Quantitative microscopy of fluorescent adenovirus entry

Aptamer-based approaches for the detection of waterborne pathogens

Waterborne ailments pose a serious threat to public health and are a huge economic burden. Lack of hygiene in drinking and recreational water is the chief source of microbial pathogens in developing countries. Poor water quality and sanitation account for more than 3.4 million deaths a year worldwide. This has urged authorities and researchers to explore different avenues of pathogen detection. There is a growing demand for rapid and reliable sensor technologies, in particular those that can detect in situ and perform in harsh conditions. Some of the major waterborne pathogens include Vibrio cholerae, Leptospira interrogans, Campylobacter jejuni, Shigella spp., enterotoxigenic Escherichia coli, Clostridium difficile, Cryptosporidium parvum, Entamoeba histolytica, and Hepatitis A virus. While conventional methods of pathogen detection like serodiagnosis and microbiological methods have been superseded by nucleic acid amplification methods, there is still potential for improvement. This review provides an insight into aptamers and their utility in the form of aptasensors. It discusses how aptamer-based approaches have emerged as a novel strategy and its advantages over more resource-intensive and complex biochemical approaches.

Adenovirus Entry: From Infection to Immunity

More than 80 different adenovirus (AdV) types infect humans through the respiratory, ocular, or gastrointestinal tracts. They cause acute clinical mani-festations or persist under humoral and cell-based immunity. Immuno-suppressed individuals are at risk of death from an AdV infection. Concepts about cell entry of AdV build on strong foundations from molecular and cellular biology-and increasingly physical virology. Here, we discuss how virions enter and deliver their genome into the nucleus of epithelial cells. This process breaks open the virion at distinct sites because the particle has nonisometric mechanical strength and reacts to specific host factors along the entry pathway. We further describe how macrophages and dendritic cells resist AdV infection yet enhance productive entry into polarized epithelial cells. A deep understanding of the viral mechanisms and cell biological and biophysical principles will continue to unravel how epithelial and antigen-presenting cells respond to AdVs and control inflammation and persistence in pathology and therapy.

Impact of dynamin 2 on adenovirus nuclear entry

The large GTPase dynamin 2 controls both endosomal fission and microtubule acetylation. Here we report that dynamin 2 alters microtubules and regulates the trafficking of human adenovirus type 37. Dynamin 2 knockdown by siRNA in infected cells resulted in accumulation of acetylated tubulin, repositioning of microtubule organizing centers (MTOCs) closer to cell nuclei, increased virus in the cytosol (with a compensatory decrease in endosomal virus), reduced proinflammatory cytokine induction, and increased binding of virus to the nucleoporin, Nup358. These events led to increased viral DNA nuclear entry and viral replication. Overexpression of dynamin 2 generated opposite effects. Therefore, dynamin 2 inhibits adenovirus replication and promotes innate immune responses by the infected cell. MTOC transposition in dynamin 2 knockdown promotes a closer association with nuclear pore complexes to facilitate viral DNA delivery. Dynamin 2 plays a key role in adenoviral trafficking and influences host responses to infection.

Virus-Receptor Interactions: The Key to Cellular Invasion

Virus–receptor interactions play a key regulatory role in viral host range, tissue tropism, and viral pathogenesis. Viruses utilize elegant strategies to attach to one or multiple receptors, overcome the plasma membrane barrier, enter, and access the necessary host cell machinery. The viral attachment protein can be viewed as the “key” that unlocks host cells by interacting with the “lock”—the receptor—on the cell surface, and these lock-and-key interactions are critical for viruses to successfully invade host cells. Many common themes have emerged in virus–receptor utilization within and across virus families demonstrating that viruses often target particular classes of molecules in order to mediate these events. Common viral receptors include sialylated glycans, cell adhesion molecules such as immunoglobulin superfamily members and integrins, and phosphatidylserine receptors. The redundancy in receptor usage suggests that viruses target particular receptors or “common locks” to take advantage of their cellular function and also suggests evolutionary conservation. Due to the importance of initial virus interactions with host cells in viral pathogenesis and the redundancy in viral receptor usage, exploitation of these strategies would be an attractive target for new antiviral therapeutics.

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Viral receptors are key regulators of host range, tissue tropism, and viral pathogenesis.

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Many viruses utilize common viral receptors including sialic acid, cell adhesion molecules such as immunoglobulin superfamily members and integrins, and phosphatidylserine receptors.

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Detailed molecular interactions between viruses and receptors have been defined through elegant biochemical analyses including glycan array screens, structural–functional analyses, and cell-based approaches providing tremendous insights into these initial events in viral infection.

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Commonalities in virus–receptor interactions present promising targets for the development of broad-spectrum antiviral therapies.

Imaging, Tracking and Computational Analyses of Virus Entry and Egress with the Cytoskeleton

Viruses have a dual nature: particles are “passive substances” lacking chemical energy transformation, whereas infected cells are “active substances” turning-over energy. How passive viral substances convert to active substances, comprising viral replication and assembly compartments has been of intense interest to virologists, cell and molecular biologists and immunologists. Infection starts with virus entry into a susceptible cell and delivers the viral genome to the replication site. This is a multi-step process, and involves the cytoskeleton and associated motor proteins. Likewise, the egress of progeny virus particles from the replication site to the extracellular space is enhanced by the cytoskeleton and associated motor proteins. This overcomes the limitation of thermal diffusion, and transports virions and virion components, often in association with cellular organelles. This review explores how the analysis of viral trajectories informs about mechanisms of infection. We discuss the methodology enabling researchers to visualize single virions in cells by fluorescence imaging and tracking. Virus visualization and tracking are increasingly enhanced by computational analyses of virus trajectories as well as in silico modeling. Combined approaches reveal previously unrecognized features of virus-infected cells. Using select examples of complementary methodology, we highlight the role of actin filaments and microtubules, and their associated motors in virus infections. In-depth studies of single virion dynamics at high temporal and spatial resolutions thereby provide deep insight into virus infection processes, and are a basis for uncovering underlying mechanisms of how cells function.

The adenovirus major core protein VII is dispensable for virion assembly but is essential for lytic infection

The Adenovirus (Ad) genome within the capsid is tightly associated with a virus-encoded, histone-like core protein—protein VII. Two other Ad core proteins, V and X/μ, also are located within the virion and are loosely associated with viral DNA. Core protein VII remains associated with the Ad genome during the early phase of infection. It is not known if naked Ad DNA is packaged into the capsid, as with dsDNA bacteriophage and herpesviruses, followed by the encapsidation of viral core proteins, or if a unique packaging mechanism exists with Ad where a DNA-protein complex is simultaneously packaged into the virion. The latter model would require an entirely new molecular mechanism for packaging compared to known viral packaging motors. We characterized a virus with a conditional knockout of core protein VII. Remarkably, virus particles were assembled efficiently in the absence of protein VII. No changes in protein composition were evident with VII⁻virus particles, including the abundance of core protein V, but changes in the proteolytic processing of some capsid proteins were evident. Virus particles that lack protein VII enter the cell, but incoming virions did not escape efficiently from endosomes. This greatly diminished all subsequent aspects of the infectious cycle. These results reveal that the Ad major core protein VII is not required to condense viral DNA within the capsid, but rather plays an unexpected role during virus maturation and the early stages of infection. These results establish a new paradigm pertaining to the Ad assembly mechanism and reveal a new and important role of protein VII in early stages of infection.

The Ad major core protein VII protects the viral genome from recognition by a cellular DNA damage response during the early stages of infection and alters cellular chromatin to block innate signaling mechanisms. The packaging of the Ad genome into the capsid is thought to follow the paradigm of dsDNA bacteriophage where viral DNA is inserted into a preassembled capsid using a packaging motor. How this process occurs if Ad packages a DNA-core protein complex is unknown. We analyzed an Ad mutant that lacks core protein VII and demonstrated that virus assembly and DNA packaging takes place normally, but that the mutant is deficient in the maturation of several capsid proteins and displays a defect in the escape of virions from the endosome. These results have profound implications for the Ad assembly mechanism and for the role of protein VII during infection.

A dynamic cell entry pathway of respiratory syncytial virus revealed by tracking the quantum dot-labeled single virus

Studying the cell entry pathway at the single-particle level can provide detailed and quantitative information for the dynamic events involved in virus entry. Indeed, the viral entry dynamics cannot be monitored by static staining methods used in cell biology, and thus virus dynamic tracking could be useful in the development of effective antiviral strategies. Therefore, the aim of this work was to use a quantum dot-based single-particle tracking approach to monitor the cell entry behavior of the respiratory syncytial virus (RSV) in living cells. The time-lapse fluorescence imaging and trajectory analysis of the quantum dot-labeled RSV showed that RSV entry into HEp-2 cells consisted of a typical endocytosis trafficking process. Three critical events during RSV entry were observed according to entry dynamic and fluorescence colocalization analysis. Firstly, RSV was attached to lipid rafts of the cell membrane, and then it was efficiently delivered into the perinuclear region within 2 h post-infection, mostly moving and residing into the lysosome compartment. Moreover, the relatively slow velocity of RSV transport across the cytoplasm and the formation of the actin tail indicated actin-based RSV motility, which was also confirmed by the effects of cytoskeletal inhibitors. Taken together, these findings provided new insights into the RSV entry mechanism and virus-cell interactions in RSV infection that could be beneficial in the development of antiviral drugs and vaccines.

Multiscale modeling of virus replication and spread

Replication and spread of human viruses is based on the simultaneous exploitation of many different host functions, bridging multiple scales in space and time. Mathematical modeling is essential to obtain a systems-level understanding of how human viruses manage to proceed through their life cycles. Here, we review corresponding advances for viral systems of large medical relevance, such as human immunodeficiency virus-1 (HIV-1) and hepatitis C virus (HCV). We will outline how the combination of mathematical models and experimental data has advanced our quantitative knowledge about various processes of these pathogens, and how novel quantitative approaches promise to fill remaining gaps.

Quantum dot labelling of adenovirus allows highly sensitive single cell flow and imaging cytometry

A quantum dot method for highly efficient labelling of single adenoviral particles is developed. The technique has no impact on viral fitness and allows the imaging and tracking of virus binding and internalisation events using a variety of techniques including imaging cytometry and confocal microscopy. The method is applied to characterise the tropism of different adenoviral vectors.

Tracking viral genomes in host cells at single-molecule resolution

Viral DNA trafficking in cells has large impacts on physiology and disease development. Current methods lack the resolution and accuracy to visualize and quantify viral DNA trafficking at single-molecule resolution. We developed a noninvasive protocol for accurate quantification of viral DNA-genome (vDNA) trafficking in single cells. Ethynyl-modified nucleosides were used to metabolically label newly synthesized adenovirus, herpes virus, and vaccinia virus vDNA, without affecting infectivity. Superresolution microscopy and copper(I)-catalyzed azide-alkyne cycloaddition (click) reactions allowed visualization of infection at single vDNA resolution within mammalian cells. Analysis of adenovirus infection revealed that a large pool of capsid-free vDNA accumulated in the cytosol upon virus uncoating, indicating that nuclear import of incoming vDNA is a bottleneck. The method described here is applicable for the entire replication cycle of DNA viruses and offers opportunities to localize cellular and viral effector machineries on newly replicated viral DNA, or innate immune sensors on cytoplasmic viral DNA.

Particle invasion, survival, and non-ergodicity in 2D diffusion processes with space-dependent diffusivity

We study the thermal Markovian diffusion of tracer particles in a 2D medium with spatially varying diffusivity D(r), mimicking recently measured, heterogeneous maps of the apparent diffusion coefficient in biological cells. For this heterogeneous diffusion process (HDP) we analyse the mean squared displacement (MSD) of the tracer particles, the time averaged MSD, the spatial probability density function, and the first passage time dynamics from the cell boundary to the nucleus. Moreover we examine the non-ergodic properties of this process which are important for the correct physical interpretation of time averages of observables obtained from single particle tracking experiments. From extensive computer simulations of the 2D stochastic Langevin equation we present an in-depth study of this HDP. In particular, we find that the MSDs along the radial and azimuthal directions in a circular domain obey anomalous and Brownian scaling, respectively. We demonstrate that the time averaged MSD stays linear as a function of the lag time and the system thus reveals a weak ergodicity breaking. Our results will enable one to rationalise the diffusive motion of larger tracer particles such as viruses or submicron beads in biological cells.

Investigating endocytic pathways to the endoplasmic reticulum and to the cytosol using SNAP-trap

Cholera toxin enters cells via an unusual pathway that involves trafficking through endosomes to the endoplasmic reticulum (ER). Whether the toxin induces its own pathway or travels along a physiological retrograde route is not known. To study its trafficking, we labeled cholera toxin B (CTB) or endogenous plasma membrane proteins with a small chemical compound, benzylguanine, which covalently reacts with the protein SNAP-tag. Using ER-targeted SNAP-tag as reporter, we found that transport of CTB to the ER depends on dynamin-2 and syntaxin 5. Plasma membrane proteins and a fluid-phase marker added to the medium were also transported to the ER. This flux was not affected by exposing cells to CTB but was inhibited by depleting syntaxin 5 and increased by depleting dynamin-2. As a control for confined intracellular localization of ER-targeted SNAP-tag we used adenovirus-5, which traffics to endosomes and then escapes into the cytosol. The virus did not react with ER-targeted SNAP but with cytosolic SNAP. Together, our results establish a new method (SNAP-trap) to study trafficking of different cargo to the ER and the cytosol and provide evidence for the existence of a constitutive pathway from the cell surface to the ER.

Reduced infectivity of adenovirus type 5 particles and degradation of entering viral genomes associated with incomplete processing of the preterminal protein

To investigate further the contribution of the adenovirus type 5 (Ad5) E1B 55-kDa protein to genome replication, viral DNA accumulation was examined in primary human fibroblasts and epithelial cells infected with Ad5 or the E1B 55-kDa-null mutant Hr6. Unexpectedly, all cell types were observed to contain a significantly higher concentration of entering Hr6 than of Ad5 DNA, as did an infectious unit of Hr6. However, the great majority of the Hr6 genomes were degraded soon after entry. As this unusual phenotype cannot be ascribed to the Hr6 E1B frameshift mutation (J. S. Chahal and S. J. Flint, J. Virol. 86:3064-3072, 2012), the sequences of the Ad5 and Hr6 genomes were compared by using high-throughput sequencing. Seven previously unrecognized mutations were identified in the Hr6 genome, two of which result in substitutions in virion proteins, G315V in the preterminal protein (preTP) and A406V in fiber protein IV. Previous observations and the visualization by immunofluorescence of greater numbers of viral genomes entering the cytosol of Hr6-infected cells than of Ad5-infected cells indicated that the fiber mutation could not be responsible for the low-infectivity phenotype of Hr6. However, comparison of the forms of terminal protein present in purified virus particles indicated that the production of mature terminal protein from a processing intermediate is impaired in Hr6 particles. We therefore propose that complete processing of preTP within virus particles is necessary for the ability of viral genomes to become localized at appropriate sites and persist in infected cells.

Cell-free transmission of human adenovirus by passive mass transfer in cell culture simulated in a computer model

Viruses spread between cells, tissues, and organisms by cell-free and cell-cell transmissions. Both mechanisms enhance disease development, but it is difficult to distinguish between them. Here, we analyzed the transmission mode of human adenovirus (HAdV) in monolayers of epithelial cells by wet laboratory experimentation and a computer simulation. Using live-cell fluorescence microscopy and replication-competent HAdV2 expressing green fluorescent protein, we found that the spread of infection invariably occurred after cell lysis. It was affected by convection and blocked by neutralizing antibodies but was independent of second-round infections. If cells were overlaid with agarose, convection was blocked and round plaques developed around lytic infected cells. Infected cells that did not lyse did not give rise to plaques, highlighting the importance of cell-free transmission. Key parameters for cell-free virus transmission were the time from infection to lysis, the dose of free viruses determining infection probability, and the diffusion of single HAdV particles in aqueous medium. With these parameters, we developed an in silico model using multiscale hybrid dynamics, cellular automata, and particle strength exchange. This so-called white box model is based on experimentally determined parameters and reproduces viral infection spreading as a function of the local concentration of free viruses. These analyses imply that the extent of lytic infections can be determined by either direct plaque assays or can be predicted by calculations of virus diffusion constants and modeling.

Viral weapons of membrane destruction: variable modes of membrane penetration by non-enveloped viruses

Significant progress has recently been obtained in our understanding of cellular entry by nonenveloped viruses (NEVs). A key step in the entry process involves the disruption or remodeling of the limiting cell membrane allowing the virus to gain access to the cellular replication machinery. Biochemical, genetic and structural data from diverse virus groups have shed light on the process of membrane penetration thereby revealing both the conservation and divergence of the mechanisms and principles governing this process. In general, membrane breach is achieved via the highly regulated spatiotemporal exposure of a virally encoded membrane lytic factor, resulting in the transfer of the viral genome or nucleocapsid into the cytosol.

Optimization of HEK-293S cell cultures for the production of adenoviral vectors in bioreactors using on-line OUR measurements

The culture of HEK-293S cells in a stirred tank bioreactor for adenoviral vectors production for gene therapy is studied. Process monitoring using oxygen uptake rate (OUR) was performed. The OUR was determined on-line by the dynamic method, providing good information of the process evolution. OUR enabled cell activity monitoring, facilitating as well the determination of the feeding rate in perfusion cultures and when to infect the culture. Batch cultures were used to validate the monitoring methodology. A cell density of 10×10(5)cell/mL was infected, producing 1.3×10(9) infectious viral particles/mL (IVP/mL). To increase cell density values maintaining cell specific productivity, perfusion cultures, based on tangential flow filtration, were studied. In this case, OUR measurements were used to optimize the dynamic culture medium feeding strategy, addressed to avoid any potential nutrient limitation. Furthermore, the infection protocol was defined in order to optimize the use of the viral inoculum, minimizing the uncontrolled release of particles through the filter unit mesh. All these developments enabled an infection at 78×10(5)cell/mL with the consequent production of 44×10(9)IVP/mL, representing a cell specific productivity 4.3 times higher than for the batch culture.

Development and improvement of a serum-free suspension process for the production of recombinant adenoviral vectors using HEK293 cells

Human Embryonic Kidney 293 (HEK293) cells were adapted into a serum-free suspension medium through steps of gradual serum weaning for the production of adenoviral (AdV) gene therapy vectors. The presence of sodium heparin in the medium formulation reduced cell clumping dramatically in suspension culture. The adapted cells were ready to grow either in serum-containing medium as an attached culture or in serum-free medium in suspension culture. A scalable production process was developed in shake flasks and was then evaluated in stirred tank bioreactors. This process includes a growth phase in batch-mode followed by a production phase involving medium perfusion and supplementation. Fortification with calcium chloride post viral inoculation resulted in an increase in virus production by at least one fold. Addition of stimulating agents such as sodium butyrate, N-acetyl-L-cysteine (NAC), dimethyl sulfoxide(DMSO), or ethyl alcohol post infection was shown to further improve virus production in a dose-dependent manner. The serum-free suspension process described here should be suitable for the manufacturing of other E1-deleted AdV vectors and could potentially be used for the production of recombinant proteins by HEK293 cells.

Stepwise loss of fluorescent core protein V from human adenovirus during entry into cells

Human adenoviruses (Ads) replicate and assemble particles in the nucleus. They organize a linear double-strand DNA genome into a condensed core with about 180 nucleosomes, by the viral proteins VII (pVII), pX, and pV attaching the DNA to the capsid. Using reverse genetics, we generated a novel, nonconditionally replicating Ad reporter by inserting green fluorescent protein (GFP) at the amino terminus of pV. Purified Ad2-GFP-pV virions had an oversized complete genome and incorporated about 38 GFP-pV molecules per virion, which is about 25% of the pV levels in Ad2. GFP-pV cofractionated with the DNA core, like pV, and newly synthesized GFP-pV had a subcellular localization indistinguishable from that of pV, indicating that GFP-pV is a valid reporter for pV. Ad2-GFP-pV completed the replication cycle, although at lower yields than Ad2. Incoming GFP-pV (or pV) was not imported into the nucleus. Virions lost GFP-pV at two points during the infection process: at entry into the cytosol and at the nuclear pore complex, where capsids disassemble. Disassembled capsids, positive for the conformation-specific antihexon antibody R70, were devoid of GFP-pV. The loss of GFP-pV was reduced by the macrolide antibiotic leptomycin B (LMB), which blocks nuclear export and adenovirus attachment to the nuclear pore complex. LMB inhibited the appearance of R70 epitopes on Ad2 and Ad2-GFP-pV, indicating that the loss of GFP-pV from Ad2-GFP-pV is an authentic step in the adenovirus uncoating program. Ad2-GFP-pV is genetically complete and hence enables detailed analyses of infection and spreading dynamics in cells and model organisms or assessment of oncolytic adenoviral potential.

Particle tracking of intracellular trafficking of octaarginine-modified liposomes: a comparative study with adenovirus

It is previously reported that octaarginine (R8)-modified liposome (R8-Lip) was taken up via macropinocytosis, and subsequently delivered to the nuclear periphery. In the present study, we investigated the mechanism for the cytoplasmic transport of R8-Lips, comparing with that for adenovirus. Treatment with microtubule-disruption reagent (nocodazole) inhibited the transfection activity of plasmid DNA (pDNA)-encapsulating R8-Lip more extensively than that of adenovirus. The directional transport of R8-Lips along green fluorescent protein (GFP)-tagged microtubules was observed; however, the velocity was slower than those for adenovirus or endosomes that were devoid of R8-Lips. These directional motions were abrogated in R8-Lips by nocodazole treatment, whereas adenovirus continued to undergo random motion. This finding suggests that the nuclear access of R8-Lip predominantly involves microtubule-dependent transport, whereas an apparent diffusive motion is also operative in nuclear access of adenovirus. Furthermore, quantum dot-labeled pDNA underwent directional motion concomitantly with rhodamine-labeled lipid envelopes, indicating that the R8-Lips were subject to microtubule-dependent transport in the intact form. Dual particle tracking of carriers and endosomes revealed that R8-Lip was directionally transported, associated with endosomes, whereas this occurs after endosomal escape in adenovirus. Collectively, the findings reported herein indicate that vesicular transport is a key factor in the cytoplasmic transport of R8-Lips.

Adenovirus

Of the 53 different human adenovirus (HAdV) serotypes belonging to species A-G, a significant number are associated with acute respiratory, gastrointestinal and ocular infections. Replication-defective HAdV-5-based vectors also continue to play a significant role in gene transfer trials and in vaccine delivery efforts in the clinic. Although significant progress has been made from studies of AdV biology, we still have an incomplete understanding of AdV's structure as well as its multifactorial interactions with the host. Continuing efforts to improve knowledge in these areas, as discussed in this chapter, will be crucial for revealing the mechanisms of AdV pathogenesis and for allowing optimal use of AdV vectors for biomedical applications.

A stochastic model for microtubule motors describes the in vivo cytoplasmic transport of human adenovirus

Cytoplasmic transport of organelles, nucleic acids and proteins on microtubules is usually bidirectional with dynein and kinesin motors mediating the delivery of cargoes in the cytoplasm. Here we combine live cell microscopy, single virus tracking and trajectory segmentation to systematically identify the parameters of a stochastic computational model of cargo transport by molecular motors on microtubules. The model parameters are identified using an evolutionary optimization algorithm to minimize the Kullback-Leibler divergence between the in silico and the in vivo run length and velocity distributions of the viruses on microtubules. The present stochastic model suggests that bidirectional transport of human adenoviruses can be explained without explicit motor coordination. The model enables the prediction of the number of motors active on the viral cargo during microtubule-dependent motions as well as the number of motor binding sites, with the protein hexon as the binding site for the motors.

Molecular motors, due to their transportation function, are essential to the cell, but they are often hijacked by viruses to reach their replication site. Imaging of virus trajectories provides information about the patterns of virus transport in the cytoplasm, leading to improved understanding of the underlying mechanisms. In turn improved understanding may suggest actions that can be taken to interfere with the transport of pathogens in the cell. In this work we use in vivo imaging of virus trajectories to develop a computational model of virus transport in the cell. The model parameters are identified by an optimization procedure to minimize the discrepancy between in vivo and in silico trajectories. The model explains the in vivo trajectories as the result of a stochastic interaction between motors. Furthermore it enables predictions on the number of motors and binding sites on pathogens, quantities that are difficult to obtain experimentally. Beyond the understanding of mechanisms involved in pathogen transport, the present paper introduces a systematic parameter identification algorithm for stochastic models using in vivo imaging. The discrete and noisy characteristics of biological systems have led to increased attention in stochastic models and this work provides a methodology for their systematic development.

Shape reconstruction of subcellular structures from live cell fluorescence microscopy images

Live imaging of subcellular structures is indispensible to advance our understanding of cellular processes. The blurred digital images acquired in light microscopy are, however, complex to analyze, and identification and reconstruction of subcellular structures from such images remains a major challenge. We present a novel, model-based image analysis algorithm to reconstruct outlines of subcellular structures using a sub-pixel representation. The algorithm explicitly accounts for the optical properties of the microscope. We validate the reconstruction performance on synthetic data and apply the new method to fluorescence microscopy images of endosomes identified by the GTPase EGFP-Rab5. The benefits of the new algorithm are outlined by comparison to standard techniques. We demonstrate that the new algorithm leads to better discrimination between different endosomal virus entry pathways and to more robust, accurate, and self-consistent quantification of endosome shape features. This allows establishing a set of features that quantify endosome morphology and robustly capture the dynamics of endosome fusion.

Subversion of CtBP1-controlled macropinocytosis by human adenovirus serotype 3

Endocytosis supports cell communication, growth, and pathogen infection. The species B human adenovirus serotype 3 (Ad3) is associated with epidemic conjunctivitis, and fatal respiratory and systemic disease. Here we show that Ad3 uses dynamin-independent endocytosis for rapid infectious entry into epithelial and haematopoietic cells. Unlike Ad5, which uses dynamin-dependent endocytosis, Ad3 endocytosis spatially and temporally coincided with enhanced fluid-phase uptake. It was sensitive to macropinocytosis inhibitors targeting F-actin, protein kinase C, the sodium-proton exchanger, and Rac1 but not Cdc42. Infectious Ad3 macropinocytosis required viral activation of p21-activated kinase 1 (PAK1) and the C-terminal binding protein 1 of E1A (CtBP1), recruited to macropinosomes. These macropinosomes also contained the Ad3 receptors CD46 and alpha v integrins. CtBP1 is a phosphorylation target of PAK1, and is bifunctionally involved in membrane traffic and transcriptional repression of cell cycle, cancer, and innate immunity pathways. Phosphorylation-defective S147A-CtBP1 blocked Ad3 but not Ad5 infection, providing a direct link between PAK1 and CtBP1. The data show that viruses induce macropinocytosis for infectious entry, a pathway used in antigen presentation and cell migration.

The contributions of microtubule stability and dynamic instability to adenovirus nuclear localization efficiency

Adenoviruses (Ads) utilize host cell microtubules to traverse the intracellular space and reach the nucleus in a highly efficient manner. Previous studies have shown that Ad infection promotes the formation of stable, posttranslationally modified microtubules by a RhoA-dependent mechanism. Ad infection also shifts key parameters of microtubule dynamic instability by a Rac1-dependent mechanism, resulting in microtubules with lower catastrophe frequencies, persistent growth phases, and a bias toward net growth compared to microtubules in uninfected cells. Until now it was unclear whether changes in RhoGTPase activity or microtubule dynamics had a direct impact on the efficiency of Ad microtubule-dependent nuclear localization. Here we have performed synchronous Ad infections and utilized confocal microscopy to analyze the individual contributions of RhoA activation, Rac1 activation, microtubule stability, dynamic behavior, and posttranslational modifications on Ad nuclear localization efficiency (NLE). We found that drug-induced suppression of microtubule dynamics impaired Ad NLE by disrupting the radial organization of the microtubule array. When the microtubule array was maintained, the suppression or enhancement of microtubule turnover did not significantly affect Ad NLE. Furthermore, RhoA activation or the formation of acetylated microtubules did not enhance Ad NLE. In contrast, active Rac1 was required for efficient Ad nuclear localization. Because Rac1 mediates persistent growth of microtubules to the lamellar regions of cells, we propose that Ad-induced activation of Rac1 enhances the ability of microtubules to "search and capture" incoming virus particles.

Viral strategies for intracellular trafficking: motors and microtubules

To overcome barriers to diffusion, many viruses utilize the microtubule-associated molecular motor cytoplasmic dynein 1 to drive transport towards the nucleus of a target cell. Cytoplasmic dynein 1 generates movement towards the minus end of microtubules located at the microtubule organizing centre (MTOC), a structure that is typically in close proximity to the nucleus. Physiological cargoes for cytoplasmic dynein include membranous organelles, protein complexes and aggregates of misfolded protein. In this review, we discuss the study of microtubule-based translocation of viruses and raise questions about the mechanisms for association with and then dissociation from cytoplasmic dynein with a goal of understanding whether viruses are seen by the intracellular trafficking machinery as functional protein complexes or misfolded protein aggregates.

A novel supervised trajectory segmentation algorithm identifies distinct types of human adenovirus motion in host cells

Biological trajectories can be characterized by transient patterns that may provide insight into the interactions of the moving object with its immediate environment. The accurate and automated identification of trajectory motifs is important for the understanding of the underlying mechanisms. In this work, we develop a novel trajectory segmentation algorithm based on supervised support vector classification. The algorithm is validated on synthetic data and applied to the identification of trajectory fingerprints of fluorescently tagged human adenovirus particles in live cells. In virus trajectories on the cell surface, periods of confined motion, slow drift, and fast drift are efficiently detected. Additionally, directed motion is found for viruses in the cytoplasm. The algorithm enables the linking of microscopic observations to molecular phenomena that are critical in many biological processes, including infectious pathogen entry and signal transduction.

Default assembly of early adenovirus chromatin

In adenovirus particles, the viral nucleoprotein is organized into a highly compacted core structure. Upon delivery to the nucleus, the viral nucleoprotein is very likely to be remodeled to a form accessible to the transcription and replication machinery. Viral protein VII binds to intra-nuclear viral DNA, as do at least two cellular proteins, SET/TAF-Ibeta and pp32, components of a chromatin assembly complex that is implicated in template remodeling. We showed previously that viral DNA-protein complexes released from infecting particles were sensitive to shearing after cross-linking with formaldehyde, presumably after transport of the genome into the nucleus. We report here the application of equilibrium-density gradient centrifugation to the analysis of the fate of these complexes. Most of the incoming protein VII was recovered in a form that was not cross-linked to viral DNA. This release of protein VII, as well as the binding of SET/TAF-Ibeta and cellular transcription factors to the viral chromatin, did not require de novo viral gene expression. The distinct density profiles of viral DNA complexes containing protein VII, compared to those containing SET/TAF-Ibeta or transcription factors, were consistent with the notion that the assembly of early viral chromatin requires both the association of SET/TAF-1beta and the release of protein VII.

Core labeling of adenovirus with EGFP

The study of adenovirus could greatly benefit from diverse methods of virus detection. Recently, it has been demonstrated that carboxy-terminal EGFP fusions of adenovirus core proteins Mu, V, and VII properly localize to the nucleus and display novel function in the cell. Based on these observations, we hypothesized that the core proteins may serve as targets for labeling the adenovirus core with fluorescent proteins. To this end, we constructed various chimeric expression vectors with fusion core genes (Mu-EGFP, V-EGFP, preVII-EGFP, and matVII-EGFP) while maintaining expression of the native proteins. Expression of the fusion core proteins was suboptimal using E1 expression vectors with both conventional CMV and modified (with adenovirus tripartite leader sequence) CMV5 promoters, resulting in non-labeled viral particles. However, robust expression equivalent to the native protein was observed when the fusion genes were placed in the deleted E3 region. The efficient Ad-wt-E3-V-EGFP and Ad-wt-E3-preVII-EGFP expression vectors were labeled allowing visualization of purified virus and tracking of the viral core during early infection. The vectors maintained their viral function, including viral DNA replication, viral DNA encapsidation, cytopathic effect, and thermostability. Core labeling offers a means to track the adenovirus core in vector targeting studies as well as basic adenovirus virology.

Quantitative Comparison of Intracellular Trafficking and Nuclear Transcription between Adenoviral and Lipoplex Systems

To develop nonviral gene vectors that are sufficient for clinical application, it is necessary to understand why and to what extent nonviral vectors are inferior to viral vectors, which in general show a more efficient transfection activity. This study describes a systematic and quantitative comparison of the cellular uptake and subsequent intracellular distribution (e.g., endosome/lysosome, cytosol, and nucleus) of exogenous DNA transfected by viral and nonviral vectors in living cells, using a combination of TaqMan PCR and a recently developed confocal image-assisted three-dimensionally integrated quantification method. As a model, adenovirus (Ad) and Lipofectamine Plus (LFN) were used for comparison since they are highly potent and widely used viral and nonviral vectors, respectively. The findings indicate that the efficiency of cellular uptake for LFN is significantly higher than that for Ad. Once taken up by a cell, Ad exhibited comparable endosomal escape and slightly higher nuclear transfer efficiency compared with LFN. In contrast, LFN requires 3 orders of magnitude more intranuclear gene copies to exhibit a transgene expression comparable to that of the Ad, suggesting that the difference in transfection efficiency principally arises from differences in nuclear transcription efficiency and not from a difference in intracellular trafficking between Ad and LFN.

Antiviral cyclic D,L-alpha-peptides: targeting a general biochemical pathway in virus infections

Diverse virus families have evolved to exploit the acidification of endosomal compartments to gain entry into cells. We describe a supramolecular approach for selectively targeting and inhibiting viral infections through this central biochemical pathway. Using adenovirus as a model non-enveloped virus, we have determined that an eight-residue cyclic D,L-alpha-peptide, selected from a directed combinatorial library, can specifically prevent the development of low pH in endocytic vesicles, arrest the escape of virions from the endosome, and abrogate adenovirus infection without an apparent adverse effect on cell viability. The likely generality of this approach against other pH-dependent viral infections is supported by the inhibition of type-A influenza virus escape from endosomes in the presence of the same peptide. Our studies suggest that self-assembling cyclic D,L-alpha-peptides hold considerable potential as a new rational supramolecular approach toward the design and discovery of broad-spectrum antiviral agents.

A model for intracellular trafficking of adenoviral vectors

Here we develop an integrative computational framework to model biophysical processes involved in viral gene delivery. The model combines reaction-diffusion-advection equations that describe intracellular trafficking with kinetic equations that describe transcription and translation of the exogenous DNA. It relates molecular-level trafficking events to whole-cell distribution of viruses. The approach makes use of the current understanding of cellular processes and data from single-particle single-cell imaging experiments. The model reveals two important parameters that characterize viral transport at the population level, namely, the effective velocity, V(eff), and the effective diffusion coefficient, D(eff). V(eff) measures virus's net movement rate and D(eff) represents the total dispersion rate. We employ the model to study the influence of microtubule-mediated movements on nuclear targeting and gene expression of adenoviruses of type 2 and type 5 in HeLa and A549 cells. Effects of microtubule organization and the presence of microtubule-destabilizing drugs on viral transport were analyzed and quantified. Model predictions agree well with experimental data available in literature. The paper serves as a guide for future theoretical and experimental efforts to understand viral gene delivery.

Nuclear targeting of adenovirus type 2 requires CRM1-mediated nuclear export

Incoming adenovirus type 2 (Ad2) and Ad5 shuttle bidirectionally along microtubules, biased to the microtubule-organizing center by the dynein/dynactin motor complex. It is unknown how the particles reach the nuclear pore complex, where capsids disassemble and viral DNA enters the nucleus. Here, we identified a novel link between nuclear export and microtubule-mediated transport. Two distinct inhibitors of the nuclear export factor CRM1, leptomycin B (LMB) and ratjadone A (RJA) or CRM1-siRNAs blocked adenovirus infection, arrested cytoplasmic transport of viral particles at the microtubule-organizing center or in the cytoplasm and prevented capsid disassembly and nuclear import of the viral genome. In mitotic cells where CRM1 is in the cytoplasm, adenovirus particles were not associated with microtubules but upon LMB treatment, they enriched at the spindle poles implying that CRM1 inhibited microtubule association of adenovirus. We propose that CRM1, a nuclear factor exported by CRM1 or a protein complex containing CRM1 is part of a sensor mechanism triggering the unloading of the incoming adenovirus particles from microtubules proximal to the nucleus of interphase cells.

Adenovirus type-5 entry and disassembly followed in living cells by FRET, fluorescence anisotropy, and FLIM

We have used fluorescence resonance energy transfer (FRET) to follow the process of capsid disassembly for adenovirus (Ad) serotype 5 (Ad5) in living CHO-CAR cells. Ad5 were weakly labeled on their capsid proteins with FRET donor and acceptor fluorophores. A progressive decrease in FRET efficiency recorded during Ad5 uptake revealed that the time course of Ad5 capsid disassembly has two sequential protein dissociation rates with half-times of 3 and 60 min. Fluorescence anisotropy measurements of the segmental motions of fluorophores on Ad5 indicate that the first rate is linked to the detachment from the capsid of the protruding, flexible fiber proteins. The second rate was shown to report on the combined dissociation of protein IX, penton base, and hexons, which form the rigid icosahedral capsid shell. Fluorescence lifetime imaging microscopy measurements using a pH-sensitive probe provided information on the pH of the microenvironment of Ad5 particles during intracellular trafficking, and confirmed that the fast fiber dissociation step occurred at the onset of endocytosis. The slower dissociation phase was shown to coincide with the escape of Ad5 from endocytic compartments into the cytosol, and its arrival at the nuclear membrane. These results demonstrate a rapid, quantitative live-cell assay for the investigation of virus-cell interactions and capsid disassembly.

The human membrane cofactor CD46 is a receptor for species B adenovirus serotype 3

Many human adenovirus (Ad) serotypes use the coxsackie B virus-Ad receptor (CAR). Recently, CD46 was suggested to be a receptor of species B Ad serotype 11 (Ad11), Ad14, Ad16, Ad21, Ad35, and Ad50. Using Sindbis virus-mediated cDNA library expression, we identify here the membrane cofactor protein CD46 as a surface receptor of species B Ad3. All four major CD46 transcripts and one minor CD46 transcript expressed in nucleated human cells were isolated. Rodent BHK cells stably expressing the BC1 form of CD46 bound radiolabeled Ad3 with a dissociation constant of 0.3 nM, identical to that of CD46-positive HeLa cells expressing twice as many Ad3 binding sites. Pull-down experiments with recombinant Ad3 fibers and a soluble form of the CD46 extracellular domain linked to the Fc portion of human immunoglobulin G (CD46ex-Fc) indicated direct interactions of the Ad3 fiber knob with CD46ex-Fc but not CARex-Fc (Fc-linked extracellular domain of CAR). Ad3 colocalized with cell surface CD46 in both rodent and human cells at the light and electron microscopy levels. Anti-CD46 antibodies and CD46ex-Fc inhibited Ad3 binding to CD46-expressing BHK cells more than 10-fold and to human cells 2-fold. In CD46-expressing BHK cells, wild-type Ad3 and a chimeric Ad consisting of the Ad5 capsid and the Ad3 fiber elicited dose-dependent cytopathic effects and transgene expression, albeit less efficiently than in human cells. Together, our results show that all of the major splice forms of CD46 are predominant and functional binding sites of Ad3 on CD46-expressing rodent and human cells but may not be the sole receptor of species B Ads on human cells. These results have implications for understanding viral pathogenesis and therapeutic gene delivery.

Cholesterol is required for endocytosis and endosomal escape of adenovirus type 2

The species C adenovirus type 2 (Ad2) and Ad5 bind the coxsackievirus B Ad receptor and alphav integrin coreceptors and enter epithelial cells by clathrin-mediated endocytosis. This pathway is rapid and efficient. It leads to cell activation and the cholesterol-dependent formation of macropinosomes. Macropinosomes are triggered to release their contents when incoming Ad2 escapes from endosomes. Here, we show that cholesterol extraction of epithelial cells by methyl-beta-cyclodextrin (mbetaCD) treatment reduced Ad5-mediated luciferase expression approximately 4-fold. The addition of cholesterol to normal cells increased gene expression in a dose-dependent manner up to threefold, but it did not restore gene expression in mbetaCD-treated cells. mbetaCD had no effect in the presence of excess cholesterol, indicating that the inhibition of gene expression was due specifically to cholesterol depletion. Cholesterol depletion inhibited rapid Ad2 endocytosis, endosomal escape, and nuclear targeting, consistent with the notion that clathrin-dependent endocytosis of Ad2 is cholesterol dependent. In cholesterol-reduced cells, Ad2 internalized at a low rate, suggestive of an alternative, clathrin-independent, low-capacity entry pathway. While exogenous cholesterol completely restored rapid Ad2 endocytosis, macropinocytosis, and macropinosome disruption, it did not, surprisingly, restore viral escape from endosomes. Our results indicate that macropinosome disruption and endosomal escape of Ad2 are independent events in cells depleted of and then refilled with cholesterol, suggesting that viral escape from endosomes requires lipid-controlled membrane homeostasis, trafficking, or signaling.

Fluorescently Labeled Adenovirus with pIX-EGFP for Vector Detection

Adenoviruses are extensively studied in terms of their use as gene therapy vectors and pathogenesis. These vectors have been targeted on both transcriptional and transductional levels to achieve cell-specific gene delivery. Current detection strategies, including reporter gene expression, viral component detection, and vector labeling with fluorophores, have been applied to analyze adenoviral vectors; however, these methods are inadequate for assessing transductional targeting. As an alternative to conventional vector detection techniques, we developed a specific genetic labeling system whereby an adenoviral vector incorporates a fusion between capsid protein IX and EGFP. DNA packaging and thermostability were marginally hampered by the modification while DNA replication, cytopathic effect, and CAR-dependent binding were not affected. The fluorescent label was associated with the virus capsid and conferred a fluorescent property useful in detecting adenoviral particles in flow cytometry, tracking, and tissue sections. We believe our genetic adenovirus labeling system has important implications for vector development, detecting adenovirus vectors in targeting schemes, and studying adenovirus biology. In addition, this technique has potential utility for dynamic monitoring of adenovirus replication and spread.

Insider information: what viruses tell us about endocytosis

Viruses have long served as tools in molecular and cellular biology to study a variety of complex cellular processes. Currently, there is a revived interest in virus entry into animal cells because it is evident that incoming viruses make use of numerous endocytic pathways that are otherwise difficult to study. Besides the classical clathrin-mediated uptake route, viruses use caveolae-mediated endocytosis, lipid-raft-mediated endocytic pathways, and macropinocytosis. Some of these are subject to regulation, involve novel endocytic organelles, and some of them connect organelles that were previously not known to communicate by membrane traffic.

Adenovirus type 5 DNA-protein complexes from formaldehyde cross-linked cells early after infection

We report here the properties of viral DNA-protein complexes that purify with cellular chromatin following formaldehyde cross-linking of intact cells early after infection. The cross-linked viral DNA fractionated into shear-sensitive (S) and shear- resistant (R) components that were separable by sedimentation, which allowed independent characterization. The R component had the density and sedimentation properties expected for DNA-protein complexes and contained intact viral DNA. It accounted for about 50% of the viral DNA recovered at 1.5 h after infection but less than 20% by 4.5 h. The proportion of R component was independent of multiplicity of infection, even at less than one particle per cell. Viral hexon and protein VII, but not protein VI, were detected in the fractions containing the R component. These properties are consistent with those of partially uncoated virions associated with the nuclear envelope. A substantial proportion of the S component viral DNA had the same density as cellular chromatin. Protein VII was the most abundant viral protein present in gradient fractions that contained the S component. Complexes containing USF transcription factor cross-linked to the adenovirus major late promoter were detected by viral chromatin immunoprecipitation of the fractions containing S component. The S component probably contained uncoated nuclear viral DNA that assembles into early viral transcription complexes.

Non-classical export of an adenovirus structural protein

The icosahedral capsids of Adenoviruses (Ads) consist of the hexon and stabilizing proteins building the facettes, and of the vertex protein penton base (Pb) anchoring the protruding fibers. The fibers bind to the Coxsackie virus B Ad cell surface receptor (CAR) and Pb to integrins. Here we describe a novel property of the Ad2 Pb. Pb was found to leave the infected cell and, upon exit, it attached to the surrounding noninfected cells forming a radial gradient with highest Pb levels on cells adjacent to the infected cell. The producer cells remained intact until at least 30 h post infection. At this point, Pb was not recovered from the extracellular medium, suggesting that its cell-cell spread might not involve free Pb. When viral particles were released at late stages of infection, soluble Pb was found in the extracellular medium and it randomly bound to noninfected cells. Nonlytic export of Pb occurred upon transient transfection with plasmid DNA, but plasmid-encoded fiber was not exported, indicating that cell-cell spread of Pb is autonomous of infection. Pb export was not affected by Brefeldin A-induced disruption of the Golgi apparatus, suggesting that it occurred via a nonclassical mechanism. Interestingly, the coexpression of Pb and fiber leads to both Pb and fiber export, termed 'protein abduction'. We suggest that fiber abduction might support viral dissemination in infected tissues by interfering with tissue integrity.

Intact microtubules support adenovirus and herpes simplex virus infections

Capsids and the enclosed DNA of adenoviruses, including the species C viruses adenovirus type 2 (Ad2) and Ad5, and herpesviruses, such as herpes simplex virus type 1 (HSV-1), are targeted to the nuclei of epithelial, endothelial, fibroblastic, and neuronal cells. Cytoplasmic transport of fluorophore-tagged Ad2 and immunologically detected HSV-1 capsids required intact microtubules and the microtubule-dependent minus-end-directed motor complex dynein-dynactin. A recent study with epithelial cells suggested that Ad5 was transported to the nucleus and expressed its genes independently of a microtubule network. To clarify the mechanisms by which Ad2 and, as an independent control, HSV-1 were targeted to the nucleus, we treated epithelial cells with nocodazole (NOC) to depolymerize microtubules and measured viral gene expression at different times and multiplicities of infections. Our results indicate that in NOC-treated cells, viral transgene expression was significantly reduced at up to 48 h postinfection (p.i.). A quantitative analysis of subcellular capsid localization indicated that NOC blocked the nuclear targeting of Ad2 and also HSV-1 by more than 90% at up to 7 h p.i. About 10% of the incoming Texas Red-coupled Ad2 (Ad2-TR) was enriched at the nucleus in microtubule-depleted cells at 5 h p.i. This result is consistent with earlier observations that Ad2-TR capsids move randomly in NOC-treated cells at less than 0.1 micro m/s and over distances of less than 5 micro m, characteristic of Brownian motion. We conclude that fluorophore-tagged Ad2 and HSV-1 particles are infectious and that microtubules play a prominent role in efficient nuclear targeting during entry and gene expression of species C Ads and HSV-1.

Adenovirus triggers macropinocytosis and endosomal leakage together with its clathrin-mediated uptake

Adenovirus type 2 (Ad2) binds the coxsackie B virus Ad receptor and is endocytosed upon activation of the alphav integrin coreceptors. Here, we demonstrate that expression of dominant negative clathrin hub, eps15, or K44A-dynamin (dyn) inhibited Ad2 uptake into epithelial cells, indicating clathrin-dependent viral endocytosis. Surprisingly, Ad strongly stimulated the endocytic uptake of fluid phase tracers, coincident with virus internalization but without affecting receptor-mediated transferrin uptake. A large amount of the stimulated endocytic activity was macropinocytosis. Macropinocytosis depended on alphav integrins, PKC, F-actin, and the amiloride-sensitive Na+/H+ exchanger, which are all required for Ad escape from endosomes and infection. Macropinocytosis stimulation was not a consequence of viral escape, since it occurred in K44A-dyn-expressing cells. Surprisingly, 30-50% of the endosomal contents were released into the cytosol of control and also K44A-dyn-expressing cells, and the number of fluid phase-positive endosomes dropped below the levels of noninfected cells, indicating macropinosomal lysis. The release of macropinosomal contents was Ad dose dependent, but the presence of Ad particles on macropinosomal membranes was not sufficient for contents release. We conclude that Ad signaling from the cell surface controls the induction of macropinosome formation and leakage, and this correlates with viral exit to the cytosol and infection.

Unhampered prion neuroinvasion despite impaired fast axonal transport in transgenic mice overexpressing four-repeat tau

Transmissible spongiform encephalopathies often are caused by peripheral uptake of infectious prions, and the peripheral nervous system is involved in prion spread to the brain. Although the cellular prion protein is subjected to fast axonal transport, the mechanism of intranerval transport of infectious prions is unclear. Here we administered prions intranervally to transgenic mice overexpressing the four-repeat human tau protein, which exhibit defective fast axonal transport. These mice showed unaltered neuroinvasion, suggesting that transport mechanisms distinct from fast axonal transport effect prion neuroinvasion along peripheral nerves. Surprisingly, scrapie-sick tau transgenic mice accumulated intraneuronal deposits of hyperphosphorylated tau protein. The coincidence of tau and prion pathology resembled Gerstmann-Sträussler-Scheinker syndrome. These findings identify tau pathology as a possible end stretch of prion-induced neurodegeneration.

Enhanced microtubule-dependent trafficking and p53 nuclear accumulation by suppression of microtubule dynamics

The tumor suppressor protein p53 localizes to microtubules (MT) and, in response to DNA damage, is transported to the nucleus via the MT minus-end-directed motor protein dynein. Dynein is also responsible for MT-mediated nuclear targeting of adenovirus type 2 (Ad2). Here we show that treatment with low concentrations of MT-targeting compounds (MTCs) that do not disrupt the MT network but are known to suppress MT dynamics enhanced p53 nuclear accumulation, and the activation of the p53-downstream target genes. p53 nuclear accumulation required binding of MTCs to MTs and enhanced the induction of p53-up-regulated modulator of apoptosis (PUMA) mRNA and apoptosis on challenging cells with the DNA-damaging drug adriamycin. Low concentrations of MTCs enhanced the rate of movement of fluorescent Ad2 to the nucleus and increased the nuclear targeting efficiency of Ad2. We propose that suppression of MT dynamics by low concentrations of MTCs enhances MT-dependent trafficking toward the minus ends of MTs and facilitates nuclear targeting.

Gene transfer to salivary glands

This article provides a review of the application of gene transfer technology to studies of salivary glands. Salivary glands provide an uncommon target site for gene transfer but offer many experimental situations likely of interest to the cell biologist. The reader is provided with a concise overview of salivary biology, along with a general discussion of the strategies available for gene transfer to any tissue. In particular, adenoviral vectors have been useful for proof of concept studies with salivary glands. Several examples are given, using adenoviral-mediated gene transfer, for addressing both biological and clinical questions. Additionally, benefits and shortcomings affecting the utility of this technology are discussed.

Import of adenovirus DNA involves the nuclear pore complex receptor CAN/Nup214 and histone H1

Adenovirus type 2 (Ad2) imports its DNA genome through the nuclear pore complex (NPC) of cells in interphase for viral production. Here we identify the NPC-filament protein CAN/Nup214 as a docking site for incoming Ad2 capsids. Binding to CAN is independent of cytosolic factors. Capsids disassemble at NPCs to free their DNA for import. This process requires binding of nuclear histone H1 to the stably docked capsids and involves H1-import factors, restricting this irreversible process to the proximity of the nucleus. Our results provide a molecular mechanism for disassembly of Ad2 and reveal an unexpected function of histone H1 in virus-mediated DNA import.

Caveolar endocytosis of simian virus 40 reveals a new two-step vesicular-transport pathway to the ER

Simian virus 40 (SV40) is unusual among animal viruses in that it enters cells through caveolae, and the internalized virus accumulates in a smooth endoplasmic reticulum (ER) compartment. Using video-enhanced, dual-colour, live fluorescence microscopy, we show the uptake of individual virus particles in CV-1 cells. After associating with caveolae, SV40 leaves the plasma membrane in small, caveolin-1-containing vesicles. It then enters larger, peripheral organelles with a non-acidic pH. Although rich in caveolin-1, these organelles do not contain markers for endosomes, lysosomes, ER or Golgi, nor do they acquire ligands of clathrin-coated vesicle endocytosis. After several hours in these organelles, SV40 is sorted into tubular, caveolin-free membrane vesicles that move rapidly along microtubules, and is deposited in perinuclear, syntaxin 17-positive, smooth ER organelles. The microtubule-disrupting agent nocodazole inhibits formation and transport of these tubular carriers, and blocks viral infection. Our results demonstrate the existence of a two-step transport pathway from plasma-membrane caveolae, through an intermediate organelle (termed the caveosome), to the ER. This pathway bypasses endosomes and the Golgi complex, and is part of the productive infectious route used by SV40.

Adenovirus-activated PKA and p38/MAPK pathways boost microtubule-mediated nuclear targeting of virus

Nuclear targeting of adenovirus is mediated by the microtubule-dependent, minus-end-directed motor complex dynein/dynactin, in competition with plus- end-directed motility. We demonstrate that adenovirus transiently activates two distinct signaling pathways to enhance nuclear targeting. The first pathway activates integrins and cAMP-dependent protein kinase A (PKA). The second pathway activates the p38/MAP kinase and the downstream MAPKAP kinase 2 (MK2), dependent on the p38/MAPK kinase MKK6, but independent of integrins and PKA. Motility measurements in PKA-inhibited, p38-inhibited or MK2-lacking (MK2(-/-)) cells indicate that PKA and p38 stimulated both the frequency and velocity of minus-end-directed viral motility without affecting the perinuclear localization of transferrin-containing endosomal vesicles. p38 also suppressed lateral viral motilities and MK2 boosted the frequency of minus-end-directed virus transport. Nuclear targeting of adenovirus was rescued in MK2(-/-) cells by overexpression of hsp27, an MK2 target that enhances actin metabolism. Our results demonstrate that complementary activities of PKA, p38 and MK2 tip the transport balance of adenovirus towards the nucleus and thus enhance infection.

The first step of adenovirus type 2 disassembly occurs at the cell surface, independently of endocytosis and escape to the cytosol

Disassembly is a key event of virus entry into cells. Here, we have investigated cellular requirements for the first step of adenovirus type 2 (Ad2) disassembly, the release of the fibers. Although fiber release coincides temporally with virus uptake, fiber release is not required for Ad2 endocytosis. It is, however, inhibited by actin-disrupting agents or soluble RGD peptides, which interfere with integrin-dependent endocytosis of Ad2. Fiber release occurs at the cell surface. Actin stabilization with jasplakinolide blocks Ad2 entry at extended cell surface invaginations and efficiently promotes fiber release, indicating that fiber release and virus endocytosis are independent events. Fiber release is not sufficient for Ad2 escape from endosomes, since inhibition of protein kinase C (PKC) prevents Ad2 escape from endosomes but does not affect virus internalization or fiber release. PKC-inhibited cells accumulate Ad2 in small vesicles near the cell periphery, indicating that PKC is also required for membrane trafficking of virus. Taken together, our data show that fiber release from incoming Ad2 requires integrins and filamentous actin. Together with correct subcellular transport of Ad2-containing endosomes, fiber release is essential for efficient delivery of virus to the cytosol. We speculate that fiber release at the surface might extend the host range of Ad2 since it is associated with the separation of a small fraction of incoming virus from the target cells.