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.     

The role of glycosylation in ionotropic glutamate receptor ligand binding, function, and trafficking

Members of the ionotropic glutamate receptor (iGluR) family have between 4 and 12 consensus asparagine (N)-linked glycosylation sites. They are localized on the extracellular N-termini, and the loop between the penultimate and last transmembrane domains. These regions also contain the essential elements for formation of the ligand binding site. N-linked glycosylation does not appear to be essential for formation of the ligand binding site per se, but there are demonstrated interactions between glycosylation state and ligand binding affinity, receptor physiology, susceptibility to allosteric modulation and, in some cases, trafficking. There is no indication of a general role for N-linked glycosylation in iGluRs; instead the effects of glycosylation vary among glutamate receptor subtypes and splice variants, with specific effects on structure or function with different subunits.     

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.     

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.     

The active role of astrocytes in synaptic transmission

In the central nervous system, astrocytes form an intimately connected network with neurons, and their processes closely enwrap synapses. The critical role of these cells in metabolic and trophic support to neurons, ion buffering and clearance of neurotransmitters is well established. However, recent accumulating evidence suggests that astrocytes are active partners of neurons in additional and more complex functions. In particular, astrocytes express a repertoire of neurotransmitter receptors mirroring that of neighbouring synapses. Such receptors are stimulated during synaptic activity and start calcium signalling into the astrocyte network. Intracellular oscillations and intercellular calcium waves represent the astrocyte's own form of excitability, as they trigger release of transmitter (i.e. glutamate) via a novel process sensitive to blockers of exocytosis and involving cyclooxygenase eicosanoids. Astrocyte-released glutamate activates receptors on the surrounding neurons and modifies their electrical and intracellular calcium ([Ca2+]i) state. These exciting new findings reveal an active participation of astrocytes in synaptic transmission and the involvement of neuronastrocyte circuits in the processing of information in the brain.     

The role of chemokines and their receptors in angiogenesis

Chemokines are a vertebrate-specific group of small molecules that regulate cell migration and behaviour in diverse contexts. So far, around 50 chemokines have been identified in humans, which bind to 18 different chemokine receptors. These are members of the seven-transmembrane receptor family. Initially, chemokines were identified as modulators of the immune response. Subsequently, they were also shown to regulate cell migration during embryonic development. Here, we discuss the influence of chemokines and their receptors on angiogenesis, or the formation of new blood vessels. We highlight recent advances in our understanding of how chemokine signalling might directly influence endothelial cell migration. We furthermore examine the contributions of chemokine signalling in immune cells during this process. Finally, we explore possible implications for disease settings, such as chronic inflammation and tumour progression.     

The role of sialic acid in 5-HT binding to synaptic membranes

Summary The high affinity binding of [¹⁴C]5-HT to nerve ending membranes isolated from rat brain is not affected by neuraminidase treatment. The specificity of ligand receptor interaction was demonstrated by displacement studies with tryptamine derivatives, noradrenaline, and acetylcholine.The expert technical assistance of Miss Christiane von Winterfeld is appreciated. This work was supported by a grant of the Deutsche Forschungsgemeinschaft.     

Age-dependent changes in hippocampal synaptic transmission and plasticity in the PLB1Triple Alzheimer mouse

Several genetically engineered models exist that mimic aspects of the pathological and cognitive hallmarks of Alzheimer’s disease (AD). Here we report on a novel mouse model generated by targeted knock-in of transgenes containing mutated human amyloid precursor protein (APP) and microtubule-associated protein tau genes, inserted into the HPRT locus and controlled by the CaMKIIα regulatory element. These mice were crossed with an asymptomatic presenilin1_A246E overexpressing line to generate PLB1_Triple mice. Gene expression analysis and in situ hybridization confirmed stable, forebrain-specific, and gene-dose-dependent transgene expression. Brain tissue harvested from homozygous, heterozygous, and wild-type cohorts aged between 3 and 24 months was analyzed immunohistochemically and electrophysiologically. Homozygous PLB1_Triple offspring presented with mostly intracellular cortical and hippocampal human APP/amyloid, first detected reliably at 6 months. Human tau was already uncovered at 3 months (phospho-tau at 6 months) and labeling intensifying progressively with age. Gene-dose dependence was confirmed in age-matched heterozygous females that accumulated less tau and amyloid protein. General excitability of hippocampal neurones was not altered in slices from PLB1_Triple mice up to 12 months, but 2-year-old homozygous PLB1_Triple mice had smaller synaptically evoked postsynaptic potentials compared with wild types. Synaptic plasticity (paired-pulse depression/facilitation and long-term potentiation) of synaptic CA1 pyramidal cell responses was deficient from 6 months of age. Long-term depression was not affected at any age or in any genotype. Therefore, despite comparatively subtle gene expression and protein build-up, PLB1_Triple mice develop age-dependent progressive phenotypes, suggesting that aggressive protein accumulation is not necessary to reconstruct endophenotypes of AD.     

The role of sialic acid in 5-HT binding to synaptic membranes.

The high affinity binding of [14C]5-HT to nerve ending membranes isolated from rat brain is not affected by neuraminidase treatment. The specificity of ligand receptor interaction was demonstrated by displacement studies with tryptamine derivatives, noradrenaline, and acetylcholine.     

The role of VEGF receptors in angiogenesis; complex partnerships

Vascular endothelial growth factors (VEGFs) regulate blood and lymphatic vessel development and homeostasis but also have profound effects on neural cells. VEGFs are predominantly produced by endothelial, hematopoietic and stromal cells in response to hypoxia and upon stimulation with growth factors such as transforming growth factors, interleukins or platelet-derived growth factor. VEGFs bind to three variants of type III receptor tyrosine kinases, VEGF receptor 1, 2 and 3. Each VEGF isoform binds to a particular subset of these receptors giving rise to the formation of receptor homo- and heterodimers that activate discrete signaling pathways. Signal specificity of VEGF receptors is further modulated upon recruitment of coreceptors, such as neuropilins, heparan sulfate, integrins or cadherins. Here we summarize the knowledge accumulated since the discovery of these proteins more than 20 years ago with the emphasis on the signaling pathways activated by VEGF receptors in endothelial cells during cell migration, growth and differentiation. Received 15 September 2005; received after revision 11 November; accepted 24 November 2005     

Evaluation of monoaminergic receptors in the genetically epilepsy prone rat

Summary The intensity of sound-induced convulsions in the genetically epilepsy-prone rat (GEPR) was reduced in a dose related fashion by intracerebroventricular administration of dobutamine, (₁ agonist), terbutaline (₂ agonist) or phenylephrine (₁ agonist). BHT-920 (₂ agonist) did not cause a dose-related decrease in sound-induced convulsion intensity. Binding studies showed that whole brain and receptor densities (B_max) were normal while the K_d was increased for the ligand in GEPR brain.Acknowledgment. We are most grateful to Boehringer Ingelheim for generously supplying BHT 920. We are also indebted to Ciba-Geigy Corporation for the gift of terbutaline hydrochloride and phentolamine hydrochloride. The work was supported in part by NIH grant NS 16829.     

Evaluation of monoaminergic receptors in the genetically epilepsy prone rat

The intensity of sound-induced convulsions in the genetically epilepsy-prone rat (GEPR) was reduced in a dose related fashion by intracerebroventricular administration of dobutamine, (beta 1 agonist), terbutaline (beta 2 agonist) or phenylephrine (alpha 1 agonist). BHT-920 (alpha 2 agonist) did not cause a dose-related decrease in sound-induced convulsion intensity. Binding studies showed that whole brain alpha and beta receptor densities (Bmax) were normal while the Kd was increased for the beta ligand in GEPR brain.     

The innate immunity of the central nervous system in chronic pain: The role of Toll-like receptors

Toll-like receptors (TLRs) are a family of pattern recognition receptors that mediate innate immune responses to stimuli from pathogens or endogenous signals. Under various pathological conditions, the central nervous system (CNS) mounts a well-organized innate immune response, in which glial cells, in particular microglia, are activated. Further, the innate immune system has emerged as a promising target for therapeutic control of development and persistence of chronic pain. Especially, microglial cells respond to peripheral and central infection, injury, and other stressor signals arriving at the CNS and initiate a CNS immune activation that might contribute to chronic pain facilitation. In the orchestration of this limited immune reaction, TLRs on microglia appear to be most relevant in triggering and tailoring microglial activation, which might be a driving force of chronic pain. New therapeutic approaches targeting the CNS innate immune system may achieve the essential pharmacological control of chronic pain. Received 21 November 2006; received after revision 8 January 2007; accepted 7 February 2007     

Interferon receptors and their role in interferon action

Interferon (IFN) proteins interact with cells through specific cell surface receptors, some of which have been purified and cloned. The alpha-IFNs and beta-IFN bind to a common receptor (type I), whereas gamma-IFN binds to a separate receptor (type II). Both types of high-affinity receptors have been demonstrated on a variety of receptors and the ways in which IFNs may affect cellular physiology and gene expression is discussed.     

Interferon receptors and their role in interferon action

Summary Interferon (IFN)_proteins interact with cells through specific cell surface receptors, some of which have been purified and cloned. The alpha-IFNs and beta-IFN bind to a common receptor (type I), whereas gamma-IFN binds to a separate receptor (type II). Both types of high-affinity receptors have been demonstrated on a variety of different kinds of cells but in relatively low numbers (10²–10⁴/cell). The relationship between IFN binding to receptors and the ways in which IFNs may affect cellular physiology and gene expression is discussed.     

The role of mammalian antimicrobial peptides and proteins in awakening of innate host defenses and adaptive immunity

Since we live in a dirty environment, we have developed many host defenses to contend with microorganisms. The epithelial lining of our skin, gastrointestinal tract and bronchial tree produces a number of antibacterial peptides, and our phagocytic neutrophils rapidly ingest and enzymatically degrade invading organisms, as well as produce peptides and enzymes with antimicrobial activities. Some of these antimicrobial moieties also appear to alert host cells involved in both innate host defense and adaptive immune responses. The epithelial cells are a source of constitutively produced beta defensin (HBD1) and proinflammatory cytokine-inducible beta defensins (HBD2 and -3) and cathelicidin (LL37). The neutrophils-derived antimicrobial peptides are released on demand from their cytoplasmic granules. They include the enzymes cathepsin G and chymase, azurocidin, a defensins and cathelicidin. In contrast, C5a and C3b are produced by activation of the serum complement cascade. The antimicrobial moieties direct the migration and activate target cells by interacting with selected G-protein-coupled seven-transmembrane receptors (GPCRs) on cell surfaces. The beta defensins interact with the CCR6 chemokine GPCRs, whereas cathelicidins interact with the low-affinity FPRL-1 receptors. The neutrophil-derived cathepsin G acts on the high-affinity FMLP receptor (GPCR) known as FPR, while the receptors for chymase and azurocidin have not been identified as yet. The serum-derived C5a uses a GPCR known as C5aR to mediate its chemotactic and cell-activating effects. Consequently, all these ligand-receptor interactions in addition to mediating chemotaxis also activate receptor-expressing cells to produce other mediators of inflammation.     

The possible role of isoforms of cytochrome c oxidase subunit VIa in mammalian thermogenesis

A single cDNA of cytochrome c oxidase subunit VIa was characterised from liver, heart and the thermogenic organ of the partially endotherm tuna fish. The amino acid sequence revealed high identity with subunit VIa from carp and trout, but low identity to subunits VIaL (liver type) and VIaH (heart type) of mammalian cytochrome c oxidase. In reconstituted cytochrome c oxidase from bovine heart, the H ⁺/e⁻ stoichiometry is decreased from 1.0 to 0.5 at high intraliposomal ATP/ADP ratios via exchange of bound ADP by ATP at the matrix domain of the transmembraneous subunit VIaH. Reconstituted cytochrome c oxidase from bovine liver and kidney, containing subunit VIaL, revealed H ⁺/e⁻ ratios below 0.5, independent of the ATP/ADP ratio. The results suggest the evolution of three types of subunit VIa. Subunits VIaH and VIaL are postulated to participate in mammalian thermogenesis. Received 3 May 1999; received after revision 10 June 1999; accepted 29 June 1999     

Mononuclear phagocyte-mediated antifungal immunity: the role of chemotactic receptors and ligands

Over the past two decades, fungal infections have emerged as significant causes of morbidity and mortality in patients with hematological malignancies, hematopoietic stem cell or solid organ transplantation and acquired immunodeficiency syndrome. Besides neutrophils and CD4⁺ T lymphocytes, which have long been known to play an indispensable role in promoting protective antifungal immunity, mononuclear phagocytes are now being increasingly recognized as critical mediators of host defense against fungi. Thus, a recent surge of research studies has focused on understanding the mechanisms by which resident and recruited monocytes, macrophages and dendritic cells accumulate and become activated at the sites of fungal infection. Herein, we critically review how a variety of G-protein coupled chemoattractant receptors and their ligands mediate mononuclear phagocyte recruitment and effector function during infection by the most common human fungal pathogens.