Activation of the A3 adenosine receptor affects cell cycle progression and cell growth

The A3 adenosine receptor has been implicated in modulation of cell growth. As a first step to the characterization of the underlying mechanisms, we exposed Chinese hamster ovary (CHO) cells transfected with the human A3 receptor (A3R-CHO) to selective A3 receptor ligands. At micromolar concentrations, the A3 agonists N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (IB-MECA) and its 2-chloro derivative Cl-IB-MECA reduced cell number, with no effects on either parental CHO cells (not expressing any adenosine receptor), or CHO cells transfected with the human A1 receptor. Cl-IB-MECA also reduced cell number in the human HEK293 cell line transfected with the human A3 receptor cDNA as opposed to the respective untransfected wild-type cells. In A3R-CHO, agonist-induced effects were antagonized by nanomolar concentrations of A3 antagonists, including the triazoloquinazoline derivative MRS 1220, the dihydropyridine derivative MRS 1191, and the triazolonaphthyridine derivative L-249,313. A3 agonist-induced effects were not due to modulation of cell adhesion, nor to necrosis or apoptosis. Growth curves revealed highly impeded growth, and flow-cytometric analysis showed markedly reduced bromodeoxyuridine incorporation into nuclei. The effect on cell cycle was completely antagonized by MRS1191. Hence, activation of the human A3 receptor in A3R-CHO results in markedly impaired cell cycle progression, suggesting an important role for this adenosine receptor subtype in cell cycle regulation and cell growth.

AlphaCaMKII binding to the C-terminal tail of NMDA receptor subunit NR2A and its modulation by autophosphorylation

Ca2+/calmodulin-dependent protein kinase II (CaMKII), a multifunctional, widely distributed enzyme, is enriched in post-synaptic densities (PSDs). Here, we demonstrate that CaMKII binds to a discrete C-terminal region of the NR2A subunit of NMDA receptors and promotes the phosphorylation of a Ser residue of this NMDA receptor subunit. Glutathione S-transferase (GST)-NR2A(1349-1464) binds native CaMKII from solubilised hippocampal PSDs in 'pull-out' and overlay experiments and this binding is competed by recombinant alphaCaMKII(1-315). The longer GST-NR2A(1244-1464), although containing the CaMKII phosphosite Ser-1289, binds the kinase with a lower efficacy. CaMKII association to NR2A(1349-1464) is positively modulated by kinase autophosphorylation in the presence of Ca2+/calmodulin. These data provide direct evidence for a mechanism modulating the synaptic strength.

Characterization of [3H]thiocolchicoside binding sites in rat spinal cord and cerebral cortex

Thiocolchicoside, a semi-synthetic derivative of the naturally occurring compound colchicoside with a relaxant effect on skeletal muscle, has been found to displace both [3H]gamma-aminobutyric acid ([3H]GABA) and [3H]strychnine binding, suggesting an interaction with both GABA and strychnine-sensitive glycine receptors. In order to gain further insight into the interaction of thiocolchicoside with these receptors, the binding of [3H]thiocolchicoside in rat spinal cord-brainstem and cortical synaptic membranes was characterized. [3H]Thiocolchicoside binding was saturable in both tissues examined. In spinal cord-brainstem membranes, we found a K(D) of 254 +/- 47 nM and a Bmax of 2.39 +/- 0.36 pmol/mg protein, whereas in cortical membranes, a K(D) of 176 nM and a Bmax of 4.20 pmol/mg protein was observed. A similar K(D) value was found in kinetic experiments performed in spinal cord-brainstem membranes. Heterologous displacement experiments showed that GABA and strychnine displaced the binding in a dose-dependent manner, whereas glycine was ineffective. [3H]Thiocolchicoside binding was also displaced by several GABA(A) receptor agonists and antagonists, but not by baclofen, flunitrazepam, guvacine, picrotoxin or by other drugs unrelated to GABA transmission. In spinal cord-brainstem, and to a lower extent, in cortical membranes, GABA and its analogs were not able to completely displace [3H]thiocolchicoside specific binding indicating that, besides GABA(A) receptors, thiocolchicoside can bind to another unidentified site. Unlabelled thiocolchicoside, however, completely displaced [3H]muscimol binding both in cortical and in spinal cord-brainstem synaptic membranes with an IC50 in the low microM range. Neurosteroids were found to modulate the binding in cortical but not in spinal cord-brainstem synaptic membranes. We conclude that [3H]thiocolchicoside binding shows a pharmacological profile indicating an interaction with the GABA(A) receptor. The different affinities for the GABA(A) receptor agonists and antagonists and sensitivity to neurosteroids obtained in the cerebral cortex and in the spinal cord may indicate a preferential interaction of the compound with a subtype of the GABA(A) receptor. The data also indicate that [3H]thiocolchicoside binds to another site(s), whose nature remains to be elucidated.

Pathophysiological implications of the structural organization of the excitatory synapse

The glutamatergic synapse is the key structure in the development of activity-dependent synaptic plasticity in the central nervous system. The analysis of the complex biochemical mechanisms at the basis of the long-term changes in synaptic efficacy have received a tremendous impulse by the observation that the post-synaptic constituents of the synapse can be separated and purified through a simple procedure involving detergent treatment of synaptosomes and differential centrifugation. In this fraction, called post-synaptic density (PSD), the functional interactions of its constituents are preserved. The various subunits of ionotropic glutamate receptors are held in register with the presynaptic active zone through their interaction with linker proteins. N-methyl-D-aspartate (NMDA) subunits NR2A and NR2B, bind to the PSD protein called PSD-95, which in turn binds neuroligins, providing a handle for interacting with neurexin, located in the plasma membrane at the presynaptic active zone. Additional clustering of NMDA receptors is provided through the binding of NRI subunits to the cytoskeletal protein alpha-actinin-2. AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) and kainate receptors are other important constituents of PSDs and bind to different anchoring proteins. Phosphorylation processes have long been known to modulate NMDA receptor functional activity: the finding that several protein kinases, particularly Ca2+/Calmodulin-dependent protein kinase II and protein tyrosine kinases of the src family, are major constituents of PSDs has allowed to demonstrate that these enzymes are localized in a strategic position of the glutamatergic synapse, so that their activation provides a means for NMDA receptor function regulation upon its activation. The relevance of these mechanisms has been demonstrated in experimental models of pathologies involving deficits in synaptic plasticity, such as in streptozotocin-induced diabetes and in an animal model of prenatal induced ablation of hippocampal neurons. Both animal models display disturbances in long-term potentiation and cognitive deficits, thus providing in vivo models to study pathology related changes in both the structure and the function of the excitatory synapse.

NMDA receptor subunits are modified transcriptionally and post-translationally in the brain of streptozotocin-diabetic rats

AIMS/HYPOTHESIS

Moderate disturbances of learning and memory were recognized as a complication of diabetes mellitus in patients. The streptozotocin-diabetic rat, an animal model of insulin-dependent diabetes, shows impairments in spatial memory and in long-term potentiation expression. We have studied the effect of experimental diabetes on expression of post-synaptic glutamate N-Methyl-D-Aspartate ionotropic receptors and of other key proteins regulating synaptic transmission at the post-synaptic compartment.

METHODS

In situ hybridization and Western blot analysis were used to assess expression and protein concentration of N-Methyl-D-Aspartate receptors and alpha-calcium-calmodulin-dependent kinase II. Receptor subunits alphaCaMKII-dependent phosphorylation was studied in post-synaptic densities obtained from the hippocampus and cortex of control, streptozotocin-diabetic and insulin-treated rats.

RESULTS

The transcript levels of NR1 and NR2A subunits of N-Methyl-D-Aspartate were unchanged in rats with a diabetic duration of 3 months when compared with age-matched control rats. Accordingly, NR1 and NR2A as well as GluR1, GluR2/3, PSD-95 and alphaCaMKII protein concentrations in post-synaptic densities were the same in both control and diabetic rats, whereas the immunoreactivity for NR2B was reduced by about 40%. In addition, the activity of alphaCaMKII on exogenous substrates, such as syntide-2, and the phosphorylation of NR2A/B subunits of N-Methyl-D-Aspartate receptor was reduced in hippocampal post-synaptic densities of streptozotocin-diabetic rats as compared with control rats. Furthermore, we show that insulin intervention for 3 months after diabetic duration partially restored both alphaCaMKII activity and NR2B levels.

CONCLUSION/INTERPRETATION

N-Methyl-D-Aspartate receptor expression and phosphorylation is possibly involved in behavioural and electrophysiological abnormalities observed in streptozotocin-diabetic rats.

Prenatal methylazoxymethanol treatment in rats produces brain abnormalities with morphological similarities to human developmental brain dysgeneses

A double methylazoxymethanol (MAM) intraperitoneal injection was prenatally administered to pregnant rats at gestational day 15 to induce developmental brain dysgeneses. Thirty adult rats from 8 different progenies were investigated with a combined electrophysiological and neuroanatomical analysis. The offspring of treated dams was characterized by extensive cortical layering abnormalities, subpial bands of heterotopic neurons in layer I, and subcortical nodules of heterotopic neurons extending from the periventricular region to the hippocampus and neocortex. The phenotype of cell subpopulations within the heterotopic structures was analyzed by means of antibodies raised against glial and neuronal markers, calcium binding proteins, GABA, and AMPA glutamate receptors. Neurons within the subcortical heterotopic nodules were characterized by abnormal firing properties, with sustained repetitive bursts of action potentials. The subcortical nodules were surrounded by cell clusters with ultrastructural features of young migrating neurons. The immunocytochemical data suggested, moreover, that the subcortical heterotopia were formed by neurons originally committed to the neocortex and characterized by morphological features similar to those found in human periventricular nodular heterotopia. The present study demonstrates that double MAM treatment at gestational day 15 induces in rats developmental brain abnormalities whose anatomical and physiological features bear resemblance to those observed in human brain dysgeneses associated with intractable epilepsy. Therefore, MAM treated rats could be considered as useful tools in investigating the pathogenic mechanisms involved in human developmental brain dysgeneses.

CaMKII-dependent phosphorylation of NR2A and NR2B is decreased in animals characterized by hippocampal damage and impaired LTP

The calcium-calmodulin-dependent protein kinase II (CaMKII) subserves activity-dependent plasticity in central neurons. To examine in vivo the implication of CaMKII activity in synaptic plasticity, we used an animal model characterized by developmentally induced targeted neuronal ablation within the cortex and the hippocampus, and showing, at presynaptic level, molecular alterations leading to facilitation of glutamate release in hippocampal synapses (methylazoxymethanol-treated rats, MAM-rats). We report here that at the postsynaptic side, the activity of CaMKII is markedly decreased in MAM-rats when compared to controls, although the concentration of the enzyme in Post Synaptic Density (PSD) is not altered. This effect is confined to PSD-associated CaMKII, as enzyme activity tested in the soluble fraction is unchanged in MAM-rats. In addition, the decreased activity is not due to inhibition by autophosphorylation in specific sites within the calmodulin-binding domain, as preincubation with purified phosphatases 1 and 2A failed to restore CaMKII activity in PSD of MAM-rats. The CaMKII-dependent phosphorylation of NR2A/B subunits of NMDA receptor is lower in MAM-rats when compared to controls (51.77 +/- 7.39% of controls level), as revealed in back-phosphorylation experiments. In addition, a treatment able to restore long-term potentiation (LTP) in hippocampal slices from MAM-rats, e.g. exposure to D-serine, is able to restore CaMKII activity to the control value.

Calcium/calmodulin-dependent protein kinase II is associated with NR2A/B subunits of NMDA receptor in postsynaptic densities

NMDA receptors and Ca2+/calmodulin-dependent kinase II (CaMKII) have been reported to be highly concentrated in the postsynaptic density (PSD). Although the possibility that CaMKII in PSD might be associated with specific proteins has been put forward, the protein or proteins determining the targeting of the kinase in PSD have not yet been identified. Here we report that CaMKII binds to NR2A and NR2B subunits of NMDA receptors in PSD isolated from cortex and hippocampus. The association of NMDA receptor subunits and CaMKII was assessed by immunoprecipitating PSD proteins with antibodies specific for NR2A/B and CaMKII: CaMKII coprecipitated with NR2A/B and NR1 but not with other glutamate ionotropic receptor subunits, such as GluR1 and GluR2-3. A direct association between CaMKII and NR2A/B subunits was further confirmed by overlay experiments using either 32P-autophosphorylated CaMKII or 32P-NR2A/B and by evaluating the formation of a CaMKII-NR2A/B complex by means of the cross-linker disuccimidyl suberate. These data demonstrate an association between the NMDA receptor complex and CaMKII in the postsynaptic compartment, suggesting that this colocalization may be relevant for synaptic plasticity.

Differential level of platelet amyloid beta precursor protein isoforms: an early marker for Alzheimer disease

OBJECTIVE

To determine whether a differential level of platelet amyloid beta precursor protein (APP) isoforms is specifically related to Alzheimer disease (AD) and whether it shows a correlation with the progression of clinical symptoms.

DESIGN

After subjects were grouped according to diagnosis and severity of dementia, APP isoform levels in platelets were compared.

SETTING

University medical centers.

PATIENTS

Thirty-two patients who fulfilled diagnostic criteria for probable AD, 25 age-matched control subjects, and 16 patients with non-AD dementia.

MAIN OUTCOME MEASURE

The levels of APP isoforms were evaluated by means of Western blot analysis and immunostaining of whole platelets. Messenger RNAs for APP transcripts were also evaluated by means of reverse transcriptase polymerase chain reaction.

RESULTS

The ratio between the intensity of the 130-kd and 106- to 110-kd APP isoforms was significantly lower in the AD group (0.31 +/- 0.15, mean +/- SD) compared with both controls (0.84 +/- 0.2) and non-AD subjects (0.97 +/- 0.4). The ratio of platelet APP isoforms in patients with AD grouped by Clinical Diagnostic Rating score significantly correlated with the severity of the disease (Pearson correlation coefficient, followed by Bonferroni correction, P = .01). Reverse transcriptase polymerase chain reaction experiments showed that APP transcripts in all experimental groups were equally expressed.

CONCLUSIONS

The pattern of platelet APP isoforms is specifically altered in patients with AD. In addition, the alteration of platelet APP isoforms shows a positive correlation with the progression of clinical symptoms, supporting the possibility to consider this peripheral parameter as a marker of progression of the disease. These alterations are not related to abnormalities of APP isoforms messenger RNAs in platelets.

Altered connections between neocortical and heterotopic areas in methylazoxymethanol-treated rat

We are currently investigating various treatments which could determine, in the rat brain, structural abnormalities mimicking those reported in human brain dysgeneses. We can induce the formation of neuronal heterotopia in the progeny of rats by means of a double injection of the cytotoxic agent methylazoxymethanol acetate (MAM) on embryonic day 15. We have now investigated the anatomical connections of these heterotopia by means of anterograde and retrograde tract tracing techniques. The induced heterotopia along the border of the lateral ventricles shared common anatomical features with the periventricular nodules in human periventricular or subcortical nodular heterotopia (PNH). The tract tracing data demonstrated the existence of reciprocal connections between the neuronal heterotopia and the ipsilateral and contralateral cortical areas, and the presence of abnormal cortico-hippocampal and cortico-cortical connections. On the basis of the connectivity patterns, it may be speculated that some cells in the heterotopia could be neurons originally committed to the cortex, that were interrupted in their migration by the MAM treatment. Given the common morphological features seen in human PNH and MAM-induced brain heterotopia, the anatomical and developmental analysis of MAM-treated rats may shed light on the mechanisms by which human brain dysgeneses develop in human patients.

Expression of hexokinase mRNA in human hippocampus

The expression of hexokinase messenger RNA was evaluated in human hippocampus using in situ hybridization technique. The message showed an uneven distribution with high levels present in the granular cell layer of the dentate gyrus and CA3 region. The detection of specific transcripts was also observed in the lateral geniculate nucleus, the dentate polymorphic cell layer and the parahippocampal gyrus. The data suggest that, in the hippocampus, the expression of hexokinase is higher in neurons than in glial cells and that the rate of glucose metabolism may display considerable variations in the different subregions of this area.

Apoptosis by 2-chloro-2'-deoxy-adenosine and 2-chloro-adenosine in human peripheral blood mononuclear cells

Adenosine has profound effects on immune cells and has been implicated in the intrathymic apoptotic deletion of T-cells during development. In order to characterize adenosine effects on quiescent peripheral blood mononuclear cells (PBMC), we have evaluated the ability of the previously characterized adenosine receptor agonist 2-chloro-adenosine (2CA; Ceruti, Barbieri et al., 1997) and of the antineoplastic drug 2-chloro-2'-deoxy-adenosine (2CdA, cladribine) to trigger apoptosis of PBMC. Apoptosis was assessed by morphological changes, DNA fragmentation by agarose gel electrophoresis and appearance of hypodiploid DNA peak by flow cytometry. 2CA (10 microM) and 2CdA (1 microM) induced apoptosis in human PBMC, which are relatively insensitive to apoptosis. For both agents, the effect was concentration- and time-dependent, although 2CdA induced apoptosis more potently than 2CA. Evaluation of mitochondrial function in parallel samples using the mitochondrial membrane-potential-specific dye JC-1 showed that mitochondrial damage followed the same kinetics as apoptosis, hence an early damage of mitochondria is likely not responsible for adenosine-induced death of PBMC. The effect of 2CA was partially prevented by addition of dipyridamole (DP), a nucleoside transport inhibitor, hence some of the apoptotic effect of this nucleoside is, at least in part, due to intracellular action. Alternatively, DP did not affect 2CdA-induced apoptosis, suggesting that 2CdA may enter cells via a DP-insensitive transporter. 5-Iodotubercidin (5-Itu), a nucleoside kinase inhibitor, was also able to partially prevent the action of 2CA and was not able to affect 2CdA-induced apoptosis, suggesting a different role for phosphorylation in 2CA- vs 2CdA-induced apoptosis. To test the role of P1 receptors, agonists and antagonists selective at various P1 receptor subtypes were used. Data suggest that, for 2CA, apoptosis is partially sustained by activation of the A2A receptor subtype, whereas no role is exerted by P1 receptors in 2CdA-dependent apoptosis. Moreover, in these cells, apoptosis could also be triggered through intense activation of the A3 receptor via selective agonists such as 2-chloro-N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide (Cl-IB-MECA), but this mechanism plays no role in either 2CA- or 2CdA-induced apoptosis. On the whole, our results suggest that 2CA and 2CdA follow different pathways in inducing apoptosis of immune cells. Moreover, our data also suggest that there are at least three different ways by which adenosine derivatives may induce apoptosis of human PBMC: (i) through an A2A-like extracellular membrane receptor; (ii) through entry of nucleosides into cells and direct activation of intracellular events involved in the apoptotic process; or (iii) through activation of the A3 receptor.

The A3 adenosine receptor mediates cell spreading, reorganization of actin cytoskeleton, and distribution of Bcl-XL: studies in human astroglioma cells

The pathophysiological role of the adenosine A3 receptor in the central nervous system is largely unknown. We have investigated the effects of the selective A3 receptor agonist 2-chloro-N6-(3-iodobenzyl)-adenosine, Cl-IB-MECA, in cells of the astroglial lineage (human astrocytoma ADF cells). A marked reorganization of the cytoskeleton, with appearance of stress fibers and numerous cell protrusions, was found following exposure of cells to low (nM) concentrations of Cl-IB-MECA. These "trophic" effects were accompanied by induction of the expression of Rho, a small GTP-binding protein, which was virtually absent in control cells, and by changes of the intracellular distribution of the antiapoptotic protein Bcl-XL, that, in agonist-exposed cells, became specifically associated to cell protrusions. This is the first demonstration that the intracellular organization of Bcl-XL can be modulated by the activation of a G-protein-coupled membrane receptor, such as the A3 adenosine receptor. Moreover, modulation of the astrocytic cytoskeleton by adenosine may have intriguing implications in both nervous system development and in the response of the brain to trauma and ischemia.

Phosphorylation of the casein kinase II domain of B-50 (GAP-43) in rat cortical growth cones

Growth-associated phosphoprotein B-50 is a neural protein kinase C (PKC) substrate enriched in nerve growth cones that has been implicated in growth cone plasticity. Here we investigated whether B-50 is a physiological substrate for casein kinase II (CKII) in purified rat cortical growth cone preparations. Using site-specific proteolysis and known modulators of PKC, in combination with immunoprecipitation, mass spectrometry, and phosphoamino acid analysis, we demonstrate that endogenous growth cone B-50 is phosphorylated at multiple sites, on both serine and threonine residues. Consistent with previous reports, stimulation of PKC activity increased the phosphorylation of only those proteolytic fragments containing Ser41. Under basal conditions, however, phosphorylation was predominantly associated with fragments not containing Ser41. Mass spectrometry of tryptic digests of B-50, which had been immunoprecipitated from untreated growth cones, revealed that in situ phosphorylation occurs within peptides B-50(181-198) and B-50(82-98). These peptides contain the major and minor in vitro CKII phosphosites, respectively. In addition, cyanogen bromide digestion of immunoprecipitated chick B-50 generated a 4-kDa C-terminal B-50 phosphopeptide, confirming that phosphorylation of the CKII domain occurs across evolutionary diverse species. We conclude that B-50 in growth cones is not only a substrate for PKC, but also for CKII.

Calcium-dependent mitochondrial formation of species mediating DNA single strand breakage in U937 cells exposed to sublethal concentrations of tert-butylhydroperoxide

Treatment of U937 cells with a sublethal albeit DNA-damaging concentration of tert-butylhydroperoxide (tB-OOH) enhanced mitochondrial Ca++ uptake and ruthenium red (RR), a polycation that inhibits the calcium uniporter of mitochondria, significantly reduced the extent of DNA cleavage generated by the hydroperoxide. Release of Ca++ from the ryanodine(Ry)/caffeine(Cf)-sensitive stores further increased mitochondrial Ca++ uptake and elicited a parallel enhancement in DNA strand scission induced by tB-OOH that was prevented by both Ry and RR. DNA damage caused by tB-OOH alone or associated with either Cf or RR was prevented by iron chelators, insensitive to antioxidants and repaired with kinetics superimposable with those observed after treatment with H2O2. Cf enhanced the DNA-damaging effects of tB-OOH in permeabilized cells as well, and similar effects were observed upon addition of CaCl2. Cf did not further increase the formation of DNA lesions elicited by tB-OOH in the presence of CaCl2. The enhancing effects of Cf were prevented by RR and ryanodine, whereas those mediated by exogenous calcium were prevented only by RR. DNA strand scission caused by tB-OOH alone or associated with Cf in the permeabilized cell system was severely inhibited by ethylene glycol-bis(beta-aminoethyl ether)-N, N,N',N'-tetraacetic acid. The mechanism(s) whereby Ca++ promotes the mitochondrial formation of species that will ultimately result in the formation of DNA lesions was subsequently analyzed using intact as well as permeabilized cells. Hydrogen peroxide was identified to be one of these species.

Characterization of the Ca2+ responses evoked by ATP and other nucleotides in mammalian brain astrocytes

1. This study was aimed at characterizing ATP-induced rises in cytosolic free calcium ion, [Ca2+]i, in a population of rat striatal astrocytes loaded with the fluorescent Ca2+ probe Fura2, by means of fluorescence spectrometry. 2. ATP triggered a fast and transient elevation of [Ca2+]i in a concentration-dependent manner. The responses of the purine analogues 2-methylthio-ATP (2-meSATP), adenosine-5'-O-(2-thiodiphosphate) (ADP beta S), as well as uridine-5'-triphosphate (UTP) resembled that of ATP, while alpha, beta-methylene-ATP (alpha, beta-meATP) and beta, gamma-methylene-ATP (beta, gamma-meATP) were totally ineffective. 3. Suramin (50 microM) had only a minor effect on the ATP response, whereas pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (5 microM) significantly depressed the maximum response. 4. Extracellular Ca2+ did not contribute to the observed [Ca2+]i rise: removing calcium from the extracellular medium (with 1 mM EGTA) or blocking its influx by means of either Ni2+ (1 mM) or Mn2+ (1 mM) did not modify the nucleotide responses. 5. Furthermore, after preincubation with 10 microM thapsigargin, the nucleotide-evoked [Ca2+]i increments were completely abolished. In contrast, 10 mM caffeine did not affect the responses, suggesting that thapsigargin-, but not caffeine/ryanodine-sensitive stores are involved. 6. Both application of the G-protein blocker guanosine-5'-O-(2-thiodiphosphate) (GDP beta S) (1 mM) and preincubation with pertussis toxin (PTx) (350 ng ml-1) partially inhibited the nucleotide-mediated responses. Moreover, the phospholipase C (PLC) inhibitor U-73122, but not its inactive stereoisomer U-73343 (5 microM), significantly reduced the ATP-evoked [Ca2+]i rise. 7. In conclusion, our results suggest that, in rat striatal astrocytes, ATP-elicited elevation of [Ca2+]i is due solely to release from intracellular stores and is mediated by a G-protein-linked P2Y receptor, partially sensitive to PTx and coupled to PLC.

Characterization of the signalling pathways involved in ATP and basic fibroblast growth factor-induced astrogliosis

1. A brief challenge of rat astrocytes with either alpha, beta-methyleneATP (alpha, beta-meATP) or basic fibroblast growth factor (bFGF) resulted, three days later, in morphological differentiation of cells, as shown by marked elongation of astrocytic processes. The P2 receptor antagonist suramin prevented alpha, beta-meATP- but not bFGF-induced astrocytic elongation. Similar effects on astrocytic elongation were also observed with ATP and other P2 receptor agonists (beta, gamma meATP, ADP beta S, 2meSATP and, to a lesser extent, UTP). 2. Pertussis toxin completely abolished alpha, beta-meATP- but not bFGF-induced effects. No effects were exerted by alpha, beta-meATP on cyclic AMP production; similarly, neomycin had no effects on elogation of processes induced by the purine analogue, suggesting that adenylyl cyclase and phospholipase C are probably not involved in alpha, beta-meATP-induced effects (see also the accompanying paper by Centemeri et al., 1997). The tyrosine-kinase inhibitor genistein greatly reduced bFGF- but not alpha, beta-meATP-induced astrocytic elongation. 3. Challenge of cultures with alpha, beta-meATP rapidly and concentration-dependently increased [3H]-arachidonic acid (AA) release from cells, suggesting that activation of phospholipase A2 (PLA2) may be involved in the long-term functional effects evoked by purine analogues. Consistently, exogenously added AA markedly elongated astrocytic processes. Moreover, various PLA2 inhibitors (e.g. mepacrine and dexamethasone) prevented both the early alpha, beta-meATP-induced [3H]-AA release and/or the associated long-term morphological changes, without affecting the astrocytic elongation induced by bFGF. Finally, the protein kinase C (PKC) inhibitor H7 fully abolished alpha, beta-meATP- but not bFGF-induced effects. 4. Both alpha, beta-meATP and bFGF rapidly and transiently induced the nuclear accumulation of Fos and Jun. Both c-fos and c-jun induction by the purine analogue could be fully prevented by pretreatment with suramin. In contrast, the effects of bFGF were unaffected by this P2 receptor antagonist. 5. It was concluded that alpha, beta-meATP- and bFGF-morphological differentiation of astrocytes occurs via independent transductional pathways. For the purine analogue, signalling involves a Gi/G(o) protein-coupled P2Y-receptor which may be linked to activation of PLA2 (involvement of an arachidonate-sensitive PKC is speculated); for bFGF, a tyrosine kinase receptor is involved. Both pathways merge on some common intracellular target, as suggested by induction of primary response genes, which in turn may regulate late response genes mediating long-term phenotypic changes of astroglial cells. 6. These findings implicate P2 receptors as novel targets for the pharmacological regulation of reactive astrogliosis, which has intriguing implications in nervous system diseases characterized by degenerative events.

Increased secretion of the amino-terminal fragment of amyloid precursor protein in brains of rats with a constitutive up-regulation of protein kinase C

Protein kinase C (PKC) activation stimulates release of secreted amyloid precursor protein (APPs) in several cell lines. To ascertain the role of PKC in regulating APP metabolism in vivo, we used an animal model (methylazoxymethanol-treated rats; MAM rats) in which PKC is permanently hyperactivated in selected brain areas, i.e., cortex and hippocampus. A significant decrease in membrane-bound APP concentration was found in synaptosomes derived from cortex and hippocampus of MAM rats, where PKC is up-regulated, with a concomitant increase in APPs production in soluble fractions of the same brain areas. In contrast, in a brain area not affected by MAM treatment (i.e., cerebellum), APP secretion is similar in control and MAM rats, indicating that altered metabolism of APP is restricted to only those areas in which the PKC system is up-regulated. In addition, phorbol esters or H-7 modulate APPs release in hippocampal slices from both control and MAM rats, further supporting an in vivo role for this enzyme in regulating metabolism of mature APP.

Alternative mechanisms for hydroperoxide-induced DNA single strand breakage

The results presented in this study point out the existence of similarities as well as differences in the DNA-damaging effects of organic vs. inorganic hydroper-oxides in human myeloid leukemia U937 cells. On the one hand, the formation of DNA single strand breaks (SSBs) induced by either hydrogen peroxide (H2O2) or tert-butylhydroperoxide (tBu-OOH) was prevented by iron chelators, was not affected by antioxidants or glucose omission before and during peroxide exposure and was enhanced by prior catalase depletion. Furthermore, H2O2- and tBu-OOH-induced DNA strand scission were also detected after treatment at 0 degree C. On the other hand, H2O2, but not tBu-OOH or cumene hydroperoxide (cum-OOH), produced DNA strand scission in isolated nuclei and post-lysed DNA samples. In addition, lowering the basal intracellular calcium concentration with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) markedly reduced the DNA-damaging efficiency of tBu-OOH while promoting only a slight decline in the number of DNA SSBs induced by H2O2. Taken together, these results are consistent with the commonly held theory that DNA damage caused by H2O2 is mediated by the formation of hydroxyl radicals. tBu-OOH-induced DNA single strand breakage appears to involve both the formation of H2O2 and a rise in cytosolic calcium ions.

Increased presynaptic protein kinase C activity and glutamate release in rats with a prenatally induced hippocampal lesion

We have previously shown that protein kinase C (PKC) activity is up-regulated in nerve terminals of animals that have been subjected to targeted cellular ablation of cortical and hippocampal neurons by treatment with methylazoxymethanol (MAM), which results in impaired long-term potentiation (LTP) and cognitive deficit. In this study we investigated the consequences of increased membrane-bound PKC in the regulation of release of glutamate, the major excitatory transmitter involved in LTP. We show that nerve terminals of MAM-treated rats show higher PKC activity, as monitored by the in situ phosphorylation of B-50/GAP-43, in both basal and phorbol ester-stimulated conditions. In these animals, hippocampal nerve endings release a greater amount of glutamate than those of controls, both in basal conditions and when synaptosomes are stimulated with KCl or 3,4-diaminopyridine. The potentiation observed in MAM-treated rats was counteracted by the PKC blocker H-7 and the clostridial tetanus toxin. On the contrary, GABA release was not significantly up-regulated, either in basal or in depolarization-evoked conditions. Therefore our data show that the increase in synaptosomal PKC activity is paralleled by increased glutamate but not GABA release in this animal model. Whether this reflects specific up-regulation of membrane PKC activity in glutamatergic terminals or an alteration in the regulation of glutamate release remains to be determined.