA release at any of the ACTH-(1-24) concentrations tested. ACTH-(1-24) had no effect on B-50 phosphorylation in intact synaptosomes, whereas it concentration-dependently inhibited B-50 phosphorylation in permeated cortical synaptosomes (IC50 value of 100 microM). ACTH-(1-24) inhibited (IC50 value of 10 microM) B-50/calmodulin binding in vitro. We conclude that the effects of high concentrations of ACTH-(1-24) on various biochemical B-50 related parameters are not likely to represent the mechanisms underlying the action of ACTH-(1-24) on neurotransmitter release.

Noradrenaline release from permeabilized synaptosomes is inhibited by the light chain of tetanus toxin

Noradrenaline release from rat brain cortical synaptosomes permeabilized with streptolysin O can be triggered by microM concentrations of free Ca2+. This process was inhibited within minutes by tetanus toxin and its isolated light chain, but not by its heavy chain. The data demonstrate that the effect of tetanus toxin on NA release from purified synaptosomes is caused by the intraterminal action of its light chain.

Depolarization-induced phosphorylation of the protein kinase C substrate B-50 (GAP-43) in rat cortical synaptosomes

We studied the molecular events underlying K(+)-induced phosphorylation of the neuron-specific protein kinase C substrate B-50. Rat cortical synaptosomes were prelabelled with 32P-labelled orthophosphate. B-50 phosphorylation was measured by an immunoprecipitation assay. In this system, various phorbol esters, as well as a synthetic diacylglycerol derivative, enhance B-50 phosphorylation. K+ depolarization induces a transient enhancement of B-50 phosphorylation, which is totally dependent on extracellular Ca2+. Also, the application of the Ca2+ ionophore A23187 induces B-50 phosphorylation, but the magnitude and kinetics of A23187-induced B-50 phosphorylation differ from those induced by depolarization. The protein kinase inhibitors 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), and staurosporine antagonize K(+)- as well as PDB-induced B-50 phosphorylation, whereas trifluoperazine and calmidazolium are ineffective under both conditions. We suggest that elevation of the intracellular Ca2+ level after depolarization is a trigger for activation of protein kinase C, which subsequently phosphorylates its substrate B-50. This sequence of events could be of importance for the mechanism of depolarization-induced transmitter release.