Protein tyrosine phosphorylation: implications for synaptic function

Tyrosine 402 phosphorylation of Pyk2 is involved in ionomycin-induced neurotransmitter release

Protein tyrosine kinases, which are highly expressed in the central nervous system, are implicated in many neural processes. However, the relationship between protein tyrosine kinases and neurotransmitter release remains unknown. In this study, we found that ionomycin, a Ca²⁺ ionophore, concurrently induced asynchronous neurotransmitter release and phosphorylation of a non-receptor protein tyrosine kinase, proline-rich tyrosine kinase 2 (Pyk2), in clonal rat pheochromocytoma PC12 cells and cerebellar granule cells, whereas introduction of Pyk2 siRNA dramatically suppressed ionomycin-induced neurotransmitter release. Further study indicated that Tyr-402 (Y402) in Pyk2, instead of other tyrosine sites, underwent rapid phosphorylation after ionomycin induction in 1 min to 2 min. We demonstrated that the mutant of Pyk2 Y402 could abolish ionomycin-induced dopamine (DA) release by transfecting cells with recombinant Pyk2 and its mutants (Y402F, Y579F, Y580F, and Y881F). In addition, Src inhibition could prolong phosphorylation of Pyk2 Y402 and increase DA release. These findings suggested that Pyk2 was involved in ionomycin-induced neurotransmitter release through phosphorylation of Y402.

Myosin VI small insert isoform maintains exocytosis by tethering secretory granules to the cortical actin

Before undergoing neuroexocytosis, secretory granules (SGs) are mobilized and tethered to the cortical actin network by an unknown mechanism. Using an SG pull-down assay and mass spectrometry, we found that myosin VI was recruited to SGs in a Ca(2+)-dependent manner. Interfering with myosin VI function in PC12 cells reduced the density of SGs near the plasma membrane without affecting their biogenesis. Myosin VI knockdown selectively impaired a late phase of exocytosis, consistent with a replenishment defect. This exocytic defect was selectively rescued by expression of the myosin VI small insert (SI) isoform, which efficiently tethered SGs to the cortical actin network. These myosin VI SI-specific effects were prevented by deletion of a c-Src kinase phosphorylation DYD motif, identified in silico. Myosin VI SI thus recruits SGs to the cortical actin network, potentially via c-Src phosphorylation, thereby maintaining an active pool of SGs near the plasma membrane.

Increased activity of the vesicular soluble N-ethylmaleimide-sensitive factor attachment protein receptor TI-VAMP/VAMP7 by tyrosine phosphorylation in the Longin domain

Vesicular (v)- and target (t)-SNAREs play essential roles in intracellular membrane fusion through the formation of cytoplasmic α-helical bundles. Several v-SNAREs have a Longin N-terminal extension that, by promoting a closed conformation, plays an autoinhibitory function and decreases SNARE complex formation and membrane fusion efficiency. The molecular mechanism leading to Longin v-SNARE activation is largely unknown. Here we find that exocytosis mediated by the Longin v-SNARE TI-VAMP/VAMP7 is activated by tonic treatment with insulin and insulin-like growth factor-1 but not by depolarization and intracellular calcium rise. In search of a potential downstream mechanism, we found that TI-VAMP is phosphorylated in vitro by c-Src kinase on tyrosine 45 of the Longin domain. Accordingly, a mutation of tyrosine 45 into glutamate, but not phenylalanine, activates both t-SNARE binding and exocytosis. Activation of TI-VAMP-mediated exocytosis thus relies on tyrosine phosphorylation.

Regulation of c-Src activity in glutamate-induced neurodegeneration

c-Src is heavily expressed in the brain and in human neural tissues. Our pursuit for characterization of the neuroprotective mechanisms of tocotrienols led to the first evidence demonstrating that rapid c-Src activation plays a central role in executing glutamate-induced neurodegeneration. It is now known that Src deficiency or blockade of Src activity in mice provides cerebral protection following stroke. Here, we sought to examine the mechanisms that regulate inducible c-Src activity in glutamate-challenged HT4 neural cells and primary cortical neurons. Knockdown of c-Src protected cells against glutamate-induced loss of viability. Consistently, microinjection of siRNA against c-Src protected cells against glutamate. Using overexpression and knockdown approaches, we noted that SHP-1 may be implicated in glutamate-induced c-Src activation. Following such activation, Cbp and caveolin-1 were phosphorylated and associated with Csk. Csk was translocated to the membrane where it down-regulated glutamate-induced c-Src activity by catalyzing the inhibitory phosphorylation of a tyrosine residue in c-Src. Findings of this study present a new paradigm that addresses the regulation of c-Src under neurodegenerative conditions.

CCK1 and 2 receptors are expressed in immortalized rat brain neuroblasts: Intracellular signals after cholecystokinin stimulation

Cholecystokinin (CCK) is one of the most abundant neuropeptides in the central nervous system (CNS) where it promotes important functions by activation of receptors CCK1 and CCK2. Our aim was to investigate CCK receptors expression and their downstream intracellular signaling in immortalized rat brain neuroblasts. Results show that CCK1 and CCK2 receptor mRNAs and CCK2 receptor protein are expressed in neuroblasts. CCK incubation of neuroblasts leads to stimulation in a time-dependent manner of several signaling pathways, such as tyrosine phosphorylation of adaptor proteins paxillin and p130(Cas), phosphorylation of p44/p42 ERKs as well as PKB (Ser473). Moreover, CCK-8 stimulates the DNA-binding activity of the transcription factor AP-1. The CCK2 receptor agonist gastrin stimulates ERK1/2 phosphorylation in a comparable degree as CCK does. ERK1/2 phosphorylation activated by CCK-8 was markedly inhibited by the CCK2 receptor antagonist CR2945. Incubation for 48 h with CCK-8 increases neuroblasts viability in a similar degree as EGF. In summary, our data clearly identify CCK1 and CCK2 receptor mRNAs and CCK2 receptor protein in brain neuroblasts and show that incubation with CCK promotes cell proliferation and activates the phosphorylation of survival transduction pathways. Stimulation of ERK1/2 phosphorylation by CCK is mainly mediated by the CCK2 receptor. Moreover, this work might provide a novel model of proliferating neuronal cells to further study the biochemical mechanisms by which the neuropeptide CCK exerts its actions in the CNS.

Ablation of gene expression of N-methyl-D-aspartate receptor one by antisense oligonucleotides in striatal neurons in culture

In the present study, a twenty-mer antisense oligonucleotide specific for N-methyl-D-aspartate receptor one (ANR1) was applied to striatal neurons in primary cell culture. The ANR1 was found to be specific and nontoxic. Significant reductions in expression of NR1 mRNA and proteins were resulted after a single dose of ANR1 transcripts. Interestingly, there were reductions in total NR1 proteins but two phosphorylated forms of NR1 proteins at serine 896 and 897 residues were not reduced. There was also no change in the pattern of distribution of NR1 immunoreactivity in the striatal neurons. In addition, significant reductions of NMDA-mediated peak inward current were found after application of a higher concentration of ANR1 (20-100 microM) by patch clamp recordings. The present results indicate that ANR1 is a useful agent in reducing NMDA receptor functions. The present data thus provide detailed cellular and molecular mechanisms to explain our previous findings of amelioration of motor symptoms in a rat model of Parkinson's disease. More importantly, application of ANR1 was also found to display neuroprotective effects of striatal neurons against NMDA-induced excitotoxic cell death. The findings have implications in development of new approach in prevention of cell death in neurodegenerative diseases and new treatments for these diseases.

Tyrosine phosphorylation of synaptophysin in synaptic vesicle recycling

The integral SV (synaptic vesicle) protein synaptophysin was one of the first nerve terminal proteins identified. However its role, if any, in the SV life cycle remains undetermined. One of the most prominent features of synaptophysin is that its cytoplasmic C-terminus largely consists of pentapeptide repeats initiated by a tyrosine residue. Synaptophysin is heavily phosphorylated by tyrosine kinases in the nerve terminal, suggesting that this phosphorylation is central to its function. This review will cover the evidence for tyrosine phosphorylation of synaptophysin and how this phosphorylation may control its function in the SV life cycle.

Tyrosine phosphorylation of synaptophysin in synaptic vesicle recycling

The integral SV (synaptic vesicle) protein synaptophysin was one of the first nerve terminal proteins identified. However its role, if any, in the SV life cycle remains undetermined. One of the most prominent features of synaptophysin is that its cytoplasmic C-terminus largely consists of pentapeptide repeats initiated by a tyrosine residue. Synaptophysin is heavily phosphorylated by tyrosine kinases in the nerve terminal, suggesting that this phosphorylation is central to its function. This review will cover the evidence for tyrosine phosphorylation of synaptophysin and how this phosphorylation may control its function in the SV life cycle.

Analysis of transcriptional responses in the mouse dorsal striatum following acute 3,4-methylenedioxymethamphetamine (ecstasy): identification of extracellular signal-regulated kinase-controlled genes

3,4-Methylenedioxymethamphetamine (ecstasy), a widely used recreational drug with psychoactive properties, induces both serotonin and dopamine release in the brain. However, little is known about its intracellular effects. We previously showed that 3,4-methylenedioxymethamphetamine rewarding effects in mice were dependent upon extracellular signal-regulated kinase activation and that dorsal striatum was a critical region for mediating extracellular signal-regulated kinase-dependent Egr1 3,4-methylenedioxymethamphetamine-induced transcription. Here, we extend these findings by showing that 3,4-methylenedioxymethamphetamine is indeed able to activate extracellular signal-regulated kinase within this structure. To identify genes regulated by acute 3,4-methylenedioxymethamphetamine in the mice dorsal striatum, and selectively controlled by this kinase, we performed microarray experiments by using a selective inhibitor of extracellular signal-regulated kinase activation, SL327. Of the approximately 24,000 genes from the microarray, 27 showed altered expression after exposure to 3,4-methylenedioxymethamphetamine, and among these, 59% were partially or totally inhibited by SL327 pretreatment. Our results showed that the genes regulated by 3,4-methylenedioxymethamphetamine encode proteins that belong to transcription factors family, signaling pathways (phosphatases, cytoskeleton regulation), and synaptic functions. These early changes, and especially those controlled by extracellular signal-regulated kinase activation might play significant roles in the expression of many of the behaviors that occur following 3,4-methylenedioxymethamphetamine taking.

Soy isoflavones and cognitive function

There is growing interest in the physiological functions of soy isoflavones, especially in whether they affect cognitive function and have beneficial effects on neurodegenerative diseases. Here we review the recent evidence from clinical and experimental studies supporting a role for soy isoflavones in cognitive function. Soy isoflavones may mimic the actions and functions of estrogens on brain, and they have been shown to have positive effects on the cognitive function in females; however, studies on their effects on spatial memory have not provided consistent results in males. Although data from humans, cultures, and animal models are currently insufficient for elucidating the metabolism of soy isoflavone actions on cognitive function and the nervous system, we suggest two putative pathways; (1) an estrogen receptor-mediated pathway and (2) via the inhibition of tyrosine kinase, in particular by genistein, which is one of the soy isoflavones. Although soy isoflavones appear to have a positive effect on brain function, further research is needed to determine not only the efficacy but also the safety of soy isoflavones on the nervous system and cognitive function.

Novel pharmacological strategies for motor complications in Parkinson’s disease

In advanced Parkinson's disease, the combination of disease progression and levodopa therapy leads to the development of motor problems complicating the therapeutic response, known as motor response complications. The nonphysiological, pulsatile stimulation produced by most currently available dopaminergic therapies triggers a complicated series of responses resulting in the dysregulation of glutamate receptors and many other neurotransmitter systems on striatal neurons. Although a number of novel compounds that provide a more continuous dopaminergic stimulation are becoming available, no practical way to accomplish this in a truly physiological manner currently exists. Novel strategies for pharmacological intervention with the use of nondopaminergic treatments, with drugs targeting selected transmitter receptors expressed on striatal neurons appear more promising. These include NMDA or AMPA antagonists, or drugs acting on 5-hydroxytryptamine subtype 2A, alpha2-adrenergic, adenosine A2A and cannabinoid CB1 receptors. Future strategies may also target pre- and postsynaptic components that regulate firing pattern, like synaptic vesicle proteins, or nonsynaptic gap junction communication mechanisms, or drugs with actions at the signal transduction systems that modulate the phosphorylation state of NMDA receptors. These new therapeutic strategies, alone or in combination, hold the promise of providing effective control or reversal of motor response complications.

Characterization of multiple transcripts and isoforms derived from the mouse protein tyrosine phosphatase gene Ptprr

The use of alternative splice sites, promoters and translation start sites considerably adds to the complexity of organisms. Four mouse cDNAs (PTPBR7, PTP-SL, PTPPBSgamma+ and PTPPBSgamma-) have been cloned that contain different 5' parts but encode identical protein tyrosine phosphatase PTPRR catalytic domains. We investigated the genomic origin and coding potential of these transcripts to elucidate their interrelationship. Mouse gene Ptprr exons were identified within a 260 kbp segment on chromosome 10, revealing PTP-SL- and PTPPBSgamma-specific transcription start sites within introns two and four, respectively, relative to the 14 PTPBR7 exons. Northern and RT-PCR analyses demonstrated differential expression patterns for these promoters. Furthermore, transfection studies and AUG codon mutagenesis demonstrated that in PTP-SL and PTPPBSgamma messengers multiple translation initiation sites are being used. Resulting 72, 60, 42 and 37 kDa PTPRR protein isoforms differ not only in the length of their N-terminal part but also in their subcellular localization, covering all major PTP subtypes; receptor-like, membrane associated and cytosolic. In summary, mouse gene Ptprr gives rise to multiple isoforms through the use of distinct promoters, alternative splicing and differential translation starts. These results set the stage for further investigations on the physiological roles of PTPRR proteins.

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.

Differential expression and ser897 phosphorylation of striatal N -methyl- d -aspartate receptor subunit NR1 in animal models of Parkinson's disease

Parkinson's disease (PD) is associated with complex compensatory changes in the functional and neurochemical anatomy of the basal ganglia resulting from striatal dopamine deficiency. Available evidence suggests that glutamatergic corticostriatal pathway becomes overactive following nigrostriatal dopaminergic denervation. Since N-methyl-d-aspartate (NMDA) receptors are abundant and functionally important in the striatum, we examined the effects of striatal dopamine deficiency on the distribution and density of NMDA receptor subunit 1 (NR1) and serine phosphorylation (ser897) of NR1 in the striatum. Phosphorylation at this residue is believed to be dependent on protein kinase A. In both 6-hydroxydopamine-lesioned rats and MPTP-treated monkeys, striatal dopamine denervation resulted in down-regulation of NR1 expression and an increase in the expression of ser897-phosphorylated NR1 by striatal neurons. The decreased NR1 expression may be a compensatory response to overactive glutamatergic inputs, which appear to occur in response to striatal dopamine denervation in animal models of PD. Furthermore, the alterations observed in protein kinase A-dependent phosphorylation of NR1 are suggestive of receptor internalization and subcellular trafficking of NR1 in response to increased striatal glutamatergic activity.

The cholecystokinin system in the rat retina: receptor expression and in vivo activation of tyrosine phosphorylation pathways

High levels of endogenous cholecystokinin (CCK) are present in the rat retina and restricted to amacrine cells. Two types of CCK receptors exist but their expression and intracellular transduction pathways coupled in the rat retina are unknown. The aims of this study were to investigate CCK receptors expression in rat retina and to study downstream tyrosine phosphorylation pathways. For this purpose, total mRNA isolated from rat retina was subjected to RT-PCR analysis. Isolated rat retinas were incubated in presence of CCK. Soluble proteins in retinal homogenates were immunoprecipitated with anti-phoshpotyrosine or anti-p130(Cas) specific monoclonal antibodies and subjected to SDS-PAGE, followed by Western blotting analysis. Both types of CCK receptor mRNAs, A and B, are present in the rat retina. Incubation of retina with CCK induced a rapid increase in several phosphotyrosine-containing bands with molecular masses greater than 30 kDa. Western Blotting and immunoprecipitation with a specific monoclonal antibody identified one of the phosphotyrosine bands as the adapter protein p130(Cas). Tyrosine phosphorylation of p130(Cas) induced by CCK in rat retina was time and concentration dependent: CCK induced tyrosine phosphorylation of p130(Cas) occurred rapidly with the maximum effect observed at 2.5 min incubation with 1 microM CCK. Our data clearly identified CCK-A and CCK-B receptor mRNAs in the rat retina and demonstrated that they are functional, stimulating tyrosine phosphorylation pathways. Our results provide novel biochemical information to further understand the physiological role of CCK A and B receptors in rat retina.

NGF signaling in sensory neurons: evidence that early endosomes carry NGF retrograde signals

Target-derived NGF promotes the phenotypic maintenance of mature dorsal root ganglion (DRG) nociceptive neurons. Here, we provide in vivo and in vitro evidence for the presence within DRG neurons of endosomes containing NGF, activated TrkA, and signaling proteins of the Rap1/Erk1/2, p38MAPK, and PI3K/Akt pathways. Signaling endosomes were shown to be retrogradely transported in the isolated sciatic nerve in vitro. NGF injection in the peripheral target of DRG neurons increased the retrograde transport of p-Erk1/2, p-p38, and pAkt in these membranes. Conversely, NGF antibody injections decreased the retrograde transport of p-Erk1/2 and p-p38. Our results are evidence that signaling endosomes, with the characteristics of early endosomes, convey NGF signals from the target of nociceptive neurons to their cell bodies.

Striatal glutamatergic mechanisms and extrapyramidal movement disorders

The nonphysiologic stimulation of striatal dopaminergic receptors, as a result of disease- or drug-related denervation or intermittent excitation, triggers adaptive responses in the basal ganglia which contribute to the appearance of parkinsonian symptoms and later to the dyskinesias and other alterations in motor response associated with dopaminergic therapy. Current evidence suggests that these altered responses involve activation of signal transduction cascades in striatal medium spiny neurons linking dopaminergic to coexpressed ionotropic glutamatergic receptors of the N-methyl-D-aspartate (NMDA) and Alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) classes. These intraneuronal signaling pathways appear capable of modifying the phosphorylation state of NMDA and AMPA receptor subunits; resultant sensitization enhances cortical glutamatergic input which in turn modifies striatal output in ways that compromise motor behavior. The regulation of these spiny neuron glutamate receptors can also be affected by the activation state of coexpressed nondopaminergic receptors as well as by changes associated with Huntington's disease. These observations lend new insight into molecular mechanisms contributing to the integration of synaptic inputs to spiny neurons. They also suggest novel approaches to the pharmacotherapy of extrapyramidal motor dysfunction.

Analysis of ecstasy (MDMA)-induced transcriptional responses in the rat cortex

3,4-methylenedioxymethamphetamine (MDMA, ecstasy) is a popular drug of abuse. MDMA is pharmacologically classified as an entactogen because of its affinities to classical hallucinogens and stimulants. Oral ingestion of a single dose of the drug is associated with euphoria, elevated self-confidence, and heightened sensory awareness in humans. Evidence for neurotoxicity in the human serotonin (5-HT) system has been provided. In rats, a single injection of MDMA induces hyperthermia and formation of reactive oxygen species. These effects may cause MDMA-associated, long-term 5-HT depletion, with the cortex being quite sensitive to the biochemical effects of MDMA. It has been suggested that these MDMA effects may be associated with molecular changes in this brain region. To test these ideas, we have made use of the cDNA array analysis, which can provide a more global view of the molecular changes secondary to MDMA injections. Our results show that the genes regulated by MDMA encode proteins that belong to signaling pathways, transcription regulators, or xenobiotic metabolism. Our observations indicate that cortical cells respond to the acute administration of MDMA by modulating transcription of several genes that might lead to long-term changes in the brain.

Tyrosine phosphorylation of the metabotropic glutamate receptor mGluR5 in striatal neurons

Protein phosphorylation, controlled by the coordinated actions of phosphatases and kinases, is an important regulatory mechanism in synaptic transmission and other neurophysiological processes. Ionotropic glutamate receptors are known targets of phosphorylation on serine, threonine and tyrosine residues, with functional consequences for cell excitability, plasticity and toxicity. While phosphorylation of metabotropic glutamate receptors (mGluRs) also impacts critical cellular processes, there has been no evidence for direct tyrosine phosphorylation of mGluRs. In the present study, anti-phosphotyrosine and specific mGluR antibodies were used to detect tyrosine-phosphorylated mGluRs in rat brain. In particular, we found that mGluR5 is an abundant phosphotyrosine protein in vivo as well as in primary striatal neurons and tissue slices in vitro. The protein phosphatase inhibitor pervanadate robustly increased the amount of tyrosine-phosphorylated mGluR5, suggesting the receptor is subject to an endogenous, active cycle of phosphorylation and dephosphorylation. Furthermore, NMDA treatment also increased the amount of tyrosine-phosphorylated mGluR5, suggesting these endogenous phosphorylation regulatory mechanisms can be used to mediate crosstalk between synaptic glutamate receptors. While mGluR5-stimulated phosphoinositide hydrolysis appears to be unaltered by pervanadate treatment, tyrosine phosphorylation of mGluR5 may be important in trafficking, anchoring, or signaling of the receptor through G protein-independent pathways.

alpha-Synuclein interacts with phospholipase D isozymes and inhibits pervanadate-induced phospholipase D activation in human embryonic kidney-293 cells

alpha-Synuclein has been implicated in the pathogenesis of many neurodegenerative diseases, including Parkinson's disease and Alzheimer's disease. Although the function of alpha-synuclein remains largely unknown, recent studies have demonstrated that this protein can interact with phospholipids. To address the role of alpha-synuclein in neurodegenerative disease, we have investigated whether it binds phospholipase D (PLD) and affects PLD activity in human embryonic kidney (HEK)-293 cells overexpressing wild type alpha-synuclein or the mutant forms of alpha-synuclein (A53T, A30P) associated with Parkinson's disease. Tyrosine phosphorylation of alpha-synuclein appears to play a modulatory role in the inhibition of PLD, because mutation of Tyr(125) to Phe slightly increases inhibitory effect of alpha-synuclein on PLD activity. Treatment with pervanadate or phorbol myristate acetate inhibits PLD more in HEK 293 cells overexpressing alpha-synuclein than in control cells. Binding of alpha-synuclein to PLD requires phox and pleckstrin homology domain of PLD and the amphipathic repeat region and non-Abeta component of alpha-synuclein. Although biologically important, co-transfection studies indicate that the interaction of alpha-synuclein with PLD does not influence the tendency of alpha-synuclein to form pathological inclusions. These results suggest that the association of alpha-synuclein with PLD, and modulation of PLD activity, is biologically important, but PLD does not appear to play an essential role in the pathophysiology of alpha-synuclein.

A src family tyrosine kinase inhibits neurotransmitter release from neuronal cells

Tyrosine kinases are expressed in many tissues, particularly in the central nervous system, and regulate various cellular functions. We report here that a src family tyrosine kinase-specific inhibitor, PP2, enhances neurotransmitter release from PC12 cells and primary cultured neurons. PP2 enhances only Ca(2+)-dependent release; it does not affect basal release. These effects result from an enhancement of vesicular exocytosis and not from the reuptake or refilling of neurotransmitters because Ca(2+)-dependent secretion of an exogenously expressed reporter protein, the human growth hormone (hGH), is also enhanced by PP2. Overexpression of constitutive active v-src, but not of a kinase-inactive mutant, suppressed Ca(2+)-dependent release. In PP2-treated cells, Pyk2, paxillin, and some other proteins showed a decrease in tyrosine phosphorylation, and the enhancement of tyrosine phosphorylation of these proteins in response to Ca(2+) influx was also reduced. Electron and fluorescence microscopy showed that PP2 treatment induced morphological change and decreased phalloidin reactivity at the filopodium-like structures on the processes of PC12 cells. Interestingly, inhibition of actin polymerization with cytochalasin D and latrunculin A enhanced Ca(2+)-dependent, but not basal, release. It is possible that a src family tyrosine kinase, through the regulation of actin dynamics, has an inhibitory function to regulate neurotransmitter release.

Effects of infusions of the tyrosine kinase inhibitor radicicol into the hippocampus on short- and long-term memory of the inhibitory avoidance task

The aim of the present work was to test the role of protein tyrosine kinases (PTKs) on both the short-term memory (STM) and long-term memory (LTM) of the inhibitory avoidance task in rats using the inhibitor of tyrosine kinase, radicicol. Rats implanted with cannulae in the CA1 area of the dorsal hippocampus received a 0.5 microl infusion of radicicol (0.5, 1, 5, 10, 20 microg/ml) or vehicle (water) at different times after training and were tested for STM (1.5 or 3 h) and LTM (24 h). Additionally, one group received radicicol 10 min prior to the test for LTM. Radicicol depressed both STM and LTM when infused before and immediately after training and had no effect on either form of memory when infused 30 or 90 min after training. Radicicol also depressed the retrieval of LTM. Our results indicate that memory formation and retrieval in the hippocampus can involve PTK activity, but the present findings should be taken merely as a possible starting point for future investigations.

alpha-synuclein is phosphorylated by members of the Src family of protein-tyrosine kinases

alpha-Synuclein (alpha-Syn) is implicated in the pathogenesis of Parkinson's Disease, genetically through missense mutations linked to early onset disease and pathologically through its presence in Lewy bodies. alpha-Syn is phosphorylated on serine residues; however, tyrosine phosphorylation of alpha-Syn has not been established (, ). A comparison of the protein sequence between Synuclein family members revealed that all four tyrosine residues of alpha-Syn are conserved in all orthologs and beta-Syn paralogs described to date, suggesting that these residues may be of functional importance (). For this reason, experiments were performed to determine whether alpha-Syn could be phosphorylated on tyrosine residue(s) in human cells. Indeed, alpha-Syn is phosphorylated within 2 min of pervanadate treatment in alpha-Syn-transfected cells. Tyrosine phosphorylation occurs primarily on tyrosine 125 and was inhibited by PP2, a selective inhibitor of Src protein-tyrosine kinase (PTK) family members at concentrations consistent with inhibition of Src function (). Finally, we demonstrate that alpha-Syn can be phosphorylated directly both in cotransfection experiments using c-Src and Fyn expression vectors and in in vitro kinase assays with purified kinases. These data suggest that alpha-Syn can be a target for phosphorylation by the Src family of PTKs.

Pyrantel resistance alters nematode nicotinic acetylcholine receptor single-channel properties

Resistance to the anthelmintics pyrantel ((E)-1,4,5, 6-tetrahydro-1-methyl-2-[2-(2thienyl)ethenyl]pyrimidine) and levamisole ((S)-2,3,5,6-tetrahydro-6-phenylimidazo[2,1-b]thiazole) is an increasingly widespread problem in gastro-intestinal nematode infestations. Both compounds act on the nicotinic acetylcholine receptors on the surface of nematode somatic muscle. The patch-clamp technique was used to measure nematode nicotinic acetylcholine receptor properties at 75, 50, -50 and -75 mV in a pyrantel-resistant isolate of Oesophagostomum dentatum. Patch pipettes contained 30 microM levamisole as agonist. We found that 28. 1% of membrane patches contained active receptors. At -50 mV, the single-channel conductance was 36.2+/-1.4 pS, the mean open-time (tau) was 1.45+/-0.14 ms and the mean probability of opening (P(o)) was 0.004+/-0.002. We compared these results with previous work on an anthelmintic sensitive isolate and a levamisole-resistant isolate [Robertson, A.P., Bjorn, H.E., Martin, R.J., 1999. Levamisole resistance resolved at the single-channel level. FASEB J. 13, 749-760.]. We found that pyrantel-resistant parasites had a reduced percentage of active patches and a reduced P(o) value when compared to anthelmintic sensitive worms. We concluded that pyrantel resistance is associated with a modification of the target nicotinic receptor properties.

Altered association of protein tyrosine kinases with postsynaptic densities after transient cerebral ischemia in the rat brain

Transient cerebral ischemia results in an increase in the tyrosine phosphorylation of proteins associated with postsynaptic densities (PSDs). The authors investigated the possible mechanisms behind this increase by analyzing isolated PSDs for protein tyrosine kinase activity and for the presence of specific tyrosine kinases. Transient (15 minutes) global ischemia was produced in adult rats by four-vessel occlusion, and PSDs were isolated immediately after ischemia or after 20 minutes or 6 hours of reperfusion. Tyrosine phosphorylation of several PSD proteins, including the N-methyl-D-aspartate (NMDA) receptor subunits NR2A and NR2B, was enhanced relative to shams after 20 minutes of reperfusion and underwent a further increase between 20 minutes and 6 hours. The ability of intrinsic PSD tyrosine kinase to phosphorylate PSD proteins, including the NMDA receptor, increased threefold after ischemia. Whereas PSD-associated proline-rich tyrosine kinase 2 (PYK2) and gp145TrkB were elevated immediately after the ischemic event, increases in Src and Fyn were not apparent until 6 hours of reperfusion. The level of PSD-associated pp125FAK decreased after ischemia. The results demonstrate that ischemia results in selective changes in the association of protein tyrosine kinases with the PSD which may account for ischemia-induced increases in the tyrosine phosphorylation of PSD proteins.

Heat shock proteins Hsp27 and Hsp32 localize to synaptic sites in the rat cerebellum following hyperthermia

Stressful stimuli activate the heat shock (stress) response in which a set of heat shock proteins (hsps) is induced, which play roles in cellular repair and protective mechanisms. Most studies in the mammalian nervous system have focused on Hsp70, however, the present investigation targets other members of the induced set, namely Hsp27 and Hsp32. In response to hyperthermia, these hsps are strongly induced in Bergmann glial cells in the rat brain and transported into their radial fibers, which project into the 'synaptic-enriched' molecular layer of the cerebellum. Using subcellular fractionation and immunoelectron microscopy, hyperthermia-induced Hsp27 and Hsp32 were detected in synaptic elements and in perisynaptic glial processes. These results suggest that stress-induced Hsp27 and Hsp32 may contribute to repair and protective mechanisms at the synapse.

Localization of the heat-shock protein Hsp70 to the synapse following hyperthermic stress in the brain

Heat-shock proteins are induced in response to cellular stress. Although heat-shock proteins are known to function in repair and protective mechanisms, their relationship to critical neural processes, such as synaptic function, has received little attention. Here we investigate whether the major heat-shock protein Hsp70 localizes to the synapse following a physiologically relevant increase in temperature in the mammalian nervous system. Our results indicate that hyperthermia-induced Hsp70 is associated with pre- and postsynaptic elements, including the postsynaptic density. The positioning of Hsp70 at the synapse could facilitate the repair of stress-induced damage to synaptic proteins and also contribute to neuroprotective events at the synapse.

Regulation of signaling by protein-tyrosine phosphatases: potential roles in the nervous system

During neuronal development, cells respond to a variety of environmental cues through cell surface receptors that are coupled to a signaling transduction machinery based on protein tyrosine phosphorylation and dephosphorylation. Receptor and non-receptor tyrosine kinases have received a great deal of attention; however, in the last few years, receptor (plasma membrane associated) and non-receptor protein-tyrosine phosphatases (PTPs) have also been shown to play important roles in development of the nervous system. In many cases PTPs have provocative distribution patterns or have been shown to be associated with specific cell adhesion and growth factor receptors. Additionally, altering PTP expression levels or activity impairs neuronal behavior. In this review we outline what is currently known about the role of PTPs in development, differentiation and neuronal physiology.

Neurotoxic and synaptic effects of okadaic acid, an inhibitor of protein phosphatases

Protein phosphorylation and dephosphorylation reactions, catalyzed by kinases and phosphatases, are involved in the regulation of a wide variety of physiological processes. In the nervous system, such reactions seem to modulate the function of several proteins crucial in synaptic transmission, including voltage-gated and ligand-gated channels, neurotransmitter release, and neurotransmitter transporters. On the other hand, hyperphosphorylation of certain cytoskeletal proteins or receptors may lead to neuronal death. In the present work we review the neurotoxic effect of okadaic acid (OKA), a potent and specific inhibitor of the serine/threonine protein phosphatases 1 and 2A, as well as its action on synaptic function. We analyze recent findings demonstrating that the microinjection of OKA in rat hippocampus induces neuronal stress, hyperexcitation and neurodegeneration, and discuss their possible relationships to alterations of protein phosphorylation-dephosphorylation observed in Alzheimer's disease brain. These results suggest that protein hyperphosphorylation due to inhibition of phosphatases in vivo induces neuronal stress and subsequent neurodegeneration.

FAK+ and PYK2/CAKbeta, two related tyrosine kinases highly expressed in the central nervous system: similarities and differences in the expression pattern

Focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2/cell adhesion kinase beta (PYK2/CAKbeta) are related, non-receptor, cytoplasmic tyrosine kinases, highly expressed in the central nervous system (CNS). In addition, FAK+ is a splice isoform of FAK containing a 3-amino acid insertion in the carboxy-terminal region. In rat hippocampal slices, FAK+ and PYK2/CAKbeta are differentially regulated by neurotransmitters and depolarization. We have studied the regional and cellular distribution of these kinases in adult rat brain and during development. Whereas PYK2/CAKbeta expression increased with postnatal age and was maximal in the adult, FAK+ levels were stable. PYK2/CAKbeta mRNAs, detected by in situ hybridization, were expressed at low levels in the embryonic brain, and became very abundant in the adult forebrain. Immunocytochemistry of the adult brain showed a widespread neuronal distribution of FAK+ and PYK2/CAKbeta immunoreactivities (ir). PYK2/CAKbeta appeared to be particularly abundant in the hippocampus. In hippocampal neurons in culture at early stages of development, FAK+ and PYK2/CAKbeta were enriched in the perikarya and growth cones. FAK+ extended to the periphery of the growth cones tips, whereas PYK2/CAKbeta appeared to be excluded from the lamellipodia. During the establishment of polarity, a proximal-distal gradient of increasing PYK2/CAKbeta-ir could be observed in the growing axon. In most older neurons, FAK+-ir was confined to the cell bodies, whereas PYK2/CAKbeta-ir was also present in the processes. In vitro and in vivo, a subpopulation of neurons displayed neurites with intense FAK+-ir. Thus, FAK+ and PYK2/CAKbeta are differentially regulated during development yet they are both abundantly expressed in the adult brain, with distinctive but overlapping distributions.

The mouse Ptprr gene encodes two protein tyrosine phosphatases, PTP-SL and PTPBR7, that display distinct patterns of expression during neural development

The protein tyrosine phosphatases PTP-SL and PTPBR7 differ only in the length of their N-terminal domain. We show here that PTP-SL and PTPBR7 are isoforms derived from a single gene (Ptprr) through developmentally regulated use of alternative promoters. Isoform-specific reverse transcriptase-polymer chain reaction (RT-PCR) and RNA in situ hybridization experiments reveal that PTPBR7 is expressed during early embryogenesis in spinal ganglia cells as well as in developing Purkinje cells. Post-natally, PTPBR7 is expressed in various regions of the adult mouse brain, but expression in Purkinje cells has ceased and is replaced by the PTP-SL-specific transcript. In transient transfection experiments it is confirmed that PTPBR7 is a type I transmembrane protein tyrosine phosphatase (PTPase). PTP-SL, however, appears to be a cytosolic membrane-associated PTPase that is located at perinuclear vesicular structures that partly belong to the endosomal compartment. Thus, during maturation of Purkinje cells, a gene-promoter switch results in the replacement of a receptor-type PTPase by a cytosolic vesicle-associated isoform.

Shaping excitation at glutamatergic synapses

Glutamatergic synapses vary, exhibiting EPSCs of widely different magnitudes and timecourses. The main contributors to this variability are: presynaptic factors, including release probability, quantal content and vesicle composition; factors that modulate the concentration and longevity of glutamate in the cleft, including diffusion and the actions of glutamate transporters; and postsynaptic factors, including the types and locations of ionotropic glutamate receptors, their numbers, and the nature and locations of associated intracellular signalling systems.

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).

Effect of dopamine denervation and dopamine agonist administration on serine phosphorylation of striatal NMDA receptor subunits

Sensitization of striatal N-methyl-d-aspartate (NMDA) receptors has been implicated in the pathogenesis of the response alterations associated with dopaminomimetic treatment of parkinsonian animals and patients. To determine whether serine phosphorylation of NMDA receptor subunits by activation of Ca2+/calmodulin-dependent protein-kinase II (CaMKII) contributes to this process, we examined the effects of unilateral nigrostriatal ablation with 6-hydroxydopamine and subsequent treatment with levodopa, SKF 38393 (D1-preferring dopamine agonist), or quinpirole (D2-preferring agonist) on motor responses and phosphorylation states. Three weeks of twice-daily levodopa administration to rats shortened the duration of their rotational response to levodopa or SKF 38393 challenge, but prolonged the duration of quinpirole-induced rotation. At the same time, levodopa treatment elevated serine phosphorylation of striatal NR2A (p<0.02), but not that of NR2B subunits, without associated changes in subunit protein levels. Chronic treatment with SKF 38393 increased NR2A (p<0.0001) but decreased NR2B (p<0.004) serine phosphorylation. In contrast, chronic quinpirole treatment had no effect on NR2A but increased NR2B phosphorylation (p<0.0001). The acute intrastriatal injection of the CaMKII inhibitor KN93 (1.0 micrograms) not only normalized the levodopa-induced motor response alterations but also attenuated the D1 and D2 receptor-mediated serine phosphorylation of NR2A and NR2B subunits, respectively (p<0.02). These results suggest that a CaMKII-mediated rise in serine phosphorylation of NMDA receptor subunits induced by intermittent stimulation of D1 or D2 dopaminergic receptors contributes to the apparent enhancement in striatal NMDA receptor sensitivity and thus to the dopaminergic response plasticity in levodopa-treated parkinsonian rats.

Modulation of neurotransmitter release by dihydropyridine-sensitive calcium channels involves tyrosine phosphorylation

Cultured rat cerebellar granule cells depolarized by high KCl, display a large component of Ca2+ influx through L-type voltage-dependent Ca2+ channels as defined by a sensitivity to 1 microM nifedipine. This Ca2+ influx is not coupled to neurotransmitter exocytosis but has implications for neuronal development. KCl stimulation in the absence of external Ca2+ followed by the readdition of Ca2+ allows the coupling of a class of L-type Ca2+ channels to neurotransmitter exocytosis as assessed by loading of glutamatergic pools with [3H]-D-aspartate. KCl stimulation in the absence of external Ca2+ ('predepolarization') enhances tyrosine phosphorylation of several cellular proteins, and inhibitors of tyrosine kinases block both phosphorylation and the neurotransmitter release coupled to the L-type Ca2+ channel. More specifically, an inhibitor of src family tyrosine kinases, PP1, blocks the effects of predepolarization suggesting a role for a src family kinase in the process. Furthermore, L-type Ca2+ channel recruitment and modulation of release could be activated with the tyrosine phosphatase inhibitor sodium orthovanadate. The phosphoproteins enhanced by predepolarization, which include the cytoskeletal proteins focal adhesion kinase (FAK) and vinculin, are also highly phosphorylated early on in culture when neurite outgrowth occurs. As the neurons develop a network of neurites, both tyrosine phosphorylation and L-type Ca2+ channel activity decrease. These results show a novel mechanism for the recruitment of L-type Ca2+ channels and their coupling to neurotransmitter release which involves tyrosine phosphorylation. This phenomenon has a role in cerebellar granule cell development.

Enhanced tyrosine phosphorylation of striatal NMDA receptor subunits: effect of dopaminergic denervation and L-DOPA administration

Sensitization of striatal N-methyl-D-aspartate receptors (NMDAR) has been linked to events leading to the motor response changes associated with the administration of dopaminomimetics to parkinsonian animals and patients. To determine whether tyrosine phosphorylation of NMDAR subunits contributes to the apparent long-term enhancement in synaptic efficacy of these receptors, we examined the effect of unilateral nigrostriatal dopamine system ablation with 6-hydroxydopamine followed by twice-daily treatment with l-DOPA on the phosphorylation state of rat striatal NR2A and NR2B subunits. Three weeks of intermittent l-DOPA administration produced a shortening in the duration of the rotational response to dopaminergic challenge and other changes mimicking those occurring in patients with Parkinson's disease. Concurrently, tyrosine phosphorylation of NR2A and especially of NR2B subunits increased ipsilateral to the lesion (20+/-5% and 46+/-7% of intact striatum, respectively; p<0.01) without attendant changes in subunit protein levels. Selective blockade of NR2B subunits with ACEA 10-1244, but not of NR2A subunits with MDL 100,453, reversed the l-DOPA-induced response alterations. The intrastriatal injection of a tyrosine kinase inhibitor, genistein, at a dose (2.0 microg) that normalized the response shortening, attenuated the NR2A and NR2B phosphorylation increase by about 12% and 24%, respectively (p<0.01). Taken together, these results suggest that augmented tyrosine phosphorylation of NR2B subunits, alone or in combination with the smaller rise in NR2A subunit phosphorylation, contributes to the apparent enhancement in striatal NMDAR sensitivity and thus to the plastic alterations in dopaminergic responses in l-DOPA-treated parkinsonian rats.

Neurotrophin-induced activation of casein kinase 2 in rat hippocampal slices

Casein kinase 2 is present in the brain, including the hippocampus. It is associated with long-term potentiation and is known to be involved in phosphorylation of proteins potentially important for neuroplasticity, but regulation of its activity in neuronal cells is not yet known. In the present work, it was found that brain-derived neurotrophic factor and neurotrophin-4 control the activity of casein kinase 2 in hippocampal slices of adult rat. It is shown that: (i) treatment of slices for 4 h with the neurotrophins results in a five-fold increase in the activity of cytosolic casein kinase 2; (ii) this effect does not require protein synthesis. In addition, using calcium chelators, phospholipase inhibitors and protein kinase inhibitors, evidence is provided that: (i) neurotrophin-induced activation of casein kinase 2 is dependent on the availability of intracellular calcium due to stimulation of phospholipase C; (ii) both a tyrosine kinase(s) and a serine/threonine kinase(s) convey the signal of calcium. Since there is now accumulating evidence for involvement of brain-derived neurotrophic factor, intracellular calcium, tyrosine kinases and serine/threonine kinases in the regulation of synaptic plasticity, it is suggested that the signalling cascade detected here might contribute to control of synaptic strength in the hippocampus.