Cargo trafficking between endosomes and the trans-Golgi network

Identification of lamprey variable lymphocyte receptors that target the brain vasculature

The blood–brain barrier (BBB) represents a significant bottleneck for the delivery of therapeutics to the central nervous system. In recent years, the promise of coopting BBB receptor-mediated transport systems for brain drug delivery has increased in large part due to the discovery and engineering of BBB-targeting antibodies. Here we describe an innovative screening platform for identification of new BBB targeting molecules from a class of lamprey antigen recognition proteins known as variable lymphocyte receptors (VLRs). Lamprey were immunized with murine brain microvessel plasma membranes, and the resultant repertoire cloned into the yeast surface display system. The library was screened via a unique workflow that identified 16 VLR clones that target extracellular epitopes of in vivo-relevant BBB membrane proteins. Of these, three lead VLR candidates, VLR-Fc-11, VLR-Fc-30, and VLR-Fc-46 selectively target the brain vasculature and traffic within brain microvascular endothelial cells after intravenous administration in mice, with VLR-Fc-30 being confirmed as trafficking into the brain parenchyma. Epitope characterization indicates that the VLRs, in part, recognize sialylated glycostructures. These promising new targeting molecules have the potential for brain targeting and drug delivery with improved brain vascular specificity.

The Cellular and Chemical Biology of Endocytic Trafficking and Intracellular Delivery-The GL-Lect Hypothesis

Lipid membranes are common to all forms of life. While being stable barriers that delimitate the cell as the fundamental organismal unit, biological membranes are highly dynamic by allowing for lateral diffusion, transbilayer passage via selective channels, and in eukaryotic cells for endocytic uptake through the formation of membrane bound vesicular or tubular carriers. Two of the most abundant fundamental fabrics of membranes-lipids and complex sugars-are produced through elaborate chains of biosynthetic enzymes, which makes it difficult to study them by conventional reverse genetics. This review illustrates how organic synthesis provides access to uncharted areas of membrane glycobiology research and its application to biomedicine. For this Special Issue on Chemical Biology Research in France, focus will be placed on synthetic approaches (i) to study endocytic functions of glycosylated proteins and lipids according to the GlycoLipid-Lectin (GL-Lect) hypothesis, notably that of Shiga toxin; (ii) to mechanistically dissect its endocytosis and intracellular trafficking with small molecule; and (iii) to devise intracellular delivery strategies for immunotherapy and tumor targeting. It will be pointed out how the chemical biologist's view on lipids, sugars, and proteins synergizes with biophysics and modeling to "look" into the membrane for atomistic scale insights on molecular rearrangements that drive the biogenesis of endocytic carriers in processes of clathrin-independent endocytosis.

Human Respiratory Syncytial Virus Infection in a Human T Cell Line Is Hampered at Multiple Steps

Human respiratory syncytial virus (HRSV) is the most frequent cause of severe respiratory disease in children. The main targets of HRSV infection are epithelial cells of the respiratory tract, and the great majority of the studies regarding HRSV infection are done in respiratory cells. Recently, the interest on respiratory virus infection of lymphoid cells has been growing, but details of the interaction of HRSV with lymphoid cells remain unknown. Therefore, this study was done to assess the relationship of HRSV with A3.01 cells, a human CD4⁺ T cell line. Using flow cytometry and fluorescent focus assay, we found that A3.01 cells are susceptible but virtually not permissive to HRSV infection. Dequenching experiments revealed that the fusion process of HRSV in A3.01 cells was nearly abolished in comparison to HEp-2 cells, an epithelial cell lineage. Quantification of viral RNA by RT-qPCR showed that the replication of HRSV in A3.01 cells was considerably reduced. Western blot and quantitative flow cytometry analyses demonstrated that the production of HRSV proteins in A3.01 was significantly lower than in HEp-2 cells. Additionally, using fluorescence in situ hybridization, we found that the inclusion body-associated granules (IBAGs) were almost absent in HRSV inclusion bodies in A3.01 cells. We also assessed the intracellular trafficking of HRSV proteins and found that HRSV proteins colocalized partially with the secretory pathway in A3.01 cells, but these HRSV proteins and viral filaments were present only scarcely at the plasma membrane. HRSV infection of A3.01 CD4⁺ T cells is virtually unproductive as compared to HEp-2 cells, as a result of defects at several steps of the viral cycle: Fusion, genome replication, formation of inclusion bodies, recruitment of cellular proteins, virus assembly, and budding.

Amino acids stimulate the endosome-to-Golgi trafficking through Ragulator and small GTPase Arl5

The endosome-to-Golgi or endocytic retrograde trafficking pathway is an important post-Golgi recycling route. Here we show that amino acids (AAs) can stimulate the retrograde trafficking and regulate the cell surface localization of certain Golgi membrane proteins. By testing components of the AA-stimulated mTORC1 signaling pathway, we demonstrate that SLC38A9, v-ATPase and Ragulator, but not Rag GTPases and mTORC1, are essential for the AA-stimulated trafficking. Arl5, an ARF-like family small GTPase, interacts with Ragulator in an AA-regulated manner and both Arl5 and its effector, the Golgi-associated retrograde protein complex (GARP), are required for the AA-stimulated trafficking. We have therefore identified a mechanistic connection between the nutrient signaling and the retrograde trafficking pathway, whereby SLC38A9 and v-ATPase sense AA-sufficiency and Ragulator might function as a guanine nucleotide exchange factor to activate Arl5, which, together with GARP, a tethering factor, probably facilitates the endosome-to-Golgi trafficking.

Amino acid levels are known to regulate anabolic and catabolic pathways. Here, the authors report that amino acids also affect membrane trafficking by stimulating endosome-to-Golgi retrograde trafficking and regulating cell surface localization of certain Golgi proteins through Ragulator and Arl5.

Most rotavirus strains require the cation-independent mannose-6-phosphate receptor, sortilin-1, and cathepsins to enter cells

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Rotaviruses require the TGN to LE transporter CI-M6PR for cell entry.

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Sortilin-1 was identified as a cell factor involved in rotavirus replication.

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Rotaviruses require cathepsins also to enter Caco-2 cells.

Cathepsins, endosomal acid proteases, are transported from the trans-Golgi network to late endosomes by the mannose-6-phosphate receptor (M6PR). We have previously demonstrated that some rotavirus strains, like UK, Wa, WI61, DS-1, and YM, require the cation-dependent (CD-) M6PR and cathepsins to enter from late endosomes to the cytoplasm in MA104 cells, while other strains, like the simian strain RRV, which enter cells from maturing endosomes, do not. However, the role of other trans-Golgi network-late endosome transporters, such as the cation-independent (CI-) M6PR and sortillin-1, has not been evaluated. In this work, we found that several rotavirus strains that require the CD-M6PR for cell entry are also dependent on CI-M6PR and sortilin-1. Furthermore, we showed that the infectivity of all these rotavirus strains also requires cathepsins to enter not only MA104 cells, but also human intestinal Caco-2 cells. This study identifies sortilin-1 as a novel cell factor necessary for the infectivity of a virus; in addition, our results strongly suggest that cathepsins could be common cell factors needed for the infectivity of most rotavirus strains.

Endocytosis of tight junction proteins and the regulation of degradation and recycling

Internalization of tight junction (TJ) proteins from the plasma membrane is a pivotal mechanism regulating TJ plasticity and function in both epithelial and endothelial barrier tissues. Once internalized, the TJ proteins enter complex vesicular machinery, where further trafficking is directly dependent on the initiating stimulus and downstream signaling pathways that regulate the sorting and destiny of TJ proteins, as well as on cell and barrier responses. The destiny of internalized TJ proteins is recycling to the plasma membrane or sorting to late endosomes and degradation. This review highlights recent advances in our knowledge of endocytosis and vesicular trafficking of TJ proteins in both epithelial and endothelial cells. A greater understanding of these processes may allow for the development of methods to modulate barrier permeability for drug delivery or prevent barrier dysfunction in disease states.

Cell surface recycling in yeast: mechanisms and machineries

Sorting internalized proteins and lipids back to the cell surface controls the supply of molecules throughout the cell and regulates integral membrane protein activity at the surface. One central process in mammalian cells is the transit of cargo from endosomes back to the plasma membrane (PM) directly, along a route that bypasses retrograde movement to the Golgi. Despite recognition of this pathway for decades we are only beginning to understand the machinery controlling this overall process. The budding yeastSaccharomyces cerevisiae, a stalwart genetic system, has been routinely used to identify fundamental proteins and their modes of action in conserved trafficking pathways. However, the study of cell surface recycling from endosomes in yeast is hampered by difficulties that obscure visualization of the pathway. Here we briefly discuss how recycling is likely a more prevalent process in yeast than is widely appreciated and how tools might be built to better study the pathway.

Bidirectional traffic between the Golgi and the endosomes - machineries and regulation

The bidirectional transport between the Golgi complex and the endocytic pathway has to be finely regulated in order to ensure the proper delivery of newly synthetized lysosomal enzymes and the return of sorting receptors from degradative compartments. The high complexity of these routes has led to experimental difficulties in properly dissecting and separating the different pathways. As a consequence, several models have been proposed during the past decades. However, recent advances in our understanding of endosomal dynamics have helped to unify these different views. We provide here an overview of the current insights into the transport routes between Golgi and endosomes in mammalian cells. The focus of the Commentary is on the key molecules involved in the trafficking pathways between these intracellular compartments, such as Rab proteins and sorting receptors, and their regulation. A proper understanding of the bidirectional traffic between the Golgi complex and the endolysosomal system is of uttermost importance, as several studies have demonstrated that mutations in the factors involved in these transport pathways result in various pathologies, in particular lysosome-associated diseases and diverse neurological disorders, such as Alzheimer's and Parkinson's disease.

Post-Golgi anterograde transport requires GARP-dependent endosome-to-TGN retrograde transport

GARP (tethering factor)- and VAMP4 (v-SNARE)-dependent endosome-to-TGN retrograde transport is required for the efficient post-Golgi anterograde transport of cell-surface integral membrane proteins. Golgi-resident membrane proteins TMEM87A and TMEM87B are involved in endosome-to-TGN retrograde transport.

The importance of endosome-to–trans-Golgi network (TGN) retrograde transport in the anterograde transport of proteins is unclear. In this study, genome-wide screening of the factors necessary for efficient anterograde protein transport in human haploid cells identified subunits of the Golgi-associated retrograde protein (GARP) complex, a tethering factor involved in endosome-to-TGN transport. Knockout (KO) of each of the four GARP subunits, VPS51–VPS54, in HEK293 cells caused severely defective anterograde transport of both glycosylphosphatidylinositol (GPI)-anchored and transmembrane proteins from the TGN. Overexpression of VAMP4, v-SNARE, in VPS54-KO cells partially restored not only endosome-to-TGN retrograde transport, but also anterograde transport of both GPI-anchored and transmembrane proteins. Further screening for genes whose overexpression normalized the VPS54-KO phenotype identified TMEM87A, encoding an uncharacterized Golgi-resident membrane protein. Overexpression of TMEM87A or its close homologue TMEM87B in VPS54-KO cells partially restored endosome-to-TGN retrograde transport and anterograde transport. Therefore GARP- and VAMP4-dependent endosome-to-TGN retrograde transport is required for recycling of molecules critical for efficient post-Golgi anterograde transport of cell-surface integral membrane proteins. In addition, TMEM87A and TMEM87B are involved in endosome-to-TGN retrograde transport.

Visualizing and quantitating sequence-dependent GPCR recycling

Recent advances in direct imaging have given us a new appreciation of the spatial and temporal dynamics of membrane trafficking processes, and have allowed us to ask questions that were difficult to address with traditional methods. A relevant example of this is protein sorting in the endosome, which serves as the primary sorting station for proteins internalized from the cell surface. In this chapter, we discuss fluorescence imaging protocols to directly visualize and quantitate the recycling of G protein-coupled receptors (GPCRs)-a highly physiologically relevant family of signaling receptors-in real time in living cells. The protocols allow direct visualization and quantitation of both GPCR exit from the endosome and GPCR delivery to the cell surface. The methods may be extended to study the endolysosomal sorting of many proteins that undergoes endocytic cycling, and may be adapted to other organelles and systems where proteins are sorted.

Retromer-Mediated Trafficking of Transmembrane Receptors and Transporters

Transport between the endoplasmatic reticulum, the Golgi-network, the endo-lysosomal system and the cell surface can be categorized as anterograde or retrograde, describing traffic that goes forward or backward, respectively. Traffic going from the plasma membrane to endosomes and lysosomes or the trans-Golgi network (TGN) constitutes the major retrograde transport routes. Several transmembrane proteins undergo retrograde transport as part of a recycling mechanism that contributes to reutilization and maintenance of a steady-state protein localization. In addition, some receptors are hijacked by exotoxins and used for entry and intracellular transport. The physiological relevance of retrograde transport cannot be overstated. Retrograde trafficking of the amyloid precursor protein determines the distribution between organelles, and hence the possibility of cleavage by γ-secretase. Right balancing of the pathways is critical for protection against Alzheimer’s disease. During embryonic development, retrograde transport of Wntless to the TGN is essential for the following release of Wnt from the plasma membrane. Furthermore, overexpression of Wntless has been linked to oncogenesis. Here, we review relevant aspects of the retrograde trafficking of mammalian transmembrane receptors and transporters, with focus on the retromer-mediated transport between endosomes and the TGN.

Cellular uptake and intracellular trafficking of oligonucleotides

Oligonucleotides manifest much promise as potential therapeutic agents. However, understanding of how oligonucleotides function within living organisms is still rather limited. A major concern in this regard is the mechanisms of cellular uptake and intracellular trafficking of both 'free' oligonucleotides and oligonucleotides associated with various polymeric or nanocarrier delivery systems. Here we review basic aspects of the mechanisms of endocytosis and intracellular trafficking and how insights from these processes can be used to understand oligonucleotide delivery. In particular we discuss opportunities for escape of oligonucleotides from endomembrane compartments and describe recent studies using small molecules to enhance oligonucleotide effects.

Differential effects of c-Src and c-Yes on the endocytic vesicle-mediated trafficking events at the Sertoli cell blood-testis barrier: an in vitro study

The blood-testis barrier (BTB) is one of the tightest blood-tissue barriers in the mammalian body. However, it undergoes cyclic restructuring during the epithelial cycle of spermatogenesis in which the "old" BTB located above the preleptotene spermatocytes being transported across the immunological barrier is "disassembled," whereas the "new" BTB found behind these germ cells is rapidly "reassembled," i.e., mediated by endocytic vesicle-mediated protein trafficking events. Thus, the immunological barrier is maintained when preleptotene spermatocytes connected in clones via intercellular bridges are transported across the BTB. Yet the underlying mechanism(s) in particular the involving regulatory molecules that coordinate these events remains unknown. We hypothesized that c-Src and c-Yes might work in contrasting roles in endocytic vesicle-mediated trafficking, serving as molecular switches, to effectively disassemble and reassemble the old and the new BTB, respectively, to facilitate preleptotene spermatocyte transport across the BTB. Following siRNA-mediated specific knockdown of c-Src or c-Yes in Sertoli cells, we utilized biochemical assays to assess the changes in protein endocytosis, recycling, degradation and phagocytosis. c-Yes was found to promote endocytosed integral membrane BTB proteins to the pathway of transcytosis and recycling so that internalized proteins could be effectively used to assemble new BTB from the disassembling old BTB, whereas c-Src promotes endocytosed Sertoli cell BTB proteins to endosome-mediated protein degradation for the degeneration of the old BTB. By using fluorescence beads mimicking apoptotic germ cells, Sertoli cells were found to engulf beads via c-Src-mediated phagocytosis. A hypothetical model that serves as the framework for future investigation is thus proposed.

The Mysterious Ways of ErbB2/HER2 Trafficking

The EGFR- or ErbB-family of receptor tyrosine kinases consists of EGFR/ErbB1, ErbB2/HER2, ErbB3/HER3 and ErbB4/HER4. Receptor activation and downstream signaling are generally initiated upon ligand-induced receptor homo- or heterodimerization at the plasma membrane, and endocytosis and intracellular membrane transport are crucial for regulation of the signaling outcome. Among the receptors, ErbB2 is special in several ways. Unlike the others, ErbB2 has no known ligand, but is still the favored dimerization partner. Furthermore, while the other receptors are down-regulated either constitutively or upon ligand-binding, ErbB2 is resistant to down-regulation, and also inhibits down-regulation of its partner upon heterodimerization. The reason(s) why ErbB2 is resistant to down-regulation are the subject of debate. Contrary to other ErbB-proteins, mature ErbB2 needs Hsp90 as chaperone. Several data suggest that Hsp90 is an important regulator of factors like ErbB2 stability, dimerization and/or signaling. Hsp90 inhibitors induce degradation of ErbB2, but whether Hsp90 directly makes ErbB2 endocytosis resistant is unclear. Exposure to anti-ErbB2 antibodies can also induce down-regulation of ErbB2. Down-regulation induced by Hsp90 inhibitors or antibodies does at least partly involve internalization and endosomal sorting to lysosomes for degradation, but also retrograde trafficking to the nucleus has been reported. In this review, we will discuss different molecular mechanisms suggested to be important for making ErbB2 resistant to down-regulation, and review how membrane trafficking is involved when down-regulation and/or relocalization of ErbB2 is induced.

From endosomes to the trans-Golgi network

The retrograde trafficking from endosomes to the trans-Golgi network (TGN) is one of the major endocytic pathways to divert proteins and lipids away from lysosomal degradation. Retrograde transported cargos enter the TGN via two itineraries from either the early endosome/recycling endosome or the late endosome and involve various machinery components such as retromer, sorting nexins, clathrin, small GTPases, tethering factors and SNAREs. Recently, the pathway has been recognized for its role in signal transduction, physiology and pathogenesis of human diseases.

The Histochem Cell Biol conspectus: the year 2013 in review

Herein, we provide a brief synopsis of all manuscripts published in Histochem Cell Biol in the year 2013. For ease of reference, we have divided the manuscripts into the following categories: Advances in Methodologies; Molecules in Health and Disease; Organelles, Subcellular Structures and Compartments; Golgi Apparatus; Intermediate Filaments and Cytoskeleton; Connective Tissue and Extracellular Matrix; Autophagy; Stem Cells; Musculoskeletal System; Respiratory and Cardiovascular Systems; Gastrointestinal Tract; Central Nervous System; Peripheral Nervous System; Excretory Glands; Kidney and Urinary Bladder; and Male and Female Reproductive Systems. We hope that the readership will find this annual journal synopsis of value and serve as a quick, categorized reference guide for “state-of-the-art” manuscripts in the areas of histochemistry, immunohistochemistry, and cell biology.

Rotaviruses reach late endosomes and require the cation-dependent mannose-6-phosphate receptor and the activity of cathepsin proteases to enter the cell

Rotaviruses (RVs) enter cells through different endocytic pathways. Bovine rotavirus (BRV) UK uses clathrin-mediated endocytosis, while rhesus rotavirus (RRV) employs an endocytic process independent of clathrin and caveolin. Given the differences in the cell internalization pathway used by these viruses, we tested if the intracellular trafficking of BRV UK was the same as that of RRV, which is known to reach maturing endosomes (MEs) to infect the cell. We found that BRV UK also reaches MEs, since its infectivity depends on the function of Rab5, the endosomal sorting complex required for transport (ESCRT), and the formation of endosomal intraluminal vesicles (ILVs). However, unlike RRV, the infectivity of BRV UK was inhibited by knocking down the expression of Rab7, indicating that it has to traffic to late endosomes (LEs) to infect the cell. The requirement for Rab7 was also shared by other RV strains of human and porcine origin. Of interest, most RV strains that reach LEs were also found to depend on the activities of Rab9, the cation-dependent mannose-6-phosphate receptor (CD-M6PR), and cathepsins B, L, and S, suggesting that cellular factors from the trans-Golgi network (TGN) need to be transported by the CD-M6PR to LEs to facilitate RV cell infection. Furthermore, using a collection of UK × RRV reassortant viruses, we found that the dependence of BRV UK on Rab7, Rab9, and CD-M6PR is associated with the spike protein VP4. These findings illustrate the elaborate pathway of RV entry and reveal a new process (Rab9/CD-M6PR/cathepsins) that could be targeted for drug intervention.

IMPORTANCE

Rotavirus is an important etiological agent of severe gastroenteritis in children. In most instances, viruses enter cells through an endocytic pathway that delivers the viral particle to vesicular organelles known as early endosomes (EEs). Some viruses reach the cytoplasm from EEs, where they start to replicate their genome. However, other viruses go deeper into the cell, trafficking from EEs to late endosomes (LEs) to disassemble and reach the cytoplasm. In this work, we show that most RV strains have to traffic to LEs, and the transport of endolysosomal proteases from the Golgi complex to LEs, mediated by the mannose-6-phosphate receptor, is necessary for the virus to exit the vesicular compartment and efficiently start viral replication. We also show that this deep journey into the cell is associated with the virus spike protein VP4. These findings illustrate the elaborate pathway of RV entry that could be used for drug intervention.

Reduction in Golgi apparatus dimension in the absence of a residential protein, N-acetylglucosaminyltransferase V

Various proteins are involved in the generation and maintenance of the membrane complex known as the Golgi apparatus. We have used mutant Chinese hamster ovary (CHO) cell lines Lec4 and Lec4A lacking N-acetylglucosaminyltransferase V (GlcNAcT-V, MGAT5) activity and protein in the Golgi apparatus to study the effects of the absence of a single glycosyltransferase on the Golgi apparatus dimension. Quantification of immunofluorescence in serial confocal sections for Golgi α-mannosidase II and electron microscopic morphometry revealed a reduction in Golgi volume density up to 49 % in CHO Lec4 and CHO Lec4A cells compared to parental CHO cells. This reduction in Golgi volume density could be reversed by stable transfection of Lec4 cells with a cDNA encoding Mgat5. Inhibition of the synthesis of β1,6-branched N-glycans by swainsonine had no effect on Golgi volume density. In addition, no effect on Golgi volume density was observed in CHO Lec1 cells that contain enzymatically active GlcNAcT-V, but cannot synthesize β1,6-branched glycans due to an inactive GlcNAcT-I in their Golgi apparatus. These results indicate that it may be the absence of the GlcNAcT-V protein that is the determining factor in reducing Golgi volume density. No dimensional differences existed in cross-sectioned cisternal stacks between Lec4 and control CHO cells, but significantly reduced Golgi stack hits were observed in cross-sectioned Lec4 cells. Therefore, the Golgi apparatus dimensional change in Lec4 and Lec4A cells may be due to a compaction of the organelle.