Pharmacological characterization of six trkB antibodies reveals a novel class of functional agents for the study of the BDNF receptor

BACKGROUND AND PURPOSE

By interacting with trkB receptors, brain-derived neurotrophic factor (BDNF) triggers various signalling pathways responsible for neurone survival, differentiation and modulation of synaptic transmission. Numerous reports have implicated BDNF and trkB in the pathogenesis of various central nervous system affections and in cancer, thus representing trkB as a promising therapeutic target. In this study, we used an antibody-based approach to search for trkB-selective functional reagents.

EXPERIMENTAL APPROACH

Six commercially available polyclonal and monoclonal antibodies were tested on recombinant and native, human and rodent trkB receptors. Functional and pharmacological characterization was performed using a modified version of the KIRA-elisa method and radioligand binding studies. Western blot analyses and neurite outgrowth assays were carried out to determine the specificity and selectivity of antibody effects. The survival properties of one antibody were further assessed on cultured neurones in a serum-deprived paradigm.

KEY RESULTS

The functional trkB-selective antibodies showed distinct pharmacological profiles, ranging from partial agonists to antagonists, acting on trkB receptors through allosteric modulations. The same diversity of effects was observed on the mitogen-activated protein kinase signalling pathway downstream of trkB and on the subsequent neurite outgrowth. One antibody with partial agonist activity demonstrated cell survival properties by activating the Akt pathway. Finally, these antibodies were functionally validated as true trkB-selective ligands because they failed activating trkA or trkC, and contrary to BDNF, none of them bind to p75(NTR).

CONCLUSIONS AND IMPLICATIONS

These trkB-selective antibodies represent a novel class of pharmacological tools to explore the pathophysiological roles of trkB and its potential therapeutic relevance for the treatment of various disorders.

In vitro effects of 17 beta-oestradiol on the sensitivity of receptors coupled to adenylate cyclase on striatal neurons in primary culture

Pretreatment of intact striatal neurons from the mouse embryo in primary culture with 17 beta-oestradiol (10(-9) M), 24 hours) enhanced the stimulation of adenylate cyclase activity induced by either dopamine (D1 receptors), isoproterenol, serotonin or 2-chloroadenosine (maximal effective concentrations) but suppressed inhibitory responses evoked by agonists of D2-dopaminergic or enkephalin (mu and delta) receptors. Binding studies indicated that some of these effects are (beta 1) or are not (D1 and D2) associated with changes in the number of receptors. Similar effects were partially seen with testosterone but not with 17 alpha-oestradiol, progesterone or dexamethasone and those induced by 17 beta-oestradiol were abolished when cells were exposed to inhibitors of mRNA transcription (alpha-amanitin) or protein synthesis (cycloheximide). Modifications in the properties of Gs or Go,i proteins were postulated because the number of adenylate cyclase catalytic subunits was not affected by 17 beta-oestradiol pretreatment. Results of ADP-ribosylation experiments with cholera toxin or pertussis toxin and of immunoblot experiments with anti-G alpha o and anti-G beta sera led us to suggest that 17 beta-oestradiol induces qualitative modifications in Go,i proteins leading to a stabilization of the associated form of the heterotrimer G alpha o,i beta gamma. In fact, pretreatment with pertussis toxin (which impairs G alpha o,i beta gamma dissociation) mimics the effects of 17 beta-oestradiol on responses of adenylate cyclase to stimulatory and inhibitory agonists.

N-Methyl-D-aspartate receptor activation inhibits protein synthesis in cortical neurons independently of its ionic permeability properties

Transient cerebral ischemia, which is accompanied by a sustained release of glutamate, strongly depresses protein synthesis. We have previously demonstrated in cortical neurons that a glutamate-induced increase in intracellular Ca(2+) is likely responsible for the blockade of the elongation step of protein synthesis. In this study, we provide evidence indicating that NMDA mobilizes a thapsigargin-sensitive pool of intracellular Ca(2+). Exposure of cortical neurons to NMDA, in the absence of external Ca(2+), produced a transient rise in intracellular Ca(2+) that was suppressed by pretreatment with thapsigargin. This rise in intracellular Ca(2+) did not result from an influx of Na(+) via reversal of the mitochondrial Na(+)/Ca(2+) exchanger since it persisted in a Na(+)-free medium or in the presence of CGP 37157, an inhibitor of the exchanger. Moreover, the NMDA-induced increase in intracellular Ca(2+) required the presence of D-serine, was blocked by D(-)-2-amino-5-phosphonopentanoic acid, but was not reduced in the presence of external Mg(2+). This unexpected non-ionotropic effect of NMDA was associated with an inhibition of protein synthesis that was also insensitive to the absence of external Ca(2+) or Na(+), or presence of Mg(2+). NMDA treatment resulted in an increase in the phosphorylation of eEF-2 in the absence or presence of external Ca(2+). The initiation step of protein synthesis was not blocked by NMDA since the phosphorylation of initiation factor eIF-2alpha subunit was not altered by NMDA treatment. In conclusion, we provide evidence indicating that NMDA can inhibit protein synthesis in cortical neurons through a process that involves the mobilization of intracellular Ca(2+) stores via a mechanism that is not linked to the ionic properties of NMDA receptors.

AMPA receptor activation induces association of G-beta protein with the alpha subunit of the sodium channel in neurons

Glutamatergic transmission is mediated by ionotropic receptors that directly gate cationic channels and metabotropic receptors that are coupled to second messenger generating systems and to ionic channels via heterotrimeric guanine-nucleotide binding- (G) proteins. This distinction cannot be made for the ionotropic receptor subclass activated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), which has been shown to be physically associated with the alpha-subunit of Gi1 protein and activates this G-protein. Here, we report that, in addition to a Ca2+ influx, AMPA induces the mobilization of Ca2+ from the mitochondrial pool by reversing the mitochondrial Na+/Ca2+ exchanger in mouse neurons in primary culture. Both processes required the activation of tetrodotoxin-sensitive Na+ channels. AMPA receptor activation modified the gating properties of the Na+ channel, independently of the AMPA current, suggesting a G-protein-mediated process. Indeed, co-immunoprecipitation experiments indicated that AMPA receptor activation induced the association of Gbeta with the alpha-subunit of the Na+ channel. These results suggest that, in addition to its ionic channel function, the AMPA receptor is coupled to Na+ channels through G-proteins and that this novel metabotropic function is involved in the control of neuronal excitability.

Sphingosine-1-phosphate induces proliferation of astrocytes: regulation by intracellular signalling cascades

Sphingosine-1-phosphate (S1P) is a potent lysophospholipid mediator mostly released by activated platelets. It is involved in several functions in peripheral tissues, but its effects in the central nervous system are poorly documented. Therefore, we have examined the effects of S1P on the proliferation of striatal astrocytes from the mouse embryo. These cells have been found to express mRNAs for the S1P receptors, Edg-1 and Edg-3. S1P stimulated thymidine incorporation and induced activation of extracellular signal-regulated kinases (Erks). Both effects were prevented by U0126, an Erk kinase inhibitor. The S1P-evoked activation of Erk1 was totally blocked in astrocytes pretreated with a combination of either phorbol ester (24 h) and LY294002, or phorbol ester (24 h) and pertussis toxin (PTX). Each individual treatment only partially inhibited Erk1 activation. This suggests that several separate mechanisms mediate this process, one involving protein kinase C and another involving Gi/Go proteins and phosphatidylinositol 3-kinase. In contrast, the stimulatory effect of S1P on astrocyte proliferation was totally blocked by either PTX or LY294002, but not by a downregulation of protein kinase C. S1P dramatically inhibited the evoked production of cyclic AMP, a response that was impaired by PTX. Finally, S1P stimulated the production of inositol phosphates and increased intracellular calcium by mobilization from thapsigargin-sensitive stores. These latter effects were mainly insensitive to PTX. Probably, Gi/Go protein activation and phosphoinositide hydrolysis are early events that regulate the activation of Erks by S1P. Altogether, these observations show that astrocytes are targets for S1P. Their proliferation in response to S1P could have physiopathological consequences at sites of brain lesions and alterations of the blood-brain barrier.

Antiproliferative properties of sphingosine-1-phosphate in human hepatic myofibroblasts

Sphingosine-1-phosphate (S1P) is a potent lysophospholipid mediator mostly released by activated platelets. It is involved in several functions in peripheral tissues, but its effects in the central nervous system are poorly documented. Therefore, we have examined the effects of S1P on the proliferation of striatal astrocytes from the mouse embryo. These cells have been found to express mRNAs for the S1P receptors, Edg-1 and Edg-3. S1P stimulated thymidine incorporation and induced activation of extracellular signal-regulated kinases (Erks). Both effects were prevented by U0126, an Erk kinase inhibitor. The S1P-evoked activation of Erk1 was totally blocked in astrocytes pretreated with a combination of either phorbol ester (24 h) and LY294002, or phorbol ester (24 h) and pertussis toxin (PTX). Each individual treatment only partially inhibited Erk1 activation. This suggests that several separate mechanisms mediate this process, one involving protein kinase C and another involving Gi/Go proteins and phosphatidylinositol 3-kinase. In contrast, the stimulatory effect of S1P on astrocyte proliferation was totally blocked by either PTX or LY294002, but not by a downregulation of protein kinase C. S1P dramatically inhibited the evoked production of cyclic AMP, a response that was impaired by PTX. Finally, S1P stimulated the production of inositol phosphates and increased intracellular calcium by mobilization from thapsigargin-sensitive stores. These latter effects were mainly insensitive to PTX. Probably, Gi/Go protein activation and phosphoinositide hydrolysis are early events that regulate the activation of Erks by S1P. Altogether, these observations show that astrocytes are targets for S1P. Their proliferation in response to S1P could have physiopathological consequences at sites of brain lesions and alterations of the blood-brain barrier.

Inhibition of protein synthesis in cortical neurons during exposure to hydrogen peroxide

Transient cerebral ischemia, which is accompanied by a sustained release of glutamate and zinc, as well as H(2)O(2) formation during the reperfusion period, strongly depresses protein synthesis. We have previously demonstrated that the glutamate-induced increase in cytosolic Ca(2+) is likely responsible for blockade of the elongation step of protein synthesis, whereas Zn(2+) preferentially inhibits the initiation step. In this study, we provide evidence indicating that H(2)O(2) and thapsigargin mobilized a common intracellular Ca(2+) pool. H(2)O(2) treatment stimulated a slow increase in intracellular Ca(2+), and precluded the effect of thapsigargin on Ca(2+) mobilization. H(2)O(2) stimulated the phosphorylation of both eIF-2alpha and eEF-2, in a time- and dose-dependent manner, suggesting that both the blockade of the elongation and of the initiation step are responsible for the H(2)O(2)-induced inhibition of protein synthesis. However, kinetic data indicated that, at least during the first 15 min of H(2)O(2) treatment, the inhibition of protein synthesis resulted mainly from the phosphorylation of eEF-2. In conclusion, H(2)O(2) inhibits protein translation in cortical neurons by a process that involves the phosphorylation of both eIF-2alpha and eEF-2 and the relative contribution of these two events depends on the duration of H(2)O(2) treatment.

Routes of zinc entry in mouse cortical neurons: role in zinc-induced neurotoxicity

Exposure of central neurons to Zn2+ triggers neuronal death. The routes of Zn2+ entry were investigated in living cortical neurons from the mouse using the specific Zn2+ fluorescent dye N-(6-methoxy-8-quinolyl)-p-toluene sulphonamide (TSQ), which preferentially detects membrane-bound Zn2+. Exposure of cortical neurons to increasing concentrations of Zn2+ (1-100 microM) induced a progressive increase in the fluorescence of TSQ. This fluorescence signal was not attenuated by the permeation of plasma membrane with digitonin. Accordingly, the major part of TSQ fluorescence (two-thirds) was associated to the particulate fraction of cortical neurons exposed to Zn2+. These results suggest that Zn2+ detected with TSQ in neurons is mainly bound to membranes. TSQ fluorescence measured in neurons exposed to 3 microM Zn2+ was enhanced by Na+-pyrithione, a Zn2+ ionophore, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), N-methyl-D-aspartate (NMDA) or KCl-induced depolarization. However, in the absence of any treatment, TSQ labelling of neurons exposed to 3 microM Zn2+ was only decreased by NMDA receptor antagonists, whereas it remained unaltered in the presence of antagonists of AMPA receptors or L-type voltage-gated Ca2+ channels. Zn2+ entry through NMDA receptors did not contribute to Zn2+-induced neuronal death, as it was prevented by antagonists of NMDA receptors only when they were added after the Zn2+ exposure. Finally, Zn2+ induced a delayed accumulation of extracellular glutamate which might be responsible for the delayed NMDA receptor activation that leads to neuronal death.

Zinc inhibits protein synthesis in neurons. Potential role of phosphorylation of translation initiation factor-2alpha

In the central nervous system, Zn(2+) is concentrated in the cerebral cortex and hippocampus and has been found to be toxic to neurons. In this study, we show that exposure of cultured cortical neurons from mouse to increasing concentrations of Zn(2+) (10-300 microM) induces a progressive decrease in global protein synthesis. The potency of Zn(2+) was increased by about 2 orders of magnitude in the presence of Na(+)-pyrithione, a Zn(2+) ionophore. The basal rate of protein synthesis was restored 3 h after Zn(2+) removal. Zn(2+) induced a sustained increase in phosphorylation of the alpha subunit of the translation eukaryotic initiation factor-2 (eIF-2alpha), whereas it triggered a transient increase in phosphorylation of eukaryotic elongation factor-2 (eEF-2). Protein synthesis was still depressed 60 min after the onset of Zn(2+) exposure while the state of eEF-2 phosphorylation had already returned to its basal level. Moreover, Zn(2+) was less effective than glutamate to increase eEF-2 phosphorylation, whereas it induced a more profound inhibition of protein synthesis. These results suggest that Zn(2+)-induced inhibition of protein synthesis mainly correlates with the increase in eIF-2alpha phosphorylation. Supporting further that Zn(2+) acts at the initiation step of protein synthesis, it strongly decreased the amount of polyribosomes.

Pyruvate and lactate protect striatal neurons against N-methyl-D-aspartate-induced neurotoxicity

A sustained release of glutamate contributes to neuronal loss during cerebral ischaemia. Using cultured mouse striatal neurons, we observed that glucose deprivation, which occurs in this pathological process, enhanced the N-Methyl-D-aspartate (NMDA)- or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-induced neurotoxicity. The end products of glycolysis, lactate and pyruvate, strongly protected neurons from these neurotoxic effects. The neuroprotective effect of pyruvate (which is more prominent in the absence of glucose) was not related to its ability to react with H2O2 by a decarboxylation process. Pyruvate and L-lactate strongly counteracted the deep decrease in the neuronal ATP content induced by NMDA, indicating that they might protect striatal neurons by rescuing cellular energy charge. Addition of MK-801 after the NMDA withdrawal completely protected neurons, suggesting that NMDA neurotoxicity resulted from a delayed NMDA receptor activation probably linked to a delayed release of an endogenous agonist in the extracellular medium. The strong accumulation of extracellular glutamate which was found in both sham and NMDA-treated cultures was markedly decreased by pyruvate. Thus, pyruvate might also exert its protecting activity by decreasing the delayed accumulation of glutamate which seemed to be neurotoxic only after a preexposure of neurons to NMDA.

Increase in external glutamate and NMDA receptor activation contribute to H2O2-induced neuronal apoptosis

The present study aims to investigate the role of extracellular glutamate and NMDA receptor stimulation in the neuronal death induced by a transient exposure to H2O2 of cultured neurons originating from mouse cerebral cortex. Most of the neuronal loss following a transient exposure to H2O2 of cortical neurons results from an apoptotic process involving a secondary stimulation of NMDA receptors, which occurs after H2O2 washout. Indeed, (a) the neurotoxic effect of H2O2 was strongly reduced by antagonists of NMDA receptors, (b) the neurotoxic effect of H2O2 was enhanced in the absence of Mg2+, (c) the protective effect of MK-801 progressively decayed when it was applied with increasing delay time after H2O2 exposure, and (d), finally, the extracellular concentration of glutamate was increased after H2O2 exposure. The major part of H2O2-induced neurotoxicity is mediated by the formation of hydroxyl radicals, which might be involved in (a) the delayed accumulation of extracellular glutamate and NMDA receptor activation and (b) the poly(ADP-ribose) polymerase activation and the related NAD content decrease. The combination of these two mechanisms could lead to both an increase in ATP consumption and a decrease of ATP synthesis. The resulting large decrease in ATP content might be finally responsible for the neuronal death.

Gap junctional communication and pharmacological heterogeneity in astrocytes cultured from the rat striatum

Indo-1 and fluo-3 imaging techniques were used to investigate the role of gap junctions in the changes in cytosolic calcium concentrations ([Ca2+]i) induced by several receptor agonists. Subpopulations of confluent cultured astrocytes from the rat striatum were superfused with submaximal concentrations of endothelin-1 (Et1) and the alpha 1-adrenergic and muscarinic receptor agonists, methoxamine and carbachol, respectively. 2. Combined binding and autoradiographic studies indicated that all striatal astrocytes possess binding sites for Et1. In contrast, alpha 1-adrenergic and muscarinic binding sites were found to be heterogeneously distributed. In agreement with these findings, Et1 induced fast calcium responses in all cells while only subsets of striatal astrocytes responded to the application of methoxamine or carbachol. 3. Halothane, heptanol and octanol, which are commonly used as gap junction inhibitors, drastically reduced the amplitude of Et1-induced calcium responses. In contrast, 18-alpha-glycyrrhetinic acid (alpha GA) used at a concentration known to block gap junction permeability in astrocytes had no significant effect on the amplitude of these calcium responses. 4. As demonstrated by quantitative and topological analysis, Et1 application similarly increased [Ca2+]i levels in all astrocytes in both the absence and presence of alpha GA. 5. In control conditions, subpopulations of cells responding to methoxamine or carbachol exhibited two main types of calcium responses which differed in their shape and kinetic characteristics. In the presence of alpha GA the number of cells responding to these receptor agonists was significantly reduced. Indeed, responses characterized by their long latency, slow rise time and weak amplitude disappeared in the presence of alpha GA while responses with short latency and fast rise time were preserved. 6. These results indicate that permeable gap junction channels tend to attenuate the pharmacological and functional heterogeneity of populations of astrocytes, while their inhibition restricts calcium responses in astrocytes expressing high densities of transmitter receptors coupled to phospholipase C.

Pyruvate protects neurons against hydrogen peroxide-induced toxicity

Hydrogen peroxide (H2O2) is suspected to be involved in numerous brain pathologies such as neurodegenerative diseases or in acute injury such as ischemia or trauma. In this study, we examined the ability of pyruvate to improve the survival of cultured striatal neurons exposed for 30 min to H2O2, as estimated 24 hr later by the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazoliumbromide assay. Pyruvate strongly protected neurons against both H2O2 added to the external medium and H2O2 endogenously produced through the redox cycling of the experimental quinone menadione. The neuroprotective effect of pyruvate appeared to result rather from the ability of alpha-ketoacids to undergo nonenzymatic decarboxylation in the presence of H2O2 than from an improvement of energy metabolism. Indeed, several other alpha-ketoacids, including alpha-ketobutyrate, which is not an energy substrate, reproduced the neuroprotective effect of pyruvate. In contrast, lactate, a neuronal energy substrate, did not protect neurons from H2O2. Optimal neuroprotection was achieved with relatively low concentrations of pyruvate (</=1 mM), whereas at high concentration (10 mM) pyruvate was ineffective. This paradox could result from the cytosolic acidification induced by the cotransport of pyruvate and protons into neurons. Indeed, cytosolic acidification both enhanced the H2O2-induced neurotoxicity and decreased the rate of pyruvate decarboxylation by H2O2. Together, these results indicate that pyruvate efficiently protects neurons against both exogenous and endogenous H2O2. Its low toxicity and its capacity to cross the blood-brain barrier open a new therapeutic perspective in brain pathologies in which H2O2 is involved.

Glutamate-dependent phosphorylation of elongation factor-2 and inhibition of protein synthesis in neurons

Postischemic delayed neuronal death is attributed to excitotoxic activation of glutamate receptors. It is preceded by a persistent inhibition of protein synthesis, the molecular basis of which is not known. Here we have examined in cortical neurons in culture the regulation by glutamate of phosphorylation of eukaryotic elongation factor-2 (eEF-2) by eEF-2 kinase, a Ca2+/calmodulin-dependent enzyme. Using a phosphorylation state-specific antibody, we show that glutamate, which triggers a large influx of Ca2+, enhances dramatically the phosphorylation of eEF-2. On the basis of kinetic and pharmacological analysis, we demonstrate a close correlation among the increase in cytosolic Ca2+ concentration, the degree of eEF-2 phosphorylation, and the inhibition of protein synthesis. A 30 min treatment with NMDA induced a transient phosphorylation of eEF-2 and delayed neuronal death. However, pharmacological inhibition of protein translation was not neurotoxic by itself and protected neurons against the toxicity evoked by low concentrations of NMDA. Thus, phosphorylation of eEF-2 and the resulting depression of protein translation may have protective effects against excitotoxicity and open new perspectives for understanding long-term effects of glutamate.

Interleukin-1 enhances the ATP-evoked release of arachidonic acid from mouse astrocytes

During neuropathological states associated with inflammation, the levels of cytokines such as interleukin-1beta (IL-1beta) are increased. Several studies have suggested that the neuronal damage observed in pathogenesis implicating IL-1beta are caused by an alteration in the neurochemical interactions between neurons and astrocytes. We report here that treating striatal astrocytes in primary culture with IL-1beta for 22-24 hr enhances the ATP-evoked release of arachidonic acid (AA) with no effect on the ATP-induced accumulation of inositol phosphates. The molecular mechanism responsible for this effect involves the expression of P2Y2 receptors (a subtype of purinoceptor activated by ATP) and cytosolic phospholipase A2 (cPLA2, an enzyme that mediates AA release). Indeed, P2Y2 antisense oligonucleotides reduce the ATP-evoked release of AA only from IL-1beta-treated astrocytes. Further, both the amount of cPLA2 (as assessed by Western blotting) and the release of AA resulting from direct activation of cPLA2 increased fourfold in cells treated with IL-1beta. We also report evidence indicating that the coupling of newly expressed P2Y2 receptors to cPLA2 is dependent on PKC activity. These results suggest that during inflammatory conditions, IL-1beta reveals a functional P2Y2 signaling pathway in astrocytes that results in a dramatic increase in the levels of free AA. This pathway may thus contribute to the neuronal loss associated with cerebral ischemia or traumatic brain injury.

Oxygen free radicals enhance the nitric oxide-induced covalent NAD(+)-linkage to neuronal glyceraldehyde-3-phosphate dehydrogenase

Nitric oxide (NO) induces a covalent modification of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from various tissues. This phenomenon, which has previously been interpreted as an auto-ADP-ribosylation, is in fact a covalent binding of NAD+ to the enzyme. In the present study, we show that 3-morpholino-sydnonimine (SIN-1) is much more efficient than sodium nitroprusside (SNP) in stimulating the covalent labelling of GAPDH from cultured striatal neurones in the presence of [adenylate-32P]NAD+ (877 +/- 110 and 266 +/- 33% increase in NAD(+)-labelling induced by maximally effective concentrations of SIN-1 and SNP respectively). The difference in the efficacy of both NO-generating compounds could be due to the additional release of superoxide by SIN-1, since superoxide dismutase and the nitrone 5,5'-dimethyl pyrroline-1-oxide markedly inhibited the SIN-1-induced covalent binding of NAD+ to GAPDH. Catalase and selective scavengers of hydroxyl radicals, mannitol and dimethyl sulphoxide, did not alter the SIN-1-induced covalent modification of GAPDH, ruling out the involvement of hydroxyl radicals in this phenomenon. Supporting further a role of oxygen free radicals in the NAD+ linkage to GAPDH, pyrogallol, a superoxide generator, which alone was ineffective, potentiated the SNP-evoked response. The NAD+ linkage to neuronal GAPDH measured in the presence of NO and superoxide probably involves sulphydryl groups, since the radiolabelling of the protein was reversed by exposure to HgCl2 and prevented by pretreatment with the alkylating agent N-ethylmaleimide. Moreover, the NO-induced inhibition of GAPDH activity was enhanced by pyrogallol, which was ineffective alone. In conclusion, the present study indicates that superoxide anions potentiate NO-induced covalent NAD(+)-linkage to GAPDH and enzyme inactivation.

Glucose regulates glutamate-evoked arachidonic acid release from cultured striatal neurons

L-Glutamate stimulates the liberation of arachidonic acid from mouse striatal neurons via the activation of N-methyl-D-aspartic acid (NMDA) receptors and by the joint stimulation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and metabotropic receptors. In this study, we investigated whether starving cultured mouse striatal neurons of glucose would modify glutamatergic receptor-mediated arachidonic acid release. Glucose deprivation for 30 min led to enhancement of the NMDA-evoked release of arachidonic acid, compared with that observed in the presence of glucose. This enhanced response depended on both the concentration of glucose and the length of time of glucose deprivation. The enhanced NMDA response appeared to result from both a release of glutamate and the subsequent additional release of arachidonic acid due to the activation of AMPA and metabotropic receptors. Indeed, the increased NMDA response was completely reversed when extracellular glutamate was enzymatically removed. Moreover, glucose deprivation potentiated the combined AMPA/metabotropic receptor-evoked release of arachidonic acid, even in the absence of extracellular glutamate. However, removing glucose did not improve the calcium rise induced by AMPA or NMDA. The ATP-evoked release of arachidonic acid from striatal astrocytes was not altered by glucose starvation. In summary, glucose deprivation affected two properties of striatal neurons: (a) it induced an NMDA-evoked release of glutamate from striatal neurons and (b) it selectively potentiated the AMPA/(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid-evoked release of [3H]arachidonic acid without altering the authentic NMDA-mediated response.

Synergistic effects of acetylcholine and glutamate on the release of arachidonic acid from cultured striatal neurons

The activation of muscarinic and NMDA receptors by carbachol and NMDA, respectively, stimulated the release of [3H]arachidonic acid ([3H]AA) from cultured striatal neurons. Striking synergistic effects were observed when both agonists were coapplied. This synergistic response was suppressed by atropine or (5R, 10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-im ine hydrogen maleate and inhibited by magnesium. It was markedly reduced in the absence of external calcium and suppressed by mepacrine. NMDA strongly elevated the intracellular calcium concentration ([Ca2+]i), but carbachol was ineffective. Ionomycin, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, or potassium depolarization, which increased [Ca2+]i but was ineffective on [3H]AA release, also potentiated the carbachol response. Sphingosine and Ro 31-8220 suppressed the responses evoked by carbachol, NMDA, or both agonists. However, no synergistic responses could be observed when phorbol 12-myristate 13-acetate was associated with either carbachol or NMDA. Together, these results suggest that both the massive influx of calcium induced by NMDA and the coupling of muscarinic receptors with a putative phospholipase A2 are required for the strong synergistic effects of carbachol and NMDA on [3H]AA release. Synergistic effects were also observed with acetylcholine and glutamate in the presence of magnesium, further revealing the physiological relevance of this process.

Nicotine protects cultured striatal neurones against N-methyl-D-aspartate receptor-mediated neurotoxicity

The role of cholinergic mechanisms in N-methyl-D-aspartate (NMDA)-mediated neuronal death was investigated using mouse striatal neurones in primary culture. A 30 min exposure of striatal neurones to increasing concentrations of NMDA resulted 24 h later in dramatic neuronal degeneration as assessed by MTT staining, crystal violet incorporation and determination of microtubule-associated protein 2. The NMDA-induced neurodegeneration was strongly inhibited by the co-application of two non-selective cholinergic agonists, acetylcholine or carbachol. This protective effect appears to be mediated by nicotinic receptors since it was insensitive to the muscarinic antagonist atropine but mimicked by nicotine, nornicotine and 1,1-dimethyl-4-phenyl-piperazinium. Moreover, the nicotine-evoked neuroprotection was inhibited by the central nicotinic antagonist hexamethonium. Therefore, this study suggests that cholinergic interneurones play an important role in neuronal survival in the striatum.

Antagonism by riluzole of entry of calcium evoked by NMDA and veratridine in rat cultured granule cells: evidence for a dual mechanism of action

1. Intracellular calcium levels were measured in cultured cerebellar granule cells of the rat by use of the fluorescent dye, indo-1/AM. 2. Intracellular calcium levels were increased by depolarizing stimuli such as N-methyl-D-aspartate (NMDA) (100 microM), glutamic acid (20 microM), and veratridine (10 microM). This increase was essentially due to entry of external calcium. 3. Riluzole (10 microM) blocked responses to all the depolarizing agents. 4. Riluzole could still block the increase in intracellular calcium evoked by NMDA or glutamic acid when sodium channels were blocked by tetrodotoxin, suggesting that this effect is not mediated by a direct action of riluzole on the voltage-dependent sodium channel. 5. Pretreatment of the cells with pertussis toxin (0.1 micrograms ml-1) did not modify the increases in intracellular calcium evoked by NMDA, glutamic acid or veratridine. 6. In pertussis toxin-treated cells, riluzole could no longer block responses to excitatory amino acids, but still blocked responses to veratridine. 7. It is concluded that riluzole has a dual action on cerebellar granule cells, both blocking voltage-dependent sodium channels and interfering with NMDA receptor-mediated responses via a pertussis toxin-sensitive mechanism. Furthermore, these two processes have been shown to be independent.

Is protein kinase C activity required for the N-methyl-D-aspartate-evoked rise in cytosolic Ca2+ in mouse striatal neurons?

The present study investigates the roles of protein kinase C (PKC) and A (PKA) activities in NMDA-mediated Ca2+ entry in primary cultures of mouse striatal neurons. Inhibitors of protein kinases, such as sphingosine, RO 31-8220 and staurosporine inhibited the NMDA- but also the KCl-induced rise in cytosolic Ca2+. However, the PKA antagonist Rp-adenosine-3',5'monophosphothioate (Rp-cAMPS) did not alter the NMDA+D-serine response, whereas it completely suppressed the KCl response. The NMDA+D-serine-evoked rise in cytosolic Ca2+, observed in the absence of external Mg2+, was potentiated by the PKC activator phorbol 12-myristate 13-acetate (PMA) only when submaximal effective concentrations of this agonist and co-agonist were used. In addition, the PKC activator did not alter the NMDA+D-serine-evoked response in the presence of varying concentrations of Mg2+. Confirming the dependence on PKC activity, desensitization of PKC resulting from long-term PMA treatment led to an impairment of the NMDA response, leaving the KCl-induced response intact. We therefore propose that PKC not only potentiates but is also required for the NMDA-evoked elevation in cytosolic Ca2+ in mouse striatal neurons.

Glutamate-evoked release of arachidonic acid from mouse brain astrocytes

Brain astrocytes in primary culture from the rat or the mouse have been shown to possess ionotropic and metabotropic glutamatergic receptors. The activation of both types of receptors is responsible for a rise in the cytosolic concentration of calcium, while the stimulation of metabotropic receptors induces the accumulation of inositol phosphates. In the present study, it is demonstrated that in striatal astrocytes from mouse embryos, glutamate evokes a release of arachidonic acid. The nonionotropic receptors involved in this effect appeared to be pharmacologically distinct from those coupled to phospholipase C: (1) glutamate displayed different dose-response curves for the production of inositol phosphates (biphasic: EC50 = 25 and 300 microM) and the release of arachidonic acid (monophasic: EC50 = 200 microM); (2) L(+)-2-amino-4-phosphonobutyric acid (AP4) only antagonized the glutamate-evoked release of arachidonic acid without altering the production of inositol phosphates; (3) when used at a concentration of 0.1 mM, quisqualate induced a higher formation of inositol phosphates than glutamate (2 mM) while, in contrast to glutamate, it only weakly stimulated arachidonic acid release when used either at 0.1 mM or 1 mM. L(+)-2-amino-3-phosphonopropionic acid (AP3) suppressed both responses. The glutamate-evoked release of arachidonic acid seems to be oppositely regulated by protein kinases A and C. Indeed, the stimulation of adenylate cyclase by the beta-adrenergic agonist isoproterenol, vasoactive intestinal peptide, or pretreatment of striatal astrocytes with cholera toxin decreased the glutamate-evoked release of arachidonic acid. In contrast, ATP, which markedly stimulated inositol phosphate production, strongly potentiated the glutamate-evoked release of arachidonic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of arachidonic acid and glutamate in the formation of inositol phosphates induced by noradrenalin in striatal astrocytes

The noradrenalin-evoked production of [3H]inositol phosphates in mouse striatal astrocytes in primary culture appeared to be the result of the combined stimulation of alpha 1- and alpha 2-adrenergic receptors. Indeed, the noradrenalin (100 microM) response was only partially reproduced by a maximally effective concentration of methoxamine (100 microM), a selective agonist of alpha 1-adrenergic receptors. In addition, the noradrenalin (100 microM)-induced production of [3H]inositol phosphates, which was completely suppressed by the alpha 1-adrenergic antagonist prazosin (1 microM), was also partially inhibited by yohimbine, a selective antagonist of alpha 2-adrenoceptors (maximum inhibition = -57 +/- 11%, measured in the presence of 10 microM yohimbine; six experiments). Finally, UK14.304, a selective alpha 2-adrenergic agonist that was ineffective alone, enhanced the methoxamine-evoked production of [3H] inositol phosphates (EC50 = 86 +/- 21 nM; three experiments). These results suggest that the stimulation of alpha 1-adrenergic receptors is required for the alpha 2-adrenergic receptor-mediated enhancement of phospholipase C activity. The increased production of [3H]inositol phosphates resulting from the stimulation of alpha 2-adrenergic receptors involved pertussis toxin-sensitive G proteins (Gi/o) and depended on extracellular calcium. As shown using the fluorescent dye indo-1, noradrenalin (100 microM) induced a long-lasting increase in cytosolic calcium in striatal astrocytes. Moreover, noradrenalin (100 microM) stimulated [3H]arachidonic acid release from these cells. These two latter responses may result from synergistic effects due to the combined stimulation of alpha 1- and alpha 2-adrenergic receptors, because they were inhibited by either prazosin (1 microM) or yohimbine (10 microM). Finally, the noradrenalin-evoked production of [3H]inositol phosphates seems to result partly from an inhibition by arachidonic acid of glutamate uptake into astrocytes, leading to the stimulation of glutamate metabotropic receptors coupled to phospholipase C. Indeed, the alpha 2-adrenergic component of the noradrenalin response was suppressed by either enzymatic removal of external glutamate or addition of 2-amino-3-phosphonopropionic acid (1 mM), an antagonist of glutamate metabotropic receptors that blocked the glutamate-evoked production of [3H]inositol phosphates in striatal astrocytes, and was reproduced by the direct application of either glutamate or an inhibitor of glutamate uptake, beta-methyl-DL-aspartic acid.

Synergistic regulation of cytosolic Ca2+ concentration in mouse astrocytes by NK1 tachykinin and adenosine agonists

The effects on cytosolic Ca2+ concentration of 2-chloroadenosine and [L-Pro9]-substance P, a selective agonist of NK1 receptors, were investigated on astrocytes from embryonic mice in primary culture. Cells responded to [L-Pro9]-substance P with a transitory increase in cytosolic Ca2+ which was of shorter duration when external Ca2+ was removed. A transient response to 2-chloroadenosine alone occurred. When simultaneously applied, [L-Pro9]-substance P and 2-chloroadenosine evoked a prolonged elevation of cytosolic Ca2+ (up to 30 min). This phenomenon was dependent on the presence of extracellular Ca2+, but insensitive to dihydropyridines, La3+, and Co2+, excluding the implication of voltage-operated Ca2+ channels. Arachidonic acid also induced a sustained elevation of cytosolic Ca2+, but did not increase further the response evoked by [L-Pro9]-substance P and 2-chloroadenosine. The activation of protein kinase C by a diacylglycerol analogue mimicked the effect of [L-Pro9]-substance P in potentiating the 2-chloroadenosine-evoked response. Like 2-chloroadenosine, pinacidil, which hyperpolarizes the cells by opening K+ channels, prolonged the elevation of cytosolic Ca2+ concentration induced by [L-Pro9]-substance P. Conversely, depolarization with 50 mM KCl canceled the effects of either pinacidil or 2-chloroadenosine applied with [L-Pro9]-substance P. Pertussis toxin pretreatment suppressed all the effects induced by 2-chloroadenosine.

Are several G proteins involved in the different effects of endothelin-1 in mouse striatal astrocytes?

High-affinity specific receptors of endothelin (ET-1) were identified on primary cultures of mouse embryo striatal astrocytes by binding experiments performed with 125I-ET-1. Stimulation of production of inositol phosphates, a biphasic increase of the intracellular calcium concentration, and inhibition of cyclic AMP accumulation were observed in the same cells under ET-1 stimulation. Pretreatment of these cells with Bordetella pertussis toxin affected these effects to different extends, an observation suggesting that they are mediated by multiple transduction pathways, possibly involving several guanine nucleotide-binding proteins.

Treatment of intact striatal neurones with cholera toxin or 8-bromoadenosine 3',5'-(cyclic)phosphate decreases the ability of pertussis toxin to ADP-ribosylate the alpha-subunits of inhibitory and other guanine-nucleotide-binding regulatory proteins, Gi and Go. Evidence for two distinct mechanisms

Using primary cultures of striatal neurones from the mouse embryo, we showed that treatment of intact cells with cholera toxin (5 micrograms/ml, 22 h) decreases the subsequent ADP-ribosylation of the alpha subunit of the guanine-nucleotide-binding regulatory protein Go (Go alpha) and the alpha subunit of the inhibitory guanine-nucleotide-binding regulatory protein (Gi alpha) of adenylate cyclase, which is catalyzed in vitro on neuronal membranes by pertussis toxin. The inhibitory effect of cholera toxin could not only be attributed to an increased production of cAMP in neurones. Treatment of cells with 0.1 microM 8-bromoadenosine 3',5'-(cyclic)phosphate (BrcAMP) for 16 h, or with 0.1 mM BrcAMP for 5 min, mimicked the effect of cholera toxin on the ADP-ribosylation of Go alpha and Gi alpha in vitro. However, the two agents seem to act through distinct mechanisms. The protein kinase inhibitor 1-(5-isoquinolinesulfonyl)-2-methylpiperazine prevented the action of Br8cAMP but not that of cholera toxin. In addition, measurements of the pI of the Go alpha deduced from immunoblots of two-dimensional gels performed using a specific antibody directed against Go alpha suggest that treatment of neurones with cholera toxin induces ADP-ribosylation of Go alpha in intact cells, while BrcAMP does not.

Immortalization of bipotential and plastic glio-neuronal precursor cells

Permanent clonal cell lines from newborn mouse striatum have been established after transfer of the simian virus 40 large tumor oncogene by means of a retroviral vector. Some of the lines obtained displayed properties of bipotential and plastic glio-neuronal precursors. Depending on the culture conditions, these cells express either the glial fibrillary acidic protein or neurofilaments. In addition, the cells can display adrenergic, D1 and D2 dopaminergic, muscarinic, and 5-hydroxytryptamine type 2 serotoninergic receptors, which are coupled either to the adenylate cyclase or to the phosphatidylinositol signaling pathways. The panel of receptors for neurotransmitters exhibited by these lines closely resembles that of primary striatal neurons. Results suggest that plastic common precursors of astrocytes and neurons persist in the striatum at a late developmental stage. As these permanent cell lines constitute an unlimited source of homogenous cell material, we suggest that they should be useful for molecular and pharmacological studies on the mechanisms and regulation of signal transduction as well as the commitment, plasticity, and differentiation of neural cells.

Pharmacological and functional heterogeneity of astrocytes: regional differences in phospholipase C stimulation by neuromediators

Carbamylcholine stimulated phospholipase C activity in astrocytes in primary culture from the mesencephalon but not from the striatum or cerebral cortex of the mouse embryo. An alpha 1-adrenergic-mediated response was observed in all astrocyte populations. 2-Chloroadenosine potentiated the alpha 1-adrenergic response in mesencephalic and striatal astrocytes but not in cortical astrocytes. It also stimulated the carbamylcholine-evoked response in mesencephalic astrocytes. Through cell-cell cooperation, 2-chloroadenosine potentiated the neuronal carbamylcholine-evoked activation of phospholipase C in homotopic cocultures (neuro-glial) from the striatum but not in homotopic cocultures (neuro-glial) from the cerebral cortex or in heterotopic cocultures (cortical astrocytes-striatal neurons; striatal astrocytes-cortical neurons.

Two simian virus 40 (SV40)-transformed cell lines from the mouse striatum and mesencephalon presenting astrocytic characters. I. Immunological and pharmacological properties

Dissociate cultures were initiated from embryonic rostral mesencephalic and striatal tissues dissected from the mouse brain and previously incubated with a simian virus 40 (SV40) suspension. After several weeks in culture foci of fastly dividing cells were resuspended and cloned by successive dilutions. Several clones expressing the SV40 nuclear T antigen were obtained by these procedures and two of them, one mesencephalic (F7-Mes) and one striatal (F12-Str) were screened for the expression of glial or neuronal characters. Both clones possess adenylate cyclase-linked beta 2-adrenergic receptors. They also take up and synthesize gamma-aminobutyric acid (GABA) in amounts compatible with a glial origin. As is the case for astrocytes, the uptake of GABA is inhibited by beta-alanine and rather insensitive to the presence of diaminobutyric acid (DABA), a specific inhibitor of the neuronal GABA carrier. The most convincing evidence that F7-Mes and F12-Str belong to the astrocytic lineage comes from the fact that the two cell lines synthesize glial fibrillary acidic protein (GFAP) as demonstrated by immunofluorescence and immunoblotting. In an accompanying paper we also show that these lines behave like astrocytes when considered from the point of view of neuroglial interactions.

Modulation by monoamines of somatostatin-sensitive adenylate cyclase on neuronal and glial cells from the mouse brain in primary cultures

Primary cultures of mouse embryonic neuronal or glial cells from the cerebral cortex, striatum, and mesencephalon were used to identify and determine the cellular localization of somatostatin receptors coupled to an adenylate cyclase. Somatostatin inhibited basal adenylate cyclase activity on neuronal but not on glial crude membranes in the three structures examined. The somatostatin-inhibitory effect on neuronal crude membranes was still observed in the presence of (-)-isoproterenol, 3,4-dihydroxyphenylethylamine (dopamine, DA), or 5-hydroxytryptamine (5-HT, serotonin) used at a concentration (10(-5) M) inducing maximal adenylate cyclase activation. In addition, in most cases biogenic amines modified the pattern of the somatostatin-inhibitory effect, triggering either an increase in the peptide apparent affinity for its receptors or an increase in the maximal reduction of adenylate cyclase activity or both. However, 5-HT did not modify the somatostatin-inhibitory response on striatal and cortical neuronal crude membranes. The changes in somatostatin-inhibitory responses were interpreted as a colocalization of the amine and the peptide receptors on subtypes of neuronal cell populations. Finally, somatostatin was shown to inhibit adenylate cyclase activity following its activation by (-)-isoproterenol on glial crude membranes of the striatum and the mesencephalon but not on those of the cerebral cortex.

Vasoactive intestinal polypeptide receptors linked to an adenylate cyclase, and their relationship with biogenic amine- and somatostatin-sensitive adenylate cyclases on central neuronal and glial cells in primary cultures

The presence of vasoactive intestinal polypeptide (VIP) receptors coupled to an adenylate cyclase was demonstrated on membranes of neurons or glial cells grown in primary cultures originating from the cerebral cortex, striatum, and mesencephalon of mouse embryos. A biphasic pattern of activation was observed in all these cell types, involving distinct high- and low-apparent-affinity mechanisms. The absence of additive effects of VIP and 3,4-dihydroxyphenylethylamine (DA, dopamine), isoproterenol (ISO), and 5-hydroxytryptamine (5-HT, serotonin) suggests that the peptide receptors are colocated with each of the corresponding amine receptors on neuronal membranes of the three structures studied. The nonadditivity between the VIP- and ISO-induced responses on cortical and striatal glial membranes reveals as well a colocation of VIP and beta-adrenergic-sensitive adenylate cyclases on the same cells. A subpopulation of mesencephalic glia could possess only one of the two types of receptors, as a partial additivity of the VIP and ISO responses was seen. In addition, VIP modified the characteristics of the somatostatin inhibitory effect on adenylate cyclase activity of neuronal membranes from the cerebral cortex and striatum but not from those of the mesencephalon. On striatal and mesencephalic glial membranes the somatostatin inhibitory effect was observed only in the presence of VIP. However, as previously seen with ISO, the presence of VIP did not allow the appearance of a somatostatin inhibitory response on cortical glial membranes. This suggests that cortical glia are devoid of somatostatin receptors.

Topographical distribution of dopaminergic innervation and dopaminergic receptors of the anterior cerebral cortex of the rat

The quantitative topographical distribution of the dopaminergic innervation and the DA-sensitive adenylate cyclase were estimated in the anterior cerebral cortex of the rat. The high affinity uptake of [3H]DA and endogenous levels of DA were used as markers of the dopaminergic innervation. [3H]DA uptake, DA levels and DA-sensitive adenylate cyclase were estimated in microdiscs of tissues punched out from frozen serial frontal slices. The uptake of [3H]DA was measured on sucrose homogenates prepared from such microdiscs. The ventral part of the frontal cortex contained the highest DA concentration and DA-sensitive adenylate cyclase activity; the other structures rich in DA and in DA receptors were the cingular (close to the corpus callsoum) and the rhinal cortices. All of these cortical areas were rich in [3H]DA uptake sites. However, curiously, the dorsal part of the frontal cortex, which contained only moderate amounts of DA and of DA-sensitive adenylate cyclase, presented the highest number of [3H]DA uptake sites. Nevertheless, the uptake of [3H]DA in this region decreased by 60% after bilateral electrolytical lesions of the ventral tegmental area (A10 group). The parietal cortex was practically devoid of dopaminergic innervation and of DA-sensitive adenylate cyclase. The activity of the DA-sensitive adenylate cyclase in the frontal, cingular and rhinal cortices was 10-fold higher than that found in the striatum when compared to their respective DA levels.