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The refinement of immature neuronal networks into efficient mature ones is critical to nervous system development and function. This process of synapse refinement is driven by the neuronal activity–dependent competition of converging synaptic inputs, resulting in the elimination of weak inputs and the stabilization of strong ones. Neuronal activity, whether in the form of spontaneous activity or experience-evoked activity, is known to drive synapse refinement in numerous brain regions. More recent studies are now revealing the manner and mechanisms by which neuronal activity is detected and converted into molecular signals that appropriately regulate the elimination of weaker synapses and stabilization of stronger ones. Here, we highlight how spontaneous activity and evoked activity instruct neuronal activity–dependent competition during synapse refinement. We then focus on how neuronal activity is transformed into the molecular cues that determine and execute synapse refinement. A comprehensive understanding of the mechanisms underlying synapse refinement can lead to novel therapeutic strategies in neuropsychiatric diseases characterized by aberrant synaptic function.

Growth factor signaling in the brain is implicated in many neuropsychiatric disorders, including depression, autism, and epilepsy. Fibroblast growth factor 22 is a growth factor that regulates excitatory synapse development and neurogenesis in the brain. We have previously shown that adult mice in which fibroblast growth factor 22 is constitutively inactivated in all cells throughout life (fibroblast growth factor 22-null mice) show anhedonia, a core feature of depression in humans, suggesting that fibroblast growth factor 22 signaling contributes to the regulation of affective behavior. Here we asked (1) whether inactivation of fibroblast growth factor 22 specifically in neurons is sufficient to induce anhedonia in mice and (2) whether fibroblast growth factor 22 signaling is important during development or in adults for the regulation of affective behavior. To address these questions, we performed the sucrose preference test, which is used as an indicator of anhedonia, with neuron-specific conditional fibroblast growth factor 22 knockout mice, in which fibroblast growth factor 22 is inactivated in neurons at birth (neonatal-fibroblast growth factor 22-knockout mice) or in adults (adult-fibroblast growth factor 22-knockout mice). We found that neonatal-fibroblast growth factor 22-knockout mice show anhedonia (decreased preference for sucrose), while adult-fibroblast growth factor 22-knockout mice do not. Therefore, neuronal fibroblast growth factor 22 signaling is critical during development, and not in adults, for the regulation of affective behavior. Our work also implies that defects in growth factor-dependent synapse development, neurogenesis, or both may underlie depression of a developmental origin.

More is not always better Mutations in X-linked genes usually affect male individuals more than females, but the opposite characterizes the Protocadherin-19 gene ( PCDH19 ) on the X chromosome. Mutations in the PCDH19 cell-adhesion molecule cause cognitive impairment, affecting females more than males. Hoshina et al. studied mice with PCDH19 mutations, showing that a mismatch between PCDH19 and another cell-adhesion molecule causes trouble when mossy fibers of the brain are forming synapses (see the Perspective by Shohayeb and Cooper). In the heterozygous setting, mutant PCDG19 sequesters the partner cell-adhesion molecule into dysfunctional complexes. In the hemizygous setting, as in males, enough of that partner cell-adhesion molecule roams free to make functional interactions. Science , this issue p. eaaz3893 ; see also p. 235

Developmental synaptic remodeling is important for the formation of precise neural circuitry and its disruption has been linked to neurodevelopmental disorders such as autism and schizophrenia. Microglia prune synapses, but integration of this synapse pruning with overlapping and concurrent neurodevelopmental processes remains elusive. Adhesion G protein-coupled receptor ADGRG1/GPR56 controls multiple aspects of brain development in a cell type-specific manner: in neural progenitor cells, GPR56 regulates cortical lamination, whereas in oligodendrocyte progenitor cells, GPR56 controls developmental myelination and myelin repair. Here, we show that microglial GPR56 maintains appropriate synaptic numbers in several brain regions in a time- and circuit-dependent fashion. Phosphatidylserine (PS) on pre-synaptic elements binds GPR56 in a domain-specific manner, and microglia-specific deletion of Gpr56 leads to increased synapses as a result of reduced microglial engulfment of PS+ pre-synaptic inputs. Remarkably, a particular alternatively spliced isoform of GPR56 is selectively required for microglia-mediated synaptic pruning. Our present data provide a ligand- and isoform-specific mechanism underlying microglial GPR56-mediated synapse pruning in the context of complex neurodevelopmental processes.

Synapse maturation is a neural activity–dependent process during brain development, in which active synapses preferentially undergo maturation to establish efficient neural circuits in the brain. Defects in this process are implicated in various neuropsychiatric disorders. We have previously reported that a postsynaptic transmembrane protein, signal regulatory protein-α (SIRPα), plays an important role in activity-dependently directing synapse maturation. In the presence of synaptic activity, the ectodomain of SIRPα is cleaved and released and then acts as a retrograde signal to induce presynaptic maturation. However, how SIRPα detects synaptic activity to promote its ectodomain cleavage and synapse maturation is unknown. Here, we show that activity-dependent tyrosine phosphorylation of SIRPα is critical for SIRPα cleavage and synapse maturation. We found that during synapse maturation and in response to neural activity, SIRPα is highly phosphorylated on its tyrosine residues in the hippocampus, a structure critical for learning and memory. Tyrosine phosphorylation of SIRPα was necessary for SIRPα cleavage and presynaptic maturation, as indicated by the fact that a phosphorylation-deficient SIRPα variant underwent much less cleavage and could not drive presynaptic maturation. However, SIRPα phosphorylation did not affect its synaptic localization. Finally, we show that inhibitors of the Src and JAK kinase family suppress neural activity–dependent SIRPα phosphorylation and cleavage. Together, our results indicate that SIRPα phosphorylation serves as a mechanism for detecting synaptic activity and linking it to the ectodomain cleavage of SIRPα, which in turn drives synapse maturation in an activity-dependent manner.

To establish functional neural circuits in the brain, synaptic connections are refined by neural activity during development, where active connections are maintained while inactive ones are eliminated. However, the molecular signals that regulate synapse refinement remain to be elucidated. When we inactivate a subset of neurons in the mouse cingulate cortex, their callosal connections are eliminated through activity-dependent competition. Using this system, we identify JAK2 tyrosine kinase as a key regulator of inactive synapse elimination. We show: JAK2 is necessary and sufficient for the elimination of inactive connections; JAK2 is activated at inactive synapses in response to signals from other active synapses; STAT1, a substrate of JAK2, mediates inactive synapse elimination; JAK2 signaling is critical for physiological refinement of synapses during normal development; and JAK2 regulates synapse refinement in multiple brain regions. We propose that JAK2 is an activity-dependent switch that serves as a determinant of inactive synapse elimination.

Neurons in the brain must establish a balanced network of excitatory and inhibitory synapses during development for the brain to function properly. An imbalance between these synapses underlies various neurological and psychiatric disorders. The formation of excitatory and inhibitory synapses requires precise molecular control. In the hippocampus, the structure crucial for learning and memory, fibroblast growth factor 22 (FGF22) and FGF7 specifically promote excitatory or inhibitory synapse formation, respectively. Knockout of either Fgf gene leads to excitatory-inhibitory imbalance in the mouse hippocampus and manifests in an altered susceptibility to epileptic seizures, underscoring the importance of FGF-dependent synapse formation. However, the receptors and signaling mechanisms by which FGF22 and FGF7 induce excitatory and inhibitory synapse differentiation are unknown. Here, we show that distinct sets of overlapping FGF receptors (FGFRs), FGFR2b and FGFR1b, mediate excitatory or inhibitory presynaptic differentiation in response to FGF22 and FGF7. Excitatory presynaptic differentiation is impaired in Fgfr2b and Fgfr1b mutant mice; however, inhibitory presynaptic defects are only found in Fgfr2b mutants. FGFR2b and FGFR1b are required for an excitatory presynaptic response to FGF22, whereas only FGFR2b is required for an inhibitory presynaptic response to FGF7. We further find that FGFRs are required in the presynaptic neuron to respond to FGF22, and that FRS2 and PI3K, but not PLCγ, mediate FGF22-dependent presynaptic differentiation. Our results reveal the specific receptors and signaling pathways that mediate FGF-dependent presynaptic differentiation, and thereby provide a mechanistic understanding of precise excitatory and inhibitory synapse formation in the mammalian brain.

Synapses are specialized sites of communication between neurons and their targets. The presynaptic and postsynaptic compartments must be properly aligned and maintained in order for the optimal functioning of the nervous system. Synaptogenesis consists of four major steps: (1) neurite elongation and guidance (in which axons and dendrites of partner neurons grow into apposition with each other); (2) synaptic differentiation (in which the appropriate transmission machinery is assembled); (3) synaptic refinement (in which functional connections are maintained and extraneous connections eliminated); and (4) synaptic maturation and maintenance (an ongoing process through age that ensures the stability of the brain). Disruption at any phase leads to synaptic defects that can have neurological/psychiatric consequences. The general process of synaptic development is largely conserved across species. Here, we highlight what is known about mammalian synaptogenesis, and how failures in synaptic development can result in a variety of neurological and psychiatric disorders.

Functional synapse formation requires tight coordination between pre- and post-synaptic termini. Previous studies have shown that postsynaptic expression of heparan sulfate proteoglycan syndecan-2 (SDC2) induces dendritic spinogenesis. Those SDC2-induced dendritic spines are frequently associated with presynaptic termini. However, how postsynaptic SDC2 accelerates maturation of corresponding presynaptic termini is unknown. Because fibroblast growth factor 22 (FGF22), a heparan sulfate binding growth factor, has been shown to act as a presynaptic organizer released from the postsynaptic site, it seems possible that postsynaptic SDC2 presents FGF22 to the presynaptic FGF receptor to promote presynaptic differentiation. Here, we show that postsynaptic SDC2 uses its ectodomain to interact with and facilitate dendritic filopodial targeting of FGF22, triggering presynaptic maturation. Since SDC2 also enhances filopodial targeting of NMDAR via interaction with the CASK-mLIN7-MINT1 adaptor complex, presynaptic maturation promoted by FGF22 further feeds back to activate NMDAR at corresponding postsynaptic sites through increased neurotransmitter release and, consequently, promotes the dendritic filopodia-spines (F-S) transition. Meanwhile, via regulation of the KIF17 motor, CaMKII (activated by the NMDAR pathway) may further facilitate FGF22 targeting to dendritic filopodia that receive presynaptic stimulation. Our study suggests a positive feedback that promotes the coordination of postsynaptic and presynaptic differentiation.

Microglia regulate synaptic circuit remodeling and phagocytose synaptic material in the healthy brain; however, the mechanisms directing microglia to engulf specific synapses and avoid others remain unknown. Here, we demonstrate that an innate immune signaling pathway protects synapses from inappropriate removal. The expression patterns of “don’t eat me” signal CD47 and its receptor, SIRPα, correlated with peak pruning in the developing retinogeniculate system, and mice lacking these proteins exhibited increased microglial engulfment of retinogeniculate inputs and reduced synapse numbers in the dorsal lateral geniculate nucleus. CD47-deficient mice also displayed increased functional pruning as measured by electrophysiology. Additionally, CD47 was found to be required for neuronal activity-mediated changes in engulfment, as microglia in CD47 knockout mice failed to display preferential engulfment of less active inputs. Together, these results demonstrate that CD47-SIRPα signaling prevents excess microglial phagocytosis, and show that molecular brakes can be regulated by activity to protect specific inputs.

Specific growth factors induce formation and differentiation of excitatory and inhibitory synapses, and are essential for brain development and function. Fibroblast growth factor 22 (FGF22) is important for specifying excitatory synapses during development, including in the hippocampus. Mice with a genetic deletion of FGF22 (FGF22KO) during development subsequently have fewer hippocampal excitatory synapses in adulthood. As a result, FGF22KO mice are resistant to epileptic seizure induction. In addition to playing a key role in learning, the hippocampus is known to mediate mood and anxiety. Here, we explored whether loss of FGF22 alters affective, anxiety or social cognitive behaviors in mice. We found that relative to control mice, FGF22KO mice display longer duration of floating and decreased latency to float in the forced swim test, increased immobility in the tail suspension test, and decreased preference for sucrose in the sucrose preference test, which are all suggestive of a depressive-like phenotype. No differences were observed between control and FGF22KO mice in other behavioral assays, including motor, anxiety, or social cognitive tests. These results suggest a novel role for FGF22 specifically in affective behaviors.

How are excitatory (glutamatergic) and inhibitory (GABAergic) synapses established? Do distinct molecular mechanisms direct differentiation of glutamatergic and GABAergic synapses? In the brain, glutamatergic and GABAergic synaptic connections are formed with specific patterns. To establish such precise synaptic patterns, neurons pass through multiple checkpoints during development, such as cell fate determination, cell migration and localization, axonal guidance and target recognition, and synapse formation. Each stage offers key molecules for neurons/synapses to obtain glutamatergic or GABAergic specificity. Some mechanisms are based on intrinsic systems to induce gene expression, whereas others are based on extrinsic systems mediated by cell-cell or axon-target interactions. Recent studies indicate that specific formation of glutamatergic and GABAergic synapses is controlled by the expression or activation of different sets of molecules during development. In this review, the authors outline stages critical to the determination of glutamatergic or GABAergic specificity and describe molecules that act as determinants of specificities in each stage, with a particular focus on the synapse formation stage. They also discuss possible mechanisms underlying glutamatergic and GABAergic synapse formation via synapse-type specific synaptic organizers.

SummaryEfficient memory formation relies on the establishment of functional hippocampal circuits. It has been proposed that synaptic connections are refined by neural activity to form functional brain circuitry. However, it is not known whether and how hippocampal connections are refined by neural activity in vivo. Using a mouse genetic system in which restricted populations of neurons in the hippocampal circuit are inactivated, we show that inactive axons are eliminated after they develop through a competition with active axons. Remarkably, in the dentate gyrus, which undergoes neurogenesis throughout life, axon refinement is achieved by a competition between mature and young neurons. These results demonstrate that activity-dependent competition plays multiple roles in the establishment of functional memory circuits in vivo.

A critical step in synaptic development is the differentiation of presynaptic and postsynaptic compartments. This complex process is regulated by a variety of secreted factors that serve as synaptic organizers. Specifically, fibroblast growth factors, Wnts, neurotrophic factors and various other intercellular signaling molecules are proposed to regulate presynaptic and/or postsynaptic differentiation. Many of these factors appear to function at both the neuromuscular junction and in the central nervous system, although the specific function of the molecules differs between the two. Here we review secreted molecules that organize the synaptic compartments and discuss how these molecules shape synaptic development, focusing on mammalian in vivo systems. Their critical role in shaping a functional neural circuit is underscored by their possible link to a wide range of neurological and psychiatric disorders both in animal models and by mutations identified in human patients.

Major depressive and bipolar disorders are serious illnesses that affect millions of people. Growing evidence implicates glutamate signalling in depression, though the molecular mechanism by which glutamate signalling regulates depression-related behaviour remains unknown. In this study, we provide evidence suggesting that tyrosine phosphorylation of the NMDA receptor, an ionotropic glutamate receptor, contributes to depression-related behaviour. The NR2A subunit of the NMDA receptor is tyrosine-phosphorylated, with Tyr 1325 as its one of the major phosphorylation site. We have generated mice expressing mutant NR2A with a Tyr-1325-Phe mutation to prevent the phosphorylation of this site in vivo. The homozygous knock-in mice show antidepressant-like behaviour in the tail suspension test and in the forced swim test. In the striatum of the knock-in mice, DARPP-32 phosphorylation at Thr 34, which is important for the regulation of depression-related behaviour, is increased. We also show that the Tyr 1325 phosphorylation site is required for Src-induced potentiation of the NMDA receptor channel in the striatum. These data argue that Tyr 1325 phosphorylation regulates NMDA receptor channel properties and the NMDA receptor-mediated downstream signalling to modulate depression-related behaviour.

Fibroblast growth factors (FGFs) are a large family of secreted growth factors that are involved in the development, regeneration and repair of various tissues. In the nervous system, FGFs have been implicated in early developmental processes, such as neural induction, proliferation and patterning. Accumulating data indicate that FGFs are also important for the formation of functional neural networks. The role of FGFs in axon guidance, target recognition and synaptic differentiation as target-derived factors, and how they cooperate with cell adhesion molecules that are also involved in the wiring of the nervous system are the focus of this review.

Formation of chemical synapses requires exchange of organizing signals between the synaptic partners. Using synaptic vesicle aggregation in cultured neurons as a marker of presynaptic differentiation, we purified candidate presynaptic organizers from mouse brain. A major bioactive species was the extracellular domain of signal regulatory protein alpha (SIRP-alpha), a transmembrane immunoglobulin superfamily member concentrated at synapses. The extracellular domain of SIRP-alpha is cleaved and shed in a developmentally regulated manner. The presynaptic organizing activity of SIRP-alpha is mediated in part by CD47. SIRP-alpha homologues, SIRP-beta and -gamma also have synaptic vesicle clustering activity. The effects of SIRP-alpha are distinct from those of another presynaptic organizer, FGF22: the two proteins induced vesicle clusters of different sizes, differed in their ability to promote neurite branching, and acted through different receptors and signaling pathways. SIRP family proteins may act together with other organizing molecules to pattern synapses.

Target-derived factors organize synaptogenesis by promoting differentiation of nerve terminals at synaptic sites. Several candidate organizing molecules have been identified based on their bioactivities in vitro, but little is known about their roles in vivo. Here, we show that three sets of organizers act sequentially to pattern motor nerve terminals: FGFs, beta2 laminins, and collagen alpha(IV) chains. FGFs of the 7/10/22 subfamily and broadly distributed collagen IV chains (alpha1/2) promote clustering of synaptic vesicles as nerve terminals form. beta2 laminins concentrated at synaptic sites are dispensable for embryonic development of nerve terminals but are required for their postnatal maturation. Synapse-specific collagen IV chains (alpha3-6) accumulate only after synapses are mature and are required for synaptic maintenance. Thus, multiple target-derived signals permit discrete control of the formation, maturation, and maintenance of presynaptic specializations.

Communication between pre- and postsynaptic cells promotes the initial organization of synaptic specializations, but subsequent synaptic stabilization requires transcriptional regulation. Here we show that fibroblast growth factor 22 (FGF22), a target-derived presynaptic organizer in the mouse hippocampus, induces the expression of insulin-like growth factor 2 (IGF2) for the stabilization of presynaptic terminals. FGF22 is released from CA3 pyramidal neurons and organizes the differentiation of excitatory nerve terminals formed onto them. Local application of FGF22 on the axons of dentate granule cells (DGCs), which are presynaptic to CA3 pyramidal neurons, induces IGF2 in the DGCs. IGF2, in turn, localizes to DGC presynaptic terminals and stabilizes them in an activity-dependent manner. IGF2 application rescues presynaptic defects of Fgf22-/- cultures. IGF2 is dispensable for the initial presynaptic differentiation, but is required for the following presynaptic stabilization both in vitro and in vivo. These results reveal a novel feedback signal that is critical for the activity-dependent stabilization of presynaptic terminals in the mammalian hippocampus. DOI: http://dx.doi.org/10.7554/eLife.12151.001, eLife digest Nerve cells in the developing brain must organize themselves into complex networks by forming appropriate connections with one another. These connections are known as synapses, and they assemble via two critical stages. First, a new synapse forms, and then it stabilizes. This first stage is a localized event that involves the contact site between the two nerve cells, while the stabilization of a synapse requires the expression of genes in a nerve cell’s nucleus. Furthermore, only active synapses may be stabilized. Many synapses form in a region of the brain called the hippocampus, which plays a key role in learning and memory. A protein called fibroblast growth factor 22 (or FGF22 for short) helps synapses to initially form within the hippocampus. However, much less is known about the signals that regulate the stabilization of synapses and the genes that are involved. It is also not clear if these genes might be controlled by FGF22 signaling. To address these questions, Terauchi et al. searched the mouse hippocampus for genes with expression that depended on FGF22 signaling. One gene in particular, which encodes a protein called insulin-like growth factor 2 (IGF2), was much less expressed in mice that lack FGF22 compared to normal mice. Further experiments revealed that only active nerve cells transport IGF2 to synapses, and that IGF2 helps to stabilize these structures. By contrast, IGF2 is not required for synapse to initially form. This indicates that FGF22 controls both the formation and stabilization of synapses, and that it controls the first stage directly, and the second stage indirectly via its effects on IGF2 expression. Terauchi et al. also showed that FGF22-IGF2 signaling is not involved in the stabilization of all synapses in the mouse hippocampus. Instead, synapses between different types of nerve cell appear to use distinct signals for synapse formation and stabilization. A key topic for future studies will be to understand these specific signals and how they cooperate in the brain to establish precise networks of nerve cells. DOI: http://dx.doi.org/10.7554/eLife.12151.002

Synapses form after growing axons recognize their appropriate targets. The subsequent assembly of aligned pre and postsynaptic specializations is critical for synaptic function. This highly precise apposition of presynaptic elements (i.e. active zones) to postsynaptic specializations (i.e. neurotransmitter receptor clusters) strongly suggests that communication between the axon and target is required for synaptic differentiation. What trans-synaptic factors drive such differentiation at vertebrate synapses? First insights into the answers to this question came from studies at the neuromuscular junction (NMJ), where axon-derived agrin and muscle-derived laminin beta2 induce post and presynaptic differentiation, respectively. Recent work has suggested that axon- and target-derived factors similarly drive synaptic differentiation at central synapses. Specifically, WNT-7a, neuroligin, synaptic cell adhesion molecule (SynCAM) and fibroblast growth factor-22 (FGF-22) have all been identified as target-derived presynaptic organizers, whereas axon-derived neuronal activity regulated pentraxin (Narp), ephrinB and neurexin reciprocally co-ordinate postsynaptic differentiation. In addition to these axon- and target-derived inducers of synaptic differentiation, factors released from glial cells have also been implicated in regulating synapse assembly. Together, these recent findings have profoundly advanced our understanding of how precise appositions are established during vertebrate nervous system development.

In mammals, fibroblast growth factors (FGFs) are encoded by 22 genes. FGFs bind and activate alternatively spliced forms of four tyrosine kinase FGF receptors (FGFRs 1–4). The spatial and temporal expression patterns of FGFs and FGFRs and the ability of specific ligand-receptor pairs to actively signal are important factors regulating FGF activity in a variety of biological processes. FGF signaling activity is regulated by the binding specificity of ligands and receptors and is modulated by extrinsic cofactors such as heparan sulfate proteoglycans. In previous studies, we have engineered BaF3 cell lines to express the seven principal FGFRs and used these cell lines to determine the receptor binding specificity of FGFs 1–9 by using relative mitogenic activity as the readout. Here we have extended these semiquantitative studies to assess the receptor binding specificity of the remaining FGFs 10–23. This study completes the mitogenesis-based comparison of receptor specificity of the entire FGF family under standard conditions and should help in interpreting and predicting in vivo biological activity.

Phosphorylation of neural proteins in response to a diverse array of external stimuli is one of the main mechanisms underlying dynamic changes in neural circuitry. The NR2B subunit of the NMDA receptor is tyrosine-phosphorylated in the brain, with Tyr-1472 its major phosphorylation site. Here, we generate mice with a knockin mutation of the Tyr-1472 site to phenylalanine (Y1472F) and show that Tyr-1472 phosphorylation is essential for fear learning and amygdaloid synaptic plasticity. The knockin mice show impaired fear-related learning and reduced amygdaloid long-term potentiation. NMDA receptor-mediated CaMKII signaling is impaired in YF/YF mice. Electron microscopic analyses reveal that the Y1472F mutant of the NR2B subunit shows improper localization at synapses in the amygdala. We thus identify Tyr-1472 phosphorylation as a key mediator of fear learning and amygdaloid synaptic plasticity.

The delta2 glutamate receptor (GluRdelta2) is predominantly expressed in the postsynaptic densities of parallel fiber-Purkinje cell synapses and plays a crucial role in cerebellar function. However, the mechanisms by which GluRdelta2 participates in cerebellar functions are largely unknown because GluRdelta2 does not bind glutamate analogs. We investigated the possibility that GluRdelta2 may be involved in channel formation together with other glutamate receptor families. We transiently expressed lurcher mutant AMPA receptor GluR1(Lc) and kainate receptor GluR6(Lc) in HEK293 cells. Cells expressing these constitutively active channels displayed a rectifying current-voltage (I-V) relationship. However, when cells were co-transfected with GluRdelta2(Lc), which had the arginine residue in the channel pore region, cells displayed a linear I-V relationship, a result that indicates GluRdelta2(Lc) formed functional heteromeric channels with GluR1(Lc) or GluR6(Lc). Assembly of GluRdelta2 with GluR1 or GluR6 was further confirmed by co-immunoprecipitation assays in HEK293 cells. In addition, GluRdelta2 receptors were partially co-immunoprecipitated from cerebellar synaptosomal fractions by antibodies against GluR2 or KA2. In contrast to lurcher channels, expression of wild-type GluRdelta2 significantly reduced the glutamate-induced current of the wild-type GluR1 receptors without affecting channel properties, such as current kinetics, dose-response relationship, and single-channel conductance. Thus, the heteromeric channel created by the association of wild-type GluR1 and GluRdelta2 may not be gated by glutamate and does not participate in glutamate-induced currents. These results suggest that GluRdelta2 and AMPA or kainate receptors can assemble to form heteromeric receptors in vitro and could modify glutamate signaling in vivo. These findings may help explain the role of GluRdelta2.

The 5-HT1A receptor is a G protein coupled receptor (GPCR) that activates G proteins of the Gαi/o family. 5-HT1A receptors expressed in the raphe, hippocampus and prefrontal cortex are implicated in the control of mood and are targets for anti-depressant drugs. Regulators of G protein signaling (RGS) proteins are members of a large family that play important roles in signal transduction downstream of G protein coupled receptors (GPCRs). The main role of RGS proteins is to act as GTPase accelerating proteins (GAPs) to dampen or negatively regulate GPCR-mediated signaling. We have shown that a mouse expressing Gαi2 that is insensitive to all RGS protein GAP activity has an anti-depressant-like phenotype due to increased signaling of postsynaptic 5-HT1A receptors, thus implicating the 5-HT1A receptor–Gαi2 complex as an important target. Here we confirm that RGS proteins act as GAPs to regulate signaling to adenylate cyclase and the mitogen-activated protein kinase (MAPK) pathway downstream of the 5-HT1A receptor, using RGS-insensitive Gαi2 protein expressed in C6 cells. We go on to use short hairpin RNA (shRNA) to show that RGS19 is responsible for the GAP activity in C6 cells and also that RGS19 acts as a GAP for 5-HT1A receptor signaling in human neuroblastoma SH-SY5Y cells and primary hippocampal neurons. In addition, in both cell types the synergy between 5-HT1A receptor and the fibroblast growth factor receptor 1 in stimulating the MAPK pathway is enhanced following shRNA reduction of RGS19 expression. Thus RGS19 may be a viable new target for anti-depressant medications.

Specific formation of excitatory and inhibitory synapses is crucial for proper functioning of the brain. Fibroblast growth factor 22 (FGF22) and FGF7 are postsynaptic-cell-derived presynaptic organizers necessary for excitatory and inhibitory presynaptic differentiation, respectively, in the hippocampus. For the establishment of specific synaptic networks, these FGFs must localize to appropriate synaptic locations – FGF22 to excitatory and FGF7 to inhibitory postsynaptic sites. Here, we show that distinct motor and adaptor proteins contribute to intracellular microtubule transport of FGF22 and FGF7. Excitatory synaptic targeting of FGF22 requires the motor proteins KIF3A and KIF17 and the adaptor protein SAP102 (also known as DLG3). By contrast, inhibitory synaptic targeting of FGF7 requires the motor KIF5 and the adaptor gephyrin. Time-lapse imaging shows that FGF22 moves with SAP102, whereas FGF7 moves with gephyrin. These results reveal the basis of selective targeting of the excitatory and inhibitory presynaptic organizers that supports their different synaptogenic functions. Finally, we found that knockdown of SAP102 or PSD95 (also known as DLG4), which impairs the differentiation of excitatory synapses, alters FGF7 localization, suggesting that signals from excitatory synapses might regulate inhibitory synapse formation by controlling the distribution of the inhibitory presynaptic organizer.

Bone morphogenetic protein (BMP) controls osteoblast proliferation and differentiation through Smad proteins. Here we show that Tob, a member of the emerging family of antiproliferative proteins, is a negative regulator of BMP/Smad signaling in osteoblasts. Mice carrying a targeted deletion of the tob gene have a greater bone mass resulting from increased numbers of osteoblasts. Orthotopic bone formation in response to BMP2 is elevated in tob-deficient mice. Overproduction of Tob represses BMP2-induced, Smad-mediated transcriptional activation. Finally, Tob associates with receptor-regulated Smads (Smad1, 5, and 8) and colocalizes with these Smads in the nuclear bodies upon BMP2 stimulation. The results indicate that Tob negatively regulates osteoblast proliferation and differentiation by suppressing the activity of the receptor-regulated Smad proteins.

BACKGROUND The NMDA receptors (NMDARs) are ion channels through which Ca2+ influx triggers various intracellular responses. Tyrosine phosphorylation of NMDARs regulates NMDA channel activities, which may be important in neuronal plasticity. The biological significance of the tyrosine phosphorylation events, however, differs among NMDAR subunits: tyrosine phosphorylation of NMDARepsilon1 increases NMDA channel activities, but that of NMDARepsilon2 does not. Since signal transductions from various cell surface receptors are mediated by protein-protein interaction through phosphotyrosine and the Src homology 2 (SH2) domain, we examined the possibility that phosphotyrosines in NMDARepsilon2 contribute to the intracellular signalling events. RESULTS We first show that Fyn is deeply involved in the phosphorylation of NMDARepsilon2 and second that a phosphotyrosine in NMDARepsilon2 interacts with the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI3-kinase). Both the level of tyrosine phosphorylation on NMDARepsilon2 and the amounts of the p85 subunit (p85) bound to NMDARepsilon2 are decreased in Fyn-deficient mice. Moreover, we show that ischaemia stimulates the binding of p85 to phosphorylated NMDARepsilon2, suggesting a physiological role of the phosphotyrosine/SH2-based interaction between NMDARepsilon2 and p85 in the brain. CONCLUSIONS The tyrosine phosphorylation event on NMDARs is important in not only the regulation of its channel activity but also intracellular signalling mediated through the interaction of the NMDAR with SH2 domain-containing molecules.

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. The ionotropic glutamate receptors are classified into two groups, NMDA (N-methyl-D-aspartate) receptors and AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) receptors. The AMPA receptor is a ligand-gated cation channel that mediates the fast component of excitatory postsynaptic currents in the central nervous system. Here we report that AMPA receptors function not only as ion channels but also as cell-surface signal transducers by means of their interaction with the Src-family non-receptor protein tyrosine kinase Lyn. In the cerebellum, Lyn is physically associated with the AMPA receptor and is rapidly activated following stimulation of the receptor. Activation of Lyn is independent of Ca2+ and Na+ influx through AMPA receptors. As a result of activation of Lyn, the mitogen-activated protein kinase (MAPK) signalling pathway is activated, and the expression of brain-derived neurotrophic factor (BDNF) messenger RNA is increased in a Lyn-kinase-dependent manner. Thus, AMPA receptors generate intracellular signals from the cell surface to the nucleus through the Lyn-MAPK pathway, which may contribute to synaptic plasticity by regulating the expression of BDNF.

Using a polymerase chain reaction-mediated cloning procedure, we have identified a novel member, termed ANA (from Abundant in Neuroepithelium Area), of Tob/BTG1 family of antiproliferative genes. Molecular cloning and analysis of cDNAs revealed that the human and mouse ANA encoded a protein of 252 amino acids. The amino-terminal half of ANA was homologous to the previously characterized antiproliferative gene products, BTG1, PC3/TIS21/BTG2, and Tob. The human ANA gene was localized at chromosome 21q11.2-q21.1. ANA was expressed in a variety of tissues and cell lines, its expression being high in the ovary, testis, prostate, thymus, and lung. Further analysis revealed that ANA expression was high in the ventricular zone of the developing central nervous system. Finally, overexpression of ANA impaired serum-induced cell cycle progression from the G0/G1 to S phase. In conclusion, ANA is a fourth member of the Tob/BTG1 family that might play roles in neurogenesis in the central nervous system.

Formation of appropriate synaptic connections is critical for proper functioning of the brain. After initial synaptic differentiation, active synapses are stabilized by neural activity-dependent signals to establish functional synaptic connections. However, the molecular mechanisms underlying activity-dependent synapse maturation remain to be elucidated. Here we show that activity-dependent ectodomain shedding of signal regulatory protein-α (SIRPα) mediates presynaptic maturation. Two target-derived molecules, fibroblast growth factor 22 and SIRPα, sequentially organize the glutamatergic presynaptic terminals during the initial synaptic differentiation and synapse maturation stages, respectively, in the mouse hippocampus. SIRPα drives presynaptic maturation in an activity-dependent fashion. Remarkably, neural activity cleaves the extracellular domain of SIRPα, and the shed ectodomain in turn promotes the maturation of the presynaptic terminal. This process involves calcium/calmodulin-dependent protein kinase, matrix metalloproteinases and the presynaptic receptor CD47. Finally, SIRPα-dependent synapse maturation has an impact on synaptic function and plasticity. Thus, ectodomain shedding of SIRPα is an activity-dependent trans-synaptic mechanism for the maturation of functional synapses.

To identify novel signal transducers involved in signaling mediated by the Src-family protein tyrosine kinases (PTKs), we used a yeast two-hybrid system with a probe corresponding to the regulatory region of p56lyn, a member of Src-family PTKs. One of the isolated clones contained the COOH-terminal 470 amino acid residues of p120c-cbl, the product of the cellular homologue of the v-cbl retroviral oncogene. p120c-cbl is a cytoplasmic protein with nuclear protein-like motifs. Here we show in vivo association of p120c-cbl with p53/56lyn. After stimulation of the B cell antigen receptor (BCR), p120c-cbl was rapidly tyrosine phosphorylated. Studies with lyn- or syk-negative chicken B cells demonstrated that p53/56lyn, but not p72syk, was crucial for tyrosine phosphorylation of p120c-cbl upon stimulation of the BCR. We also show the importance of p59fyn in tyrosine phosphorylation of p120c-cbl in the T-cell receptor-mediated signaling using fyn-overexpressing T cell hybridomas and splenic T cells from fyn-deficient mice. These results suggest that p120c-cbl is an important substrate of Src-family PTKs in the intracellular signaling mediated by the antigen receptors

Brain development involves multiple levels of molecular coordination in forming a functional nervous system. The hippocampus is a brain area that is important for memory formation and spatial reasoning. During early postnatal development of the hippocampal circuit, Fibroblast growth factor 22 (FGF22) and FGF7 act to establish a balance of excitatory and inhibitory tone. Both FGFs are secreted from CA3 dendrites, acting on excitatory or inhibitory axon terminals formed onto CA3 dendrites, respectively. Mechanistically, FGF22 utilizes FGFR2b and FGFR1b to induce synaptic vesicle recruitment within axons of dentate granule cells (DGCs), and FGF7 utilizes FGFR2b to induce synaptic vesicle recruitment within interneuron axons. FGF signaling eventually induces gene expression in the presynaptic neurons; however, the effects of FGF22-induced gene expression within DGCs and FGF7-induced gene expression within interneurons in the context of a developing hippocampal circuit have yet to be explored. Here, we propose one hypothetical mechanism of FGF22-induced gene expression in controlling adult neurogenesis.

One of the most common types of epilepsy in adults is temporal lobe epilepsy. Temporal lobe epilepsy is often resistant to pharmacological treatment, requiring urgent understanding of its molecular and cellular mechanisms. It is generally accepted that an imbalance between excitatory and inhibitory inputs is related to epileptogenesis. We have recently identified that fibroblast growth factor (FGF) 7 is critical for inhibitory synapse formation in the developing hippocampus. Remarkably, FGF7 knockout mice are prone to epileptic seizures induced by chemical kindling (Terauchi et al., 2010). Here we show that FGF7 knockout mice exhibit epileptogenesis-related changes in the hippocampus even without kindling induction. FGF7 knockout mice show mossy fiber sprouting and enhanced dentate neurogenesis by 2 months of age, without apparent spontaneous seizures. These results suggest that FGF7-deficiency impairs inhibitory synapse formation, which results in mossy fiber sprouting and enhanced neurogenesis during development, making FGF7 knockout mice vulnerable to epilepsy.

Crosslinking of membrane-bound immunoglobulins, which are B-cell antigen receptors, causes proliferation and differentiation of B cells or inhibition of their growth. The receptor-mediated signaling involves tyrosine phosphorylation of cellular proteins and rapid activation of Src-like kinases. The amino acid sequences of five proteolytic peptides of p75, a major substrate of protein-tyrosine(s) in the signaling, showed that p75 is the human HS1 gene product. The HS1 gene is expressed specifically in hematopoietic cells and encodes p75HS1, which carries both helix-turn-helix and Src homology 3 motifs. p75HS1 showed rapid tyrosine phosphorylation and association with a Src-like kinase, Lyn, after crosslinking of membrane-bound IgM. Thus, p75HS1 may be an important substrate of Lyn and possibly other protein-tyrosine kinases upon B-cell antigen receptor-mediated signaling.

Background Anxiety disorders are a highly prevalent and disabling class of psychiatric disorders. There is growing evidence implicating the glutamate system in the pathophysiology and treatment of anxiety disorders, though the molecular mechanism by which the glutamate system regulates anxiety-like behavior remains unclear. Results In this study, we provide evidence suggesting that tyrosine phosphorylation of the NMDA receptor, an ionotropic glutamate receptor, contributes to anxiety-like behavior. The GluN2B subunit of the NMDA receptor is tyrosine-phosphorylated: Tyr-1472 is the major phosphorylation site. Homozygous knock-in mice that express a Tyr-1472-Phe mutant of GluN2B, which prevents phosphorylation of this site, show enhanced anxiety-like behavior in the elevated plus-maze test. Expression of corticotropin-releasing factor (CRF), which is important for the regulation of anxiety-like behavior, is increased in the amygdala of the knock-in mice. Furthermore, injection of CRF receptor antagonist attenuated the enhanced anxiety-like behavior of the knock-in mice. We also show that elevated plus-maze exposure simultaneously induced de-phosphorylation of Tyr-1472 and increased CRF expression. Conclusions These data suggest that Tyr-1472 phosphorylation on GluN2B is important for anxiety-like behavior by negative regulation of CRF expression in the amygdala.

A critical step in synaptic development is the differentiation of presynaptic and postsynaptic compartments. This complex process is regulated by a variety of secreted factors that serve as synaptic organizers. Specifically, fibroblast growth factors, Wnts, neurotrophic factors, and various other intercellular signaling molecules are proposed to regulate presynaptic and/or postsynaptic differentiation. Many of these factors appear to function at both the neuromuscular junction and in the central nervous system, although the specific function of the molecules differs between the two. Here we review secreted molecules that organize the synaptic compartments and discuss how these molecules shape synaptic development, focusing on mammalian in vivo systems. Their critical role in shaping a functional neural circuit is underscored by their possible link to a wide range of neurological and psychiatric disorders both in animal models and by mutations identified in human patients.

The Src-like protein-tyrosine kinases Fyn and Lyn are expressed in lymphocytes. Fyn is expressed in T cells at elevated levels and is associated with the T cell antigen receptor complex, whereas Lyn is expressed in B cells and is associated with membrane-bound immunoglobulin. Thus, these kinases are suggested to participate in antigen-mediated signal transduction in lymphocytes. Previous report showed that fyn was also expressed in brain, but its cellular distribution was not examined. Expression of Lyn in neural tissues was not previously reported. Here we report that both fyn and lyn are expressed in discrete regions of the brain. To throw light on their functions in the brain, we investigated their expressions during brain ontogenesis in mice. In situ hybridization analysis showed that Fyn mRNA was specifically expressed in neurons of embryos and newborn mice. In adult animals, fyn mRNA was expressed in oligodendrocytes as well as neurons. In contrast, the expression of lyn mRNA was relatively low in brains of embryos and newborn mice, but in adults the transcript was specifically expressed in the granular layer of the cerebellum. Therefore, the Fyn and Lyn kinases may regulate distinct functions of specific cells during brain development. The specific expressions of Fyn and Lyn in both lymphatic and neural tissues could suggest common signalling mechanisms in the immune system and central nervous system.

The differential formation of excitatory (glutamate-mediated) and inhibitory (GABA-mediated) synapses is a critical step for the proper functioning of the brain. An imbalance in these synapses may lead to various neurological disorders such as autism, schizophrenia, Tourette's syndrome and epilepsy. Synapses are formed through communication between the appropriate synaptic partners. However, the molecular mechanisms that mediate the formation of specific synaptic types are not known. Here we show that two members of the fibroblast growth factor (FGF) family, FGF22 and FGF7, promote the organization of excitatory and inhibitory presynaptic terminals, respectively, as target-derived presynaptic organizers. FGF22 and FGF7 are expressed by CA3 pyramidal neurons in the hippocampus. The differentiation of excitatory or inhibitory nerve terminals on dendrites of CA3 pyramidal neurons is specifically impaired in mutants lacking FGF22 or FGF7. These presynaptic defects are rescued by postsynaptic expression of the appropriate FGF. FGF22-deficient mice are resistant to epileptic seizures, and FGF7-deficient mice are prone to them, as expected from the alterations in excitatory/inhibitory balance. Differential effects of FGF22 and FGF7 involve both their distinct synaptic localizations and their use of different signalling pathways. These results demonstrate that specific FGFs act as target-derived presynaptic organizers and help to organize specific presynaptic terminals in the mammalian brain.

Fibroblast growth factors (FGFs) are a large family of secreted growth factors that are involved in the development, regeneration and repair of various tissues. In the nervous system, FGFs have been implicated in early developmental processes, such as neural induction, proliferation and patterning. Accumulating data indicate that FGFs are also important for the formation of functional neural networks. The role of FGFs in axon guidance, target recognition and synaptic differentiation as target-derived factors, and how they cooperate with cell adhesion molecules that are also involved in the wiring of the nervous system are the focus of this review.

A Short History of the Synapse - Golgi Versus Ramon y Cajal.- Cell Adhesion Molecules at the Drosophila Neuromuscular Junction.- Development of the Vertebrate Neuromuscular Junction.- Synapse Formation in the Mammalian Central Nervous System.- Developmental Axonal Pruning and Synaptic Plasticity.- Cell Adhesion Molecules in Synaptopathies.- The Cadherin Superfamily in Synapse Formation and Function.- Nectins and Nectin-Like Molecules in the Nervous System.- The Down Syndrome Cell Adhesion Molecule.- Molecular Basis of Lamina-Specific Synaptic Connections in the Retina: Sidekick Immunoglobulin Superfamily Molecules.- SYG/Nephrin/IrreC Family of Adhesion Proteins Mediate Asymmetric Cell-Cell Adhesion in Development.- L1-Type Cell Adhesion Molecules: Distinct Roles in Synaptic Targeting, Organization, and Function.- Cell Adhesion Molecules of the NCAM Family and Their Roles at Synapses.- MHC Class I Function at the Neuronal Synapse.- Pathfinding Molecules Branch Out: Semaphorin Family Members Regulate Synapse Development.- Ephrins and Eph Receptor Tyrosine Kinases in Synapse Formation.- Neurexins and Neuroligins: A Synaptic Code for Neuronal Wiring That Is Implicated in Autism.- Synaptic Adhesion-Like Molecules (SALMs).- The Role of Integrins at Synapses.- Extracellular Matrix Molecules in Neuromuscular Junctions and Central Nervous System Synapses.- Gap Junctions as Electrical Synapses.

The NMDA receptor regulates spine morphological plasticity by modulating Rho GTPases. However, the molecular mechanisms for NMDA receptor-mediated regulation of Rho GTPases remain elusive. In this study, we show that p250GAP, an NMDA receptor-associated RhoGAP, regulates spine morphogenesis by modulating RhoA activity. Knock-down of p250GAP increased spine width and elevated the endogenous RhoA activity in primary hippocampal neurons. The increased spine width by p250GAP knock-down was suppressed by the expression of a dominant-negative form of RhoA. Furthermore, p250GAP is involved in NMDA receptor-mediated RhoA activation. In response to NMDA receptor activation, exogenously expressed green fluorescent protein (GFP)-tagged p250GAP was redistributed. Thus, these data suggest that p250GAP plays an important role in NMDA receptor-mediated regulation of RhoA activity leading to spine morphological plasticity.

N-Methyl-d-aspartate (NMDA) receptors regulate structural plasticity by modulating actin organization within dendritic spines. Herein, we report identification and characterization of p250GAP, a novel GTPase-activating protein for Rho family proteins that interacts with the GluRϵ2 (NR2B) subunit of NMDA receptors in vivo. The p250GAP mRNA was enriched in brain, with high expression in cortex, corpus striatum, hippocampus, and thalamus. Within neurons, p250GAP was highly concentrated in the postsynaptic density and colocalized with the GluRϵ2 (NR2B) subunit of NMDA receptors and with postsynaptic density-95. p250GAP promoted GTP hydrolysis of Cdc42 and RhoA in vitro and in vivo. When overexpressed in neuroblastoma cells, p250GAP suppressed the activities of Rho family proteins, which resulted in alteration of neurite outgrowth. Finally, NMDA receptor stimulation led to dephosphorylation and redistribution of p250GAP in hippocampal slices. Together, p250GAP is likely to be involved in NMDA receptor activity-dependent actin reorganization in dendritic spines.

Tob inhibits bone morphogenetic protein (BMP) signaling by interacting with receptor-regulated Smads in osteoblasts. Here we provide evidence that Tob also interacts with the inhibitory Smads 6 and 7. A yeast two-hybrid screen identified Smad6 as a protein interacting with Tob. Tob co-localizes with Smad6 at the plasma membrane and enhances the interaction between Smad6 and activated BMP type I receptors. Furthermore, we have isolated Xenopus Tob2, and show that it cooperates with Smad6 in inducing secondary axes when expressed in early Xenopus embryos. Finally, Tob and Tob2 cooperate with Smad6 to inhibit endogenous BMP signaling in Xenopus embryonic explants and in cultured mammalian cells. Our results provide both in vitro and in vivo evidence that Tob inhibits endogenous BMP signaling by facilitating inhibitory Smad functions.

The N-methyl-d-aspartate (NMDA) receptor, an ionotropic glutamate receptor, is implicated in motor activity that is regulated in the striatum and nucleus accumbens of the brain. A Src family kinase Lyn is highly expressed in striatum, cortex, thalamus, and cerebellum in the brain. Here we show that spontaneous motor activity is suppressed in lyn−/− mice. S.c. injection of methylphenidate, which causes accumulation of dopamine in synapses, reveals that dopaminergic pathway is normal in lyn−/− mice. After blocking the NMDA receptor, motor activity of lyn−/− mice increased to the same level as that of wild type mice. Therefore, the NMDA receptor-mediated signaling is enhanced in lyn−/− mice, indicating that Lyn regulates the NMDA receptor pathway negatively. Intriguingly, the activity of protein kinase C (PKC), an enzyme regulated downstream of NMDA receptors, is increased in lyn−/− mice. The present data suggest that the NMDA receptor signal that is enhanced in the absence of Lyn suppresses the motor activity, probably through inhibition of dopaminergic pathway at striatum. We conclude that Lyn contributes to coordination of motor activity through regulation of the NMDA pathway. It appears that this negative regulation involves suppression of downstream signaling of NMDA receptor such as those mediated by PKC.

The N-methyl-d-aspartate (NMDA) receptors play critical roles in synaptic plasticity, neuronal development, and excitotoxicity. Tyrosine phosphorylation of NMDA receptors by Src-family tyrosine kinases such as Fyn is implicated in synaptic plasticity. To precisely address the roles of NMDA receptor tyrosine phosphorylation, we identified Fyn-mediated phosphorylation sites on the GluR epsilon 2 (NR2B) subunit of NMDA receptors. Seven out of 25 tyrosine residues in the C-terminal cytoplasmic region of GluR epsilon 2 were phosphorylated by Fyn in vitro. Of these 7 residues, Tyr-1252, Tyr-1336, and Tyr-1472 in GluR epsilon 2 were phosphorylated in human embryonic kidney fibroblasts when co-expressed with active Fyn, and Tyr-1472 was the major phosphorylation site in this system. We then generated rabbit polyclonal antibodies specific to Tyr-1472-phosphorylated GluR epsilon 2 and showed that Tyr-1472 of GluR epsilon 2 was indeed phosphorylated in murine brain using the antibodies. Importantly, Tyr-1472 phosphorylation was greatly reduced in fyn mutant mice. Moreover, Tyr-1472 phosphorylation became evident when hippocampal long term potentiation started to be observed, and its magnitude became larger in murine brain. Finally, Tyr-1472 phosphorylation was significantly enhanced after induction of long term potentiation in the hippocampal CA1 region. These data suggest that Tyr-1472 phosphorylation of GluR epsilon 2 is important for synaptic plasticity.

Glutamate receptor (GluR) delta2 is selectively expressed in cerebellar Purkinje cells and plays a crucial role in cerebellum-dependent motor learning. Although GluRdelta2 belongs to an ionotropic GluR family, little is known about its pharmacological features and downstream signaling cascade. To study molecular mechanisms underlying GluRdelta2-dependent motor learning, we employed yeast two-hybrid screening to isolate GluRdelta2-interacting molecules and identified protein-tyrosine phosphatase PTPMEG. PTPMEG is a family member of band 4.1 domain-containing protein-tyrosine phosphatases and is expressed prominently in brain. Here, we showed by in situ hybridization analysis that the PTPMEG mRNA was enriched in mouse thalamus and Purkinje cells. We also showed that PTPMEG interacted with GluRdelta2 as well as with N-methyl-d-aspartate receptor GluRepsilon1 in cultured cells and in brain. PTPMEG bound to the putative C-terminal PDZ target sequence of GluRdelta2 and GluRepsilon1 via its PDZ domain. Examination of the effect of PTPMEG on tyrosine phosphorylation of GluRepsilon1 unexpectedly revealed that PTPMEG enhanced Fyn-mediated tyrosine phosphorylation of GluRepsilon1 in its PTPase activity-dependent manner. Thus, we conclude that PTPMEG associates directly with GluRdelta2 and GluRepsilon1. Moreover, our data suggest that PTPMEG plays a role in signaling downstream of the GluRs and/or in regulation of their activities through tyrosine dephosphorylation.