神经肌肉接头突触发育信号机制研究进展

神经肌肉接头是目前研究较为深入的一种经典外周胆碱能化学突触。神经肌肉接头突触形成依赖于运动神经元与骨骼肌细胞之间的精细相互作用和复杂信号传递。此外,胶质细胞在神经肌肉接头突触发育和成熟过程中亦发挥重要功能。现将主要围绕近年来神经肌肉接头发育过程中若干重要信号通路以及相关神经肌肉系统疾病的主要研究进展进行综述。     

神经肌肉接头研究世纪回顾

神经肌肉接头作为突触解剖、生理和发育的模型已被研究了一个多世纪。成像技术提供了诸多关于神经肌肉接头的信息,其中一些技术是专为观察该突触的结构和功能而发展起来的。本文回顾了神经肌肉接头研究中几个重要方面的发展史,包括其结构、N型乙酰胆碱受体的分布、突触小泡释放,以及神经肌肉接头的发育。     

神经细胞粘连分子和多聚唾液酸在神经发育和再生中的作用

神经系统的形成依赖于细胞间的互相粘连。本文综述了神经细胞粘连分子（ＮＣＡＭ）及其多聚唾液酸（ＰＳＡ）组份对神经发育和再生的作用。ＮＣＡＭ的基本功能是介导细胞粘连,ＰＳＡ则由于其特殊的分子结构而降低细胞间的粘连。研究表明,鸡胚的发育过程中,ＰＳＡ含量在三个关键时期表达的高低决定了运动神经元能否准确地识别和支配肌肉。成年大鼠周围神经损伤后,肌肉内ＮＣＡＭ含量的高低决定于该肌肉的神经支配状况。成年大鼠脑内,切断内嗅皮层与海马的神经联系,发现齿回外分子层ＰＳＡ含量显著增加,并至少可持续６０天。已有的研究资料提示在去神经靶区域ＰＳＡ的重新表达可能有利于移植神经元轴突的生长并与宿主重建突触联系。     

突触细胞黏附分子在应激中作用的研究进展

突触细胞黏附分子是一类介导突触前、后膜结构和功能互作的膜表面糖蛋白,可以动态调节突触活动和可塑性,其表达与功能受到环境因素调控。突触细胞黏附分子亦是应激反应重要的效应分子之一,可介导应激对认知和情绪的不良影响。本文综述近年来突触细胞黏附分子在应激中作用的研究进展,旨在为应激相关障碍的分子机制研究和药物研发提供思路。     

免疫突触的形成及其介导的分子识别

T细胞和APC细胞相互作用形成免疫突触涉及到连续发生的一系列的分子识别事件,最初APC细胞在趋化因子的作用下向T细胞移动,相遇后在抗原非依赖性的弱的黏附力作用下发生最初的黏附,同时伴随着TCR在APC表面俘获特异性抗原;抗原识别之后,由多种机制使T细胞和APC紧密接触并维持一段时间,随后分开,最终引起T细胞的增殖和分化。对免疫突触形成过程中的分子识别机制目前尚无定论,拓扑模式和数学模式的解释,脂筏和细胞骨架蛋白的重排以及接头蛋白的连接为免疫突触形成中分子的识别提供了一定的依据。     

neurexin家族在突触发生和突触传递中作用的研究进展

neurexin家族属于神经细胞表面蛋白,参与细胞识别和细胞黏附,可能介导细胞信号转导。最近研究表明,neurexins在突触发生和突触传递等过程中发挥重要作用,并可能影响学习记忆功能。这些研究进展对于进一步揭示neurexins在神经突触可塑性及其在学习记忆过程中的可能作用具有重要意义。本文主要对neurexin家族的研究概况、NRXN1在突触发生和突触传递中的功能及其在学习记忆功能中的可能作用进行简要综述。     

突触泡蛋白2研究进展

突触泡蛋白2(SV2)是一类跨膜糖蛋白,定位于脊椎动物神经元及内分泌细胞,与神经递质的释放、内分泌泡胞吐作用、突触泡稳态的维持、神经肌肉接头的形成及肾上腺素能受体α2C的定位密切相关。最近还发现SV2是肉毒神经毒素BoNT/A的受体,介导BoNT/A进入神经元。SV2可作为突触泡标记蛋白,广泛应用于生物学研究及肿瘤诊断。此外,SV2还是抗癫痫药物的作用靶标。     

集聚蛋白:神经突触和免疫突触中的共同分子

集聚蛋白是一种细胞外基质蛋白其编码基因全长8kb左右,蛋白质分子量为200-600kD。先后发现于电鳐,大鼠,鸡,小鼠及人等机体内,在海马,神经肌接头,雪旺氏细胞,心肌,肾脏等组织均有表达,其在神经突触中的主要作用为引起神经突触后膜的AchR发生集聚和膜结构的稳定,促进神经突触的发育和形成,近来Rupp等人发现集聚蛋白亦在免疫系统中表达,活化T细胞产生的集聚蛋白对T细胞突触的形成,TCR,CD28,CD3等免疫分子和脂筏的集聚起着重要的作用并能明显降低特异性抗原的刺激域值。     

血纤维蛋白溶酶原激活因子及其抑制因子在神经系统的作用

血纤维蛋白溶酶原激活因子和血纤维蛋白溶酶原激活因子的抑制因子与神经肌肉系统的生长发育有十分密切关系。ＰＡ对神经细胞的增殖、迁移、神经的变性与再生和对肌肉卫星细胞的融全以及对神经肌肉间突触的成和可塑性都有明显的作用。     

体内跟踪单个冲经细胞及其突触分布

近日,科学家开发了一项能够在成体水平小鼠脑区的复杂神经网络系统中特异性标记单个神经元细胞及其神经突触分布形式的技术。该项技术利用转基因方法在小鼠不同脑区的神经元细胞可控地表达不同的荧光蛋白和突触囊泡蛋白,应用不同分子标记对神经元细胞及其突触进行特异性标记,能够在单个神经元细胞水平上提供详尽的三维神经突触分布信息,     

Synapse formation regulated by protein tyrosine phosphatase receptor T through interaction with cell adhesion molecules and Fyn

The receptor‐type protein tyrosine phosphatases (RPTPs) have been linked to signal transduction, cell adhesion, and neurite extension. PTPRT/RPTPρ is exclusively expressed in the central nervous system and regulates synapse formation by interacting with cell adhesion molecules and Fyn protein tyrosine kinase. Overexpression of PTPRT in cultured neurons increased the number of excitatory and inhibitory synapses by recruiting neuroligins that interact with PTPRT through their ecto‐domains. In contrast, knockdown of PTPRT inhibited synapse formation and withered dendrites. Incubation of cultured neurons with recombinant proteins containing the extracellular region of PTPRT reduced the number of synapses by inhibiting the interaction between ecto‐domains. Synapse formation by PTPRT was inhibited by phosphorylation of tyrosine 912 within the membrane–proximal catalytic domain of PTPRT by Fyn. This tyrosine phosphorylation reduced phosphatase activity of PTPRT and reinforced homophilic interactions of PTPRT, thereby preventing the heterophilic interaction between PTPRT and neuroligins. These results suggest that brain‐specific PTPRT regulates synapse formation through interaction with cell adhesion molecules, and this function and the phosphatase activity are attenuated through tyrosine phosphorylation by the synaptic tyrosine kinase Fyn.     

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.     

Cell adhesion molecules in the central nervous system

Cell-cell adhesion molecules play key roles at the intercellular junctions of a wide variety of cells, including interneuronal synapses and neuron-glia contacts. Functional studies suggest that adhesion molecules are implicated in many aspects of neural network formation, such as axon-guidance, synapse formation, regulation of synaptic structure and astrocyte-synapse contacts. Some basic cell biological aspects of the assembly of junctional complexes of neurons and glial cells resemble those of epithelial cells. However, the neuron specific junctional machineries are required to exert neuronal functions, such as synaptic transmission and plasticity. In this review, we describe the distribution and function of cell adhesion molecules at synapses and at contacts between synapses and astrocytes.Key words: synapses, cell adhesion molecules, cadherin superfamily, immunoglobulin superfamily, nerve tissue proteins, axons     

Ligand-Induced Cell Adhesion as a New Mechanism to Promote Synapse Formation

The formation of neuronal synapses is a finely organized process that involves the presynaptic assembly of the machinery responsible for neurotransmitter release and the postsynaptic recruitment of neurotransmitter receptors and scaffold proteins to the postsynaptic density (PSD). The molecular cues guiding the establishment of synaptic connections are now beginning to be identified. Recent evidences indicate that cell adhesion molecules (CAMs) participate prominently in the key steps of synapse formation, inducing trans-synaptic adhesion and promoting a precise alignment of pre- and postsynaptic terminals. This addendum describes a new mechanism of cell-cell interaction that combines features of both diffusible and membrane-bound synaptogenic factors. It particularly points out the key role played by GDNF triggering trans-homophilic binding between GFRα1 molecules and cell adhesion between GFRα1-expressing cells. In this model GFRα1 functions as a ligand-induced cell adhesion molecule (LICAM) to establish precise synaptic contacts and promote the assembly of presynaptic terminals. In this overview, I summarize the current concepts of synapse formation in the limelight of this new mechanism of ligand-induced cell adhesion.     

Cell adhesion molecules and actin cytoskeleton at immune synapses and kinapses

The immunological synapse is a stable adhesive junction between a polarized immune effector cell and an antigen-bearing cell. Immunological synapses are often observed to have a striking radial symmetry in the plane of contact with a prominent central cluster of antigen receptors surrounded by concentric rings of adhesion molecules and actin-rich projections. There is a striking similarity between the radial zones of the immunological synapse and the dynamic actinomyosin modules employed by migrating cells. Breaking the symmetry of an immunological synapse generates a moving adhesive junction that can be defined as a kinapse, which facilitates signal integration by immune cells while moving over the surface of antigen-presenting cells.     

The Extent of Synaptic Stripping of Motoneurons after Axotomy Is Not Correlated to Activation of Surrounding Glia or Downregulation of Postsynaptic Adhesion Molecules

Synapse elimination in the adult central nervous system can be modelled by axotomy of spinal motoneurons which triggers removal of synapses from the cell surface of lesioned motoneurons by processes that remain elusive. Proposed candidate mechanisms are removal of synapses by reactive microglia and astrocytes, based on the remarkable activation of these cell types in the vicinity of motoneurons following axon lesion, and/or decreased expression of synaptic adhesion molecules in lesioned motoneurons. In the present study, we investigated glia activation and adhesion molecule expression in motoneurons in two mouse strains with deviant patterns of synapse elimination following axotomy. Mice deficient in complement protein C3 display a markedly reduced loss of synapses from axotomized motoneurons, whereas mice with impaired function of major histocompatibility complex (MHC) class Ia display an augmented degree of stripping after axotomy. Activation of microglia and astrocytes was assessed by semiquantative immunohistochemistry for Iba 1 (microglia) and GFAP (astrocytes), while expression of synaptic adhesion molecules was determined by in situ hybridization. In spite of the fact that the two mouse strains display very different degrees of synapse elimination, no differences in terms of glial activation or in the downregulation of the studied adhesion molecules (SynCAM1, neuroligin-2,-3 and netrin G-2 ligand) could be detected. We conclude that neither glia activation nor downregulation of synaptic adhesion molecules are correlated to the different extent of the synaptic stripping in the two studied strains. Instead the magnitude of the stripping event is most likely a consequence of a precise molecular signaling, which at least in part is mediated by immune molecules.     

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.     

MHC class II-peptide complexes and APC lipid rafts accumulate at the immunological synapse

Activation of CD4(+) Th cells requires their cognate interaction with APCs bearing specific relevant MHC class II-peptide complexes. This cognate interaction culminates in the formation of an immunological synapse that contains the various proteins and lipids required for efficient T cell activation. We now show that APC lipid raft membrane microdomains contain specific class II-peptide complexes and serve as platforms that deliver these raft-associated class II molecules to the immunological synapse. APC rafts are required for T cell:APC conjugate formation and T cell activation at low densities of relevant class II-peptide complexes, a requirement that can be overcome at high class II-peptide density. Analysis of confocal microscopy images revealed that over time APC lipid rafts, raft-associated relevant class II-peptide complexes, and even immunologically irrelevant class II molecules accumulate at the immunological synapse. As the immunological synapse matures, relevant class II-peptide complexes are sorted to a central region of the interface, while irrelevant class II molecules are excluded from this site. We propose that T cell activation is facilitated by recruitment of MHC class II-peptide complexes to the immunological synapse by virtue of their constitutive association with lipid raft microdomains.     

The role of cell adhesion molecules in synaptic plasticity and memory.

Studies in the past few years suggest that cell adhesion molecules may play signaling as well as structural roles at adult synapses during plasticity. The observation that many adhesion molecules are expressed both pre-synaptically and post-synaptically raises the possibility that information about synaptic activity might simultaneously be communicated to both sides of the synapse, circumventing the need for distinct anterograde and retrograde messengers.     

Latrophilins Function as Heterophilic Cell-adhesion Molecules by Binding to Teneurins: REGULATION BY ALTERNATIVE SPLICING*

Latrophilin-1, -2, and -3 are adhesion-type G protein-coupled receptors that are auxiliary α-latrotoxin receptors, suggesting that they may have a synaptic function. Using pulldowns, we here identify teneurins, type II transmembrane proteins that are also candidate synaptic cell-adhesion molecules, as interactors for the lectin-like domain of latrophilins. We show that teneurin binds to latrophilins with nanomolar affinity and that this binding mediates cell adhesion, consistent with a role of teneurin binding to latrophilins in trans-synaptic interactions. All latrophilins are subject to alternative splicing at an N-terminal site; in latrophilin-1, this alternative splicing modulates teneurin binding but has no effect on binding of latrophilin-1 to another ligand, FLRT3. Addition to cultured neurons of soluble teneurin-binding fragments of latrophilin-1 decreased synapse density, suggesting that latrophilin binding to teneurin may directly or indirectly influence synapse formation and/or maintenance. These observations are potentially intriguing in view of the proposed role for Drosophila teneurins in determining synapse specificity. However, teneurins in Drosophila were suggested to act as homophilic cell-adhesion molecules, whereas our findings suggest a heterophilic interaction mechanism. Thus, we tested whether mammalian teneurins also are homophilic cell-adhesion molecules, in addition to binding to latrophilins as heterophilic cell-adhesion molecules. Strikingly, we find that although teneurins bind to each other in solution, homophilic teneurin-teneurin binding is unable to support stable cell adhesion, different from heterophilic teneurin-latrophilin binding. Thus, mammalian teneurins act as heterophilic cell-adhesion molecules that may be involved in trans-neuronal interaction processes such as synapse formation or maintenance.