Similar uptake but different trafficking and escape routes of reovirus virions and infectious subvirion particles imaged in polarized Madin-Darby canine kidney cells

Polarized epithelial cells that line the digestive, respiratory, and genitourinary tracts form a barrier that many viruses must breach to infect their hosts. Current understanding of cell entry by mammalian reovirus (MRV) virions and infectious subvirion particles (ISVPs), generated from MRV virions by extracellular proteolysis in the digestive tract, are mostly derived from in vitro studies with nonpolarized cells. Recent live-cell imaging advances allow us for the first time to visualize events at the apical surface of polarized cells. In this study, we used spinning-disk confocal fluorescence microscopy with high temporal and spatial resolution to follow the uptake and trafficking dynamics of single MRV virions and ISVPs at the apical surface of live polarized Madin-Darby canine kidney cells. Both types of particles were internalized by clathrin-mediated endocytosis, but virions and ISVPs exhibited strikingly different trafficking after uptake. While virions reached early and late endosomes, ISVPs did not and instead escaped the endocytic pathway from an earlier location. This study highlights the broad advantages of using live-cell imaging combined with single-particle tracking for identifying key steps in cell entry by viruses.

CAR-associated vesicular transport of an adenovirus in motor neuron axons

Axonal transport is responsible for the movement of signals and cargo between nerve termini and cell bodies. Pathogens also exploit this pathway to enter and exit the central nervous system. In this study, we characterised the binding, endocytosis and axonal transport of an adenovirus (CAV-2) that preferentially infects neurons. Using biochemical, cell biology, genetic, ultrastructural and live-cell imaging approaches, we show that interaction with the neuronal membrane correlates with coxsackievirus and adenovirus receptor (CAR) surface expression, followed by endocytosis involving clathrin. In axons, long-range CAV-2 motility was bidirectional with a bias for retrograde transport in nonacidic Rab7-positive organelles. Unexpectedly, we found that CAR was associated with CAV-2 vesicles that also transported cargo as functionally distinct as tetanus toxin, neurotrophins, and their receptors. These results suggest that a single axonal transport carrier is capable of transporting functionally distinct cargoes that target different membrane compartments in the soma. We propose that CAV-2 transport is dictated by an innate trafficking of CAR, suggesting an unsuspected function for this adhesion protein during neuronal homeostasis.

Adenoviruses commonly cause subclinical morbidity in the ocular, respiratory, and gastrointestinal tracts, and less frequently, adenovirus-induced disease can be fatal for newborns and immunocompromised hosts. In addition, adenoviruses can reach the central nervous system (CNS) and cause associated encephalitis and tumours. On the flip side, during the last two decades, adenovirus vectors have become powerful tools to treat and address diseases of the CNS. Despite the fact that axonal transport of adenoviruses was reported more than 15 years ago, nothing was known concerning how adenoviruses access the CNS. The characterization of their interactions with brain cells was therefore long overdue. In this study, we describe the axonal trafficking of an adenovirus that preferentially infects neurons and reaches the CNS through long-range axonal transport. We show that this adenovirus exploits an endogenous vesicular pathway used by the adhesion molecule CAR (coxsackievirus and adenovirus receptor). Our study characterizes this endogenous route of access, which is likely to be crucial to neuronal survival, neurodegenerative diseases, gene transfer vectors, and adenovirus-induced morbidity.

Dynamic Interaction of HIV-1 Nef with the Clathrin-Mediated Endocytic Pathway at the Plasma Membrane

The HIV-1 Nef protein perturbs the trafficking of membrane proteins such as CD4 by interacting with clathrin-adaptor complexes. We previously reported that Nef alters early/recycling endosomes, but its role at the plasma membrane is poorly documented. Here, we used total internal reflection fluorescence microscopy, which restricts the analysis to a approximately 100 nm region of the adherent surface of the cells, to focus on the dynamic of Nef at the plasma membrane relative to that of clathrin. Nef colocalized both with clathrin spots (CS) that remained static at the cell surface, corresponding to clathrin-coated pits (CCPs), and with approximately 50% of CS that disappeared from the cell surface, corresponding to forming clathrin-coated vesicles (CCVs). The colocalization of Nef with clathrin required the di-leucine motif essential for Nef binding to AP complexes and was independent of CD4 expression. Furthermore, analysis of Nef mutants showed that the capacity of Nef to induce internalization and downregulation of CD4 in T lymphocytes correlated with its localization into CCPs. In conclusion, this analysis shows that Nef is recruited into CCPs and into forming CCVs at the plasma membrane, in agreement with a model in which Nef uses the clathrin-mediated endocytic pathway to induce internalization of some membrane proteins from the surface of HIV-1-infected T cells.

HIV-1 Tat enters T cells using coated pits before translocating from acidified endosomes and eliciting biological responses

The HIV-1 Tat protein is secreted by infected cells. Extracellular Tat can affect bystander uninfected T cells and induce numerous biological responses such as apoptosis and cytokine secretion. Tat is likely involved in several immune disorders during AIDS. Nevertheless, it is not known whether Tat triggers cell responses directly upon binding to signaling receptors at the plasma membrane or after delivery to the cytosol. The pathway that enables Tat to reach the cytosol is also unclear. Here we visualized Tat within T-cell-coated pits and endosomes. Moreover, inhibitors of clathrin/AP-2-mediated uptake such as chlorpromazine, activated RhoA, or dominant-negative mutants of Eps15, intersectin, dynamin, or rab5 impaired Tat delivery to the cytosol by preventing its endocytosis. Molecules neutralizing low endosomal pH or Hsp90 inhibitors abolished Tat entry at a later stage by blocking its endosomal translocation, as directly shown using a cell-free translocation assay. Finally, endosomal pH neutralization prevented Tat from inducing T-cell responses such as NF-kappaB activation, apoptosis, and interleukin secretion, indicating that cytosolic delivery is required for Tat signaling. Hence, Tat enters T cells essentially like diphtheria toxin, using clathrin-mediated endocytosis before low-pH-induced and Hsp90-assisted endosomal translocation. Cell responses are then induced from the cytosol.

Caveolae as an additional route for influenza virus endocytosis in MDCK cells

Clathrin-mediated endocytosis has been described as the primary internalization pathway for many viruses, including the influenza virus. However, caveolae, an alternative clathrin-independent endocytotic pathway, has also been described as mediating the entry of some molecules, including viruses. To address the question of pathway selection by the influenza virus, we have investigated whether the virus is internalized via clathrin-coated pits and/or caveolae in Madin Darby canine kidney (MDCK) cells. By applying pharmacological manipulations to selectively disrupt the cell internalization pathways, we found that, in MDCK cells, the influenza virus may be internalized via caveolae in addition to entry by clathrin-mediated endocytosis. However, a small contribution by another mode of entry, as recently proposed, cannot be excluded.

In vivo, dendritic cells can cross-present virus-like particles using an endosome-to-cytosol pathway

Recombinant parvovirus-like particles (PPV-VLPs) are particulate exogenous Ags that induce strong CTL response in the absence of adjuvant. In the present report to decipher the mechanisms responsible for CTL activation by such exogenous Ag, we analyzed ex vivo and in vitro the mechanisms of capture and processing of PPV-VLPs by dendritic cells (DCs). In vivo, PPV-VLPs are very efficiently captured by CD8alpha- and CD8alpha+ DCs and then localize in late endosomes of DCs. Macropinocytosis and lipid rafts participate in PPV-VLPs capture. Processing of PPV-VLPs does not depend upon recycling of MHC class I molecules, but requires vacuolar acidification as well as proteasome activity, TAP translocation, and neosynthesis of MHC class I molecules. This study therefore shows that in vivo DCs can cross-present PPV-VLPs using an endosome-to-cytosol processing pathway.

Cellular uptake and infection by canine parvovirus involves rapid dynamin-regulated clathrin-mediated endocytosis, followed by slower intracellular trafficking

Canine parvovirus (CPV) is a small, nonenveloped virus that is a host range variant of a virus which infected cats and changes in the capsid protein control the ability of the virus to infect canine cells. We used a variety of approaches to define the early stages of cell entry by CPV. Electron microscopy showed that virus particles concentrated within clathrin-coated pits and vesicles early in the uptake process and that the infecting particles were rapidly removed from the cell surface. Overexpression of a dominant interfering mutant of dynamin in the cells altered the trafficking of capsid-containing vesicles. There was a 40% decrease in the number of CPV-infected cells in mutant dynamin-expressing cells, as well as a approximately 40% decrease in the number of cells in S phase of the cell cycle, which is required for virus replication. However, there was also up to 10-fold more binding of CPV to the surface of mutant dynamin-expressing cells than there was to uninduced cells, suggesting an increased receptor retention on the cell surface. In contrast, there was little difference in virus binding, virus infection rate, or cell cycle distribution between induced and uninduced cells expressing wild-type dynamin. CPV particles colocalized with transferrin in perinuclear endosomes but not with fluorescein isothiocyanate-dextran, a marker for fluid-phase endocytosis. Cells treated with nanomolar concentrations of bafilomycin A1 were largely resistant to infection when the drug was added either 30 min before or 90 min after inoculation, suggesting that there was a lag between virus entering the cell by clathrin-mediated endocytosis and escape of the virus from the endosome. High concentrations of CPV particles did not permeabilize canine A72 or mink lung cells to alpha-sarcin, but canine adenovirus type 1 particles permeabilized both cell lines. These data suggest that the CPV entry and infection pathway is complex and involves multiple vesicular components.

Early stages of influenza virus entry into Mv-1 lung cells: involvement of dynamin

Viruses generally have one of two mechanisms for entry and uncoating. They can enter the cell either by endocytosis or by direct fusion at the plasma membrane. We have established a novel mink lung (Mv-1) cell line that expresses a dominant-interfering form of dynamin-1 (K44A) under the control of a tetracycline-responsive element and studied the early events in influenza infection using these cells. We found that influenza virus binds equally to both induced and uninduced cells, but in K44A-expressing cells, electron microscopy showed viruses trapped in deep coated pits and irregular-shaped tubular structures that contain discrete coated regions. We also show by immunofluorescence and confocal microscopy that entry of incoming virus into the nucleus is blocked in K44A-expressing cells. Virus replication was assayed by immunofluorescence microscopy and was strongly inhibited at both early and late times postinfection in K44A-expressing cells. Virus infectivity was inhibited by approximately 2 log units in cells expressing K44A dynamin when analyzed by influenza plaque assay. Overall these data show that dynamin is required for efficient influenza virus entry, presumably due to its function in release of vesicles from coated pits.

The paramyxovirus simian virus 5 hemagglutinin-neuraminidase glycoprotein, but not the fusion glycoprotein, is internalized via coated pits and enters the endocytic pathway

The hemagglutinin-neuraminidase (HN) and fusion (F) glycoproteins of the paramyxovirus simian virus 5 (SV5) are expressed on the surface of virus-infected cells. Although the F protein was found to be expressed stably, the HN protein was internalized from the plasma membrane. HN protein lacks known internalization signals in its cytoplasmic domain that are common to many integral membrane proteins that are internalized via clathrin-coated pits. Thus, the cellular pathway of HN protein internalization was examined. Biochemical analysis indicated that HN was lost from the cell surface with a t1/2 of approximately 45-50 min and turned over with a t1/2 of approximately 2 h. Immunofluorescent analysis showed internalized SV5 HN in vesicle-like structures in a juxtanuclear pattern coincident with the localization of ovalbumin. In contrast the SV5 F glycoprotein and the HN glycoprotein of the highly related parainfluenza virus 3 (hPIV-3) were found only on the cell surface. Immunogold staining of HN on the surface of SV5-infected CV-1 cells and examination using electron microscopy, showed heavy surface labeling that gradually decreased with time. Concomitantly, gold particles were detected in the endosomal system and with increasing time, gold-labeled structures having the morphology of lysosomes were observed. On the plasma membrane approximately 5% of the gold-labeled HN was found in coated pits. The inhibition of the pinching-off of coated pits from the plasma membrane by cytosol acidification significantly reduced HN internalization. Internalized HN was co-localized with gold-conjugated transferrin, a marker for the early endosomal compartments, and with gold-conjugated bovine serum albumin, a marker for late endosomal compartments. Taken together, these data strongly suggest that the HN glycoprotein is internalized via clathrin-coated pits and delivered to the endocytic pathway.

Dansylcadaverine and cytochalasin D enhance rotavirus infection of murine L cells

Although murine L cells bind and internalize rotavirus as well as permissive cell lines, L cells are essentially nonpermissive for rotaviruses. In nonpermissive cell lines such as L cells, internalized rotavirus fails to uncoat and remains as infectious, double-shelled particles. This block in the infectious cycle can be overcome by direct lipofection of viral particles into the L cell cytoplasm. We hypothesized that the internalized rotavirus particles within L cells are sequestered in the endocytic pathway and are unable to initiate infection. L cells were pretreated with a variety of inhibitors of endocytosis prior to infection with rhesus rotavirus. While agents which inhibit acidification of endosomes had no effect on rotavirus infection, two potential direct inhibitors of vesicular transport, dansylcadaverine and cytochalasin D, enhanced rotavirus infection of L cells 5- to 10-fold. All of the drugs, including both inhibitors of endocytosis and lysosomotrophic agents, significantly reduced infection of L cells by serotype 1 reovirus which is known to infect L cells by the endocytic pathway. Time course studies demonstrated that the drugs were effective in promoting rotavirus infection of L cells in only the early phases of infection. Pretreatment of L cells with dansylcadaverine significantly decreased the number of intact, double-shelled rotavirus particles sequestered within the cells. Inhibition of endocytosis may increase the efficiency of infection of L cells by rotavirus by allowing an increased proportion of attached rotavirus virions to enter cells by a productive route which is probably direct membrane penetration.