Mechanisms of Synapse Assembly and Disassembly

Impaired synapse function during postnatal development in the absence of CALEB, an EGF-like protein processed by neuronal activity

In an attempt to characterize the molecular components by which electric activity influences the development of synapses, we searched for cell surface proteins modulated by calcium influx and glutamate receptor activity. Here, we report that neuronal depolarization facilitates the conversion of CALEB, which results in a truncated transmembrane form with an exposed EGF domain. To characterize the role of CALEB in synapse development, synaptic features were investigated in slices of the colliculus superior from CALEB-deficient mice. In the absence of CALEB, the number of synapses and their morphological characteristics remained unchanged. However, in CALEB-deficient mice, synapses displayed higher paired-pulse ratios, less depression during prolonged repetitive activation, a lower rate of spontaneous postsynaptic currents, and a lower release probability at early but not mature postnatal stages. Our findings indicate that CALEB provides a molecular basis for maintaining normal release probability at early developmental stages.

Development of an Inhibitory Interneuronal Circuit in the Embryonic Spinal Cord

Locally projecting inhibitory interneurons play a crucial role in the patterning and timing of network activity. However, because of their relative inaccessibility, little is known about their development or incorporation into circuits. In this study, we characterized the functional onset, neurotransmitters, rostrocaudal spread, and funicular distribution of one such spinal interneuronal circuit during development. The R-interneuron is the avian homologue of the mammalian Renshaw cell. Both cell types receive input from motoneuron recurrent collaterals and make direct connections back onto motoneurons. By stimulating motoneurons projecting in a given ventral root and recording the response in adjacent ventral roots, we demonstrate that the R-interneuron circuit becomes functional between embryonic day 6 (E6) and E7. This ventral root response is observed at E11 and at E14 until it can no longer be detected at E16. Using bath-applied neurotransmitter receptor antagonists, we were able to demonstrate that the circuit is predominately nicotinic and GABAergic from E7.5 to E15. We also found a glutamatergic component to the pathway throughout this developmental period. The R-interneuron projects three or more segments both rostrally and caudally through the ventrolateral funiculus. The distribution of this circuit may become more locally focused between E7.5 and E15.

Mechanistic and epidemiologic considerations in the evaluation of adverse birth outcomes following gestational exposure to statins

The cholesterol-lowering "statin" drugs are contraindicated in pregnancy, but few data exist on their safety in human gestation. We reviewed case reports for patterns suggesting drug-related effects on prenatal development and considered a variety of mechanisms by which such effects, if confirmed, might occur. This uncontrolled case series included all FDA reports of statin exposures during gestation, as well as others from the literature and from manufacturers. Exposures and outcomes were reviewed and were tabulated by individual drug. Age-specific rates of exposure to each drug among women of child-bearing age were estimated. Of 214 ascertained pregnancy exposures, 70 evaluable reports remained after excluding uninformative cases. Among 31 adverse outcomes were 22 cases with structural defects, 4 cases of intrauterine growth restriction, and 5 cases of fetal demise. There were two principal categories of recurrent structural defects: cerivastatin and lovastatin were associated with four reports of severe midline CNS defects; simvastatin, lovastatin, and atorvastatin were all associated with reports of limb deficiencies, including two similar complex lower limb defects reported following simvastatin exposure. There were also two cases of VACTERL association among the limb deficiency cases. All adverse outcomes were reported following exposure to cerivastatin, simvastatin, lovastatin, or atorvastatin, which are lipophilic and equilibrate between maternal and embryonic compartments. None were reported following exposure to pravastatin, which is minimally present in the embryo. Statins reaching the embryo may down-regulate biosynthesis of cholesterol as well as many important metabolic intermediates, and may have secondary effects on sterol-dependent morphogens such as Sonic Hedgehog. The reported cases display patterns consistent with dysfunction of cholesterol biosynthesis and Sonic Hedgehog activity. Controlled studies are needed to investigate the teratogenicity of individual drugs in this class.

Control of Excitatory and Inhibitory Synapse Formation by Neuroligins

The normal function of neural networks depends on a delicate balance between excitatory and inhibitory synaptic inputs. Synapse formation is thought to be regulated by bidirectional signaling between pre- and postsynaptic cells. We demonstrate that members of the Neuroligin family promote postsynaptic differentiation in cultured rat hippocampal neurons. Down-regulation of neuroligin isoform expression by RNA interference results in a loss of excitatory and inhibitory synapses. Electrophysiological analysis revealed a predominant reduction of inhibitory synaptic function. Thus, neuroligins control the formation and functional balance of excitatory and inhibitory synapses in hippocampal neurons.

Dissection of synaptic excitability phenotypes by using a dominant-negative Shaker K+ channel subunit

During nervous system development, synapses undergo morphological change as a function of electrical activity. In Drosophila, enhanced activity results in the expansion of larval neuromuscular junctions. We have examined whether these structural changes involve the pre- or postsynaptic partner by selectively enhancing electrical excitability with a Shaker dominant-negative (SDN) potassium channel subunit. We find that the SDN enhances neurotransmitter release when expressed in motoneurons, postsynaptic potential broadening when expressed in muscles and neurons, and selectively suppresses fast-inactivating, Shaker-mediated IA currents in muscles. SDN expression also phenocopies the canonical behavioral phenotypes of the Sh mutation. At the neuromuscular junction, we find that activity-dependent changes in arbor size occur only when SDN is expressed presynaptically. This finding indicates that elevated postsynaptic membrane excitability is by itself insufficient to enhance presynaptic arbor growth. Such changes must minimally involve increased neuronal excitability.

Cadherins and catenins in synapse development

Cadherin-catenin complexes have been well established as key regulators of cell adhesion. Recent work has elucidated a pivotal role for these molecules in synaptic assembly, remodelling and plasticity. Far from being mere adhesive scaffolds, cadherins might directly regulate cell signalling to modulate synaptic connectivity.

Endocytosis and degradative sorting of NMDA receptors by conserved membrane-proximal signals

Regulation of the abundance of NMDA receptors (NMDARs) at excitatory synapses is critical during changes in synaptic efficacy underlying learning and memory as well as during synapse formation throughout neural development. However, the molecular signals that govern NMDAR delivery, maintenance, and internalization remain unclear. In this study, we identify a conserved family of membrane-proximal endocytic signals, two within the NMDAR type 1 (NR1) subunit and one within the NR2A and NR2B subunits, necessary and sufficient to drive the internalization of NMDARs. These endocytic motifs reside in the region of NMDAR subunits immediately after the fourth membrane segment, a region implicated in use-dependent rundown and NMDA channel inactivation. Although endocytosis driven by the distal C-terminal domain of NR2B is followed by rapid recycling, internalization mediated by membrane-proximal motifs selectively targets receptors to late endosomes and accelerates degradation. These results define a novel conserved signature of NMDARs regulating internalization and postendocytic trafficking.

Synaptic targeting of neuroligin is independent of neurexin and SAP90/PSD95 binding

Synaptic cell adhesion and synaptogenesis are thought to involve the interaction of neuroligin, a postsynaptic transmembrane protein, with its presynaptic ligand neurexin. Neuroligin also interacts with SAP90/PSD95, a multidomain scaffolding protein thought to recruit proteins to postsynaptic sites. Using expression of GFP-tagged versions of neuroligin in cultured hippocampal neurons, we find that neuroligin is targeted to synapses via intracellular sequences distinct from its SAP90/PSD95 binding site. A neuroligin mutant lacking the intracellular domain fails to target to synapses. These data indicate that postsynaptic targeting of neuroligin does not rely on the scaffolding action of SAP90/PSD95 and is not induced by binding to presynaptic neurexin. Neuroligin is rather targeted to synapses via a postsynaptic mechanism, which may precede and be necessary for subsequent recruitment of neurexin and other neuroligin interactors such as SAP90/PSD95, suggesting a pivotal position for neuroligin in a putative hierarchy of interactions assembling or stabilizing synapses.

Selective capability of SynCAM and neuroligin for functional synapse assembly

Synaptic cell adhesion is central for synapse formation and function. Recently, the synaptic cell adhesion molecules neuroligin 1 (NL1) and SynCAM were shown to induce presynaptic differentiation in cocultured neurons when expressed in a non-neuronal cell. However, it is uncertain how similar the resulting artificial synapses are to regular synapses. Are these molecules isofunctional, or do all neuronal cell adhesion molecules nonspecifically activate synapse formation? To address these questions, we analyzed the properties of artificial synapses induced by NL1 and SynCAM, compared the actions of these molecules with those of other neuronal cell adhesion molecules, and examined the functional effects of NL1 and SynCAM overexpression in neurons. We found that only NL1 and SynCAM specifically induced presynaptic differentiation in cocultured neurons. The induced nerve terminals were capable of both spontaneous and evoked neurotransmitter release, suggesting that a full secretory apparatus was assembled. By all measures, SynCAM- and NL1-induced artificial synapses were identical. Overexpression in neurons demonstrated that only SynCAM, but not NL1, increased synaptic function in immature developing excitatory neurons after 8 d in vitro. Tests of chimeric molecules revealed that the dominant-positive effect of SynCAM on synaptic function in developing neurons was mediated by its intracellular cytoplasmic tail. Interestingly, morphological analysis of neurons overexpressing SynCAM or NL1 showed the opposite of the predictions from electrophysiological results. In this case, only NL1 increased the synapse number, suggesting a role for NL1 in morphological synapse induction. These results suggest that both NL1 and SynCAM act similarly and specifically in artificial synapse induction but that this process does not reflect a shared physiological function of these molecules.

Dynamics and pathology of dendritic spines

Dendritic spines are key players in information processing in the brain. Changes in spine shape and wholesale spine turnover provide mechanisms for modifying existing synaptic connections and altering neuronal connectivity. Although neuronal cell death in acute and chronic neurodegenerative diseases is clearly an important factor in decline of cognitive or motor function, loss of dendritic spines, in the absence of cell death, may also contribute to impaired brain function in these diseases, as well as in psychiatric disorders and aging. Because spines can function in neuroprotection in vitro, advances toward a molecular understanding of spine maintenance might one day aid in the design of therapies to minimize neurological damage following excitotoxic injury. In addition, progress in defining the biochemical basis of spine development and stabilization may yield insights into mental retardation and psychiatric disorders.

AMPA receptor-dependent clustering of synaptic NMDA receptors is mediated by Stargazin and NR2A/B in spinal neurons and hippocampal interneurons

Under standard conditions, cultured ventral spinal neurons cluster AMPA- but not NMDA-type glutamate receptors at excitatory synapses on their dendritic shafts in spite of abundant expression of the ubiquitous NMDA receptor subunit NR1. We demonstrate here that the NMDA receptor subunits NR2A and NR2B are not routinely expressed in cultured spinal neurons and that transfection with NR2A or NR2B reconstitutes the synaptic targeting of NMDA receptors and confers on exogenous application of the immediate early gene product Narp the ability to cluster both AMPA and NMDA receptors. The use of dominant-negative mutants of GluR2 further showed that the synaptic targeting of NMDA receptors is dependent on the presence of synaptic AMPA receptors and that synaptic AMPA and NMDA receptors are linked by Stargazin and a MAGUK protein. This system of AMPA receptor-dependent synaptic NMDA receptor localization was preserved in hippocampal interneurons but reversed in hippocampal pyramidal neurons.

Genetic analysis of synaptic target recognition and assembly

Formation of synapses by neurons onto specific targets is essential to the function of a nervous system. The isolation and analysis of Caenorhabditis elegans and Drosophila mutants with synaptogenesis defects has provided insight into the functions of evolutionarily conserved molecules at single-synapse resolution. Importantly, such studies have uncovered novel molecules and signaling mechanisms. Here, recent progress on synaptic target recognition and synaptic assembly are reviewed.

Activity-dependent transcription regulation of PSD-95 by neuregulin-1 and Eos

Neuregulin-1 (Nrg-1) contains an intracellular domain (Nrg-ICD) that translocates into the nucleus, where it may regulate gene expression upon neuronal depolarization. However, the identity of its target promoters and the mechanisms by which it regulates transcription have been elusive. Here we report that, in the mouse cochlea, synaptic activity increases the level of nuclear Nrg-ICD and upregulates postsynaptic density protein-95 (PSD-95), a scaffolding protein that is enriched in post-synaptic structures. Nrg-ICD enhances the transcriptional activity of the PSD-95 promoter by binding to a zinc-finger transcription factor, Eos. The Nrg-ICD-Eos complex induces endogenous PSD-95 expression in vivo through a signaling pathway that is mostly independent of gamma-secretase regulation. This upregulation of PSD-95 expression by the Nrg-ICD-Eos complex provides a molecular basis for activity-dependent synaptic plasticity.

Dap160/intersectin scaffolds the periactive zone to achieve high-fidelity endocytosis and normal synaptic growth

Dap160/Intersectin is a multidomain adaptor protein that colocalizes with endocytic machinery in the periactive zone at the Drosophila NMJ. We have generated severe loss-of-function mutations that eliminate Dap160 protein from the NMJ. dap160 mutant synapses have decreased levels of essential endocytic proteins, including dynamin, endophilin, synaptojanin, and AP180, while other markers of the active zone and periactive zone are generally unaltered. Functional analyses demonstrate that dap160 mutant synapses are unable to sustain high-frequency transmitter release, show impaired FM4-64 loading, and show a dramatic increase in presynaptic quantal size consistent with defects in synaptic vesicle recycling. The dap160 mutant synapse is grossly malformed with abundant, highly ramified, small synaptic boutons. We present a model in which Dap160 scaffolds both endocytic machinery and essential synaptic signaling systems to the periactive zone to coordinately control structural and functional synapse development.

Neural activity and the dynamics of central nervous system development

Recent imaging studies show that the formation of neural connections in the central nervous system is a highly dynamic process. The iterative formation and elimination of synapses and neuronal branches result in the formation of a much larger number of trial connections than is maintained in the mature brain. Neural activity modulates development through biasing this process of formation and elimination, promoting the formation and stabilization of appropriate synaptic connections on the basis of functional activity patterns.

Presynaptic function

Changing the strength of synapses is key to the adaptive modifications of what neuronal circuits compute. Unsurprisingly, many different mechanisms have evolved to alter synaptic strength. Some of these mechanisms depend on the history of synaptic use, others reflect the activity of modulatory neurons that are controlled through neural computations, and still others involve more global measures of neural activity. The molecular machinery synapses use to convey information from one neuron to the next not only plays an essential part in brain function but also is at the basis of processes that are vital to all cells. Because membrane fusion events at synapses are so precisely controlled, synapses offer an especially favorable system in which to study these basic processes. Here, I review some of the recent progress that has been made in understanding both how synaptic strength is regulated and how fundamental cell biological mechanisms are used to accomplish neuronal intercommunication.

Postsynaptic density assembly is fundamentally different from presynaptic active zone assembly

The cellular mechanisms involved in the formation of the glutamatergic postsynaptic density (PSD) are mainly unknown. Previous studies have indicated that PSD assembly may occur in situ by a gradual recruitment of postsynaptic molecules, whereas others have suggested that the PSD may be assembled from modular transport packets assembled elsewhere. Here we used cultured hippocampal neurons and live cell imaging to examine the process by which PSD molecules from different layers of the PSD are recruited to nascent postsynaptic sites. GFP-tagged NR1, the essential subunit of the NMDA receptor, and ProSAP1/Shank2 and ProSAP2/Shank3, scaffolding molecules thought to reside at deeper layers of the PSD, were recruited to new synaptic sites in gradual manner, with no obvious involvement of discernible discrete transport particles. The recruitment kinetics of these three PSD molecules were remarkably similar, which may indicate that PSD assembly rate is governed by a common upstream rate-limiting process. In contrast, the presynaptic active zone (AZ) molecule Bassoon was observed to be recruited to new presynaptic sites by means of a small number of mobile packets, in full agreement with previous studies. These findings indicate that the assembly processes of PSDs and AZs may be fundamentally different.

The role of ECM molecules in activity-dependent synaptic development and plasticity

Growth and guidance of neurites (axons and dendrites) during development is the prerequisite for the establishment of functional neural networks in the adult organism. In the adult, mechanisms similar to those used during development may regulate plastic changes that underlie important nervous system functions, such as memory and learning. There is now ever-increasing evidence that extracellular matrix (ECM)-associated factors are critically involved in the formation of neuronal connections during development, and their plastic changes in the adult. Here, we review the current literature on the role of ECM components in activity-dependent synaptic development and plasticity, with the major focus on the thrombospondin type I repeat (TSR) domain-containing proteins. We propose that ECM components may modulate neuronal development and plasticity by: 1) regulating cellular motility and morphology, thus contributing to structural alterations that are associated with the expression of synaptic plasticity, 2) coordinating transsynaptic signaling during plasticity via their cell surface receptors, and 3) defining the physical parameters of the extracellular space, thereby regulating diffusion of soluble signaling molecules in the extracellular space (ECS).

Neuronal Polarity and Trafficking

Among the most morphologically complex cells, neurons are masters of membrane specialization. Nowhere is this more striking than in the division of cellular labor between the axon and the dendrites. In morphology, signaling properties, cytoskeletal organization, and physiological function, axons and dendrites (or more properly, the somatodendritic compartment) are radically different. Such polarization of neurons into domains specialized for either receiving (dendrites) or transmitting (axons) cellular signals provides the underpinning for all neural circuitry. The initial specification of axonal and dendritic identity occurs early in neuronal life, persists for decades, and is manifested by the presence of very different sets of cell surface proteins. Yet, how neuronal polarity is established, how distinct axonal and somatodendritic domains are maintained, and how integral membrane proteins are directed to dendrites or accumulate in axons remain enduring and formidable questions in neuronal cell biology.