Macropinocytotic uptake and infection of human epithelial cells with species B2 adenovirus type 35

Cellular uptake and fate of cationic polymer-coated nanodiamonds delivering siRNA: a mechanistic study

Gene silencing using small interfering RNAs (siRNAs) is a selective and promising approach for treatment of numerous diseases. However, broad applications of siRNAs are compromised by their low stability in a biological environment and limited ability to penetrate cells. Nanodiamonds (NDs) coated with cationic polymers can enable cellular delivery of siRNAs. Recently, we developed a new type of ND coating based on a random copolymer consisting of (2-dimethylaminoethyl) methacrylate (DMAEMA) and N-(2-hydroxypropyl) methacrylamide (HPMA) monomers. These hybrid ND-polymer particles (Cop+-FND) provide near-infrared fluorescence, form stable complexes with siRNA in serum, show low toxicity, and effectively deliver siRNA into cells in vitro and in vivo. Here, we present data on the mechanism of cellular uptake and cell trafficking of Cop+-FND : siRNA complexes and their ability to selectively suppress mRNA levels, as well as their cytotoxicity, viability and colloidal stability. We identified clathrin-mediated endocytosis as the predominant entry mechanism for Cop+-FND : siRNA into U-2 OS human bone osteosarcoma cells, with a substantial fraction of Cop+-FND : siRNA following the lysosome pathway. Cop+-FND : siRNA potently inhibited the target GAPDH gene with negligible toxicity and sufficient colloidal stability. Based on our results, we suggest that Cop+-FND : siRNA can serve as a suitable in vivo delivery system for siRNA.

NGR-bearing human adenovirus type 5 infects cells in flotillin- or caveolin-mediated manner depending on the NGR insertion site

Human adenoviruses represent attractive candidates for the design of cancer gene therapy vectors. Modification of adenovirus tropism by incorporating a targeting ligand into the adenovirus capsid proteins allows retargeting of adenovirus towards the cells of interest. Human adenovirus type 5 (HAdV-C5) bearing NGR containing peptide (CNGRCVSGCAGRC) inserted into the fiber (AdFNGR) or the hexon (AdHNGR) protein demonstrated an increased transduction of endothelial cells showing expression of aminopeptidase N, also known as CD13, and αvβ3 integrin both present on tumor vasculature, indicating that NGR-bearing adenoviruses could be used as tools for anti-angiogenic cancer therapy. Here we investigated how AdFNGR and AdHNGR infect cells lacking HAdV-C5 primary receptor, coxsackie and adenovirus receptor, and we showed that both AFNGR and AdHNGR enter cells by dynamin- and lipid raft-mediated endocytosis, while clathrin is not required for endocytosis of these viruses. We present evidence that productive infection of both AdFNGR and AdHNGR involves lipid rafts, with usage of flotillin-mediated cell entry for AdFNGR and limited role of caveolin in AdHNGR transduction efficiency. Lipid rafts play important role in angiogenesis and process of metastasis. Therefore, the ability of AdFNGR and AdHNGR to use lipid raft-dependent endocytosis, involving respectively flotillin- or caveolin-mediated pathway, could give them an advantage in targeting tumor cells lacking HAdV-C5 primary receptor.

Toward the understanding of DSG2 and CD46 interaction with HAdV-11 fiber, a super-complex analysis

IMPORTANCE

The main limitation of oncolytic vectors is neutralization by blood components, which prevents intratumoral administration to patients. Enadenotucirev, a chimeric HAdV-11p/HAdV-3 adenovirus identified by bio-selection, is a low seroprevalence vector active against a broad range of human carcinoma cell lines. At this stage, there's still some uncertainty about tropism and primary receptor utilization by HAdV-11. However, this information is very important, as it has a direct influence on the effectiveness of HAdV-11-based vectors. The aim of this work is to determine which of the two receptors, DSG2 and CD46, is involved in the attachment of the virus to the host, and what role they play in the early stages of infection.

Mechanistic-Based Classification of Endocytosis-Related Inhibitors: Does It Aid in Assigning Drugs against SARS-CoV-2?

Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) canonically utilizes clathrin-mediated endocytosis (CME) and several other endocytic mechanisms to invade airway epithelial cells. Endocytic inhibitors, particularly those targeting CME-related proteins, have been identified as promising antiviral drugs. Currently, these inhibitors are ambiguously classified as chemical, pharmaceutical, or natural inhibitors. However, their varying mechanisms may suggest a more realistic classification system. Herein, we present a new mechanistic-based classification of endocytosis inhibitors, in which they are segregated among four distinct classes including: (i) inhibitors that disrupt endocytosis-related protein-protein interactions, and assembly or dissociation of complexes; (ii) inhibitors of large dynamin GTPase and/or kinase/phosphatase activities associated with endocytosis; (iii) inhibitors that modulate the structure of subcellular components, especially the plasma membrane, and actin; and (iv) inhibitors that cause physiological or metabolic alterations in the endocytosis niche. Excluding antiviral drugs designed to halt SARS-CoV-2 replication, other drugs, either FDA-approved or suggested through basic research, could be systematically assigned to one of these classes. We observed that many anti-SARS-CoV-2 drugs could be included either in class III or IV as they interfere with the structural or physiological integrity of subcellular components, respectively. This perspective may contribute to our understanding of the relative efficacy of endocytosis-related inhibitors and support the optimization of their individual or combined antiviral potential against SARS-CoV-2. However, their selectivity, combined effects, and possible interactions with non-endocytic cellular targets need more clarification.

Fowl adenovirus serotype 4 enters leghorn male hepatocellular cells via the clathrin-mediated endocytosis pathway

Hepatitis-hydropericardium syndrome (HHS) induced by fowl adenovirus serotype-4 (FAdV-4) has caused large economic losses to the world poultry industry in recent years. HHS is characterized by pericardial effusion and hepatitis, manifesting as a swollen liver with focal necroses and petechial haemorrhage. However, the process of FAdV-4 entry into hepatic cells remains largely unknown. In this paper, we present a comprehensive study on the entry mechanism of FAdV-4 into leghorn male hepatocellular (LMH) cells. We first observed that FAdV-4 internalization was inhibited by chlorpromazine and clathrin heavy chain (CHC) knockdown, suggesting that FAdV-4 entry into LMH cells depended on clathrin. By using the inhibitor dynasore, we showed that dynamin was required for FAdV-4 entry. In addition, we found that FAdV-4 entry was dependent on membrane cholesterol, while neither the knockdown of caveolin nor the inhibition of a tyrosine kinase-based signalling cascade affected FAdV-4 infection. These results suggested that FAdV-4 entry required cholesterol but not caveolae. We also found that macropinocytosis played a role, and phosphatidylinositol 3-kinase (PI3K) was required for FAdV-4 internalization. However, inhibitors of endosomal acidification did not prevent FAdV-4 entry. Taken together, our findings demonstrate that FAdV-4 enters LMH cells through dynamin- and cholesterol-dependent clathrin-mediated endocytosis, accompanied by the involvement of macropinocytosis requiring PI3K. Our work potentially provides insight into the entry mechanisms of other avian adenoviruses.

Human Adenovirus Type 26 Infection Mediated by αvβ3 Integrin Is Caveolin-1-Dependent

Human adenovirus type 26 (HAdV26) has been recognized as a promising platform for vaccine vector development, and very recently vaccine against COVID-19 based on HAdV26 was authorized for emergency use. Nevertheless, basic biology of this virus, namely, pathway which HAdV26 uses to enter the cell, is still insufficiently known. We have shown here that HAdV26 infection of human epithelial cells expressing low amount of αvβ3 integrin involves clathrin and is caveolin-1-independent, while HAdV26 infection of cells with high amount of αvβ3 integrin does not involve clathrin but is caveolin-1-dependent. Thus, this study demonstrates that caveolin-1 is limiting factor in αvβ3 integrin-mediated HAdV26 infection. Regardless of αvβ3 integrin expression, HAdV26 infection involves dynamin-2. Our data provide for the first-time description of HAdV26 cell entry pathway, hence increase our knowledge of HAdV26 infection. Knowing that functionality of adenovirus vector is influenced by its cell entry pathway and intracellular trafficking, our results will contribute to better understanding of HAdV26 immunogenicity and antigen presentation when used as vaccine vector.

IMPORTANCE In order to fulfill its role as a vector, adenovirus needs to successfully deliver its DNA genome to the host nucleus, a process highly influenced by adenovirus intracellular translocation. Thus, cell entry pathway and intracellular trafficking determine functionality of human adenovirus-based vectors. Endocytosis of HAdV26, currently extensively studied as a vaccine vector, has not been described so far. We present here that HAdV26 infection of human epithelial cells with high expression of αvβ3 integrin, one of the putative HAdV26 receptors, is caveolin-1- and partially dynamin-2-dependent. Since caveolin containing domains provide a unique environment for specific signaling events and participate in inflammatory signaling one can imagine that directing HAdV26 cell entry toward caveolin-1-mediate pathway might play role in immunogenicity of this virus. Therefore, our results contribute to better understanding of HAdV26 infection pathway, hence, can be helpful in explaining induction of immune response and antigen presentation by HAdV26-based vaccine vector.

Human adenovirus type 26 basic biology and its usage as vaccine vector

Due to their nature, adenoviruses have been recognised as promising candidates for vaccine vector development. Since they mimic natural infection, recombinant adenovirus vectors have been proven as ideal shuttles to deliver foreign transgenes aiming at inducing both humoral and cellular immune response. In addition, a potent adjuvant effect can be exerted due to the adenovirus inherent stimulation of various elements of innate and adaptive immunity. Due to its low seroprevalence in humans as well as induction of favourable immune response to inserted transgene, human adenovirus type 26 (HAdV-D26) has been recognised as a promising platform for vaccine vector development and is studied in number of completed or ongoing clinical studies. Very recently HAdV-D26 based Ebola and Covid-19 vaccines were approved for medical use. In this review, current state of the art regarding HAdV-D26 basic biology and its usage as vaccine vector will be discussed.

Revealing macropinocytosis using nanoparticles

Endocytosis mechanisms are one of the methods that cells use to interact with their environments. Endocytosis mechanisms vary from the clathrin-mediated endocytosis to the receptor independent macropinocytosis. Macropinocytosis is a niche of endocytosis that is quickly becoming more relevant in various fields of research since its discovery in the 1930s. Macropinocytosis has several distinguishing factors from other receptor-mediated forms of endocytosis, including: types of extracellular material for uptake, signaling cascade, and niche uses between cell types. Nanoparticles (NPs) are an important tool for various applications, including drug delivery and disease treatment. However, surface engineering of NPs could be tailored to target them inside the cells exploiting different endocytosis pathways, such as endocytosis versus macropinocytosis. Such surface engineering of NPs mainly, size, charge, shape and the core material will allow identification of new adapter molecules regulating different endocytosis process and provide further insight into how cells tweak these pathways to meet their physiological need. In this review, we focus on the description of macropinocytosis, a lesser studied endocytosis mechanism than the conventional receptor mediated endocytosis. Additionally, we will discuss nanoparticle endocytosis (including macropinocytosis), and how the physio-chemical properties of the NP (size, charge, and surface coating) affect their intracellular uptake and exploiting them as tools to identify new adapter molecules regulating these processes.

Purification of infection-associated macropinosomes by magnetic isolation for proteomic characterization

Macropinocytosis refers to the nonselective uptake of extracellular molecules into many different types of eukaryotic cells within large fluid-filled vesicles named macropinosomes. Macropinosomes are relevant for a wide variety of cellular processes, such as antigen sampling in immune cells, homeostasis in the kidney, cell migration or pathogen uptake. Understanding the molecular composition of the different macropinosomes formed during these processes has helped to differentiate their regulations from other endocytic events. Here, we present a magnetic purification protocol that segregates scarce macropinosomes from other endocytic vesicles at a high purity and in a low-cost and unbiased manner. Our protocol takes advantage of moderate-sized magnetic beads of 100 nm in diameter coupled to mass-spectrometry-based proteomic analysis. Passing the cell lysate through a table-top magnet allows the quick retention of the bead-containing macropinosomes. Unlike other cell-fractionation-based methodologies, our protocol minimizes sample loss and production cost without prerequisite knowledge of the macropinosomes and with minimal laboratory experience. We describe a detailed procedure for the isolation of infection-associated macropinosomes during bacterial invasion and the optimization steps to readily adapt it to various studies. The protocol can be performed in 3 d to provide highly purified and enriched macropinosomes for qualitative proteomic composition analysis.

The Revolving Door of Adenovirus Cell Entry: Not All Pathways Are Equal

Adenoviruses represent exceptional candidates for wide-ranging therapeutic applications, from vectors for gene therapy to oncolytics for cancer treatments. The first ever commercial gene therapy medicine was based on a recombinant adenovirus vector, while most recently, adenoviral vectors have proven critical as vaccine platforms in effectively controlling the global coronavirus pandemic. Here, we discuss factors involved in adenovirus cell binding, entry, and trafficking; how they influence efficiency of adenovirus-based vectors; and how they can be manipulated to enhance efficacy of genetically modified adenoviral variants. We focus particularly on endocytosis and how different adenovirus serotypes employ different endocytic pathways to gain cell entry, and thus, have different intracellular trafficking pathways that subsequently trigger different host antiviral responses. In the context of gene therapy, the final goal of the adenovirus vector is to efficiently deliver therapeutic transgenes into the target cell nucleus, thus allowing its functional expression. Aberrant or inefficient endocytosis can impede this goal, therefore, it should be considered when designing and constructing adenovirus-based vectors.

Potential Diagnostic and Prognostic Biomarkers for Adenovirus Respiratory Infection in Children and Young Adults

Human Adenoviruses (HAdV) are known to be potentially associated with strong inflammatory responses and morbidity in pediatric patients. Although most of the primary infections are self-limiting, the severity of clinical presentation, the elevation of the white blood cell count and inflammatory markers often mimic a bacterial infection and lead to an inappropriate use of antibiotics. In infections caused by HAdV, rapid antigen detection kits are advisable but not employed routinely; costs and feasibility of rapid syndromic molecular diagnosis may limit its use in the in-hospital setting; lymphocyte cultures and two-sampled serology are time consuming and impractical when considering the use of antibiotics. In this review, we aim to describe the principal diagnostic tools and the immune response in HAdV infections and evaluate whether markers based on the response of the host may help early recognition of HAdV and avoid inappropriate antimicrobial prescriptions in acute airway infections.

Dynamic interactions and intracellular fate of label-free, thin graphene oxide sheets within mammalian cells: role of lateral sheet size

Graphene oxide (GO) holds great potential for biomedical applications, however fundamental understanding of the way it interacts with biological systems is still lacking even though it is essential for successful clinical translation. In this study, we exploit intrinsic fluorescent properties of thin GO sheets to establish the relationship between lateral dimensions of the material, its cellular uptake mechanisms and intracellular fate over time. Label-free GO with distinct lateral dimensions, small (s-GO) and ultra-small (us-GO) were thoroughly characterised both in water and in biologically relevant cell culture medium. Interactions of the material with a range of non-phagocytic mammalian cell lines (BEAS-2B, NIH/3T3, HaCaT, 293T) were studied using a combination of complementary analytical techniques (confocal microscopy, flow cytometry and TEM). The uptake mechanism was initially interrogated using a range of pharmaceutical inhibitors and validated using polystyrene beads of different diameters (0.1 and 1 μm). Subsequently, RNA-Seq was used to follow the changes in the uptake mechanism used to internalize s-GO flakes over time. Regardless of lateral dimensions, both types of GO were found to interact with the plasma membrane and to be internalized by a panel of cell lines studied. However, s-GO was internalized mainly via macropinocytosis while us-GO was mainly internalized via clathrin- and caveolae-mediated endocytosis. Importantly, we report the shift from macropinocytosis to clathrin-dependent endocytosis in the uptake of s-GO at 24 h, mediated by upregulation of mTORC1/2 pathway. Finally, we show that both s-GO and us-GO terminate in lysosomal compartments for up to 48 h. Our results offer an insight into the mechanism of interaction of GO with non-phagocytic cell lines over time that can be exploited for the design of biomedically-applicable 2D transport systems.

Multifactorial Traits of SARS-CoV-2 Cell Entry Related to Diverse Host Proteases and Proteins

The most effective way to control newly emerging infectious disease, such as the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, is to strengthen preventative or therapeutic public health strategies before the infection spreads worldwide. However, global health systems remain at the early stages in anticipating effective therapeutics or vaccines to combat the SARS-CoV-2 pandemic. While maintaining social distance is the most crucial metric to avoid spreading the virus, symptomatic therapy given to patients on the clinical manifestations helps save lives. The molecular properties of SARS-CoV-2 infection have been quickly elucidated, paving the way to therapeutics, vaccine development, and other medical interventions. Despite this progress, the detailed biomolecular mechanism of SARS-CoV-2 infection remains elusive. Given virus invasion of cells is a determining factor for virulence, understanding the viral entry process can be a mainstay in controlling newly emerged viruses. Since viral entry is mediated by selective cellular proteases or proteins associated with receptors, identification and functional analysis of these proteins could provide a way to disrupt virus propagation. This review comprehensively discusses cellular machinery necessary for SARS-CoV-2 infection. Understanding multifactorial traits of the virus entry will provide a substantial guide to facilitate antiviral drug development.

Key principles and methods for studying the endocytosis of biological and nanoparticle therapeutics

Endocytosis is a critical step in the process by which many therapeutic nanomedicines reach their intracellular targets. Our understanding of cellular uptake mechanisms has developed substantially in the past five years. However, these advances in cell biology have not fully translated to the nanoscience and therapeutics literature. Misconceptions surrounding the role of different endocytic pathways and how to study these pathways are hindering progress in developing improved nanoparticle therapies. Here, we summarize the latest insights into cellular uptake mechanisms and pathways. We highlight limitations of current systems to study endocytosis, particularly problems with non-specific inhibitors. We also summarize alternative genetic approaches to robustly probe these pathways and discuss the need to understand how cells endocytose particles in vivo. We hope that this critical assessment of the current methods used in studying nanoparticle uptake will guide future studies at the interface of cell biology and nanomedicine.

Influenza A Virus Nucleoprotein Activates the JNK Stress-Signaling Pathway for Viral Replication by Sequestering Host Filamin A Protein

Influenza A virus (IAV) poses a major threat to global public health and is known to employ various strategies to usurp the host machinery for survival. Due to its fast-evolving nature, IAVs tend to escape the effect of available drugs and vaccines thus, prompting the development of novel antiviral strategies. High-throughput mass spectrometric screen of host-IAV interacting partners revealed host Filamin A (FLNA), an actin-binding protein involved in regulating multiple signaling pathways, as an interaction partner of IAV nucleoprotein (NP). In this study, we found that the IAV NP interrupts host FLNA-TRAF2 interaction by interacting with FLNA thus, resulting in increased levels of free, displaced TRAF2 molecules available for TRAF2-ASK1 mediated JNK pathway activation, a pathway critical to maintaining efficient viral replication. In addition, siRNA-mediated FLNA silencing was found to promote IAV replication (87% increase) while FLNA-overexpression impaired IAV replication (65% decrease). IAV NP was observed to be a crucial viral factor required to attain FLNA mRNA and protein attenuation post-IAV infection for efficient viral replication. Our results reveal FLNA to be a host factor with antiviral potential hitherto unknown to be involved in the IAV replication cycle thus, opening new possibilities of FLNA-NP interaction as a candidate anti-influenza drug development target.

Entry of Epidemic Keratoconjunctivitis-Associated Human Adenovirus Type 37 in Human Corneal Epithelial Cells

Purpose

Ocular infection by human adenovirus species D type 37 (HAdV-D37) causes epidemic keratoconjunctivitis, a severe, hyperacute condition. The corneal component of epidemic keratoconjunctivitis begins upon infection of corneal epithelium, and the mechanism of viral entry dictates subsequent proinflammatory gene expression. Therefore, it is important to understand the specific pathways of adenoviral entry in these cells.

Methods

Transmission electron microscopy of primary and tert-immortalized human corneal epithelial cells infected with HAdV-D37 was performed to identify the means of viral entry. Confocal microscopy was used to determine intracellular trafficking. The results of targeted small interfering RNA and specific chemical inhibitors were analyzed by quantitative PCR, and Western blot.

Results

By transmission electron microscopy, HAdV-D37 was seen to enter by both clathrin-coated pits and macropinocytosis; however, entry was both pH and dynamin 2 independent. Small interfering RNA against clathrin, AP2A1, and lysosome-associated membrane protein 1, but not early endosome antigen 1, decreased early viral gene expression. Ethyl-isopropyl amiloride, which blocks micropinocytosis, did not affect HAdV-D37 entry, but IPA, an inhibitor of p21-activated kinase, and important to actin polymerization, decreased viral entry in a dose-dependent manner.

Conclusions

HAdV-D37 enters human corneal epithelial cells by a noncanonical clathrin-mediated pathway involving lysosome-associated membrane protein 1 and PAK1, independent of pH, dynamin, and early endosome antigen 1. We showed earlier that HAdV-D37 enters human keratocytes through caveolae. Therefore, epidemic keratoconjunctivitis-associated viruses enter different corneal cell types via disparate pathways, which could account for a relative paucity of proinflammatory gene expression upon infection of corneal epithelial cells compared with keratocytes, as seen in prior studies.

A brain tumor-homing tetra-peptide delivers a nano-therapeutic for more effective treatment of a mouse model of glioblastoma

Organ-specific cell-penetrating peptides (CPPs) are a class of molecules that can be highly effective at delivering therapeutic cargoes, and they are currently of great interest in cancer treatment strategies. Herein, we describe a new CPP (amino acid sequence serine-isoleucine-tyrosine-valine, or SIWV) that homes to glioblastoma multiforme (GBM) brain tumor tissues with remarkable specificity in vitro and in vivo. The SIWV sequence was identified from an isoform of annexin-A3 (AA3H), a membrane-interacting human protein. The mechanism of intracellular permeation is proposed to follow a caveolin-mediated endocytotic pathway, based on in vitro and in vivo receptor inhibition and genetic knockdown studies. Feasibility as a targeting agent for therapeutics is demonstrated in a GBM xenograft mouse model, where porous silicon nanoparticles (pSiNPs) containing the clinically relevant anticancer drug SN-38 are grafted with SIWV via a poly-(ethylene glycol) (PEG) linker. The formulation shows enhanced in vivo targeting ability relative to a formulation employing a scrambled control peptide, and significant (P < 0.05) therapeutic efficacy relative to free SN-38 in the GBM xenograft animal model.

From Cell Entry to Engraftment of Exogenous Mitochondria

Mitochondrial transfer has been recognized to play a role in a variety of processes, ranging from fertilization to cancer and neurodegenerative diseases as well as mammalian horizontal gene transfer. It is achieved through either exogeneous or intercellular mitochondrial transfer. From the viewpoint of evolution, exogeneous mitochondrial transfer is quite akin to the initial process of symbiosis between α-protobacterium and archaea, although the progeny have developed more sophisticated machinery to engulf environmental materials, including nutrients, bacteria, and viruses. A molecular-based knowledge of endocytosis, including macropinocytosis and endosomal escape involving bacteria and viruses, could provide mechanistic insights into exogeneous mitochondrial transfer. We focus on exogeneous mitochondrial transfer in this review to facilitate the clinical development of the use of isolated mitochondria to treat various pathological conditions. Several kinds of novel procedures to enhance exogeneous mitochondrial transfer have been developed and are summarized in this review.

Understanding SARS-CoV-2 endocytosis for COVID-19 drug repurposing

The quest for the effective treatment against coronavirus disease 2019 pneumonia caused by the severe acute respiratory syndrome (SARS)‐coronavirus 2(CoV‐2) coronavirus is hampered by the lack of knowledge concerning the basic cell biology of the infection. Given that most viruses use endocytosis to enter the host cell, mechanistic investigation of SARS‐CoV‐2 infection needs to consider the diversity of endocytic pathways available for SARS‐CoV‐2 entry in the human lung epithelium. Taking advantage of the well‐established methodology of membrane trafficking studies, this research direction allows for the rapid characterisation of the key cell biological mechanism(s) responsible for SARS‐CoV‐2 infection. Furthermore, 11 clinically approved generic drugs are identified as potential candidates for repurposing as blockers of several potential routes for SARS‐CoV‐2 endocytosis. More broadly, the paradigm of targeting a fundamental aspect of human cell biology to protect against infection may be advantageous in the context of future pandemic outbreaks.

Host-Directed Antiviral Therapy

Antiviral drugs have traditionally been developed by directly targeting essential viral components. However, this strategy often fails due to the rapid generation of drug-resistant viruses. Recent genome-wide approaches, such as those employing small interfering RNA (siRNA) or clustered regularly interspaced short palindromic repeats (CRISPR) or those using small molecule chemical inhibitors targeting the cellular "kinome," have been used successfully to identify cellular factors that can support virus replication. Since some of these cellular factors are critical for virus replication, but are dispensable for the host, they can serve as novel targets for antiviral drug development. In addition, potentiation of immune responses, regulation of cytokine storms, and modulation of epigenetic changes upon virus infections are also feasible approaches to control infections. Because it is less likely that viruses will mutate to replace missing cellular functions, the chance of generating drug-resistant mutants with host-targeted inhibitor approaches is minimized. However, drug resistance against some host-directed agents can, in fact, occur under certain circumstances, such as long-term selection pressure of a host-directed antiviral agent that can allow the virus the opportunity to adapt to use an alternate host factor or to alter its affinity toward the target that confers resistance. This review describes novel approaches for antiviral drug development with a focus on host-directed therapies and the potential mechanisms that may account for the acquisition of antiviral drug resistance against host-directed agents.

Reovirus uses macropinocytosis-mediated entry and fast axonal transport to infect neurons

Several barriers protect the central nervous system (CNS) from pathogen invasion. Yet viral infections of the CNS are common and often debilitating. Understanding how neurotropic viruses co-opt host machinery to overcome challenges to neuronal entry and transmission is important to combat these infections. Neurotropic reovirus disseminates through neural routes and invades the CNS to cause lethal encephalitis in newborn animals. To define mechanisms of reovirus neuronal entry and directional transport, we used primary neuron cultures, which reproduce in vivo infection patterns displayed by different reovirus serotypes. Treatment of neurons with small-molecule inhibitors of different endocytic uptake pathways allowed us to discover that the cellular machinery mediating macropinocytosis is required for reovirus neuronal entry. This mechanism of reovirus entry differs from clathrin-mediated endocytosis, which is used by reovirus to invade non-neuronal cells. Analysis of reovirus transport and release from isolated soma or axonal termini of neurons cultivated in microfluidic devices indicates that reovirus is capable of retrograde but only limited anterograde neuronal transmission. The dynamics of retrograde reovirus movement are consistent with fast axonal transport coordinated by dynein along microtubules. Further analysis of viral transport revealed that multiple virions are transported together in axons within non-acidified vesicles. Reovirus-containing vesicles acidify after reaching the soma, where disassembly of virions and release of the viral core into the cytoplasm initiates replication. These results define mechanisms of reovirus neuronal entry and transport and establish a foundation to identify common host factors used by neuroinvasive viruses. Furthermore, our findings emphasize consideration of cell type-specific entry mechanisms in the tailored design of neurotropic viruses as tracers, oncolytic agents, and delivery vectors.

Viral infections of the central nervous system (CNS) cause a significant health burden globally and compel a better mechanistic understanding of neural invasion by viruses to develop effective interventions. Neurotropic reovirus disseminates through neural routes to infect the CNS and serves as a tractable model to study neural invasion by viruses. Despite knowledge of reovirus neurotropism for decades, mechanisms mediating reovirus neuronal infection remain undefined. We used primary neurons cultured in microfluidic devices to study entry and directional transport of reovirus. We discovered that reovirus uses macropinocytosis for neuronal entry as opposed to the use of a clathrin-mediated pathway in non-neuronal cells. We are unaware of another virus using macropinocytosis to enter neurons. Following internalization, reovirus spreads in the retrograde direction using dynein-mediated fast axonal transport but exhibits limited anterograde spread. We further demonstrate that reovirus disassembly and replication occur in the neuronal soma subsequent to axonal transport. Remarkably, these entry and transport mechanisms mirror those used by misfolded proteins implicated in neurodegenerative diseases. Our findings establish the mechanics of reovirus neuronal uptake and spread and provide clues about therapeutic targets to limit neuropathology inflicted by pathogens and misfolded proteins.

Single-Virus Tracking: From Imaging Methodologies to Virological Applications

Uncovering the mechanisms of virus infection and assembly is crucial for preventing the spread of viruses and treating viral disease. The technique of single-virus tracking (SVT), also known as single-virus tracing, allows one to follow individual viruses at different parts of their life cycle and thereby provides dynamic insights into fundamental processes of viruses occurring in live cells. SVT is typically based on fluorescence imaging and reveals insights into previously unreported infection mechanisms. In this review article, we provide the readers a broad overview of the SVT technique. We first summarize recent advances in SVT, from the choice of fluorescent labels and labeling strategies to imaging implementation and analytical methodologies. We then describe representative applications in detail to elucidate how SVT serves as a valuable tool in virological research. Finally, we present our perspectives regarding the future possibilities and challenges of SVT.

Disparate Entry of Adenoviruses Dictates Differential Innate Immune Responses on the Ocular Surface

Human adenovirus infection of the ocular surface is associated with severe keratoconjunctivitis and the formation of subepithelial corneal infiltrates, which may persist and impair vision for months to years following infection. Long term pathology persists well beyond the resolution of viral replication, indicating that the prolonged immune response is not virus-mediated. However, it is not clear how these responses are sustained or even initiated following infection. This review discusses recent work from our laboratory and others which demonstrates different entry pathways specific to both adenovirus and cell type. These findings suggest that adenoviruses may stimulate specific pattern recognition receptors in an entry/trafficking-dependent manner, leading to distinct immune responses dependent on the virus/cell type combination. Additional work is needed to understand the specific connections between adenoviral entry and the stimulation of innate immune responses by the various cell types present on the ocular surface.

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.

Polyvalent Diazonium Polymers Provide Efficient Protection of Oncolytic Adenovirus Enadenotucirev from Neutralizing Antibodies while Maintaining Biological Activity In Vitro and In Vivo

Oncolytic viruses offer many advantages for cancer therapy when administered directly to confined solid tumors. However, the systemic delivery of these viruses is problematic because of the host immune response, undesired interactions with blood components, and inherent targeting to the liver. Efficacy of systemically administered viruses has been improved by masking viral surface proteins with polymeric materials resulting in modulation of viral pharmacokinetic profile and accumulation in tumors in vivo. Here we describe a new class of polyvalent reactive polymer based on poly( N-(2-hydroxypropyl)methacrylamide) (polyHPMA) with diazonium reactive groups and their application in the modification of the chimeric group B oncolytic virus enadenotucirev (EnAd). A series of six copolymers with different chain lengths and density of reactive groups was synthesized and used to coat EnAd. Polymer coating was found to be extremely efficient with concentrations as low as 1 mg/mL resulting in complete (>99%) ablation of neutralizing antibody binding. Coating efficiency was found to be dependent on both chain length and reactive group density. Coated viruses were found to have reduced transfection activity both in vitro and in vivo, with greater protection against neutralizing antibodies resulting in lower transgene production. However, in the presence of neutralizing antibodies, some in vivo transgene expression was maintained for coated virus compared to the uncoated control. The decrease in transgene expression was found not to be solely due to lower cellular uptake but due to reduced unpackaging of the virus within the cells and reduced replication, indicating that the polymer coating does not cause permanent inactivation of the virus. These data suggest that virus activity may be modulated by the appropriate design of coating polymers while retaining protection against neutralizing antibodies.

Targeting p21-activated kinase 1 inhibits growth and metastasis via Raf1/MEK1/ERK signaling in esophageal squamous cell carcinoma cells

Background

p21-activated kinase 1 (PAK1) plays a fundamental role in promoting the development and progression of several cancers and is a potential therapeutic target. However, the biological function and underlying mechanism of PAK1 in esophageal squamous cell carcinoma (ESCC) remain unclear.

Methods

The expression of PAK1 was detected in both ESCC cell lines and clinical samples. Cell growth was measured by MTT, focus formation and soft agar assays. Cell migration and invasion were detected by wound healing and transwell assays. Animal models of subcutaneous tumourigenicity and tail vein metastasis were performed to determine the inhibitory effect of pharmacological inhibitor IPA-3 on tumor growth and metastasis of ESCC cells.

Results

We found that PAK1 was frequently overexpressed in ESCC. Ectopic expression of PAK1 promoted cellular growth, colony formation and anchorage-independent growth. Overexpressing PAK1 also enhanced migration, invasion and the expression of MMP-2 and MMP-9 in ESCC cells. In contrast, silencing PAK1 by lentiviral knockdown or a specific inhibitor IPA-3 resulted in a contrary effect. Subsequent investigations revealed that Raf1/MEK1/ERK signaling pathway was involved in PAK1-mediated effect. Enhanced expression of Raf1 attenuated the inhibitory functions of PAK1 shRNA. Whereas blocking of Raf1 by shRNA or specific inhibition of MEK1 by U0126 antagonized the oncogenetic effect of PAK1 on ESCC cells. More importantly, Pharmacological inhibition of PAK1 by IPA-3 significantly suppressed tumor growth and lung metastasis of ESCC cells in vivo.

Conclusions

These data support that PAK1 is an ideal target for the development of potential therapeutic drugs for ESCC patients even with metastasis.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0343-5) contains supplementary material, which is available to authorized users.

Infectious bronchitis virus entry mainly depends on clathrin mediated endocytosis and requires classical endosomal/lysosomal system

Although several reports suggest that the entry of infectious bronchitis virus (IBV) depends on lipid rafts and low pH, the endocytic route and intracellular trafficking are unclear. In this study, we aimed to shed greater light on early steps in IBV infection. By using chemical inhibitors, RNA interference, and dominant negative mutants, we observed that lipid rafts and low pH was indeed required for virus entry; IBV mainly utilized the clathrin mediated endocytosis (CME) for entry; GTPase dynamin 1 was involved in virus containing vesicle scission; and the penetration of IBV into cells led to active cytoskeleton rearrangement. By using R18 labeled virus, we found that virus particles moved along with the classical endosome/lysosome track. Functional inactivation of Rab5 and Rab7 significantly inhibited IBV infection. Finally, by using dual R18/DiOC labeled IBV, we observed that membrane fusion was induced after 1 h.p.i. in late endosome/lysosome.

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Intact lipid rafts is involved in IBV entry.

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Low pH in intracyplasmic vesicles is required for IBV entry.

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IBV penetrates cells via clathrin mediated endocytosis.

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IBV moves along with the classical endosome/lysosome track, finally fuses with late endosome/lysosome.

αvβ3 Integrin Is Required for Efficient Infection of Epithelial Cells with Human Adenovirus Type 26

Human adenoviruses (HAdVs) are being explored as vectors for gene transfer and vaccination. Human adenovirus type 26 (HAdV26), which belongs to the largest subgroup of adenoviruses, species D, has a short fiber and a so-far-unknown natural tropism. Due to its low seroprevalence, HAdV26 has been considered a promising vector for the development of vaccines. Despite the fact that the in vivo safety and immunogenicity of HAdV26 have been extensively studied, the basic biology of the virus with regard to receptor use, cell attachment, internalization, and intracellular trafficking is poorly understood. In this work, we investigated the roles of the coxsackievirus and adenovirus receptor (CAR), CD46, and αv integrins in HAdV26 infection of human epithelial cell lines. By performing different gain- and loss-of-function studies, we found that αvβ3 integrin is required for efficient infection of epithelial cells by HAdV26, while CAR and CD46 did not increase the transduction efficiency of HAdV26. By studying intracellular trafficking of fluorescently labeled HAdV26 in A549 cells and A549-derived cell clones with stably increased expression of αvβ3 integrin, we observed that HAdV26 colocalizes with αvβ3 integrin and that increased αvβ3 integrin enhances internalization of HAdV26. Thus, we conclude that HAdV26 uses αvβ3 integrin as a receptor for infecting epithelial cells. These results give us new insight into the HAdV26 infection pathway and will be helpful in further defining HAdV-based vector manufacturing and vaccination strategies.IMPORTANCE Adenovirus-based vectors are used today for gene transfer and vaccination. HAdV26 has emerged as a promising candidate vector for development of vaccines due to its relatively low seroprevalence and its ability to induce potent immune responses against inserted transgenes. However, data regarding the basic biology of the virus, like receptor usage or intracellular trafficking, are limited. In this work, we found that efficient infection of human epithelial cell lines by HAdV26 requires the expression of the αvβ3 integrin. By studying intracellular trafficking of fluorescently labeled HAdV26 in a cell clone with stably increased expression of αvβ3 integrin, we observed that HAdV26 colocalizes with αvβ3 integrin and confirmed that αvβ3 integrin expression facilitates efficient HAdV26 internalization. These results will allow further improvement of HAdV26-based vectors for gene transfer and vaccination.

Cluster of Differentiation 46 Is the Major Receptor in Human Blood-Brain Barrier Endothelial Cells for Uptake of Exosomes Derived from Brain-Metastatic Melanoma Cells (SK-Mel-28)

Brain metastasis is a frequent complication of cancer and may be mediated, at least in part, by the internalization of cancer-cell-derived exosomes into brain capillary endothelial cells. Clarifying the mechanism(s) of this internalization is of interest because it could help us to develop ways to block brain metastasis, as well as affording a potential new route for drug delivery into the brain. Therefore, the purpose of the present study was to address this issue by identifying the receptors involved in the internalization of exosomes derived from a brain-metastatic cancer cell line (SK-Mel-28) into human blood-brain barrier endothelial cells (hCMEC/D3 cells). The combination of sulfo-SBED-based cross-linking and comprehensive proteomics yielded 20 proteins as exosome receptor candidates in hCMEC/D3 cells. The uptake of PKH67-labeled exosomes by hCMEC/D3 cells measured at 37 °C was significantly reduced by 95.6% at 4 °C and by 15.3% in the presence of 1 mM RGD peptide, an integrin ligand. Therefore, we focused on the identified RGD receptors, integrin α5 and integrin αV, and CD46, which is reported to act as an adenovirus receptor, together with integrin αV. A mixture of neutralizing antibodies against integrin α5 and integrin αV significantly decreased the exosome uptake by 11.8%, while application of CD46 siRNA reduced it by 39.0%. Immunohistochemical analysis confirmed the presence of CD46 in human brain capillary endothelial cells. These results suggest that CD46 is a major receptor for the uptake of SK-Mel-28-derived exosomes by human blood-brain barrier endothelial cells (hCMEC/D3 cells).

Glycomics and Proteomics Approaches to Investigate Early Adenovirus-Host Cell Interactions

Adenoviruses as most viruses rely on glycan and protein interactions to attach to and enter susceptible host cells. The Adenoviridae family comprises more than 80 human types and they differ in their attachment factor and receptor usage, which likely contributes to the diverse tropism of the different types. In the past years, methods to systematically identify glycan and protein interactions have advanced. In particular sensitivity, speed and coverage of mass spectrometric analyses allow for high-throughput identification of glycans and peptides separated by liquid chromatography. Also, developments in glycan microarray technologies have led to targeted, high-throughput screening and identification of glycan-based receptors. The mapping of cell surface interactions of the diverse adenovirus types has implications for cell, tissue, and species tropism as well as drug development. Here we review known adenovirus interactions with glycan- and protein-based receptors, as well as glycomics and proteomics strategies to identify yet elusive virus receptors and attachment factors. We finally discuss challenges, bottlenecks, and future research directions in the field of non-enveloped virus entry into host cells.

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.

Peste des Petits Ruminants Virus Enters Caprine Endometrial Epithelial Cells via the Caveolae-Mediated Endocytosis Pathway

Peste des petits ruminants virus (PPRV) causes an acute and highly contagious disease of sheep and goats and has spread with alarming speed around the world. The pathology of Peste des petits ruminants is linked to retrogressive changes and necrotic lesions in lymphoid tissues and epithelial cells. However, the process of PPRV entry into host epithelial cells remains largely unknown. Here, we performed a comprehensive study of the entry mechanism of PPRV into caprine endometrial epithelial cells (EECs). We clearly demonstrated that PPRV internalization was inhibited by chloroquine and ammonium chloride, which elevate the pH of various organelles. However, PPRV entry was not affected by chlorpromazine and knockdown of the clathrin heavy chain in EECs. In addition, we found that the internalization of PPRV was dependent on dynamin and membrane cholesterol and was suppressed by silencing of caveolin-1. Macropinocytosis did not play a role, but phosphatidylinositol 3-kinase (PI3K) was required for PPRV internalization. Cell type and receptor-dependent differences indicated that PPRV entry into caprine fetal fibroblast cells (FFCs) occurred via a different route. Taken together, our findings demonstrate that PPRV enters EECs through a cholesterol-dependent caveolae-mediated uptake mechanism that is pH-dependent and requires dynamin and PI3K but is independent of clathrin. This potentially provides insight into the entry mechanisms of other morbilliviruses.

A comparative review of viral entry and attachment during large and giant dsDNA virus infections

Viruses enter host cells via several mechanisms, including endocytosis, macropinocytosis, and phagocytosis. They can also fuse at the plasma membrane and can spread within the host via cell-to-cell fusion or syncytia. The mechanism used by a given viral strain depends on its external topology and proteome and the type of cell being entered. This comparative review discusses the cellular attachment receptors and entry pathways of dsDNA viruses belonging to the families Adenoviridae, Baculoviridae, Herpesviridae and nucleocytoplasmic large DNA viruses (NCLDVs) belonging to the families Ascoviridae, Asfarviridae, Iridoviridae, Phycodnaviridae, and Poxviridae, and giant viruses belonging to the families Mimiviridae and Marseilleviridae as well as the proposed families Pandoraviridae and Pithoviridae. Although these viruses have several common features (e.g., topology, replication and protein sequence similarities) they utilize different entry pathways to infect wide-range of hosts, including humans, other mammals, invertebrates, fish, protozoa and algae. Similarities and differences between the entry methods used by these virus families are highlighted, with particular emphasis on viral topology and proteins that mediate viral attachment and entry. Cell types that are frequently used to study viral entry are also reviewed, along with other factors that affect virus-host cell interactions.

Porcine Hemagglutinating Encephalomyelitis Virus Enters Neuro-2a Cells via Clathrin-Mediated Endocytosis in a Rab5-, Cholesterol-, and pH-Dependent Manner

Porcine hemagglutinating encephalomyelitis virus (PHEV) is a highly neurovirulent coronavirus that invades the central nervous system (CNS) in piglets. Although important progress has been made toward understanding the biology of PHEV, many aspects of its life cycle remain obscure. Here we dissected the molecular mechanism underlying cellular entry and intracellular trafficking of PHEV in mouse neuroblastoma (Neuro-2a) cells. We first performed a thin-section transmission electron microscopy (TEM) assay to characterize the kinetics of PHEV, and we found that viral entry and transfer occur via membranous coating-mediated endo- and exocytosis. To verify the roles of distinct endocytic pathways, systematic approaches were used, including pharmacological inhibition, RNA interference, confocal microscopy analysis, use of fluorescently labeled virus particles, and overexpression of a dominant negative (DN) mutant. Quantification of infected cells showed that PHEV enters cells by clathrin-mediated endocytosis (CME) and that low pH, dynamin, cholesterol, and Eps15 are indispensably involved in this process. Intriguingly, PHEV invasion leads to rapid actin rearrangement, suggesting that the intactness and dynamics of the actin cytoskeleton are positively correlated with viral endocytosis. We next investigated the trafficking of internalized PHEV and found that Rab5- and Rab7-dependent pathways are required for the initiation of a productive infection. Furthermore, a GTPase activation assay suggested that endogenous Rab5 is activated by PHEV and is crucial for viral progression. Our findings demonstrate that PHEV hijacks the CME and endosomal system of the host to enter and traffic within neural cells, providing new insights into PHEV pathogenesis and guidance for antiviral drug design.

IMPORTANCE Porcine hemagglutinating encephalomyelitis virus (PHEV), a nonsegmented, positive-sense, single-stranded RNA coronavirus, invades the central nervous system (CNS) and causes neurological dysfunction. Neural cells are its targets for viral progression. However, the detailed mechanism underlying PHEV entry and trafficking remains unknown. PHEV is the etiological agent of porcine hemagglutinating encephalomyelitis, which is an acute and highly contagious disease that causes numerous deaths in suckling piglets and enormous economic losses in China. Understanding the viral entry pathway will not only advance our knowledge of PHEV infection and pathogenesis but also open new approaches to the development of novel therapeutic strategies. Therefore, we employed systematic approaches to dissect the internalization and intracellular trafficking mechanism of PHEV in Neuro-2a cells. This is the first report to describe the process of PHEV entry into nerve cells via clathrin-mediated endocytosis in a dynamin-, cholesterol-, and pH-dependent manner that requires Rab5 and Rab7.

Measles Virus Enters Breast and Colon Cancer Cell Lines through a PVRL4-Mediated Macropinocytosis Pathway

Measles virus (MeV) is a member of the family Paramixoviridae that causes a highly contagious respiratory disease but has emerged as a promising oncolytic platform. Previous studies of MeV entry focused on the identification of cellular receptors. However, the endocytic and trafficking pathways utilized during MeV entry remain poorly described. The contribution of each endocytic pathway has been examined in cells that express the MeV receptors SLAM (signaling lymphocyte-activating molecule) and PVRL4 (poliovirus receptor-like 4) (nectin-4). Recombinant MeVs expressing either firefly luciferase or green fluorescent protein together with a variety of inhibitors were used. The results showed that MeV uptake was dynamin independent in the Vero.hPVRL4, Vero.hSLAM, and PVRL4-positive MCF7 breast cancer cell lines. However, MeV infection was blocked by 5-(N-ethyl-N-propyl)amiloride (EIPA), the hallmark inhibitor of macropinocytosis, as well as inhibitors of actin polymerization. By using phalloidin staining, MeV entry was shown to induce actin rearrangements and the formation of membrane ruffles accompanied by transient elevated fluid uptake. Small interfering RNA (siRNA) knockdown of p21-activated kinase 1 (PAK1) demonstrated that MeV enters both Vero.hPVRL4 and Vero.hSLAM cells in a PAK1-independent manner using a macropinocytosis-like pathway. In contrast, MeV entry into MCF7 human breast cancer cells relied upon Rac1 and its effector PAK1 through a PVRL4-mediated macropinocytosis pathway. MeV entry into DLD-1 colon and HTB-20 breast cancer cells also appeared to use the same pathway. Overall, these findings provide new insight into the life cycle of MeV, which could lead to therapies that block virus entry or methods that improve the uptake of MeV by cancer cells during oncolytic therapy.IMPORTANCE In the past decades, measles virus (MeV) has emerged as a promising oncolytic platform. Previous studies concerning MeV entry focused mainly on the identification of putative receptors for MeV. Nectin-4 (PVRL4) was recently identified as the epithelial cell receptor for MeV. However, the specific endocytic and trafficking pathways utilized during MeV infections are poorly documented. In this study, we demonstrated that MeV enters host cells via a dynamin-independent and actin-dependent endocytic pathway. Moreover, we show that MeV gains entry into MCF7, DLD-1, and HTB-20 cancer cells through a PVRL4-mediated macropinocytosis pathway and identified the typical cellular GTPase and kinase involved. Our findings provide new insight into the life cycle of MeV, which may lead to the development of therapies that block the entry of the virus into the host cell or alternatively promote the uptake of oncolytic MeV into cancer cells.

Imaging macropinosomes during Shigella infections

Macropinocytosis is the uptake of extracellular fluid within vesicles of varying size that takes place during numerous cellular processes in a large variety of cells. A growing number of pathogens, including viruses, parasites, and bacteria are known to induce macropinocytosis during their entry into targeted host cells. We have recently discovered that the human enteroinvasive, bacterial pathogen Shigella causes in situ macropinosome formation during its entry into epithelial cells. These infection-associated macropinosomes are not generated to ingest the bacteria, but are instead involved in Shigella's intracellular niche formation. They make contacts with the phagocytosed shigellae to promote vacuolar membrane rupture and their cytosolic release. Here, we provide an overview of the different imaging approaches that are currently used to analyze macropinocytosis during infectious processes with a focus on Shigella entry. We detail the advantages and disadvantages of genetically encoded reporters as well as chemical probes to trace fluid phase uptake. In addition, we report how such reporters can be combined with ultrastructural approaches for correlative light electron microscopy either in thin sections or within large volumes. The combined imaging techniques introduced here provide a detailed characterization of macropinosomes during bacterial entry, which, apart from Shigella, are relevant for numerous other ones, including Salmonella, Brucella or Mycobacteria.

Subverting Host Cell P21-Activated Kinase: A Case of Convergent Evolution across Pathogens

Intracellular pathogens have evolved a wide range of strategies to not only escape from the immune systems of their hosts, but also to directly exploit a variety of host factors to facilitate the infection process. One such strategy is to subvert host cell signalling pathways to the advantage of the pathogen. Recent research has highlighted that the human serine/threonine kinase PAK, or p21-activated kinase, is a central component of host-pathogen interactions in many infection systems involving viruses, bacteria, and eukaryotic pathogens. PAK paralogues are found in most mammalian tissues, where they play vital roles in a wide range of functions. The role of PAKs in cell proliferation and survival, and their involvement in a number of cancers, is of great interest in the context of drug discovery. In this review we discuss the latest insights into the surprisingly central role human PAK1 plays for the infection by such different infectious disease agents as viruses, bacteria, and parasitic protists. It is our intention to open serious discussion on the applicability of PAK inhibitors for the treatment, not only of neoplastic diseases, which is currently the primary objective of drug discovery research targeting these enzymes, but also of a wide range of infectious diseases.

Viruses That Exploit Actin-Based Motility for Their Replication and Spread

The actin cytoskeleton is a crucial part of the eukaryotic cell. Viruses depend on host cells for their replication, and, as a result, many have developed ways of manipulating the actin network to promote their spread. This chapter reviews the various ways in which viruses utilize the actin cytoskeleton at discrete steps in their life cycle, from entry into the host cell, replication, and assembly of new progeny to virus release. Various actin inhibitors that function in different ways to affect proper actin dynamics can be used to parse the role of actin at these steps.

African horse sickness virus infects BSR cells through macropinocytosis

Cellular pathways involved in cell entry by African horse sickness virus (AHSV), a member of the Orbivirus genus within the Reoviridae family, have not yet been determined. Here, we show that acidic pH is required for productive infection of BSR cells by AHSV-4, suggesting that the virus is likely internalized by an endocytic pathway. We subsequently analyzed the major endocytic routes using specific inhibitors and determined the consequences for AHSV-4 entry into BSR cells. The results indicated that virus entry is dynamin dependent, but clathrin- and lipid raft/caveolae-mediated endocytic pathways were not used by AHSV-4 to enter and infect BSR cells. Instead, binding of AHSV-4 to BSR cells stimulated uptake of a macropinocytosis-specific cargo and inhibition of Na(+)/H(+) exchangers, actin polymerization and cellular GTPases and kinases involved in macropinocytosis significantly inhibited AHSV-4 infection. Altogether, the data suggest that AHSV-4 infects BSR cells by utilizing macropinocytosis as the primary entry pathway.

Principles of Virus Uncoating: Cues and the Snooker Ball

Viruses are spherical or complex shaped carriers of proteins, nucleic acids and sometimes lipids and sugars. They are metastable and poised for structural changes. These features allow viruses to communicate with host cells during entry, and to release the viral genome, a process known as uncoating. Studies have shown that hundreds of host factors directly or indirectly support this process. The cell provides molecules that promote stepwise virus uncoating, and direct the virus to the site of replication. It acts akin to a snooker player who delivers accurate and timely shots (cues) to the ball (virus) to score. The viruses, on the other hand, trick (snooker) the host, hijack its homeostasis systems, and dampen innate immune responses directed against danger signals. In this review, we discuss how cellular cues, facilitators, and built‐in viral mechanisms promote uncoating. Cues come from receptors, enzymes and chemicals that act directly on the virus particle to alter its structure, trafficking and infectivity. Facilitators are defined as host factors that are involved in processes which indirectly enhance entry or uncoating. Unraveling the mechanisms of virus uncoating will continue to enhance understanding of cell functions, and help counteracting infections with chemicals and vaccines.

Co-option of Membrane Wounding Enables Virus Penetration into Cells

During cell entry, non-enveloped viruses undergo partial uncoating to expose membrane lytic proteins for gaining access to the cytoplasm. We report that adenovirus uses membrane piercing to induce and hijack cellular wound removal processes that facilitate further membrane disruption and infection. Incoming adenovirus stimulates calcium influx and lysosomal exocytosis, a membrane repair mechanism resulting in release of acid sphingomyelinase (ASMase) and degradation of sphingomyelin to ceramide lipids in the plasma membrane. Lysosomal exocytosis is triggered by small plasma membrane lesions induced by the viral membrane lytic protein-VI, which is exposed upon mechanical cues from virus receptors, followed by virus endocytosis into leaky endosomes. Chemical inhibition or RNA interference of ASMase slows virus endocytosis, inhibits virus escape to the cytosol, and reduces infection. Ceramide enhances binding of protein-VI to lipid membranes and protein-VI-induced membrane rupture. Thus, adenovirus uses a positive feedback loop between virus uncoating and lipid signaling for efficient membrane penetration.

Influence of cell physiological state on gene delivery to T lymphocytes by chimeric adenovirus Ad5F35

Adoptive transfer of genetically-modified T cells is a promising approach for treatment of both human malignancies and viral infections. Due to its ability to efficiently infect lymphocytes, the chimeric adenovirus Ad5F35 is potentially useful as an immunotherapeutic for the genetic modification of T cells. In previous studies, it was found that the infection efficiency of Ad5F35 was significantly increased without enhanced expression of the viral receptor after T cell stimulation; however, little is known about the underlying mechanism. Nonetheless, cell physiology has long been thought to affect viral infection. Therefore, we aimed to uncover the physiologic changes responsible for the increased infection efficiency of Ad5F35 following T cell stimulation. Given the complexity of intracellular transport we analyzed viral binding, entry, and escape using a Jurkat T cell model and found that both cell membrane fluidity and endosomal escape of Ad5F35 were altered under different physiological states. This, in turn, resulted in differences in the amount of virus entering cells and reaching the cytoplasm. These results provide additional insight into the molecular mechanisms underlying Ad5F35 infection of T cells and consequently, will help further the clinical application of genetically-modified T cells for immunotherapy.

Productive Entry of Foot-and-Mouth Disease Virus via Macropinocytosis Independent of Phosphatidylinositol 3-Kinase

Virus entry is an attractive target for therapeutic intervention. Here, using a combination of electron microscopy, immunofluorescence assay, siRNA interference, specific pharmacological inhibitors, and dominant negative mutation, we demonstrated that the entry of foot-and-mouth disease virus (FMDV) triggered a substantial amount of plasma membrane ruffling. We also found that the internalization of FMDV induced a robust increase in fluid-phase uptake, and virions internalized within macropinosomes colocalized with phase uptake marker dextran. During this stage, the Rac1-Pak1 signaling pathway was activated. After specific inhibition on actin, Na(+)/H(+) exchanger, receptor tyrosine kinase, Rac1, Pak1, myosin II, and protein kinase C, the entry and infection of FMDV significantly decreased. However, inhibition of phosphatidylinositol 3-kinase (PI3K) did not reduce FMDV internalization but increased the viral entry and infection to a certain extent, implying that FMDV entry did not require PI3K activity. Results showed that internalization of FMDV exhibited the main hallmarks of macropinocytosis. Moreover, intracellular trafficking of FMDV involves EEA1/Rab5-positive vesicles. The present study demonstrated macropinocytosis as another endocytic pathway apart from the clathrin-mediated pathway. The findings greatly expand our understanding of the molecular mechanisms of FMDV entry into cells, as well as provide potential insights into the entry mechanisms of other picornaviruses.

Polyaminoacid–doxorubicin prodrug micelles as highly selective therapeutics for targeted cancer therapy

An amphiphilic copolymer carrying high-dose doxorubicin (21% on a weight basis), PHEA–EDA–P,C–Doxo, was prepared by coupling doxorubicin with a biocompatible polyaminoacid through a pH-sensitive spacer.

Polycaprolactone/maltodextrin nanocarrier for intracellular drug delivery: formulation, uptake mechanism, internalization kinetics, and subcellular localization

Prostate cancer (PCa) disease progression is associated with significant changes in intracellular and extracellular proteins, intracellular signaling mechanism, and cancer cell phenotype. These changes may have direct impact on the cellular interactions with nanocarriers; hence, there is the need for a much-detailed understanding, as nanocarrier cellular internalization and intracellular sorting mechanism correlate directly with bioavailability and clinical efficacy. In this study, we report the differences in the rate and mechanism of cellular internalization of a biocompatible polycaprolactone (PCL)/maltodextrin (MD) nanocarrier system for intracellular drug delivery in LNCaP, PC3, and DU145 PCa cell lines. PCL/MD nanocarriers were designed and characterized. PCL/MD nanocarriers significantly increased the intracellular concentration of coumarin-6 and fluorescein isothiocyanate-labeled bovine serum albumin, a model hydrophobic and large molecule, respectively. Fluorescence microscopy and flow cytometry analysis revealed rapid internalization of the nanocarrier. The extent of nanocarrier cellular internalization correlated directly with cell line aggressiveness. PCL/MD internalization was highest in PC3 followed by DU145 and LNCaP, respectively. Uptake in all PCa cell lines was metabolically dependent. Extraction of endogenous cholesterol by methyl-β-cyclodextrin reduced uptake by 75%±4.53% in PC3, 64%±6.01% in LNCaP, and 50%±4.50% in DU145, indicating the involvement of endogenous cholesterol in cellular internalization. Internalization of the nanocarrier in LNCaP was mediated mainly by macropinocytosis and clathrin-independent pathways, while internalization in PC3 and DU145 involved clathrin-mediated endocytosis, clathrin-independent pathways, and macropinocytosis. Fluorescence microscopy showed a very diffused and non-compartmentalized subcellular localization of the PCL/MD nanocarriers with possible intranuclear localization and minor colocalization in the lysosomes with time.

Signalling at tight junctions during epithelial differentiation and microbial pathogenesis

Tight junctions are a component of the epithelial junctional complex, and they form the paracellular diffusion barrier that enables epithelial cells to create cellular sheets that separate compartments with different compositions. The assembly and function of tight junctions are intimately linked to the actomyosin cytoskeleton and, hence, are under the control of signalling mechanisms that regulate cytoskeletal dynamics. Tight junctions not only receive signals that guide their assembly and function, but transmit information to the cell interior to regulate cell proliferation, migration and survival. As a crucial component of the epithelial barrier, they are often targeted by pathogenic viruses and bacteria, aiding infection and the development of disease. In this Commentary, we review recent progress in the understanding of the molecular signalling mechanisms that drive junction assembly and function, and the signalling processes by which tight junctions regulate cell behaviour and survival. We also discuss the way in which junctional components are exploited by pathogenic viruses and bacteria, and how this might affect junctional signalling mechanisms.

Macropinocytosis is the entry mechanism of amphotropic murine leukemia virus

The entry mechanism of murine amphotropic retrovirus (A-MLV) has not been unambiguously determined. We show here that A-MLV is internalized not by caveolae or other pinocytic mechanisms but by macropinocytosis. Thus, A-MLV infection of mouse embryonic fibroblasts deficient for caveolin or dynamin, and NIH 3T3 cells knocked down for caveolin expression, was unaffected. Conversely, A-MLV infection of NIH 3T3 and HeLa cells was sensitive to amiloride analogues and actin-depolymerizing drugs that interfere with macropinocytosis. Further manipulation of the actin cytoskeleton through conditional expression of dominant positive or negative mutants of Rac1, PAK1, and RhoG, to increase or decrease macropinocytosis, similarly correlated with an augmented or inhibited infection with A-MLV, respectively. The same experimental perturbations affected the infection of viruses that use clathrin-coated-pit endocytosis or other pathways for entry only mildly or not at all. These data agree with immunofluorescence studies and cryo-immunogold labeling for electron microscopy, which demonstrate the presence of A-MLV in protrusion-rich areas of the cell surface and in cortical fluid phase (dextran)-filled macropinosomes, which also account for up to a half of the cellular uptake of the cell surface-binding lectin concanavalin A. We conclude that A-MLV use macropinocytosis as the predominant entry portal into cells.

IMPORTANCE

Binding and entry of virus particles into mammalian cells are the first steps of infection. Understanding how pathogens and toxins exploit or divert endocytosis pathways has advanced our understanding of membrane trafficking pathways, which benefits development of new therapeutic schemes and methods of drug delivery. We show here that amphotropic murine leukemia virus (A-MLV) pseudotyped with the amphotropic envelope protein (which expands the host range to many mammalian cells) gains entry into host cells by macropinocytosis. Macropinosomes form as large, fluid-filled vacuoles (up to 10 μm) following the collapse of cell surface protrusions and membrane scission. We used drugs or the introduction of mutant proteins that affect the actin cytoskeleton and cell surface dynamics to show that macropinocytosis and A-MLV infection are correlated, and we provide both light- and electron-microscopic evidence to show the localization of A-MLV in macropinosomes. Finally, we specifically exclude some other potential entry portals, including caveolae, previously suggested to internalize A-MLV.

Recombinant adeno-associated virus utilizes cell-specific infectious entry mechanisms

Understanding the entry and trafficking mechanism(s) of recombinant adeno-associated virus (rAAV) into host cells can lead to evolution in capsid and vector design and delivery methods, resulting in enhanced transduction and therapeutic gene expression. Variability of findings regarding the early entry pathway of rAAV supports the possibility that rAAV, like other viruses, can utilize more than one infectious entry pathway. We tested whether inhibition of macropinocytosis impacted rAAV transduction of HeLa cells compared to hepatocellular carcinoma cell lines. We found that macropinocytosis inhibitor cytochalasin D blocked rAAV transduction of HeLa cells (>2-fold) but enhanced (10-fold) transduction in HepG2 and Huh7 lines. Similar results were obtained with another macropinocytosis inhibitor, 5-(N-ethyl-N-isopropyl) amiloride (EIPA). The augmented transduction was due to neither viral binding nor promoter activity, affected multiple rAAV serotypes (rAAV2, rAAV2-R585E, and rAAV8), and influenced single-stranded and self-complementary virions to comparable extents. Follow-up studies using CDC42 inhibitor ML141 and p21-activated kinase 1 (PAK1) siRNA knockdown also resulted in enhanced HepG2 transduction. Microscopy revealed that macropinocytosis inhibition correlated with expedited nuclear entry of the rAAV virions into HepG2 cells. Enhancement of hepatocellular rAAV transduction extended to the mouse liver in vivo (4-fold enhancement) but inversely blocked heart tissue transduction (13-fold). This evidence of host cell-specific rAAV entry pathways confers a potent means for controlling and enhancing vector delivery and could help unify the divergent accounts of rAAV cellular entry mechanisms.

IMPORTANCE

There is a recognized need for improved rAAV vector targeting strategies that result in delivery of fewer total particles, averting untoward toxicity and/or an immune response against the vector. A critical step in rAAV transduction is entry and early trafficking through the host cellular machinery, the mechanisms of which are under continued study. However, should the early entry and trafficking mechanisms of rAAV differ across virus serotype or be dependent on host cell environment, this could expand our ability to target particular cells and tissue for selective transduction. Thus, the observation that inhibiting macropinocytosis leads to cell-specific enhancement or inhibition of rAAV transduction that extends to the organismic level exposes a new means of modulating vector targeting.