Postnatal age and protein tyrosine phosphorylation at synapses in the developing rat brain

Tetrahydroxystilbene glucoside improves the learning and memory of amyloid-β(₁₋₄₂)-injected rats and may be connected to synaptic changes in the hippocampus

The aim of this study was to evaluate the protective effects of 2,3,5,4'-tetrahydroxystilbene-2-O-β-D-glucoside (TSG), an active component extracted from Polygonum multiflorum, on learning/memory deficits in Alzheimer's disease (AD). We randomly divided 24 male Sprague-Dawley rats among 4 groups: (i) the sham-operated group (control); (ii) sham-operated group also treated with TSG (sham+TSG); (iii) beta amyloid treated group (Aβ); and (iv) Aβ treatment group also treated with TSG (Aβ+TSG). Rats in the Aβ and Aβ+TSG groups were treated with Aβ₁₋₄₂ intracerebroventricularly, whereas the control and sham+TSG groups were given phosphate-buffered saline. Rats in the sham+TSG and Aβ+TSG groups were then treated intragastrically with TSG (50 mg·(kg body mass)⁻¹·day⁻¹) for 4 weeks, and rats in the Aβ and control groups were treated with saline. The results from Morris water maze tests, electron microscopy, real-time polymerase chain reaction, and Western blotting demonstrated that Aβ₁₋₄₂ induced impairment in learning and memory, degeneration in synaptic structures, and downregulation of Src and NR2B at the gene and protein level, respectively. These alterations were reversed by the administration of TSG, suggesting that TSG exerts anti-AD properties by protecting synaptic structure and function. TSG-induced upregulation of Src and NR2B may be responsible for this process.

Arrested maturation of excitatory synapses in autosomal dominant lateral temporal lobe epilepsy

A subset of central glutamatergic synapses are coordinatelypruned and matured by unresolved mechanisms during early postnatal life. We report that human epilepsy gene LGI1, mutated in autosomal dominant lateral temporal lobe epilepsy (ADLTE), mediates this process in hippocampus. We introduced full-length genes encoding (1) ADLTE truncated mutant LGI1 (835delC) and (2) excess wild-type LGI1 proteins into transgenic mice. We discovered that the normal postnatal Kv1 channel-dependent down-regulation of presynaptic release probability and Src kinase-related decrease of postsynaptic NR2B/NR2A ratio were arrested by ADLTE mutant LGI1, and contrastingly, were magnified by excess wild-type LGI1. Concurrently, mutant LGI1 inhibited dendritic pruning and increased the spine density to markedly increase excitatory transmission. Inhibitory transmission, by contrast, was unaffected. Furthermore, mutant LGI1 promoted epileptiform discharge in vitro and kindling epileptogenesis in vivo with partial GABA_A receptor blockade. Thus, LGI1 represents the first human gene mutated to promote epilepsy through impaired glutamatergic circuit maturation.

Developmental changes in the association of NMDA receptors with lipid rafts

Lipid rafts (LR) are lipid microdomains present in the cell surface membrane that are organizational platforms involved in protein trafficking and formation of cell signaling complexes. In the adult brain, NMDA receptors (NMDAR) and receptor-associated proteins such as membrane-associated guanylate kinases (PSD-95 and SAP102), are distributed between the postsynaptic density (PSD) and lipid rafts. However, the time course of the association of NMDAR with LR during neural development is not known. We therefore investigated the effect of development on the association of NMDAR with LR prepared from rat brains ranging in postnatal age from 1-35 days and compared this with their expression in PSDs. LR and PSD fractions were prepared by extraction of P2 membranes with Tx-100 followed by sucrose density gradient centrifugation. The yield of LR, as reflected by levels of protein, Thy-1, and flotillin-1 increased during postnatal development. NR2A was associated predominantly with the lipid raft fraction at all ages examined whereas NR2B underwent a gradual shift from PSDs to lipid rafts during the first 3 weeks after birth. These changes in the distribution of NR2A and NR2B were paralleled by changes in the distribution of PSD-95 and SAP102 respectively. Tyrosine-phosphorylated proteins, including NR2A and NR2B, were preferentially associated with lipid rafts in older, as compared to younger, animals. These results show that the association of NMDAR with LR is regulated developmentally.

Microarray analysis of the developing cortex

Abnormal development of the prefrontal cortex (PFC) is associated with a number of neuropsychiatric disorders that have an onset in childhood or adolescence. Although the basic laminar structure of the PFC is established in utero, extensive remodeling continues into adolescence. To map the overall pattern of changes in cortical gene transcripts during postnatal development, we made serial measurements of mRNA levels in mouse PFC using oligonucleotide microarrays. We observed changes in mRNA transcripts consistent with known postnatal morphological and biochemical events. Overall, most transcripts that changed significantly showed a progressive decrease in abundance after birth, with the majority of change between postnatal weeks 2 and 4. Genes with cell proliferative, cytoskeletal, extracellular matrix, plasma membrane lipid/transport, protein folding, and regulatory functions had decreases in mRNA levels. Quantitative PCR verified the microarray results for six selected genes: DNA methyltransferase 3A (Dnmt3a), procollagen, type III, alpha 1 (Col3a1), solute carrier family 16 (monocarboxylic acid transporters), member 1 (Slc16a1), MARCKS-like 1 (Marcksl1), nidogen 1 (Nid1) and 3-hydroxybutyrate dehydrogenase (heart, mitochondrial) (Bdh).

Hypoxia-ischemia in the immature brain

The immature brain has long been considered to be resistant to the damaging effects of hypoxia and hypoxia-ischemia (H/I). However, it is now appreciated that there are specific periods of increased vulnerability, which relate to the developmental stage at the time of the insult. Although much of our knowledge of the pathophysiology of cerebral H/I is based on extensive experimental studies in adult animal models, it is important to appreciate the major differences in the immature brain that impact on its response to, and recovery from, H/I. Normal maturation of the mammalian brain is characterized by periods of limitations in glucose transport capacity and increased use of alternative cerebral metabolic fuels such as lactate and ketone bodies, all of which are important during H/I and influence the development of energy failure. Cell death following H/I is mediated by glutamate excitotoxicity and oxidative stress, as well as other events that lead to delayed apoptotic death. The immature brain differs from the adult in its sensitivity to all of these processes. Finally, the ultimate outcome of H/I in the immature brain is determined by the impact on the ensuing cerebral maturation. A hypoxic-ischemic insult of insufficient severity to result in rapid cell death and infarction can lead to prolonged evolution of tissue damage.

c-Src kinase activation regulates preprotachykinin gene expression and substance P secretion in rat sensory ganglia

Increased synthesis of substance P (SP) in the dorsal root ganglia (DRG) and enhanced axonal transport to and secretion from the primary afferent sensory neurons might enhance pain signalling in the spinal dorsal horn by modifying pronociceptive pathways. IL-1beta increases SP synthesis by enhancing the expression of preprotachykinin (PPT) mRNA encoding for SP and other tachykinins in the DRG. Stimulation of IL-1 receptor by IL-1beta may induce the phosphorylation of tyrosine residues in many effector proteins through the activation of p60c-src kinase. The hypothesis that the synthesis of SP in and secretion from the primary sensory ganglia are regulated by the activation of p60c-src kinase induced by IL-1beta was tested. Pretreatment of DRG neurons in culture with herbimycin A, genistein or PP2, three structurally different nonreceptor tyrosine kinase inhibitors that act by different mechanisms, decreased the kinase activity of p60c-src induced by the activation of IL-1 receptor. PP3, a negative control for the Src family of tyrosine kinase inhibitor PP2 had no effect. Herbimycin A and genistein also decreased IL-1beta-induced expression of PPT mRNA-encoding transcripts and the levels of SP-li synthesized in the cells and secreted into the culture medium in a concentration-dependent manner. SB 203580 [a p38 mitogen-activated protein kinase (p38 MAPK) inhibitor] and PD 98059 (a p44/42 MAPK kinase inhibitor) were ineffective in modulating IL-1beta-induced SP synthesis and secretion, and p60c-src kinase activity in DRG neurons. Whereas, IL-1 receptor antagonist and cycloheximide inhibited IL-1beta-evoked secretion of SP-like immunoreactivity (SP-li), actinomycin D decreased it significantly but did not entirely abolish it. These findings show that phosphorylation of specific protein tyrosine residue(s) following IL-1 receptor activation might play a key role in IL-1beta signalling to modulate PPT gene expression and SP secretion in sensory neurons. In view of the role of SP as an immunomodulator, these studies provide a new insight into neural-immune intercommunication in pain regulation in the sensory ganglia through the IL-1beta-induced p60c-src activation.

Inhibition of protein kinase C reduces ischemia-induced tyrosine phosphorylation of the N-methyl-d-aspartate receptor

The role of protein kinase C (PKC) in tyrosine phosphorylation of the N-methyl-d-aspartate receptor (NMDAR) following transient cerebral ischemia was investigated. Transient (15 min) cerebral ischemia was produced in adult rats by four-vessel occlusion and animals allowed to recover for 15 or 45 min. Following ischemia, tyrosine phosphorylation of NR2A and NR2B and activated Src-family kinases (SFKs) and Pyk2 were increased in post-synaptic densities (PSDs). Phosphorylation of NR2B on Y1472 by PSDs isolated from post-ischemic forebrains was inhibited by the SFK specific inhibitor PP2, and by the PKC inhibitors GF109203X (GF), Gö6976 and calphostin C. Intravenous injection of GF immediately following the ischemic challenge resulted in decreased phosphorylation of NR1 on PKC phosphorylation sites and reduced ischemia-induced increases in tyrosine phosphorylation of NR2A and NR2B without affecting the increase in total tyrosine phosphorylation of hippocampal proteins. Ischemia-induced increases in activated Pyk2 and SFKs in PSDs, but not the translocation of PKC, Pyk2 or Src to the PSD, were also inhibited by GF. The inactive homologue of GF, bisindolylmaleimide V, had no effect on these parameters. The results are consistent with a role for PKC in the ischemia-induced increase in tyrosine phosphorylation of the NMDAR, via a pathway involving Pyk2 and Src-family kinases.

Phosphorylation of c-Cbl protooncogene product following ethanol administration in rat cerebellum: possible involvement of Fyn kinase

We have previously shown that ethanol administration results in tyrosine phosphorylation of the 130 kDa protein in rat brain, and identified the protein as Cas, the crk-associated src substrate. In the present study, we demonstrate that Cbl of a 120 kDa protein is also tyrosine-phosphorylated in the cerebellum in response to ethanol administration. We also investigated whether Fyn kinase was involved in ethanol-induced Cbl phosphorylation. Immunoprecipitation experiments showed that the amount of coimmunoprecipitated Fyn kinase with an anti-Cbl antibody increased in extracts from ethanol-administered rats compared to those from saline-administered rats. Exogenous Fyn kinase was shown to phosphorylate on tyrosine residue(s) of Cbl from the cerebellum in vitro. Furthermore, Fyn kinase and Cbl were demonstrated immunohistochemically to be coexpressed in white matter in the cerebellum. These findings indicate that Cbl is tyrosine-phosphorylated in rat cerebellum in response to ethanol administration, and also raise the possibility that Fyn kinase may be involved in the process.

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.

Group I mGluRs coupled to G proteins are regulated by tyrosine kinase in dopamine neurons of the rat midbrain

Metabotropic glutamate receptors (mGluRs) modulate neuronal function via different transduction mechanisms that are either dependent or independent on G-protein function. Here we investigated, using whole cell patch-clamp recordings in combination with fluorimetric measurements of intracellular calcium concentration ([Ca(2+)](i)), the metabolic pathways involved in the responses induced by group I mGluRs in dopamine neurons of the rat midbrain. The inward current and the [Ca(2+)](i) increase caused by the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG, 100 microM) were permanently activated and subsequently abolished in cells loaded with the nonhydrolizable GTP-analogue GTP-gamma-S (600 microM). In addition, when GDP-beta-S (600 microM) was dialyzed into the cells to produce the blockade of the G proteins, the DHPG-dependent responses were reduced. When the tissue was bathed with the phospholipase C inhibitor 1-[6[[(17 beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]exyl]-1H-pyrrole-2,5-dione (10 microM), the DHPG-induced calcium transients slightly diminished but the associated inward currents were not affected. Interestingly, a substantial depression of the DHPG-induced inward current and transient increase of [Ca(2+)](i) was caused by the protein tyrosine kinase inhibitors tyrphostin B52 (40 microM) and 4',5,7-trihydroxyisoflavone (genistein; 40 microM), whereas genistein's inactive analogue 4',5,7-trihydroxyisoflavone-7-glucoside (40 microM) was ineffective. The blockade of the Src family of tyrosine kinase by 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (20 microM), mitogen-activated protein kinase by 2'-amino-3' methoxyflavone (50 microM), and protein kinase C by staurosporine (1 microM) had no effect on the cellular responses caused by DHPG. The mGluR5-selective antagonist 2-methyl-6-(phenylethynyl)-pyridine (10--100 microM) did not affect the actions of DHPG. Thus our results indicate that the responses, mainly mediated by mGluRs1 in dopamine neurons, are activated by intracellular mechanisms coupled to G proteins and regulated by tyrosine kinases.

Possible involvement of Fyn kinase in ethanol-stimulated Cas tyrosine phosphorylation in rat cerebellum and cerebral cortex

In the present study, we have investigated the effect of intraperitoneal injection of ethanol (3.5 g/kg) on tyrosine phosphorylation in rat brain. Immunoblot analysis using an antiphosphotyrosine antibody revealed that a 130-kDa protein band was detected in the brain extract in response to ethanol administration. This ethanol-stimulated tyrosine phosphorylation of the 130-kDa protein was found in the brain but not in the heart, liver or thymus. The 130-kDa phosphotyrosine-containing protein was identified by immunoprecipitation to be Cas, a crk-associated src substrate. This ethanol-stimulated tyrosine phosphorylation of Cas was observed most prominently in the cerebellum and the cerebral cortex. We further examined the possible involvement of Fyn kinase in ethanol-stimulated Cas tyrosine phosphorylation. Immunecomplex kinase assay showed that Fyn was activated in the cerebellum and cerebral cortex of ethanol-administered rats. Immunoprecipitation experiments also showed that Fyn was co-immunoprecipitated with an anti-Cas antibody in these regions from ethanol-administered rats. Furthermore, exogenous Fyn was shown to phosphorylate Cas from cerebellum and cerebral cortex in vitro. These findings indicate that ethanol stimulates tyrosine phosphorylation of Cas in rat cerebellum and cerebral cortex, and that Fyn may be involved in the process.

Early effects of protein kinase modulators on DNA synthesis in rat cerebral cortex

By using tissue miniunits, protein kinase modulators, and topoisomerase inhibitors in short-term incubation (0-90 min) we studied (1) the role of protein phosphorylation in the immediate control of DNA replication in the developing rat cerebral cortex and (2) the mechanism of action for genistein-mediated DNA synthesis inhibition. Genistein decreased the DNA synthesis within less than 30 min. None of the other protein kinase inhibitors examined (herbimycin A, staurosporine, calphostin-C) or the protein phosphatase inhibitor sodium orthovanadate inhibited DNA synthesis and they did not affect the genistein-mediated inhibition. The selective topoisomerase inhibitors camptothecin and etoposide decreased the DNA synthesis to an extent similar to that of genistein and within less than 30 min. In addition, the effects of these substances on topoisomerase I and II were studied. Etoposide and genistein but not herbimycin A, staurosporine, or calphostin-C strongly inhibited the activity of topoisomerase II. Our results (1) strongly suggest that the net rate of DNA replication during the S phase of the cell cycle is independent of protein phosphorylation and (2) indicate that the early inhibitory effect of genistein on DNA synthesis is mediated by topoisomerase II inhibition rather than protein tyrosine kinase inhibition.

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.

Platelet-activating factor stimulation of p125(FAK) and p130(Cas) tyrosine phosphorylation in brain

The effect of platelet-activating factor (PAF) on protein tyrosine phosphorylation was studied in rat brain slices. PAF induced a time- and concentration-dependent increase in tyrosine phosphorylation of a doublet of approximately 125 kDa. These proteins were identified by immunoprecipitation as p125(FAK) and p130(Cas), using monoclonal antibodies. This effect was mediated by PAF receptors, as shown by its inhibition by the action of a PAF antagonist. The tyrosine phosphorylation evoked by PAF was dependent, at least in part, on external calcium. The involvement of protein kinase C was demonstrated by the synergistic effect of TPA on PAF-stimulated tyrosine phosphorylation. The finding that PAF stimulates tyrosine phosphorylation of both focal adhesion protein p125(FAK) and p130(Cas) suggests that PAF might modulate the integrin mediated signal transduction in the brain.

Src, a molecular switch governing gain control of synaptic transmission mediated by N-methyl-D-aspartate receptors

The N-methyl-D-aspartate (NMDA) receptor is a principal subtype of glutamate receptor mediating fast excitatory transmission at synapses in the dorsal horn of the spinal cord and other regions of the central nervous system. NMDA receptors are crucial for the lasting enhancement of synaptic transmission that occurs both physiologically and in pathological conditions such as chronic pain. Over the past several years, evidence has accumulated indicating that the activity of NMDA receptors is regulated by the protein tyrosine kinase, Src. Recently it has been discovered that, by means of up-regulating NMDA receptor function, activation of Src mediates the induction of the lasting enhancement of excitatory transmission known as long-term potentiation in the CA1 region of the hippocampus. Also, Src has been found to amplify the up-regulation of NMDA receptor function that is produced by raising the intracellular concentration of sodium. Sodium concentration increases in neuronal dendrites during high levels of firing activity, which is precisely when Src becomes activated. Therefore, we propose that the boost in NMDA receptor function produced by the coincidence of activating Src and raising intracellular sodium may be important in physiological and pathophysiological enhancement of excitatory transmission in the dorsal horn of the spinal cord and elsewhere in the central nervous system.

Endothelin stimulates tyrosine phosphorylation of p125FAK and p130Cas in rat cerebral cortex

Stimulation of rat cerebral cortex with endothelin-1 (ET-1) caused an increase in the tyrosine phosphorylation of several proteins. Two of these phosphoproteins were identified by the immunoprecipitation assays as being the focal adhesion kinase p125FAK and crk-associated substrate p130Cas. This effect was time- and dose-dependent, with an EC50 value of 3.9 x 10(-8) M. In addition, the cerebral cortex ET receptor subtype involved in this action was determined by using BQ-123 and BQ-788, which are ET(A) and ET(B) receptor antagonists respectively. Our results indicate that the ET-1 effect on protein tyrosine phosphorylation occurred through ET(B) receptors. The requirement for extracellular Ca2+ on ET-1 action was also studied. ET-1-stimulated tyrosine phosphorylation of both p125FAK and p130Cas was abolished in the absence of external Ca2+ or in the presence of nimodipine, a Ca2+ channel-blocker. These results suggest that the ET-1-stimulated protein tyrosine phosphorylation was secondary to Ca2+ influx through the dihydropyridine Ca2+-channel. In slices where protein kinase C was inhibited, ET-1-stimulated tyrosine phosphorylation of both proteins was reduced. These results indicate that ET-1 modulates the tyrosine phosphorylation of specific proteins, which may be involved in adhesion processes in the brain.

Overexpression of striatal enriched phosphatase (STEP) promotes the neurite outgrowth induced by a cAMP analogue in PC12 cells

A cytoplasmic protein tyrosine phosphatase (PTPase) designated as striatal enriched phosphatase with a molecular weight of 46 kDa (STEP46) is highly expressed in striatal neurons with dopamine D1-receptors. To examine the hypothesis that STEP46 is involved in the neuronal functions modulated by the cyclic adenosine 3', 5'-monophosphate (cAMP)-signaling system, we introduced the complementary DNA of STEP46 into the pheochromocytoma cell line PC12, which exhibits neuronal differentiation characterized by neurite outgrowth in response to cAMP and nerve growth factor stimulation, and we established subclonal cell lines that constitutively overexpress STEP46 protein with PTPase activity. The subclones expressing STEP46 showed increased neurite outgrowth during differentiation induced by a cAMP analogue (dibutyryl cAMP). The positive regulatory role of STEP46 in the cAMP-induced neuronal differentiation of PC12 cells indicates that STEP46 may play a role in neuronal processes modulated by the cAMP-signaling cascade as a PTPase.

Effect of pre- and postnatal ethanol exposure on protein tyrosine kinase activity and its endogenous substrates in rat cerebral cortex

The rat brain contains high levels of tyrosine-specific protein kinases (PTKs) that specifically phosphorylate the tyrosine-containing synthetic peptide poly(Glu4Tyr1). Using this peptide as a substrate, we have measured the protein tyrosine kinase activity in membrane and cytosolic fractions from the cerebral cortices of pre- and postnatal ethanol-exposed rats at time intervals of 8, 30, and 90 days. During the course of development of the cerebral cortex, PTK activity decreased both in the membrane and cytosolic fractions from 8 and 90 days of age. Maximum activity was associated at the age of 8 days and gradually declined in the later ages (30 and 90 days) of postnatal development. However, PTK activity in the ethanol exposed rat cerebral cortex was further decreased when compared to controls in all the ages of postnatal development in membrane as well as in cytosolic fractions. In the presence of vanadate, a specific inhibitor of protein tyrosine phosphatases (PTPs), the PTK activity increased, indicating that the balance between protein tyrosine kinase and protein tyrosine phosphatase might be lost during ethanol exposure. In addition, when using an antibody specific for phosphotyrosine, endogenous substrates for protein tyrosine kinases were identified on an immunoblot of membrane and cytosolic fractions from the ethanol-exposed rat cerebral cortex. The immunoblot showed several phosphotyrosine-containing proteins with molecular weights of 114, 70, 36, 34, 32, 20, and 14 kDa that were present in the cerebral cortex. However, higher levels of immunoreactivity of these proteins were found in the ethanol-exposed membrane fractions when compared to control fractions-particularly at the age of 30 and 90 days. Two phosphotyrosine proteins with molecular weights of 38 and 40 kDa showed decreased immunoreactivity at the age of 90 days in the cytosolic fraction of an ethanol-exposed rat's cerebral cortex. The differences in tyrosine-specific protein kinase activity and in phosphotyrosine-containing proteins observed during pre- and postnatal ethanol exposure may reflect specific functional defects in the cerebral cortex which could possibly underlie the mechanism contributing to fetal alcohol syndrome (FAS).

Presence of a voltage-dependent anion channel 1 in the rat postsynaptic density fraction

Little is known about the molecular organization and functions of the postsynaptic density (PSD), a cytoskeletal specialization on the postsynaptic membrane. In an attempt to elucidate the protein composition of PSD, we have sequenced a 35 kDa protein of the rat forebrain PSD fraction. Amino acid sequence information of the tryptic peptides and immunoblot analyses revealed that the protein is a voltage-dependent anion channel 1 (VDAC1). VDAC1 was enriched in the PSD fraction and was partially soluble in 1% n-octyl glucoside (NOG) or Triton X-100. Our data indicate that VDAC1, which is originally found in the outer mitochondrial membrane, is also present in the central nervous system (CNS) synapses in association with the PSD 'core'.

Src, N-methyl-D-aspartate (NMDA) receptors, and synaptic plasticity

The protein tyrosine kinase Src is expressed widely in the central nervous system and is abundant in neurons. Over the past several years, evidence has accumulated showing that one function of Src is to regulate the activity of N-methyl-D-aspartate (NMDA) receptors and other ion channels. NMDA receptors are a principal subtype of glutamate receptor that mediates fast excitatory transmission at most central synapses. Recently it has been discovered that, by means of up-regulating the function of NMDA receptors, Src mediates the induction of long-term potentiation (LTP) in the CA1 region of the hippocampus. This finding led to a new model for induction of LTP whereby tetanic stimulation produces a rapid activation of Src, causing enhanced NMDA receptor function. This enhanced NMDA receptor function boosts the entry of Ca2+, which may thereby trigger the downstream signalling cascade, ending in potentiation of non-NMDA receptors. This functional role for Src may be important in physiological and pathophysiological processes in the central nervous system.

A peptide activator of endogenous tyrosine kinase enhances synaptic currents mediated by NMDA receptors

N-methyl-D-aspartic acid (NMDA) receptor currents in cultured cells or expression systems are increased by the addition of purified tyrosine kinases. However, there is no direct demonstration of this effect at NMDA receptors in intact synapses of rat brain slices. Transmitters which might be used to activate tyrosine kinases in situ are unlikely to have a sufficiently selective action to allow a clear interpretation of their effects. Therefore, we used a phosphotyrosine-containing decapeptide which can be included in recording electrodes to activate postsynaptic src-family tyrosine kinases. This peptide enhanced NMDA responses in dissociated hippocampal CA1 neurons. These effects were not reproduced by a non-phosphorylated peptide or a scrambled-sequence phosphopeptide. The enhancement of NMDA responses was blocked by a tyrosine kinase inhibitor. In brain slices the phosphopeptide, but not control peptide, increased NMDA receptor-mediated synaptic current indicating that endogenous tyrosine kinase can upregulate the response of NMDA receptors at glutamatergic synapses in the hippocampus.

Fast and sensitive method for simultaneous measurement of cell proliferation rate and drug sensitivity in rat cerebral cortex

A proliferation assay based on the production of mini-units of tissue was adopted and modified for the simultaneous determination of cell proliferation rate and the effect of genistein in rat cerebral cortex. Mini-units of tissue were produced from rat cerebral cortex immediately after killing the animal and incubated with culture medium containing 3H-methyl-thymidine during 90 min. The proliferation rate was assessed by measurement of 3H-methyl-thymidine incorporation into trichloroacetic acid insoluble material/mg of protein/min. The mini-unit method preserves the neural-cell topological relation existing in vivo and, in addition, has several additional advantages: (1) the short incubation time required limits the metabolic changes, (2) the sensitivity to drugs can be assessed simultaneously with the cell proliferation rate, (3) the complete procedure can be performed within 4-6 h, and (4) many experiments can be performed with the tissue from one animal. Genistein in doses from 10 to 100 microM inhibited cell proliferation in a concentration-dependent manner. The percentage of inhibition was highest in young animals and decreased with increasing age. This method is a powerful tool for the study of drugs with short-time onset mechanisms of action and can be useful for the screening of new drugs.

Protein tyrosine phosphorylation: implications for synaptic function

The phosphorylation of proteins on tyrosine residues, initially believed to be primarily involved in cell growth and differentiation, is now recognized as having a critical role in regulating the function of mature cells. The brain exhibits one of the highest levels of tyrosine kinase activity in the adult animal and the synaptic region is particularly rich in tyrosine kinases and tyrosine phosphorylated proteins. Recent studies have described the effects of tyrosine phosphorylation on the activities of a number of proteins which are potentially involved in the regulation of synaptic function. Furthermore, it is becoming apparent that tyrosine phosphorylation is involved in the modification of synaptic activity, such as occurs during depolarization, the induction of long-term potentiation or long-term depression, and ischemia. Changes in the activities of tyrosine kinases and/or protein tyrosine phosphatases which are associated with synaptic structures may result in altered tyrosine phosphorylation of proteins located at the synapse leading to both short-term and long-lasting changes in synaptic and neuronal function.

A possible role for tyrosine kinases in the regulation of the neuronal dopamine transporter in mouse striatum

The present investigation was undertaken to test the hypothesis that a reduction in the activity of protein tyrosine kinases would result in an alteration in dopamine transport. Genistein, a broad-spectrum inhibitor of protein tyrosine kinases, inhibited dopamine uptake into mouse striatal homogenates with an IC50 of 18 microM. The inhibition by genistein was rapid, reversible and somewhat selective, in that genistein did not inhibit the uptake of choline or GABA under similar conditions. Kinetic analyses indicated that genistein was a non-competitive inhibitor. Another protein tyrosine kinase inhibitor, tyrphostin 23, also inhibited transport but was significantly less potent than genistein. Tyrphostin 25 and lavendustin A were without major effect on dopamine uptake. In addition, the inactive structural analog of genistein, genistein, had no significant effect on dopamine uptake. The inhibition of dopamine transport by 50 microM genistein was accompanied by a reduction in the level of a 110-kDa band of tyrosine phosphoprotein. It is suggested that protein tyrosine kinases play a role in the cascade of events which ultimately lead to regulation of neuronal dopamine transport.

Differential surface expression and phosphorylation of the N-methyl-D-aspartate receptor subunits NR1 and NR2 in cultured hippocampal neurons

The trafficking and phosphorylation of the NR1 and NR2 subunits of the N-methyl-D-aspartate-type glutamate receptor complex were studied in cultured rat hippocampal neurons. Surface expression was examined by modifying surface receptors via treatment of intact neurons with either the protease chymotrypsin or the cross-linking reagent bis(sulfosuccinimidyl)suberate, followed by quantification of anti-NR1 and anti-NR2B Western blot immunostaining. These studies revealed that only 40-50% of total NR1 immunoreactivity is found at the cell surface, as compared to more than 90% of total NR2B immunoreactivity. Metabolic labeling of the cultures with 32P revealed that NR2 subunits are highly phosphorylated under basal conditions, whereas basal phosphorylation of NR1 subunits is barely detectable. Following stimulation of the cultures with glutamate/glycine or phorbol esters, NR1 phosphorylation was found to be enhanced by 3-5-fold, whereas phosphorylation of NR2 subunits was enhanced by less than 2-fold. To determine whether the difference in the basal NR1 versus NR2 phosphorylation could be due to tyrosine phosphorylation of NR2, phosphoamino acid analyses of NR2 were performed. These analyses revealed phosphorylation on serine but not on threonine or tyrosine; immunoprecipitation and deglycosylation experiments using anti-phosphotyrosine antibodies confirmed that NR2 subunits in the primary hippocampal cultures are not detectably phosphorylated on tyrosine residues. These results demonstrate that the NR1 and NR2 subunits, which assemble into heteromeric complexes to form functional N-methyl-D-aspartate receptors, are trafficked in neurons with differential efficiency to the plasma membrane and exhibit different levels of basal versus stimulated serine phosphorylation.

STEP61: a member of a family of brain-enriched PTPs is localized to the endoplasmic reticulum

The STEP family of protein tyrosine phosphatases is highly enriched within the CNS. Members of this family are alternatively spliced to produce both transmembrane and cytosolic variants. This manuscript describes the distinctive intracellular distribution and enzymatic activity of the membrane-associated isoform STEP61. Transfection experiments in fibroblasts, as well as subcellular fractionations, sucrose density gradients, immunocytochemical labeling, and electron microscopy in brain tissue, show that STEP61 is an intrinsic membrane protein of striatal neurons and is associated with the endoplasmic reticulum. In addition, structural analysis of the novel N-terminal region of STEP61 reveals several motifs not present in the cytosolic variant STEP46. These include two putative transmembrane domains, two sequences rich in Pro, Glu, Asp, Ser, and Thr (PEST sequences), and two polyproline-rich domains. Like STEP46, STEP61 is enriched in the brain, but the recombinant protein has less enzymatic activity than STEP46. Because STEP46 is contained in its entirety within STEP61 and differs only in the extended N terminus of STEP61, this amino acid sequence is responsible for the association of STEP61 with membrane compartments and may also regulate its enzymatic activity.

Enhanced tyrosine phosphorylation of the 2B subunit of the N-methyl-D-aspartate receptor in long-term potentiation

Both serine/threonine and tyrosine phosphorylation of receptor proteins have been implicated in the process of long-term potentiation (LTP), but there has been no direct demonstration of a change in receptor phosphorylation after LTP induction. We show that, after induction of LTP in the dentate gyrus of anesthetized adult rats, there is an increase in the tyrosine phosphorylation of the 2B subunit of the N-methyl-D-aspartate (NMDA) receptor (NR2B), as well as several other unidentified proteins. Tyrosine phosphorylation of NR2B was measured in two ways: binding of antiphosphotyrosine antibodies (PY20) to glycoprotein(s) of 180 kDa (GP180) purified on Con A-Sepharose and binding of anti-NR2B antibodies to tyrosine-phosphorylated proteins purified on PY20-agarose. Three hours after LTP induction, anti-NR2B binding to tyrosine phosphorylated proteins, expressed as a ratio of tetanized to control dentate (Tet/Con), was 2.21 +/- 0.50 and PY20 binding to GP180 was 1.68 +/- 0.16. This increase in the number of tyrosine phosphorylated NR2B subunits occurred without a change in the total number of NR2B subunits. When the induction of LTP was blocked by pretreatment of the animal with the NMDA receptor antagonist MK801, the increase in PY20 binding to GP180 was also blocked (Tet/Con = 1.09 +/- 0.26). The increased PY20 binding to GP180 was also apparent 15 min after LTP induction (Tet/Con = 1.41 +/- 0.16) but not detectable 5 min after LTP induction (Tet/Con = 1.01 +/- 0.19). These results suggest that tyrosine phosphorylation of the NMDA receptor contributes to the maintenance of LTP.

Transient compartmental expression of a family of protein tyrosine phosphatases in the developing striatum

The expression of a family of intracellular protein tyrosine phosphatases (STEP) was studied in the striatum of rats during ontogeny. Links between the formation of dopamine islands and STEP immunoreactive patches in the striatum were examined since previous work had suggested that STEP isoforms were selectively expressed in dopaminoceptive brain regions. STEP protein and mRNAs were distributed in a patchy manner during the first postnatal week. By 2 weeks, STEP immunoreactivity was homogeneous, indicating that both patch and matrix neurons express STEP by maturity. Two-color immunofluorescent staining was also performed to compare STEP with specific markers for patch and matrix. Tyrosine hydroxylase immunoreactive fibers from the substantia nigra form distinctive dopamine islands in the striatum during late embryonic development, and occupy the sites of future patches [23,37,38,54]. These fiber islands align with STEP immunoreactive neuronal patches during the first two postnatal weeks, suggesting that STEP is a marker for patch neurons in early postnatal development. When STEP's distribution was compared with other markers for patch (substance P) or matrix (calbindin), STEP co-localized with substance P in most striatal neurons on postnatal days 1 through 7. However, STEP was also expressed within a subset of calbindin-positive neurons in the lateral striatum, but not with these neurons elsewhere in the striatum. By adulthood, STEP colocalized with both markers. These results suggest that STEP is expressed first within patch neurons but not matrix, and subsequently within both. The expression of STEP may be triggered by the arrival of striatal afferents or other regulatory factors.

Focal adhesion kinase in the brain: novel subcellular localization and specific regulation by Fyn tyrosine kinase in mutant mice

Signaling by tyrosine kinases is required for the induction of synaptic plasticity in the central nervous system. Comparison of fyn, src, yes, and abl nonreceptor tyrosine kinase mutant mice shows a specific requirement for Fyn in the induction of long-term potentiation at CA1 synapses in the hippocampus. To identify components of a Fyn-dependent pathway that may be involved with hippocampus function we examined tyrosine-phosphorylated proteins in kinase mutant mice. We found that nine proteins were hypophosphorylated specifically in fyn mutants. One of the hypophosphorylated proteins was focal adhesion tyrosine kinase (FAK). FAK also showed reduced activity in immunocomplex kinase assays only in fyn mutants. FAK is expressed at very high levels in the brain but in contrast to non-neural cells, FAK was not restricted to focal adhesion contacts. FAK was found in axons, dendrites, and the intermediate filament cytoskeleton of astrocytes. Brain extracts from the mutants also show specific patterns of compensatory changes in the activity of the remaining Src family kinases. Tyrosine phosphorylation is a critical regulator of FAK, and impairments in FAK signal transduction in fyn mutants may contribute to the mutant neural phenotype.

Modulation of protein tyrosine phosphorylation in rat insular cortex after conditioned taste aversion training

Protein tyrosine phosphorylation is a major signal transduction pathway involved in cellular metabolism, growth, and differentiation. Recent data indicate that tyrosine phosphorylation also plays a role in neuronal plasticity. We are using conditioned taste aversion, a fast and robust associative learning paradigm subserved among other brain areas by the insular cortex, to investigate molecular correlates of learning and memory in the rat cortex. In conditioned taste aversion, rats learn to associate a novel taste (e.g., saccharin) with delayed poisoning (e.g., by LiCl injection). Here we report that after conditioned taste aversion training, there is a rapid and marked increase in tyrosine phosphorylation of a set of proteins in the insular cortex but not in other brain areas. A major protein so modulated, of 180 kDa, is abundant in a membrane fraction and remains modulated for more than an hour after training. Exposure of the rats to the novel taste alone results in only a small modulation of the aforementioned proteins whereas administration of the malaise-inducing agent per se has no effect. To the best of our knowledge, this is the first demonstration of modulation of protein tyrosine phosphorylation in the brain after a behavioral experience.

The major tyrosine-phosphorylated protein in the postsynaptic density fraction is N-methyl-D-aspartate receptor subunit 2B

The postsynaptic density (PSD) is a specialization of the submembranous cytoskeleton that is visible in the electron microscope on the cytoplasmic face of the postsynaptic membrane. A subcellular fraction enriched in structures with the morphology of PSDs contains signal-transduction molecules thought to regulate receptor localization and function in the central nervous system. We have purified a prominent tyrosine-phosphorylated glycoprotein of apparent molecular mass 180 kDa, termed PSD-gp180, that is highly enriched in the rat forebrain PSD fraction. The sequences of four tryptic peptides generated from the protein reveal that it is the 2B subunit of the N-methyl-D-aspartate (NMDA) type glutamate receptor. We have confirmed the identity of PSD-gp180 by showing that it reacts with antibodies raised against a unique fragment of the 2B subunit of the NMDA receptor. We also show that the 2B subunit is the most prominently tyrosine-phosphorylated protein in the PSD fraction based upon recognition by an anti-phosphotyrosine antibody. Two types of NMDA receptor subunits have been identified by molecular cloning [Nakanishi, S. (1992) Science 258, 597-603]. The single type 1 subunit is expressed throughout the brain and is necessary for formation of the receptor channel. The four type 2 subunits (2A, 2B, 2C, and 2D) are expressed in discrete brain regions, contain unusually long unique C termini, and confer distinct kinetic properties on NMDA receptors that contain them. Our findings suggest that, in the forebrain, NMDA receptor subunit 2B may serve to anchor NMDA receptors at the postsynaptic membrane through its interaction with the PSD. The prominent presence of tyrosine phosphate further suggests that the NMDA receptor may be regulated by tyrosine phosphorylation or that it may participate in signaling through tyrosine phosphorylation and through its ion channel.

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 and neurotransmitters increase neuronal protein tyrosine phosphorylation

In rat hippocampal slices and in neurons in primary culture, K(+)-induced depolarization increased markedly and rapidly tyrosine phosphorylation of a 110-kDa protein (pp110) and, to a lesser degree, of a 120-kDa protein (pp120), in a calcium-dependent fashion. Glutamate, 1-aminocyclopentane-trans-1,3-dicarboxylic acid (an agonist of metabotropic glutamate receptors), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (an agonist of ionotropic glutamate receptors) stimulated also tyrosine phosphorylation of pp110 and pp120. These effects were not observed in astrocytes in primary culture. In hippocampal slices tyrosine phosphorylation of pp110 and pp120 was stimulated by Ca(2+)-ionophores and by phorbol esters and antagonized by a chelator of intracellular Ca2+ and by drugs that inhibit protein kinase C. Stimulation of muscarinic and alpha 1-adrenergic receptors increased also tyrosine phosphorylation of pp110 and pp120. These results demonstrate that membrane depolarization and stimulation of neurotransmitter receptors activate a tyrosine phosphorylation pathway in neurons. This pathway involves an increase in intracellular Ca2+ concentrations and the activation of protein kinase C. It may provide a biochemical basis for some neurotrophic effects of electrical activity and neurotransmitters and may contribute to the role of tyrosine phosphorylation in long-term potentiation.

Phosphotyrosine-containing proteins are concentrated in differentiating cells during chicken embryonic development

Protein tyrosine phosphorylation may be an important indicator of both the proliferative status and differentiation status of cells during embryonic development. To determine how each of these factors contributes to the level of phosphotyrosine-containing proteins detectable in embryonic tissues we have used immunohistochemistry with anti-phosphotyrosine antibodies on sections of developing chicken embryos. In contrast to an earlier study (Takata and Singer, 1988) we found proteins phosphorylated on tyrosine residues to be present in many different cells of the developing chicken embryo. The successful detection of phosphotyrosine-containing proteins in many cell types required the presence of sodium orthovanadate, a phosphotyrosine phosphatase inhibitor, during fixation. Despite the fact that the majority of tyrosine kinases identified to date are growth factor receptors, the highest levels of phosphotyrosine-containing proteins in many tissues were localized to populations of cells which were differentiating or migrating rather than dividing.

Impaired long-term potentiation, spatial learning, and hippocampal development in fyn mutant mice

Mice with mutations in four nonreceptor tyrosine kinase genes, fyn, src, yes, and abl, were used to study the role of these kinases in long-term potentiation (LTP) and in the relation of LTP to spatial learning and memory. All four kinases were expressed in the hippocampus. Mutations in src, yes, and abl did not interfere with either the induction or the maintenance of LTP. However, in fyn mutants, LTP was blunted even though synaptic transmission and two short-term forms of synaptic plasticity, paired-pulse facilitation and post-tetanic potentiation, were normal. In parallel with the blunting of LTP, fyn mutants showed impaired spatial learning, consistent with a functional link between LTP and learning. Although fyn is expressed at mature synapses, its lack of expression during development resulted in an increased number of granule cells in the dentate gyrus and of pyramidal cells in the CA3 region. Thus, a common tyrosine kinase pathway may regulate the growth of neurons in the developing hippocampus and the strength of synaptic plasticity in the mature hippocampus.

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.

Stimulation of protein-tyrosine phosphorylation in rat striatum after lesion of dopamine neurons or chronic neuroleptic treatment

Even though the short-term actions of dopamine on postsynaptic receptors are well-characterized, the molecular bases for long-term trophic interactions between dopamine neurons and their targets remain unclear. Since protein-tyrosine phosphorylation plays a key role in the action of trophic factors, we have investigated its possible involvement in the interactions between dopamine neurons and their striatal targets. Lesioning rat nigrostriatal dopamine neurons by using 6-hydroxydopamine increased the phosphorylation on tyrosine of several proteins, including a major 180-kDa protein (pp180) in the ipsilateral striatum. Protein-tyrosine kinase activity was also increased in the striatum ipsilateral to the lesion, whereas no change in phosphotyrosine phosphatase activity was detected. The stimulation of pp180 phosphorylation was observed 1, 2, and 8 weeks after 6-hydroxydopamine lesion, was selective for the destruction of dopamine neurons, and was mimicked by chronic blockade of dopamine receptors with neuroleptics. Additional lesion experiments and subcellular fractionation showed that pp180 is located in neuronal postsynaptic densities, suggesting that pp180 is a postsynaptic component of corticostriatal synapses. Our results indicate that lesion of specific afferent fibers can activate tyrosine phosphorylation in central neurons and suggest that tyrosine phosphorylation is involved in the long-term consequences of dopamine deficiency and may play a role in synaptic plasticity.