Simian virus 40 infection via MHC class I molecules and caveolae

Mouse polyomavirus utilizes recycling endosomes for a traffic pathway independent of COPI vesicle transport

Mouse polyomavirus enters host cells internalized, similar to simian virus 40 (SV40), in smooth monopinocytic vesicles, the movement of which is associated with transient actin disorganization. The major capsid protein (VP1) of the incoming polyomavirus accumulates on membranes around the cell nucleus. Here we show that unlike SV40, mouse polyomavirus infection is not substantially inhibited by brefeldin A, and colocalization of VP1 with beta-COP during early stages of polyomavirus infection in mouse fibroblasts was observed only rarely. Thus, these viruses obviously use different traffic routes from the plasma membrane toward the cell nucleus. At approximately 3 h postinfection, a part of VP1 colocalized with the endoplasmic reticulum marker BiP, and a subpopulation of virus was found in perinuclear areas associated with Rab11 GTPase and colocalized with transferrin, a marker of recycling endosomes. Earlier postinfection, a minor subpopulation of virions was found to be associated with Rab5, known to be connected with early endosomes, but the cell entry of virus was slower than that of transferrin or cholera toxin B-fragment. Neither Rab7, a marker of late endosomes, nor LAMP-2 lysosomal glycoprotein was found to colocalize with polyomavirus. In situ hybridization with polyomavirus genome-specific fluorescent probes clearly demonstrated that, regardless of the multiplicity of infection, only a few virions delivered their genomic DNA into the cell nucleus, while the majority of viral genomes (and VP1) moved back from the proximity of the nucleus to the cytosol, apparently for their degradation.

High cloning capacity of in vitro packaged SV40 vectors with no SV40 virus sequences

In vitro packaging of plasmid DNA using recombinant SV40 capsid proteins is a potentially useful procedure that overcomes some restrictions of the other SV40 systems such as the requirement for SV40 sequences and the limitation in size of DNA that can be packaged. The in vitro packaging system uses the four SV40 proteins (VP1, VP2, VP3, and agno) or VP1 only. The ability to confer drug resistance by three ABC transporter genes (MDR 1, MRP 1, or MXR) was determined using the surrogate fluorescent substrates rhodamine-123 or calcein AM and their specific inhibitors, or by using specific antibodies to the transporters to detect cell surface expression by fluorescence-activated cell sorter analysis (FACS). A green fluorescent protein plasmid (EGFP-C1) was also used to monitor gene transfer. The packaged plasmids ranged in size from 4.2 to 17.6 kb, and only slightly affected particle size as determined by electron microscopy. When 9.5 kb and larger plasmids were packaged using all SV40 proteins, MDR1 expression was decreased compared to VP1 alone. The size of the 15.2 kb DNA after packaging was the same as the original DNA. Packaging with SV40 capsid proteins in vitro does not require any SV40 sequences. Using either the MDR1 or the GFP gene we could demonstrate enhanced expression when cells were pretreated with phorbol 12-myristate 13-acetate (PMA) at low concentrations. Interferon-gamma did not alter expression. We conclude that in vitro packaging is more flexible then previously realized, permitting packaging of at least 17 kb plasmid DNA without the requirement for any viral sequences. This system combines efficient gene delivery of the SV40 viral vector with the presumed safety of nonviral vectors.

Sphingolipid and cholesterol dependence of alphavirus membrane fusion. Lack of correlation with lipid raft formation in target liposomes

Semliki Forest virus (SFV) and Sindbis virus (SIN) are enveloped viruses that infect their host cells by receptor-mediated endocytosis and subsequent fusion from within acidic endosomes. Fusion of the viral envelope requires the presence of both cholesterol and sphingolipids in the target membrane. This is suggestive of a possible involvement of sphingolipid-cholesterol microdomains, or "lipid rafts," in the membrane fusion and cell entry process of the virus. In this study, large unilamellar vesicles (LUVs) were prepared from synthetic sphingolipids and sterols that vary with respect to their capacity to promote microdomain formation, as assessed by gradient flotation analysis in the presence of Triton X-100. SFV and SIN fused with LUVs irrespective of the presence or absence of Triton X-100-insoluble microdomains. These results suggest that SFV and SIN do not require the presence of lipid rafts for fusion with target membranes. Furthermore, it is not necessary for sphingolipids to reside in a detergent-insoluble complex with cholesterol to promote SFV or SIN fusion.

Dissecting virus entry via endocytosis

Numerous virus families utilize endocytosis to infect host cells, mediating virus internalization as well as trafficking to the site of replication. Recent research has demonstrated that viruses employ the full endocytic capabilities of the cell. The endocytic pathways utilized include clathrin-mediated endocytosis, caveolae, macropinocytosis and novel non-clathrin, non-caveolae pathways. The tools to study endocytosis and, consequently, virus entry are becoming more effective and specific as the amount of information on endocytic component structure and function increases. The use of inhibitory drugs, although still quite common, often leads to non-specific disruptions in the cell. Molecular inhibitors in the form of dominant-negative proteins have surpassed the use of chemical inhibitors in terms of specificity to individual pathways. Dominant-negative molecules are derived from both structural proteins of endocytosis, such as dynamin and caveolin, and regulatory proteins, primarily small GTPases and kinases. This review focuses on the experimental approaches taken to examine virus entry and provides both classic examples and recent research on a variety of virus families.

Caveolar endocytosis of simian virus 40 is followed by brefeldin A-sensitive transport to the endoplasmic reticulum, where the virus disassembles

Simian virus 40 (SV40) enters cells by atypical endocytosis mediated by caveolae that transports the virus to the endoplasmic reticulum (ER) instead of to the endosomal-lysosomal compartment, which is the usual destination for viruses and other cargo that enter by endocytosis. We show here that SV4O is transported to the ER via an intermediate compartment that contains beta-COP, which is best known as a component of the COPI coatamer complexes that are required for the retrograde retrieval pathway from the Golgi to the ER. Additionally, transport of SV40 to the ER, as well as infection, is sensitive to brefeldin A. This drug acts by specifically inhibiting the ARF1 GTPase, which is known to regulate assembly of COPI coat complexes on Golgi cisternae. Moreover, some beta-COP colocalizes with intracellular caveolin-1, which was previously shown to be present on a new organelle (termed the caveosome) that is an intermediate in the transport of SV40 to the ER (L. Pelkmans, J. Kartenbeck, and A. Helenius, Nat. Cell Biol. 3:473-483, 2001). We also show that the internal SV40 capsid proteins VP2 and VP3 become accessible to immunostaining starting at about 5 h. Most of that immunostaining overlays the ER, with some appearing outside of the ER. In contrast, immunostaining with anti-SV40 antisera remains confined to the ER.

Inhibitors of COP-mediated transport and cholera toxin action inhibit simian virus 40 infection

Simian virus 40 (SV40) is a nonenveloped virus that has been shown to pass from surface caveolae to the endoplasmic reticulum in an apparently novel infectious entry pathway. We now show that the initial entry step is blocked by brefeldin A and by incubation at 20 degrees C. Subsequent to the entry step, the virus reaches a domain of the rough endoplasmic reticulum by an unknown pathway. This intracellular trafficking pathway is also brefeldin A sensitive. Infection is strongly inhibited by expression of GTP-restricted ADP-ribosylation factor 1 (Arf1) and Sar1 mutants and by microinjection of antibodies to betaCOP. In addition, we demonstrate a potent inhibition of SV40 infection by the dipeptide N-benzoyl-oxycarbonyl-Gly-Phe-amide, which also inhibits late events in cholera toxin action. Our results identify novel inhibitors of SV40 infection and show that SV40 requires COPI- and COPII-dependent transport steps for successful infection.

Internalization of echovirus 1 in caveolae

Echovirus 1 (EV1) is a human pathogen which belongs to the Picornaviridae family of RNA viruses. We have analyzed the early events of infection after EV1 binding to its receptor alpha 2 beta 1 integrin and elucidated the route by which EV1 gains access to the host cell. EV1 binding onto the cell surface and subsequent entry resulted in conformational changes of the viral capsid as demonstrated by sucrose gradient sedimentation analysis. After 15 min to 2 h postinfection (p.i.) EV1 capsid proteins were seen in vesicular structures that were negative for markers of the clathrin-dependent endocytic pathway. In contrast, immunofluorescence confocal microscopy showed that EV1, alpha 2 beta 1 integrin, and caveolin-1 were internalized together in vesicular structures to the perinuclear area. Electron microscopy showed the presence of EV1 particles inside caveolae. Furthermore, infective EV1 could be isolated with anti-caveolin-1 beads 15 min p.i., confirming a close association with caveolin-1. Finally, the expression of dominant negative caveolin in cells markedly inhibited EV1 infection, indicating the importance of caveolae for the viral replication cycle of EV1.

In vitro-packaged SV40 pseudovirions as highly efficient vectors for gene transfer

A procedure for in vitro packaging of plasmid DNA in recombinant SV40 capsid proteins was developed by Sandalon et al. (1997). Here, we report the highly efficient transduction into different human, murine and monkey cell lines using a scaled-up protocol for producing SV40 pseudovirions, packaged in vitro, carrying the human multidrug-resistance gene MDR1 encoding P-glycoprotein (P-gp) or the green fluorescent protein reporter gene (GFP) under control of SV40 and cytomegalovirus (CMV) promoters. The percentage of expressing cells was proportional to the number of transducing particles, with close to 100% of cells transduced at optimal ratios of transducing particles to cells. The ability to confer multidrug resistance was evaluated by measuring dye efflux and cell-surface expression in infected cells. The relative level of expression of P-gp driven by the different promoters varied among different cell lines. In human lymphoblastoid cells, which express high levels of major histocompatibility complex (MHC) class I (a surface receptor for SV40), constructs that carry an intron yield the highest expression. Our experiments further demonstrate that MDR1 and GFP expression driven by these promoters is transient; however, transduced cells remain MDR1-positive if selected in colchicine. Thus, the SV40 vectors are well suited to situations in which only short-term expression is required or expression is selected, such as for bone marrow protection during chemotherapy.

Caveolae are involved in the trafficking of mouse polyomavirus virions and artificial VP1 pseudocapsids toward cell nuclei

Electron and confocal microscopy were used to observe the entry and the movement of polyomavirus virions and artificial virus-like particles (VP1 pseudocapsids) in mouse fibroblasts and epithelial cells. No visible differences in adsorption and internalization of virions and VP1 pseudocapsids ("empty" or containing DNA) were observed. Viral particles entered cells internalized in smooth monopinocytic vesicles, often in the proximity of larger, caveola-like invaginations. Both "empty" vesicles derived from caveolae and vesicles containing viral particles were stained with the anti-caveolin-1 antibody, and the two types of vesicles often fused in the cytoplasm. Colocalization of VP1 with caveolin-1 was observed during viral particle movement from the plasma membrane throughout the cytoplasm to the perinuclear area. Empty vesicles and vesicles with viral particles moved predominantly along microfilaments. Particle movement was accompanied by transient disorganization of actin stress fibers. Microfilaments decorated by the VP1 immunofluorescent signal could be seen as concentric curves, apparently along membrane structures that probably represent endoplasmic reticulum. Colocalization of VP1 with tubulin was mostly observed in areas close to the cell nuclei and on mitotic tubulin structures. By 3 h postinfection, a strong signal of the VP1 (but no viral particles) had accumulated in the proximity of nuclei, around the outer nuclear membrane. However, the vast majority of VP1 pseudocapsids did not enter the nuclei.

Human immunodeficiency virus type 1 entry into macrophages mediated by macropinocytosis

Whereas human immunodeficiency virus (HIV) infects various cell types by fusion at the plasma membrane, we observed a different entry route in human primary macrophages, in which macropinocytosis is active. Shortly after exposure of macrophages to HIV-1 and irrespective of viral envelope-receptor interactions, particles were visible in intracellular vesicles, which were identified as macropinosomes. Most virions appeared subsequently degraded. However, fusion leading to capsid release in the cytosol and productive infection could take place inside vesicles when particles were properly enveloped. These observations provide new insights into HIV-1 interactions with a cell target relevant to pathogenesis. They may have implications for the design of soluble inhibitors aimed at interfering with the fusion or entry processes.

Endocytosis without clathrin coats

Endocytosis is involved in an enormous variety of cellular processes. To date, most studies on endocytosis in mammalian cells have focused on pathways that start with uptake through clathrin-coated pits. Recently, new techniques and reagents have allowed a wider range of endocytic pathways to begin to be characterized. Various non-clathrin endocytic mechanisms have been identified, including uptake through caveolae, macropinosomes and via a separate constitutive pathway. Many markers for clathrin-independent endocytosis are found in detergent-resistant membrane fractions, or lipid rafts. We will discuss these emerging new findings and their implications for the nature of lipid rafts themselves, as well as for the potential roles of non-clathrin endocytic pathways in remodeling of the plasma membrane and in regulating the membrane composition of specific intracellular organelles.

Caveolae in the uptake and targeting of infectious agents and secreted toxins

A variety of microbial pathogens, including viruses, intracellular bacteria, and prions, as well as certain secreted bacterial toxins, can now be added to the list of ligands that enter cells via caveolae or caveolae-like membrane domains. In general, the caveolae-mediated entry pathway results in transport of these microbes and toxins to intracellular destinations that are different from that of cargo entering by other means. As a result, the caveolae-mediated entry pathway can profoundly affect the host cell-pathogen interaction long after entry has occurred. Furthermore, some microbes such as SV40 that enter via cavolae will be valuable as probes to analyze certain poorly understood intracellular trafficking pathways, such as retrograde transport to the ER. Also, viruses that enter via caveolae may have unique potential as gene and drug delivery vectors. In addition, some extracellular microbial pathogens, such as Pneumocystis carinii, may also interact with host cells via caveolae. Finally, caveolae may play a role in host immune defense mechanisms.

Co-option of endocytic functions of cellular caveolae by pathogens

It is increasingly becoming clear that various immune cells are infected by the very pathogens that they are supposed to attack. Although many mechanisms for microbial entry exist, it appears that a common route of entry shared by certain bacteria, viruses and parasites involves cellular lipid-rich microdomains sometimes called caveolae. These cellular entities, which are characterized by their preferential accumulation of glycosylphosphatidylinositol (GPI)-anchored molecules, cholesterol and various glycolipids, and a distinct protein (caveolin), are present in many effector cells of the immune system including neutrophils, macrophages, mast cells and dendritic cells. These structures have an innate capacity to endocytoze various ligands and traffic them to different intracellular sites and sometimes, back to the extracellular cell surface. Because caveolae do not typically fuse with lysosomes, the ligands borne by caveolar vesicles are essentially intact, which is in marked contrast to ligands endocytozed via the classical endosome-lysosome pathway. A number of microbes or their exotoxins co-opt the unique features of caveolae to enter and traffic, without any apparent loss of viability and function, to different sites within immune and other host cells. In spite of their wide disparity in size and other structural attributes, we predict that a common feature among caveolae-utilizing pathogens and toxins is that their cognate receptor(s) are localized within plasmalemmal caveolae of the host cell.

Microbial pathogenesis: lipid rafts as pathogen portals

The route of initial entry influences how host cells respond to intracellular pathogens. Recent studies have demonstrated that a wide variety of pathogens target lipid microdomains in host cell membranes, known as lipid rafts, to enter host cells as an infectious strategy.

DNA-carrier proteins for targeted gene delivery

The development of vectors for cell-specific gene delivery is a major goal of gene therapeutic strategies. Significant progress has been made in the construction of non-viral vectors that combine different functions required for gene transfer in an artificial complex. To some extent this can be achieved by complexing plasmid DNA with synthetic compounds such as lipids and polycations. Alternative approaches rely on the activities of natural or recombinant DNA-carrier proteins to achieve uptake and intracellular delivery of plasmid DNA. Nuclear proteins such as histones and members of the high mobility group protein family have been shown to condense DNA and transfect cultured cells. Some structural proteins of DNA viruses spontaneously assemble with plasmid DNA and form transfection-competent pseudocapsids. In addition, chimeric fusion proteins have been engineered that incorporate in a single polypeptide chain heterologous protein domains which facilitate binding to plasmid DNA, specific recognition of target cells, induction of receptor-mediated endocytosis, and DNA transport through intracellular compartments.