Synaptic BMAL1 phosphorylation controls circadian hippocampal plasticity

The time of day strongly influences adaptive behaviors like long-term memory, but the correlating synaptic and molecular mechanisms remain unclear. The circadian clock comprises a canonical transcription-translation feedback loop (TTFL) strictly dependent on the BMAL1 transcription factor. We report that BMAL1 rhythmically localizes to hippocampal synapses in a manner dependent on its phosphorylation at Ser42 [pBMAL1(S42)]. pBMAL1(S42) regulates the autophosphorylation of synaptic CaMKIIα and circadian rhythms of CaMKIIα-dependent molecular interactions and LTP but not global rest/activity behavior. Therefore, our results suggest a model in which repurposing of the clock protein BMAL1 to synapses locally gates the circadian timing of plasticity.

The projection-specific signals that establish functionally segregated dopaminergic synapses

Dopaminergic projections regulate various brain functions and are implicated in many neuropsychiatric disorders. There are two anatomically and functionally distinct dopaminergic projections connecting the midbrain to striatum: nigrostriatal, which controls movement, and mesolimbic, which regulates motivation. However, how these discrete dopaminergic synaptic connections are established is unknown. Through an unbiased search, we identify that two groups of antagonistic TGF-β family members, bone morphogenetic protein (BMP)6/BMP2 and transforming growth factor (TGF)-β2, regulate dopaminergic synapse development of nigrostriatal and mesolimbic neurons, respectively. Projection-preferential expression of their receptors contributes to specific synapse development. Downstream, Smad1 and Smad2 are specifically activated and required for dopaminergic synapse development and function in nigrostriatal vs. mesolimbic projections. Remarkably, Smad1 mutant mice show motor defects, whereas Smad2 mutant mice show lack of motivation. These results uncover the molecular logic underlying the proper establishment of functionally segregated dopaminergic synapses and may provide strategies to treat relevant, projection-specific disease symptoms by targeting specific BMPs/TGF-β and/or Smads.

Activity-Dependent Synapse Refinement: From Mechanisms to Molecules

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.

The molecular signals that regulate activity-dependent synapse refinement in the brain

The formation of appropriate synaptic connections is critical for the proper functioning of the brain. Early in development, neurons form a surplus of immature synapses. To establish efficient, functional neural networks, neurons selectively stabilize active synapses and eliminate less active ones. This process is known as activity-dependent synapse refinement. Defects in this process have been implicated in neuropsychiatric disorders such as schizophrenia and autism. Here we review the manner and mechanisms by which synapse elimination is regulated through activity-dependent competition. We propose a theoretical framework for the molecular mechanisms of synapse refinement, in which three types of signals regulate the refinement. We then describe the identity of these signals and discuss how multiple molecular signals interact to achieve appropriate synapse refinement in the brain.

CDKL5 sculpts functional callosal connectivity to promote cognitive flexibility

Functional and structural connectivity alterations in short- and long-range projections have been reported across neurodevelopmental disorders (NDD). Interhemispheric callosal projection neurons (CPN) represent one of the major long-range projections in the brain, which are particularly important for higher-order cognitive function and flexibility. However, whether a causal relationship exists between interhemispheric connectivity alterations and cognitive deficits in NDD remains elusive. Here, we focused on CDKL5 Deficiency Disorder (CDD), a severe neurodevelopmental disorder caused by mutations in the X-linked Cyclin-dependent kinase-like 5 (CDKL5) gene. We found an increase in homotopic interhemispheric connectivity and functional hyperconnectivity across higher cognitive areas in adult male and female CDKL5-deficient mice by resting-state functional MRI (rs-fMRI) analysis. This was accompanied by an increase in the number of callosal synaptic inputs but decrease in local synaptic connectivity in the cingulate cortex of juvenile CDKL5-deficient mice, suggesting an impairment in excitatory synapse development and a differential role of CDKL5 across excitatory neuron subtypes. These deficits were associated with significant cognitive impairments in CDKL5 KO mice. Selective deletion of CDKL5 in the largest subtype of CPN likewise resulted in an increase of functional callosal inputs, without however significantly altering intracortical cingulate networks. Notably, such callosal-specific changes were sufficient to cause cognitive deficits. Finally, when CDKL5 was selectively re-expressed only in this CPN subtype, in otherwise CDKL5-deficient mice, it was sufficient to prevent the cognitive impairments of CDKL5 mutants. Together, these results reveal a novel role of CDKL5 by demonstrating that it is both necessary and sufficient for proper CPN connectivity and cognitive function and flexibility, and further validates a causal relationship between CPN dysfunction and cognitive impairment in a model of NDD.

ASD/OCD-Linked Protocadherin-10 Regulates Synapse, But Not Axon, Development in the Amygdala and Contributes to Fear- and Anxiety-Related Behaviors

The Protocadherin-10 (PCDH10) gene is associated with autism spectrum disorder (ASD), obsessive-compulsive disorder (OCD), and major depression (MD). The PCDH10 protein is a homophilic cell adhesion molecule that belongs to the δ2-protocadherin family. PCDH10 is highly expressed in the developing brain, especially in the basolateral nucleus of the amygdala (BLA). However, the role of PCDH10 in vivo has been debatable: one paper reported that a Pcdh10 mutant mouse line showed changes in axonal projections; however, another Pcdh10 mutant mouse line was reported to have failed to detect axonal phenotypes. Therefore, the actual roles of PCDH10 in the brain remain to be elucidated. We established a new Pcdh10 KO mouse line using the CRISPR/Cas9 system, without inserting gene cassettes to avoid nonspecific effects, examined the roles of PCDH10 in the brain, and studied the behavioral consequences of Pcdh10 inactivation. Here, we show that Pcdh10 KO mice do not show defects in axonal development. Instead, we find that Pcdh10 KO mice exhibit impaired development of excitatory synapses in the dorsal BLA. We further demonstrate that male Pcdh10 KO mice exhibit reduced anxiety-related behaviors, impaired fear conditioning, decreased stress-coping responses, and mildly impaired social recognition and communication. These results indicate that PCDH10 plays a critical role in excitatory synapse development, but not axon development, in the dorsal BLA and that PCDH10 regulates anxiety-related, fear-related, and stress-related behaviors. Our results reveal the roles of PCDH10 in the brain and its relationship to relevant psychiatric disorders such as ASD, OCD, and MD.SIGNIFICANCE STATEMENTProtocadherin-10 (PCDH10) encodes a cell adhesion molecule and is implicated in autism spectrum disorder (ASD), obsessive-compulsive disorder (OCD), and major depression (MD). PCDH10 is highly expressed in the basolateral nucleus of the amygdala (BLA). However, the phenotypes of previously published Pcdh10 mutant mice are debatable, and some are possibly because of the nonspecific effects of the LacZ/Neo cassette inserted in the mice. We have generated a new Pcdh10 mutant mouse line without the LacZ/Neo cassette. Using our new mouse line, we reveal the roles of PCDH10 for excitatory synapse development in the BLA. The mutant mice exhibit anxiety-related, fear-related, and stress-related behaviors, which are relevant to ASD, OCD, and MD, suggesting a possible treatment strategy for such psychiatric disorders.

In utero intraocular AAV injection for early gene expression in the developing rodent retina

The visual system is the best system to study activity-dependent sensory circuit development. The connections from the retina to the dorsal lateral geniculate nucleus, the retinogeniculate connections, undergo extensive remodeling during early postnatal life. Thus, techniques that allow the expression of transgenes early in the developing retina are essential to study visual system development. Here, we describe a protocol to express genes-of-interest in the developing mouse retina via in utero intraocular adeno-associated virus injections. For complete details on the use and execution of this protocol, please refer to Yasuda et al. (2021).

Female-specific synaptic dysfunction and cognitive impairment in a mouse model of PCDH19 disorder

Protocadherin-19 (PCDH19) mutations cause early-onset seizures and cognitive impairment. The PCDH19 gene is on the X-chromosome. Unlike most X-linked disorders, PCDH19 mutations affect heterozygous females (PCDH19^HET♀ ) but not hemizygous males (PCDH19^HEMI♂ ); however, the reason why remains to be elucidated. We demonstrate that PCDH19, a cell-adhesion molecule, is enriched at hippocampal mossy fiber synapses. Pcdh19^HET♀ but not Pcdh19^HEMI♂ mice show impaired mossy fiber synaptic structure and physiology. Consistently, Pcdh19^HET♀ but not Pcdh19^HEMI♂ mice exhibit reduced pattern completion and separation abilities, which require mossy fiber synaptic function. Furthermore, PCDH19 appears to interact with N-cadherin at mossy fiber synapses. In Pcdh19^HET♀ conditions, mismatch between PCDH19 and N-cadherin diminishes N-cadherin-dependent signaling and impairs mossy fiber synapse development; N-cadherin overexpression rescues Pcdh19^HET♀ phenotypes. These results reveal previously unknown molecular and cellular mechanisms underlying the female-specific PCDH19 disorder phenotype.

A splicing isoform of GPR56 mediates microglial synaptic refinement via phosphatidylserine binding

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 presynaptic 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+ presynaptic 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.

Optimizing Nervous System-Specific Gene Targeting with Cre Driver Lines: Prevalence of Germline Recombination and Influencing Factors

The Cre-loxP system is invaluable for spatial and temporal control of gene knockout, knockin, and reporter expression in the mouse nervous system. However, we report varying probabilities of unexpected germline recombination in distinct Cre driver lines designed for nervous system-specific recombination. Selective maternal or paternal germline recombination is showcased with sample Cre lines. Collated data reveal germline recombination in over half of 64 commonly used Cre driver lines, in most cases with a parental sex bias related to Cre expression in sperm or oocytes. Slight differences among Cre driver lines utilizing common transcriptional control elements affect germline recombination rates. Specific target loci demonstrated differential recombination; thus, reporters are not reliable proxies for another locus of interest. Similar principles apply to other recombinase systems and other genetically targeted organisms. We hereby draw attention to the prevalence of germline recombination and provide guidelines to inform future research for the neuroscience and broader molecular genetics communities.

CD47 Protects Synapses from Excess Microglia-Mediated Pruning during Development

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 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. In addition, 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. Taken 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.

Tyrosine phosphorylation of the transmembrane protein SIRPα: Sensing synaptic activity and regulating ectodomain cleavage for synapse maturation

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.

Selective Inactivation of Fibroblast Growth Factor 22 (FGF22) in CA3 Pyramidal Neurons Impairs Local Synaptogenesis and Affective Behavior Without Affecting Dentate Neurogenesis

Various growth factors regulate synapse development and neurogenesis, and are essential for brain function. Changes in growth factor signaling are implicated in many neuropsychiatric disorders such as depression, autism and epilepsy. We have previously identified that fibroblast growth factor 22 (FGF22) is critical for excitatory synapse formation in several brain regions including the hippocampus. Mice with a genetic deletion of FGF22 (FGF22 null mice) have fewer excitatory synapses in the hippocampus. We have further found that as a behavioral consequence, FGF22 null mice show a depression-like behavior phenotype such as increased passive stress-coping behavior and anhedonia, without any changes in motor, anxiety, or social cognitive tests, suggesting that FGF22 is specifically important for affective behavior. Thus, addressing the precise roles of FGF22 in the brain will help understand how synaptogenic growth factors regulate affective behavior. In the hippocampus, FGF22 is expressed mainly by CA3 pyramidal neurons, but also by a subset of dentate granule cells. We find that in addition to synapse formation, FGF22 also contributes to neurogenesis in the dentate gyrus: FGF22 null mice show decreased dentate neurogenesis. To understand the cell type-specific roles of FGF22, we generated and analyzed CA3-specific FGF22 knockout mice (FGF22-CA3KO). We show that FGF22-CA3KO mice have reduced excitatory synapses on CA3 pyramidal neurons, but do not show changes in dentate neurogenesis. Behaviorally, FGF22-CA3KO mice still show increased immobility and decreased latency to float in the forced swim test and decreased preference for sucrose in the sucrose preference test, which are suggestive of a depressive-like phenotype similar to FGF22 null mice. These results demonstrate that: (i) CA3-derived FGF22 serves as a target-derived excitatory synaptic organizer in CA3 in vivo; (ii) FGF22 plays important roles in dentate neurogenesis, but CA3-derived FGF22 is not involved in neurogenesis; and (iii) a depression-like phenotype can result from FGF22 inactivation selectively in CA3 pyramidal neurons. Our results link the role of CA3-derived FGF22 in synapse development, and not in neurogenesis, to affective behavior.

Activity-dependent proteolytic cleavage of cell adhesion molecules regulates excitatory synaptic development and function

Activity-dependent remodeling of neuronal connections is critical to nervous system development and function. These processes rely on the ability of synapses to detect neuronal activity and translate it into the appropriate molecular signals. One way to convert neuronal activity into downstream signaling is the proteolytic cleavage of cell adhesion molecules (CAMs). Here we review studies demonstrating the mechanisms by which proteolytic processing of CAMs direct the structural and functional remodeling of excitatory glutamatergic synapses during development and plasticity. Specifically, we examine how extracellular proteolytic cleavage of CAMs switches on or off molecular signals to 1) permit, drive, or restrict synaptic maturation during development and 2) strengthen or weaken synapses during adult plasticity. We will also examine emerging studies linking improper activity-dependent proteolytic processing of CAMs to neurological disorders such as schizophrenia, brain tumors, and Alzheimer's disease. Together these findings suggest that the regulation of activity-dependent proteolytic cleavage of CAMs is vital to proper brain development and lifelong function.

A microRNA negative feedback loop downregulates vesicle transport and inhibits fear memory

The SNARE-mediated vesicular transport pathway plays major roles in synaptic remodeling associated with formation of long-term memories, but the mechanisms that regulate this pathway during memory acquisition are not fully understood. Here we identify miRNAs that are up-regulated in the rodent hippocampus upon contextual fear-conditioning and identify the vesicular transport and synaptogenesis pathways as the major targets of the fear-induced miRNAs. We demonstrate that miR-153, a member of this group, inhibits the expression of key components of the vesicular transport machinery, and down-regulates Glutamate receptor A1 trafficking and neurotransmitter release. MiR-153 expression is specifically induced during LTP induction in hippocampal slices and its knockdown in the hippocampus of adult mice results in enhanced fear memory. Our results suggest that miR-153, and possibly other fear-induced miRNAs, act as components of a negative feedback loop that blocks neuronal hyperactivity at least partly through the inhibition of the vesicular transport pathway.

DOI:http://dx.doi.org/10.7554/eLife.22467.001

Postsynaptic SDC2 induces transsynaptic signaling via FGF22 for bidirectional synaptic formation

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.

Retrograde fibroblast growth factor 22 (FGF22) signaling regulates insulin-like growth factor 2 (IGF2) expression for activity-dependent synapse stabilization in the mammalian brain

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.

Buttressing a balanced brain: Target-derived FGF signaling regulates excitatory/inhibitory tone and adult neurogenesis within the maturating hippocampal network

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.

Deletion of fibroblast growth factor 22 (FGF22) causes a depression-like phenotype in adult mice

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.

Excitability governs neural development in a hippocampal region-specific manner

Neuronal activity, including intrinsic neuronal excitability and synaptic transmission, is an essential regulator of brain development. However, how the intrinsic neuronal excitability of distinct neurons affects their integration into developing circuits remains poorly understood. To investigate this problem, we created several transgenic mouse lines in which intrinsic excitability is suppressed, and the neurons are effectively silenced, in different excitatory neuronal populations of the hippocampus. Here we show that CA1, CA3 and dentate gyrus neurons each have unique responses to suppressed intrinsic excitability during circuit development. Silenced CA1 pyramidal neurons show altered spine development and synaptic transmission after postnatal day 15. By contrast, silenced CA3 pyramidal neurons seem to develop normally. Silenced dentate granule cells develop with input-specific decreases in spine density starting at postnatal day 11; however, a compensatory enhancement of neurotransmitter release onto these neurons maintains normal levels of synaptic activity. The synaptic changes in CA1 and dentate granule neurons are not observed when synaptic transmission, rather than intrinsic excitability, is blocked in these neurons. Thus, our results demonstrate a crucial role for intrinsic neuronal excitability in establishing hippocampal connectivity and reveal that neuronal development in each hippocampal region is distinctly regulated by excitability.

Distinct sets of FGF receptors sculpt excitatory and inhibitory synaptogenesis

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.

FGF22 signaling regulates synapse formation during post-injury remodeling of the spinal cord

The remodeling of axonal circuits after injury requires the formation of new synaptic contacts to enable functional recovery. Which molecular signals initiate such axonal and synaptic reorganisation in the adult central nervous system is currently unknown. Here, we identify FGF22 as a key regulator of circuit remodeling in the injured spinal cord. We show that FGF22 is produced by spinal relay neurons, while its main receptors FGFR1 and FGFR2 are expressed by cortical projection neurons. FGF22 deficiency or the targeted deletion of FGFR1 and FGFR2 in the hindlimb motor cortex limits the formation of new synapses between corticospinal collaterals and relay neurons, delays their molecular maturation, and impedes functional recovery in a mouse model of spinal cord injury. These results establish FGF22 as a synaptogenic mediator in the adult nervous system and a crucial regulator of synapse formation and maturation during post-injury remodeling in the spinal cord.

Selective synaptic targeting of the excitatory and inhibitory presynaptic organizers FGF22 and FGF7

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.

5-HT1A receptor-mediated phosphorylation of extracellular signal-regulated kinases (ERK1/2) is modulated by regulator of G protein signaling protein 19

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.

The best-laid plans go oft awry: synaptogenic growth factor signaling in neuropsychiatric disease

Growth factors play important roles in synapse formation. Mouse models of neuropsychiatric diseases suggest that defects in synaptogenic growth factors, their receptors, and signaling pathways can lead to disordered neural development and various behavioral phenotypes, including anxiety, memory problems, and social deficits. Genetic association studies in humans have found evidence for similar relationships between growth factor signaling pathways and neuropsychiatric phenotypes. Accumulating data suggest that dysfunction in neuronal circuitry, caused by defects in growth factor-mediated synapse formation, contributes to the susceptibility to multiple neuropsychiatric diseases, including epilepsy, autism, and disorders of thought and mood (e.g., schizophrenia and bipolar disorder, respectively). In this review, we will focus on how specific synaptogenic growth factors and their downstream signaling pathways might be involved in the development of neuropsychiatric diseases.

Synapse maturation by activity-dependent ectodomain shedding of SIRPα

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 SIRPα mediates presynaptic maturation. Two target-derived molecules, FGF22 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 CaM kinase, matrix metalloproteinases, and the presynaptic receptor CD47. Finally, SIRPα-dependent synapse maturation has significant impacts on synaptic function and plasticity. Thus, ectodomain shedding of SIRPα is an activity-dependent trans-synaptic mechanism for the maturation of functional synapses.

Suppression of epileptogenesis-associated changes in response to seizures in FGF22-deficient mice

In the developing hippocampus, fibroblast growth factor (FGF) 22 promotes the formation of excitatory presynaptic terminals. Remarkably, FGF22 knockout (KO) mice show resistance to generalized seizures in adults as assessed by chemical kindling, a model that is widely used to study epileptogenesis (Terauchi et al., 2010). Repeated injections of low dose pentylenetetrazol (PTZ) induce generalized seizures ("kindled") in wild type (WT) mice. With additional PTZ injections, FGF22KO mice do show moderate seizures, but they do not kindle. Thus, analyses of how FGF22 impacts seizure susceptibility will contribute to the better understanding of the molecular and cellular mechanisms of epileptogenesis. To decipher the roles of FGF22 in the seizure phenotype, we examine four pathophysiological changes in the hippocampus associated with epileptogenesis: enhancement of dentate neurogenesis, hilar ectopic dentate granule cells (DGCs), increase in hilar cell death, and formation of mossy fiber sprouting (MFS). Dentate neurogenesis is enhanced, hilar ectopic DGCs appeared, and hilar cell death is increased in PTZ-kindled WT mice relative to PBS-injected WT mice. Even in WT mice with fewer PTZ injections, which showed only mild seizures (so were not kindled), neurogenesis, hilar ectopic DGCs, and hilar cell death are increased, suggesting that mild seizures are enough to induce these changes in WT mice. In contrast, PTZ-injected FGF22KO mice do not show these changes despite having moderate seizures: neurogenesis is rather suppressed, hilar ectopic DGCs do not appear, and hilar cell death is unchanged in PTZ-injected FGF22KO mice relative to PBS-injected FGF22KO mice. These results indicate that FGF22 plays important roles in controlling neurogenesis, ectopic migration of DGCs, and hilar cell death after seizures, which may contribute to the generalized seizure-resistant phenotype of FGF22KO mice and suggests a possibility that inhibition of FGF22 may alleviate epileptogenesis.

Specific sets of intrinsic and extrinsic factors drive excitatory and inhibitory circuit formation

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.

Fibroblast growth factor 22 contributes to the development of retinal nerve terminals in the dorsal lateral geniculate nucleus

At least three forms of signaling between pre- and postsynaptic partners are necessary during synapse formation. First, “targeting” signals instruct presynaptic axons to recognize and adhere to the correct portion of a postsynaptic target cell. Second, trans-synaptic “organizing” signals induce differentiation in their synaptic partner so that each side of the synapse is specialized for synaptic transmission. Finally, in many regions of the nervous system an excess of synapses are initially formed, therefore “refinement” signals must either stabilize or destabilize the synapse to reinforce or eliminate connections, respectively. Because of both their importance in processing visual information and their accessibility, retinogeniculate synapses have served as a model for studying synaptic development. Molecular signals that drive retinogeniculate “targeting” and “refinement” have been identified, however, little is known about what “organizing” cues are necessary for the differentiation of retinal axons into presynaptic terminals. To identify such “organizing” cues, we used microarray analysis to assess whether any target-derived “synaptic organizers” were enriched in the mouse dorsal lateral geniculate nucleus (dLGN) during retinogeniculate synapse formation. One candidate “organizing” molecule enriched in perinatal dLGN was FGF22, a secreted cue that induces the formation of excitatory nerve terminals in muscle, hippocampus, and cerebellum. In FGF22 knockout mice, the development of retinal terminals in dLGN was impaired. Thus, FGF22 is an important “organizing” cue for the timely development of retinogeniculate synapses.

Multiple forms of activity-dependent competition refine hippocampal circuits in vivo

Efficient 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.

Orchestrating the synaptic network by tyrosine phosphorylation signalling

The establishment of a functional brain requires coordinated and stereotyped formation of synapses between neurons. For this, trans-synaptic molecular cues (synaptic organizers) are exchanged between a neuron and its target to organize appropriate synapses. The understanding of signalling mechanisms by which such synaptic organizers lead to synapse formation is just being elucidated. However, recent studies revealed that some of these cues act through receptor protein tyrosine kinases (RPTKs) or phosphatases (RPTPs). Synaptogenic RPTKs and RPTPs pattern synaptic network through affecting local protein-protein binding dynamics, changing the phosphorylation state of signalling cascades, or promoting gene expression. Each RPTK or RPTP has distinct roles in synapse formation, serving at different synapses or showing differential synaptogenic effects. Thus, tyrosine phosphorylation signalling plays critical roles in building the orchestrated synaptic circuitry in the brain.

NMDAR2B tyrosine phosphorylation regulates anxiety-like behavior and CRF expression in the amygdala

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.

Secreted factors as synaptic organizers

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.

Distinct FGFs promote differentiation of excitatory and inhibitory synapses

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.

Involvement of NMDAR2A tyrosine phosphorylation in depression-related behaviour

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.

Weaving the neuronal net with target-derived fibroblast growth factors

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.

Signal regulatory proteins (SIRPS) are secreted presynaptic organizing molecules

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.

Regulation of dendritic spine morphology by an NMDA receptor-associated Rho GTPase-activating protein, p250GAP

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.

Distinct target-derived signals organize formation, maturation, and maintenance of motor nerve terminals

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.

Receptor specificity of the fibroblast growth factor family. The complete mammalian FGF family

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.

Seeking long-term relationship: axon and target communicate to organize synaptic differentiation

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.

NR2B tyrosine phosphorylation modulates fear learning as well as amygdaloid synaptic plasticity

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.

FGF22 and its close relatives are presynaptic organizing molecules in the mammalian brain

Target-derived cues promote local differentiation of axons into nerve terminals at sites of synaptic contact. Using clustering of synaptic vesicles in cultured neurons as an assay, we purified putative target-derived presynaptic organizing molecules from mouse brain and identified FGF22 as a major active species. FGF7 and FGF10, the closest relatives of FGF22, share this activity; other FGFs have distinct effects. FGF22 is expressed by cerebellar granule cells during the period when they receive synapses. Its receptor, FGFR2, is expressed by pontine and vestibular neurons when their axons (mossy fibers) are making synapses on granule cells. Neutralization of FGF7, -10, and -22 inhibits presynaptic differentiation of mossy fibers at sites of contact with granule cells in vivo. Inactivation of FGFR2 has similar effects. These results indicate that FGF22 and its relatives are presynaptic organizing molecules in the mammalian brain and suggest new functions for this family of signaling molecules.

Impairment of N-methyl-D-aspartate receptor-controlled motor activity in LYN-deficient mice

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.

p250GAP, a novel brain-enriched GTPase-activating protein for Rho family GTPases, is involved in the N-methyl-d-aspartate receptor signaling

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 GluRepsilon2 (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 GluRepsilon2 (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.

Mice lacking a transcriptional corepressor Tob are predisposed to cancer

tob is a member of antiproliferative family genes. Mice lacking tob are prone to spontaneous formation of tumors. The occurrence rate of diethylnitrosamine-induced liver tumors is higher in tob(-/-) mice than in wild-type mice. tob(-/-)p53(-/-) mice show accelerated tumor formation in comparison with single null mice. Expression of cyclin D1 mRNA is increased in the absence of Tob and is reduced by Tob. Tob acts as a transcriptional corepressor and suppresses the cyclin D1 promoter activity through an interaction with histone deacetylase. Levels of tob mRNA are often decreased in human cancers, implicating tob in cancer development.

Tob proteins enhance inhibitory Smad-receptor interactions to repress BMP signaling

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.

Heteromer formation of delta2 glutamate receptors with AMPA or kainate receptors

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.