Reduced NMDA receptor tyrosine phosphorylation in PTPalpha-deficient mouse synaptosomes is accompanied by inhibition of four src family kinases and Pyk2: an upstream role for PTPalpha in NMDA receptor regulation

Homocysteine-induced sustained GluN2A NMDA receptor stimulation leads to mitochondrial ROS generation and neurotoxicity

Homocysteine, a sulfur-containing amino acid derived from methionine metabolism, is a known agonist of N-methyl-D-aspartate receptor (NMDAR) and is involved in neurotoxicity. Our previous findings showed that neuronal exposure to elevated homocysteine levels leads to sustained low-level increase in intracellular Ca2+, which is dependent on GluN2A subunit-containing NMDAR (GluN2A-NMDAR) stimulation. These studies further showed a role of ERK MAPK in homocysteine-GluN2A-NMDAR-mediated neuronal death. However, the intracellular mechanisms associated with such sustained GluN2A-NMDAR stimulation and subsequent Ca2+ influx have remained unexplored. Using live-cell imaging with Fluo3-AM and biochemical approaches, we show that homocysteine-GluN2A NMDAR-induced initial Ca2+ influx triggers sequential phosphorylation and subsequent activation of the proline rich tyrosine kinase 2 (Pyk2) and Src family kinases, which in turn phosphorylates GluN2A-Tyr1325 residue of GluN2A-NMDARs to maintain channel activity. The continuity of this cycle of events leads to sustained Ca2+ influx through GluN2A-NMDAR. Our findings also show that lack of activation of the regulatory tyrosine phosphatase STEP, which can limit Pyk2 and Src family kinase activity further contributes to the maintenance of this cycle. Additional studies using live-cell imaging of neurons expressing a redox-sensitive GFP targeted to the mitochondrial matrix show that treatment with homocysteine leads to a progressive increase in mitochondrial reactive oxygen species generation, which is dependent on GluN2A-NMDAR-mediated sustained ERK MAPK activation. This later finding demonstrates a novel role of GluN2A-NMDAR in homocysteine-induced mitochondrial ROS generation and highlights the role of ERK MAPK as the intermediary signaling pathway between GluN2A-NMDAR stimulation and mitochondrial reactive oxygen species generation.

Simulating neuronal development: exploring potential mechanisms for central nervous system metastasis in acute lymphoblastic leukemia

Background

Acute lymphoblastic leukemia (ALL) is prone to metastasize to the central nervous system (CNS), which is an important cause of poor treatment outcomes and unfavorable prognosis. However, the pathogenesis of CNS metastasis of ALL cells has not been fully illuminated. Recent reports have shed some light on the correlation between neural mechanisms and ALL CNS metastasis. These progressions prompt us to study the relationship between ALL central nervous system metastasis and neuronal development, exploring potential biomarkers and therapeutic targets of CNS metastasis.

Materials and methods

ALL central nervous system metastasis- and neuronal development-related differentially expressed genes (DEGs) were identified by analyzing gene expression datasets GSE60926 and GSE13715. Target prediction and network analysis methods were applied to assess protein-protein interaction networks. Gene Ontology (GO) terms and pathway enrichment for DEGs were assessed. Co-expressed differentially expressed genes (co-DEGs) coupled with corresponding predicted microRNAs (miRNAs) were studied as well. Reverse transcription-polymerase chain reaction (RT-PCR) and flow cytometry were employed for the validation of key co-DEGs in primary ALL cells. Furthermore, ALL cells were treated with a vascular endothelial growth factor (VEGF) inhibitor to block neuronal development and assess changes in the co-DEGs.

Results

We identified 216, 208, and 204 DEGs in ALL CNS metastasis specimens and neuronal development samples (GSE60926 and GSE13715). CD2, CD3G, CD3D, and LCK may be implicated in ALL CNS metastasis. LAMB1, MATN3, IGFBP3, LGALS1, and NEUROD1 may be associated with neuronal development. Specifically, four co-DEGs (LGALS1, TMEM71, SHISA2, and S100A11) may link ALL central nervous system metastasis and neuronal development process. The miRNAs for each co-DEG could be potential biomarkers or therapeutic targets for ALL central nervous system metastasis, especially hsa-miR-22-3p, hsa-miR-548t-5p, and hsa-miR-6134. Additionally, four co-DEGs (LGALS1, TMEM71, SHISA2, and S100A11) were validated in CNS-infiltrated ALL cells. The VEGF inhibitor demonstrated a suppressive effect on mRNA and protein expression of key co-DEGs.

Conclusion

The bioinformatic survey and key gene validation suggest a possible correlation between ALL CNS metastasis and the neuronal development process. Simulating the neuronal development process might be a possible strategy for CNS metastasis in ALL. LGALS1, TMEM71, SHISA2, and S100A11 genes are promising and novel biomarkers and targets in ALL CNS metastasis.

A Comprehensive Review of Receptor-Type Tyrosine-Protein Phosphatase Gamma (PTPRG) Role in Health and Non-Neoplastic Disease

Protein tyrosine phosphatase receptor gamma (PTPRG) is known to interact with and regulate several tyrosine kinases, exerting a tumor suppressor role in several type of cancers. Its wide expression in human tissues compared to the other component of group 5 of receptor phosphatases, PTPRZ expressed as a chondroitin sulfate proteoglycan in the central nervous system, has raised interest in its role as a possible regulatory switch of cell signaling processes. Indeed, a carbonic anhydrase-like domain (CAH) and a fibronectin type III domain are present in the N-terminal portion and were found to be associated with its role as [HCO3-] sensor in vascular and renal tissues and a possible interaction domain for cell adhesion, respectively. Studies on PTPRG ligands revealed the contactins family (CNTN) as possible interactors. Furthermore, the correlation of PTPRG phosphatase with inflammatory processes in different normal tissues, including cancer, and the increasing amount of its soluble form (sPTPRG) in plasma, suggest a possible role as inflammatory marker. PTPRG has important roles in human diseases; for example, neuropsychiatric and behavioral disorders and various types of cancer such as colon, ovary, lung, breast, central nervous system, and inflammatory disorders. In this review, we sum up our knowledge regarding the latest discoveries in order to appreciate PTPRG function in the various tissues and diseases, along with an interactome map of its relationship with a group of validated molecular interactors.

The Non-receptor Tyrosine Kinase Pyk2 in Brain Function and Neurological and Psychiatric Diseases

Pyk2 is a non-receptor tyrosine kinase highly enriched in forebrain neurons. Pyk2 is closely related to focal adhesion kinase (FAK), which plays an important role in sensing cell contacts with extracellular matrix and other extracellular signals controlling adhesion and survival. Pyk2 shares some of FAK’s characteristics including recruitment of Src-family kinases after autophosphorylation, scaffolding by interacting with multiple partners, and activation of downstream signaling pathways. Pyk2, however, has the unique property to respond to increases in intracellular free Ca²⁺, which triggers its autophosphorylation following stimulation of various receptors including glutamate NMDA receptors. Pyk2 is dephosphorylated by the striatal-enriched phosphatase (STEP) that is highly expressed in the same neuronal populations. Pyk2 localization in neurons is dynamic, and altered following stimulation, with post-synaptic and nuclear enrichment. As a signaling protein Pyk2 is involved in multiple pathways resulting in sometimes opposing functions depending on experimental models. Thus Pyk2 has a dual role on neurites and dendritic spines. With Src family kinases Pyk2 participates in postsynaptic regulations including of NMDA receptors and is necessary for specific types of synaptic plasticity and spatial memory tasks. The diverse functions of Pyk2 are also illustrated by its role in pathology. Pyk2 is activated following epileptic seizures or ischemia-reperfusion and may contribute to the consequences of these insults whereas Pyk2 deficit may contribute to the hippocampal phenotype of Huntington’s disease. Pyk2 gene, PTK2B, is associated with the risk for late-onset Alzheimer’s disease. Studies of underlying mechanisms indicate a complex contribution with involvement in amyloid toxicity and tauopathy, combined with possible functional deficits in neurons and contribution in microglia. A role of Pyk2 has also been proposed in stress-induced depression and cocaine addiction. Pyk2 is also important for the mobility of astrocytes and glioblastoma cells. The implication of Pyk2 in various pathological conditions supports its potential interest for therapeutic interventions. This is possible through molecules inhibiting its activity or increasing it through inhibition of STEP or other means, depending on a precise evaluation of the balance between positive and negative consequences of Pyk2 actions.

The Implication of STEP in Synaptic Plasticity and Cognitive Impairments in Alzheimer's Disease and Other Neurological Disorders

STriatal-Enriched protein tyrosine Phosphatase (STEP) is a tyrosine phosphatase that has been implicated in Alzheimer’s disease (AD), the most common form of dementia, and many other neurological diseases. The protein level and activity of STEP have been found to be elevated in most of these disorders, and specifically in AD as a result of dysregulation of different pathways including PP2B/DARPP32/PP1, PKA as well as impairments of both proteasomal and lysosomal systems. The upregulation in STEP leads to increased binding to, and dephosphorylation of, its substrates which are mainly found to be synaptic plasticity and thus learning and memory related proteins. These proteins include kinases like Fyn, Pyk2, ERK1/2 and both NMDA and AMPA receptor subunits GluN2B and GluA2. The dephosphorylation of these molecules results in inactivation of these kinases and internalization of NMDA and AMPA receptor complexes leading to synapse loss and cognitive impairments. In this study, we aim to review STEP regulation and its implications in AD as well as other neurological disorders and then summarize data on targeting STEP as therapeutic strategy in these diseases.

Rare variants in Protein tyrosine phosphatase, receptor type A (PTPRA) in schizophrenia: Evidence from a family based study

The contribution of both common and rare risk variants to the genetic architecture of schizophrenia (SZ) has been documented in genome-wide association studies, whole exome and whole genome sequencing approaches. As SZ is highly heritable and segregates in families, highly penetrant rare variants are more likely to be identified through analyses of multiply affected families. Further, much of the gene mapping studies in SZ have utilized individuals of Caucasian ancestry. Analysis of other ethnic groups may be informative. In this study, we aimed at identification of rare, penetrant risk variants utilizing whole exome sequencing (WES) in a three-generation Indian family with multiple members affected. Filtered data from WES, combined with in silico analyses revealed a novel heterozygous missense variant (NM_080841:c.1730C>G:p.T577R; exon18) in Protein tyrosine phosphatase, receptor type A (PTPRA 20p13). The variant was located in an evolutionarily conserved position and predicted to be damaging. Screening for variants in this gene in the WES data of an independent SZ cohort (n = 350) of matched ethnicity, identified five additional rare missense variants with MAF < 0.003, which were also predicted to be damaging. In conclusion, the rare missense variants in PTPRA identified in this study could confer risk for SZ. This has also derived support from concordant data from prior linkage and association, as well as animal studies which indicated a role for PTPRA in glutamate function.

Loss-of-function of PTPR γ and ζ, observed in sporadic schizophrenia, causes brain region-specific deregulation of monoamine levels and altered behavior in mice

RATIONALE

The receptor protein tyrosine phosphatase PTPRG has been genetically associated with psychiatric disorders and is a ligand for members of the contactin family, which are themselves linked to autism spectrum disorders.

OBJECTIVE

Based on our finding of a phosphatase-null de novo mutation in PTPRG associated with a case of sporadic schizophrenia, we used PTPRG knockout (KO) mice to model the effect of a loss-of-function mutation. We compared the results with loss-of-function in its close paralogue PTPRZ, previously associated with schizophrenia. We tested PTPRG ^-/- , PTPRZ ^-/- , and wild-type male mice for effects on social behavior, forced swim test, and anxiety, as well as on regional brain neurochemistry.

RESULTS

The most notable behavioral consequences of PTPRG gene inactivation were reduced immobilization in the forced swim test, suggestive of some negative symptoms of schizophrenia. By contrast, PTPRZ ^-/- mice demonstrated marked social alteration with increased aggressivity, reminiscent of some positive symptoms of schizophrenia. Both knockouts showed elevated dopamine levels in prefrontal cortex, hippocampus, and most particularly amygdala, but not striatum, accompanied by reduced dopamine beta hydroxylase activity only in amygdala. In addition, PTPRG KO elicited a distinct increase in hippocampal serotonin level not observed in PTPRZ KO.

CONCLUSION

PTPRG and PTPRZ gene loss therefore induces distinct patterns of behavioral change and region-specific alterations in neurotransmitters, highlighting their usefulness as models for neuropsychiatric disorder mechanisms and making these receptors attractive targets for therapy.

Structural Basis for the Interaction between Pyk2-FAT Domain and Leupaxin LD Repeats

Proline-rich tyrosine kinase 2 (Pyk2) is a nonreceptor tyrosine kinase and belongs to the focal adhesion kinase (FAK) family. Like FAK, the C-terminal focal adhesion-targeting (FAT) domain of Pyk2 binds to paxillin, a scaffold protein in focal adhesions; however, the interaction between the FAT domain of Pyk2 and paxillin is dynamic and unstable. Leupaxin is another member in the paxillin family and was suggested to be the native binding partner of Pyk2; Pyk2 gene expression is strongly correlated with that of leupaxin in many tissues including primary breast cancer. Here, we report that leupaxin interacts with Pyk2-FAT. Leupaxin has four leucine–aspartate (LD) motifs. The first and third LD motifs of leupaxin preferably target the two LD-binding sites on the Pyk2-FAT domain, respectively. Moreover, the full-length leupaxin binds to Pyk2-FAT as a stable one-to-one complex. Together, we propose that there is an underlying selectivity between leupaxin and paxillin for Pyk2, which may influence the differing behavior of the two proteins at focal adhesion sites.

RPTPs in axons, synapses and neurology

Receptor-like protein tyrosine phosphatases represent a large protein family related to cell adhesion molecules, with diverse roles throughout neural development in vertebrates and invertebrates. This review focuses on their roles in axon growth, guidance and repair, as well as more recent findings demonstrating their key roles in pre-synaptic and post-synaptic maturation and function. These enzymes have been linked to memory and neuropsychiatric defects in loss-of-function rodent models, highlighting their potential as future drug targets.

Structural and mechanistic insights into the interaction between Pyk2 and paxillin LD motifs

Proline-rich tyrosine kinase 2 (Pyk2) is a member of the focal adhesion kinase (FAK) subfamily of cytoplasmic tyrosine kinases. The C-terminal Pyk2-focal adhesion targeting (FAT) domain binds to paxillin, an adhesion molecule. Paxillin has five leucine-aspartate (LD) motifs (LD1-LD5). Here, we show that the second LD motif of paxillin, LD2, interacts with Pyk2-FAT, similar to the known Pyk2-FAT/LD4 interaction. Both LD motifs can target two ligand binding sites on Pyk2-FAT. Interestingly, they also share similar binding affinity for Pyk2-FAT with preferential association to one site relative to the other. Nevertheless, the LD2-LD4 region of paxillin (paxillin(133-290)) binds to Pyk2-FAT as a 1:1 complex. However, our data suggest that the Pyk2-FAT and paxillin complex is dynamic and it appears to be a mixture of two distinct conformations of paxillin that almost equally compete for Pyk2-FAT binding. These studies provide insight into the underlying selectivity of paxillin for Pyk2 and FAK that may influence the differing behavior of these two closely related kinases in focal adhesion sites.

Late-Onset Alzheimer's Disease Genes and the Potentially Implicated Pathways

Late-onset Alzheimer's disease (LOAD) is a devastating neurodegenerative disease with no effective treatment or cure. In addition to APOE, recent large genome-wide association studies have identified variation in over 20 loci that contribute to disease risk: CR1, BIN1, INPP5D, MEF2C, TREM2, CD2AP, HLA-DRB1/HLA-DRB5, EPHA1, NME8, ZCWPW1, CLU, PTK2B, PICALM, SORL1, CELF1, MS4A4/MS4A6E, SLC24A4/RIN3,FERMT2, CD33, ABCA7, CASS4. In addition, rare variants associated with LOAD have also been identified in APP, TREM2 and PLD3 genes. Previous research has identified inflammatory response, lipid metabolism and homeostasis, and endocytosis as the likely modes through which these gene products participate in Alzheimer's disease. Despite the clustering of these genes across a few common pathways, many of their roles in disease pathogenesis have yet to be determined. In this review, we examine both general and postulated disease functions of these genes and consider a comprehensive view of their potential roles in LOAD risk.

Protein tyrosine phosphatase α in the dorsomedial striatum promotes excessive ethanol-drinking behaviors

We previously found that excessive ethanol drinking activates Fyn in the dorsomedial striatum (DMS) (Wang et al., 2010; Gibb et al., 2011). Ethanol-mediated Fyn activation in the DMS leads to the phosphorylation of the GluN2B subunit of the NMDA receptor, to the enhancement of the channel's activity, and to the development and/or maintenance of ethanol drinking behaviors (Wang et al., 2007, 2010). Protein tyrosine phosphatase α (PTPα) is essential for Fyn kinase activation (Bhandari et al., 1998), and we showed that ethanol-mediated Fyn activation is facilitated by the recruitment of PTPα to synaptic membranes, the compartment where Fyn resides (Gibb et al., 2011). Here we tested the hypothesis that PTPα in the DMS is part of the Fyn/GluN2B pathway and is thus a major contributor to the neuroadaptations underlying excessive ethanol intake behaviors. We found that RNA interference (RNAi)-mediated PTPα knockdown in the DMS reduces excessive ethanol intake and preference in rodents. Importantly, no alterations in water, saccharine/sucrose, or quinine intake were observed. Furthermore, downregulation of PTPα in the DMS of mice significantly reduces ethanol-mediated Fyn activation, GluN2B phosphorylation, and ethanol withdrawal-induced long-term facilitation of NMDAR activity without altering the intrinsic features of DMS neurons. Together, these results position PTPα upstream of Fyn within the DMS and demonstrate the important contribution of the phosphatase to the maladaptive synaptic changes that lead to excessive ethanol intake.

Therapeutic implications for striatal-enriched protein tyrosine phosphatase (STEP) in neuropsychiatric disorders

Striatal-enriched protein tyrosine phosphatase (STEP) is a brain-specific phosphatase that modulates key signaling molecules involved in synaptic plasticity and neuronal function. Targets include extracellular-regulated kinase 1 and 2 (ERK1/2), stress-activated protein kinase p38 (p38), the Src family tyrosine kinase Fyn, N-methyl-D-aspartate receptors (NMDARs), and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). STEP-mediated dephosphorylation of ERK1/2, p38, and Fyn leads to inactivation of these enzymes, whereas STEP-mediated dephosphorylation of surface NMDARs and AMPARs promotes their endocytosis. Accordingly, the current model of STEP function posits that it opposes long-term potentiation and promotes long-term depression. Phosphorylation, cleavage, dimerization, ubiquitination, and local translation all converge to maintain an appropriate balance of STEP in the central nervous system. Accumulating evidence over the past decade indicates that STEP dysregulation contributes to the pathophysiology of several neuropsychiatric disorders, including Alzheimer's disease, schizophrenia, fragile X syndrome, epileptogenesis, alcohol-induced memory loss, Huntington's disease, drug abuse, stroke/ischemia, and inflammatory pain. This comprehensive review discusses STEP expression and regulation and highlights how disrupted STEP function contributes to the pathophysiology of diverse neuropsychiatric disorders.

Src family kinases: modulators of neurotransmitter receptor function and behavior

Src family kinases (SFKs) are non-receptor-type protein tyrosine kinases that were originally identified as the products of proto-oncogenes and were subsequently implicated in the regulation of cell proliferation and differentiation in the developing mammalian brain. Recent studies using transgenic mouse models have demonstrated that SFKs that are highly expressed in the adult brain regulate neuronal plasticity and behavior through tyrosine phosphorylation of key substrates such as neurotransmitter receptors. Here, we provide an overview of these recent studies, as well as discussing how modulation of the endocytosis of neurotransmitter receptors by SFKs contributes, in part, to this regulation. Deregulation of SFK-dependent tyrosine phosphorylation of such substrates might underlie certain brain disorders.

Ethanol-induced increase in Fyn kinase activity in the dorsomedial striatum is associated with subcellular redistribution of protein tyrosine phosphatase α

In vivo exposure of rodents to ethanol leads to a long-lasting increase in Fyn kinase activity in the dorsomedial striatum (DMS). In this study, we set out to identify a molecular mechanism that contributes to the enhancement of Fyn activity in response to ethanol in the DMS. Protein tyrosine phosphatase α (PTPα) positively regulates the activity of Fyn, and we found that repeated systemic administration or binge drinking of ethanol results in an increase in the synaptic localization of PTPα in the DMS, the same site where Fyn resides. We also demonstrate that binge drinking of ethanol leads to an increase in Fyn activity and to the co-localization of Fyn and PTPα in lipid rafts in the DMS. Finally, we show that the level of tyrosine phosphorylated (and thus active) PTPα in the synaptic fractions is increased in response to contingent or non-contingent exposure of rats to ethanol. Together, our results suggest that the redistribution of PTPα in the DMS into compartments where Fyn resides is a potential mechanism by which the activity of the kinase is increased upon ethanol exposure. Such neuroadaptations could be part of a mechanism that leads to the development of excessive ethanol consumption.

Loss of function studies in mice and genetic association link receptor protein tyrosine phosphatase α to schizophrenia

BACKGROUND

Solid evidence links schizophrenia (SZ) susceptibility to neurodevelopmental processes involving tyrosine phosphorylation-mediated signaling. Mouse studies implicate the Ptpra gene, encoding protein tyrosine phosphatase RPTPα, in the control of radial neuronal migration, cortical cytoarchitecture, and oligodendrocyte differentiation. The human gene encoding RPTPα, PTPRA, maps to a chromosomal region (20p13) associated with susceptibility to psychotic illness.

METHODS

We characterized neurobehavioral parameters, as well as gene expression in the central nervous system, of mice with a null mutation in the Ptpra gene. We searched for genetic association between polymorphisms in PTPRA and schizophrenia risk (two independent cohorts, 1420 cases and 1377 controls), and we monitored PTPRA expression in prefrontal dorsolateral cortex of SZ patients (35 cases, 2 control groups of 35 cases).

RESULTS

We found that Ptpra⁻/⁻ mice reproduce neurobehavioral endophenotypes of human SZ: sensitization to methamphetamine-induced hyperactivity, defective sensorimotor gating, and defective habituation to a startle response. Ptpra loss of function also leads to reduced expression of multiple myelination genes, mimicking the hypomyelination-associated changes in gene expression observed in postmortem patient brains. We further report that a polymorphism at the PTPRA locus is genetically associated with SZ, and that PTPRA mRNA levels are reduced in postmortem dorsolateral prefrontal cortex of subjects with SZ.

CONCLUSIONS

The implication of this well-studied signaling protein in SZ risk and endophenotype manifestation provides novel entry points into the etiopathology of this disease.

Substrate specificity of protein tyrosine phosphatases 1B, RPTPα, SHP-1, and SHP-2

We determined the substrate specificities of the protein tyrosine phosphatases (PTPs) PTP1B, RPTPα, SHP-1, and SHP-2 by on-bead screening of combinatorial peptide libraries and solution-phase kinetic analysis of individually synthesized phosphotyrosyl (pY) peptides. These PTPs exhibit different levels of sequence specificity and catalytic efficiency. The catalytic domain of RPTPα has very weak sequence specificity and is approximately 2 orders of magnitude less active than the other three PTPs. The PTP1B catalytic domain has modest preference for acidic residues on both sides of pY, is highly active toward multiply phosphorylated peptides, but disfavors basic residues at any position, a Gly at the pY-1 position, or a Pro at the pY+1 position. By contrast, SHP-1 and SHP-2 share similar but much narrower substrate specificities, with a strong preference for acidic and aromatic hydrophobic amino acids on both sides of the pY residue. An efficient SHP-1/2 substrate generally contains two or more acidic residues on the N-terminal side and one or more acidic residues on the C-terminal side of pY but no basic residues. Subtle differences exist between SHP-1 and SHP-2 in that SHP-1 has a stronger preference for acidic residues at the pY-1 and pY+1 positions and the two SHPs prefer acidic residues at different positions N-terminal to pY. A survey of the known protein substrates of PTP1B, SHP-1, and SHP-2 shows an excellent agreement between the in vivo dephosphorylation pattern and the in vitro specificity profiles derived from library screening. These results suggest that different PTPs have distinct sequence specificity profiles and the intrinsic activity/specificity of the PTP domain is an important determinant of the enzyme's in vivo substrate specificity.

PTPalpha activates Lyn and Fyn and suppresses Hck to negatively regulate FcepsilonRI-dependent mast cell activation and allergic responses

Mast cell activation via FcεRI involves activation of the Src family kinases (SFKs) Lyn, Fyn, and Hck that positively or, in the case of Lyn, negatively regulate cellular responses. Little is known of upstream activators of these SFKs in FcεRI-dependent signaling. We investigated the role of receptor protein tyrosine phosphatase (PTP)α, a well-known activator of SFKs in diverse signaling systems, FcεRI-mediated mast cell activation, and IgE-dependent allergic responses in mice. PTPα(-/-) bone marrow-derived mast cells hyperdegranulate and exhibit increased cytokine and cysteinyl leukotriene secretion, and PTPα(-/-) mice display enhanced IgE-dependent anaphylaxis. At or proximal to FcεRI, PTPα(-/-) cells have reduced IgE-dependent activation of Lyn and Fyn, as well as reduced FcεRI and SHIP phosphorylation. In contrast, Hck and Syk activation is enhanced. Syk hyperactivation correlated with its increased phosphorylation at positive regulatory sites and defective phosphorylation at a negative regulatory site. Distal to FcεRI, we observed increased activation of PI3K and MAPK pathways. These findings demonstrate that PTPα activates the FcεRI-coupled kinases Lyn and Fyn and suppresses Hck activity. Furthermore, the findings indicate that hyperactivation of PTPα(-/-) mast cells and enhanced IgE-dependent allergic responses of PTPα(-/-) mice are due to the ablated function of PTPα as a critical regulator of Lyn negative signaling.

Mechanisms of Group I mGluR-Dependent Long-Term Depression of NMDA Receptor–Mediated Transmission at Schaffer Collateral–CA1 Synapses

The mechanisms underlying group I metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) of N-methyl-d-aspartate receptor (NMDAR)-mediated synaptic currents (EPSCsNMDAR) are poorly understood. Here we investigated the effects of ( R,S)-3,5-dihydroxyphenylglycine (DHPG), a selective agonist of group I mGluRs, on the EPSCsNMDAR in area CA1 of acute hippocampal slices from 6- to 8-wk Sprague-Dawley rats. DHPG acutely and persistently depressed the isolated EPSCNMDAR and transiently slowed its decay rate. Combined antagonism of mGluR1 and mGluR5 blocked the effects of DHPG. Strong calcium buffering with intracellular BAPTA did not reduce the acute depression or LTD, making the involvement of elevated postsynaptic calcium unlikely. The acute depression and LTD were not mediated by activation of tyrosine kinases or phosphatases, nor were they dependent on protein synthesis. However, the LTD was prevented by the intracellular actin-stabilizer jasplakinolide, raising the possibility that it was associated with a lateral movement of NMDARs. Supporting this hypothesis, when the effective spatial spread of synaptically released glutamate was increased using the glutamate transporter inhibitor TBOA, the resultant EPSCNMDAR did not undergo LTD in response to DHPG. Importantly, isolation of the extrasynaptic EPSCNMDAR by blockade of synaptic NMDARs with MK-801 showed that this was not due to a potentiation of the preexisting extrasynaptic component. These findings indicate that LTD of NMDAR-mediated synaptic transmission occurs via lateral movement of receptors away from the synapse.

Activation of c-Src and Fyn kinases by protein-tyrosine phosphatase RPTPalpha is substrate-specific and compatible with lipid raft localization

Src family tyrosine kinases (SFKs) function in multiple signaling pathways, raising the question of how appropriate regulation and substrate choice are achieved. SFK activity is modulated by several protein-tyrosine phosphatases, among which RPTPalpha and SHP2 are the best established. We studied how RPTPalpha affects substrate specificity and regulation of c-Src and Fyn in response to epidermal growth factor and platelet-derived growth factor. We find that RPTPalpha, in a growth factor-specific manner, directs the specificity of these kinases toward a specific subset of SFK substrates, particularly the focal adhesion protein Paxillin and the lipid raft scaffolding protein Cbp/PAG. A significant fraction of RPTPalpha is present in lipid rafts, where its targets Fyn and Cbp/PAG reside, and growth factor-mediated SFK activation within this compartment is strictly dependent on RPTPalpha. Forced concentration of RPTPalpha into lipid rafts is compatible with activation of Fyn. Finally, RPTPalpha-induced phosphorylation of Paxillin and Cbp/PAG induces recruitment of the SFK inhibitory kinase Csk, indicative of negative feedback loops limiting SFK activation by RPTPalpha. Our findings indicate that individual SFK-controlling PTPs play important and specific roles in dictating SFK substrate specificity and regulatory mechanism.

Protein-tyrosine phosphatase alpha regulates stem cell factor-dependent c-Kit activation and migration of mast cells

The role of protein-tyrosine phosphatase alpha (PTPalpha) in mast cell function was investigated in tissues and cells from PTPalpha-deficient mice. Bone marrow-derived mast cells (BMMCs) lacking PTPalpha exhibit defective stem cell factor (SCF)-dependent polarization and migration. Investigation of the molecular basis for this reveals that SCF/c-Kit-stimulated activation of the Fyn tyrosine kinase is impaired in PTPalpha(-/-) BMMCs, with a consequent inhibition of site-specific c-Kit phosphorylation at tyrosines 567/569 and 719. Although c-Kit-mediated activation of phosphatidylinositol 3-kinase and Akt is unaffected, profound defects occur in the activation of downstream signaling proteins, including mitogen-activated protein kinases and Rho GTPases. Phosphorylation and interaction of Fyn effectors Gab2 and Shp2, which are linked to Rac/JNK activation in mast cells, are impaired in PTPalpha(-/-) BMMCs. Thus, PTPalpha is required for SCF-induced c-Kit and Fyn activation, and in this way regulates a Fyn-based c-Kit signaling axis (Fyn/Gab2/Shp2/Vav/PAK/Rac/JNK) that mediates mast cell migration. These defective signaling events may underlie the altered tissue-resident mast cell populations found in PTPalpha(-/-) mice.

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

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

Neural recognition molecules CHL1 and NB-3 regulate apical dendrite orientation in the neocortex via PTP alpha

Apical dendrites of pyramidal neurons in the neocortex have a stereotypic orientation that is important for neuronal function. Neural recognition molecule Close Homolog of L1 (CHL1) has been shown to regulate oriented growth of apical dendrites in the mouse caudal cortex. Here we show that CHL1 directly associates with NB-3, a member of the F3/contactin family of neural recognition molecules, and enhances its cell surface expression. Similar to CHL1, NB-3 exhibits high-caudal to low-rostral expression in the deep layer neurons of the neocortex. NB-3-deficient mice show abnormal apical dendrite projections of deep layer pyramidal neurons in the visual cortex. Both CHL1 and NB-3 interact with protein tyrosine phosphatase alpha (PTPalpha) and regulate its activity. Moreover, deep layer pyramidal neurons of PTPalpha-deficient mice develop misoriented, even inverted, apical dendrites. We propose a signaling complex in which PTPalpha mediates CHL1 and NB-3-regulated apical dendrite projection in the developing caudal cortex.

Protective up-regulation of CK2 by mutant huntingtin in cells co-expressing NMDA receptors

Huntington's disease is caused by a polyglutamine expansion in the huntingtin (htt) protein, and previous data indicate that over-activation of NMDA receptors (NMDARs) may be involved in the selective degeneration of cells expressing NR1/NR2B NMDARs. We used Kinetworkstrade mark multi-immunoblotting screens to examine expression of 76 protein kinases, 18 protein phosphatases, 25 heat shock/stress proteins, and 27 apoptosis proteins in human embryonic kidney 293 cells transfected with NR1/NR2B and htt containing 15 (htt-15Q; wild-type) or 138 (htt-138Q; mutant) glutamine repeats. Follow-up experiments revealed several proteins involved in the heat-shock response pathway to be up-regulated in the soluble fraction from cells expressing htt-138Q, including protein phosphatase 5 and cyclin-dependent kinase 5. Increased expression in the soluble fraction of htt-138Q-expressing cells was also noted for the stress- and calcium-activated protein-serine/threonine kinase casein kinase 2, a change which was confirmed in striatal tissue of yeast artificial chromosome transgenic mice expressing full-length mutant htt. Inhibition of casein kinase 2 activity in cultured striatal neurons from these mice significantly exacerbated NMDAR-mediated toxicity, as assessed by labeling of apoptotic nuclei. Our findings are consistent with up-regulation of components of the stress response pathway in the presence of polyglutamine-expanded htt and NR1/NR2B which may reflect an attempt at the cellular level to ameliorate the detrimental effects of mutant htt expression.

RPTPalpha is required for rigidity-dependent inhibition of extension and differentiation of hippocampal neurons

Receptor-like protein tyrosine phosphatase alpha (RPTPalpha)-knockout mice have severe hippocampal abnormalities similar to knockouts of the Src family kinase Fyn. These enzymes are linked to the matrix-rigidity response in fibroblasts, but their function in neurons is unknown. The matrix-rigidity response of fibroblasts appears to differ from that of neuronal growth cones but it is unknown whether the rigidity detection mechanism or response pathway is altered. Here, we report that RPTPalpha is required for rigidity-dependent reinforcement of fibronectin (FN)-cytoskeleton bonds and the rigidity response in hippocampal neuron growth cones, like in fibroblasts. In control neurons, rigid FN surfaces inhibit neurite extension and neuron differentiation relative to soft surfaces. In RPTPalpha(-/-) neurons, no inhibition of extension and differentiation is found on both rigid and soft surfaces. The RPTPalpha-dependent rigidity response in neurons is FN-specific, and requires clustering of alpha(v)beta(6) integrin at the leading edge of the growth cones. Further, RPTPalpha is necessary for the rigidity-dependent concentration of Fyn and p130Cas phosphorylation at the leading edge of the growth cone, like it is in fibroblasts. Although neurons respond to rigid FN surfaces in the opposite way to fibroblasts, we suggest that the mechanism of detecting FN rigidity is similar and involves rigidity-dependent RPTPalpha recruitment of Fyn.