Eph/ephrin对树突棘的调控与中枢神经系统疾病

树突棘是神经元树突上的功能性突起结构,通常作为突触后成份与投射来的轴突共同构成完整的突触连接。树突棘的形态与结构具有明显的可塑性,其变化通常会引起突触功能的改变。Eph受体酪氨酸激酶家族分子与其配体ephrin都是重要的神经导向因子,同时对树突棘结构也有直接的调控作用。Eph受体的活化可以促进树突棘的发生并影响树突棘的形态及内部结构;而Eph受体的异常也往往会损害正常的突触功能,甚至导致许多与树突棘结构异常相关的神经系统病变的发生。     

受体酪氨酸激酶依赖的信号转导在突触可塑性中的研究

突触可塑性对于脑发育过程中的神经环路重构以及学习记忆等脑的高级功能是非常重要的。许多受体酪氨酸激酶家族成员,包括TrkB、ErbB和Eph在神经连接的建立和重构过程中起到核心作用。比如,突触后EphB依赖的信号会导致树突棘的产生和神经递质受体的聚集,而ephrinA引起的EphA4激活可以导致树突棘的回缩。但是,目前对EphA4依赖的树突棘重组和对神经递质受体的调节背后的机制还知之甚少。本文将集中探讨EphA4及其下游的信号通路在神经肌肉接头和中枢神经的突触中,对神经递质受体的调节功能。     

神经元的突触可塑性与学习和记忆

大量研究表明,神经元的突触可塑性包括功能可塑性和结构可塑性,与学习和记忆密切相关.最近,在经过训练的动物海马区,记录到了学习诱导的长时程增强(long term potentiation,LTP),如果用激酶抑制剂阻断晚期LTP,就会使大鼠丧失训练形成的记忆.这些结果指出,LTP可能是形成记忆的分子基础.因此,进一步研究哺乳动物脑内突触可塑性的分子机制,对揭示学习和记忆的神经基础有重要意义.此外,在精神迟滞性疾病和神经退行性疾病患者脑内记录到异常的LTP,并发现神经元的树突棘数量减少,形态上产生畸变或萎缩,同时发现,产生突变的基因大多编码调节突触可塑性的信号通路蛋白,故突触可塑性研究也将促进精神和神经疾病的预防和治疗.综述了突触可塑性研究的最新进展,并展望了其发展前景.     

NMDA受体参与小鼠的前额扣带回的神经突触传递

谷氨酸性突触是哺乳动物神经系统的主要兴奋性突触。在正常条件下,大多数的突触反应是由谷氨酸的AMPA受体传递的。NMDA受体在静息电位下为镁离子抑制。在被激活时,NMDA受体主要参与突触的可塑性变化。但是,许多NMDA受体拮抗剂在全身或局部注射时能产生行为效应,提示NMDA受体可能参与静息状态的生理功能。此文中,我们在离体的前额扣带回脑片上进行电生理记录,发现NMDA受体参与前额扣带回的突触传递。在重复刺激或近于生理性温度时,NMDA受体传递的反应更为明显。本文直接显示了NMDA受体参与前额扣带回的突触传递,并提示NMDA受体在前额扣带回中起着调节神经元兴奋的重要作用。     

NMDA受体与Ⅰ组代谢型谷氨酸受体的相互作用

谷氨酸是哺乳动物中枢神经系统重要兴奋性神经递质,参与学习、记忆、药物依赖成瘾及神经系统退行性疾病等多种病理生理过程。谷氨酸通过激活离子型（iGluRs）和代谢型谷氨酸受体（mGluRs）发挥作用。业已有研究提示iGluRs和mGluRs之间存在相互作用,但具体机制尚待阐明。本文从蛋白分子结构、突触可塑性、相互作用可能涉及的信号分子和通路等方面综述了NMDAR与Ⅰ组mGluRs之间的相互作用,旨在为深入研究谷氨酸受体之间的相互作用提供线索。     

谷氨酸性突触在痛觉和记忆中的突触和分子机制

谷氨酸是哺乳动物脑中的兴奋性递质。中枢神经系统的谷氨酸性突触广泛参与痛觉传递,突触可塑性和递质的调节。谷氨酸的NMDA受体参与前脑相关的学习及功能。在这篇综述中,我们提出前脑的NMDA受体通过增强谷氨酸性突触传递导致长期性的炎痛。具有增强NMDA受体功能的小鼠会产生更多的慢性痛。NMDA NR2B受体抑制剂在未来可能被用来控制人类的慢性痛。     

细胞间黏附分子5在树突棘发育和突触可塑性中的研究进展

诸多神经精神性疾病的发生均伴有树突棘发育异常。免疫球蛋白超家族成员细胞间黏附分子5(intercellular adhesion molecule 5,ICAM5)是一个通过抑制树突棘成熟,将其维持在丝状形态的跨膜蛋白,它只表达于端脑兴奋性神经元,可能与树突棘发育、突触可塑性乃至学习记忆密切相关。现综述了ICAM5的发现和特征、分子结构、基因结构、在树突棘发育过程中的作用,以及与脆性X综合征等疾病的关系,试图为阐明发育阶段脑神经元异常树突棘形成的机制提供线索。     

外周输入依赖的嗅球颗粒细胞的突触结构可塑性

活动依赖的突触结构可塑性是学习和记忆的基础.哺乳动物,尤其是啮齿类动物,具有高度发达的嗅觉系统和惊人的气味学习和记忆能力.本研究以CNGA2敲除而导致外周输入缺失的小鼠为模型,研究嗅球内活动依赖的突触结构可塑性.利用特异性的突触前和突触后标记物,发现外周输入缺失减少了突触标记蛋白突触素(synaptophysin)和抑制性突触标记蛋白桥蛋白(gephyrin)在嗅球外网状层和颗粒细胞层中的表达;兴奋性突触标记蛋白囊泡谷氨酸转运蛋白1(VGluT1)的表达水平只在外网状层中有显著下降,而在颗粒细胞层中没有明显变化.进一步通过活体质粒电转标记嗅球颗粒细胞后发现,CNGA2敲除小鼠颗粒细胞上位于外网状层中的远端树突棘密度显著减小,而位于颗粒细胞层中的近端树突棘密度没有明显变化.这些结果表明颗粒细胞上的树-树突触具有对外周活动依赖的结构可塑性,而轴-树突触则无.     

NMDA受体信号复合体中蛋白质的相互作用

谷氨酸能兴奋性突触的突触后密集区(postsynaptic density,PSD)包含多种受体蛋白、骨架蛋白和信号蛋白,它们通过分子中特定的结构域相互识别并动态地结合,形成多个信号复合体,参与突触后受体功能的调节及其下游特异性信号转导通路的激活。其中,NMDA受体信号复合体中蛋白质-蛋白质的相互作用及其调控机制的阐明,对于深入了解神经发育、突触可塑性、兴奋性毒性等生理病理的分子机制有重要意义。     

受体、突触和记忆

大脑中神经元突触间的信号传递是由许多神经递质受体介导的。在过去,Richard L．Huganir实验室一直致力于神经递质受体功能调节的分子机制。而最近,该实验室又聚焦到大脑中一种最主要的兴奋性受体的研究——谷氨酸受体。谷氨酸受体主要可以分为两大类：AMPA受体和NMDA受体。AMPA受体主要介导了快速的兴奋性突触传递;而NMDA受体则在神经可塑性和发育中起到重要作用。实验发现,AMPA受体和NMDA受体都可以被一系列的蛋白激酶磷酸化,而磷酸化的水平则直接影响了这些受体的功能特性,包括通道电导和受体膜定位等。AMPA受体磷酸化的水平同时还在学习和记忆的细胞模型中发生改变,如长时程增强（LTP）和长时程抑制（LTD）。此外,AMPA受体中GluR1亚单位的磷酸化对于各种形式的可塑性以及空间记忆的维持有重要的作用。实验室主要研究突触部位谷氨酸受体在亚细胞水平的定位和聚集的分子机制。最近,一系列可以直接或间接与AMPA和NMDA受体相互作用的蛋白质得以发现,其中包括一个新发现的蛋白家族GRIPs（glutamate receptor interacting proteins）。GRIPs可以直接和AMPA受体的GluR2／3亚单位的C端结合。GRIPs包含7个PDZ结构域,可以介导蛋白与蛋白直接的相互连接,从而把各个AMPA受体交互连接在一起并与其他蛋白相连。另外,GluR2亚单位的c端还可以和兴奋性突触中的蛋白激酶C结合蛋白（PICK1）的PDZ结构域相互作用。另外,GluR2亚单位的C端也可以与一种参与膜融合的蛋白NSF相互作用。这些与AMPA受体相互作用的蛋白质对于受体在膜上的运输以及定位有至关重要的作用。同时,受体与PICK1和GRIP的结合对于小脑运动学习中的LTD有重要作用。总体上说,该实验室发现了一系列可以调节神经递质受体功能的分子机制,这些工作提示受体功能的调节可能是?     

Role of actin cytoskeleton in dendritic spine morphogenesis

Dendritic spines are the postsynaptic receptive regions of most excitatory synapses, and their morphological plasticity play a pivotal role in higher brain functions, such as learning and memory. The dynamics of spine morphology is due to the actin cytoskeleton concentrated highly in spines. Filopodia, which are thin and headless protrusions, are thought to be precursors of dendritic spines. Drebrin, a spine-resident side-binding protein of filamentous actin (F-actin), is responsible for recruiting F-actin and PSD-95 into filopodia, and is suggested to govern spine morphogenesis. Interestingly, some recent studies on neurological disorders accompanied by cognitive deficits suggested that the loss of drebrin from dendritic spines is a common pathognomonic feature of synaptic dysfunction. In this review, to understand the importance of actin-binding proteins in spine morphogenesis, we first outline the well-established knowledge pertaining to the actin cytoskeleton in non-neuronal cells, such as the mechanism of regulation by small GTPases, the equilibrium between globular actin (G-actin) and F-actin, and the distinct roles of various actin-binding proteins. Then, we review the dynamic changes in the localization of drebrin during synaptogenesis and in response to glutamate receptor activation. Because side-binding proteins are located upstream of the regulatory pathway for actin organization via other actin-binding proteins, we discuss the significance of drebrin in the regulatory mechanism of spine morphology through the reorganization of the actin cytoskeleton. In addition, we discuss the possible involvement of an actin-myosin interaction in the morphological plasticity of spines.     

Protein kinase D promotes plasticity-induced F-actin stabilization in dendritic spines and regulates memory formation

Actin turnover in dendritic spines influences spine development, morphology, and plasticity, with functional consequences on learning and memory formation. In nonneuronal cells, protein kinase D (PKD) has an important role in stabilizing F-actin via multiple molecular pathways. Using in vitro models of neuronal plasticity, such as glycine-induced chemical long-term potentiation (LTP), known to evoke synaptic plasticity, or long-term depolarization block by KCl, leading to homeostatic morphological changes, we show that actin stabilization needed for the enlargement of dendritic spines is dependent on PKD activity. Consequently, impaired PKD functions attenuate activity-dependent changes in hippocampal dendritic spines, including LTP formation, cause morphological alterations in vivo, and have deleterious consequences on spatial memory formation. We thus provide compelling evidence that PKD controls synaptic plasticity and learning by regulating actin stability in dendritic spines.     

Structural and functional plasticity of dendritic spines – root or result of behavior?

Dendritic spines are multifunctional integrative units of the nervous system and are highly diverse and dynamic in nature. Both internal and external stimuli influence dendritic spine density and morphology on the order of minutes. It is clear that the structural plasticity of dendritic spines is related to changes in synaptic efficacy, learning and memory and other cognitive processes. However, it is currently unclear whether structural changes in dendritic spines are primary instigators of changes in specific behaviors, a consequence of behavioral changes, or both. In this review, we first examine the basic structure and function of dendritic spines in the brain, as well as laboratory methods to characterize and quantify morphological changes in dendritic spines. We then discuss the existing literature on the temporal and functional relationship between changes in dendritic spines in specific brain regions and changes in specific behaviors mediated by those regions. Although technological advancements have allowed us to better understand the functional relevance of structural changes in dendritic spines that are influenced by environmental stimuli, the role of spine dynamics as an underlying driver or consequence of behavior still remains elusive. We conclude that while it is likely that structural changes in dendritic spines are both instigators and results of behavioral changes, improved research tools and methods are needed to experimentally and directly manipulate spine dynamics in order to more empirically delineate the relationship between spine structure and behavior.     

Rapid induction of dendritic spine morphogenesis by trans-synaptic ephrinB-EphB receptor activation of the Rho-GEF kalirin

The morphogenesis of dendritic spines, the major sites of excitatory synaptic transmission in the brain, is important in synaptic development and plasticity. We have identified an ephrinB-EphB receptor trans-synaptic signaling pathway which regulates the morphogenesis and maturation of dendritic spines in hippocampal neurons. Activation of the EphB receptor induces translocation of the Rho-GEF kalirin to synapses and activation of Rac1 and its effector PAK. Overexpression of dominant-negative EphB receptor, catalytically inactive kalirin, or dominant-negative Rac1, or inhibition of PAK eliminates ephrin-induced spine development. This novel signal transduction pathway may be critical for the regulation of the actin cytoskeleton controlling spine morphogenesis during development and plasticity.     

alpha5 integrin signaling regulates the formation of spines and synapses in hippocampal neurons

The actin-based dynamics of dendritic spines play a key role in synaptic plasticity, which underlies learning and memory. Although it is becoming increasingly clear that modulation of actin is critical for spine dynamics, the upstream molecular signals that regulate the formation and plasticity of spines are poorly understood. In non-neuronal cells, integrins are critical modulators of the actin cytoskeleton, but their function in the nervous system is not well characterized. Here we show that alpha5 integrin regulates spine morphogenesis and synapse formation in hippocampal neurons. Knockdown of alpha5 integrin expression using small interfering RNA decreased the number of dendritic protrusions, spines, and synapses. Expression of constitutively active or dominant negative alpha5 integrin also resulted in alterations in the number of dendritic protrusions, spines, and synapses. alpha5 integrin signaling regulates spine morphogenesis and synapse formation by a mechanism that is dependent on Src kinase, Rac, and the signaling adaptor GIT1. Alterations in the activity or localization of these molecules result in a significant decrease in the number of spines and synapses. Thus, our results point to a critical role for integrin signaling in regulating the formation of dendritic spines and synapses in hippocampal neurons.     

Loss of the actin regulator cyclase-associated protein 1 (CAP1) modestly affects dendritic spine remodeling during synaptic plasticity

Dendritic spines form the postsynaptic compartment of most excitatory synapses in the vertebrate brain. Morphological changes of dendritic spines contribute to major forms of synaptic plasticity such as long-term potentiation (LTP) or depression (LTD). Synaptic plasticity underlies learning and memory, and defects in synaptic plasticity contribute to the pathogeneses of human brain disorders. Hence, deciphering the molecules that drive spine remodeling during synaptic plasticity is critical for understanding the neuronal basis of physiological and pathological brain function. Since actin filaments (F-actin) define dendritic spine morphology, actin-binding proteins (ABP) that accelerate dis-/assembly of F-actin moved into the focus as critical regulators of synaptic plasticity. We recently identified cyclase-associated protein 1 (CAP1) as a novel actin regulator in neurons that cooperates with cofilin1, an ABP relevant for synaptic plasticity. We therefore hypothesized a crucial role for CAP1 in structural synaptic plasticity. By exploiting mouse hippocampal neurons, we tested this hypothesis in the present study. We found that induction of both forms of synaptic plasticity oppositely altered concentration of exogenous, myc-tagged CAP1 in dendritic spines, with chemical LTP (cLTP) decreasing and chemical LTD (cLTD) increasing it. cLTP induced spine enlargement in CAP1-deficient neurons. However, it did not increase the density of large spines, different from control neurons. cLTD induced spine retraction and spine size reduction in control neurons, but not in CAP1-KO neurons. Together, we report that postsynaptic myc-CAP1 concentration oppositely changed during cLTP and cTLD and that CAP1 inactivation modestly affected structural plasticity.     

Actin filaments and microtubules in dendritic spines

Dendritic spines are small protrusions emerging from their parent dendrites, and their morphological changes are involved in synaptic plasticity. These tiny structures are composed of thousands of different proteins belonging to several subfamilies such as membrane receptors, scaffold proteins, signal transduction proteins, and cytoskeletal proteins. Actin filaments in dendritic spines consist of double helix of actin protomers decorated with drebrin and ADF/cofilin, and the balance of the two is closely related to the actin dynamics, which may govern morphological and functional synaptic plasticity. During development, the accumulation of drebrin‐binding type actin filaments is one of the initial events occurring at the nascent excitatory postsynaptic site, and plays a pivotal role in spine formation as well as small GTPases. It has been recently reported that microtubules transiently appear in dendritic spines in correlation with synaptic activity. Interestingly, it is suggested that microtubule dynamics might couple with actin dynamics. In this review, we will summarize the contribution of both actin filaments and microtubules to the formation and regulation of dendritic spines, and further discuss the role of cytoskeletal deregulation in neurological disorders.     

Local protein synthesis, actin dynamics, and LTP consolidation

Modulation of local protein synthesis in neuronal dendrites plays a key role in the production of long-term, activity-dependent changes in synapse structure and functional efficacy. Such long-term changes also require regulation of actin dynamics in dendritic spines. Recent evidence couples local protein synthesis to regulation of actin dynamics in long-term synaptic plasticity. Translation of the dendritically localized mRNA, Arc, is required for consolidation of LTP and stabilization of nascent polymerized actin. BDNF signaling activates Arc-dependent LTP consolidation and is required for actin polymerization and stable expansion of dendritic spines during LTP. Regulation of actin pools within dendritic spines modulates spine size and enlargement, organization of the postsynaptic density, receptor trafficking, and localization of the translational machinery.     

Regulation of neuronal PKA signaling through AKAP targeting dynamics

Central to organization of signaling pathways are scaffolding, anchoring and adaptor proteins that mediate localized assembly of multi-protein complexes containing receptors, second messenger-generating enzymes, kinases, phosphatases, and substrates. At the postsynaptic density (PSD) of excitatory synapses, AMPA (AMPAR) and NMDA (NMDAR) glutamate receptors are linked to signaling proteins, the actin cytoskeleton, and synaptic adhesion molecules on dendritic spines through a network of scaffolding proteins that may play important roles regulating synaptic structure and receptor functions in synaptic plasticity underlying learning and memory. AMPARs are rapidly recruited to dendritic spines through NMDAR activation during induction of long-term potentiation (LTP) through pathways that also increase the size and F-actin content of spines. Phosphorylation of AMPAR-GluR1 subunits by the cAMP-dependent protein kinase (PKA) helps stabilize AMPARs recruited during LTP. In contrast, induction of long-term depression (LTD) leads to rapid calcineurin-protein phosphatase 2B (CaN) mediated dephosphorylation of PKA-phosphorylated GluR1 receptors, endocytic removal of AMPAR from synapses, and a reduction in spine size. However, mechanisms for coordinately regulating AMPAR localization, phosphorylation, and synaptic structure by PKA and CaN are not well understood. A kinase-anchoring protein (AKAP) 79/150 is a PKA- and CaN-anchoring protein that is linked to NMDARs and AMPARs through PSD-95 and SAP97 membrane-associated guanylate kinase (MAGUK) scaffolds. Importantly, disruption of PKA-anchoring in neurons and functional analysis of GluR1-MAGUK-AKAP79 complexes in heterologous cells suggests that AKAP79/150-anchored PKA and CaN may regulate AMPARs in LTD. In the work presented at the "First International Meeting on Anchored cAMP Signaling Pathways" (Berlin-Buch, Germany, October 15-16, 2005), we demonstrate that AKAP79/150 is targeted to dendritic spines by an N-terminal basic region that binds phosphatidylinositol-4,5-bisphosphate (PIP(2)), F-actin, and actin-linked cadherin adhesion molecules. Thus, anchoring of PKA and CaN as well as physical linkage of the AKAP to both cadherin-cytoskeletal and MAGUK-receptor complexes could play roles in coordinating changes in synaptic structure and receptor signaling functions underlying plasticity. Importantly, we provide evidence showing that NMDAR-CaN signaling pathways implicated in AMPAR regulation during LTD lead to a disruption of AKAP79/150 interactions with actin, MAGUKs, and cadherins and lead to a loss of the AKAP and anchored PKA from postsynapses. Our studies thus far indicate that this AKAP79/150 translocation depends on activation of CaN, F-actin reorganization, and possibly Ca(2+)-CaM binding to the N-terminal basic regions. Importantly, this tranlocation of the AKAP79/150-PKA complex from spines may shift the balance of PKA kinase and CaN/PP1 phosphatase activity at the postsynapse in favor of the phosphatases. This loss of PKA could then promote actions of CaN and PP1 during induction of LTD including maintaining AMPAR dephosphorylation, promoting AMPAR endocytosis, and preventing AMPAR recycling. Overall, these findings challenge the accepted notion that AKAPs are static anchors that position signaling proteins near fixed target substrates and instead suggest that AKAPs can function in more dynamic manners to regulate local signaling events.