Mechanisms of synaptic plasticity: from membrane to intracellular AMPAR trafficking

Brief periods of repetitive neural firing onto adjacent neurons can lead to changes in synaptic plasticity, that is, changes in the make-up of macromolecular complexes located at synapses. This process includes the regulated trafficking of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) to synaptic membranes. Little is known, however, about how the AMPARs are regulated before they are shuttled to the membrane. Greger et al. have found that the length of the cytoplasmic tails of constituent subunits of a given AMPAR is determined by editing [at a glutamine (Q) or an arginine (R) codon] near their C termini. Tail length, in turn, dictates whether AMPARs will be retained or quickly released from the endoplasmic reticulum.     

Regulation of intracellular membrane transport.

A number of proteins that are necessary for membrane transport have been identified using cell-free assays and yeast genetics. Although our knowledge of transport mechanisms remains limited, common themes are clearly emerging. In particular, specific GTP-binding proteins appear to be involved, not only at all steps of membrane traffic but also at more than one check-point within each step. The ordered sequence of events occurring during vesicle formation, targeting and fusion may be regulated in a stepwise manner by specific GTP-dependent switches, which act as modular elements of the transport mechanism.     

Plant sulphate transporters: co-ordination of uptake, intracellular and long-distance transport

Proton/sulphate co-transport in the plasma membrane of root cells is the first step for the uptake of sulphate from the environment by plants. Further intracellular, cell-to-cell and long-distance transport must fulfil the requirements for sulphate assimilation and source/sink demands within the plant. A gene family of sulphate transporters, which may be subdivided into five groups, has been identified with examples from many different plant species. For at least two groups, proton/sulphate co-transport activity has been confirmed. It appears that each group represents sulphate transporters with distinct kinetic properties, patterns of expression, and cell/tissue specificity related to specific roles in the uptake and allocation of sulphate. High-affinity sulphate uptake and low-affinity vascular transport, as well as vacuolar efflux, are controlled by the nutritional status of the plant. Most notably there is an apparent increase in capacity for cellular sulphate uptake and vacuolar efflux when sulphur supply is limiting. Within the groups, the individual sulphate transporters may be further subdivided by differences in temporal, cellular and tissue expression. Many of the transporters are regulated by the nutritional status of the individual tissues, to optimize sulphate movement within and between sink and source organs.     

Probing intracellular vesicle trafficking and membrane remodelling by cryo-EM

Protein transport between the membranous compartments of the eukaryotic cells is mediated by the constant fission and fusion of the membrane-bounded vesicles from a donor to an acceptor membrane. While there are many membrane remodelling complexes in eukaryotes, COPII, COPI, and clathrin-coated vesicles are the three principal classes of coat protein complexes that participate in vesicle trafficking in the endocytic and secretory pathways. These vesicle-coat proteins perform two key functions: deforming lipid bilayers into vesicles and encasing selective cargoes. The three trafficking complexes share some commonalities in their structural features but differ in their coat structures, mechanisms of cargo sorting, vesicle formation, and scission. While the structures of many of the proteins involved in vesicle formation have been determined in isolation by X-ray crystallography, elucidating the proteins' structures together with the membrane is better suited for cryogenic electron microscopy (cryo-EM). In recent years, advances in cryo-EM have led to solving the structures and mechanisms of several vesicle trafficking complexes and associated proteins.     

GPI-anchor remodeling: potential functions of GPI-anchors in intracellular trafficking and membrane dynamics

Glycosylphosphatidylinositol (GPI) anchoring of proteins is a conserved post-translational modification in eukaryotes. GPI is synthesized and transferred to proteins in the endoplasmic reticulum. GPI-anchored proteins are then transported from the endoplasmic reticulum to the plasma membrane through the Golgi apparatus. GPI-anchor functions as a sorting signal for transport of GPI-anchored proteins in the secretory and endocytic pathways. After GPI attachment to proteins, the structure of the GPI-anchor is remodeled, which regulates the trafficking and localization of GPI-anchored proteins. Recently, genes required for GPI remodeling were identified in yeast and mammalian cells. Here, we describe the structural remodeling and function of GPI-anchors, and discuss how GPI-anchors regulate protein sorting, trafficking, and dynamics. This article is part of a Special Issue entitled Lipids and Vesicular Transport.     

Shaping tubular carriers for intracellular membrane transport

The particular compositions of the intracellular membrane organelles rely on the proteins and lipids received frequently through membrane trafficking. The delivery of these molecules is driven by the membrane-bound organelles known as transport carriers (TCs). Advanced microscopy approaches have revealed that TC morphology ranges from small vesicles to complex tubular membrane structures. These tubular TCs (TTCs) support effectively both sorting and transport events within the biosynthetic and endocytic pathways, while a coherent picture of the processes that define the formation and further fate of TTCs is still missing. Here, we present an overview of the mechanisms operating during the TTC life cycle, as well as of the emerging role of tubular carriers in different intracellular transport routes.     

Regulation of intracellular membrane trafficking and cell dynamics by syntaxin-6

Intracellular membrane trafficking along endocytic and secretory transport pathways plays a critical role in diverse cellular functions including both developmental and pathological processes. Briefly, proteins and lipids destined for transport to distinct locations are collectively assembled into vesicles and delivered to their target site by vesicular fusion. SNARE (soluble N-ethylmaleimide-sensitive factor-attachment protein receptor) proteins are required for these events, during which v-SNAREs (vesicle SNAREs) interact with t-SNAREs (target SNAREs) to allow transfer of cargo from donor vesicle to target membrane. Recently, the t-SNARE family member, syntaxin-6, has been shown to play an important role in the transport of proteins that are key to diverse cellular dynamic processes. In this paper, we briefly discuss the specific role of SNAREs in various mammalian cell types and comprehensively review the various roles of the Golgi- and endosome-localized t-SNARE, syntaxin-6, in membrane trafficking during physiological as well as pathological conditions.     

Candida albicans actively modulates intracellular membrane trafficking in mouse macrophage phagosomes

The intracellular trafficking/survival strategies of the opportunistic human pathogen Candida albicans are poorly understood. Here we investigated the infection of RAW264.7 macrophages with a virulent wild-type (WT) filamentous C. albicans strain and a hyphal signalling-defective mutant ( efg1 Δ /cph1 Δ). A comparative analysis of the acquisition by phagosomes of actin, and of early/late endocytic organelles markers of the different fungal strains was performed and related to Candida's survival inside macrophages. Our results show that both fungal strains have evolved a similar mechanism to subvert the 'lysosomal' system, as seen by the inhibition of the phagosome fusion with compartments enriched in the lysobisphosphatidic acid and the vATPase, and thereby the acquisition of a low pH from the outset of infection. Besides, the virulent WT strain displayed additional specific survival strategies to prevent its targeting to compartmentsdisplaying late endosomal/lysosomal features, such as induction of active recycling out of phagosomes of the lysosomal membrane protein LAMP-1, the lysosomal protease cathepsin D and preinternalized colloidal gold. Finally, both virulent and efg1 Δ /cph1 Δ mutant fungal strains actively suppressed the production of macrophage nitric oxide (NO), although their cell wall extracts were potent inducers of NO.     

The role of calcium in intracellular trafficking

The molecular mechanism of membrane fusion essential to vital cellular activities such as intracellular transport, hormone secretion, enzyme release, or neurotransmission, involve the assembly and disassembly of a specialized set of proteins in opposing bilayers. Recent evidences shed new light on the role Ca(2+) has in the regulation of this mechanism in which the Golgi apparatus works as a central station; from here, Ca(2+) ions are released into and recovered from the cytosol during the different steps of the cargo progression. In fact, transient cytosolic Ca(2+) fluctuations take a crucial role to recruit proteins and enzymes Ca(2+)-sensitive on Golgi membranes where they are involved in membranes remodelling which is fundamental process for the fusion events that allow protein trafficking. Here I provide an overview of the role Ca(2+) plays in intra-Golgi trafficking underlying some interesting aspects to clarify the mechanisms of cargo progression.     

PIST regulates the intracellular trafficking and plasma membrane expression of Cadherin 23

Background  

The atypical cadherin protein cadherin 23 (CDH23) is crucial for proper function of retinal photoreceptors and inner ear hair cells. As we obtain more and more information about the specific roles of cadherin 23 in photoreceptors and hair cells, the regulatory mechanisms responsible for the transport of this protein to the plasma membrane are largely unknown.     

The early stages of the intracellular transport of membrane proteins: clinical and pharmacological implications

Intracellular transport mechanisms ensure that integral membrane proteins are delivered to their correct subcellular compartments. Efficient intracellular transport is a prerequisite for the establishment of both cell architecture and function. In the past decade, transport processes of proteins have also drawn the attention of clinicians and pharmacologists since many diseases have been shown to be caused by transport-deficient proteins. Membrane proteins residing within the plasma membrane are transported via the secretory (exocytotic) pathway. The general transport routes of the secretory pathway are well established. The transport of membrane proteins starts with their integration into the ER membrane. The ribosomes synthesizing membrane proteins are targeted to the ER membrane, and the nascent chains are co-translationally integrated into the bilayer, i.e., they are inserted while their synthesis is in progress. During ER insertion, the orientation (topology) of the proteins in the membrane is determined. Proteins are folded, and their folding state is checked by a quality control system that allows only correctly folded forms to leave the ER. Misfolded or incompletely folded forms are retained, transported back to the cytosol and finally subjected to proteolysis. Correctly folded proteins are transported in the membranes of vesicles through the ER/Golgi intermediate compartment (ERGIC) and the individual compartments of the Golgi apparatus (cis, medial, trans) to the plasma membrane. In this review, the current knowledge of the first stages of the intracellular trafficking of membrane proteins will be summarized. This early secretory pathway includes the processes of ER insertion, topology determination, folding, quality control and the transport to the Golgi apparatus. Mutations in the genes of membrane proteins frequently lead to misfolded forms that are recognized and retained by the quality control system. Such mutations may cause inherited diseases like cystic fibrosis or retinitis pigmentosa. In the second part of this review, the clinical implications of the early secretory pathway will be discussed. Finally, new pharmacological strategies to rescue misfolded and transport-defective membrane proteins will be outlined.Abbreviations AP1 Clathrin-associated adaptor protein complex 1 - AQP Aquaporin - ARF ADP-ribosylation factor - AVP 8-Arginine-vasopressin;BiP immunoglobulin heavy chain binding protein - CFTR Cystic fibrosis transmembrane conductance regulator - CLQTS Congenital long QT syndrome - CMT Charcot-Marie-Tooth syndrome - CNX Calnexin - COPI Coat protein complex I - COPII Coat protein complex II - CPX 8-Cyclopentyl-1,2-dipropylxanthine - CRT Calreticulin - CSID Congenital sucrose-isomaltase deficiency - Cx Connexin - cGMP Cyclic 3:5 guanosine monophosphate - ECL Extracellular loop - EndoH Endoglycosidase H - ER Endoplasmic reticulum - ERAD ER-associated degradation - ERGIC ER/Golgi intermediate compartment - ERp ER protein - ET_BR Human endothelin B receptor - FH Familial hypercholesterolemia - GABA Gamma amino butyric acid - GFP Green fluorescent protein - GH Growth hormone - GHIS Growth hormone insensitivity syndrome - GLCase Glucosidase - GlcNac N-acetylglucosamine - GPCR G protein-coupled receptor - GPI Glycosylphosphatidylinositol - G protein GTP-binding protein - GRP Glucose-regulated protein - HA Hemagglutinin - Hdj-2 Human DnaJ-2 protein - HFE Human hemochromatosis protein - HH Hereditary hemochromatosis - HEK 293 cells Human embryonic kidney 293 cells - HERG Human ether-a-go-go-related protein - Hsc70 Heat shock cognate 70 protein - ICL Intracellular loop - IGF-I Insulin-like growth factor-1 - I_Kr Rapidly activating delayed rectifier potassium current - I_Ks Slowly activating delayed rectifier potassium current - JAK Janus kinase - LDL Low-density lipoprotein - LH Luteinizing hormone/choriogonadotropin - LS Laron syndrome - MATP Membrane associated transporter protein - MDCK cells Madin-Darby canine kidney epithelial cells - MHC Major histocompatibility complex - MiRP1 minK-related peptide 1 - NDI Congenital nephrogenic diabetes insipidus - NMDA N-methyl-d-aspartate - OCA Oculocutaneous albinism - PDI Protein disulfide isomerase - Pgp P-glycoprotein - PKA Protein kinase A - PLP Proteolipid protein - PMP22 Peripheral myelin protein 22 - RP Primary retinitis pigmentosa - SI Sucrase-isomaltase - SNARE Ethylmaleimide-sensitive factor attachment protein - SRP Signal recognition particle - TCR T-cell antigen receptor - TM Transmembrane domain - TRAM Translocating chain-associated membrane protein - Tyr Tyrosinase - Tyrp1 Tyrosinase-related protein-1 - UGGT UDP-glucose:glycoprotein glucosyltransferase - VIP Vesicular-integral membrane protein - V₂R Vasopressin V₂ receptor - VSV Vesicular stomatitis virus     

The epithelial cell cytoskeleton and intracellular trafficking. I. Shiga toxin B-subunit system: retrograde transport,intracellular vectorization,and more

Many intracellular transport routes are still little explored. This is particularly true for retrograde transport between the plasma membrane and the endoplasmic reticulum. Shiga toxin B subunit has become a powerful tool to study this pathway, and recent advances on the molecular mechanisms of transport in the retrograde route and on its physiological function(s) are summarized. Furthermore, it is discussed how the study of retrograde transport of Shiga toxin B subunit allows one to design new methods for the intracellular delivery of therapeutic compounds.     

Arabidopsis plasma membrane proteomics identifies components of transport, signal transduction and membrane trafficking

In order to identify integral proteins and peripheral proteins associated with the plasma membrane, highly purified Arabidopsis plasma membranes from green tissue (leaves and petioles) were analyzed by mass spectrometry. Plasma membranes were isolated by aqueous two-phase partitioning, which yields plasma membrane vesicles with a cytoplasmic-side-in orientation and with a purity of 95%. These vesicles were turned inside-out by treatment with Brij 58 to remove soluble contaminating proteins enclosed in the vesicles and to remove loosely bound contaminating proteins. In total, 238 putative plasma membrane proteins were identified, of which 114 are predicted to have transmembrane domains or to be glycosyl phosphatidylinositol anchored. About two-thirds of the identified integral proteins have not previously been shown to be plasma membrane proteins. Of the 238 identified proteins, 76% could be classified according to function. Major classes are proteins involved in transport (17%), signal transduction (16%), membrane trafficking (9%) and stress responses (9%). Almost a quarter of the proteins identified in the present study are functionally unclassified and more than half of these are predicted to be integral.