Clathrin-Coated Pits: Vive La Différence?

Sorting it out in endosomes: an emerging concept in renal epithelial cell transport regulation

Ion and water transport by the kidney is continually adjusted in response to physiological cues. Selective endocytosis and endosomal trafficking of ion transporters are increasingly appreciated as mechanisms to acutely modulate renal function. Here, we discuss emerging paradigms in this new area of investigation.

Dynamin GTPase regulation is altered by PH domain mutations found in centronuclear myopathy patients

The large GTPase dynamin has an important membrane scission function in receptor-mediated endocytosis and other cellular processes. Self-assembly on phosphoinositide-containing membranes stimulates dynamin GTPase activity, which is crucial for its function. Although the pleckstrin-homology (PH) domain is known to mediate phosphoinositide binding by dynamin, it remains unclear how this promotes activation. Here, we describe studies of dynamin PH domain mutations found in centronuclear myopathy (CNM) that increase dynamin's GTPase activity without altering phosphoinositide binding. CNM mutations in the PH domain C-terminal α-helix appear to cause conformational changes in dynamin that alter control of the GTP hydrolysis cycle. These mutations either 'sensitize' dynamin to lipid stimulation or elevate basal GTPase rates by promoting self-assembly and thus rendering dynamin no longer lipid responsive. We also describe a low-resolution structure of dimeric dynamin from small-angle X-ray scattering that reveals conformational changes induced by CNM mutations, and defines requirements for domain rearrangement upon dynamin self-assembly at membrane surfaces. Our data suggest that changes in the PH domain may couple lipid binding to dynamin GTPase activation at sites of vesicle invagination.

Characterization of endocytosis and exocytosis of cationic nanoparticles in airway epithelium cells

A major challenge of drug delivery using colloids via the airway is to understand the mechanism implied in their interactions with epithelial cells. The purpose of this work was to characterize the process of endocytosis and exocytosis of cationic nanoparticles (NPs) made of maltodextrin which were developed as a delivery system for antigens in vaccine applications. Confocal microscopy demonstrated that these NP are rapidly endocytosed after as little as 3 min incubation, and that the endocytosis was also faster than NP binding since most of the NPs were found in the middle of the cells around the nuclei. A saturation limit was observed after a 40 min incubation, probably due to an equilibrium becoming established between endocytosis and exocytosis. Endocytosis was dramatically reduced at 4 degrees C compared with 37 degrees C, or by NaN(3) treatment, both results suggesting an energy dependent process. Protamine pretreatment of the cells inhibited NPs uptake and we found that clathrin pathway is implied in their endocytosis. Cholesterol depletion increased NP uptake by 300% and this phenomenon was explained by the fact that cholesterol depletion totally blocked NP exocytosis. These results suggest that these cationic NPs interact with anionic sites, are quickly endocytosed via the clathrin pathway and that their exocytosis is cholesterol dependent, and are similar to those obtained in other studies with viruses such as influenza.

Native type IV collagen induces cell migration through a CD9 and DDR1-dependent pathway in MDA-MB-231 breast cancer cells

CD9 is a member of the tetraspanin family and is widely expressed in the plasma membrane of several cell types as well as malignant cells. CD9 associates with a number of transmembrane proteins, which facilitates biological processes, including cell signaling, adhesion, migration and proliferation. DDR1 is activated by native type IV collagen and overexpressed in human breast cancer. Type IV collagen is the main component of basement membranes, and may interact with cell surface biomolecules, promoting adhesion and motility. However, the role of DDR1 and type IV collagen in the regulation of CD9-cell surface levels and migration in breast cancer cells has not been studied in detail. We demonstrate here that native type IV collagen induces a transient increase of CD9-cell surface levels through a DDR1-dependent pathway in MDA-MB-231 breast cancer cells, as revealed by flow cytometry and Western blotting using specific antibodies that recognize CD9. In contrast, type IV collagen does not induce any increase of CD9-cell surface levels in the mammary non-tumorigenic epithelial cells MCF10A and MCF12A. Transient increase of CD9-cell surface levels is coupled with clathrin-mediated endocytosis and it is dependent of DDR1 expression. In addition, type IV collagen induces cell migration through a DDR1 and CD9-dependent pathway. In summary, our data demonstrate, for the first time, that native type IV collagen induces a transient increase of CD9-cell surface levels and cell migration through a DDR1 and CD9-dependent pathway in MDA-MB-231 breast cancer cells.

On some aspects of the thermodynamic of membrane recycling mediated by fluid phase endocytosis: evaluation of published data and perspectives

The theoretical and experimental description of fluid phase endocytosis (FPE) requires an asymmetry in phospholipid number between the two leaflets of the cell membrane, which provides the biomechanical torque needed to generate membrane budding. Although the motor force behind FPE is defined, its kinetic has yet to be determined. Based on a body of evidences suggesting that the mean surface tension is unlikely to be involved in endocytosis we decided to determine whether the cytosolic hydrostatic pressure could be involved, by considering a constant energy exchanged between the cytosol and the cell membrane. The theory is compared to existing experimental data obtained from FPE kinetic studies in living cells where altered phospholipid asymmetry or changes in the extracellular osmotic pressure have been investigated. The model demonstrates that FPE is dependent on the influx and efflux of vesicular volumes (i.e. vesicular volumes recycling) rather than the membrane tension of cells. We conclude that: (i) a relationship exists between membrane lipid number asymmetry and resting cytosolic pressure and (ii) the validity of Laplace's law is limited to cells incubated in a definite hypotonic regime. Finally, we discuss how the model could help clarifying elusive observations obtained from different fields and including: (a) the non-canonical shuttling of aquaporin in cells, (b) the relationship between high blood pressure and inflammation and (c) the mechanosensitivity of the sodium/proton exchanger.

Biopharmaceutical parameters to consider in order to alter the fate of nanocarriers after oral delivery

Oral route is the most common route for the delivery of drugs because it is simple to implement and improves patient compliance and quality of life. However, oral absorption is limited by various physiological barriers and remains a scientific challenge. Nanometric-sized drug delivery systems are being extensively studied and provide promising potential for oral drug delivery. Many different technological solutions have been proposed to enhance the bioavailability or the targeting of drug after oral administration. To reach these goals, it is important to analyze the biopharmaceutical parameters to consider in order to alter the fate of nanocarriers after oral delivery. In the present review, the gastrointestinal barrier and physiological stress factors with regard to nanocarriers' performance or integrity issues are first described. Second, the different characteristics offered by the nanocarriers (size, surface composition and properties mediated by external factors such as ligands) and their effect on the optimal transport of drug into the bloodstream are discussed. Finally, the integrity issue is discussed in function of the expected role of the nanocarriers: bioavailability enhancement or pharmacological targeting.

The ciliary pocket: an endocytic membrane domain at the base of primary and motile cilia

Cilia and flagella are eukaryotic organelles involved in multiple cellular functions. The primary cilium is generally non motile and found in numerous vertebrate cell types where it controls key signalling pathways. Despite a common architecture, ultrastructural data suggest some differences in their organisation. Here, we report the first detailed characterisation of the ciliary pocket, a depression of the plasma membrane in which the primary cilium is rooted. This structure is found at low frequency in kidney epithelial cells (IMCD3) but is associated with virtually all primary cilia in retinal pigment epithelial cells (RPE1). Transmission and scanning electron microscopy, immunofluorescence analysis and videomicroscopy revealed that the ciliary pocket establishes closed links with the actin-based cytoskeleton and that it is enriched in active and dynamic clathrin-coated pits. The existence of the ciliary pocket was confirmed in mouse tissues bearing primary cilia (cumulus), as well as motile cilia and flagella (ependymal cells and spermatids). The ciliary pocket shares striking morphological and functional similarities with the flagellar pocket of Trypanosomatids, a trafficking-specialised membrane domain at the base of the flagellum. Our data therefore highlight the conserved role of membrane trafficking in the vicinity of cilia.

Unique leptin trafficking by a tailless receptor

Impairment in blood-to-brain transport of leptin is a major cause as well as consequence of obesity. Leptin crosses the blood-brain barrier by transcytosis rather than undergoing intracellular degradation. Results from previous studies have indicated that the membrane juxtapositional cytoplasmic sequence of the leptin receptor ObR is responsible for leptin transport. To identify the specific structural domains, we generated a series of ObR truncates with different lengths of the intracellular sequence, overexpressed them in 3 types of mammalian cells including cerebral endothelia, and quantified leptin binding and endocytosis. All mutant ObRs were able to bind and mediate the internalization of leptin. Surprisingly, ObR860, a construct with no cytoplasmic sequence, could act like the classical ObRa transporter in internalizing leptin. There were some cell type-dependent variations in the intracellular trafficking of Alexa-labeled leptin when mediated by ObR860 or ObRa because of differential involvement of membrane microdomains, as shown by use of the clathrin inhibitor chlorpromazine and the dynamin inhibitor Dynasore. The clathrin- and dynamin-mediated endocytosis of leptin contrasts with the lack of effect of the caveolae inhibitors nystatin and filipin. Thus, leptin-induced internalization of the ligand-receptor complex can occur without specific sorting signals in the cytoplasmic region of ObR. This novel finding may have significant implications for leptin transport.

Endocytic internalization routes required for delta/notch signaling

The internalization of transmembrane receptors from the cell surface plays a central role in signal regulation. Receptor internalization can occur through different routes; however, because of the difficulty in selectively blocking these routes in vivo, their roles in signaling are poorly understood. Here we use null mutations in Drosophila dynamin, clathrin, and AP-2 adaptor subunits to analyze internalization requirements for the Delta ligand and its receptor, Notch. Bulk Notch is internalized via AP-2-dependent endocytosis, but signaling by Notch requires AP-2-independent clathrin-dependent endocytosis, highlighting a distinction between Notch endocytic routes required for degradation versus signaling activation. Signaling by Delta requires dynamin, but whether this generates a pulling force of Delta on Notch or allows for Delta entry into a recycling pathway to gain signaling competence is widely debated. Surprisingly, we show that signaling by Delta in germline cells can occur by clathrin-independent endocytosis, when endosomal entry is blocked, and when activity of Rab11 or its effectors is reduced, suggesting that Delta need not pass through a recognized recycling pathway to achieve signaling competence. The absolute requirement for dynamin-dependent endocytosis but not endosomal entry or Rab11 activity supports "pulling force" rather than "recycling" models for Delta activation.

A genomic analysis of transcytosis in the pea aphid, Acyrthosiphon pisum, a mechanism involved in virus transmission

Aphids are the primary vectors of plant viruses. Transmission can occur via attachment to the cuticle lining of the insect (non-circulative transmission) or after internalization in the insect cells with or without replication (circulative transmission). In this paper, we have focused on the circulative and non-propagative mode during which virions enter the cell following receptor-mediated endocytosis, are transported across the cell in vesicles and released by exocytosis without replicating. The correct uptake, transport and delivery of the vesicles cargo relies on the participation of proteins from different families which have been identified in the Acyrthosiphon pisum genome. Assemblage of this annotated dataset provides a useful basis to improve our understanding of the molecules and mechanisms involved in virus transmission by A. pisum and other aphid species.

Role of TI-VAMP and CD82 in EGFR cell-surface dynamics and signaling

The v-SNARE TI-VAMP (VAMP7) mediates exocytosis during neuritogenesis, phagocytosis and lysosomal secretion. It localizes to endosomes and lysosomes but also to the trans-Golgi network. Here we show that depletion of TI-VAMP enhances the endocytosis of activated EGF receptor (EGFR) without affecting constitutive endocytosis of EGFR, or transferrin uptake. This increased EGFR internalization is mainly clathrin dependent. Searching for defects in EGFR regulators, we found that TI-VAMP depletion reduces the cell surface amount of CD82, a tetraspanin known to control EGFR localization in microdomains. We further show that TI-VAMP is required for secretion from the Golgi apparatus to the cell surface, and that TI-VAMP-positive vesicles transport CD82. Quantum dots video-microscopy indicates that depletion of TI-VAMP, or its cargo CD82, restrains EGFR diffusion and the area explored by EGFR at the cell surface. Both depletions also impair MAPK signaling and enhance endocytosis of activated EGFR by increased recruitment of AP-2. These results highlight the role of TI-VAMP in the secretory pathway of a tetraspanin, and support a model in which CD82 allows EGFR entry in microdomains that control its clathrin-dependent endocytosis and signaling.

The clathrin heavy chain isoform CHC22 functions in a novel endosomal sorting step

Clathrin heavy chain 22 (CHC22) is an isoform of the well-characterized CHC17 clathrin heavy chain, a coat component of vesicles that mediate endocytosis and organelle biogenesis. CHC22 has a distinct role from CHC17 in trafficking glucose transporter 4 (GLUT4) in skeletal muscle and fat, though its transfection into HEK293 cells suggests functional redundancy. Here, we show that CHC22 is eightfold less abundant than CHC17 in muscle, other cell types have variably lower amounts of CHC22, and endogenous CHC22 and CHC17 function independently in nonmuscle and muscle cells. CHC22 was required for retrograde trafficking of certain cargo molecules from endosomes to the trans-Golgi network (TGN), defining a novel endosomal-sorting step distinguishable from that mediated by CHC17 and retromer. In muscle cells, depletion of syntaxin 10 as well as CHC22 affected GLUT4 targeting, establishing retrograde endosome-TGN transport as critical for GLUT4 trafficking. Like CHC22, syntaxin 10 is not expressed in mice but is present in humans and other vertebrates, implicating two species-restricted endosomal traffic proteins in GLUT4 transport.

Endocytosis unplugged: multiple ways to enter the cell

Endocytosis occurs at the cell surface and involves internalization of the plasma membrane (PM) along with its constituent membrane proteins and lipids. Endocytosis is involved in sampling of the extracellular milieu and also serves to regulate various processes initiated at the cell surface. These include nutrient uptake, signaling from cell-surface receptors, and many other processes essential for cell and tissue functioning in metazoans. It is also central to the maintenance of PM lipid and protein homeostasis. There are multiple means of internalization that operate concurrently, at the cell surface. With advancement in high-resolution visualization techniques, it is now possible to track multiple endocytic cargo at the same time, revealing a remarkable diversity of endocytic processes in a single cell. A combination of live cell imaging and efficient genetic manipulations has also aided in understanding the functional hierarchy of molecular players in these mechanisms of internalization. Here we provide an account of various endocytic routes, their mechanisms of operation and occurrence across phyla.

Internalization and intracellular sorting of the EGF receptor: a model for understanding the mechanisms of receptor trafficking

The epidermal growth factor receptor (EGFR; also known as ErbB1) is one of four related receptor tyrosine kinases. These receptors (EGFR, ErbB2, ErbB3 and ErbB4) are frequently overexpressed in cancer and such overexpression is associated with poor clinical outcome. Understanding the mechanisms involved in growth-factor-receptor downregulation is medically important, as several drugs that interfere with the function and trafficking of ErbB proteins are currently being developed or are already in clinical trials. EGFR has become a model protein for understanding the biology and endocytosis of related growth-factor receptors, and the mechanisms involved in its endocytosis and degradation have been scrutinized for several decades. Nevertheless, the details and principles of these processes are still poorly understood and often controversial. In particular, the literature describing how the ubiquitylation and recruitment of EGFR to clathrin-coated pits are connected is inconsistent and confusing. In this Opinion article, we discuss the impact of signaling motifs, kinase activity and ubiquitylation on clathrin-dependent endocytosis and lysosomal sorting of EGFR. In addition, we discuss potential explanations for contradicting reports, and propose models for the recruitment of ligand-activated EGFR to clathrin-coated pits as well as for lysosomal sorting of ligand-activated EGFR.

Endocytic events in TCR signaling: focus on adapters in microclusters

Although the critical role of T-cell receptor (TCR) microclusters in T-cell activation is now widely accepted, the mechanisms of regulation of these TCR-rich structures, which also contain enzymes, adapters, and effectors, remain poorly defined. Soon after microcluster formation, several signaling proteins rapidly dissociate from the TCR. Recent studies from our laboratory demonstrated that the movement of the adapters linker for activation of T cells (LAT) and Src homology 2 domain-containing leukocyte protein of 76 kDa (SLP-76) away from initial microcluster formation sites represents endocytic events. Ubiquitylation, Cbl proteins, and multiple endocytic pathways are involved in the internalization events that disassemble signaling microclusters. Several recent studies have indicated that microcluster movement and centralization plays an important role in signal termination. We suggest that microcluster movement is directly linked to endocytic events, thus implicating endocytosis of microclusters as a means to regulate signaling output of the T cell.

HIV-1 accessory protein Vpu internalizes cell-surface BST-2/tetherin through transmembrane interactions leading to lysosomes

Bone marrow stromal antigen 2 (BST-2, also known as tetherin) is a recently identified interferon-inducible host restriction factor that can block the production of enveloped viruses by trapping virus particles at the cell surface. This antiviral effect is counteracted by the human immunodeficiency virus type 1 (HIV-1) accessory protein viral protein U (Vpu). Here we show that HIV-1 Vpu physically interacts with BST-2 through their mutual transmembrane domains and leads to the degradation of this host factor via a lysosomal, not proteasomal, pathway. The degradation is partially controlled by a cellular protein, beta-transducin repeat-containing protein (betaTrCP), which is known to be required for the Vpu-induced degradation of CD4. Importantly, targeting of BST-2 by Vpu occurs at the plasma membrane followed by the active internalization of this host protein by Vpu independently of constitutive endocytosis. Thus, the primary site of action of Vpu is the plasma membrane, where Vpu targets and internalizes cell-surface BST-2 through transmembrane interactions, leading to lysosomal degradation, partially in a betaTrCP-dependent manner. Also, we propose the following configuration of BST-2 in tethering virions to the cell surface; each of the dimerized BST-2 molecules acts as a bridge between viral and cell membranes.

G. S, Kumari S, Lakshminarayan R, Dey G, Mayor S. Analysis of endocytic pathways in Drosophila cells reveals a conserved role for GBF1 in internalization via GEECs

In mammalian cells, endocytosis of the fluid phase and glycosylphosphatidylinositol-anchored proteins (GPI-APs) forms GEECs (GPI-AP enriched early endosomal compartments) via an Arf1- and Cdc42-mediated, dynamin independent mechanism. Here we use four different fluorescently labeled probes and several markers in combination with quantitative kinetic assays, RNA interference and high resolution imaging to delineate major endocytic routes in Drosophila cultured cells. We find that the hallmarks of the pinocytic GEEC pathway are conserved in Drosophila and identify garz, the fly ortholog of the GTP exchange factor GBF1, as a novel component of this pathway. Live confocal and TIRF imaging reveals that a fraction of GBF1 GFP dynamically associates with ABD RFP (a sensor for activated Arf1 present on nascent pinosomes). Correspondingly, a GTP exchange mutant of GBF1 has altered ABD RFP localization in the evanescent field and is impaired in fluid phase uptake. Furthermore, GBF1 activation is required for the GEEC pathway even in the presence of Brefeldin A, implying that, like Arf1, it has a role in endocytosis that is separable from its role in secretion.

Endocytosis and intracellular trafficking of human natural killer cell receptors

Natural killer (NK) cells play a vital role in the defense against viral infections and tumor development. NK cell function is primarily regulated by the sum of signals from a broad array of activation and inhibitory receptors. Key to generating the input level of either activating or inhibitory signals is the maintenance of receptor expression levels on the cell surface. Although the mechanisms of endocytosis and trafficking for some cell surface receptors, such as transferrin receptor and certain immune receptors, are very well known, that is not the situation for receptors expressed by NK cells. Recent studies have uncovered that endocytosis and trafficking routes characteristic for specific activation and inhibitory receptors can regulate the functional responses of NK cells. In this review, we summarize the current knowledge of receptor endocytosis and trafficking, and integrate this with our current understanding of NK cell receptor trafficking.

Ca2+ regulation of dynamin-independent endocytosis in cortical astrocytes

Astrocytes release ATP and glutamate through vesicular exocytosis to mediate neuron-glial interactions. In contrast to exocytosis, the endocytic pathways in astroglial cells are poorly understood. Here, we identify a constitutive endocytic pathway in cultured astrocytes that is dependent on neither clathrin nor dynamin. This dynamin-independent endocytic pathway is regulated by Rab5, an early endosome protein. The endocytosed vesicles show fast transition from early endosomes to late endosomes and lysosomes within a few minutes. Interestingly, this clathrin- and dynamin-independent endocytosis in astrocytes is potently regulated by intracellular Ca(2+). ATP and glutamate greatly enhance the dynamin-independent endocytosis through elevating the intracellular Ca(2+). In addition, amyloid-beta peptide (A beta) also enhances the dynamin-independent endocytosis by inducing Ca(2+) transients in astrocytes. These results demonstrate a novel endocytic pathway in glial cells that is dynamin independent but tightly regulated by intracellular Ca(2+). The regulation by ATP, glutamate, and A beta suggests an important role of the dynamin-independent endocytosis in both physiological and pathological conditions.

Inhibition of dynamin prevents CCL2-mediated endocytosis of CCR2 and activation of ERK1/2

The magnitude and duration of G protein-coupled receptor (GPCR) signals are regulated through desensitization mechanisms. In leukocytes, ligand binding to chemokine receptors leads to Ca2+ mobilization and ERK activation through pertussis toxin-sensitive G proteins, as well as to phosphorylation of the GPCR. After interaction with the endocytic machinery (clathrin, adaptin), the adaptor beta-arrestin recognizes the phosphorylated GPCR tail and quenches signaling to receptors. The molecular mechanisms that lead to receptor endocytosis are not universal amongst the GPCR, however, and the precise spatial and temporal events in the internalization of the CCR2 chemokine receptor remain unknown. Here we show that after ligand binding, CCR2 internalizes rapidly and reaches early endosomes, and later, lysosomes. Knockdown of clathrin by RNA interference impairs CCR2 internalization, as does treatment with the dynamin inhibitor, dynasore. Our results show that CCR2 internalization uses a combination of clathrin-dependent and -independent pathways, as observed for other chemokine receptors. Moreover, the use of dynasore allowed us to confirm the existence of a dynamin-sensitive element that regulates ERK1/2 activation. Our results indicate additional complexity in the link between receptor internalization and cell signaling.

Endocytosis and signalling: a meeting with mathematics

Although endocytosis has traditionally been understood as a signal attenuation mechanism, an emerging view considers endocytosis as an integral part of signal propagation and processing. On the short time scale, trafficking of endocytic vesicles contributes to signal propagation from the surface to distant targets, with bi-directional communication between signalling and trafficking. Mathematical modelling helps combine the mechanistic, molecular knowledge with rigorous analysis of the complex output dynamics of endocytosis in time and space. Simulations reveal novel roles for endocytosis, including the control of cell polarity, enhancing the spatial signal propagation, and controlling the signal magnitudes, kinetics, and synchronization with stimulus dynamics.

Vesicle formation and endocytosis: function, machinery, mechanisms, and modeling

Vesicle formation provides a means of cellular entry for extracellular substances and for recycling of membrane constituents. Mechanisms governing the two primary endocytic pathways (i.e., caveolae- and clathrin-mediated endocytosis, as well as newly emerging vesicular pathways) have become the focus of intense investigation to improve our understanding of nutrient, hormone, and drug delivery, as well as opportunistic invasion of pathogens. In this review of endocytosis, we broadly discuss the structural and signaling proteins that compose the molecular machinery governing endocytic vesicle formation (budding, invagination, and fission from the membrane), with some regard for the specificity observed in certain cell types and species. Important biochemical functions of endocytosis and diseases caused by their disruption also are discussed, along with the structures of key components of endocytic pathways and their known mechanistic contributions. The mechanisms by which principal components of the endocytic machinery are recruited to the plasma membrane, where they interact to induce vesicle formation, are discussed, together with computational approaches used to simulate simplified versions of endocytosis with the hope of clarifying aspects of vesicle formation that may be difficult to determine experimentally. Finally, we pose several unanswered questions intended to stimulate further research interest in the cell biology and modeling of endocytosis.

Plasma membrane area increases with spread area by exocytosis of a GPI-anchored protein compartment

The role of plasma membrane (PM) area as a critical factor during cell motility is poorly understood, mainly due to an inability to precisely follow PM area dynamics. To address this fundamental question, we developed static and dynamic assays to follow exocytosis, endocytosis, and PM area changes during fibroblast spreading. Because the PM area cannot increase by stretch, spreading proceeds by the flattening of membrane folds and/or by the addition of new membrane. Using laser tweezers, we found that PM tension progressively decreases during spreading, suggesting the addition of new membrane. Next, we found that exocytosis increases the PM area by 40-60% during spreading. Reducing PM area reduced spread area, and, in a reciprocal manner, reducing spreadable area reduced PM area, indicating the interconnection between these two parameters. We observed that Golgi, lysosomes, and glycosylphosphatidylinositol-anchored protein vesicles are exocytosed during spreading, but endoplasmic reticulum and transferrin receptor-containing vesicles are not. Microtubule depolymerization blocks lysosome and Golgi exocytosis but not the exocytosis of glycosylphosphatidylinositol-anchored protein vesicles or PM area increase. Therefore, we suggest that fibroblasts are able to regulate about half of their original PM area by the addition of membrane via a glycosylphosphatidylinositol-anchored protein compartment.

The dyslexia-associated protein KIAA0319 interacts with adaptor protein 2 and follows the classical clathrin-mediated endocytosis pathway

Recently, genetic studies have implicated KIAA0319 in developmental dyslexia, the most common of the childhood learning disorders. The first functional data indicated that the KIAA0319 protein is expressed on the plasma membrane and may be involved in neuronal migration. Further analysis of the subcellular distribution of the overexpressed protein in mammalian cells indicates that KIAA0319 can colocalize with the early endosomal marker early endosome antigen 1 (EEA1) in large intracellular vesicles, suggesting that it is endocytosed. Antibody internalization assays with full-length KIAA0319 and deletion constructs confirmed that KIAA0319 is internalized and showed the importance of the cytoplasmic juxtamembranal region in this process. The present study has identified the medium subunit (μ2) of adaptor protein 2 (AP-2) as a binding partner of KIAA0319 in a yeast two-hybrid screen. Using Rab5 mutants or depletion of the μ-subunit of AP-2 or clathrin heavy chain by RNA interference, we demonstrate that KIAA0319 follows a clathrin-mediated endocytic pathway. We also identify tyrosine-995 of KIAA0319 as a critical amino acid required for the interaction with AP-2 and subsequent internalization. These results suggest the surface expression of KIAA0319 is regulated by endocytosis, supporting the idea that the internalization and recycling of the protein may be involved in fine tuning its role in neuronal migration.

Mechanisms of endocytosis

Endocytic mechanisms control the lipid and protein composition of the plasma membrane, thereby regulating how cells interact with their environments. Here, we review what is known about mammalian endocytic mechanisms, with focus on the cellular proteins that control these events. We discuss the well-studied clathrin-mediated endocytic mechanisms and dissect endocytic pathways that proceed independently of clathrin. These clathrin-independent pathways include the CLIC/GEEC endocytic pathway, arf6-dependent endocytosis, flotillin-dependent endocytosis, macropinocytosis, circular doral ruffles, phagocytosis, and trans-endocytosis. We also critically review the role of caveolae and caveolin1 in endocytosis. We highlight the roles of lipids, membrane curvature-modulating proteins, small G proteins, actin, and dynamin in endocytic pathways. We discuss the functional relevance of distinct endocytic pathways and emphasize the importance of studying these pathways to understand human disease processes.

SNX9 - a prelude to vesicle release

The sorting nexin SNX9 has, in the past few years, been singled out as an important protein that participates in fundamental cellular activities. SNX9 binds strongly to dynamin and is partly responsible for the recruitment of this GTPase to sites of endocytosis. SNX9 also has a high capacity for modulation of the membrane and might therefore participate in the formation of the narrow neck of endocytic vesicles before scission occurs. Once assembled on the membrane, SNX9 stimulates the GTPase activity of dynamin to facilitate the scission reaction. It has also become clear that SNX9 has the ability to activate the actin regulator N-WASP in a membrane-dependent manner to coordinate actin polymerization with vesicle release. In this Commentary, we summarize several aspects of SNX9 structure and function in the context of membrane remodeling, discuss its interplay with various interaction partners and present a model of how SNX9 might work in endocytosis.

Role of antibodies in controlling dengue virus infection

The incidence and disease burden of arthropod-borne flavivirus infections have dramatically increased during the last decades due to major societal and economic changes, including massive urbanization, lack of vector control, travel, and international trade. Specifically, in the case of dengue virus (DENV), the geographical spread of all four serotypes throughout the subtropical regions of the world has led to larger and more severe outbreaks. Many studies have established that recovery from infection by one DENV serotype provides immunity against that serotype, whereas reinfection with another serotype may result in severe disease. Pre-existing antibodies thus play a critical role in controlling viral infection. Both neutralization and enhancement of DENV infection by antibodies are thought to be related to the natural route of viral entry into cells. In this review, we will describe the current knowlegde on the mechanisms involved in flavivirus cell entry and discuss how antibodies may influence the course of infection towards neutralization or enhancement of viral disease.

Internalization of swine vesicular disease virus into cultured cells: a comparative study with foot-and-mouth disease virus

We performed a comparative analysis of the internalization mechanisms used by three viruses causing important vesicular diseases in animals. Swine vesicular disease virus (SVDV) internalization was inhibited by treatments that affected clathrin-mediated endocytosis and required traffic through an endosomal compartment. SVDV particles were found in clathrin-coated pits by electron microscopy and colocalized with markers of early endosomes by confocal microscopy. SVDV infectivity was significantly inhibited by drugs that raised endosomal pH. When compared to foot-and-mouth disease virus (FMDV), which uses clathrin-mediated endocytosis, the early step of SVDV was dependent on the integrity of microtubules. SVDV-productive endocytosis was more sensitive to plasma membrane cholesterol extraction than that of FMDV, and differential cell signaling requirements for virus infection were also found. Vesicular stomatitis virus, a model virus internalized by clathrin-mediated endocytosis, was included as a control of drug treatments. These results suggest that different clathrin-mediated routes are responsible for the internalization of these viruses.

Macropinocytosis in Shiga toxin 1 uptake by human intestinal epithelial cells and transcellular transcytosis

Shiga toxin 1 and 2 production is a cardinal virulence trait of enterohemorrhagic Escherichia coli infection that causes a spectrum of intestinal and systemic pathology. However, intestinal sites of enterohemorrhagic E. coli colonization during the human infection and how the Shiga toxins are taken up and cross the globotriaosylceramide (Gb3) receptor-negative intestinal epithelial cells remain largely uncharacterized. We used samples of human intestinal tissue from patients with E. coli O157:H7 infection to detect the intestinal sites of bacterial colonization and characterize the distribution of Shiga toxins. We further used a model of largely Gb3-negative T84 intestinal epithelial monolayers treated with B-subunit of Shiga toxin 1 to determine the mechanisms of non-receptor-mediated toxin uptake. We now report that E. coli O157:H7 were found at the apical surface of epithelial cells only in the ileocecal valve area and that both toxins were present in large amounts inside surface and crypt epithelial cells in all tested intestinal samples. Our in vitro data suggest that macropinocytosis mediated through Src activation significantly increases toxin endocytosis by intestinal epithelial cells and also stimulates toxin transcellular transcytosis. We conclude that Shiga toxin is taken up by human intestinal epithelial cells during E. coli O157:H7 infection regardless of the presence of bacterial colonies. Macropinocytosis might be responsible for toxin uptake by Gb3-free intestinal epithelial cells and transcytosis. These observations provide new insights into the understanding of Shiga toxin contribution to enterohemorrhagic E. coli-related intestinal and systemic diseases.

Endocytosis as a mechanism of regulating natural killer cell function: unique endocytic and trafficking pathway for CD94/NKG2A

Natural killer (NK) cells are lymphocytes generally recognized as sentinels of the innate immune system due to their inherent capacity to deal with diseased (stressed) cells, including malignant and infected. This ability to recognize many potentially pathogenic situations is due to the expression of a diverse panel of activation receptors. Because NK cell activation triggers an aggressive inflammatory response, it is important to have a means of throttling this response. Hence, NK cells also express a panel of inhibitory receptors that recognize ligands expressed by "normal" cells. Little or nothing is known about the endocytosis and trafficking of NK cell receptors, which are of great relevance to understanding how NK cells maintain the appropriate balance of activating and inhibitory receptors on their cell surface. In this review, we focus on the ITIM-containing inhibitory receptor CD94/NKG2A showing that it is endocytosed by a previously undescribed macropinocytic-like process that may be related to the maintenance of its surface expression.

Elimination of a bacterial pore-forming toxin by sequential endocytosis and exocytosis

Staphylococcus aureus alpha-toxin is the archetype of bacterial pore forming toxins and a key virulence factor secreted by the majority of clinical isolates of S. aureus. Toxin monomers bind to target cells and oligomerize to form small beta-barrel pores in the plasma membrane. Many nucleated cells are able to repair a limited number of lesions by unknown, calcium-independent mechanisms. Here we show that cells can internalize alpha-toxin, that uptake is essential for cellular survival, and that pore-complexes are not proteolytically degraded, but returned to the extracellular milieu in the context of exosome-like structures, which we term toxosomes.

Polar targeting and endocytic recycling in auxin-dependent plant development

Plant development is characterized by a profound phenotypic plasticity that often involves redefining of the developmental fate and polarity of cells within differentiated tissues. The plant hormone auxin and its directional intercellular transport play a major role in these processes because they provide positional information and link cell polarity with tissue patterning. This plant-specific mechanism of transport-dependent auxin gradients depends on subcellular dynamics of auxin transport components, in particular on endocytic recycling and polar targeting. Recent insights into these cellular processes in plants have revealed important parallels to yeast and animal systems, including clathrin-dependent endocytosis, retromer function, and transcytosis, but have also emphasized unique features of plant cells such as diversity of polar targeting pathways; integration of environmental signals into subcellular trafficking; and the link between endocytosis, cell polarity, and cell fate specification. We review these advances and focus on the translation of the subcellular dynamics to the regulation of whole-plant development.

Clathrin- and caveolin-independent entry of human papillomavirus type 16--involvement of tetraspanin-enriched microdomains (TEMs)

Background

Infectious entry of human papillomaviruses into their host cells is an important step in the viral life cycle. For cell binding these viruses use proteoglycans as initial attachment sites. Subsequent transfer to a secondary receptor molecule seems to be involved in virus uptake. Depending on the papillomavirus subtype, it has been reported that entry occurs by clathrin- or caveolin-mediated mechanisms. Regarding human papillomavirus type 16 (HPV16), the primary etiologic agent for development of cervical cancer, clathrin-mediated endocytosis was described as infectious entry pathway.

Methodology/Principal Findings

Using immunofluorescence and infection studies we show in contrast to published data that infectious entry of HPV16 occurs in a clathrin- and caveolin-independent manner. Inhibition of clathrin- and caveolin/raft-dependent endocytic pathways by dominant-negative mutants and siRNA-mediated knockdown, as well as inhibition of dynamin function, did not impair infection. Rather, we provide evidence for involvement of tetraspanin-enriched microdomains (TEMs) in HPV16 endocytosis. Following cell attachment, HPV16 particles colocalized with the tetraspanins CD63 and CD151 on the cell surface. Notably, tetraspanin-specific antibodies and siRNA inhibited HPV16 cell entry and infection, confirming the importance of TEMs for infectious endocytosis of HPV16.

Conclusions/Significance

Tetraspanins fulfill various roles in the life cycle of a number of important viral pathogens, including human immunodeficiency virus (HIV) and hepatitis C virus (HCV). However, their involvement in endocytosis of viral particles has not been proven. Our data indicate TEMs as a novel clathrin- and caveolin-independent invasion route for viral pathogens and especially HPV16.

Kinetic regulation of coated vesicle secretion

The secretion of vesicles for intracellular transport often relies on the aggregation of specialized membrane-bound proteins into a coat able to curve cell membranes. The nucleation and growth of a protein coat is a kinetic process that competes with the energy-consuming turnover of coat components between the membrane and the cytosol. We propose a generic kinetic description of coat assembly and the formation of coated vesicles and discuss its implication to the dynamics of COP vesicles that traffic within the Golgi and with the endoplasmic reticulum. We show that stationary coats of fixed area emerge from the competition between coat growth and the recycling of coat components, in a fashion resembling the treadmilling of cytoskeletal filaments. We further show that the turnover of coat components allows for a highly sensitive switching mechanism between a quiescent and a vesicle producing membrane, upon a slowing down of the exchange kinetics. We claim that the existence of this switching behavior, also triggered by factors, such as the presence of cargo and variation of the membrane mechanical tension, allows for efficient regulation of vesicle secretion. We propose a model, supported by different experimental observations, in which vesiculation of secretory membranes is impaired by the energy-consuming desorption of coat proteins, until the presence of cargo or other factors triggers a dynamical switch into a vesicle producing state.

Gyrating clathrin: highly dynamic clathrin structures involved in rapid receptor recycling

We report here detection of novel intracellular clathrin-coated structures revealed by continuous high-speed imaging of cells expressing green fluorescent protein fusion proteins. These structures, which we operationally term 'gyrating clathrin' (G-clathrin), are characterized by localized but extremely rapid movement, leading to the hypothesis that they are coated buds on waving membrane tubules. G-clathrin structures have structurally and functionally distinct features. They lack detectable adaptor proteins AP-1 and AP-2 but contain GGA1 [Golgi-localized, gamma-ear-containing, Arf (ADP-ribosylation factor)-binding protein] as well as the cation-dependent mannose-6-phosphate receptor. While they accumulate internalized transferrin (Tf), they do not contain detectable levels of cargos targeted for the late endosome/lysosome pathway such as EGF and dextran. Pulse-chase studies indicate that Tf appears in G-clathrin structures in the cell periphery after sorting endosomes (SEs), but before filling of the perinuclear endocytic recycling compartment. Furthermore, the inhibitors LY294002 and wortmannin, which inhibit direct recycling of Tf from SEs to the plasma membrane, also block its appearance in G-clathrin. These observations suggest that peripheral G-clathrin contributes to rapid recycling, a kinetically defined compartment that has largely eluded structural identification. More generally, the rapid continuous live cell imaging reported here reveals new aspects of membrane trafficking.

Mediation, modulation, and consequences of membrane-cytoskeleton interactions

Elements of the cytoskeleton interact intimately and communicate bidirectionally with cellular membranes. Such interactions are critical for a host of cellular processes. Here we focus on the many types of interactions that exist between the cytoskeleton and the plasma membrane to illustrate why these cellular components can never truly be studied in isolation in vivo. We discuss how membrane-cytoskeleton interactions are mediated and modulated, and how many proteins involved in these interactions are disrupted in human disease. We then highlight key molecular and physical variables that must be considered in order to mechanistically dissect events associated with changes in plasma membrane morphology. These considerations are integrated into the context of cell migration, filopodia formation, and clathrin-mediated endocytosis to show how a holistic view of the plasma membrane-cytoskeleton interface can allow for the appropriate interpretation of experimental findings and provide novel mechanistic insight into these important cellular events.

Focusing on clathrin-mediated endocytosis

Investigations into the mechanisms which regulate entry of integral membrane proteins, and associated ligands, into the cell through vesicular carriers (endocytosis) have greatly benefited from the application of live-cell imaging. Several excellent recent reviews have detailed specific aspects of endocytosis, such as entry of particular cargo, or the different routes of internalization. The aim of the present review is to highlight how advances in live-cell fluorescence microscopy have affected the study of clathrin-mediated endocytosis. The last decade has seen a tremendous increase in the development and dissemination of methods for imaging endocytosis in live cells, and this has been followed by a dramatic shift in the way this critical cellular pathway is studied and understood. The present review begins with a description of the technical advances which have permitted new types of experiment to be performed, as well as potential pitfalls of these new technologies. Subsequently, advances in the understanding of three key endocytic proteins will be addressed: clathrin, dynamin and AP-2 (adaptor protein 2). Although great strides have clearly been made in these areas in recent years, as is often the case, each answer has bred numerous questions. Furthermore, several examples are highlighted where, because of seemingly minor differences in experimental systems, what appear at first to be very similar studies have, at times, yielded vastly differing results and conclusions. Thus this is an exceedingly exciting time to study endocytosis, and this area serves as a clear demonstration of the power of applying live-cell imaging to answer fundamental biological questions.

The cytoplasmic domain of the hyaluronan receptor for endocytosis (HARE) contains multiple endocytic motifs targeting coated pit-mediated internalization

The hyaluronic acid (HA) receptor for endocytosis (HARE) is the primary scavenger receptor for HA and chondroitin sulfates in mammals. The two human isoforms of HARE (full-length 315-kDa and a 190-kDa proteolytic cleavage product), which are type I single-pass membrane proteins, are highly expressed in sinusoidal endothelial cells of lymph nodes, liver, and spleen. Their identical HARE cytoplasmic domains contain four candidate AP-2/clathrin-mediated endocytic signaling motifs as follows: YSYFRI(2485), FQHF(2495), NPLY(2519), and DPF(2534) (315-HARE numbering). Stably transfected cells expressing 190-HARE(DeltaYSYFRI), 190-HARE(DeltaFQHF), or 190-HARE(DeltaNPLY) (lacking Motifs 1, 2, or 3) had decreased (125)I-HA endocytosis rates of approximately 49, approximately 39, and approximately 56%, respectively (relative to wild type). In contrast, 190-HARE(DeltaDPF) cells (lacking Motif 4) showed no change in HA endocytic rate. Deletions of motifs 1 and 2 or of 1, 2, and 4 decreased the rate of HA endocytosis by only approximately 41%. Endocytosis was approximately 95% decreased in mutants lacking all four motifs. Cells expressing a 190-HARE(Y2519A) mutant of the NPLY motif retained 85-90% of wild type endocytosis, whereas this mutation in the triple motif deletant decreased endocytosis to approximately 7% of wild type. Tyr in NPLY(2519) is thus important for endocytosis. All HARE mutants showed similar HA binding and degradation of the internalized HA, indicating that altering endocytic motifs did not affect ectodomain binding of HA or targeting of internalized HA to lysosomes. We conclude that, although NPLY may be the most important motif, it functions together with two other endocytic motifs; thus three signal sequences (YSYFRI, FQHF, and NPLY) provide redundancy to mediate coated pit targeting and endocytosis of HARE.

Uncommon endocytic and trafficking pathway of the natural killer cell CD94/NKG2A inhibitory receptor

The CD94/NKG2A inhibitory receptor, expressed by natural killer and T cells, is constantly exposed to its HLA-E ligand expressed by surrounding cells. Ligand exposure often induces receptor downregulation. For CD94/NKG2A, this could potentiate activation receptor(s) induced responses to normal bystander cells. We investigated CD94/NKG2A endocytosis and found that it occurs by an amiloride-sensitive, Rac1-dependent macropinocytic-like process; however, it does not require clathrin, dynamin, ADP ribosylation factor-6, phosphoinositide-3 kinase or the actin cytoskeleton. Once endocytosed, CD94/NKG2A traffics to early endosomal antigen 1(+), Rab5(+) early endosomes. It does appear in Rab4(+) early/sorting endosome, but, in the time period examined, fails to reach Rab11(+) recycling or Rab7(+) late endosomes or lysosome-associated membrane protein-1(+) lysosomes. These results indicate that CD94/NKG2A utilizes a previously undescribed endocytic mechanism coupled with an abbreviated trafficking pattern, perhaps to insure surface expression.

Imaging and imagination: understanding the endo-lysosomal system

Lysosomes are specialized compartments for the degradation of endocytosed and intracellular material and essential regulators of cellular homeostasis. The importance of lysosomes is illustrated by the rapidly growing number of human disorders related to a defect in lysosomal functioning. Here, we review current insights in the mechanisms of lysosome biogenesis and protein sorting within the endo-lysosomal system. We present increasing evidence for the existence of parallel pathways for the delivery of newly synthesized lysosomal proteins directly from the trans-Golgi network (TGN) to the endo-lysosomal system. These pathways are either dependent or independent of mannose 6-phosphate receptors and likely involve multiple exits for lysosomal proteins from the TGN. In addition, we discuss the different endosomal intermediates and subdomains that are involved in sorting of endocytosed cargo. Throughout our review, we highlight some examples in the literature showing how imaging, especially electron microscopy, has made major contributions to our understanding of the endo-lysosomal system today.

Clathrin-independent endocytosis: from nonexisting to an extreme degree of complexity

Today it is generally accepted that there are several endocytic mechanisms, both the clathrin-dependent one and mechanisms which operate without clathrin and with different requirements when it comes to dynamin, small GTP-binding proteins of the Rho family and specific lipids. It should be noted that clathrin-independent endocytosis can occur even when the cholesterol level in the membrane has been reduced to so low levels that caveolae are gone and clathrin-coated membrane areas are flat. Although new investigators in the field take it for granted that there is a multitude of entry mechanisms, it has taken a long time for this to become accepted. However, more work needs to be done, because one can still ask the question: How many endocytic mechanisms does a cell have, what are their function, and how are they regulated? This article describes some of the history of endocytosis research and attempts to give an overview of the complexity of the mechanisms and their regulation.

Regulation of raft-dependent endocytosis

Raft-dependent endocytosis is in large part defined as the cholesterol-sensitive, clathrin-independent internalization of ligands and receptors from the plasma membrane. It encompasses the endocytosis of caveolae, smooth plasmalemmal vesicles that form a subdomain of cholesterol and sphingolipid-rich lipid rafts and that are enriched for caveolin-1. While sharing common mechanisms, like cholesterol sensitivity, raft endocytic routes show differential regulation by various cellular components including caveolin-1, dynamin-2 and regulators of the actin cytoskeleton. Dynamin-dependent raft pathways, mediated by caveolae and morphologically equivalent non-caveolin vesicular intermediates, are referred to as caveolae/raft-dependent endocytosis. In contrast, dynamin-independent raft pathways are mediated by non-caveolar intermediates. Raft-dependent endocytosis is regulated by tyrosine kinase inhibitors and, through the regulation of the internalization of various ligands, receptors and effectors, is also a determinant of cellular signaling. In this review, we characterize and discuss the regulation of raft-dependent endocytic pathways and the role of key regulators such as caveolin-1.