Molecules involved in the formation of synaptic connections in muscle and brain.

Synapses are highly specialized structures designed to guarantee precise and efficient communication between neurons and their target cells. Molecules of the extracellular matrix have an instructive role in the formation of the neuromuscular junction, the best-characterized synapse. In this review, the molecular mechanisms underlying these instructive signals will be discussed with particular emphasis on the receptors involved. Additionally, recent evidence for the involvement of specific adhesion complexes in the formation and modulation of synapses in the central nervous system will be reviewed. Synapses are specialized junctions between neurons and their target cells where information is transferred from the pre- to the postsynaptic cell. At most vertebrate synapses, this transfer is accomplished by the release of a specific neurotransmitter from the presynaptic nerve terminal. The release of neurotransmitter is initiated by the action potential and the subsequent influx of Ca(2+) into the presynaptic nerve terminal. This results in the rapid fusion of vesicles with the nerve membrane and the release of the neurotransmitter into the synaptic cleft. The neurotransmitter then diffuses across the cleft and binds to specific postsynaptic receptors, resulting in a change in the membrane potential of the postsynaptic cell. This can result in the generation of an action potential. The high precision of synaptic transmission requires that pre- and postsynaptic structures are both highly organized and in juxtaposition to each other. In addition, alterations in synaptic transmission are the basis of learning and memory and are likely to be accompanied by the remodeling of synaptic structures (Toni et al., 1999). Thus, the study of how synapses are formed during development is also of relevance for the understanding of the cellular and molecular processes involved in learning and memory. This review focuses on the molecular mechanisms involved in the formation and the function of synapses.     

The problem of adequacy of the methods applying for testing of synaptic plasticity during processes of learning

Analysis of only the postsynaptic responses seems to be insufficient for studying the synaptic plasticity in learning, because they reflect not only synaptic modifications. The adequacy of brain slices application for investigation of the synaptic plasticity in learning per se has not been strictly specified. Learning processed can be adequately studied only in awake animals. However, traditional methods of field potential recording in response to stimulation of certain inputs that are well interpretable in vitro studies seem to be inadequate for in vivo testing synaptic plasticity. Single unit activity recording in pre- and postsynaptic fields during learning and direct threshold stimulation of monosynaptic inputs to a postsynaptic cell are suggested as a promising strategy for investigation of synaptic plasticity. Since the recording area is not deafferrented in a freely moving animal (as distinct from brain slices), the spontaneous activity in the neural network can interfere with responses to a testing stimulus. Computer simulation demonstrates that the interaction between spontaneous afferentation and testing stimulation can produce an illusion of synaptic modifications. Computer simulation of a neurophysiological experiment is proposed as a preliminary method for the reduction of the effect of spontaneous afferentation on the probability of the postsynaptic response.     

Synaptic proteins and the assembly of synaptic junctions

Synapses are highly specialized contact sites between neurons and their target cells where information in the form of chemical substances travels from a pre- to a postsynaptic cell. In the central nervous system of mammals, most nerve cells are innervated by functionally distinct types of synapses, each requiring a specific set of molecular constituents for proper function. Various molecular players that may be involved in the assembly of synaptic junctions have been identified recently.     

Natural patterns of activity and long-term synaptic plasticity

Long-term potentiation (LTP) of synaptic transmission is traditionally elicited by massively synchronous, high-frequency inputs, which rarely occur naturally. Recent in vitro experiments have revealed that both LTP and long-term depression (LTD) can arise by appropriately pairing weak synaptic inputs with action potentials in the postsynaptic cell. This discovery has generated new insights into the conditions under which synaptic modification may occur in pyramidal neurons in vivo. First, it has been shown that the temporal order of the synaptic input and the postsynaptic spike within a narrow temporal window determines whether LTP or LTD is elicited, according to a temporally asymmetric Hebbian learning rule. Second, backpropagating action potentials are able to serve as a global signal for synaptic plasticity in a neuron compared with local associative interactions between synaptic inputs on dendrites. Third, a specific temporal pattern of activity--postsynaptic bursting--accompanies synaptic potentiation in adults.     

Focal adhesion kinase regulates neuronal growth, synaptic plasticity and hippocampus-dependent spatial learning and memory

The focal adhesion kinase (FAK) is a non-receptor tyrosine kinase abundantly expressed in the mammalian brain and highly enriched in neuronal growth cones. Inhibitory and facilitatory activities of FAK on neuronal growth have been reported and its role in neuritic outgrowth remains controversial. Unlike other tyrosine kinases, such as the neurotrophin receptors regulating neuronal growth and plasticity, the relevance of FAK for learning and memory in vivo has not been clearly defined yet. A comprehensive study aimed at determining the role of FAK in neuronal growth, neurotransmitter release and synaptic plasticity in hippocampal neurons and in hippocampus-dependent learning and memory was therefore undertaken using the mouse model. Gain- and loss-of-function experiments indicated that FAK is a critical regulator of hippocampal cell morphology. FAK mediated neurotrophin-induced neuritic outgrowth and FAK inhibition affected both miniature excitatory postsynaptic potentials and activity-dependent hippocampal long-term potentiation prompting us to explore the possible role of FAK in spatial learning and memory in vivo. Our data indicate that FAK has a growth-promoting effect, is importantly involved in the regulation of the synaptic function and mediates in vivo hippocampus-dependent spatial learning and memory.     

Calcium/calmodulin-dependent protein kinase II and synaptic plasticity

A prominent role for calcium/calmodulin-dependent protein kinase II (CaMKII) in regulation of excitatory synaptic transmission was proposed two decades ago when it was identified as a major postsynaptic density protein. Since then, fascinating mechanisms optimized to fine-tune the magnitude and locations of CaMKII activity have been revealed. The importance of CaMKII activity and autophosphorylation to synaptic plasticity in vitro, and to a variety of learning and memory paradigms in vivo has been demonstrated. Recent progress brings us closer to understanding the regulation of dendritic CaMKII activity, localization, and expression, and its role in modulating synaptic transmission and cell morphology.     

Optogenetic analysis of synaptic function

We introduce optogenetic investigation of neurotransmission (OptIoN) for time-resolved and quantitative assessment of synaptic function via behavioral and electrophysiological analyses. We photo-triggered release of acetylcholine or gamma-aminobutyric acid at Caenorhabditis elegans neuromuscular junctions using targeted expression of Chlamydomonas reinhardtii Channelrhodopsin-2. In intact Channelrhodopsin-2 transgenic worms, photostimulation instantly induced body elongation (for gamma-aminobutyric acid) or contraction (for acetylcholine), which we analyzed acutely, or during sustained activation with automated image analysis, to assess synaptic efficacy. In dissected worms, photostimulation evoked neurotransmitter-specific postsynaptic currents that could be triggered repeatedly and at various frequencies. Light-evoked behaviors and postsynaptic currents were significantly (P 相似文献   

LAR-RPTPs: synaptic adhesion molecules that shape synapse development

The synapse is the most elementary operating unit in neurons, creating neural circuits that underlie all brain functions. Synaptic adhesion molecules initiate neuronal synapse connections, promote their stabilization and refinement, and control long-term synaptic plasticity. Leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs) have previously been implicated as essential elements in central nervous system (CNS) development. Recent studies have demonstrated that LAR-RPTP family members are also involved in diverse synaptic functions, playing a role in synaptic adhesion pathways together with a host of distinct transmembrane proteins and serving as major synaptic adhesion molecules in governing pre- and postsynaptic development, dysfunctions of which may underlie various disorders. This review highlights the emerging role of LAR-RPTPs as synapse organizers in orchestrating synapse development.     

Increased neuromuscular activity causes axonal defects and muscular degeneration

Before establishing terminal synapses with their final muscle targets, migrating motor axons form en passant synaptic contacts with myotomal muscle. Whereas signaling through terminal synapses has been shown to play important roles in pre- and postsynaptic development, little is known about the function of these early en passant synaptic contacts. Here, we show that increased neuromuscular activity through en passant synaptic contacts affects pre- and postsynaptic development. We demonstrate that in zebrafish twister mutants, prolonged neuromuscular transmission causes motor axonal extension and muscular degeneration in a dose-dependent manner. Cloning of twister reveals a novel, dominant gain-of-function mutation in the muscle-specific nicotinic acetylcholine receptor alpha-subunit, CHRNA1. Moreover, electrophysiological analysis demonstrates that the mutant subunit increases synaptic decay times, thereby prolonging postsynaptic activity. We show that as the first en passant synaptic contacts form, excessive postsynaptic activity in homozygous embryos severely impedes pre- and postsynaptic development, leading to degenerative defects characteristic of the human slow-channel congenital myasthenic syndrome. By contrast, in heterozygous embryos, transient and mild increase in postsynaptic activity does not overtly affect postsynaptic morphology but causes transient axonal defects, suggesting bi-directional communication between motor axons and myotomal muscle. Together, our results provide compelling evidence that during pathfinding, myotomal muscle cells communicate extensively with extending motor axons through en passant synaptic contacts.     

Signaling through Gs alpha is required for the growth and function of neuromuscular synapses in Drosophila

Although synapses are assembled in a highly regulated fashion, synapses once formed are not static structures but continue to expand and retract throughout the life of an organism. One second messenger that has been demonstrated to play a critical role in synaptic growth and function is cAMP. Here, we have tested the idea that signaling through the heterotrimeric G protein, Gs, plays a coincident role with increases in intracellular Ca(+2) in the regulation of adenylyl cyclases (ACs) during synaptic growth and in the function of synapses. In larvae containing a hypomorphic mutation in the dgs gene encoding the Drosophila Gs alpha protein, there is a significant decrease in the number of synaptic boutons and extent of synaptic arborization, as well as defects in the facilitation of synaptic transmission. Microscopic analysis confirmed that Gs alpha is localized at synapses both pre- and postsynaptically. Restricted expression of wild-type Gs alpha either pre- or postsynaptically rescued the mutational defects in bouton formation and defects in the facilitation of synaptic transmission, indicating that pathways activated by Gs alpha are likely to be involved in the reciprocal interactions between pre- and postsynaptic cells required for the development of mature synapses. In addition, this Gs alpha mutation interacted with fasII, dnc, and hyperexcitability mutants in a manner that revealed a coincident role for Gs alpha in the regulation of cAMP and FASII levels required during growth of these synapses. Our results demonstrate that Gs alpha-dependent signaling plays a role in the dynamic cellular reorganization that underlies synaptic growth.     

Glial cells maintain synaptic structure and function and promote development of the neuromuscular junction in vivo

To investigate the in vivo role of glial cells in synaptic function, maintenance, and development, we have developed an approach to selectively ablate perisynaptic Schwann cells (PSCs), the glial cells at the neuromuscular junction (NMJ), en masse from live frog muscles. In adults, following acute PSC ablation, synaptic structure and function were not altered. However, 1 week after PSC ablation, presynaptic function decreased by approximately half, while postsynaptic function was unchanged. Retraction of nerve terminals increased over 10-fold at PSC-ablated NMJs. Furthermore, nerve-evoked muscle twitch tension was reduced. In tadpoles, repeated in vivo observations revealed that PSC processes lead nerve terminal growth. In the absence of PSCs, growth and addition of synapses was dramatically reduced, and existing synapses underwent widespread retraction. Our findings provide in vivo evidence that glial cells maintain presynaptic structure and function at adult synapses and are vital for the growth and stability of developing synapses.     

Transsynaptic Coordination of Synaptic Growth,Function, and Stability by the L1-Type CAM Neuroglian

The precise control of synaptic connectivity is essential for the development and function of neuronal circuits. While there have been significant advances in our understanding how cell adhesion molecules mediate axon guidance and synapse formation, the mechanisms controlling synapse maintenance or plasticity in vivo remain largely uncharacterized. In an unbiased RNAi screen we identified the Drosophila L1-type CAM Neuroglian (Nrg) as a central coordinator of synapse growth, function, and stability. We demonstrate that the extracellular Ig-domains and the intracellular Ankyrin-interaction motif are essential for synapse development and stability. Nrg binds to Ankyrin2 in vivo and mutations reducing the binding affinities to Ankyrin2 cause an increase in Nrg mobility in motoneurons. We then demonstrate that the Nrg–Ank2 interaction controls the balance of synapse growth and stability at the neuromuscular junction. In contrast, at a central synapse, transsynaptic interactions of pre- and postsynaptic Nrg require a dynamic, temporal and spatial, regulation of the intracellular Ankyrin-binding motif to coordinate pre- and postsynaptic development. Our study at two complementary model synapses identifies the regulation of the interaction between the L1-type CAM and Ankyrin as an important novel module enabling local control of synaptic connectivity and function while maintaining general neuronal circuit architecture.     

Control of synaptic strength,a novel function of Akt

Akt (also known as PKB), a serine/threonine kinase involved in diverse signal-transduction pathways, is highly expressed in the brain. Akt is known to have a strong antiapoptotic action and thereby to be critically involved in neuronal survival, but its potential role in the dynamic modulation of synaptic transmission is unknown. Here we report that Akt phosphorylates, both in vitro and in vivo, the type A gamma-aminobutyric acid receptor (GABA(A)R), the principal receptor mediating fast inhibitory synaptic transmission in the mammalian brain. Akt-mediated phosphorylation increases the number of GABA(A)Rs on the plasma membrane surface, thereby increasing the receptor-mediated synaptic transmission in neurons. These results identify the GABA(A)R as a novel substrate of Akt, thereby linking Akt to the regulation of synaptic strength. This work also provides evidence for the rapid regulation of neurotransmitter receptor numbers in the postsynaptic domain by direct receptor phosphorylation as an important means of producing synaptic plasticity.     

Modulation of synaptic signalling complexes by Homer proteins

The number of neurotransmitter receptors in the postsynaptic membrane and their functional coupling to intracellular signalling cascades are important determinants of synaptic strength--and hence potential targets for plasticity related modulation. In this context, Homer/Vesl proteins have gained particular interest for three main reasons: (i) they constitute part of the molecular scaffold at postsynaptic densities of excitatory synapses in the mammalian brain; (ii) they physically link type-I metabotropic glutamate receptors to the postsynaptic density and to inositol 1,4,5-triphosphate receptors in the subsynaptic endoplasmic reticulum; and (iii) Homer-1a, which has been categorized as an immediate early gene isoform, exerts dominant-negative activity, suggesting that it is involved in activity dependent rearrangements at synaptic junctions. Although these fundamental aspects have been reviewed previously by Xiao et al., this review will address primarily more recent studies on the regulation of Homer 1a expression and on the role of Homer/Vesl proteins in spine morphogenesis and receptor targeting and signalling.     

Cadherins and synaptic plasticity

Given their trans-synaptic localization, their persistent expression at mature synapses and their distinct biochemical and adhesive properties, cadherins are uniquely poised at the synapse to mediate synaptic plasticity, the ability to change synaptic function thought to underlie learning and memory. For example recent work suggests that cadherins may recruit and stabilize AMPA receptors at the synapse via direct interactions or through complex formation, revealing cross talk between postsynaptic signaling and adhesion. Moreover, the use of small interfering RNA knockdown of cadherin, the availability of N-cadherin-deficient embryonic stem cells and the acute disruption of cadherin function with peptide application in vivo have allowed for more precise dissection of the molecular mechanisms by which cadherins function in both structural and functional plasticity.