Structural mechanisms of the agrin–LRP4–MuSK signaling pathway in neuromuscular junction differentiation

The neuromuscular junction (NMJ) is the most extensively studied model of neuronal synaptogenesis. Acetylcholine receptor (AChR) clustering on the postsynaptic membrane is a cardinal event in the differentiation of NMJs. AChR clustering and postsynaptic differentiation is orchestrated by sophisticated interactions among three proteins: the neuron-secreted proteoglycan agrin, the co-receptor LRP4, and the muscle-specific receptor tyrosine kinase MuSK. LRP4 and MuSK act as scaffolds for multiple binding partners, resulting in a complex and dynamic network of interacting proteins that is required for AChR clustering. In this review, we discuss the structural basis for NMJ postsynaptic differentiation mediated by the agrin–LRP4–MuSK signaling pathway.     

Intracellular trafficking of AMPA receptors in synaptic plasticity

Modification of ligand-gated receptor function at the postsynaptic domain is one of the most important mechanisms by which the efficacy of synaptic transmission in the nervous system is regulated. Traditionally, these types of modifications have been thought to be achieved mainly by altering the channel-gating properties or conductance of the receptors. However, recent evidence suggests that AMPA (α-amino-3-hydroxyl-5-methyl-4-isoxayolepropionic acid)-type ligand-gated glutamate receptors are continuously recycling between the plasma membrane and the intracellular compartments via vesicle-mediated plasma membrane insertion and clathrin-dependent endocytosis. Regulation of either receptor insertion or endocytosis results in a rapid change in the number of these receptors expressed on the plasma membrane surface and in the receptor-mediated responses, thereby playing an important role in mediating certain forms of synaptic plasticity. Thus, controlling the number of postsynaptic receptors by regulating the intracellular trafficking and plasma membrane expression of the postsynaptic receptors may be a common and important mechanism of synaptic plasticity in the mammalian central nervous system.     

Mechanisms of synaptic depression triggered by metabotropic glutamate receptors

Glutamate, by activation of metabotropic receptors (mGluRs), can lead to a reduction of synaptic efficacy at many synapses. These forms of synaptic plasticity are referred to as long-term depression (mGluR-LTD). We will distinguish between mGluR-LTD induced by pre- or postsynaptic receptors and mGluR-LTD induced by the locus of the expression mechanism of the synaptic depression. We will also review recent evidence that mGluR-mediated responses themselves are subject to depression, which may constitute a form of metaplasticity. Received 13 May 2008; received after revision 07 July 2008; accepted 11 July 2008     

Modification of glutamate receptors by phospholipase A2: its role in adaptive neural plasticity

Long-term potentiation (LTP) and long-term depression (LTD) are two electrophysiological models that have been studied extensively in recent years as they may represent basic mechanisms in many neuronal networks to store certain types of information. In several brain regions, it has been shown that these two forms of synaptic plasticity require sufficient dendritic depolarization, with the amplitude of the calcium signal being crucial for the generation of either LTP or LTD. The rise in calcium concentration mediated by the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors has been proposed to stimulate various calcium-dependent enzymatic processes that could convert the induction signal into long-lasting changes in synaptic structure; protein kinases and phosphatases have so far been considered predominantly with regard to LTP and LTD formation. According to several lines of experimental evidence, changes in synaptic function observed with LTP and LTD are thought to be the result of modifications of postsynaptic currents mediated by the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) subtype of glutamate receptors. Moreover, it has become apparent recently that activation of the calcium-dependent enzyme phospholipase A2 (PLA2) could be part of the molecular mechanisms involved in alterations of AMPA receptor properties during long-term changes in synaptic operation. In the present review, we will first describe the results that indicate a critical role of the phospholipases in regulating synaptic function. Next, sections will be devoted to the effects of PLA2 and phospholipids on the binding properties of glutamate receptors, and a revised biochemical model will be presented as an attempt to integrate the PLA2 enzyme into the mechanisms ( in particular kinases and phosphatases) that participate in adaptive neural plasticity. Finally, we will review data relevant to the issue of selective changes in AMPA binding after environmental enrichment and LTP.     

Dictyostelium mobile elements: strategies to amplify in a compact genome

Dictyostelium discoideum is a eukaryotic microorganism that is attractive for the study of fundamental biological phenomena such as cell-cell communication, formation of multicellularity, cell differentiation and morphogenesis. Large-scale sequencing of the D. discoideum genome has provided new insights into evolutionary strategies evolved by transposable elements (TEs) to settle in compact microbial genomes and to maintain active populations over evolutionary time. The high gene density (about 1 gene/2.6 kb) of the D. discoideum genome leaves limited space for selfish molecular invaders to move and amplify without causing deleterious mutations that eradicate their host. Targeting of transfer RNA (tRNA) gene loci appears to be a generally successful strategy for TEs residing in compact genomes to insert away from coding regions. In D. discoideum, tRNA gene-targeted retrotransposition has evolved independently at least three times by both non-long termina l repeat (LTR) retrotransposons and retrovirus-like LTR retrotransposons. Unlike the nonspecifically inserting D. discoideum TEs, which have a strong tendency to insert into preexisting TE copies and form large and complex clusters near the ends of chromosomes, the tRNA gene-targeted retrotransposons have managed to occupy 75% of the tRNA gene loci spread on chromosome 2 and represent 80% of the TEs recognized on the assembled central 6.5-Mb part of chromosome 2. In this review we update the available information about D. discoideum TEs which emerges both from previous work and current large-scale genome sequencing, with special emphasis on the fact that tRNA genes are principal determinants of retrotransposon insertions into the D. discoideum genome. Received 10 May 2002; received after revision 10 June 2002; accepted 12 June 2002 RID="*" ID="*"Corresponding author.     

The glycinergic inhibitory synapse

Glycine is one of the most important inhibitory neurotransmitters in the spinal cord and the brainstem, and glycinergic synapses have a well-established role in the regulation of locomotor behavior. Research over the last 15 years has yielded new insights on glycine neurotransmission. Glycinergic synapses are now known not to be restricted to the spinal cord and the brainstem. Presynaptic machinery for glycine release and uptake, the structure and function of postsynaptic receptors and the factors (both pre- and postsynaptic) which control the strength of glycinergic inhibition have been extensively studied. It is now established that glycinergic synapses can be excitatory in the immature brain and that some inhibitory synapses can corelease γ-aminobutyric acid (GABA) and glycine. Moreover, the presence of glycine transporters on glial cells and the capacity of these cells to release glycine suggest that glycine may also act as a neuromodulator. Extensive molecular studies have revealed the presence of distinct subtypes of postsynaptic glycine receptors with different functional properties. Mechanisms of glycine receptors aggregation at postsynaptic sites during development are better understood and functional implications of variation in receptor number between postsynaptic sites are partly elucidated. Mutations of glycine receptor subunits have been shown to underly some human locomotor disorders, including the startle disease. Clearly, recent work on glycine receptor channels and the synapses at which they mediate inhibitory signalling in both young and adult animals necessitates an update of our vision of glycinergic inhibitory transmission. Received 8 September 2000; received after revision 1 December 2000; accepted 21 December 2000     

Localization and stabilization of ionotropic glutamate receptors at synapses

Appropriate targeting and clustering of ionotropic glutamate receptors (iGluRs) is critical for the formation and maintenance of excitatory synapses. Recent studies have demonstrated that the synaptic localization of iGluR subtypes is remarkably heterogeneous and subject to regulation over time scales ranging from minutes to months. These findings, together with the identification of key protein binding partners of iGluRs, have opened a window onto the complex cell biology of iGluR membrane trafficking. In this article, we review recent findings on the cellular and molecular mechanisms involved in localizing iGluRs at synapses and discuss their implications for synaptogenesis and synaptic plasticity.     

Variation of the time constant (tau) of the decay of end-plate current during focalized transmitter release]

The variations of the time constant tau of the end-plate current (epc) have been studied at the clamped neuromuscular junction of the frog while transmitter release was focalised by local iontophoretic application of Ca++. In the absence of any anticholinesterasic drug, tau varies according to the variations of the intensity of epc (Iepc) due to pre or postsynaptic experimental procedure. It is suggested that focalisation of transmitter release induces local acetylcholinesterase inhibition by an excess of substrate; then the transmitter life time in the synaptic cleft is momentarily increased, allowing repeated binding of Ach to postsynaptic receptors.     

一种基于同步动力学模型的层次聚类方法

本文基于建模同步动力学行为的Kuramoto模型提出了一种新的有效层次聚类方法.本文提出的方法基于局部邻域的概念,能够实现稳定的局部同步聚类.通过不断扩大对象同步的邻域半径,所提出的方法能够实现层次化的同步聚类.此外,提出对象邻域闭包的概念,在对象间到达完全同步之前就能预测出聚类的形成,从而减少对象动态交互的时间.本文的方法不依赖于任何数据分布假设,无需任何手工参数设置,可以检测出任意数量、形状和大小的聚类.由于同步过程能够有效地规避离群点,该方法有较强的噪声数据抑制能力.在大量真实数据集和人工合成数据集上的实验结果表明本文的方法聚类准确率高,且运行时间较同类基准算法显著缩短.     

Molecular aspects of membrane fission in the secretory pathway

Membrane fission is essential in various intracellular dissociative transport steps. The molecular mechanisms by which endocytic vesicles detach from the plasma membrane are being rapidly elucidated. Much less is known about the fission mechanisms operating at Golgi tubular networks; these include the Golgi transport and sorting stations, the trans-Golgi and cis-Golgi networks, where the geometry and physical properties of the membranes differ from those at the cell surface. Here we discuss the lipid and protein machineries that have so far been related to the fission process, with emphasis on those acting in the Golgi complex. Received 10 May 2002; received after revision 20 June 2002; accepted 26 June 2002 RID="*" ID="*"Corresponding author.     

AMPAR trafficking in synapse maturation and plasticity

Glutamate ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPARs) mediate most fast excitatory synaptic transmission in the central nervous system. The content and composition of AMPARs in postsynaptic membranes (which determine synaptic strength) are dependent on the regulated trafficking of AMPAR subunits in and out of the membranes. AMPAR trafficking is a key mechanism that drives nascent synapse development, and is the main determinant of both Hebbian and homeostatic plasticity in mature synapses. Hebbian plasticity seems to be the biological substrate of at least some forms of learning and memory; while homeostatic plasticity (also known as synaptic scaling) keeps neuronal circuits stable by maintaining changes within a physiological range. In this review, we examine recent findings that provide further understanding of the role of AMPAR trafficking in synapse maturation, Hebbian plasticity, and homeostatic plasticity.     

Protein kinases: which one is the memory molecule?

Encoding of new experiences is likely to induce activity-dependent modifications in the brain. Studies in organisms far apart on the phylogenetic scale have shown that similar, sometimes identical, signal transduction pathways subserve plasticity in neuronal systems, and they may play pivotal roles in the formation of long-term memories. It has become evident that phosphorylation/dephosphorylation reactions are critical for the initiation of cellular mechanisms that embody, retain and modify information in neural circuits. Although physiological investigations on synaptic plasticity have had a major impact, we have concentrated our review on behavioural studies that provide direct or indirect evidence for a role of kinases in mechanisms underlying memory formation. From these, it appears that the learning event induces activation of a variety of kinases with specific time courses. For instance, the calcium/calmodulin-dependent protein kinase II seems to participate in an early phase of memory formation. Apparently, activation of both protein tyrosine kinases and mitogen-activated protein kinases is required for much longer and may thus have a particular function during transformation from short-term into long-term memory. Quite different time courses appear for protein kinase C (PKC) and protein kinase A (PKA), which may function at two different time points, shortly after training and again much later. This suggests that PKC and PKA might play a role at early and late stages of memory formation. However, we have considered some examples showing that these signalling pathways do not function in isolation but rather interact in an intricate intracellular network. This is indicative of a more complex contribution of each kinase to the fine tuning of encoding and information processing. To decipher this complexity, pharmacological, biochemical and genetic investigations are more than ever necessary to unravel the role of each kinase in the syntax of learning and memory formation.     

AMPA receptors: potential implications in development and disease

α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptors are one type of ionotropic glutamate receptor involved in rapid excitatory synaptic transmission. AMPA receptors have been increasingly implicated in long-term potentiation, and recent evidence suggests that they may play a role in disorders affecting the nervous system. The finding that early in postnatal development AMPA receptors are not expressed has lately been the focus of much attention. Resolving the factors involved in AMPA receptor expression suggests that their induction is a developmentally regulated process with the possibility that alterations in receptor expression may be correlated with pathology in neurological disorders. This paper provides an overview of factors involved in AMPA receptor induction as well as of their role in plasticity and neuronal pathologies. Received 5 December 2000; received after revision 12 January 2001; accepted 2 February 2001     

Induction of apoptosis and CD10/neutral endopeptidase expression by jaspamide in HL-60 line cells

Jaspamide (jasplakinolide) is a natural peptide isolated from marine sponges of Jaspis species and has fungicidal and growth-inhibiting activities. We characterized the jasplakinolide-induced loss of viability by programmed cell death in the HL-60 human promyelocytic leukemia cell line and found that this process was accompanied by neutral endopeptidase (NEP)/CD10 expression on the surface of the apoptotic cells. HL-60 cells do not normally express detectable amounts of NEP/CD10 on their surface or intracytoplasmically, but upon jaspamide treatment, CD10 was synthesized de novo, its expression being inhibited by cycloheximide pretreatment. Once synthesized, NEP/CD10 interfered with the jasplakinolide signal delivered to HL-60 cells. Inhibition of NEP/CD10 by the NEP inhibitor phosphoramidon or by an anti-CD10 monoclonal antibody significantly increased apoptosis induction. The appearance of CD10 on the cell surface was blocked by preincubation of the cells with the monocytic/macrophage-differentiating agents vitamin D3 and phorbol 12-myristate 13-acetate, but not by the granulocytic differentiating agents retinoic acid or dimethyl sulfoxide. Moreover, in the promonocytic U937 and mature monocytic THP-1 cell lines, jaspamide induced apoptosis but not CD10 expression. In HL-60 cells, CD10 expression was partially but not totally blocked by the broad-spectrum caspase inhibitor benzyloxacarbonyl-Val-Ala-Asp-fluoromethylketone, indicating a connection between apoptosis induction and CD10 synthesis. Our findings suggest that the CD10 expression is related to the programmed cell death induction by jaspamide, and also with the process of granulocytic differentiation in HL-60 cells. Received 22 April 2002; received after revision 8 June 2002; accepted 10 June 2002     

Memory formation and the regulation of gene expression

On a cellular level, formation of memory is based on a selective change in synaptic efficacy that is both fast and, in case of important information, long-lasting. Rapidity of cellular changes is achieved by modifying preexisting synaptic molecules (receptors, ion channels), which instantaneously alters the efficacy of synaptic transmission. Endurance, that is the formation of long-term memory (LTM), is based on transient and perhaps also long-lasting changes in protein synthesis. A number of different methods exist to interfere with the synthesis of specific proteins or proteins in general. Other methods, in turn, help to identify proteins whose synthesis is changed following learning. These mostly molecular methods are briefly described in the present review. Their successful application in a variety of memory paradigms in invertebrates and vertebrates is illustrated. The data support the importance of selective changes in gene expression for LTM. Proteins newly synthesized during memory consolidation are likely to contribute to restructuring processes at the synapse, altering the efficiency of transmission beyond the scope of STM. Increased or, less often, decreased synthesis of proteins appears during specific time windows following learning. Recent evidence supports older data suggesting that two or even more waves of protein synthesis exist during the consolidation period. It is expected that the new molecular methods will help to identify and characterize molecules whose expression changes during LTM formation even in complex vertebrate learning paradigms.     

Retinoic acid modulates gap junctional intercellular communication in hepatocytes and hepatoma cells

Gap junctional communication permits the direct exchange of small molecules and ions and has been implicated in tissue homeostasis/metabolite exchange. The lack of gap junctional intercellular communication (GJIC) plays important roles in the promotion and progression of carcinogenesis. In the present study, we demonstrate that treatment of human hepatoma Hep G2 cells with retinoic acid (RA) results in increased amounts and phosphorylation of connexins, their stabilisation in plasma membrane plaques and enhanced GJIC. In cultured fetal hepatocytes, which represent a non-transformed, proliferating and incompletely differentiated liver system, the effects of RA are limited to the establishment of connexin in areas of cell-cell contact and the improvement of GJIC. This suggests that modulation of cell-cell channel communication by RA occurs differently in these two experimental models: while RA is able to revert cell transformation in Hep G2 cells, in fetal hepatocytes it may induce the expression of a more differentiated phenotype. Received 19 June 2002; received after revision 29 July 2002; accepted 8 August 2002 RID="*" ID="*"Corresponding author.     

ERKs are the point of divergence of PKA and PKC activation by PTHrP in human skin fibroblasts

Parathyroid hormone-related peptide (PTHrP) receptors, coupled to trimeric G proteins, operate in most target cells through at least three different transduction routes: Gαs-mediated stimulation of adenylylcyclase (AC), Gαq-mediated activation of phospholipase Cβ (PLC) and mitogen-activated protein kinase (MAPK) activation. In this study we investigated the relative role of different pathways in human skin fibroblast prolifera-tion. Using chemical inhibitors and activators of signal transduction, we demonstrated that: (i) AC/cAMP and PLC/1,4,5 inositol triphosphate/diacylglycerol second-messenger systems are simultaneously activated following PTHrP binding to its receptors; (ii) the mitogenic response to PTHrP derives from a balance between two counteracting pathways – an activating route mediated by protein kinase C (PKC) and an inhibitory route mediated by protein kinase A (PKA); (iii) PTHrP mitogenic effects are largely dependent on MAPKs, whose activity can be modulate d by both PKA and PKC. Our results indicate that MAPKs are common targets of both transduction routes and, at the same time, their point of divergence in mediating PTHrP dual and opposite mitogenic effects. Received 2 August 2002; received after revision 10 September 2002; accepted 18 October 2002 RID="*" ID="*"Corresponding author.     

Roles of postsynaptic density-95/synapse-associated protein 90 and its interacting proteins in the organization of synapses

Synapses are central stages for neurotransmission. Neurotransmitters are released from the presynaptic membrane of one neuron, and bind to the receptors accumulated at the postsynaptic membrane, followed by the activation of the other neuron. The strength of a synapse is modified depending on the history of the previous neurotransmissions. This property is called synaptic plasticity and is implicated in learning and memory. Synapses contain not only the components essential for neurotransmission but also the signalling molecules involved in synaptic plasticity. The elucidation of the molecular structures of synapses is one of the key steps to understand the mechanism of learning and memory. Recent studies have revealed postsynaptic density (PSD)-95/synapse-associated protein (SAP) 90 as a core component in the architecture of synapses. In this review, we summarize up-to-date information about PSD-95/SAP90 and its interacting proteins, and the organization of synapses orchestrated     

Involvement of extrasynaptic glutamate in physiological and pathophysiological changes of neuronal excitability

Glutamate is the most abundant neurotransmitter of the central nervous system, as the majority of neurons use glutamate as neurotransmitter. It is also well known that this neurotransmitter is not restricted to synaptic clefts, but found in the extrasynaptic regions as ambient glutamate. Extrasynaptic glutamate originates from spillover of synaptic release, as well as from astrocytes and microglia. Its concentration is magnitudes lower than in the synaptic cleft, but receptors responding to it have higher affinity for it. Extrasynaptic glutamate receptors can be found in neuronal somatodendritic location, on astroglia, oligodendrocytes or microglia. Activation of them leads to changes of neuronal excitability with different amplitude and kinetics. Extrasynaptic glutamate is taken up by neurons and astrocytes mostly via EAAT transporters, and astrocytes, in turn metabolize it to glutamine. Extrasynaptic glutamate is involved in several physiological phenomena of the central nervous system. It regulates neuronal excitability and synaptic strength by involving astroglia; contributing to learning and memory formation, neurosecretory and neuromodulatory mechanisms, as well as sleep homeostasis.The extrasynaptic glutamatergic system is affected in several brain pathologies related to excitotoxicity, neurodegeneration or neuroinflammation. Being present in dementias, neurodegenerative and neuropsychiatric diseases or tumor invasion in a seemingly uniform way, the system possibly provides a common component of their pathogenesis. Although parts of the system are extensively discussed by several recent reviews, in this review I attempt to summarize physiological actions of the extrasynaptic glutamate on neuronal excitability and provide a brief insight to its pathology for basic understanding of the topic.     

Colloidal drug carriers: achievements and perspectives

Colloidal drug carriers such as liposomes and nanoparticles are able to modify the distribution of an associated substance. They can therefore be used to improve the therapeutic index of drugs by increasing their efficacy and/or reducing their toxicity. If these delivery systems are carefully designed with respect to the target and route of administration, they may provide one solution to some of the delivery problems posed by new classes of active molecules such as peptides, proteins, genes, and oligonucleotides. They may also extend the therapeutic potential of established drugs such as doxorubicin and amphotericin B. This article discusses the use of colloidal, particulate carrier systems (25 nm to 1 μm in diameter) in such applications. In particular, systems which show diminished uptake by mononuclear phagocytes are described. Specific targeting of carriers to particular tissues or cells is also considered. Received 8 April 2002; received after revision 25 June 2002; accepted 26 June 2002