Filopodia and Viruses: An Analysis of Membrane Processes in Entry Mechanisms

Chlamydia exploits filopodial capture and a macropinocytosis-like pathway for host cell entry

Pathogens hijack host endocytic pathways to force their own entry into eukaryotic target cells. Many bacteria either exploit receptor-mediated zippering or inject virulence proteins directly to trigger membrane reorganisation and cytoskeletal rearrangements. By contrast, extracellular C. trachomatis elementary bodies (EBs) apparently employ facets of both the zipper and trigger mechanisms and are only ~400 nm in diameter. Our cryo-electron tomography of C. trachomatis entry revealed an unexpectedly diverse array of host structures in association with invading EBs, suggesting internalisation may progress by multiple, potentially redundant routes or several sequential events within a single pathway. Here we performed quantitative analysis of actin organisation at chlamydial entry foci, highlighting filopodial capture and phagocytic cups as dominant and conserved morphological structures early during internalisation. We applied inhibitor-based screening and employed reporters to systematically assay and visualise the spatio-temporal contribution of diverse endocytic signalling mediators to C. trachomatis entry. In addition to the recognised roles of the Rac1 GTPase and its associated nucleation-promoting factor (NPF) WAVE, our data revealed an additional unrecognised pathway sharing key hallmarks of macropinocytosis: i) amiloride sensitivity, ii) fluid-phase uptake, iii) recruitment and activity of the NPF N-WASP, and iv) the localised generation of phosphoinositide-3-phosphate (PI3P) species. Given their central role in macropinocytosis and affinity for PI3P, we assessed the role of SNX-PX-BAR family proteins. Strikingly, SNX9 was specifically and transiently enriched at C. trachomatis entry foci. SNX9^-/- cells exhibited a 20% defect in EB entry, which was enhanced to 60% when the cells were infected without sedimentation-induced EB adhesion, consistent with a defect in initial EB-host interaction. Correspondingly, filopodial capture of C. trachomatis EBs was specifically attenuated in SNX9^-/- cells, implicating SNX9 as a central host mediator of filopodial capture early during chlamydial entry. Our findings identify an unanticipated complexity of signalling underpinning cell entry by this major human pathogen, and suggest intriguing parallels with viral entry mechanisms.

Chlamydia trachomatis remains the leading bacterial agent of sexually transmitted disease worldwide and causes a form of blindness called trachoma in Developing nations, which is recognised by the World Health Organisation as a neglected tropical disease. Despite this burden, we know comparatively little about how it causes disease at a molecular level. Chlamydia must live inside human cells to survive, and here we study the mechanism of how it enters cells, which is critical to the lifecycle. We study how the bacterium exploits signalling pathways inside the cell to its own advantage to deform the cell membrane by reorganising the underlying cell skeleton, and identify new factors involved in this process. Our findings suggest intriguing similarities with how some viruses enter cells. A better understanding of these processes may help to develop future vaccines and new treatments.

Tunneling Nanotubes as a Novel Route of Cell-to-Cell Spread of Herpesviruses

Various types of intercellular connections that are essential for communication between cells are often utilized by pathogens. Recently, a new type of cellular connection, consisting of long, thin, actin-rich membrane extensions named tunneling nanotubes (TNTs), has been shown to play an important role in cell-to-cell spread of HIV and influenza virus. In the present report, we show that TNTs are frequently formed by cells infected by an alphaherpesvirus, bovine herpesvirus 1 (BoHV-1). Viral proteins, such as envelope glycoprotein E (gE), capsid protein VP26, and tegument protein Us3, as well as cellular organelles (mitochondria) were detected by immunofluorescence and live-cell imaging of nanotubes formed by bovine primary fibroblasts and oropharynx cells (KOP cells). Time-lapse confocal studies of live cells infected with fluorescently labeled viruses showed that viral particles were transmitted via TNTs. This transfer also occurred in the presence of neutralizing antibodies, which prevented free entry of BoHV-1. We conclude that TNT formation contributes to successful cell-to-cell spread of BoHV-1 and demonstrate for the first time the participation of membrane nanotubes in intercellular transfer of a herpesvirus in live cells.IMPORTANCE Efficient transmission of viral particles between cells is an important factor in successful infection by herpesviruses. Herpesviruses can spread by the free-entry mode or direct cell-to-cell transfer via cell junctions and long extensions of neuronal cells. In this report, we show for the first time that an alphaherpesvirus can also spread between various types of cells using tunneling nanotubes, intercellular connections that are utilized by HIV and other viruses. Live-cell monitoring revealed that viral transmission occurs between the cells of the same type as well as between epithelial cells and fibroblasts. This newly discovered route of herpesviruses spread may contribute to efficient transmission despite the presence of host immune responses, especially after reactivation from latency that developed after primary infection. Long-range communication provided by TNTs may facilitate the spread of herpesviruses between many tissues and organs of an infected organism.

Infection and Transport of Herpes Simplex Virus Type 1 in Neurons: Role of the Cytoskeleton

Herpes simplex virus type 1 (HSV-1) is a neuroinvasive human pathogen that has the ability to infect and replicate within epithelial cells and neurons and establish a life-long latent infection in sensory neurons. HSV-1 depends on the host cellular cytoskeleton for entry, replication, and exit. Therefore, HSV-1 has adapted mechanisms to promote its survival by exploiting the microtubule and actin cytoskeletons to direct its active transport, infection, and spread between neurons and epithelial cells during primary and recurrent infections. This review will focus on the currently known mechanisms utilized by HSV-1 to harness the neuronal cytoskeleton, molecular motors, and the secretory and exocytic pathways for efficient virus entry, axonal transport, replication, assembly, and exit from the distinct functional compartments (cell body and axon) of the highly polarized sensory neurons.

The Alphavirus Exit Pathway: What We Know and What We Wish We Knew

Alphaviruses are enveloped positive sense RNA viruses and include serious human pathogens, such as the encephalitic alphaviruses and Chikungunya virus. Alphaviruses are transmitted to humans primarily by mosquito vectors and include species that are classified as emerging pathogens. Alphaviruses assemble highly organized, spherical particles that bud from the plasma membrane. In this review, we discuss what is known about the alphavirus exit pathway during a cellular infection. We describe the viral protein interactions that are critical for virus assembly/budding and the host factors that are involved, and we highlight the recent discovery of cell-to-cell transmission of alphavirus particles via intercellular extensions. Lastly, we discuss outstanding questions in the alphavirus exit pathway that may provide important avenues for future research.

Caveolin 1 restricts Group A Streptococcus invasion of nonphagocytic host cells

Caveolae are composed of 2 major proteins, caveolin 1 (CAV1) and cavin 1 or polymerase transcript release factor I (CAVIN1). Here, we demonstrate that CAV1 levels modulate invasion of Group A Streptococcus (GAS) into nonphagocytic mammalian cells. GAS showed enhanced internalisation into CAV1-knockout mouse embryonic fibroblasts and CAV1 knockdown human epithelial HEp-2 cells, whereas overexpression of CAV1 in HEp-2 cells reduced GAS invasion. This effect was not dependent on the expression of the GAS fibronectin binding protein SfbI, which had previously been implicated in caveolae-mediated uptake. Nor was this effect dependent on CAVIN1, as knockout of CAVIN1 in mouse embryonic fibroblasts resulted in reduced GAS internalisation. Although CAV1 restricted GAS invasion into host cells, we observed only minimal association of invading GAS (strain M1T1⁵⁴⁴⁸ ) with CAV1 by immunofluorescence and very low association of invading M1T1⁵⁴⁴⁸ with caveolae by transmission electron microscopy. These observations suggest that physical interaction with caveolae is not needed for CAV1 restriction of invading GAS. An indirect mechanism of action is also consistent with the finding that changing membrane fluidity reverses the increased invasion observed in CAV1-null cells. Together, these results suggest that CAV1 protects host cells against GAS invasion by a caveola-independent mechanism.

Infectious pancreatic necrosis virus enters CHSE-214 cells via macropinocytosis

Infectious pancreatic necrosis virus (IPNV) is a non-enveloped virus belonging to the Birnaviridae family. IPNV produces an acute disease in salmon fingerlings, with high mortality rates and persistent infection in survivors. Although there are reports of IPNV binding to various cells, the viral receptor and entry pathways remain unknown. The aim of this study was to determine the endocytic pathway that allows for IPNV entry. We observed that IPNV stimulated fluid uptake and virus particles co-localysed with the uptake marker dextran in intracellular compartments, suggesting a role for macropinocytosis in viral entry. Consistent with this idea, viral infection was significantly reduced when the Na+/H+ exchanger NHE1 was inhibited with 5-(N-Ethyl-N-isopropyl) amiloride (EIPA). Neither chlorpromazine nor filipin complex I affected IPNV infection. To examine the role of macropinocytosis regulators, additional inhibitors were tested. Inhibitors of the EGFR pathway and the effectors Pak1, Rac1 and PKC reduced viral infection. Together, our results indicate that IPNV is mainly internalized into CHSE-214 cells by macropinocytosis.

A novel host factor for human respiratory syncytial virus

Human respiratory syncytial virus (RSV) is the leading viral cause of severe lower respiratory disease in young children worldwide. As part of a genome-wide siRNA screen, we recently discovered that actin-related protein 2 (ARP2) is a host factor in the RSV replication cycle. ARP2 is a major constituent of the ARP2/3 complex, which catalyzes actin polymerization involved in cell morphology and motility. In the course of investigating this finding, we also found that RSV infection of human lung epithelial A459 cells induced filopodia formation and stimulated cell motility. The increase in filopodia formation was due, at least in part, to the expression of the RSV fusion F protein. Filopodia formation and increased cell motility appeared to shuttle RSV particles to nearby uninfected cells, facilitating virus cell-to-cell spread. ARP2 depletion did not reduce RSV entry or gene expression early in infection, but reduced subsequent virus production, filopodia formation, cell motility, and viral spread. Thus, the RSV F protein, ARP2-mediated actin nucleation, filopodia formation, and cell mobility all contribute to previously unrecognized mechanisms for RSV cell-to-cell spread that may promote RSV pathogenesis.

Improved synthesis and comparative analysis of the tool properties of new and existing D-ring modified (S)-blebbistatin analogs

(S)-Blebbistatin is a widely used research tool to study myosin II, an important regulator of many motility based diseases. Its potency is too low to be of clinical relevance, but identification of analogs with enhanced potency could deliver leads for targeted pharmacotherapeutics. This, however, requires a profound insight into the structure-activity relationship of the (S)-blebbistatin scaffold. Therefore, new D-ring modified (S)-blebbistatin derivatives were prepared to extend the existing small library of analogs. These molecules were obtained via an improved synthesis pathway and their myosin II inhibitory properties were evaluated in vitro. Finally, all new and known D-ring modified (S)-blebbistatin analogs were compared and the most potent ones underwent a screening of their physicochemical properties.

Actin-Related Protein 2 (ARP2) and Virus-Induced Filopodia Facilitate Human Respiratory Syncytial Virus Spread

Human respiratory syncytial virus (RSV) is an enveloped RNA virus that is the most important viral cause of acute pediatric lower respiratory tract illness worldwide, and lacks a vaccine or effective antiviral drug. The involvement of host factors in the RSV replicative cycle remains poorly characterized. A genome-wide siRNA screen in human lung epithelial A549 cells identified actin-related protein 2 (ARP2) as a host factor involved in RSV infection. ARP2 knockdown did not reduce RSV entry, and did not markedly reduce gene expression during the first 24 hr of infection, but decreased viral gene expression thereafter, an effect that appeared to be due to inhibition of viral spread to neighboring cells. Consistent with reduced spread, there was a 10-fold reduction in the release of infectious progeny virions in ARP2-depleted cells at 72 hr post-infection. In addition, we found that RSV infection induced filopodia formation and increased cell motility in A549 cells and that this phenotype was ARP2 dependent. Filopodia appeared to shuttle RSV to nearby uninfected cells, facilitating virus spread. Expression of the RSV F protein alone from a plasmid or heterologous viral vector in A549 cells induced filopodia, indicating a new role for the RSV F protein, driving filopodia induction and virus spread. Thus, this study identified roles for ARP2 and filopodia in RSV-induced cell motility, RSV production, and RSV cell-to-cell spread.

Tyrosine kinase inhibitor TKI-258 inhibits cell motility in oral squamous cell carcinoma in vitro

BACKGROUND

Oral squamous cell carcinoma is extremely invasive, and this behavior is regulated by binding of extracellular molecules to the cell membrane receptors. The TKI-258 inhibits phosphorylation of FGFRs VEGFRs and PDGFRs. Our aim was to analyze the effect of TKI-258 treatment in cell movement using SCC-4 cell line from human oral squamous cell carcinoma.

METHODS

F-actin was stained with rhodamine phalloidin, and confocal analysis was performed. The migration and invasion (membrane covered with Matrigel^™ ) three-dimensional assays were performed, and control and cells treated with TKI-258 that migrated through the membrane were counted after 24 h.

RESULTS

Control cells presented abundant cytoplasm with F-actin wide distributed and evident cell cortex; however, treated (1, 5 and 10 μM TKI-258) cells showed round morphology, scanty cytoplasm, F-actin disorganized and preserved cell cortex. TKI-258 (1, 5, and 10 μM) treatment inhibits migrating cells (ANOVA, F = 97.749, d.f. = 3, 10; P < 0.0001), and it was concentration dependent. Invading cell treated with 5 μM TKI-258 was significantly lower (t = 6.708, d.f. = 5, P < 0.001).

CONCLUSIONS

These results suggest that the tyrosine kinase inhibitor TKI-258 has an inhibitory effect on cell motility, affecting F-actin, cell migration, and cell invasion, and probably, these processes are regulated by signaling pathways FGFRs and/or PDGFRs and/or VEGFRs.

The missing link: does tunnelling nanotube-based supercellularity provide a new understanding of chronic and lifestyle diseases?

Tunnelling nanotubes (TNTs) are increasingly recognized as central players in a multitude of cellular mechanisms and diseases. Although their existence and functions in animal organisms are still elusive, emerging evidence suggests that they are involved in developmental processes, tissue regeneration, viral infections or pathogen transfer, stem cell differentiation, immune responses as well as initiation and progression of neurodegenerative disorders and cancer (see Sisakhtnezhad & Khosravi 2015 Eur. J. Cell Biol. 94, 429-443. (doi:10.1016/j.ejcb.2015.06.010)). A broader field of vision, including their striking functional and structural resemblance with nanotube-mediated phenomena found throughout the phylogenetic tree, from plants down to bacteria, points to a universal, conserved and tightly regulated mechanism of cellular assemblies. Based on our initial definition of TNTs as open-ended channels mediating membrane continuity between connected cells (Rustom et al. 2004 Science 303, 1007-1010. (doi:10.1126/science.1093133)), it is suggested that animal tissues represent supercellular assemblies that-besides opening discrete communication pathways-balance diverse stress factors caused by pathological changes or fluctuating physiological and environmental conditions, such as oxidative stress or nutrient shortage. By combining current knowledge about nanotube formation, intercellular transfer and communication phenomena as well as associated molecular pathways, a model evolves, predicting that the linkage between reactive oxygen species, TNT-based supercellularity and the intercellular shuttling of materials will have significant impact on diverse body functions, such as cell survival, redox/metabolic homeostasis and mitochondrial heteroplasmy. It implies that TNTs are intimately linked to the physiological and pathological state of animal cells and represent a central joint element of diverse diseases, such as neurodegenerative disorders, diabetes or cancer.