Changes in phosphorylation of the NMDA receptor in the rat hippocampus induced by status epilepticus

Systemic administration of pilocarpine preceded by lithium induces status epilepticus (SE) that results in neurodegeneration and may lead to the development of spontaneous recurrent seizures. We investigated the effect of Li/pilocarpine-induced SE on phosphorylation of the NMDA receptor in rat hippocampus. Phosphorylation of NR1 by PKC on Ser890 was decreased to 45% of control values immediately following 1 h of SE. During the first 3 h following the termination of SE, phosphorylation of Ser890 increased 4-fold before declining to control values by 24 h. Phosphorylation of NR1 by PKA was also depressed relative to controls immediately following SE and transiently increased above control values upon the termination of SE. SE was accompanied by a general increase in tyrosine phosphorylation of hippocampal proteins that lasted for several hours following the termination of seizures. Tyrosine phosphorylation of the NR2A and NR2B subunits of the NMDAR increased 3-4-fold over control values during SE, continued to increase during the first hour following SE and then declined to control levels by 24 h. SE resulted in the activation of Src and Pyk2 associated with the postsynaptic apparatus, suggesting a role for these enzymes in the SE-induced increase in tyrosine phosphorylation. Changes in phosphorylation of the NMDA receptor may play a role in the pathophysiological consequences of SE.

Differential effects of hypoxia-ischemia on subunit expression and tyrosine phosphorylation of the NMDA receptor in 7- and 21-day-old rats

The effect of cerebral hypoxia-ischemia (HI) on levels and tyrosine phosphorylation of the NMDA receptor was examined in 7- (P7) and 21 (P21)-day-old rats. Unilateral HI was administered by ligation of the right common carotid artery and exposure to an atmosphere of 8% O2/92% N2 for 2 (P7) or 1.5 (P21) h. This duration of HI produces significant infarction in nearly all of the survivors with damage being largely restricted to the cortex, striatum, and hippocampus of the hemisphere ipsilateral to the carotid artery ligation. NR2A levels in the right hemisphere of P7 pups were markedly reduced after 24 h of recovery, while NR1 and NR2B remained unchanged. In contrast, NR2B, but not NR2A, was reduced after HI at P21. At both ages, HI resulted in a transient increase in tyrosine phosphorylation of a number of forebrain proteins that peaked between 1 and 6 h of recovery. At both P7 and P21, tyrosine phosphorylation of NR2B was enhanced 1 h after HI and had returned to basal levels by 24 h. HI induced an increase in tyrosine phosphorylation of NR2A in 21 day, but not in 7-day-old animals. The differential effects of HI on the NMDA receptor at different post-natal ages may contribute to changing sensitivity to hypoxia-ischemia.

Hypoxia-ischemia in perinatal rat brain induces the formation of a low molecular weight isoform of striatal enriched tyrosine phosphatase (STEP)

Protein tyrosine phosphatases play a critical role in controlling tyrosine phosphorylation levels of proteins. Ischemia induces changes in tyrosine phosphorylation. As part of our investigations of the mechanisms responsible for these changes, we studied the effects of cerebral hypoxia-ischemia in 7-day-old (P7) and P21 rat brains on expression of the STEP (striatal enriched phosphatase) family of protein tyrosine phosphatases. P7 and P21 rats were subjected to unilateral hypoxia-ischemia, and brains were analyzed at various intervals of recovery for the presence of STEP. Hypoxia-ischemia induced the formation of a low Mr isoform of STEP, STEP33, in the ipsilateral (damaged) hemisphere but not in the contralateral (undamaged) side. STEP33 produced as a result of ischemia was located exclusively in the cell soluble fraction. In P21 rats, the ischemia-induced elevation in STEP33 was delayed relative to P7 rats. STEP33 was produced by digestion of postsynaptic densities with calpain I and by exposure of NT2/D1 cells expressing STEP to the calcium ionophore A23187. The results suggest that ischemia-induced calcium influx results in the calcium-dependent proteolysis of membrane-associated STEP61 and the concomitant release of STEP33 into the cytoplasm.

The effect of transient global ischemia on the interaction of Src and Fyn with the N-methyl-D-aspartate receptor and postsynaptic densities: possible involvement of Src homology 2 domains

Transient ischemia increases tyrosine phosphorylation of N-methyl-D-aspartate (NMDA) receptor subunits NR2A and NR2B in the rat hippocampus. The authors investigated the effects of this increase on the ability of the receptor subunits to bind to the Src homology 2 (SH2) domains of Src and Fyn expressed as glutathione-S-transferase-SH2 fusion proteins. The NR2A and NR2B bound to each of the SH2 domains and binding was increased approximately twofold after ischemia and reperfusion. Binding was prevented by prior incubation of hippocampal homogenates with a protein tyrosine phosphatase or by a competing peptide for the Src SH2 domain. Ischemia induced a marked increase in the tyrosine phosphorylation of several proteins in the postsynaptic density (PSD), including NR2A and NR2B, but had no effect on the amounts of individual NMDA receptor subunits in the PSD. The level of Src and Fyn in PSDs, but not in other subcellular fractions, was increased after ischemia. The ischemia-induced increase in the interaction of NR2A and NR2B with the SH2 domains of Src and Fyn suggests a possible mechanism for the recruitment of signaling proteins to the PSD and may contribute to altered signal transduction in the postischemic hippocampus.

Identification of lectin-purified neural glycoproteins, GPs 180, 116, and 110, with NMDA and AMPA receptor subunits: conservation of glycosylation at the synapse

The postsynaptic apparatus is associated with a number of glycoproteins with apparent molecular masses of 180, 116, and 110 kDa, which are highly concentrated in and may be uniquely associated with this structure. These glycoproteins, purified by concanavalin A lectin-affinity chromatography, showed immunoreactivity in the present study with subunit-specific antibodies to glutamate receptors as follows: GP 180, NMDA receptor subunits NR2A/NR2B; GP 116, NMDA receptor NR1 (1a); and GP 110, pan-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (pan-AMPA) receptors. Sensitivities to the glycosidases peptide N-glycosidase F and endo-beta-N-acetylglucosaminidase H on both western blots and silver-stained gels suggested that the glutamate receptors were at least major constituents of the glycoprotein bands. Similar detailed glycosylation was observed for all three glycoproteins, with neutral oligosaccharides being dominant. Oligomannosidic glycans (with from five to nine mannoses) accounted for approximately 50% of the neutral sugars, with Man 5 (at almost 20% of the neutral sugars) always the major glycan. Other abundant neutral oligosaccharides were of the complex type. Similar sensitivities to peptide N-glycosidase F and endo-beta-N-acetylglucosaminidase H were observed for cell line-expressed NMDA receptor subunits, suggesting that irrespective of the glycosylation processing available, the least highly processed oligosaccharides will be expressed. This may be indicative of glycosylation sites in these receptors that are inaccessible to the later processing enzymes and favours the oligomannosidic class of glycans in functional roles.

Transient ischemia differentially increases tyrosine phosphorylation of NMDA receptor subunits 2A and 2B

Activation of the N-methyl-D-aspartate (NMDA) receptor has been implicated in the events leading to ischemia-induced neuronal cell death. Recent studies have indicated that the properties of the NMDA receptor channel may be regulated by tyrosine phosphorylation. We have therefore examined the effects of transient cerebral ischemia on the tyrosine phosphorylation of NMDA receptor subunits NR2A and NR2B in different regions of the rat brain. Transient (15 min) global ischemia was produced by the four-vessel occlusion procedure. The tyrosine phosphorylation of NR2A and NR2B subunits was examined by immunoprecipitation with anti-tyrosine phosphate antibodies followed by immunoblotting with antibodies specific for NR2A or NR2B, and by immunoprecipitation with subunit-specific antibodies followed by immunoblotting with anti-phosphotyrosine antibodies. Transient ischemia followed by reperfusion induced large (23-29-fold relative to sham-operated controls), rapid (within 15 min of reperfusion), and sustained (for at least 24 h) increases in the tyrosine phosphorylation of NR2A and smaller increases in that of NR2B in the hippocampus. Ischemia-induced tyrosine phosphorylation of NR2 subunits in the hippocampus was higher than that of cortical and striatal NR2 subunits. The enhanced tyrosine phosphorylation of NR2A or NR2B may contribute to alterations in NMDA receptor function or in signaling pathways in the postischemic brain and may be related to pathogenic events leading to neuronal death.

The N-methyl-D-aspartate receptor subunits NR2A and NR2B bind to the SH2 domains of phospholipase C-gamma

The NMDA receptor has recently been found to be phosphorylated on tyrosine. To assess the possible connection between tyrosine phosphorylation of the NMDA receptor and signaling pathways in the postsynaptic cell, we have investigated the relationship between tyrosine phosphorylation and the binding of NMDA receptor subunits to the SH2 domains of phospholipase C-gamma (PLC-gamma). A glutathione S-transferase (GST) fusion protein containing both the N- and the C-proximal SH2 domains of PLC-gamma was bound to glutathione-agarose and reacted with synaptic junctional proteins and glycoproteins. Tyrosine-phosphorylated PSD-GP180, which has been identified as the NR2B subunit of the NMDA receptor, bound to the SH2-agarose beads in a phosphorylation-dependent fashion. Immunoblot analysis with antibodies specific for individual NMDA receptor subunits showed that both NR2A and NR2B subunits bound to the SH2-agarose. No binding occurred to GST-agarose lacking an associated SH2 domain, indicating that binding was specific for the SH2 domains. The binding of receptor subunits increased after the incubation of synaptic junctions with ATP and decreased after treatment of synaptic junctions with exogenous protein tyrosine phosphatase. Immunoprecipitation experiments confirmed that NR2A and NR2B were phosphorylated on tyrosine and further that tyrosine phosphorylation of each of the subunits was increased after incubation with ATP. The results demonstrate that NMDA receptor subunits NR2A and NR2B will bind to the SH2 domains of PLC-gamma and that isolated synaptic junctions contain endogenous protein tyrosine kinase(s) that can phosphorylate both NR2A and NR2B receptor subunits, and suggest that interaction of the tyrosine-phosphorylated NMDA receptor with proteins that contain SH2 domains may serve to link it to signaling pathways in the postsynaptic cell.

Tyrosine phosphorylation of glycoproteins in the adult and developing rat brain

The tyrosine phosphorylation of glycoproteins in the adult and developing rat brain was investigated. Immunoblotting with anti-tyr(P) antibodies identified a glycoprotein with an apparent Mr of 180,000 (GP180) as the major tyrosine-phosphorylated protein in the concanavalin A (con A)-binding fraction prepared from forebrain homogenates. This glycoprotein had the same electrophoretic mobility as the postsynaptic density (PSD)-associated glycoprotein PSD-GP180. Tyrosine-phosphorylated GP180 was enriched 24-fold in isolated PSDs relative to homogenates. Digestion with endoglycosidase F/N-glycosidase F demonstrated that GP180 present in total homogenates and PSD-GP180 present in isolated PSDs contained similar amounts of N-linked oligosaccharide suggesting that they are the same glycoprotein. The tyrosine phosphorylation of GP180 in homogenates varied between brain regions with the highest levels occurring in cortical areas and the amygdala and low or undetectable amounts being present in hindbrain regions. Incubation of homogenates with adenosine triphosphate (ATP) resulted in the tyrosine phosphorylation of GP180 in all regions examined except the cerebellum and identified a second con A-binding glycoprotein, GP110, which was phosphorylated on tyrosine. GP180 was not phosphorylated on tyrosine following the incubation of cerebellar homogenate, synaptic membranes, or PSDs and ATP. Tyr(P)-GP180 was not detected prior to the onset of synaptogenesis, increased in parallel with the formation of synapses during the first 4 weeks of postnatal development of the frontal cortex and hippocampus, and then decreased 50-60% to adult levels. The results suggest that GP180 corresponds to the PSD glycoprotein PSD-GP180 and are consistent with a role for this glycoprotein in synaptic development and signal transduction at the synapse.

Depolarization-dependent tyrosine phosphorylation in rat brain synaptosomes

Synaptosomes from rat forebrain were analyzed for the presence of phosphotyrosine-containing proteins by immunoblotting with antiphosphotyrosine antibodies. Using this technique, 10-11 phosphotyrosine-containing proteins were detected. Depolarization of synaptosomes by transfer to a high (41 mM) K+ medium resulted in increases in the phosphotyrosine content of several synaptosomal proteins, the most pronounced increase being associated with a membrane protein of M(r) 117,000 (ptp117). Additional proteins exhibiting depolarization-dependent increases in phosphotyrosine content had molecular weights of 39,000, 104,000, 135,000, and 160,000. The depolarization-dependent increase in the phosphotyrosine content of ptp117 was apparent within 30 s of the onset of depolarization, reached a maximum between 3 and 5 min, and then decreased to near control values by 30 min. The increase in tyrosine phosphorylation of ptp117 was dependent on the concentration of K+ in the depolarizing medium and was maximal with [K+] in excess of 50 mM. It was also calcium dependent and did not occur in the absence of extracellular calcium. The addition of veratridine to the incubation medium also resulted in an increase in the tyrosine phosphorylation of ptp117. The results suggest that the phosphorylation of synaptic proteins on tyrosine residues may be involved in the regulation or modulation of synaptic activity.

The effect of castanospermine on the synthesis of synaptic glycoproteins by rat brain slices

Slices were prepared from rat forebrains and the incorporation of [3H]mannose and [35S]methionine into proteins and glycoproteins determined. The incorporation of methionine continued to increase for up to 8 hours whereas mannose incorporation was maximal between 2 and 4 hours and declined thereafter. Glycopeptides prepared by pronase digestion of [3H]mannose-labeled glycoproteins were digested with endoglucosaminidase H (endo H) and analysed by gel filtration. The major endo H-sensitive oligosaccharide eluted in a position similar to standard Man8GlcNAc. In the presence of castanospermine, which inhibits glucosidase I, the first enzymatic step in the processing of N-linked oligosaccharides, a new endo H-sensitive glycan similar in size to standard Glc3Man9GlcNAc2 accumulated. Synaptic membranes (SMs) were isolated from slices which had been incubated with either [3H]mannose or [35S]methionine in the presence and absence of castanospermine. In the presence of inhibitor the relative incorporation of [3H]mannose into high-mannose glycans of synaptic glycoproteins was increased. The incorporation of newly synthesized, [35S] methionine-labeled, Con A-binding glycoproteins into SMs was not affected by the addition of inhibitor. Many of the glycoproteins synthesized in the presence of castanospermine exhibited a decreased electrophoretic mobility indicative of the presence of altered oligosaccharide chains. The results indicate that changes in oligosaccharide composition produced by castanospermine had little effect on the subsequent transport and incorporation of glycoproteins into synaptic membranes.

Phosphorylation of proteins of the postsynaptic density: effect of development on protein tyrosine kinase and phosphorylation of the postsynaptic density glycoprotein, PSD-GP180

The effect of development on the tyrosine kinase activity of postsynaptic densities (PSDs) has been determined. PSDs were prepared from the forebrains of rats ranging in postnatal age from 13 to 90 days and the phosphorylation of both exogenous and endogenous substrates by tyrosine kinase measured. PSDs exhibited tyrosine kinase activity at all ages examined. Phosphorylation of the exogenous substrates polyglutamyltyrosine (4:1) and [val5] angiotensin II increased twofold between days 17 and 22 and then decreased between days 30 and 90 to levels slightly lower than those present at 13 days. The phosphorylation of endogenous PSD proteins on tyrosine residues, assessed by alkali digestion of polyacrylamide gels of 32P-labelled PSD proteins and by measuring the formation of [32P] phosphotyrosine by PSDs incubated in the presence of [gamma-32P] ATP, closely paralleled the changes in total tyrosine kinase activity. Tyrosine phosphorylation of the PSD-specific glycoprotein, PSD-GP180, also showed a transient increase between days 22 and 30, although its concentration within the PSD continued to increase slowly up to 90 days. The results indicate that the tyrosine kinase activity of PSDs is developmentally regulated and that tyrosine phosphorylation of PSD proteins is limited by enzyme rather than substrate availability.

Synaptic protein tyrosine kinase: partial characterization and identification of endogenous substrates

The subcellular distribution of protein tyrosine kinase in rat forebrain was determined using [Val5]-angiotensin II as exogenous substrate. Enzyme activity was present in each of the fractions analyzed and was enriched in synaptic membranes (SMs) and the synaptosomal soluble fraction (2.2- and 2.5-fold over the homogenate, respectively). SMs also phosphorylated polyglutamyltyrosine (pGT; molar ratio of 4:1), the Vmax for angiotensin and pGT phosphorylation being 26.3 +/- 1.6 and 142 +/- 4 pmol/min/mg, respectively. Extraction of SMs with several different detergents resulted in enhanced enzyme activity and the solubilization of 33-37% of the angiotensin and 43-70% of the pGT-phosphorylating activity. Isolated postsynaptic densities (PSDs) contained tyrosine kinase and phosphorylated angiotensin and pGT. The Vmax values for angiotensin and pGT phosphorylation by PSDs were 17 +/- 5 and 23 +/- 1 pmol/min/mg, respectively. Six putative endogenous substrates for SM tyrosine kinase, with molecular weights of 205K, 180K, 76K, 60K, 50K, and 45K, were identified. Each of these proteins, except p76, was phosphorylated in the detergent-insoluble residue obtained following the extraction of SMs with Triton X-100 as well as in PSDs, indicating that the postsynaptic apparatus is an active site of tyrosine phosphorylation. The phosphorylation of p76 was localized to the Triton X-100 extract and also occurred in the synaptosomal soluble fraction. The results indicate that tyrosine kinase and its substrates are located in both pre- and postsynaptic compartments and suggest a role for this enzyme in synaptic function.

In vivo phosphorylation of the postsynaptic density glycoprotein gp180

Rats received intraventricular injections of [32P]PO4 and were killed after 30 min for the preparation of postsynaptic densities (PSDs). Gel electrophoretic analysis identified a number of PSD proteins that incorporated 32P under these conditions. Major proteins that were labelled with 32P had Mr of 185,000, 165,000, 140,000, 92,000, and 51,000. Of these p185, p165, and p140 were also labelled when PSDs were incubated with [gamma-32P]ATP in vitro. In contrast p92 and p51 were relatively poorly labelled under in vitro conditions. Analysis of glycoproteins isolated by chromatography on concanavalin A (Con A)-agarose demonstrated that greater than 70-80% of the 32P present in the glycoproteins eluted from Con A-agarose with alpha-methyl-D-mannopyranoside (Con A+ glycoproteins) was associated with the PSD specific glycoprotein gp180 following both in vivo and in vitro labelling. Phosphopeptide maps and phosphoamino acid analysis of gp180 indicated that similar sites were labelled in vitro and in vivo. Analysis of the subcellular distribution of glycoproteins that incorporated 32P during in vivo labelling demonstrated that gp180 was highly concentrated in PSDs, in accord with the previously suggested exclusive association of this glycoprotein with postsynaptic structures.

Synaptic junctional glycoproteins are phosphorylated by cyclic-AMP-dependent protein kinase

Synaptic junctions (SJs) isolated from rat brain are associated with protein kinase activity and a unique complement of high molecular weight gglycoproteins. Incubation of SJs with [gamma-32P]A+ glycoproteins which were retained by concanavalin A agarose (con A+ glycoproteins). Three major (apparent mol. wt. 180 K, 130 K and 110 K) and 2 minor (apparent mol. wt. 230 K and 145 K) glycoproteins were identified in the con A+ fraction. Of these, GP180 incorporated the most 32P and GP145 was not labeled. Peptide mapping experiments showed that each molecular weight class of glycoprotein was associated with a unique set of phosphorylated peptides. Cyclic AMP stimulated the incorporation of 32P into total SJ proteins and con A+ lycoproteins by 38% and 58%, respectively. GP130 showed the greatest increase in labelling in the presence of cyclic AMP (198% of control levels) although incorporation into all 4 glycoproteins was increased. Cyclic AMP selectively stimulated the incorporation of 32P into only 2 of the 6 phosphorylated peptides derived from GP130. These studies demonstrate that endogenous glycoproteins serve as substrates for intrinsic SJ protein kinases and identify this reaction as a potential means of modifying postsynaptic membrane function.