A preformed complex of postsynaptic proteins is involved in excitatory synapse development

Activity-driven mobilization of post-synaptic proteins

Synapses established during central nervous system development can be modified through synapse elimination and formation. These processes are, in part, activity dependent and require regulated trafficking of post-synaptic components. Here, we investigate the activity-driven remodeling of cultured rat hippocampal neurons at 14 days in vitro, focusing on the post-synaptic proteins PSD-95, Shank, neuroligin (NL)1 and actin. Using live imaging and photoconductive stimulation, we found that high-frequency activity altered the trajectory, but not velocity, of PSD-95-GFP and Shank-YFP clusters, whereas it reduced the speed and increased the number of NL1 clusters. Actin-CFP reorganized into puncta following activity and approximately 50% of new puncta colocalized with NL1 clusters. Actin reorganization was enhanced by the overexpression of NL1 and decreased by the expression of an NL1 mutant, NL1-R473C. These results demonstrate activity-dependent changes that may result in the formation of new post-synaptic sites and suggest that NL1 modulates actin reorganization. The results also suggest that a common mechanism underlies both the developmental and activity-dependent remodeling of excitatory synapses.

Altered neuroligin expression is involved in social deficits in a mouse model of the fragile X syndrome

The fragile X syndrome (FXS) is the most common form of inherited mental retardation. Caused by a transcriptional silencing of the fragile X mental retardation protein (FMRP), a mRNA binding protein itself, misregulated translation is thought to be the leading cause of the fragile X syndrome. Interestingly, recent results indicated several neuroligin interacting proteins to be affected by this misregulation, including neurexin1 and PSD95, which have also been implicated in autism spectrum disorders. Using co-immunoprecipitation assays and RT-PCR, FMRP is shown to interact with neuroligin1- and 2-mRNA, while no interaction with neuroligin3-mRNA is observed. In line with FMRP's role in translation regulation, Western blot as well as immunohistochemistry analysis reveal changes in protein expression levels suggesting impaired synaptic function. As increasing evidence indicates neuroligin expression to be critical for synapse maturation and function, consequences of impaired neuroligin1 expression in FXS are assessed by overexpressing HA-neuroligin1 in FMR1-/- mice, a model for FXS. Behavioural assessments demonstrate that enhanced neuroligin1 expression improves social behaviour in FMR1-/- mice, whereas no positive effect on learning and memory is seen. These results provide for the first time evidence for an involvement of a neuroligin-neurexin protein network in core symptoms of FXS.

Postsynaptic scaffolding molecules modulate the localization of neuroligins

Previous work has shown an important role for neuroligins in promoting the formation of synaptic connections in cultured cells. Although neuroligins enhance both excitatory and inhibitory synapse formation, individual neuroligin isoforms have been shown to preferentially localize to either glutamatergic or GABAergic synapses. Current evidence points to an important role for both the extracellular and intracellular domains of neuroligins in their synaptic localization. Although postsynaptic density protein 95 (PSD-95) has been shown to be involved in the recruitment of neuroligin 1 to excitatory synapses, the localization of neuroligin 2 (NL2) and neuroligin 3 (NL3) to excitatory and inhibitory synapses is less well defined. We assessed the roles of gephyrin and PSD-95, postsynaptic scaffolding molecules exclusively localized to inhibitory and excitatory synapses, respectively, in localizing NL2 and NL3 in primary neuronal cultures. We demonstrate that knockdown of gephyrin results in a significant shift of NL2 from inhibitory to excitatory synaptic contacts, while knockdown of PSD-95 leads to a partial shift of NL2 and NL3 from excitatory to inhibitory contacts. Furthermore, analysis of specific domain deletions within the C-terminal, intracellular domain of NL2 reveals that the region between amino acids 716 and 782 is required for the normal synaptic clustering of this protein. Together, these data suggest that intracellular mechanisms are involved in the targeting of different neuroligin family members to synapses (216).

Rapid assembly of functional presynaptic boutons triggered by adhesive contacts

CNS synapse assembly typically follows after stable contacts between "appropriate" axonal and dendritic membranes are made. We show that presynaptic boutons selectively form de novo following neuronal fiber adhesion to beads coated with poly-d-lysine (PDL), an artificial cationic polypeptide. As demonstrated by atomic force and live confocal microscopy, functional presynaptic boutons self-assemble as rapidly as 1 h after bead contact, and are found to contain a variety of proteins characteristic of presynaptic endings. Interestingly, presynaptic compartment assembly does not depend on the presence of a biological postsynaptic membrane surface. Rather, heparan sulfate proteoglycans, including syndecan-2, as well as others possibly adsorbed onto the bead matrix or expressed on the axon surface, are required for assembly to proceed by a mechanism dependent on the dynamic reorganization of F-actin. Our results indicate that certain (but not all) nonspecific cationic molecules like PDL, with presumably electrostatically mediated adhesive properties, can effectively bypass cognate and natural postsynaptic ligands to trigger presynaptic assembly in the absence of specific target recognition. In contrast, we find that postsynaptic compartment assembly depends on the prior presence of a mature presynaptic ending.

Rapid and long-lasting increase in sites for synapse assembly during late-phase potentiation in rat hippocampal neurons

Long-term potentiation in hippocampal neurons has stages that correspond to the stages of learning and memory. Early-phase (10–30 min) potentiation is accompanied by rapid increases in clusters or puncta of presynaptic and postsynaptic proteins, which depend on actin polymerization but not on protein synthesis. We have now examined changes in pre- and postsynaptic puncta and structures during glutamate-induced late-phase (3 hr) potentiation in cultured hippocampal neurons. We find that (1) the potentiation is accompanied by long-lasting maintenance of the increases in puncta, which depends on protein synthesis, (2) most of the puncta and synaptic structures are very dynamic, continually assembling and disassembling at sites that are more stable than the puncta or structures themselves, (3) the increase in presynaptic puncta appears to be due to both rapid and more gradual increases in the number of sites where the puncta may form, and also to the stabilization of existing puncta, (4) under control conditions, puncta of postsynaptic proteins behave similarly to puncta of presynaptic proteins and share sites with them, and (5) the increase in presynaptic puncta is accompanied by a similar increase in presumably presynaptic structures, which may form at distinct as well as shared sites. The new sites could contribute to the transition between the early and late phase mechanisms of plasticity by serving as seeds for the formation and maintenance of new synapses, thus acting as local “tags” for protein synthesis-dependent synaptic growth during late-phase plasticity.

Synaptogenesis of hippocampal neurons in primary cell culture

Hippocampal neurons in dissociated cell culture are one of the most extensively used model systems in the field of molecular and cellular neurobiology. Only limited data are however available on the normal time frame of synaptogenesis, synapse number and ultrastructure of excitatory synapses during early development in culture. Therefore, we analyzed the synaptic ultrastructure and morphology and the localization of presynaptic (Bassoon) and postsynaptic (ProSAP1/Shank2) marker proteins in cultures established from rat embryos at embryonic day 19, after 3, 7, 10, 14, and 21 days in culture. First excitatory synapses were identified at day 7 with a clearly defined postsynaptic density and presynaptically localized synaptic vesicles. Mature synapses on dendritic spines were seen from day 10 onward, and the number of synapses steeply increased in the third week. Fenestrated or multiple synapses were found after 14 or 21 days, respectively. So-called dense-core vesicles, responsible for the transport of proteins to the active zone of the presynaptic specialization, were seen on cultivation day 3 and 7 and could be detected in axons and especially in the presynaptic subcompartments. The expression and localization of the presynaptic protein Bassoon and of the postsynaptic molecule ProSAP1/Shank2 was found to correlate nicely with the ultrastructural results. This regular pattern of development and maturation of excitatory synapses in hippocampal culture starting from day 7 in culture should ease the comparison of synapse number and morphology of synaptic contacts in this widely used model system.

The origin and evolution of synapses

Understanding the evolutionary origins of behaviour is a central aim in the study of biology and may lead to insights into human disorders. Synaptic transmission is observed in a wide range of invertebrate and vertebrate organisms and underlies their behaviour. Proteomic studies of the molecular components of the highly complex mammalian postsynaptic machinery point to an ancestral molecular machinery in unicellular organisms--the protosynapse--that existed before the evolution of metazoans and neurons, and hence challenges existing views on the origins of the brain. The phylogeny of the molecular components of the synapse provides a new model for studying synapse diversity and complexity, and their implications for brain evolution.

Midline signalling systems direct the formation of a neural map by dendritic targeting in the Drosophila motor system

A fundamental strategy for organising connections in the nervous system is the formation of neural maps. Map formation has been most intensively studied in sensory systems where the central arrangement of axon terminals reflects the distribution of sensory neuron cell bodies in the periphery or the sensory modality. This straightforward link between anatomy and function has facilitated tremendous progress in identifying cellular and molecular mechanisms that underpin map development. Much less is known about the way in which networks that underlie locomotion are organised. We recently showed that in the Drosophila embryo, dendrites of motorneurons form a neural map, being arranged topographically in the antero-posterior axis to represent the distribution of their target muscles in the periphery. However, the way in which a dendritic myotopic map forms has not been resolved and whether postsynaptic dendrites are involved in establishing sets of connections has been relatively little explored. In this study, we show that motorneurons also form a myotopic map in a second neuropile axis, with respect to the ventral midline, and they achieve this by targeting their dendrites to distinct medio-lateral territories. We demonstrate that this map is "hard-wired"; that is, it forms in the absence of excitatory synaptic inputs or when presynaptic terminals have been displaced. We show that the midline signalling systems Slit/Robo and Netrin/Frazzled are the main molecular mechanisms that underlie dendritic targeting with respect to the midline. Robo and Frazzled are required cell-autonomously in motorneurons and the balance of their opposite actions determines the dendritic target territory. A quantitative analysis shows that dendritic morphology emerges as guidance cue receptors determine the distribution of the available dendrites, whose total length and branching frequency are specified by other cell intrinsic programmes. Our results suggest that the formation of dendritic myotopic maps in response to midline guidance cues may be a conserved strategy for organising connections in motor systems. We further propose that sets of connections may be specified, at least to a degree, by global patterning systems that deliver pre- and postsynaptic partner terminals to common "meeting regions."

Dendrites of cerebellar granule cells correctly recognize their target axons for synaptogenesis in vitro

Neural circuits are generated by precisely ordered synaptic connections among neurons, and this process is thought to rely on the ability of neurons to recognize specific partners. However, it is also known that neurons promiscuously form synapses with nonspecific partners, in particular when cultured in vitro, causing controversies about neural recognition mechanisms. Here we reexamined whether neurons can or cannot select particular partners in vitro. In the cerebellum, granule cell (GC) dendrites form synaptic connections specifically with mossy fibers, but not with climbing fibers. We cocultured GC neurons with pontine or inferior olivary axons, the major sources for mossy and climbing fibers, respectively, as well as with hippocampal axons as a control. The GC neurons formed synapses with pontine axons predominantly at the distal ends of their dendrites, reproducing the characteristic morphology of their synapses observed in vivo, whereas they failed to do so when combined with other axons. In the latter case, synaptic proteins could accumulate between axons and dendrites, but these synapses were randomly distributed throughout the contact sites, and also their synaptic vesicle recycling was anomalous. These observations suggest that GC dendrites can select their authentic partners for synaptogenesis even in vitro, forming the synapses with a GC-specific nature only with them.

The emerging role of synaptic cell-adhesion pathways in the pathogenesis of autism spectrum disorders

Recent advances in genetics and genomics have unveiled numerous cases of autism spectrum disorders (ASDs) associated with rare, causal genetic variations. These findings support a novel view of ASDs in which many independent, individually rare genetic variants, each associated with a very high relative risk, together explain a large proportion of ASDs. Although these rare variants impact diverse pathways, there is accumulating evidence that synaptic pathways, including those involving synaptic cell adhesion, are disrupted in some subjects with ASD. These findings provide insights into the pathogenesis of ASDs and enable the development of model systems with construct validity for specific causes of ASDs. In several neurodevelopmental disorders frequently associated with ASD, including fragile X syndrome, Rett syndrome and tuberous sclerosis, animal models have led to the development of new therapeutic approaches, giving rise to optimism with other causes of ASDs.

Dendritic spine formation and stabilization

Formation, elimination and remodeling of excitatory synapses on dendritic spines represent a continuous process that shapes the organization of synaptic networks during development. The molecular mechanisms controlling dendritic spine formation and stabilization therefore critically determine the rules of network selectivity. Recent studies have identified new molecules, such as Ephrins and Telencephalin that regulate filopodia motility and their transformation into dendritic spines. Trans-synaptic signaling involving nitric oxide, protease, adhesion molecules and Rho GTPases further controls contact formation or the structural remodeling of spines and their stability. Evidence also suggests that activity and induction of plasticity participate to the selection of persistent spines. Together these new data provide a better understanding of the mechanisms, speed and steps leading to the establishment of a stable excitatory synapse.

Wnt-5a/JNK signaling promotes the clustering of PSD-95 in hippocampal neurons

During the formation of synapses, specific regions of pre- and postsynaptic cells associate to form a single functional transmission unit. In this process, synaptogenic factors are necessary to modulate pre- and postsynaptic differentiation. In mammals, different Wnt ligands operate through canonical and non-canonical Wnt pathways, and their precise functions to coordinate synapse structure and function in the mature central nervous system are still largely unknown. Here, we studied the effect of different Wnt ligands on postsynaptic organization. We found that Wnt-5a induces short term changes in the clustering of PSD-95, without affecting its total levels. Wnt-5a promotes the recruitment of PSD-95 from a diffuse dendritic cytoplasmic pool to form new PSD-95 clusters in dendritic spines. Moreover, Wnt-5a acting as a non-canonical ligand regulates PSD-95 distribution through a JNK-dependent signaling pathway, as demonstrated by using the TAT-TI-JIP peptide in mature hippocampal neurons. Finally, using adult rat hippocampal slices, we found that Wnt-5a modulates glutamatergic synaptic transmission through a postsynaptic mechanism. Our studies indicate that the Wnt-5a/JNK pathway modulates the postsynaptic region of mammalian synapse directing the clustering and distribution of the physiologically relevant scaffold protein, PSD-95.

Shank 2 expression coincides with neuronal differentiation in the developing retina

The retinal activity for vision requires a precise synaptic connectivity. Shank proteins at postsynaptic sites of excitatory synapses play roles in signal transmission into the postsynaptic neuron. However, the correlation of Shank 2 expression with neuronal differentiation in the developing retina remains to be elucidated regardless of previous evidences of Shank 2 expression in retina. Herein, we demonstrated that with progression of development, Shank 2 is initially detected in the inner plexiform layer at P2, and then intensively detected in inner plexiform layer, outer plexiform layer, and ganglion cell layer at P14, which was closely colocalized to the neurofilament expression. Shank 2 was, however, not colocalized with glial fibrillary acidic protein. Shank 2 expression was increased in the differentiated retinoblastoma cells, which was mediated by ERK 1/2 activation. Moreover, Shank 2 expression was colocalized with neurofilament at the dendritic region of cells. In conclusion, our data suggests that Shank 2 is expressed in the neurons of the developing retina and could play a critical role in the neuronal differentiation of the developing retina.

Scribble interacts with beta-catenin to localize synaptic vesicles to synapses

An understanding of how synaptic vesicles are recruited to and maintained at presynaptic compartments is required to discern the molecular mechanisms underlying presynaptic assembly and plasticity. We have previously demonstrated that cadherin-beta-catenin complexes cluster synaptic vesicles at presynaptic sites. Here we show that scribble interacts with the cadherin-beta-catenin complex to coordinate vesicle localization. Scribble and beta-catenin are colocalized at synapses and can be coimmunoprecipitated from neuronal lysates, indicating an interaction between scribble and beta-catenin at the synapse. Using an RNA interference approach, we demonstrate that scribble is important for the clustering of synaptic vesicles at synapses. Indeed, in scribble knockdown cells, there is a diffuse distribution of synaptic vesicles along the axon, and a deficit in vesicle recycling. Despite this, synapse number and the distribution of the presynaptic active zone protein, bassoon, remain unchanged. These effects largely phenocopy those observed after ablation of beta-catenin. In addition, we show that loss of beta-catenin disrupts scribble localization in primary neurons but that the localization of beta-catenin is not dependent on scribble. Our data supports a model by which scribble functions downstream of beta-catenin to cluster synaptic vesicles at developing synapses.

Neuroligin1: a cell adhesion molecule that recruits PSD-95 and NMDA receptors by distinct mechanisms during synaptogenesis

BACKGROUND

The cell adhesion molecule pair neuroligin1 (Nlg1) and beta-neurexin (beta-NRX) is a powerful inducer of postsynaptic differentiation of glutamatergic synapses in vitro. Because Nlg1 induces accumulation of two essential components of the postsynaptic density (PSD) - PSD-95 and NMDA receptors (NMDARs) - and can physically bind PSD-95 and NMDARs at mature synapses, it has been proposed that Nlg1 recruits NMDARs to synapses through its interaction with PSD-95. However, PSD-95 and NMDARs are recruited to nascent synapses independently and it is not known if Nlg1 accumulates at synapses before these PSD proteins. Here, we investigate how a single type of cell adhesion molecule can recruit multiple types of synaptic proteins to new synapses with distinct mechanisms and time courses.

RESULTS

Nlg1 was present in young cortical neurons in two distinct pools before synaptogenesis, diffuse and clustered. Time-lapse imaging revealed that the diffuse Nlg1 aggregated at, and the clustered Nlg1 moved to, sites of axodendritic contact with a rapid time course. Using a patching assay that artificially induced clusters of Nlg, the time course and mechanisms of recruitment of PSD-95 and NMDARs to those Nlg clusters were characterized. Patching Nlg induced clustering of PSD-95 via a slow palmitoylation-dependent step. In contrast, NMDARs directly associated with clusters of Nlg1 during trafficking. Nlg1 and NMDARs were highly colocalized in dendrites before synaptogenesis and they became enriched with a similar time course at synapses with age. Patching of Nlg1 dramatically decreased the mobility of NMDAR transport packets. Finally, Nlg1 was biochemically associated with NMDAR transport packets, presumably through binding of NMDARs to MAGUK proteins that, in turn, bind Nlg1. This interaction was essential for colocalization and co-transport of Nlg1 with NMDARs.

CONCLUSION

Our results suggest that axodendritic contact leads to rapid accumulation of Nlg1, recruitment of NMDARs co-transported with Nlg1 soon thereafter, followed by a slower, independent recruitment of PSD-95 to those nascent synapses.

Assembling the presynaptic active zone

Rapid neurotransmission depends on the structural and functional integrity of synaptic connections. How synapses assemble is currently being intensely investigated to help our understanding of neuronal development and synaptic plasticity. Here we focus on the assembly of the presynaptic active zone, which regulates the synaptic vesicle exo/endo-cycle and is characterized by ultrastructural specializations and large scaffold proteins. While genetic and biochemical studies from rodents, Caenorhabditis elegans and Drosophila have started to identify proteins organizing active zone assembly, drawing a coherent picture remains challenging, with genetically established hierarchies and protein-protein interactions still to be placed into spatio-temporal and functional context. Recent advances in light and electron microscopy, together with in vivo imaging of protein traffic, will help to tackle this challenge.

Trophic factor-induced intracellular calcium oscillations are required for the expression of postsynaptic acetylcholine receptors during synapse formation between Lymnaea neurons

Nervous system functions in all animals rely upon synaptic connectivity that is established during early development. Whereas cell-cell signaling plays a critical role in establishing synapse specificity, the involvement of extrinsic growth factors cannot, however, be undermined. We have previously demonstrated that trophic factors are required for excitatory but not inhibitory synapse formation between Lymnaea neurons. Moreover, in the absence of trophic factors, neurons from a number of species establish inappropriate inhibitory synapses, which can, however, be corrected by the addition of trophic factors. The precise site of trophic factor actions (presynaptic versus postsynaptic) and the underlying mechanisms remain, however, undefined. Here, we provide the first direct evidence that the trophic factor-mediated excitatory synapse formation involves activity-induced calcium (Ca(2+)) oscillations in the postsynaptic left pedal dorsal 1 (LPeD1) but not the presynaptic visceral dorsal 4 (VD4, cholinergic) neuron. These oscillations involved Ca(2+) influx through voltage-gated Ca(2+) channels and required receptor tyrosine kinase activity which was essential for the expression of excitatory, nicotinic acetylcholine receptors in the postsynaptic cell during synapse formation. We also demonstrate that selectively blocking the electrical activity presynaptically did not perturb trophic factor-induced synapse formation between the paired cells, whereas hyperpolarizing the postsynaptic cell prevented appropriate synaptogenesis between VD4 and LPeD1 cells. Together, our data underscore the importance of extrinsic trophic factors in regulating the electrical activity of the postsynaptic but not the presynaptic cell and that the resulting Ca(2+) oscillations are essential for the expression of postsynaptic receptors during specific synapse formation.

GABAergic synapses are formed without the involvement of dendritic protrusions

Synaptogenesis and the role of dendritic protrusions in this process are well studied in glutamatergic synapses. Much less is known about the formation of GABAergic synapses, which are located predominantly on the dendritic shaft. We used genetically labeled interneurons in mature hippocampal slice cultures and two-photon laser-scanning microscopy to examine contact formation between GABAergic axons and the dendrites of CA1 pyramidal cells. Dendritic protrusions distinguished and selected between glutamatergic and GABAergic boutons. In contrast with contacts with glutamatergic boutons, which can be long lasting, the contacts of dendritic protrusions with GABAergic boutons were always short lived. Similarly, the contacts made by GABAergic axonal protrusions were always transient. New putative GABAergic synapses were formed exclusively by new boutons appearing at pre-existing axon-dendrite crossings without the involvement of any dendritic or axonal protrusions. These findings imply that fundamentally different mechanisms underlie the generation of GABAergic and glutamatergic synapses.

AMPA glutamate receptor subunits 1 and 2 regulate dendrite complexity and spine motility in neurons of the developing neocortex

Within neurons of several regions of the CNS, mature dendrite architecture is attained via extensive reorganization of arbor during the developmental period. Since dendrite morphology determines the firing patterns of the neuron, morphological refinement of dendritic arbor may have important implications for mature network activity. In the neocortex, a region of brain that is sensitive to activity-dependent structural rearrangement of dendritic arbor, the proportion of AMPA receptors increases over the developmental period. However, it is unclear whether changes in AMPA receptor expression contribute to maturation of dendritic architecture. To determine the effects of increasing AMPA receptor expression on dendrite morphology and connectivity within the neocortex, and to determine whether these effects are dependent on specific AMPA receptor subunits, we overexpressed the AMPA glutamate receptor subunit 1 (GluR1) and glutamate receptor subunit 2 (GluR2) in cultured rat neocortical neurons at the time that AMPA receptors would normally be incorporated into synapses. Following expression of GluR1 or GluR2 we observed increases in the length and complexity of dendritic arbor of cortical neurons, and a concurrent reduction in motility of spines. In addition, expression of either subunit was associated with an increased density of excitatory postsynaptic puncta. These results suggest that AMPA receptor expression is an important determinant of dendrite morphology and connectivity in neocortical neurons, and further, that contrary to other regions of the CNS, the effects of AMPA receptors on dendrite morphology are not subunit-specific.

PSD-95 promotes synaptogenesis and multiinnervated spine formation through nitric oxide signaling

Postsynaptic density 95 (PSD-95) is an important regulator of synaptic structure and plasticity. However, its contribution to synapse formation and organization remains unclear. Using a combined electron microscopic, genetic, and pharmacological approach, we uncover a new mechanism through which PSD-95 regulates synaptogenesis. We find that PSD-95 overexpression affected spine morphology but also promoted the formation of multiinnervated spines (MISs) contacted by up to seven presynaptic terminals. The formation of multiple contacts was specifically prevented by deletion of the PDZ(2) domain of PSD-95, which interacts with nitric oxide (NO) synthase (NOS). Similarly, PSD-95 overexpression combined with small interfering RNA-mediated down-regulation or the pharmacological blockade of NOS prevented axon differentiation into varicosities and multisynapse formation. Conversely, treatment of hippocampal slices with an NO donor or cyclic guanosine monophosphate analogue induced MISs. NOS blockade also reduced spine and synapse density in developing hippocampal cultures. These results indicate that the postsynaptic site, through an NOS-PSD-95 interaction and NO signaling, promotes synapse formation with nearby axons.

Synapse formation in developing neural circuits

The nervous system consists of hundreds of billions of neurons interconnected into the functional neural networks that underlie behaviors. The capacity of a neuron to innervate and function within a network is mediated via specialized cell junctions known as synapses. Synapses are macromolecular structures that regulate intercellular communication in the nervous system, and are the main gatekeepers of information flow within neural networks. Where and when synapses form determines the connectivity and functionality of neural networks. Therefore, our knowledge of how synapse formation is regulated is critical to our understanding of the nervous system and how it goes awry in neurological disorders. Synapse formation involves pairing of the pre- and postsynaptic partners at a specific neurospatial coordinate. The specificity of synapse formation requires the precise execution of multiple developmental events, including cell fate specification, cell migration, axon guidance, dendritic growth, synaptic target selection, and synaptogenesis (Juttner and Rathjen in Cell. Mol. Life Sci. 62:2811, 2005; Salie et al., in Neuron 45:189, 2005; Waites et al., in Annu. Rev. Neurosci. 28:251, 2005). Remarkably, during the development of the vertebrate nervous system, these developmental processes occur almost simultaneously in billions of neurons, resulting in the formation of trillions of synapses. How this remarkable specificity is orchestrated during development is one of the outstanding questions in the field of neurobiology, and the focus of discussion of this chapter. We center the discussion of this chapter on the early developmental events that orchestrate the process of synaptogenesis prior to activity-dependent mechanisms. We have therefore limited the discussion of important activity-dependent synaptogenic events, which are discussed in other chapters of this book. Moreover, our discussion is biased toward lessons we have learned from invertebrate systems, in particular from C. elegans and Drosophila. We did so to complement the discussions from other chapters in this book, which focus on the important findings that have recently emerged from the vertebrate literature. The chapter begins with a brief history of the field of synaptic biology. This serves as a backdrop to introduce some of the historically outstanding questions of synaptic development that have eluded us during the past century, and which are the focus of this review. We then discuss some general features of synaptic structure as it relates to its function. In particular, we will highlight evolutionarily conserved traits shared by all synaptic structures, and how these features have helped optimize these ancient cellular junctions for interneural communication. We then discuss the regulatory signals that orchestrate the precise assembly of these conserved macromolecular structures. This discussion will be framed in the context of the neurodevelopmental process. Specifically, much of our discussion will focus on how the seemingly disparate developmental processes are intimately linked at a molecular level, and how this relationship might be crucial in the developmental orchestration of circuit assembly. We hope that the discussion of the multifunctional cues that direct circuit development provides a conceptual framework into understanding how, with a limited set of signaling molecules, precise neural wiring can be coordinated between synaptic partners.

Multiple cell adhesion molecules shaping a complex nicotinic synapse on neurons

Neuroligin, SynCAM, and L1-CAM are cell adhesion molecules with synaptogenic roles in glutamatergic pathways. We show here that SynCAM is expressed in the chick ciliary ganglion, embedded in a nicotinic pathway, and, as shown previously for neuroligin and L1-CAM, acts transcellularly to promote synaptic maturation on the neurons in culture. Moreover, we show that electroporation of chick embryos with dominant negative constructs disrupting any of the three molecules in vivo reduces the total amount of presynaptic SV2 overlaying the neurons expressing the constructs. Only disruption of L1-CAM and neuroligin, however, reduces the number of SV2 puncta specifically overlaying nicotinic receptor clusters. Disrupting L1-CAM and neuroligin together produces no additional decrement, indicating that they act on the same subset of synapses. SynCAM may affect synaptic maturation rather than synapse formation. The results indicate that individual neurons can express multiple synaptogenic molecules with different effects on the same class of nicotinic synapses.

Structural plasticity with preserved topology in the postsynaptic protein network

The size, shape, and molecular arrangement of the postsynaptic density (PSD) determine the function of excitatory synapses in the brain. Here, we directly measured the internal dynamics of scaffold proteins within single living PSDs, focusing on the principal scaffold protein PSD-95. We found that individual PSDs undergo rapid, continuous changes in morphology driven by the actin cytoskeleton and regulated by synaptic activity. This structural plasticity is accompanied by rapid fluctuations in internal scaffold density over submicron distances. Using targeted photobleaching and photoactivation of PSD subregions, we show that PSD-95 is nearly immobile within the PSD, and PSD subdomains can be maintained over long periods. We propose a flexible matrix model of the PSD based on stable molecular positioning of PSD-95 scaffolds.

EphB receptors couple dendritic filopodia motility to synapse formation

Motile dendritic filopodial processes are thought to be precursors of spine synapses, but how motility relates to cell-surface cues required for axon-dendrite recognition and synaptogenesis remains unclear. We demonstrate with dynamic imaging that loss of EphBs results in reduced motility of filopodia in cultured cortical neurons and brain slice. EphB knockdown and rescue experiments during different developmental time windows show that EphBs are required for synaptogenesis only when filopodia are most abundant and motile. In the context of EphB knockdown and reduced filopodia motility, independent rescue of either motility with PAK or of Eph-ephrin binding with an EphB2 kinase mutant is not sufficient to restore synapse formation. Strikingly, the combination of PAK and kinase-inactive EphB2 rescues synaptogenesis. Deletion of the ephrin-binding domain from EphB2 precludes rescue, indicating that both motility and trans-cellular interactions are required. Our findings provide a mechanistic link between dendritic filopodia motility and synapse differentiation.

Synaptic imbalance, stereotypies, and impaired social interactions in mice with altered neuroligin 2 expression

The level of excitation in the brain is kept under control through inhibitory signals mainly exerted by GABA neurons. However, the molecular machinery that regulates the balance between excitation and inhibition (E/I) remains unclear. Candidate molecules implicated in this process are neuroligin (NL) adhesion molecules, which are differentially enriched at either excitatory or inhibitory contacts. In this study, we use transgenic mouse models expressing NL1 or NL2 to examine whether enhanced expression of specific NLs results in synaptic imbalance and altered neuronal excitability and animal behavior. Our analysis reveals several abnormalities selectively manifested in transgenic mice with enhanced expression of NL2 but not NL1. A small change in NL2 expression results in enlarged synaptic contact size and vesicle reserve pool in frontal cortex synapses and an overall reduction in the E/I ratio. The frequency of miniature inhibitory synaptic currents was also found to be increased in the frontal cortex of transgenic NL2 mice. These animals also manifested stereotyped jumping behavior, anxiety, impaired social interactions, and enhanced incidence of spike-wave discharges, as depicted by EEG analysis in freely moving animals. These findings may provide the neural basis for E/I imbalance and altered behavior associated with neurodevelopmental disorders.

Signaling by synaptogenic molecules

Multiple signaling pathways initiate and specify the formation of synapses in the central nervous system. General principles that organize nascent synapses have emerged from the studies in multiple model organisms. These include the synapse-organizing roles of dedicated synaptic adhesion molecules, synaptic signaling following receptor-ligand interactions, and the regulation of synapse formation by secreted molecules. Intracellularly, a range of effectors subsequently regulates signaling steps and cytoskeletal changes. Together, a blueprint of synapse formation is emerging into which these distinct signaling steps will need to be integrated temporally and spatially.

Capabilities of neurexins in the chick ciliary ganglion

Transcellular interactions between neuroligins (NL) and beta-neurexin have been widely documented to promote maturation and function of both glutamatergic and GABAergic synapses. Recently it has been shown that neuroligin-1 plays a similar role at nicotinic synapses on chick ciliary ganglion neurons in culture, acting from the postsynaptic side to enhance transmitter release from adjacent cholinergic terminals and boost nicotinic input to the cells. We show here that the ciliary ganglion expresses three forms of neuroligin as well as two beta-neurexins and an alpha-neurexin. Overexpression of the beta-neurexins, but not the alpha-neurexin, can induce clustering of endogenous PSD-95 in adjacent neurons, presumably engaging neuroligin in the postsynaptic cell. The trans effects of beta-neurexins are selective; though both alpha3- and alpha7-containing nicotinic receptors are available on opposing cells, beta-neurexins induce coclustering of alpha3- but not alpha7-containing nicotinic receptors. Overexpression of other putative synaptogenic molecules, including SynCAM and L1, are ineffective at trans-clustering of PSD-95 on adjacent neurons. The beta-neurexins also exert a cis effect, coclustering presynaptic markers along with beta-neurexin in neurites juxtaposed to postsynaptic proteins, consistent with organizing presynaptic components as well. Striated muscle, the synaptic target of ciliary neurons in vivo, also expresses neuroligin. The results demonstrate that NL and neurexins are present at multiple sites in nicotinic cholinergic pathways and suggest the possibility of both cis- and trans-interactions to influence nicotinic signaling.

Differential regulation of dendrite complexity by AMPA receptor subunits GluR1 and GluR2 in motor neurons

Activity-dependent developmental mechanisms in many regions of the central nervous system are thought to be responsible for shaping dendritic architecture and connectivity, although the molecular mechanisms underlying these events remain obscure. Since AMPA glutamate receptors are developmentally regulated in spinal motor neurons, we have investigated the role of activation of AMPA receptors in dendritic outgrowth of spinal motor neurons by overexpression of two subunits, GluR1 and GluR2, and find that dendrite outgrowth is differentially controlled by expression of these subunits. Overexpression of GluR1 was associated with greater numbers of filopodia, and an increase in the length and complexity of dendritic arbor. In contrast, GluR2 expression did not alter dendritic complexity, but was associated with a moderate increase in length of arbor, and decreased numbers of filopodia. Neither GluR1 nor GluR2 had any effect on the motility of filopodia. In addition, GluR1 but not GluR2 expression increased the density of dendritic puncta incorporating a GFP-labeled PSD95, suggesting that GluR1 may mediate its effect in part by augmenting the number of excitatory synapses within motor neuron dendrites. Together these results suggest that in spinal motor neurons, AMPA receptors composed of GluR1 subunits may facilitate neurotrophic mechanisms in these neurons, permitting sustained dendrite outgrowth and synaptogenesis, whereas expression of AMPA receptors containing GluR2 acts to preserve existing dendritic arbor. Thus, the observed downregulation of GluR1 in motor neurons during postnatal development may limit the formation of new dendrite segments and synapses, promoting stabilized synaptic connectivity.

Excitation Control: Balancing PSD-95 Function at the Synapse

Excitability of individual neurons dictates the overall excitation in specific brain circuits. This process is thought to be regulated by molecules that regulate synapse number, morphology and strength. Neuronal excitation is also influenced by the amounts of neurotransmitter receptors and signaling molecules retained at particular synaptic sites. Recent studies revealed a key role for PSD-95, a scaffolding molecule enriched at glutamatergic synapses, in modulation of clustering of several neurotransmitter receptors, adhesion molecules, ion channels, cytoskeletal elements and signaling molecules at postsynaptic sites. In this review we will highlight mechanisms that control targeting of PSD-95 at the synapse, and discuss how this molecule influences the retention and clustering of diverse synaptic proteins to regulate synaptic structure and strength. We will also discuss how PSD-95 may maintain a balance between excitation and inhibition in the brain and how alterations in this balance may contribute to neuropsychiatric disorders.

E-I balance and human diseases - from molecules to networking

Information transfer in the brain requires a homeostatic control of neuronal excitability. Therefore, a functional balance between excitatory and inhibitory systems is established during development. This review contains recent information about the molecular mechanisms orchestrating the establishment and maintenance of this excitation-inhibition (E-I) balance, and it reviews examples of deregulation of inhibitory and excitatory systems at a molecular, network and disease level of investigation.

Developmental patterning of glutamatergic synapses onto retinal ganglion cells

Background

Neurons receive excitatory synaptic inputs that are distributed across their dendritic arbors at densities and with spatial patterns that influence their output. How specific synaptic distributions are attained during development is not well understood. The distribution of glutamatergic inputs across the dendritic arbors of mammalian retinal ganglion cells (RGCs) has long been correlated to the spatial receptive field profiles of these neurons. Thus, determining how glutamatergic inputs are patterned onto RGC dendritic arbors during development could provide insight into the cellular mechanisms that shape their functional receptive fields.

Results

We transfected developing and mature mouse RGCs with plasmids encoding fluorescent proteins that label their dendrites and glutamatergic postsynaptic sites. We found that as dendritic density (dendritic length per unit area of dendritic field) decreases with maturation, the density of synapses along the dendrites increases. These changes appear coordinated such that RGCs attain the mature average density of postsynaptic sites per unit area (areal density) by the time synaptic function emerges. Furthermore, stereotypic centro-peripheral gradients in the areal density of synapses across the arbor of RGCs are established at an early developmental stage.

Conclusion

The spatial pattern of glutamatergic inputs onto RGCs arises early in synaptogenesis despite ensuing reorganization of dendritic structure. We raise the possibility that these early patterns of synaptic distributions may arise from constraints placed on the number of contacts presynaptic neurons are able to make with the RGCs.

WITHDRAWN: Role of NCAM in Spine Dynamics and Synaptogenesis

Increasing evidence indicates that adhesion molecules are critically involved in the regulation of mechanisms of synaptic plasticity including synapse formation, but also synaptic remodeling associated to changes in synaptic strength. Among these, the Neural Cell Adhesion Molecule (NCAM) and its polysialylated form PSA-NCAM are important candidates. Here we review recent results that point to a possible role of these two molecules in regulating the structural properties of excitatory synapses and namely the composition and stability of the postsynaptic density, thereby accounting for their contribution to mechanisms of synaptogenesis and activity-dependent synaptic plasticity.

Paralemmin-1, a modulator of filopodia induction is required for spine maturation

Dendritic filopodia are thought to participate in neuronal contact formation and development of dendritic spines; however, molecules that regulate filopodia extension and their maturation to spines remain largely unknown. Here we identify paralemmin-1 as a regulator of filopodia induction and spine maturation. Paralemmin-1 localizes to dendritic membranes, and its ability to induce filopodia and recruit synaptic elements to contact sites requires protein acylation. Effects of paralemmin-1 on synapse maturation are modulated by alternative splicing that regulates spine formation and recruitment of AMPA-type glutamate receptors. Paralemmin-1 enrichment at the plasma membrane is subject to rapid changes in neuronal excitability, and this process controls neuronal activity-driven effects on protrusion expansion. Knockdown of paralemmin-1 in developing neurons reduces the number of filopodia and spines formed and diminishes the effects of Shank1b on the transformation of existing filopodia into spines. Our study identifies a key role for paralemmin-1 in spine maturation through modulation of filopodia induction.

Gephyrin expression and clustering affects the size of glutamatergic synaptic contacts

We have recently shown that disrupting the expression and post-synaptic clustering of gephyrin in cultured hippocampal pyramidal cells, by either gephyrin RNAi (RNA interference) or over-expression of a dominant negative gephyrin-enhanced green fluorescent protein (EGFP) fusion protein, leads to decreased number of post-synaptic gephyrin and GABA(A) receptor clusters and to reduced GABAergic innervation of these cells. On the other hand, increasing gephyrin expression led to a small increase in the number of gephyrin and GABA(A) receptor clusters and to little or no effect on GABAergic innervation. We are now reporting that altering gephyrin expression and clustering affects the size but not the density of glutamatergic synaptic contacts. Knocking down gephyrin with gephyrin RNAi, or preventing gephyrin clustering by over-expression of the dominant negative gephyrin-enhanced green fluorescent protein fusion protein, leads to larger post-synaptic PSD-95 clusters and larger pre-synaptic glutamatergic terminals. On the other hand, over-expression of gephyrin leads to slightly smaller PSD-95 clusters and pre-synaptic glutamatergic terminals. The change in size of PSD-95 clusters were accompanied by a parallel change in the size of NR2-NMDA receptor clusters. It is concluded that the levels of expression and clustering of gephyrin, a protein that concentrates at the post-synaptic complex of the inhibitory synapses, not only has homotypic effects on GABAergic synaptic contacts, but also has heterotypic effects on glutamatergic synaptic contacts. We are proposing that gephyrin is a counterpart of the post-synaptic glutamatergic scaffold protein PSD-95 in regulating the number and/or size of the excitatory and inhibitory synaptic contacts.

Ephrin-B reverse signaling promotes structural and functional synaptic maturation in vivo

Ephrin-Eph signaling is involved in axon guidance during development, but it may also regulate synapse development after the axon has contacted the target cell. Here we report that the activation of ephrin-B reverse signaling in the developing Xenopus laevis optic tectum promotes morphological and functional maturation of retinotectal synapses. Elevation of ephrin-B signaling increased the number of retinotectal synapses and stabilized the axon arbors of retinal ganglion cells. It also enhanced basal synaptic transmission and activity-induced long-term potentiation (LTP) of retinotectal synapses. The functional effects were caused by a rapid enhancement of presynaptic glutamate release and a delayed increase in the postsynaptic glutamate responsiveness. The facilitated LTP induction occurred during the early phase of enhanced transmitter release and appeared to be causally related to the late-phase postsynaptic maturation via an NMDA receptor-dependent mechanism. This ephrin-B-dependent synapse maturation supports the notion that the ephrin/Eph protein families have multiple functions in neural development.

Homeostatic regulation of AMPA receptor expression at single hippocampal synapses

Homeostatic synaptic response is an important measure in confining neuronal activity within a narrow physiological range. Whether or not homeostatic plasticity demonstrates synapse specificity, a key feature characteristic of Hebbian-type plasticity, is largely unknown. Here, we report that in cultured hippocampal neurons, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid subtype glutamate receptor (AMPAR) accumulation is increased selectively in chronically inhibited single synapses, whereas the neighboring normal synapses remain unaffected. This synapse-specific homeostatic regulation depends on the disparity of synaptic activity and is mediated by GluR2-lacking AMPARs and PI3-kinase signaling. These results demonstrate the existence of synaptic specificity and the crucial role of AMPAR-gated calcium in homeostatic plasticity in central neurons.

Excitation Control: Balancing PSD-95 Function at the Synapse

Excitability of individual neurons dictates the overall excitation in specific brain circuits. This process is thought to be regulated by molecules that regulate synapse number, morphology and strength. Neuronal excitation is also influenced by the amounts of neurotransmitter receptors and signaling molecules retained at particular synaptic sites. Recent studies revealed a key role for PSD-95, a scaffolding molecule enriched at glutamatergic synapses, in modulation of clustering of several neurotransmitter receptors, adhesion molecules, ion channels, cytoskeletal elements and signaling molecules at postsynaptic sites. In this review we will highlight mechanisms that control targeting of PSD-95 at the synapse, and discuss how this molecule influences the retention and clustering of diverse synaptic proteins to regulate synaptic structure and strength. We will also discuss how PSD-95 may maintain a balance between excitation and inhibition in the brain and how alterations in this balance may contribute to neuropsychiatric disorders.

Scaffolding proteins at the postsynaptic density: shank as the architectural framework

Shank proteins are multidomain scaffold proteins of the postsynaptic density, connecting neurotransmitter receptors and other membrane proteins with signaling proteins and the actin cytoskeleton. By virtue of their protein interactions, Shank proteins assemble signaling platforms for G-protein-mediated signaling and the control of calcium homeostasis in dendritic spines. In addition, they participate in morphological changes, leading to maturation of dendritic spines and synapse formation. The importance of the Shank scaffolding function is demonstrated by genetically determined forms of mental retardation, which may be caused by haploinsufficiency for the SHANK3 gene. Consistent with its central function within the postsynaptic density, the availability of Shank is tightly controlled by local synthesis and degradation, as well as actin-dependent dynamic rearrangements within the dendritic spine.

SynCAMs organize synapses through heterophilic adhesion

Synapses are asymmetric cell junctions with precisely juxtaposed presynaptic and postsynaptic sides. Transsynaptic adhesion complexes are thought to organize developing synapses. The molecular composition of these complexes, however, remains incompletely understood, precluding us from understanding how adhesion across the synaptic cleft guides synapse development. Here, we define two immunoglobulin superfamily members, SynCAM 1 and 2, that are expressed in neurons in the developing brain and localize to excitatory and inhibitory synapses. They function as cell adhesion molecules and assemble with each other across the synaptic cleft into a specific, transsynaptic SynCAM 1/2 complex. Additionally, SynCAM 1 and 2 promote functional synapses as they increase the number of active presynaptic terminals and enhance excitatory neurotransmission. The interaction of SynCAM 1 and 2 is affected by glycosylation, indicating regulation of this adhesion complex by posttranslational modification. The SynCAM 1/2 complex is representative for the highly defined adhesive patterns of this protein family, the four members of which are expressed in neurons in divergent expression profiles. SynCAMs 1, 2, and 3 each can bind themselves, yet preferentially assemble into specific, heterophilic complexes as shown for the synaptic SynCAM 1/2 interaction and a second complex comprising SynCAM 3 and 4. Our results define SynCAM proteins as components of novel heterophilic transsynaptic adhesion complexes that set up asymmetric interactions, with SynCAM proteins contributing to synapse organization and function.

The cell biology of synaptic plasticity: AMPA receptor trafficking

The cellular processes that govern neuronal function are highly complex, with many basic cell biological pathways uniquely adapted to perform the elaborate information processing achieved by the brain. This is particularly evident in the trafficking and regulation of membrane proteins to and from synapses, which can be a long distance away from the cell body and number in the thousands. The regulation of neurotransmitter receptors, such as the AMPA-type glutamate receptors (AMPARs), the major excitatory neurotransmitter receptors in the brain, is a crucial mechanism for the modulation of synaptic transmission. The levels of AMPARs at synapses are very dynamic, and it is these plastic changes in synaptic function that are thought to underlie information storage in the brain. Thus, understanding the cellular machinery that controls AMPAR trafficking will be critical for understanding the cellular basis of behavior as well as many neurological diseases. Here we describe the life cycle of AMPARs, from their biogenesis, through their journey to the synapse, and ultimately through their demise, and discuss how the modulation of this process is essential for brain function.

Synaptic adhesion molecules and PSD-95

Synaptic adhesion molecules are known to participate in various steps of synapse development including initial contacts between dendrites and axons, formation of early synapses, and their maturation and plastic changes. Notably, a significant subset of synaptic adhesion molecules associates with synaptic scaffolding proteins, suggesting that they may act in concert to couple trans-synaptic adhesion to molecular organization of synaptic proteins. Here, we describe an emerging group of synaptic adhesion molecules that directly interact with the abundant postsynaptic scaffold PSD-95, which include neuroligins, NGLs, SALMs, and ADAM22, and discuss how these proteins and PSD-95 act together to regulate synaptic development. PSD-95 may be one of the central organizers of synaptic adhesion that recruits diverse proteins to sites of synaptic adhesion, promotes trans-synaptic signaling, and couples neuronal activity with changes in synaptic adhesion.

Neuronal sandwiches: A method for rapid and controlled initiation of synapses

Synapse formation is a fast, dynamic process that involves the assembly of many molecules following axodendritic contact. Neuronal cultures are often used to study the insertion of fluorescently tagged pre- and postsynaptic molecules in vitro. However, this task still remains challenging, since the time-point and location of newly forming synapses are largely unpredictable and rely on random contact events. We developed a technique that controls the time-point of interaction between axons and dendrites, and thus the onset of synapse formation. Dissociated hippocampal neurons were cultivated on two different coverslips, allowing for the separate outgrowth of axonal networks and of neurons with sparsely innervated dendrites. Pre- and postsynaptic partners were brought in contact as coverslips were merged. Time-lapse imaging showed clustering of GFP/PSD-95 in postsynaptic neurons within 1-3h, indicating the rapid formation of new synaptic sites. Localization of DsRed, as a control protein, remained unchanged. Imaging of neuronal activity using calcium sensitive dyes revealed that in a number of cases neurons of the pre- and postsynaptic layer were synchronously active, suggesting the functionality of newly formed synapses across layers. Therefore, our new method is a valuable tool to control synapse formation and for investigating the temporal role of signaling molecules during this process.

Postsynaptic positioning of endocytic zones and AMPA receptor cycling by physical coupling of dynamin-3 to Homer

Endocytosis of AMPA receptors and other postsynaptic cargo occurs at endocytic zones (EZs), stably positioned sites of clathrin adjacent to the postsynaptic density (PSD). The tight localization of postsynaptic endocytosis is thought to control spine composition and regulate synaptic transmission. However, the mechanisms that situate the EZ near the PSD and the role of spine endocytosis in synaptic transmission are unknown. Here, we report that a physical link between dynamin-3 and the postsynaptic adaptor Homer positions the EZ near the PSD. Disruption of dynamin-3 or its interaction with Homer uncouples the PSD from the EZ, resulting in synapses lacking postsynaptic clathrin. Loss of the EZ leads to a loss of synaptic AMPA receptors and reduced excitatory synaptic transmission that corresponds with impaired synaptic recycling. Thus, a physical link between the PSD and the EZ ensures localized endocytosis and recycling by recapturing and maintaining a proximate pool of cycling AMPA receptors.

The postsynaptic architecture of excitatory synapses: a more quantitative view

Excitatory (glutamatergic) synapses in the mammalian brain are usually situated on dendritic spines, a postsynaptic microcompartment that also harbors organelles involved in protein synthesis, membrane trafficking, and calcium metabolism. The postsynaptic membrane contains a high concentration of glutamate receptors, associated signaling proteins, and cytoskeletal elements, all assembled by a variety of scaffold proteins into an organized structure called the postsynaptic density (PSD). A complex machine made of hundreds of distinct proteins, the PSD dynamically changes its structure and composition during development and in response to synaptic activity. The molecular size of the PSD and the stoichiometry of many major constituents have been recently measured. The structures of some intact PSD proteins, as well as the spatial arrangement of several proteins within the PSD, have been determined at low resolution by electron microscopy. On the basis of such studies, a more quantitative and geometrically realistic view of PSD architecture is emerging.

Interaction of the N-Terminal Domain of the AMPA Receptor GluR4 Subunit with the Neuronal Pentraxin NP1 Mediates GluR4 Synaptic Recruitment

Synaptogenesis requires recruitment of neurotransmitter receptors to developing postsynaptic specializations. We developed a coculture system reconstituting artificial synapses between neurons and nonneuronal cells to investigate the molecular components required for AMPA-receptor recruitment to synapses. With this system, we find that excitatory axons specifically express factors that recruit the AMPA receptor GluR4 subunit to sites of contact between axons and GluR4-transfected nonneuronal cells. Furthermore, the N-terminal domain (NTD) of GluR4 is necessary and sufficient for its recruitment to these artificial synapses and also for GluR4 recruitment to native synapses. Moreover, we show that axonally derived neuronal pentraxins NP1 and NPR are required for GluR4 recruitment to artificial and native synapses. RNAi knockdown and knockout of the neuronal pentraxins significantly decreases GluR4 targeting to synapses. Our results indicate that NP1 and NPR secreted from presynaptic neurons bind to the GluR4 NTD and are critical trans-synaptic factors for GluR4 recruitment to synapses.

Hepatocyte growth factor promotes the number of PSD-95 clusters in young hippocampal neurons

Hepatocyte growth factor (HGF) and its receptor are expressed in various regions of the brain and have protective effects against excitotoxic injuries. However, their effects on synapse formation remain to be elucidated. To determine whether HGF has the ability to alter synaptic function during development, we investigated changes in the number of synapse detected by double immunostaining for NMDA receptor subunits and a presynaptic marker in cultured young hippocampal neurons. Whereas application of HGF increased the number of cluster of synapsin, a presynaptic protein, the clusters of NMDA receptor subunits NR1 and NR2B were not altered. Interestingly, colocalization of PSD-95, a scaffolding protein of the receptor, with synapsin was increased by HGF treatment without a change in the total amount of it. In addition, we investigated the expression of surface NMDA receptor, neuroligin, and neurexin, which were assessed by use of a cell-surface biotinylation assay. The application of HGF did not change the surface expression of these proteins. Furthermore, we determined the release of glutamate in response to depolarization. Treatment with HGF promoted depolarization-evoked release of glutamate. These results suggest that HGF modulates the expression of the scaffolding protein of the NMDA receptor at the synapse and promotes maturation of excitatory synapses in young hippocampal neurons.

Postsynaptic neuroligin enhances presynaptic inputs at neuronal nicotinic synapses

Neuroligins are cell adhesion molecules that interact with neurexins on adjacent cells to promote glutamatergic and GABAergic synapse formation in culture. We show here that neuroligin enhances nicotinic synapses on neurons in culture, increasing synaptic input. When neuroligin is overexpressed in neurons, the extracellular domain induces presynaptic specializations in adjacent cholinergic neurons as visualized by SV2 puncta. The intracellular domain is required to translate the SV2 puncta into synaptic input as reflected by increases in the frequency of spontaneous mini-synaptic currents. The PDZ-binding motif of neuroligin is not needed for these effects. Together, the extracellular and proximal intracellular domains of neuroligin are sufficient to induce presynaptic specializations, align them over postsynaptic receptor clusters, and increase synaptic function. Manipulation of endogenous neuroligin with beta-neurexin-expressing cells confirms its presence; repressing function with dominant negative constructs and inhibitory shRNA shows that endogenous neuroligin helps confer functionality on existing nicotinic synaptic contacts. Endogenous neuroligin does not appear to be required, however, for initial formation of the contacts, suggesting that other components under these conditions can also initiate synapse formation. The results indicate that postsynaptic neuroligin is important for functional nicotinic synapses on neurons and that the effects achieved will likely depend on neuroligin levels.

Maturation of synaptic contacts in differentiating neural stem cells

NSCs are found in the developing brain, as well as in the adult brain. They are self-renewing cells that maintain the capacity to differentiate into all major brain-specific cell types, such as glial cells and neurons. However, it is still unclear whether these cells are capable of gaining full functionality, which is one of the major prerequisites for NSC-based cell replacement strategies of neurological diseases. The ability to establish and maintain polarized excitatory synaptic contacts would be one of the basic requirements for intercellular communication and functional integration into existing neuronal networks. In primary cultures of hippocampal neurons, it has already been shown that synaptogenesis is characterized by a well-ordered, time-dependent targeting and recruitment of pre- and postsynaptic proteins. In this study, we investigated the expression and localization of important pre- and postsynaptic proteins, including Bassoon and synaptophysin, as well as proteins of the ProSAP/Shank family, in differentiating rat fetal mesencephalic NSCs. Moreover, we analyzed the ultrastructural features of neuronal cell-cell contacts during synaptogenesis. We show that NSCs express and localize cytoskeletal and scaffolding molecules of the pre- and postsynaptic specializations in a well-defined temporal order, leading to mature synaptic contacts after 14 days of differentiation. The temporal and spatial pattern of synaptic maturation is comparable to synaptogenesis of hippocampal neurons grown in primary culture. Therefore, with respect to the general ability to create mature synaptic contacts, NSCs seem to be well equipped to potentially compensate for lost or injured brain tissue. Disclosure of potential conflicts of interest is found at the end of this article.

Silencing of neuroligin function by postsynaptic neurexins

The formation of neuronal circuits during development involves a combination of synapse stabilization and elimination events. Synaptic adhesion molecules are thought to play an important role in synaptogenesis, and several trans-synaptic adhesion systems that promote the formation and maturation of synapses have been identified. The neuroligin-neurexin complex is a heterophilic adhesion system that promotes assembly and maturation of synapses through bidirectional signaling. In this protein complex, postsynaptic neuroligins are thought to interact trans-synaptically with presynaptic neurexins. However, the subcellular localization of neurexins has not been determined. Using immunoelectron microscopy, we found that endogenous neurexins and epitope-tagged neurexin-1beta are localized to axons and presynaptic terminals in vivo. Unexpectedly, neurexins are also abundant in the postsynaptic density. cis-expression of neurexin-1beta with neuroligin-1 inhibits trans-binding to recombinant neurexins, blocks the synaptogenic activity of neuroligin-1, and reduces the density of presynaptic terminals in cultured hippocampal neurons. Our results demonstrate that the function of neurexin proteins is more diverse than previously anticipated and suggest that postsynaptic cis-interactions might provide a novel mechanism for silencing the activity of a synaptic adhesion complex.