L-NAME reverses quinolinic acid-induced toxicity in rat corticostriatal slices: Involvement of src family kinases.

Quinolinic acid (QA) is an endogenous excitotoxin acting on N-methyl-d-aspartate receptors (NMDARs) that leads to the pathologic and neurochemical features similar to those observed in Huntington's disease (HD). The mechanism of QA toxicity also involves free radicals formation and oxidative stress. NMDARs are particularly vulnerable to the action of reactive oxygen species (ROS) and reactive nitrogen species (RNS) that can act as modulators of the activity of protein tyrosine kinases (PTKs) and phosphotyrosine phosphatases (PTPs). Because QA is able to activate neuronal nitric oxide synthase (nNOS) as well as to stimulate the NMDARs, we evaluated the effect of Nomega-Nitro-l-arginine-methyl ester (l-NAME), a selective nNOS inhibitor, on QA-induced neurotoxicity in rat corticostriatal slices. In electrophysiologic experiments we observed that slice perfusion with QA induced a strong reduction of field potential (FP) amplitude, followed by a partial recovery at the end of the QA washout. In the presence of l-NAME the recovery of FP amplitude was significantly increased with respect to QA alone. In synaptosomes, prepared from corticostriatal slices after the electrophysiologic recordings, we observed that l-NAME pre-incubation reversed the QA-mediated inhibitory effects on protein tyrosine phosphorylation pattern, c-src, lyn, and fyn kinase activities and tyrosine phosphorylation of NMDAR subunit NR2B, whereas the PTP activity was not recovered in the presence of l-NAME. These findings suggest that NO plays a key role in the molecular mechanisms of QA-mediated excitotoxicity in experimental model of HD.

Quinolinic acid modulates the activity of src family kinases in rat striatum: in vivo and in vitro studies.

Quinolinic acid (QA) has been shown to evoke neurotoxic events via NMDA receptor (NMDAR) overactivation and oxidative stress. NMDARs are particularly vulnerable to free radicals, which can modulate protein tyrosine kinase (PTK) and phosphotyrosine phosphatase (PTP) activities. The src family of tyrosine kinases are associated with the NMDAR complex and regulate NMDA channel function. Because QA is an NMDAR agonist as well as a pro-oxidant agent, we investigated whether it may affect the activity of PTKs and PTPs in vivo and in vitro. In synaptosomes prepared from striata dissected 15 min, 30 min or 15 days after bilateral injection of QA we observed modulation of the phosphotyrosine pattern; a significant decrease in PTP activity; and a sustained increase in c-src and lyn activity at 15 and 30 min after treatment with QA, followed by a decrease 2 weeks later. Striatal synaptosomes treated in vitro with QA showed time- and dose-dependent modulation of c-src and lyn kinase activities. Moreover, the nitric oxide synthase inhibitor NG-nitro-L-arginine-methyl ester, the NMDAR antagonist d-2-amino-5-phosphonovaleric acid and pyruvate suppressed the QA-induced modulation of c-src activity. These findings suggest a novel feature of QA in regulating src kinase activity through the formation of reactive radical species and/or NMDAR overactivation.

Protein phosphatase 1alpha is tyrosine-phosphorylated and inactivated by peroxynitrite in erythrocytes through the src family kinase fgr.

Protein serine/threonine phosphorylation is a significant component of the intracellular signal that together with tyrosine phosphorylation regulates several processes, including cell-cycle progression, muscle contraction, transcription, and neuronal signaling. Cross-talk between phosphoserine/threonine- and phosphotyrosine-mediated pathways is not yet well understood. In this study we found that peroxynitrite, a physiological oxidant formed by the fast radical-radical reaction between the nitric oxide and the superoxide anion, induced tyrosine phosphorylation of the serine/threonine protein phosphatase 1alpha (PP1alpha) in human erythrocytes through activation of src family kinases. We have previously shown in mouse red cells that upregulation of the src kinase fgr phosphorylates PP1alpha, acting as an upstream negative regulator of PP1alpha, and downregulates K-Cl cotransport. Here we found that PP1alpha is a selective substrate of peroxynitrite-activated fgr and that tyrosine phosphorylation of PP1alpha corresponds to an inhibition of its enzymatic activity. Despite fgr activation and PP1alpha downregulation, peroxynitrite stimulated in a dose-dependent fashion the function of the K-Cl cotransporter. In an attempt to understand the mechanism of K-Cl cotransport activation, we found that the effect of peroxynitrite is completely reversed by dithriothreitol, suggesting that peroxynitrite acts as an oxidizing agent by an SH-dependent and PP1alpha-independent mechanism. These findings highlight a novel function of peroxynitrite in regulating the intracellular signal transduction pathways involving serine/threonine phosphorylation and the functional role of proteins that are targets of these phosphatases.

Peroxynitrite activates kinases of the src family and upregulates tyrosine phosphorylation signaling.

The hypothesis that peroxynitrite may act as a signaling molecule able to upregulate protein tyrosine phosphorylation is discussed. This article focuses on the mechanisms for activating kinases of the src family, an important class of nonreceptor tyrosine kinases implicated in the regulation of cell communication, proliferation, migration, differentiation, and survival. Recent in vitro findings show that in erythrocytes, synaptosomes, and cerebellar primary culture cells peroxynitrite is able to inhibit phosphatases and to activate different members of the src kinase family through different mechanisms involving cysteine-dependent and -independent processes. The ability of nitrotyrosine-containing peptides with SH2 binding affinity to activate src kinases is also discussed.

Peroxynitrite affects exocytosis and SNARE complex formation and induces tyrosine nitration of synaptic proteins.

The reactive species peroxynitrite, formed via the near diffusion-limited reaction of nitric oxide and superoxide anion, is a potent oxidant that contributes to tissue damage in neurodegenerative disorders. Peroxynitrite readily nitrates tyrosine residues in proteins, producing a permanent modification that can be immunologically detected. We have previously demonstrated that in the nerve terminal, nitrotyrosine immunoreactivity is primarily associated with synaptophysin. Here we identify two other presynaptic proteins nitrated by peroxynitrite, Munc-18 and SNAP25, both of which are involved in sequential steps leading to vesicle exocytosis. To investigate whether peroxynitrite affects vesicle exocytosis, we used the fluorescent dye FM1-43 to label a recycling population of secretory vesicles within the synaptosomes. Bolus addition of peroxynitrite stimulated exocytosis and glutamate release. Notably, these effects were strongly reduced in the presence of NaHCO(3), indicating that peroxynitrite acts mainly intracellularly. Furthermore, peroxynitrite enhanced the formation of the sodium dodecyl sulfate-resistant SNARE complex in a dose-dependent manner (100-1000 microm) and induced the formation of 3-nitrotyrosine in proteins of SNARE complex. These data suggest that modification(s) of synaptic vesicle proteins induced by peroxynitrite may affect protein-protein interactions in the docking/fusion steps, thus promoting exocytosis, and that, under excessive production of superoxide and nitric oxide, neurons may up-regulate neuronal signaling.

Nitrotyrosine mimics phosphotyrosine binding to the SH2 domain of the src family tyrosine kinase lyn.

The nitration of tyrosine residues in protein occurs through the action of reactive oxygen and nitrogen species and is considered a marker of oxidative stress under pathological conditions. The most active nitrating species so far identified is peroxynitrite, the product of the reaction between nitric oxide and superoxide anion. Previously, we have reported that in erythrocytes peroxynitrite irreversibly upregulates lyn, a tyrosine kinase of the src family. In this study we investigated the possible role of tyrosine nitration in the mechanism of lyn activation. We found that tyrosine containing peptides modelled either on the C-terminal tail of src kinases or corresponding to the first 15 amino acids of human erythrocyte band 3 were able to activate lyn when the tyrosine was substituted with 3-nitrotyrosine. The activity of nitrated peptides was shared with phosphorylated but not with unphosphorylated, chlorinated or scrambled peptides. Recombinant lyn src homology 2 (SH2) domain blocked the capacity of the band 3-derived nitrotyrosine peptide to activate lyn and we demonstrated that this peptide specifically binds the SH2 domain of lyn. We propose that nitropeptides may activate src kinases through the displacement of the phosphotyrosine in the tail from its binding site in the SH2 domain. These observations suggest a new mechanism of peroxynitrite-mediated signalling that may be correlated with the upregulation of tyrosine phosphorylation observed in several pathological conditions.

Activation of src tyrosine kinases by peroxynitrite.

In this study, we demonstrate that the phosphorylation activity of five tyrosine kinases of the src family from both human erythrocytes (lyn, hck and c-fgr) and bovine synaptosomes (lyn and fyn) was stimulated by treatment with 30-250 microM peroxynitrite. This effect was not observed with syk, a non-src family tyrosine kinase. Treatment of kinase immunoprecipitates with 0.01-10 microM peroxynitrite showed that the interaction of these enzymes with the oxidant also activated the src kinases. Higher concentrations of peroxynitrite inhibited the activity of all kinases, indicating enzyme inactivation. The addition of bicarbonate (1.3 mM CO2) did not modify the upregulation of src kinases but significantly protected the kinases against peroxynitrite-mediated inhibition. Upregulation of src kinase activity by 1 microM peroxynitrite was 3.5-5-fold in erythrocytes and 1.2-2-fold in synaptosomes, but this could be the result, at least in part, of the higher basal level of src kinase activity in synaptosomes. Our results indicate that peroxynitrite can upregulate the tyrosine phosphorylation signal through the activation of src kinases.

Peroxynitrite induces tryosine nitration and modulates tyrosine phosphorylation of synaptic proteins.

Peroxynitrite, the product of the radical-radical reaction between nitric oxide and superoxide anion, is a potent oxidant involved in tissue damage in neurodegenerative disorders. We investigated the modifications induced by peroxynitrite in tyrosine residues of proteins from synaptosomes. Peroxynitrite treatment (> or =50 microM) induced tyrosine nitration and increased tyrosine phosphorylation. Synaptophysin was identified as one of the major nitrated proteins and pp60src kinase as one of the major phosphorylated substrates. Further fractionation of synaptosomes revealed nitrated synaptophysin in the synaptic vesicles, whereas phosphorylated pp60src was enriched in the postsynaptic density fraction. Tyrosine phosphorylation was increased by treatment with 50-500 microM peroxynitrite and decreased by higher concentrations, suggesting a possible activation/inactivation of kinases. Immunocomplex kinase assay proved that peroxynitrite treatment of synaptosomes modulated the pp60src autophosphorylation activity. The addition of bicarbonate (CO2 1.3 mM) produced a moderate enhancing effect on some nitrated proteins but significantly protected the activity of pp60src against peroxynitrite-mediated inhibition so that at 1 mM peroxynitrite, the kinase was still more active than in untreated synaptosomes. The phosphotyrosine phosphatase activity of synaptosomes was inhibited by peroxynitrite (> or =50 microM) but significantly protected by CO2. Thus, the increase of phosphorylation cannot be attributed to peroxynitrite-mediated inhibition of phosphatases. We suggest that peroxynitrite may regulate the posttranslational modification of tyrosine residues in pre- and postsynaptic proteins. Identification of the major protein targets gives insight into the pathways possibly involved in neuronal degeneration associated with peroxynitrite overproduction.

Decreased proliferation and altered differentiation in osteoblasts from genetically and clinically distinct craniosynostotic disorders.

Craniosynostoses are a heterogeneous group of disorders characterized by premature fusion of cranial sutures. Mutations in fibroblast growth factor receptors (FGFRs) have been associated with a number of such conditions. Nevertheless, the cellular mechanism(s) involved remain unknown. We analyzed cell proliferation and differentiation in osteoblasts obtained from patients with three genetically and clinically distinct craniosynostoses: Pfeiffer syndrome carrying the FGFR2 C342R substitution, Apert syndrome with FGFR2 P253R change, and a nonsyndromic craniosynostosis without FGFR canonic mutations, as compared with control osteoblasts. Osteoblasts from craniosynostotic patients exhibited a lower proliferation rate than control osteoblasts. P253R and nonsyndromic craniosynostosis osteoblasts showed a marked differentiated phenotype, characterized by high alkaline phosphatase activity, increased mineralization and expression of noncollagenous matrix proteins, associated with high expression and activation of protein kinase Calpha and protein kinase Cepsilon isoenzymes. By contrast, the low proliferation rate of C342R osteoblasts was not associated with a differentiated phenotype. Although they showed higher alkaline phosphatase activity than control, C342R osteoblasts failed to mineralize and expressed low levels of osteopontin and osteonectin and high protein kinase Czeta levels. Stimulation of proliferation and inhibition of differentiation were observed in all cultures on FGF2 treatment. Our results suggest that an anticipated proliferative/differentiative switch, associated with alterations of the FGFR transduction pathways, could be the causative common feature in craniosynostosis and that mutations in distinct FGFR2 domains are associated with an in vitro heterogeneous differentiative phenotype.

Bilirubin is an effective antioxidant of peroxynitrite-mediated protein oxidation in human blood plasma.

Bilirubin is a bile pigment that may have an important role as an antioxidant. Its antioxidant potential is attributed mainly to the scavenging of peroxyl radicals. We investigated the reaction of bilirubin with peroxynitrite in phosphate buffer and in blood plasma. In phosphate buffer bilirubin was rapidly oxidized by micromolar concentrations of peroxynitrite, and its oxidation yield was higher at alkaline pH with an apparent pKa = 6.9. In contrast, the major oxidation product of bilirubin in plasma was biliverdin, and the pH profile of its oxidation yield showed a slightly increased oxidation at acidic pH without a clear inflection point. The addition of NaHCO3 to bilirubin decreased the peroxynitrite-dependent oxidation, suggesting that the reactive intermediates formed in the reaction between CO2 and peroxynitrite are less efficient oxidants of bilirubin. The antioxidant role of bilirubin was investigated in some peroxynitrite-mediated plasma protein modifications that are enhanced by CO2 (tryptophan oxidation and protein tyrosine nitration) or slightly decreased by CO2 (protein carbonyl groups). Bilirubin in the micromolar concentration range afforded a significant protection against all these oxidative modifications and, notably, protected plasma proteins even when the pigment was added 5 s after peroxynitrite (i.e., when peroxynitrite is completely decomposed). The loss of tryptophan fluorescence triggered by peroxynitrite was a relatively slow process fulfilled only after a few minutes. After this time, bilirubin was unable to reduce the tryptophan loss, and it was unable to reduce previously formed nitrated albumin or previously formed carbonyls. We deduce that bilirubin in plasma cannot react to a significant extent with peroxynitrite, and we suggest that bilirubin, through a hydrogen donation mechanism, participates as a scavenger of secondary oxidants formed in the oxidative process.

Peroxynitrite modulates tyrosine-dependent signal transduction pathway of human erythrocyte band 3.

Peroxynitrite, the product of the reaction between nitric oxide and superoxide anion, is able to nitrate protein tyrosines. If this modification occurs on phosphotyrosine kinase substrates, it can down-regulate cell signaling. We investigated the effects of peroxynitrite on band 3-mediated signal transduction of human erythrocytes. Peroxynitrite treatment induced two different responses. At low concentrations (10-100 microM) it stimulated a metabolic response, leading to 1) a reversible inhibition of phosphotyrosine phosphatase activity, 2) a rise of tyrosine phosphorylation in the 22K cytoplasmic domain of band 3, 3) the release of glyceraldehyde 3-phosphate dehydrogenase from the membrane, and 4) the enhancement of lactate production. At high concentrations (200-1000 microM), peroxynitrite induced 1) cross-linking of membrane proteins, 2) inhibition of band 3 tyrosine phosphorylation, 3) nitration of tyrosines in the 22K cytoplasmic domain of band 3, 4) binding of hemoglobin to the membrane, 5) irreversible inhibition of phosphotyrosine kinase activity, 6) massive methemoglobin production, and 7) irreversible inhibition of lactate production. Our results demonstrate that at concentrations that could conceivably be achieved in vivo (10-100 microM), peroxynitrite behaves like other oxidants, i.e., it stimulates band 3 tyrosine phosphorylation and increases glucose metabolism. Thus, one plausible physiologic effect of peroxynitrite is the up-regulation of signaling through the reversible inhibition of phosphotyrosine phosphatase activity. At high concentrations of peroxynitrite, the tyrosine phosphorylation ceases in parallel with the nitration of band 3 tyrosines, but at these concentrations phosphotyrosine kinase activity and glycolysis are also irreversibly inhibited. Thus, at least in red blood cells, the postulated down-regulation of signaling by peroxynitrite cannot merely be ascribed to the nitration of tyrosine kinase targets.

Nitric oxide-dependent NAD linkage to glyceraldehyde-3-phosphate dehydrogenase: possible involvement of a cysteine thiyl radical intermediate.

Previous studies have demonstrated that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) undergoes NAD(H) linkage to an active site thiol when it comes into contact with .NO-related oxidants. We found that a free-radical generator 2,2'-azobis-(2-amidinopropane) hydrochloride (AAPH), which does not release either .NO or .NO-related species, was indeed able to induce the NAD(H) linkage to GAPDH. We performed spin-trapping studies with purified apo-GAPDH to identify a putative thiol intermediate produced by AAPH as well as by .NO-related oxidants. As .NO sources we used .NO gas and two .NO-donors, S-nitroso-N-acetyl-D,L-penicillamine and 3-morpholinosydno-nimine hydrochloride (SIN-1). Because SIN-1 produces .NO and a superoxide radical simultaneously, we also tested the effects of peroxynitrite. All the .NO-related oxidants were able to induce the linkage of NAD(H) to GAPDH and the formation of a protein free-radical identified as a thiyl radical (inhibited by N-ethylmaleimide). .NO gas and the .NO-donors required molecular oxygen to induce the formation of the GAPDH thiyl radical, suggesting the possible involvement of higher nitrogen oxides. Thiyl radical formation was decreased by the reconstitution of GAPDH with NAD+. Apo-GAPDH was a strong scavenger of AAPH radicals, but its scavenging ability was decreased when its cysteine residues were alkylated or when it was reconstituted with NAD+. In addition, after treatment with AAPH, a thiyl radical of GAPDH was trapped at high enzyme concentrations. We suggest that the NAD(H) linkage to GAPDH is mediated by a thiyl radical intermediate not specific to .NO or .NO-related oxidants. The cysteine residue located at the active site of GAPDH (Cys-149) is oxidized by free radicals to a thiyl radical, which reacts with the neighbouring coenzyme to form Cys-NAD(H) linkages. Studies with the NAD+ molecule radio-labelled in the nicotinamide or adenine portion revealed that both portions of the NAD+ molecule are linked to GAPDH.

Free radicals induce reversible membrane-cytoplasm translocation of glyceraldehyde-3-phosphate dehydrogenase in human erythrocytes.

We investigated the role of oxygen free radicals in the modulation of glyceraldehyde-3-phosphate dehydrogenase binding to the erythrocyte membrane. Previous studies have demonstrated that in vitro tyrosine phosphorylation of Band 3 prevents the binding of various glycolytic enzymes to its cytoplasmic domain. Since these enzymes are inhibited in their bound state, the functional consequence of Band 3 tyrosine phosphorylation in red blood cells should be to increase glycolysis. To generate free radicals, we used an azo-compound, the hydrophilic 2,2'-azobis(2-amidinopropane) hydrochloride, which, at 37 degrees C and in the presence of oxygen, decomposes and produces peroxyl radicals at a constant rate. The reaction of peroxyl radicals with intact red cells induced a time-dependent loss of the membrane-bound glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase, associated with a concomitant decrease in enzyme activity. At the same time, Band 3 was phosphorylated in tyrosine. These results were completely reversible in plasma after removal of the oxidative stress. The peroxyl radicals also enhanced the production of lactate in intact cells. Our data reveal a powerful mechanism of erythrocyte metabolic regulation that can boost or reduce energy production in times of special need such as during a free radical attack.

Synapsin I expression in spinal cord neurons during chick embryo development.

The cellular distribution of synapsin I in chick spinal cord has been examined during embryo development and in cultured neurons from different developmental stages. Using immunocytochemical methods we have observed that synapsin I appears lightly detectable in spinal cord of embryonic day (E)5-E8 embryos when the motor neurons have already established functional contacts with muscle fibers, and increases at E9. Until E8 synapsin I immunoreactivity appeared mainly localized in the gray matter of spinal cord; immunostaining of white matter becomes clearly evident only at E9. These observations indicate that synapsin I expression and possibly its transport to the nerve terminals may be stimulated by sequential signals. The cellular distribution of synapsin I observed in vivo is maintained in E8 and E9 spinal cord neuron cell cultures. In fact, in E8 cultured neurons, synapsin I immunostaining is observed only in the cell body, while in E9 cultured neurons both cell body and fibers are stained. The addition of muscle extracts to E8 cultures induces synapsin I decoration of fibers similar to that observed in E9 cultured neurons. Indeed Western and Northern blot analysis and in situ hybridization demonstrate an increase of synapsin I and its mRNA in spinal cord neurons kept in the presence of muscle extracts. These data suggest that synapsin I expression, as previously reported for other neuronal markers, can be modulated by soluble factors present in target cells.

Neuronal fodrin proteolysis occurs independently of excitatory amino acid-induced neurotoxicity.

In cultured cerebellar granule cells, the total amount of fodrin alpha subunit increased 3-fold between 0 and 10 days in vitro and fodrin mRNA increased 5-fold. The exposure of cerebellar neurons to NMDA induced the accumulation of a 150 kd proteolytic fragment of fodrin. The NMDA-induced breakdown of fodrin was time-, concentration-, and Ca2(+)-dependent and was inhibited by APV, Mg2+, or the calpain I inhibitor N-acetyl-Leu-Leu-norleucinal. Kainate caused fodrin proteolysis through indirect activation of NMDA receptors. Quisqualate was ineffective. The NMDA-induced degradation of fodrin occurred under conditions that did not cause degeneration of cultured cerebellar neurons. These results show that Ca2+/calpain I-dependent proteolysis of fodrin is selectively associated with NMDA receptor activation; however, fodrin proteolysis per se does not play a causal role in NMDA-induced toxicity in cerebellar granule cells.

Post-translational modification of spectrin induced by excitatory amino acids in cultured cerebellar granule cells.

We have utilized primary cultures from rat cerebellum, 95% enriched in granule cells, to investigate the expression of proteins of the cortical cytoskeleton and of the synaptic vesicles and the relationship between excitatory amino acid receptors activation and calcium dependent proteolysis of fodrin. Exposure of neuronal cell cultures to N-methyl-D-aspartate (NMDA) in the absence of Mg(2+) and in the presence of glycine causes the appearance of 150 kDa proteolytic fragment (s) of-fodrin as detected by immunoblots. The effect of NMDA on fodrin proteolysis is inhibited by the NMDA antagonist 2-amino-5-phosphonovalerate (APV), by 3 mM Mg(2+), by calpain I inhibitor and by the omission of extracellular calcium. The kainate, at the same concentration of NMDA, is less effective and no degradation of fodrin is observed after exposure of cerebellar neurons to quisqualate. These findings suggest that calcium/calpain I-dependent fodrin proteolysis is selectively associated to the functional activation of NMDA receptors.