Actin- and myosin-driven movement of viruses along filopodia precedes their entry into cells

Membrane organization of virus and target cell plays a role in HIV entry

The initial steps of the Human Immunodeficiency Virus (HIV) replication cycle play a crucial role that arbitrates viral tropism and infection efficiency. Before the release of its genome into the host cell cytoplasm, viruses operate a complex sequence of events that take place at the plasma membrane of the target cell. The first step is the binding of the HIV protein envelope (Env) to the cellular receptor CD4. This triggers conformational changes of the gp120 viral protein that allow its interaction with a co-receptor that can be either CCR5 or CXCR4, defining the tropism of the virus entering the cell. This sequential interaction finally drives the fusion of the viral and host cell membrane or to the endocytosis of the viruses. Here, we discuss how the membrane composition and organization of both the virus and the target cell can affect these steps and thus influence the capability of the viruses to infect cells.

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An overview of lipid role in HIV infection is proposed.

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We discuss the influence of lipid composition on HIV early steps of infection.

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We discuss the role of membrane organization an dynamics in HIV entry.

Three-dimensional imaging of HIV-1 virological synapses reveals membrane architectures involved in virus transmission

HIV transmission efficiency is greatly increased when viruses are transmitted at virological synapses formed between infected and uninfected cells. We have previously shown that virological synapses formed between HIV-pulsed mature dendritic cells (DCs) and uninfected T cells contain interdigitated membrane surfaces, with T cell filopodia extending toward virions sequestered deep inside invaginations formed on the DC membrane. To explore membrane structural changes relevant to HIV transmission across other types of intercellular conjugates, we used a combination of light and focused ion beam scanning electron microscopy (FIB-SEM) to determine the three-dimensional (3D) architectures of contact regions between HIV-1-infected CD4(+) T cells and either uninfected human CD4(+) T cells or human fetal astrocytes. We present evidence that in each case, membrane extensions that originate from the uninfected cells, either as membrane sheets or filopodial bridges, are present and may be involved in HIV transmission from infected to uninfected cells. We show that individual virions are distributed along the length of astrocyte filopodia, suggesting that virus transfer to the astrocytes is mediated, at least in part, by processes originating from the astrocyte itself. Mechanisms that selectively disrupt the polarization and formation of such membrane extensions could thus represent a possible target for reducing viral spread.

IMPORTANCE

Our findings lead to new insights into unique aspects of HIV transmission in the brain and at T cell-T cell synapses, which are thought to be a predominant mode of rapid HIV transmission early in the infection process.

Cell migration is another player of the minute virus of mice infection

The parvovirus minute virus of mice, prototype strain (MVMp), preferentially infects and kills cancer cells. This intrinsic MVMp oncotropism may depend in part on the early stages of MVMp infection. To test this hypothesis, we investigated the early events of MVMp infection in mouse LA9 fibroblasts and a highly invasive mouse mammary tumor cell line derived from polyomavirus middle T antigen-mediated transformation. Using a combination of fluorescence and electron microscopy, we found that various parameters of the cell migration process affect MVMp infection. We show that, after binding to the plasma membrane, MVMp particles rapidly cluster at the leading edge of migrating cells, which exhibit higher levels of MVMp uptake than non-motile cells. Moreover, promoting cell migration on a fibronectin matrix increased MVMp infection, and induction of epithelial-mesenchymal transition allowed MVMp replication in non-permissive epithelial cells. Hence, we propose that cell migration influences the early stages of MVMp infection.

Entry of a novel marine DNA virus, Singapore grouper iridovirus, into host cells occurs via clathrin-mediated endocytosis and macropinocytosis in a pH-dependent manner

Iridoviruses are nucleocytoplasmic DNA viruses which cause great economic losses in the aquaculture industry but also show significant threat to global biodiversity. However, a lack of host cells has resulted in poor progress in clarifying iridovirus behavior. We investigated the crucial events during virus entry using a combination of single-virus tracking and biochemical assays, based on the established virus-cell infection model for Singapore grouper iridovirus (SGIV). SGIV infection in host cells was strongly inhibited when cells were pretreated with drugs blocking clathrin-mediated endocytosis, including sucrose and chlorpromazine. Inhibition of key regulators of macropinocytosis, including Na(+)/H(+) exchanger, Rac1 GTPase, p21-activated kinase 1 (PAK1), protein kinase C (PKC), and myosin II, significantly reduced SGIV uptake. Cy5-labeled SGIV particles were observed to colocalize with clathrin and macropinosomes. In contrast, disruption of cellular cholesterol by methyl-β-cyclodextrin and nystatin had no effect on virus infection, suggesting that SGIV entered grouper cells via the clathrin-mediated endocytic pathway and macropinocytosis but not via caveola-dependent endocytosis. Furthermore, inhibitors of endosome acidification such as chloroquine and bafilomycin A1 blocked virus infection, indicating that SGIV entered cells in a pH-dependent manner. In addition, SGIV particles were observed to be transported along both microtubules and actin filaments, and intracellular SGIV motility was remarkably impaired by depolymerization of microtubules or actin filaments. The results of this study for the first time demonstrate that not only the clathrin-dependent pathway but also macropinocytosis are involved in fish DNA enveloped virus entry, thus providing a convenient tactic for exploring the life cycle of DNA viruses.

IMPORTANCE

Virus entry into host cells is critically important for initiating infections and is usually recognized as an ideal target for the design of antiviral strategies. Iridoviruses are large DNA viruses which cause serious threats to ecological diversity and the aquaculture industry worldwide. However, the current understanding of iridovirus entry is limited and controversial. Singapore grouper iridovirus (SGIV) is a novel marine fish DNA virus which belongs to genus Ranavirus, family Iridoviridae. Here, using single-virus tracking technology in combination with biochemical assays, we investigated the crucial events during SGIV entry and demonstrated that SGIV entered grouper cells via the clathrin-mediated endocytic pathway in a pH-dependent manner but not via caveola-dependent endocytosis. Furthermore, we propose for the first time that macropinocytosis is involved in iridovirus entry. Together, this work not only contributes greatly to understating iridovirus pathogenesis but also provides an ideal model for exploring the behavior of DNA viruses in living cells.

KSHV attachment and entry are dependent on αVβ3 integrin localized to specific cell surface microdomains and do not correlate with the presence of heparan sulfate

Cellular receptors for KSHV attachment and entry were characterized using tyramide signal amplification (TSA)-enhanced confocal microscopy. Integrins αVβ3, αVβ5 and α3β1 were detected on essentially all the actin-based cell surface microdomains that initially bind KSHV, while the presence of CD98 and heparan sulfate (HS), the putative attachment receptor, was more variable. KSHV bound to the same cell surface microdomains with and without HS indicating that initial attachment of KSHV is not dependent on HS and that receptors other than HS can mediate attachment. A human salivary gland (HSG) epithelial line was identified, which lacks αVβ3 but expresses high levels of HS, α3β1 and other putative KSHV receptors. These cells were resistant to KSHV binding and infection. Reconstitution of cell surface αVβ3 rendered HSG cells highly susceptible to KSHV infection, demonstrating a critical role for αVβ3 in the binding and entry of KSHV that is not shared with other proposed receptors.

Life without double-headed non-muscle myosin II motor proteins

Non-muscle myosin II motor proteins (myosin IIA, myosin IIB, and myosin IIC) belong to a class of molecular motor proteins that are known to transduce cellular free-energy into biological work more efficiently than man-made combustion engines. Nature has given a single myosin II motor protein for lower eukaryotes and multiple for mammals but none for plants in order to provide impetus for their life. These specialized nanomachines drive cellular activities necessary for embryogenesis, organogenesis, and immunity. However, these multifunctional myosin II motor proteins are believed to go awry due to unknown reasons and contribute for the onset and progression of many autosomal-dominant disorders, cataract, deafness, infertility, cancer, kidney, neuronal, and inflammatory diseases. Many pathogens like HIV, Dengue, hepatitis C, and Lymphoma viruses as well as Salmonella and Mycobacteria are now known to take hostage of these dedicated myosin II motor proteins for their efficient pathogenesis. Even after four decades since their discovery, we still have a limited knowledge of how these motor proteins drive cell migration and cytokinesis. We need to enrich our current knowledge on these fundamental cellular processes and develop novel therapeutic strategies to fix mutated myosin II motor proteins in pathological conditions. This is the time to think how to relieve the hijacked myosins from pathogens in order to provide a renewed impetus for patients' life. Understanding how to steer these molecular motors in proliferating and differentiating stem cells will improve stem cell based-therapeutics development. Given the plethora of cellular activities non-muscle myosin motor proteins are involved in, their importance is apparent for human life.

Murine leukemia virus Gag localizes to the uropod of migrating primary lymphocytes

B and CD4(+) T lymphocytes are natural targets of murine leukemia virus (MLV). Migrating lymphocytes adopt a polarized morphology with a trailing edge designated the uropod. Here, we demonstrate that MLV Gag localizes to the uropod in polarized B cells and CD4(+) T cells. The uropod localization of MLV Gag was dependent on plasma membrane (PM) association and multimerization of Gag but independent of the viral glycoprotein Env. Basic residues in MA that are required for MLV Gag recruitment to virological synapses between HEK293 and XC cells were dispensable for uropod localization in migrating B cells. Ultrastructural studies indicated that both wild-type and basic-residue mutant Gag localized to the outer surface of the PM at the uropod. Late-domain mutant virus particles were seen at the uropod in form of budding-arrested intermediates. Finally, uropods mediated contact between MLV-infected B cells and uninfected T cells to form virological synapses. Our results suggest that MLV, not unlike HIV, accumulates at the uropod of primary lymphocytes to facilitate viral spreading through the formation of uropod-mediated cell-cell contacts.

IMPORTANCE

Viruses have evolved mechanisms to coordinate their assembly and budding with cell polarity to facilitate their spreading. In this study, we demonstrated that the viral determinants for MLV Gag to localize to the uropod in polarized B cells are distinct from the requirements to localize to virological synapses in transformed cell lines. Basic residues in MA that are required for the Gag localization to virological synapses between HEK293 and XC cells are dispensable for Gag localization to the uropod in primary B cells. Rather, plasma membrane association and capsid-driven multimerization of Gag are sufficient to drive MLV Gag to the uropod. MLV-laden uropods also mediate contacts between MLV-infected B cells and uninfected T cells to form virological synapses. Our results indicate that MLV accumulates at the uropod of primary lymphocytes to facilitate viral spreading through the formation of uropod-mediated cell-cell contacts.

Pinpointing retrovirus entry sites in cells expressing alternatively spliced receptor isoforms by single virus imaging

Background

The majority of viruses enter host cells via endocytosis. Current knowledge of viral entry pathways is largely based upon infectivity measurements following genetic and/or pharmacological interventions that disrupt vesicular trafficking and maturation. Imaging of single virus entry in living cells provides a powerful means to delineate viral trafficking pathways and entry sites under physiological conditions.

Results

Here, we visualized single avian retrovirus co-trafficking with markers for early (Rab5) and late (Rab7) endosomes, acidification of endosomal lumen and the resulting viral fusion measured by the viral content release into the cytoplasm. Virus-carrying vesicles either merged with the existing Rab5-positive early endosomes or slowly accumulated Rab5. The Rab5 recruitment to virus-carrying endosomes correlated with acidification of their lumen. Viral fusion occurred either in early (Rab5-positive) or intermediate (Rab5- and Rab7-positive) compartments. Interestingly, different isoforms of the cognate receptor directed virus entry from distinct endosomes. In cells expressing the transmembrane receptor, viruses preferentially entered and fused with slowly maturing early endosomes prior to accumulation of Rab7. By comparison, in cells expressing the GPI-anchored receptor, viruses entered both slowly and quickly maturing endosomes and fused with early (Rab5-positive) and intermediate (Rab5- and Rab7-positive) compartments.

Conclusions

Since the rate of low pH-triggered fusion was independent of the receptor isoform, we concluded that the sites of virus entry are determined by the kinetic competition between endosome maturation and viral fusion. Our findings demonstrate the ability of this retrovirus to enter cells via alternative endocytic pathways and establish infection by releasing its content from distinct endosomal compartments.

Viral exploitation of actin: force-generation and scaffolding functions in viral infection

As a fundamental component of the host cellular cytoskeleton, actin is routinely engaged by infecting viruses. Furthermore, viruses from diverse groups, and infecting diverse hosts, have convergently evolved an array of mechanisms for manipulating the actin cytoskeleton for efficacious infection. An ongoing chorus of research now indicates that the actin cytoskeleton is critical for viral replication at many stages of the viral life cycle, including binding, entry, nuclear localization, genomic transcription and reverse transcription, assembly, and egress/dissemination. Specifically, viruses subvert the force-generating and macromolecular scaffolding properties of the actin cytoskeleton to propel viral surfing, internalization, and migration within the cell. Additionally, viruses utilize the actin cytoskeleton to support and organize assembly sites, and eject budding virions for cell-to-cell transmission. It is the purpose of this review to provide an overview of current research, focusing on the various mechanisms and themes of virus-mediated actin modulation described therein.

Baculovirus induces host cell aggregation via a Rho/Rok-dependent mechanism

Several baculoviruses can induce host cell aggregation during infection; however, the molecular basis remains unknown. The Rho family of small GTPases, including Rho1, Racs and Cdc42, plays important roles in cell migration and cell-cell contact. Activated GTPases target actin polymerization to discrete sites on the plasma membrane, thereby inducing membrane protrusions. In this study, we demonstrated that Spodoptera litura nucleopolyhedrovirus (SpltNPV) infection induced the amoeboid movement and aggregation of SpLi-221 cells in vitro. The amount of Rho1-GTP increased in the infected cells, which suggested that Rho1 was activated upon infection. RNA interference and superinfection of dominant-negative recombinants revealed that the SpltNPV-induced SpLi-221 cell aggregation was dependent on the Rho1, but not Racs or Cdc42, signalling pathway. Inhibition of Rho-associated protein kinase (Rok) activity by the inhibitor Y-27632 significantly reduced SpLi-221 cell aggregation. Silencing Rho1 expression with RNA interference decreased SpltNPV propagation by approximately 40 % in vitro, when SpLi-221 cells were infected at a low, but not high, m.o.i., suggesting that the SpltNPV-induced cell aggregation may benefit SpltNPV spread.

ROCK1 and LIM kinase modulate retrovirus particle release and cell-cell transmission events

The assembly and release of retroviruses from the host cells require dynamic interactions between viral structural proteins and a variety of cellular factors. It has been long speculated that the actin cytoskeleton is involved in retrovirus production, and actin and actin-related proteins are enriched in HIV-1 virions. However, the specific role of actin in retrovirus assembly and release remains unknown. Here we identified LIM kinase 1 (LIMK1) as a cellular factor regulating HIV-1 and Mason-Pfizer monkey virus (M-PMV) particle release. Depletion of LIMK1 reduced not only particle output but also virus cell-cell transmission and was rescued by LIMK1 replenishment. Depletion of the upstream LIMK1 regulator ROCK1 inhibited particle release, as did a competitive peptide inhibitor of LIMK1 activity that prevented cofilin phosphorylation. Disruption of either ROCK1 or LIMK1 led to enhanced particle accumulation on the plasma membrane as revealed by total internal reflection fluorescence microscopy (TIRFM). Electron microscopy demonstrated a block to particle release, with clusters of fully mature particles on the surface of the cells. Our studies support a model in which ROCK1- and LIMK1-regulated phosphorylation of cofilin and subsequent local disruption of dynamic actin turnover play a role in retrovirus release from host cells and in cell-cell transmission events.

IMPORTANCE

Viruses often interact with the cellular cytoskeletal machinery in order to deliver their components to the site of assembly and budding. This study indicates that a key regulator of actin dynamics at the plasma membrane, LIM kinase, is important for the release of viral particles for HIV as well as for particle release by a distantly related retrovirus, Mason-Pfizer monkey virus. Moreover, disruption of LIM kinase greatly diminished the spread of HIV from cell to cell. These findings suggest that LIM kinase and its dynamic modulation of the actin cytoskeleton in the cell may be an important host factor for the production, release, and transmission of retroviruses.

Rho'ing in and out of cells: viral interactions with Rho GTPase signaling

Rho GTPases are key regulators of actin and microtubule dynamics and organization. Increasing evidence shows that many viruses have evolved diverse interactions with Rho GTPase signaling and manipulate them for their own benefit. In this review, we discuss how Rho GTPase signaling interferes with many steps in the viral replication cycle, especially entry, replication, and spread. Seen the diversity between viruses, it is not surprising that there is considerable variability in viral interactions with Rho GTPase signaling. However, several largely common effects on Rho GTPases and actin architecture and microtubule dynamics have been reported. For some of these processes, the molecular signaling and biological consequences are well documented while for others we just begin to understand them. A better knowledge and identification of common threads in the different viral interactions with Rho GTPase signaling and their ultimate consequences for virus and host may pave the way toward the development of new antiviral drugs that may target different viruses.

Inhibition of dengue virus replication by a class of small-molecule compounds that antagonize dopamine receptor d4 and downstream mitogen-activated protein kinase signaling

Dengue viruses (DENV) are endemic pathogens of tropical and subtropical regions that cause significant morbidity and mortality worldwide. To date, no vaccines or antiviral therapeutics have been approved for combating DENV-associated disease. In this paper, we describe a class of tricyclic small-molecule compounds-dihydrodibenzothiepines (DHBTs), identified through high-throughput screening-with potent inhibitory activity against DENV serotype 2. SKI-417616, a highly active representative of this class, displayed activity against all four serotypes of DENV, as well as against a related flavivirus, West Nile virus (WNV), and an alphavirus, Sindbis virus (SINV). This compound was characterized to determine its mechanism of antiviral activity. Investigation of the stage of the viral life cycle affected revealed that an early event in the life cycle is inhibited. Due to the structural similarity of the DHBTs to known antagonists of the dopamine and serotonin receptors, we explored the roles of two of these receptors, serotonin receptor 2A (5HTR2A) and the D4 dopamine receptor (DRD4), in DENV infection. Antagonism of DRD4 and subsequent downstream phosphorylation of epidermal growth factor receptor (EGFR)-related kinase (ERK) were found to impact DENV infection negatively, and blockade of signaling through this network was confirmed as the mechanism of anti-DENV activity for this class of compounds.

IMPORTANCE

The dengue viruses are mosquito-borne, reemerging human pathogens that are the etiological agents of a spectrum of febrile diseases. Currently, there are no approved therapeutic treatments for dengue-associated disease, nor is there a vaccine. This study identifies a small molecule, SKI-417616, with potent anti-dengue virus activity. Further analysis revealed that SKI-417616 acts through antagonism of the host cell dopamine D4 receptor and subsequent repression of the ERK phosphorylation pathway. These results suggest that SKI-417616, or other compounds targeting the same cellular pathways, may have therapeutic potential for the treatment of dengue virus infections.

Multi-resolution 3D visualization of the early stages of cellular uptake of peptide-coated nanoparticles

A detailed understanding of the cellular uptake process is essential to the development of cellular delivery strategies and to the study of viral trafficking. However, visualization of the entire process, encompassing the fast dynamics (local to the freely diffusing nanoparticle) as well the state of the larger-scale cellular environment, remains challenging. Here, we introduce a three-dimensional multi-resolution method to capture, in real time, the transient events leading to cellular binding and uptake of peptide (HIV1-Tat)-modified nanoparticles. Applying this new method to observe the landing of nanoparticles on the cellular contour in three dimensions revealed long-range deceleration of the delivery particle, possibly due to interactions with cellular receptors. Furthermore, by using the nanoparticle as a nanoscale 'dynamics pen', we discovered an unexpected correlation between small membrane terrain structures and local nanoparticle dynamics. This approach could help to reveal the hidden mechanistic steps in a variety of multiscale processes.

Combining single RNA sensitive probes with subdiffraction-limited and live-cell imaging enables the characterization of virus dynamics in cells

The creation of fluorescently labeled viruses is currently limited by the length of imaging observation time (e.g., labeling an envelope protein) and the rescue of viral infectivity (e.g., encoding a GFP protein). Using single molecule sensitive RNA hybridization probes delivered to the cytoplasm of infected cells, we were able to isolate individual, infectious, fluorescently labeled human respiratory syncytial virus virions. This was achieved without affecting viral mRNA expression, viral protein expression, or infectivity. Measurements included the characterization of viral proteins and genomic RNA in a single virion using dSTORM, the development of a GFP fusion assay, and the development of a pulse-chase assay for viral RNA production that allowed for the detection of both initial viral RNA and nascent RNA production at designated times postinfection. Live-cell measurements included imaging and characterization of filamentous virion fusion and the quantification of virus replication within the same cell over an eight-hour period. Using probe-labeled viruses, individual viral particles can be characterized at subdiffraction-limited resolution, and viral infections can be quantified in single cells over an entire cycle of replication. The implication of this development is that MTRIP labeling of viral RNA during virus assembly has the potential to become a general methodology for the labeling and study of many important RNA viruses.

Epidermal growth factor receptor-PI3K signaling controls cofilin activity to facilitate herpes simplex virus 1 entry into neuronal cells

Herpes simplex virus type 1 (HSV-1) establishes latency in neurons and can cause severe disseminated infection with neurological impairment and high mortality. This neurodegeneration is thought to be tightly associated with virus-induced cytoskeleton disruption. Currently, the regulation pattern of the actin cytoskeleton and the involved molecular mechanisms during HSV-1 entry into neurons remain unclear. Here, we demonstrate that the entry of HSV-1 into neuronal cells induces biphasic remodeling of the actin cytoskeleton and an initial inactivation followed by the subsequent activation of cofilin, a member of the actin depolymerizing factor family that is critical for actin reorganization. The disruption of F-actin dynamics or the modulation of cofilin activity by mutation, knockdown, or overexpression affects HSV-1 entry efficacy and virus-mediated cell ruffle formation. Binding of the HSV-1 envelope initiates the epidermal growth factor receptor (EGFR)-phosphatidylinositide 3-kinase (PI3K) signaling pathway, which leads to virus-induced early cofilin phosphorylation and F-actin polymerization. Moreover, the extracellular signal-regulated kinase (ERK) kinase and Rho-associated, coiled-coil-containing protein kinase 1 (ROCK) are recruited as downstream mediators of the HSV-1-induced cofilin inactivation pathway. Inhibitors specific for those kinases significantly reduce the virus infectivity without affecting virus binding to the target cells. Additionally, lipid rafts are clustered to promote EGFR-associated signaling cascade transduction. We propose that HSV-1 hijacks cofilin to initiate infection. These results could promote a better understanding of the pathogenesis of HSV-1-induced neurological diseases.

The actin cytoskeleton is involved in many crucial cellular processes and acts as an obstacle to pathogen entry into host cells. Because HSV-1 establishes lifelong latency in neurons and because neuronal cytoskeletal disruption is thought to be the main cause of HSV-1-induced neurodegeneration, understanding the F-actin remodeling pattern by HSV-1 infection and the molecular interactions that facilitate HSV-1 entry into neurons is important. In this study, we showed that HSV-1 infection induces the rearrangement of the cytoskeleton as well as the initial inactivation and subsequent activation of cofilin. Then, we determined that activation of the EGFR-PI3K-Erk1/2 signaling pathway inactivates cofilin and promotes F-actin polymerization. We postulate that by regulating actin cytoskeleton dynamics, cofilin biphasic activation could represent the specific cellular machinery usurped by pathogen infection, and these results will greatly contribute to the understanding of HSV-1-induced early and complex changes in host cells that are closely linked to HSV-1 pathogenesis.

Lateral boundary mechanosensing by adherent cells in a collagen gel system

Cell adhesion responses to in-depth physical properties such as substrate roughness and topography are well described but little is known about the influence of lateral physical cues such as tissue boundaries on the function of adherent cells. Accordingly, we developed a model system to examine remote cell sensing of lateral boundaries. The model employs floating thin collagen gels supported by rigid grids of varying widths. The dynamics, lengths, and numbers of cell extensions were regulated by grid opening size, which in turn determined the distance of cells from rigid physical boundaries. In smaller grids (200 μm and 500 μm wide), cell-induced deformation fields extended to, and were resisted by, the grid boundaries. However, in larger grids (1700 μm wide), the deformation field did not extend to the grid boundaries, which strongly affected the mean length and number of cell extensions (∼60% reduction). The generation of cell extensions in collagen gels required expression of the β1 integrin, focal adhesion kinase and actomyosin activity. We conclude that the presence of physical boundaries interrupts the process of cell-mediated collagen compaction and fiber alignment in the collagen matrix and enhances the formation of cell extensions. This new cell culture platform provides a geometry that more closely approximates the native basement membrane and will help to elucidate the roles of cell extensions and lateral mechanosensing on extracellular matrix remodeling by invasion and degradation.

Nonmuscle myosin heavy chain IIA is a critical factor contributing to the efficiency of early infection of severe fever with thrombocytopenia syndrome virus

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a novel phlebovirus in the Bunyaviridae family. Most patients infected by SFTSV present with fever and thrombocytopenia, and up to 30% die due to multiple-organ dysfunction. The mechanisms by which SFTSV enters multiple cell types are unknown. SFTSV contains two species of envelope glycoproteins, Gn (44.2 kDa) and Gc (56 kDa), both of which are encoded by the M segment and are cleaved from a precursor polypeptide (about 116 kDa) in the endoplasmic reticulum (ER). Gn fused with an immunoglobulin Fc tag at its C terminus (Gn-Fc) bound to multiple cells susceptible to the infection of SFTSV and blocked viral infection of human umbilical vein endothelial cells (HUVECs). Immunoprecipitation assays following mass spectrometry analysis showed that Gn binds to nonmuscle myosin heavy chain IIA (NMMHC-IIA), a cellular protein with surface expression in multiple cell types. Small interfering RNA (siRNA) knockdown of NMMHC-IIA, but not the closely related NMMHC-IIB or NMMHC-IIC, reduced SFTSV infection, and NMMHC-IIA specific antibody blocked infection by SFTSV but not other control viruses. Overexpression of NMMHC-IIA in HeLa cells, which show limited susceptivity to SFTSV, markedly enhanced SFTSV infection of the cells. These results show that NMMHC-IIA is critical for the cellular entry of SFTSV. As NMMHC-IIA is essential for the normal functions of platelets and human vascular endothelial cells, it is conceivable that NMMHC-IIA directly contributes to the pathogenesis of SFTSV and may be a useful target for antiviral interventions against the viral infection.

Who's really in control: microbial regulation of protein trafficking in the epithelium

Due to evolutionary pressure, there are many complex interactions at the interface between pathogens and eukaryotic host cells wherein host cells attempt to clear invading microorganisms and pathogens counter these mechanisms to colonize and invade host tissues. One striking observation from studies focused on this interface is that pathogens have multiple mechanisms to modulate and disrupt normal cellular physiology to establish replication niches and avoid clearance. The precision by which pathogens exert their effects on host cells makes them excellent tools to answer questions about cell physiology of eukaryotic cells. Furthermore, an understanding of these mechanisms at the host-pathogen interface will benefit our understanding of how pathogens cause disease. In this review, we describe a few examples of how pathogens disrupt normal cellular physiology and protein trafficking at epithelial cell barriers to underscore how pathogens modulate cellular processes to cause disease and how this knowledge has been utilized to learn about cellular physiology.

Life cycle of phytoreoviruses visualized by electron microscopy and tomography

Rice dwarf virus and Rice gall dwarf virus, members of the genus Phytoreovirus in the family Reoviridae,are known as agents of rice disease, because their spread results in substantial economic damage in many Asian countries. These viruses are transmitted via insect vectors, and they multiply both in the plants and in the insect vectors. Structural information about the viruses and their interactions with cellular components in the life cycle are essential for understanding viral infection and replication mechanisms. The life cycle of the viruses involves various cellular events such as cell entry, synthesis of viral genome and proteins, assembly of viral components, viral egress from infected cells, and intra- and intercellular transports. This review focuses on the major events underlying the life cycle of phytoreoviruses, which has been visualized by various electron microscopy (EM) imaging techniques, including cryo-electron microscopy and tomography, and demonstrates the advantage of the advanced EM imaging techniques to investigate the viral infection and replication mechanisms.

HIV trafficking in host cells: motors wanted!

Throughout the viral replication cycle, viral proteins, complexes, and particles need to be transported within host cells. These transport events are dependent on the host cell cytoskeleton and molecular motors. However, the mechanisms by which virus is trafficked along cytoskeleton filaments and how molecular motors are recruited and regulated to guarantee successful integration of the viral genome and production of new viruses has only recently begun to be understood. Recent studies on HIV have identified specific molecular motors involved in the trafficking of these viral particles. Here we review recent literature on the transport of HIV components in the cell, provide evidence for the identity and role of molecular motors in this process, and highlight how these trafficking events may be related to those occurring with other viruses.

How filopodia pull: what we know about the mechanics and dynamics of filopodia

In recent years, the dynamic, hair-like cell protrusions called filopodia have attracted considerable attention. They have been found in a multitude of different cell types and are often called "sensory organelles," since they seem to sense the mechanical and chemical environment of a cell. Once formed, filopodia can exhibit complex behavior, they can grow and retract, push or pull, and transform into distinct structures. They are often found to make first adhesive contact with the extracellular matrix, pathogens or with adjacent cells, and to subsequently exert pulling forces. Much is known about the cytoskeletal players involved in filopodia formation, but only recently have we started to explore the mechanics of filopodia together with the related cytoskeletal dynamics. This review summarizes current advancements in our understanding of the mechanics and dynamics of filopodia, with a focus on the molecular mechanisms behind filopodial force exertion.

Live-cell imaging of Marburg virus-infected cells uncovers actin-dependent transport of nucleocapsids over long distances

Transport of large viral nucleocapsids from replication centers to assembly sites requires contributions from the host cytoskeleton via cellular adaptor and motor proteins. For the Marburg and Ebola viruses, related viruses that cause severe hemorrhagic fevers, the mechanism of nucleocapsid transport remains poorly understood. Here we developed and used live-cell imaging of fluorescently labeled viral and host proteins to characterize the dynamics and molecular requirements of nucleocapsid transport in Marburg virus-infected cells under biosafety level 4 conditions. The study showed a complex actin-based transport of nucleocapsids over long distances from the viral replication centers to the budding sites. Only after the nucleocapsids had associated with the matrix viral protein VP40 at the plasma membrane were they recruited into filopodia and cotransported with host motor myosin 10 toward the budding sites at the tip or side of the long cellular protrusions. Three different transport modes and velocities were identified: (i) Along actin filaments in the cytosol, nucleocapsids were transported at ∼200 nm/s; (ii) nucleocapsids migrated from one actin filament to another at ∼400 nm/s; and (iii) VP40-associated nucleocapsids moved inside filopodia at 100 nm/s. Unique insights into the spatiotemporal dynamics of nucleocapsids and their interaction with the cytoskeleton and motor proteins can lead to novel classes of antivirals that interfere with the trafficking and subsequent release of the Marburg virus from infected cells.

Megalocytivirus-induced proteins of turbot (Scophthalmus maximus): identification and antiviral potential

Megalocytivirus is an important fish pathogen with a broad host range that includes turbot. In this study, proteomic analysis was conducted to examine turbot proteins modulated in expression by megalocytivirus infection. Thirty five proteins from spleen were identified to be differentially expressed at 2days post-viral infection (dpi) and 7dpi. Three upregulated proteins, i.e. heat shock protein 70 (Hsp70), Mx protein, and natural killer enhancing factor (NKEF), were further analyzed for potential antiviral effect. For this purpose, turbot were administered separately with the plasmids pHsp70, pMx, and pNKEF, which express Hsp70, Mx, and NKEF respectively, before megalocytivirus infection. Viral dissemination and propagation in spleen were subsequently determined. The results showed that the viral loads in fish administered with pNKEF were significantly reduced. To examine the potential of Hsp70, Mx, and NKEF as immunological adjuvant, turbot were immunized with a DNA vaccine in the presence of pHsp70, pMx, or pNKEF. Subsequent analysis showed that the presence of pNKEF and pHsp70, but not pMx, significantly reduced viral infection and enhanced fish survival. Taken together, these results indicate that NKEF exhibits antiviral property against megalocytivirus, and that both NKEF and Hsp70 may be used in DNA vaccine-based control of megalocytivirus infection.

BIOLOGICAL SIGNIFICANCE

This study provides the first proteomic picture of turbot in response to megalocytivirus infection. We demonstrated that megalocytivirus infection modulates the expression of turbot proteins associated with various cellular functions, and that one of the upregulated proteins, NKEF, exhibits antiviral effect when overexpressed in vivo, while another upregulated protein, Hsp70, exhibits adjuvant effect when co-immunized with a DNA vaccine. These results add molecular insights into turbot immune response induced by megalocytivirus and provide candidate proteins with application potentials in the control of megalocytivirus-associated disease.

Functional complementation of a model target to study Vpu sensitivity

HIV-1 forms infectious particles with Murine Leukemia virus (MLV) Env, but not with the closely related Gibbon ape Leukemia Virus (GaLV) Env. We have determined that the incompatibility between HIV-1 and GaLV Env is primarily caused by the HIV-1 accessory protein Vpu, which prevents GaLV Env from being incorporated into particles. We have characterized the ‘Vpu sensitivity sequence’ in the cytoplasmic tail domain (CTD) of GaLV Env using a chimeric MLV Env with the GaLV Env CTD (MLV/GaLV Env). Vpu sensitivity is dependent on an alpha helix with a positively charged face containing at least one Lysine. In the present study, we utilized functional complementation to address whether all the three helices in the CTD of an Env trimer have to contain the Vpu sensitivity motif for the trimer to be modulated by Vpu. Taking advantage of the functional complementation of the binding defective (D84K) and fusion defective (L493V) MLV and MLV/GaLV Env mutants, we were able to assay the activity of mixed trimers containing both MLV and GaLV CTDs. Mixed trimers containing both MLV and GaLV CTDs were functionally active and remained sensitive to Vpu. However, trimers containing an Env with the GaLV CTD and an Env with no CTD remained functional but were resistant to Vpu. Together these data suggest that the presence of at least one GaLV CTD is sufficient to make an Env trimer sensitive to Vpu, but only if it is part of a trimeric CTD complex.

Inhibition of myosin light chain kinase can be targeted for the development of new therapies against herpes simplex virus type-1 infection

BACKGROUND

Herpes simplex virus type-1 (HSV-1) is the leading cause of infectious blindness worldwide. Through a multistep process, HSV-1 enters into naturally susceptible human corneal epithelial (HCE) cells where it establishes an optimal environment for viral replication and spread. HSV-1 employment of cytoskeletal proteins, kinases, and cell signalling pathways is crucial for the entry process.

METHODS

Here we demonstrate that non-muscle myosin IIA (NM-IIA) and/or a myosin activating kinase, myosin light chain kinase (MLCK), can be targeted for the development of new and effective therapies against HSV-1. HCE cells were incubated with MLCK inhibitors ML-7 and ML-9 and NM-IIA inhibitor blebbistatin. Following the application of inhibitors, HSV-1 entry and spread to neighbouring HCE cells was evaluated.

RESULTS

Upon application of MLCK inhibitors ML-7 and ML-9 and NM-IIA inhibitor blebbistatin, HSV-1 entry into HCE cells was significantly decreased. Furthermore, dramatic impairment of glycoprotein-mediated membrane fusion was seen in cells treated with MLCK inhibitors, thus establishing a role for MLCK activation in cell-to-cell fusion and multinucleated syncytial cell formation. These results also indicate that the activation of motor protein NM-IIA by MLCK is crucial for cytoskeletal changes required for HSV-1 infection of corneal cells.

CONCLUSIONS

We provide new evidence that NM-IIA and MLCK can be used as effective antiviral targets against ocular herpes.

Exploiting herpes simplex virus entry for novel therapeutics

Herpes Simplex virus (HSV) is associated with a variety of diseases such as genital herpes and numerous ocular diseases. At the global level, high prevalence of individuals who are seropositive for HSV, combined with its inconspicuous infection, remains a cause for major concern. At the molecular level, HSV entry into a host cell involves multiple steps, primarily the interaction of viral glycoproteins with various cell surface receptors, many of which have alternate substitutes. The molecular complexity of the virus to enter a cell is also enhanced by the existence of different modes of viral entry. The availability of many entry receptors, along with a variety of entry mechanisms, has resulted in a virus that is capable of infecting virtually all cell types. While HSV uses a wide repertoire of viral and host factors in establishing infection, current therapeutics aimed against the virus are not as diversified. In this particular review, we will focus on the initial entry of the virus into the cell, while highlighting potential novel therapeutics that can control this process. Virus entry is a decisive step and effective therapeutics can translate to less virus replication, reduced cell death, and detrimental symptoms.

In vivo imaging of virological synapses

Retroviruses such as the human immunodeficiency virus, human T-cell lymphotropic virus and murine leukaemia virus are believed to spread via sites of cell-cell contact designated virological synapses. Support for this model is based on in vitro evidence in which infected cells are observed to specifically establish long-lived cell-cell contact with uninfected cells. Whether virological synapses exist in vivo is unknown. Here we apply intravital microscopy to identify a subpopulation of B cells infected with the Friend murine leukaemia virus that form virological synapses with uninfected leucocytes in the lymph node of living mice. In vivo virological synapses are, like their in vitro counterpart, dependent on the expression of the viral envelope glycoprotein and are characterized by a prolonged polarization of viral capsid to the cell-cell interface. Our results validate the concept of virological synapses and introduce intravital imaging as a tool to visualize retroviral spreading directly in living mice.

Productive entry of HIV-1 during cell-to-cell transmission via dynamin-dependent endocytosis

HIV-1 can be transmitted as cell-free virus or via cell-to-cell contacts. Cell-to-cell transmission between CD4(+) T cells is the more efficient mode of transmission and is predominant in lymphoid tissue, where the majority of virus resides. Yet the cellular mechanisms underlying productive cell-to-cell transmission in uninfected target cells are unclear. Although it has been demonstrated that target cells can take up virus via endocytosis, definitive links between this process and productive infection remain undefined, and this route of transmission has been proposed to be nonproductive. Here, we report that productive cell-to-cell transmission can occur via endocytosis in a dynamin-dependent manner and is sensitive to clathrin-associated antagonists. These data were obtained in a number of CD4(+) T-cell lines and in primary CD4(+) T cells, using both CXCR4- and CCR5-tropic virus. However, we also found that HIV-1 demonstrated flexibility in its use of such endocytic pathways as certain allogeneic transmissions were seen to occur in a dynamin-dependent manner but were insensitive to clathrin-associated antagonists. Also, depleting cells of the clathrin accessory protein AP180 led to a viral uptake defect associated with enhanced infection. Collectively, these data demonstrate that endosomal uptake of HIV-1 during cell-to-cell transmission leads to productive infection, but they are also indicative of a flexible model of viral entry during cell-to-cell transmission, in which the virus can alter its entry route according to the pressures that it encounters.

Morphogen transport

The graded distribution of morphogens underlies many of the tissue patterns that form during development. How morphogens disperse from a localized source and how gradients in the target tissue form has been under debate for decades. Recent imaging studies and biophysical measurements have provided evidence for various morphogen transport models ranging from passive mechanisms, such as free or hindered extracellular diffusion, to cell-based dispersal by transcytosis or cytonemes. Here, we analyze these transport models using the morphogens Nodal, fibroblast growth factor and Decapentaplegic as case studies. We propose that most of the available data support the idea that morphogen gradients form by diffusion that is hindered by tortuosity and binding to extracellular molecules.

HIV-1 Gag associates with specific uropod-directed microdomains in a manner dependent on its MA highly basic region

In polarized T cells, HIV-1 Gag localizes to a rear-end protrusion known as the uropod in a multimerization-dependent manner. Gag-laden uropods participate in formation of virological synapses, intercellular contact structures that play a key role in cell-to-cell HIV-1 transmission. Our previous observations suggest that Gag associates with uropod-directed microdomains (UDMs) that eventually comigrate with Gag to the uropod over the cell surface. However, the nature of Gag multimerization required for this movement, the composition of the UDMs, and the molecular determinants for Gag association with these microdomains remain unknown. In this study, we found that Gag multimerization prior to budding but beyond dimerization is necessary for Gag localization to the uropods, indicating that uropod localization occurs early in the assembly process. We also found that prior to membrane curvature, Gag multimers associate with a specific subset of UDMs containing PSGL-1, CD43, and CD44 but not ICAM-1, ICAM-3, or CD59. Notably, upon association, Gag excludes ICAM-3 from this subset of UDMs, revealing an active and selective reorganization of these microdomains by Gag. This specific association between Gag and UDMs is dependent on the highly basic region (HBR) in the Gag matrix (MA) domain. The overall positive charge of the HBR was needed for the interaction with the specific UDM subset, while the exact HBR sequence was not, unlike that seen for MA binding to the plasma membrane phospholipid phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P2]. Taken together, these findings revealed that HIV-1 Gag associates with specific microdomains present in polarized T cells in an MA-dependent manner, which results in modification of the microdomain constituents.

Live cell imaging of viral entry

Viral entry encompasses the initial steps of infection starting from virion host cell attachment to viral genome release. Given the dynamic interactions between the virus and the host, many questions related to viral entry can be directly addressed by live cell imaging. Recent advances in fluorescent labeling of viral and cellular components, fluorescence microscopy with high sensitivity and spatiotemporal resolution, and image analysis enabled studies of a broad spectrum across many viral entry steps, including virus-receptor interactions, internalization, intracellular transport, genomic release, nuclear transport, and cell-to-cell transmission. Collectively, these live cell imaging studies have not only enriched our understandings of the viral entry mechanisms, but also provided novel insights into basic cellular biology processes.

Viral infections and sphingolipids

Besides their essential role in the immune system, sphingolipids and their metabolites are potential key regulators in the life cycle of obligatory intracellular pathogens such as viruses. They are involved in lateral and vertical segregation of receptors required for attachment, membrane fusion and endocytosis, as well as in the intracellular replication, assembly and release of viruses. Glycosphingolipids may themselves act as receptors for viruses, such as Galactosylceramide for human immunodeficiency virus (HIV). In addition, sphingolipids and their metabolites are inseparably interwoven in signal transduction processes, dynamic alterations of the cytoskeleton, and the regulation of innate and intrinsic responses of infected target cells. Depending on the nature of the intracellular pathogen, they may support or inhibit infections. Understanding of the underlying mechanisms depending on the specific virus, immune control, and type of disease may open new avenues for therapeutic interventions.

p12 tethers the murine leukemia virus pre-integration complex to mitotic chromosomes

The p12 protein of the murine leukemia virus (MLV) is a constituent of the pre-integration complex (PIC) but its function in this complex remains unknown. We developed an imaging system to monitor MLV PIC trafficking in live cells. This allowed the visualization of PIC docking to mitotic chromosomes and its release upon exit from mitosis. Docking occurred concomitantly with nuclear envelope breakdown and was impaired for PICs of viruses with lethal p12 mutations. Insertion of a heterologous chromatin binding module into p12 of one of these mutants restored PICs attachment to the chromosomes and partially rescued virus replication. Capsid dissociated from wild type PICs in mitotic cells but remained associated with PICs harboring tethering-negative p12 mutants. Altogether, these results explain, in part, MLV restriction to dividing cells and reveal a role for p12 as a factor that tethers MLV PIC to mitotic chromosomes.

Retroviruses, including the murine leukemia virus (MLV), reverse transcribe their RNA genome to a DNA copy, which travels from the cytoplasm to the nucleus as part of a ‘pre-integration complex’ (PIC), to integrate into cellular chromosomes. The viral p12 protein is a constituent of the MLV PIC, but its function in this complex has remained unknown. We developed a real-time imaging system to detect p12 and MLV PICs in live cells. This revealed that p12 tethers the MLV PIC to mitotic chromosomes. Accordingly, PICs derived from viruses with specific lethal mutations in p12 failed to attach to the chromosomes, and insertion of a heterologous chromatin binding module into p12 restored PICs attachment to the chromosomes and rescued virus replication. In addition, docking of wild type PICs to chromosomes coincided with nuclear envelope breakdown during mitosis, and detachment occurred upon exit from mitosis. Capsid - another viral component of the PIC - dissociated from wild type PICs in mitotic cells but remained associated with PICs harboring tethering-negative p12 mutants, suggesting interplay between these two proteins in regulating targeting of mitotic chromosomes by the PIC. These results highlight steps contributing to the high tropism of MLV to dividing cells.

Proteome dynamics in primary target organ of infectious bursal disease virus

Viruses induce dramatic changes in target tissue during pathogenesis, including host cellular responses that either limit or support the pathogen. The infectious bursal disease virus (IBDV) targets primarily the bursa of Fabricius (BF) of chickens, causing severe immunodeficiency. Here, we characterized the cellular proteome changes of the BF caused by IBDV replication in vivo using 2DE followed MALDI-TOF MS identification. Comparative analysis of multiple 2DE gels revealed that the majority of protein expression changes appeared between 24 and 96 h after IBDV infection. MS identified 54 altered cell proteins, 12 of which were notably upregulated by IBDV infection. Meanwhile, the other 42 cellular proteins were considerably suppressed by IBDV infection and are involved in protein degradation, energy metabolism, stress response, host macromolecular biosynthesis, and transport process. The upregulation of β-actin and downregulation of dynamin during IBDV infection were also confirmed by Western blot and immunofluorescence analysis. These altered protein expressions provide a response profile of chicken BF to virulent IBDV infection. Further functional study on these altered proteins may lead to better understanding of pathogenic mechanisms of virulent IBDV infection and to new potential therapeutic targets.

Cell-to-cell transmission of viruses

The life cycle of most viruses involves the release of particles into the extracellular space. Consequently, the study of virus egress as well as virus entry has focused almost exclusively on the biology of cell-free virus. However, cell-free virus spread is often very inefficient. Specific barriers, either located in the donor cell or in the target cell, prevent efficient spread by the cell-free mode. In contrast, viral spread by direct cell-cell contact is largely unaffected by most of these barriers resulting in preferential spread by cell-to-cell transmission. Virus cell-to-cell transmission allows an efficient coordination of several steps of the viral life cycle. It often involves complex inter-cellular adhesion, cellular polarity and intra-cellular trafficking. Because virus cell-to-cell transmission can involve transmission through zones of tight cell-cell contact that are resistant to neutralizing antibodies and reach a high local particle concentration, cell-to-cell transmission can contribute to the pathogenesis of viral infections.

Exploitation of Cellular Cytoskeletons and Signaling Pathways for Cell Entry by Kaposi's Sarcoma-Associated Herpesvirus and the Closely Related Rhesus Rhadinovirus

As obligate intracellular pathogens, viruses depend on the host cell machinery to complete their life cycle. Kaposi’s sarcoma-associated herpes virus (KSHV) is an oncogenicvirus causally linked to the development of Kaposi’s sarcoma and several other lymphoproliferative malignancies. KSHV entry into cells is tightly regulated by diverse viral and cellular factors. In particular, KSHV actively engages cellular integrins and ubiquitination pathways for successful infection. Emerging evidence suggests that KSHV hijacks both actin and microtubule cytoskeletons at different phases during entry into cells. Here, we review recent findings on the early events during primary infection of KSHV and its closely related primate homolog rhesus rhadinovirus with highlights on the regulation of cellular cytoskeletons and signaling pathways that are important for this phase of virus life cycle.

Japanese encephalitis virus infects neuronal cells through a clathrin-independent endocytic mechanism

Japanese encephalitis virus (JEV) is a mosquito-borne pathogenic flavivirus responsible for acute viral encephalitis in humans. The cellular entry of JEV is poorly characterized in terms of molecular requirements and pathways. Here we present a systematic study of the internalization mechanism of JEV in fibroblasts and neuroblastoma cells. To verify the roles of distinct pathways of cell entry, we used fluorescently labeled virus particles, a combination of pharmacological inhibitors, RNA interference (RNAi), and dominant-negative (DN) mutants of regulatory proteins involved in endocytosis. Our study demonstrates that JEV infects fibroblasts in a clathrin-dependent manner, but it deploys a clathrin-independent mechanism to infect neuronal cells. The clathrin-independent pathway requires dynamin and plasma membrane cholesterol. Virus binding to neuronal cells leads to rapid actin rearrangements and an intact and dynamic actin cytoskeleton, and the small GTPase RhoA plays an important role in viral entry. Immunofluorescence analysis of viral colocalization with endocytic markers showed that JEV traffics through Rab5-positive early endosomes and that release of the viral nucleocapsid occurs at the level of the early and not the late endosomes.

Investigation of HIV-1 assembly and release using modern fluorescence imaging techniques

The replication of HIV-1, like that of all viruses, is intimately connected with cellular structures and pathways. For many years, bulk biochemical and cell biological methods were the main approaches employed to investigate interactions between HIV-1 and its host cell. However, during the past decade advancements in fluorescence imaging technologies opened new possibilities for the direct visualization of individual steps occurring throughout the viral replication cycle. Electron microscopy (EM) methods, which have traditionally been employed for the study of viruses, are complemented by fluorescence microscopy (FM) techniques that allow us to follow the dynamics of virus-cell interaction. Subdiffraction fluorescence microscopy, as well as correlative EM/FM approaches, are narrowing the fundamental gap between the high structural resolution provided by EM and the high temporal resolution and throughput accomplished by FM. The application of modern microscopy to the study of HIV-1-host cell interactions has provided insights into the biology of the virus which could not easily, or not at all, have been gained by other methods. Here, we review how modern fluorescence imaging techniques enhanced our knowledge of the dynamic and structural changes involved in HIV-1 particle formation.

Differential pH-dependent cellular uptake pathways among foamy viruses elucidated using dual-colored fluorescent particles

Background

It is thought that foamy viruses (FVs) enter host cells via endocytosis because all FV glycoproteins examined display pH-dependent fusion activities. Only the prototype FV (PFV) glycoprotein has also significant fusion activity at neutral pH, suggesting that its uptake mechanism may deviate from other FVs. To gain new insights into the uptake processes of FV in individual live host cells, we developed fluorescently labeled infectious FVs.

Results

N-terminal tagging of the FV envelope leader peptide domain with a fluorescent protein resulted in efficient incorporation of the fluorescently labeled glycoprotein into secreted virions without interfering with their infectivity. Double-tagged viruses consisting of an eGFP-tagged PFV capsid (Gag-eGFP) and mCherry-tagged Env (Ch-Env) from either PFV or macaque simian FV (SFVmac) were observed during early stages of the infection pathway. PFV Env, but not SFVmac Env, containing particles induced strong syncytia formation on target cells. Both virus types showed trafficking of double-tagged virions towards the cell center. Upon fusion and subsequent capsid release into the cytosol, accumulation of naked capsid proteins was observed within four hours in the perinuclear region, presumably representing the centrosomes. Interestingly, virions harboring fusion-defective glycoproteins still promoted virus attachment and uptake, but failed to show syncytia formation and perinuclear capsid accumulation. Biochemical and initial imaging analysis indicated that productive fusion events occur predominantly within 4–6 h after virus attachment. Non-fused or non-fusogenic viruses are rapidly cleared from the cells by putative lysosomal degradation. Quantitative monitoring of the fraction of individual viruses containing both Env and capsid signals as a function of time demonstrated that PFV virions fused within the first few minutes, whereas fusion of SFVmac virions was less pronounced and observed over the entire 90 minutes measured.

Conclusions

The characterized double-labeled FVs described here provide new mechanistic insights into FV early entry steps, demonstrating that productive viral fusion occurs early after target cell attachment and uptake. The analysis highlights apparent differences in the uptake pathways of individual FV species. Furthermore, the infectious double-labeled FVs promise to provide important tools for future detailed analyses on individual FV fusion events in real time using advanced imaging techniques.

Analysis of virus entry and cellular membrane dynamics by single particle tracking

Viruses have evolved to mimic cellular ligands in order to gain access to their host cells for replication. Since viruses are simple in structure, they rely on host cells for all their transportation needs. Following single virus particles during the initial phase of infection, that is, virus entry into target cells, can reveal crucial information on the mechanism of pathogen infections and likewise cellular transport and membrane dynamics. Here, we give an overview on how to fluorescently label virus particles for live cell microscopy, and on how virus entry can be analyzed by single particle tracking experiments. Highlighted are strategies, on how to chemically introduce fluorophores into virions, and on how to extract quantitative information from live cell data.

Nonviral gene delivery vectors use syndecan-dependent transport mechanisms in filopodia to reach the cell surface

Lipoplexes and polyplexes, that is, assemblies of cationic lipids and polymers with nucleic acids, respectively, are popular nanocarriers for delivery of genes or siRNA into cells for therapeutic or cell biological purposes. Although endocytosis represents a major mechanism for their cellular entry, very little is known about parameters that govern early events in the initial interaction of such delivery devices with the cell surface. Here, we demonstrate that prior to entry, poly- and lipoplexes are captured by thin, actin-rich filopodial extensions, protruding from the cell surface. Subsequent additional recruitment and local clustering of filopodia-localized syndecans, presumably driven by multivalent interactions with the polycationic nanocarriers, appear instrumental in their processing to the cell body. Detailed microscopic analyses reveal that the latter relies on either directional surfing along or retraction of the filopodia. By interfering with actin polymerization or inhibiting the motor protein myosin II, localized at the base of filopodia, our data reveal that the binding of the nanocarriers to and subsequent clustering of syndecans initiates actin retrograde flow, which moves the syndecan-bound nanocarriers to the cell body. At the present experimental conditions, inhibition of this process inhibits nanocarrier-mediated transfection by 50-90%. The present findings add novel insight to our understanding of the mechanism of nanocarrier-cell surface interaction, which may be instrumental in further improving delivery efficiency. In addition, the current experimental approach may also be of relevance to improving our understanding of cellular infection by viruses and pathogenic bacteria, given a striking parallel in filopodia-mediated processing of these infectious particles and nanocarriers.

Filopodium retraction is controlled by adhesion to its tip

Filopodia are thin cell extensions sensing the environment. They play an essential role during cell migration, cell-cell or cell-matrix adhesion, by initiating contacts and conveying signals to the cell cortex. Pathogenic microorganisms can hijack filopodia to invade cells by inducing their retraction towards the cell body. Because their dynamics depend on a discrete number of actin filaments, filopodia provide a model of choice to study elementary events linked to adhesion and downstream signalling. However, the determinants controlling filopodial sensing are not well characterized. In this study, we used beads functionalized with different ligands that triggered filopodial retraction when in contact with filopodia of epithelial cells. With optical tweezers, we were able to measure forces stalling the retraction of a single filopodium. We found that the filopodial stall force depends on the coating of the bead. Stall forces reached 8 pN for beads coated with the β1 integrin ligand Yersinia Invasin, whereas retraction was stopped with a higher force of 15 pN when beads were functionalized with carboxyl groups. In all cases, stall forces increased in relation to the density of ligands contacting filopodial tips and were independent of the optical trap stiffness. Unexpectedly, a discrete and small number of Shigella type three secretion systems induced stall forces of 10 pN. These results suggest that the number of receptor-ligand interactions at the filopodial tip determines the maximal retraction force exerted by filopodia but a discrete number of clustered receptors is sufficient to induce high retraction stall forces.

HIV: cell binding and entry

The first step of the human immunodeficiency virus (HIV) replication cycle-binding and entry into the host cell-plays a major role in determining viral tropism and the ability of HIV to degrade the human immune system. HIV uses a complex series of steps to deliver its genome into the host cell cytoplasm while simultaneously evading the host immune response. To infect cells, the HIV protein envelope (Env) binds to the primary cellular receptor CD4 and then to a cellular coreceptor. This sequential binding triggers fusion of the viral and host cell membranes, initiating infection. Revealing the mechanism of HIV entry has profound implications for viral tropism, transmission, pathogenesis, and therapeutic intervention. Here, we provide an overview into the mechanism of HIV entry, provide historical context to key discoveries, discuss recent advances, and speculate on future directions in the field.

Legume lectins inhibit human parainfluenza virus type 2 infection by interfering with the entry

Three lectins with different sugar binding specificities were investigated for anti-viral activity against human parainfluenza virus type 2 (hPIV-2). The lectins, concanavalin A (Con A), lens culinaris agglutinin (LCA) and peanut agglutinin (PNA), inhibited cell fusion and hemadsorption induced by hPIV-2. Virus nucleoprotein (NP) gene synthesis was largely inhibited, but fusion (F) and hemagglutinin-neuraminidase (HN) gene syntheses were not. An indirect immunofluorescence study showed that Con A inhibited virus NP, F and HN protein syntheses, but LCA did not completely inhibit them, and that PNA inhibited only NP protein synthesis. Using a recombinant green fluorescence protein-expressing hPIV-2, without matrix protein (rghPIV-2ΔM), it was found that virus entry into the cells was not completely prevented. The lectins considerably reduced the number of viruses released compared with that of virus infected cells. The lectins bound to cell surface within 10 min, and many aggregates were observed at 30 min. Con A and LCA slightly disrupted actin microfilaments and microtubules, but PNA had almost no effect on them. These results indicated that the inhibitory effects of the lectins were caused mainly by the considerable prevention of virus adsorption to the cells by the lectin binding to their receptors.

In vivo subcellular localization of Mal de Río Cuarto virus (MRCV) non-structural proteins in insect cells reveals their putative functions

The in vivo subcellular localization of Mal de Río Cuarto virus (MRCV, Fijivirus, Reoviridae) non-structural proteins fused to GFP was analyzed by confocal microscopy. P5-1 showed a cytoplasmic vesicular-like distribution that was lost upon deleting its PDZ binding TKF motif, suggesting that P5-1 interacts with cellular PDZ proteins. P5-2 located at the nucleus and its nuclear import was affected by the deletion of its basic C-termini. P7-1 and P7-2 also entered the nucleus and therefore, along with P5-2, could function as regulators of host gene expression. P6 located in the cytoplasm and in perinuclear cloud-like inclusions, was driven to P9-1 viroplasm-like structures and co-localized with P7-2, P10 and α-tubulin, suggesting its involvement in viroplasm formation and viral intracellular movement. Finally, P9-2 was N-glycosylated and located at the plasma membrane in association with filopodia-like protrusions containing actin, suggesting a possible role in virus cell-to-cell movement and spread.