Kaposi's sarcoma-associated herpesvirus utilizes an actin polymerization-dependent macropinocytic pathway to enter human dermal microvascular endothelial and human umbilical vein endothelial cells

Newcastle disease virus induces clathrin-mediated endocytosis to establish infection through the activation of PI3K/AKT signaling pathway by VEGFR2

The phosphatidyl-inositol 3-kinase/serine-threonine kinase (PI3K/ AKT) signaling pathway constitutes a classical phosphorylation cascade that integrates tyrosine, lipid, and serine acid-threonine phosphorylation, affecting cell function. The pathway is vulnerable to viral infection. Newcastle disease virus (NDV) poses a significant threat to the global poultry industry; however, its mechanism of early viral cell invasion and pathogenesis remain unclear. Previous in vivo and in vitro studies have shown that NDV infection activates PI3K/AKT signaling; however, it remains unclear whether NDV establishes infection through endocytosis regulated by this pathway. This study aimed to examine whether different genotypes of NDV strains could activate the PI3K/AKT signaling pathway within 2 h of in vitro infection. This activation, which relies on PI3K phosphorylation, remains unaffected by the phosphorylation-phosphatase and tensin homolog/phosphatase and tensin homolog (p-PTEN/PTEN) signaling pathway. Moreover, inhibition of PI3K activity impedes NDV replication. Additionally, interfering with the PI3K regulatory subunit p85 has no significant effect on NDV replication. Conversely, the tyrosine kinase activity upstream of PI3K can influence AKT activation and viral replication, particularly through vascular endothelial growth factor receptor 2 (VEGFR2). Additionally, NDV F protein primarily mediates PI3K and AKT phosphorylation to activate the PI3K/AKT signaling pathway. NDV F and VEGFR2 proteins, along with the PI3K p85α subunit, interact and co-localize at the cell membrane. NDV-induced PI3K/AKT signaling pathway activation impacts clathrin-mediated endocytosis, with VEGFR2 playing a pivotal role. In conclusion, this study shows that NDV infection is established early through F protein binding to VEGFR2, activating the PI3K/AKT signaling pathway and inducing clathrin-mediated endocytosis, supporting infection prevention and control measures.

IMPORTANCE

Newcastle disease virus (NDV) is a threat to the global poultry industry; however, the mechanisms of NDV infection remain unclear. NDV affects the phosphatidyl-inositol 3-kinase/serine-threonine kinase (PI3K/ AKT) signaling pathway, requiring endocytosis for successful infection. Based on previous studies, we identified a close correlation between NDV infection and replication and the PI3K/AKT signaling pathway activity. This study examined the molecular mechanisms through which NDV activates the PI3K/AKT signaling pathway to regulate endocytosis and facilitate infection. This study showed that early-stage in vitro NDV infection activated the PI3K/AKT signaling pathway, enhancing clathrin-mediated endocytosis, crucial for infection onset. Notably, this process involves the interaction between NDV F protein and the vascular endothelial growth factor receptor 2 tyrosine kinase, leading to the subsequent binding and phosphorylation of the PI3K p85α regulatory subunit. This activation primes PI3K, initiating a cascade that promotes clathrin-mediated endocytosis. Our findings elucidate how NDV capitalizes on the PI3K/AKT signaling pathway to establish infection through endocytosis.

Macropinocytosis Is the Principal Uptake Mechanism of Antigen-Presenting Cells for Allergen-Specific Virus-like Nanoparticles

Virus-like nanoparticles (VNP) are regarded as efficient vaccination platforms and have proven to be useful for the non-anaphylactogenic delivery of allergen-specific immunotherapy in preclinical models previously. Herein, we sought to determine the mode of VNP uptake by antigen presenting cells (APC). Accordingly, we screened a collection of substances known to inhibit different uptake pathways by APC. The human leukemia monocytic cell line THP-1 and the murine dendritic cell line DC 2.4 were examined for the uptake of fluorescently labelled VNP in the presence or absence of inhibitors. The inhibitory effect of candidate substances that blocked VNP uptake in APC lines was subsequently evaluated in studies with primary APC present in splenocyte and lung cell homogenates in vitro and upon intratracheal application of VNP in vivo. The uptake of allergen-specific VNP in vitro and in vivo was mainly observed by macrophages and CD103+ dendritic cells and was sensitive to inhibitors that block macropinocytosis, such as hyperosmolarity induced by sucrose or the polyphenol compound Rottlerin at low micromolar concentrations but not by other inhibitors. Also, T-cell proliferation induced by allergen-specific VNP was significantly reduced by both substances. In contrast, substances that stimulate macropinocytosis, such as Heparin and phorbol myristate acetate (PMA), increased VNP-uptake and may, thus, help modulate allergen-specific T-cell responses. We have identified macropinocytosis as the principal uptake mechanism of APC for allergen-specific VNP in vitro and in vivo, paving the way for further improvement of VNP-based therapies, especially those that can be used for tolerance induction in allergy, in the future.

Ubiquitination in viral entry and replication: Mechanisms and implications

The ubiquitination process is a reversible posttranslational modification involved in many essential cellular functions, such as innate immunity, cell signaling, trafficking, protein stability, and protein degradation. Viruses can use the ubiquitin system to efficiently enter host cells, replicate and evade host immunity, ultimately enhancing viral pathogenesis. Emerging evidence indicates that enveloped viruses can carry free (unanchored) ubiquitin or covalently ubiquitinated viral structural proteins that can increase the efficiency of viral entry into host cells. Furthermore, viruses continuously evolve and adapt to take advantage of the host ubiquitin machinery, highlighting its importance during virus infection. This review discusses the battle between viruses and hosts, focusing on how viruses hijack the ubiquitination process at different steps of the replication cycle, with a specific emphasis on viral entry. We discuss how ubiquitination of viral proteins may affect tropism and explore emerging therapeutics strategies targeting the ubiquitin system for antiviral drug discovery.

Comprehensive Analysis of the Tegument Proteins Involved in Capsid Transport and Virion Morphogenesis of Alpha, Beta and Gamma Herpesviruses

Herpesviruses are enveloped and have an amorphous protein layer surrounding the capsid, which is termed the tegument. Tegument proteins perform critical functions throughout the viral life cycle. This review provides a comprehensive and comparative analysis of the roles of specific tegument proteins in capsid transport and virion morphogenesis of selected, well-studied prototypes of each of the three subfamilies of Herpesviridae i.e., human herpesvirus-1/herpes simplex virus-1 (Alphaherpesvirinae), human herpesvirus-5/cytomegalovirus (Betaherpesvirinae) and human herpesvirus -8/Kaposi's sarcomavirus (Gammaherpesvirinae). Most of the current knowledge is based on alpha herpesviruses, in particular HSV-1. While some tegument proteins are released into the cytoplasm after virus entry, several tegument proteins remain associated with the capsid and are responsible for transport to and docking at the nucleus. After replication and capsid formation, the capsid is enveloped at the nuclear membrane, which is referred to as primary envelopment, followed by de-envelopment and release into the cytoplasm. This requires involvement of at least three tegument proteins. Subsequently, multiple interactions between tegument proteins and capsid proteins, other tegument proteins and glycoproteins are required for assembly of the virus particles and envelopment at the Golgi, with certain tegument proteins acting as the central hub for these interactions. Some redundancy in these interactions ensures appropriate morphogenesis.

Effective inhibition of HCoV-OC43 and SARS-CoV-2 by phytochemicals in vitro and in vivo

OBJECTIVE

Several coronaviruses, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and human coronavirus OC43 (HCoV-OC43), can cause respiratory infections in humans. To address the need for reliable anti-coronavirus therapeutics, we screened 16 active phytochemicals selected from medicinal plants used in traditional applications for respiratory-related illnesses.

METHODS

An initial screen was completed using HCoV-OC43 to identify compounds that inhibit virus-induced cytopathic effect (CPE) and cell death inhibition. Then the top hits were validated in vitro against both HCoV-OC43 and SARS-CoV-2 by determining virus titer in cell supernatant and virus-induced cell death. Finally, the most active phytochemical was validated in vivo in the SARS-CoV-2-infected B6.Cg-Tg(K18-ACE2)2Prlmn/J mouse model.

RESULTS

The phytochemicals lycorine (LYC), capsaicin, rottlerin (RTL), piperine and chebulinic acid (CHU) inhibited HCoV-OC43-induced cytopathic effect and reduced viral titres by up to 4 log. LYC, RTL and CHU also suppressed virus replication and cell death following SARS-CoV-2 infection. In vivo, RTL significantly reduced SARS-CoV-2-induced mortality by ∼40% in human angiotensin-converting enzyme 2 (ACE2)-expressing K18 mice.

CONCLUSION

Collectively, these studies indicate that RTL and other phytochemicals have therapeutic potential to reduce SARS-CoV-2 and HCoV-OC43 infections.

Neuropilin 1 is an entry receptor for KSHV infection of mesenchymal stem cell through TGFBR1/2-mediated macropinocytosis

Kaposi's sarcoma-associated herpesvirus (KSHV) has been implicated in the pathogenesis of Kaposi's sarcoma (KS) and other malignancies. The cellular origin of KS has been suggested to be either mesenchymal stem cells (MSCs) or endothelial cells. However, receptor(s) for KSHV to infect MSCs remains unknown. By combining bioinformatics analysis and shRNA screening, we identify neuropilin 1 (NRP1) as an entry receptor for KSHV infection of MSCs. Functionally, NRP1 knockout and overexpression in MSCs significantly reduce and promote, respectively, KSHV infection. Mechanistically, NRP1 facilitated the binding and internalization of KSHV by interacting with KSHV glycoprotein B (gB), which was blocked by soluble NRP1 protein. Furthermore, NRP1 interacts with TGF-β receptor type 2 (TGFBR2) through their respective cytoplasmic domains and thus activates the TGFBR1/2 complex, which facilitates the macropinocytosis-mediated KSHV internalization via the small GTPases Cdc42 and Rac1. Together, these findings implicate that KSHV has evolved a strategy to invade MSCs by harnessing NRP1 and TGF-beta receptors to stimulate macropinocytosis.

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.

SARS-CoV-2 hijacks macropinocytosis to facilitate its entry and promote viral spike-mediated cell-to-cell fusion

Revealing the mechanisms of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) entry and cell-to-cell spread might provide insights for understanding the underlying mechanisms of viral pathogenesis, tropism, and virulence. The signaling pathways involved in SARS-CoV-2 entry and viral spike–mediated cell-to-cell fusion remain elusive. In the current study, we found that macropinocytosis inhibitors significantly suppressed SARS-CoV-2 infection at both the entry and viral spike–mediated cell-to-cell fusion steps. We demonstrated that SARS-CoV-2 entry required the small GTPase Rac1 and its effector kinase p21-activated kinase 1 by dominant-negative and RNAi assays in human embryonic kidney 293T–angiotensin-converting enzyme 2 cells and that the serine protease transmembrane serine protease 2 reversed the decrease in SARS-CoV-2 entry caused by the macropinocytosis inhibitors. Moreover, in the cell-to-cell fusion assay, we confirmed that macropinocytosis inhibitors significantly decreased viral spike–mediated cell-to-cell fusion. Overall, we provided evidence that SARS-CoV-2 utilizes a macropinocytosis pathway to enter target cells and to efficiently promote viral spike–mediated cell-to-cell fusion.

Conformational Changes in Herpes Simplex Virus Glycoprotein C

Low endosomal pH facilitates herpesvirus entry in a cell-specific manner. Herpes simplex virus 1 (HSV-1) causes significant morbidity and death in humans worldwide. HSV-1 enters cells by low-pH and neutral-pH pathways. Low-pH-induced conformational changes in the HSV envelope glycoprotein B (gB) may mediate membrane fusion during viral entry. HSV-1 gC, a 511-amino acid, type I integral membrane glycoprotein, mediates HSV-1 attachment to host cell surface glycosaminoglycans, but this interaction is not essential for viral entry. We previously demonstrated that gC regulates low-pH viral entry independent of its known role in cell attachment. Low-pH-triggered conformational changes in gB occur at a lower pH when gC is absent, suggesting that gC positively regulates gB conformational changes. Here, we demonstrate that mildly acidic pH triggers conformational changes in gC itself. Low-pH treatment of virions induced antigenic changes in distinct gC epitopes, and those changes were reversible. One of these gC epitopes is recognized by a monoclonal antibody that binds to a linear sequence that includes residues within gC amino acids 33 to 123. This antibody inhibited low-pH entry of HSV, suggesting that its gC N-terminal epitope is particularly important. We propose that gC plays a critical role in HSV entry through a low-pH endocytosis pathway, which is a major entry route in human epithelial cells. IMPORTANCE Herpesviruses are ubiquitous pathogens that cause lifelong latent infections and are characterized by multiple entry pathways. The HSV envelope gC regulates HSV entry by a low-pH entry route. The fusion protein gB undergoes pH-triggered conformational changes that are facilitated by gC. Here, we report that gC itself undergoes a conformational change at low pH. A monoclonal antibody to gC that binds to a region that undergoes pH-induced changes also selectively inhibits HSV low-pH entry, corroborating the importance of gC in the low-pH entry pathway. This study illustrates the complex role of endosomal pH during HSV entry and provides novel insights into the functions of gC.

Rottlerin plays an antiviral role at early and late steps of Zika virus infection

Infection of Zika virus (ZIKV) may cause microcephaly and other neurological disorders, while no vaccines and drugs are available. Our study revealed that rottlerin confers a broad antiviral activity against several enveloped viruses, including ZIKV, vesicular stomatitis virus, and herpes simplex virus, but not against two naked viruses (enterovirus 71 and encephalomyocarditis virus). Rottlerin does not have a direct virucidal effect on the virions, and its antiviral effect is independent of its regulation on PKCδ or ATP. Both pretreatment and post-treatment of rottlerin effectively reduce the viral replication of ZIKV. The pretreatment of rottlerin disturbs the endocytosis of enveloped viruses, while the post-treatment of rottlerin acts at a late stage through disturbing the maturation of ZIKV. Importantly, administration of rottlerin in neonatal mice significantly decreased the ZIKV replication in vivo, and alleviated the neurological symptoms caused by ZIKV. Our work suggests that rottlerin exerts an antiviral activity at two distinct steps of viral infection, and can be potentially developed as a prophylactic and therapeutic agent.

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Rottlerin confers an antiviral activity against several enveloped viruses including Zika virus.

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Rottlerin interferes with the endocytosis and maturation step of Zika virus.

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Rottlerin inhibits the ZIKV replication in vivo, and alleviates the neurological symptoms caused by Zika virus.

Interferon-Induced Transmembrane Proteins Inhibit Infection by the Kaposi's Sarcoma-Associated Herpesvirus and the Related Rhesus Monkey Rhadinovirus in a Cell-Specific Manner

The interferon-induced transmembrane proteins (IFITMs) are broad-spectrum antiviral proteins that inhibit the entry of enveloped viruses. We analyzed the effect of IFITMs on the gamma-2 herpesviruses Kaposi's sarcoma-associated herpesvirus (KSHV) and the closely related rhesus monkey rhadinovirus (RRV). We used CRISPR/Cas9-mediated gene knockout to generate A549 cells, human foreskin fibroblasts (HFF), and human umbilical vein endothelial cells (HUVEC) with combined IFITM1/2/3 knockout and identified IFITMs as cell-dependent inhibitors of KSHV and RRV infection in A549 cells and HFF but not HUVEC. IFITM overexpression revealed IFITM1 as the relevant IFITM that inhibits KSHV and RRV infection. Fluorescent KSHV particles did not pronouncedly colocalize with IFITM-positive compartments. However, we found that KSHV and RRV glycoprotein-mediated cell-cell fusion is enhanced upon IFITM1/2/3 knockout. Taken together, we identified IFITM1 as a cell-dependent restriction factor of KSHV and RRV that acts at the level of membrane fusion. Of note, our results indicate that recombinant IFITM overexpression may lead to results that are not representative for the situation at endogenous levels. Strikingly, we observed that the endotheliotropic KSHV circumvents IFITM-mediated restriction in HUVEC despite high IFITM expression, while influenza A virus (IAV) glycoprotein-driven entry into HUVEC is potently restricted by IFITMs even in the absence of interferon. Mechanistically, we found that KSHV colocalizes less with IFITM1 and IFITM2 in HUVEC than in A549 cells immediately after attachment, potentially contributing to the observed difference in restriction. IMPORTANCE IFITM proteins are the first line of defense against infection by many pathogens and may also have therapeutic importance, as they, among other effectors, mediate the antiviral effect of interferons. Neither their function against herpesviruses nor their mechanism of action is well understood. We report here that in some cells but not in, for example, primary umbilical vein endothelial cells, IFITM1 restricts KSHV and RRV and that, mechanistically, this is likely effected by reducing the fusogenicity of the cell membrane. Further, we demonstrate potent inhibition of IAV glycoprotein-driven infection of cells of extrapulmonary origin by high constitutive IFITM expression.

Host Sex Steroids Interact With Virus Infection: New Insights Into Sex Disparity in Infectious Diseases

Sex hormones are steroid hormones synthesized from the gonads of animals and tissues such as the placenta and adrenocortical reticular zone. The physiological functions of sex hormones are complex. Sex hormones are not only pathologically correlated with many diseases of the reproductive system, but are etiological factors in some viral infectious diseases, including disease caused by infections of coronaviruses, herpesviruses, hepatitis viruses, and other kinds of human viruses, which either exhibit a male propensity in clinical practice, or crosstalk with androgen receptor (AR)-related pathways in viral pathogenesis. Due to the global pandemic of coronavirus disease 2019 (COVID-19), the role of androgen/AR in viral infectious disease is highlighted again, majorly representing by the recent advances of AR-responsive gene of transmembrane protease/serine subfamily member 2 (TMPRSS2), which proteolytically activates the receptor-mediated virus entry by many coronaviruses and influenza virus, along with the role of androgen-mediated signaling for the transcription of hepatitis B virus (HBV), and the role of sex hormone responsive genes during Zika virus (ZIKV) pathogenesis, et al. Collectively, we propose to provide a comprehensive overview of the role of male sex hormones during multiple phases in the life cycle of different human viruses, which may be partly responsible for the sex-specific prevalence, severity and mortality of some diseases, therefore, may provide clues to develop more efficient prevention and treatment strategies for high-risk populations.

Fixed-Combination Eye Drops Based on Fluorometholone Nanoparticles and Bromfenac/Levofloxacin Solution Improve Drug Corneal Penetration

Purpose

The multi-instillation of three commercially available (CA) eye drops [fluorometholone (FL)-, bromfenac (BF)- and levofloxacin (LV)-eye drops] has been used to manage pain and inflammation post-intraocular surgery. However, the multi-instillation of these three eye drops causes corneal damage, and the FL drops have the disadvantage of low ocular bioavailability. To overcome these problems, we prepared fixed-combination eye drops based on FL nanoparticles (FL-NPs) and BF/LV solution (nFBL-FC), and evaluated the corneal toxicity and transcorneal penetration of the nFBL-FC eye drops.

Methods

FL powder was mixed in 2-hydroxypropyl-β-cyclodextrin solution containing benzalkonium chloride, mannitol and methylcellulose, and milled with a Bead Smash 12 (5500 rpm for 30 s×30 times). The BF/LV solution was then added to the milled-dispersions to be used as nFBL-FC. The FL, BF and LV concentrations were measured by HPLC methods, and transcorneal penetration was evaluated in rabbits.

Results

The FL particle size in nFBL-FC was 40–150 nm, with only 0.0018% in liquid form. No aggregation of FL particles in the nFBL-FC was observed for 1 month. The viability of human corneal epithelial cells treated with nFBL-FC was remarkably higher than that of cells subjected to the multi-instillation of the corresponding three CA-eye drops. In addition, the corneal penetrations (AUC) of the FL, BF and LV in nFBL-FC were 4.9-, 1.8-, and 7.1-fold those of the corresponding CA-eye drops, respectively. Moreover, the caveolae-dependent endocytosis (CavME) inhibitor (nystatin) significantly prevented the transcorneal penetration of these drugs.

Conclusion

We prepared fixed-combination eye drops based on FL-NPs and BF/LV solution (nFBL-FC), and show that high levels of FL-NPs and dissolved BF/LV (liquid drugs) can be delivered into the aqueous humor by the instillation of nFBL-FC. Further, we show that CavME is mainly related to the enhancement of transcorneal penetration of both the solid (NPs) and liquid drugs.

Monocyte chemoattractant protein-induced protein 1 directly degrades viral miRNAs with a specific motif and inhibits KSHV infection

Kaposi's sarcoma-associated herpesvirus (KSHV) expresses miRNAs during latency. However, regulation of viral miRNAs remains largely unknown. Our prior studies demonstrated that MCPIP1 regulates KSHV miRNA biogenesis by degrading most KSHV pre-miRNAs through its RNase activity. Some viral pre-miRNAs are partially resistant to degradation by MCPIP1. Here, we further characterized MCPIP1 substrate specificity and its antiviral potential against KSHV infection. In vitro cleavage assays and binding assays showed that MCPIP1 cleavage efficiency is related to binding affinity. Motif-based sequence analysis identified that KSHV pre-miRNAs that are well degraded by MCPIP1 have a 5-base motif (M5 base motif) within their terminal loops and this motif region consists of multiple pyrimidine-purine-pyrimidine (YRY) motifs. We further demonstrated that mutation of this M5 base motif within terminal loop of pre-miRNAs inhibited MCPIP1-mediated RNA degradation. We also revealed that MCPIP1 has an antiviral effect against KSHV infection. MCPIP1 can reduce the expression of Dicer, which in turn restricts KSHV infection. Conclusively, our findings demonstrated that MCPIP1 inhibited KSHV infection and suppressed viral miRNA biogenesis by directly degrading KSHV pre-miRNAs and altering the expression of miRNA biogenesis factors.

Modulation of Endosome Function, Vesicle Trafficking and Autophagy by Human Herpesviruses

Human herpesviruses are a ubiquitous family of viruses that infect individuals of all ages and are present at a high prevalence worldwide. Herpesviruses are responsible for a broad spectrum of diseases, ranging from skin and mucosal lesions to blindness and life-threatening encephalitis, and some of them, such as Kaposi’s sarcoma-associated herpesvirus (KSHV) and Epstein–Barr virus (EBV), are known to be oncogenic. Furthermore, recent studies suggest that some herpesviruses may be associated with developing neurodegenerative diseases. These viruses can establish lifelong infections in the host and remain in a latent state with periodic reactivations. To achieve infection and yield new infectious viral particles, these viruses require and interact with molecular host determinants for supporting their replication and spread. Important sets of cellular factors involved in the lifecycle of herpesviruses are those participating in intracellular membrane trafficking pathways, as well as autophagic-based organelle recycling processes. These cellular processes are required by these viruses for cell entry and exit steps. Here, we review and discuss recent findings related to how herpesviruses exploit vesicular trafficking and autophagy components by using both host and viral gene products to promote the import and export of infectious viral particles from and to the extracellular environment. Understanding how herpesviruses modulate autophagy, endolysosomal and secretory pathways, as well as other prominent trafficking vesicles within the cell, could enable the engineering of novel antiviral therapies to treat these viruses and counteract their negative health effects.

Macropinocytosis in Different Cell Types: Similarities and Differences

Macropinocytosis is a unique pathway of endocytosis characterised by the nonspecific internalisation of large amounts of extracellular fluid, solutes and membrane in large endocytic vesicles known as macropinosomes. Macropinocytosis is important in a range of physiological processes, including antigen presentation, nutrient sensing, recycling of plasma proteins, migration and signalling. It has become apparent in recent years from the study of specialised cells that there are multiple pathways of macropinocytosis utilised by different cell types, and some of these pathways are triggered by different stimuli. Understanding the physiological function of macropinocytosis requires knowledge of the regulation and fate of the macropinocytosis pathways in a range of cell types. Here, we compare the mechanisms of macropinocytosis in different primary and immortalised cells, identify the gaps in knowledge in the field and discuss the potential approaches to analyse the function of macropinocytosis in vivo.

Towards Understanding KSHV Fusion and Entry

How viruses enter cells is of critical importance to pathogenesis in the host and for treatment strategies. Over the last several years, the herpesvirus field has made numerous and thoroughly fascinating discoveries about the entry of alpha-, beta-, and gamma-herpesviruses, giving rise to knowledge of entry at the amino acid level and the realization that, in some cases, researchers had overlooked whole sets of molecules essential for entry into critical cell types. Herpesviruses come equipped with multiple envelope glycoproteins which have several roles in many aspects of infection. For herpesvirus entry, it is usual that a collective of glycoproteins is involved in attachment to the cell surface, specific interactions then take place between viral glycoproteins and host cell receptors, and then molecular interactions and triggers occur, ultimately leading to viral envelope fusion with the host cell membrane. The fact that there are multiple cell and virus molecules involved with the build-up to fusion enhances the diversity and specificity of target cell types, the cellular entry pathways the virus commandeers, and the final triggers of fusion. This review will examine discoveries relating to how Kaposi's sarcoma-associated herpesvirus (KSHV) encounters and binds to critical cell types, how cells internalize the virus, and how the fusion may occur between the viral membrane and the host cell membrane. Particular focus is given to viral glycoproteins and what is known about their mechanisms of action.

Gammaherpesvirus entry and fusion: A tale how two human pathogenic viruses enter their host cells

The prototypical human γ-herpesviruses Epstein-Barr virus (EBV) and Kaposi Sarcoma-associated herpesvirus (KSHV) are involved in the development of malignancies. Like all herpesviruses, they share the establishment of latency, the typical architecture, and the conserved fusion machinery to initiate infection. The fusion machinery reflects virus-specific adaptations due to the requirements of the respective herpesvirus. For example, EBV evolved a tropism switch involving either the B- or epithelial cell-tropism complexes to activate fusion driven by gB. Most of the EBV entry proteins and their cellular receptors have been crystallized providing molecular details of the initial steps of infection. For KSHV, a variety of entry and binding receptors has also been reported but the mechanism how receptor binding activates gB-driven fusion is not as well understood as that for EBV. However, the downstream signaling pathways that promote the early steps of KSHV entry are well described. This review summarizes the current knowledge of the key players involved in EBV and KSHV entry and the cell-type specific mechanisms that allow infection of a wide variety of cell types.

Promoting Early Diagnosis and Precise Therapy of Hepatocellular Carcinoma by Glypican-3-Targeted Synergistic Chemo-Photothermal Theranostics

The specific-targeting approach could promote the specificity of diagnosis and the accuracy of cancer treatment. The choice of a specific-targeting receptor is the key step in this approach. Glypican-3 (GPC3) is an oncofetal proteoglycan anchored on the cell membrane. It is overexpressed even in the early stage of hepatocellular carcinoma (HCC), whereas it shows almost no expression in the healthy adult liver. Therefore, GPC3 may be applied as a specific-targeting receptor for HCC theranostics. In this study, a GPC3 specific-targeting theranostics nanodevice, GPC3 targeting peptide (named G12)-modified liposomes co-loaded with sorafenib (SF) and IR780 iodide (IR780), was developed (GSI-Lip), which aims to realize early diagnosis and precise chemo-photothermal therapy of HCC. SF was the first-line chemotherapy drug for the treatment of HCC. IR780 was used for photothermal therapy and near-infrared fluorescence imaging. The evaluation of early diagnosis verified that early-stage tumors (3.45 ± 0.98 mm³, 2 days after 5 × 10⁵ H22 cells' inoculation in mice) could be clearly detected using GSI-Lip, which was significantly more sensitive than folic acid-modified liposomes ( p < 0.01, 32.90 ± 10.01 mm³, 4 days after 1 × 10⁶ H22 cells' inoculation in mice). The study of the endocytic pathway indicated that specific G12/GPC3 recognition may induce caveolae-mediated endocytosis of GSI-Lip. Notably, the accumulation of GSI-Lip in tumors was significantly increased compared with that observed with folic acid-modified liposomes ( p < 0.01). Specific-targeting endowed the precise antitumor effect of GSI-Lip. GSI-Lip showed a higher antitumor efficacy in comparison with folic acid-modified liposomes (inhibition rate: 90.52% vs 84.22%, respectively; p < 0.01). During a period of 21 days, the synergistic chemo-photothermal therapy (GSI-Lip + laser) exhibited a better antitumor effect versus GSI-Lip without laser (inhibition rate: 94.93% vs 90.52%, respectively; p < 0.01). Overall, GPC3-targeted GSI-Lip promoted the sensitivity and specificity of HCC early diagnosis and achieved synergistic efficacy of chemo-photothermal theranostics, which has potential clinical applications. Furthermore, the present study revealed that a more specific-targeting ligand could further improve the efficacy of theranostics against HCC.

Molecular Biology of KSHV in Relation to HIV/AIDS-Associated Oncogenesis

Discovered in 1994, Kaposi's sarcoma-associated herpesvirus (KSHV) has been associated with four human malignancies including Kaposi's sarcoma, primary effusion lymphoma, a subset of multicentric Castleman's disease, and KSHV inflammatory cytokine syndrome. These malignancies mostly occur in immunocompromised patients including patients with acquired immunodeficiency syndrome and often cause significant mortality because of the lack of effective therapies. Significant progresses have been made to understand the molecular basis of KSHV infection and KSHV-induced oncogenesis in the last two decades. This chapter provides an update on the recent advancements focusing on the molecular events of KSHV primary infection, the mechanisms regulating KSHV life cycle, innate and adaptive immunity, mechanism of KSHV-induced tumorigenesis and inflammation, and metabolic reprogramming in KSHV infection and KSHV-transformed cells.

Kinase inhibitors tyrphostin 9 and rottlerin block early steps of rabies virus cycle

Rabies virus (RABV) is a neurotropic virus that causes fatal encephalitis in humans and animals and still kills up to 59,000 people worldwide every year. To date, only preventive or post-exposure vaccination protects against the disease but therapeutics are missing. After screening a library of 80 kinases inhibitors, we identified two compounds as potent inhibitors of RABV infection: tyrphostin 9 and rottlerin. Mechanism of action studies show that both inhibitors interfere with an early step of viral cycle and can prevent viral replication. In presence of tyrphostin 9, the viral entry through endocytosis is disturbed leading to improper delivery of viral particles in cytoplasm, whereas rottlerin is inhibiting the transcription, most likely by decreasing intracellular ATP concentration, and therefore the replication of the viral genome.

Energy-dependent endocytosis is responsible for drug transcorneal penetration following the instillation of ophthalmic formulations containing indomethacin nanoparticles

Purpose

We previously found that ophthalmic formulations containing nanoparticles prepared by a bead mill method lead to an increase in bioavailability in comparison with traditional formulations (solution type). However, the transcorneal penetration pathway for ophthalmic formulations has not been explained yet. In this study, we investigated the mechanism of transcorneal penetration in the application of ophthalmic formulations containing indomethacin nanoparticles (IMC-NPs).

Materials and methods

IMC-NPs was prepared by the bead mill method. For the analysis of energy-dependent endocytosis, corneal epithelial (HCE-T) cell monolayers and removed rabbit cornea were thermoregulated at 4°C, where energy-dependent endocytosis is inhibited. In addition, for the analysis of different endocytosis pathways using pharmacological inhibitors, inhibitors of caveolae-mediated endocytosis (54 µM nystatin), clathrin-mediated endocytosis (40 µM dynasore), macropinocytosis (2 µM rottlerin) or phagocytosis (10 µM cytochalasin D) were used.

Results

The ophthalmic formulations containing 35–200 nm sized indomethacin nanoparticles were prepared by treatment with a bead mill, and no aggregation or degradation of indomethacin was observed in IMC-NPs. The transcorneal penetration of indomethacin was significantly decreased by the combination of nystatin, dynasore and rottlerin, and the decreased penetration levels were similar to those at 4°C in HCE-T cell monolayers and rabbit cornea. In the in vivo experiments using rabbits, dynasore and rottlerin tended to decrease the transcorneal penetration of indomethacin (area under the drug concentration – time curve in the aqueous humor [AUC_AH]), and the AUC_AH in the nystatin-treated rabbit was significantly lower than that in non-treatment group. In addition, the AUC_AH in rabbit corneas undergoing multi-treatment was obviously lower than that in rabbit corneas treated with each individual endocytosis inhibitor.

Conclusion

We found that three energy-dependent endocytosis pathways (clathrin-dependent endocytosis, caveolae-dependent endocytosis and macropinocytosis) are related to the trans-corneal penetration of indomethacin nanoparticles. In particular, the caveolae-dependent endocytosis is strongly involved.

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.

PKCδ stimulates macropinocytosis via activation of SSH1-cofilin pathway

Macropinocytosis is an actin-dependent endocytic mechanism mediating internalization of extracellular fluid and associated solutes into cells. The present study was designed to identify the specific protein kinase C (PKC) isoform(s) and downstream effectors regulating actin dynamics during macropinocytosis. We utilized various cellular and molecular biology techniques, pharmacological inhibitors and genetically modified mice to study the signaling mechanisms mediating macropinocytosis in macrophages. The qRT-PCR experiments identified PKCδ as the predominant PKC isoform in macrophages. Scanning electron microscopy and flow cytometry analysis of FITC-dextran internalization demonstrated the functional role of PKCδ in phorbol ester- and hepatocyte growth factor (HGF)-induced macropinocytosis. Western blot analysis demonstrated that phorbol ester and HGF stimulate activation of slingshot phosphatase homolog 1 (SSH1) and induce cofilin Ser-3 dephosphorylation via PKCδ in macrophages. Silencing of SSH1 inhibited cofilin dephosphorylation and macropinocytosis stimulation. Interestingly, we also found that incubation of macrophages with BMS-5, a potent inhibitor of LIM kinase, does not stimulate macropinocytosis. In conclusion, the findings of the present study demonstrate a previously unidentified mechanism by which PKCδ via activation of SSH1 and cofilin dephosphorylation stimulates membrane ruffle formation and macropinocytosis. The results of the present study may contribute to a better understanding of the regulatory mechanisms during macrophage macropinocytosis.

Soft-shelled turtle iridovirus enters cells via cholesterol-dependent, clathrin-mediated endocytosis as well as macropinocytosis

Ranaviruses are nucleoplasmic large DNA viruses that can cause major economic losses in the aquaculture industry and pose a severe threat to global ecological diversity. The available literature demonstrates that classifiable members of the genus Ranavirus enter cells via multiple and complicated routes. Here, we demonstrated the underlying cellular entry mechanism of soft-shelled turtle iridovirus (STIV) using green fluorescence tagged recombinant virus. Treatment with chlorpromazine, sucrose, ethyl-isopropyl amiloride, chloroquine or bafilomycin A1 all significantly decreased STIV infection, suggesting that STIV uses clathrin-mediated endocytosis and macropinocytosis to enter cells via a pH-dependent pathway. Depletion of cellular cholesterol with methyl-β-cyclodextrin significantly inhibited STIV entry, but neither filipin III nor nystatin did, suggesting that STIV entry was cholesterol dependent but caveola independent. Treatment with dynasore, genistein, ML-7 or cytochalasin D all significantly inhibited STIV infection, indicating that Rac GTPase and myosin II activity were required for the macropinocytosis-like pathway as well as actin polymerization. Our findings suggest that the molecular events involved in STIV entry are not identical to those of other ranavirus isolates. Our results also extend our understanding of the molecular mechanism of iridovirus entry and pathogenesis.

Exploitation of Cytoskeletal Networks during Early Viral Infection

Being dependent upon host transport systems to navigate the cytoplasm, viruses have evolved various strategies to manipulate cytoskeletal functions. Generally, viruses use the actin cytoskeleton to control entry and short-range transport at the cell periphery and exploit microtubules (MTs) for longer-range cytosolic transport, in some cases to reach the nucleus. While earlier studies established the fundamental importance of these networks to successful infection, the mechanistic details and true extent to which viruses usurp highly specialized host cytoskeletal regulators and motor adaptors is only beginning to emerge. This review outlines our current understanding of how cytoskeletal regulation contributes specifically to the early stages of viral infection, with a primary focus on retroviruses and herpesviruses as examples of recent advances in this area.

Targetable Mesoporous Silica Nanoprobes for Mapping the Subcellular Distribution of H2Se in Cancer Cells

Hydrogen selenide, a highly active reductant, is believed as a key molecule in the cytotoxicity of inorganic selenium compounds. However, the detail mechanism has hardly been studied because the distribution of H₂Se in the subcellular organelles remains unclear. Herein, we exploited a series of novel targetable mesoporous silica nanoplatforms to map the distribution of H₂Se in cytoplasm, lysosome, and mitochondria of cancer cells. The subcellular targeting moiety-conjugated mesoporous silica nanoparticles were assembled with a near-infrared fluorescent probe (NIR-H₂Se) for detecting endogenous H₂Se in the corresponding organelles. The confocal fluorescence imaging of cancer cells induced by Na₂SeO₃ found out a higher concentration of H₂Se accumulated only in mitochondria. Consequently, the H₂Se burst in mitochondria-triggered mitochondrial collapse that led to cell apoptosis. Hence, the selenite-induced cytotoxicity in cancer cells associates with the alteration in mitochondrial function caused by high level of H₂Se. These findings provide a new way to explore the tumor cell apoptosis signaling pathways induced by Na₂SeO₃, meanwhile, we propose a research strategy for tracking the biomolecules in the subcellular organelles and the correlative cellular function and related disease diagnosis.

PKCδ-Mediated Nox2 Activation Promotes Fluid-Phase Pinocytosis of Antigens by Immature Dendritic Cells

Aims

Macropinocytosis is a major endocytic pathway by which dendritic cells (DCs) internalize antigens in the periphery. Despite the importance of DCs in the initiation and control of adaptive immune responses, the signaling mechanisms mediating DC macropinocytosis of antigens remain largely unknown. The goal of the present study was to investigate whether protein kinase C (PKC) is involved in stimulation of DC macropinocytosis and, if so, to identify the specific PKC isoform(s) and downstream signaling mechanisms involved.

Methods

Various cellular, molecular and immunological techniques, pharmacological approaches and genetic knockout mice were utilized to investigate the signaling mechanisms mediating DC macropinocytosis.

Results

Confocal laser scanning microscopy confirmed that DCs internalize fluorescent antigens (ovalbumin) using macropinocytosis. Pharmacological blockade of classical and novel PKC isoforms using calphostin C abolished both phorbol ester- and hepatocyte growth factor-induced antigen macropinocytosis in DCs. The qRT-PCR experiments identified PKCδ as the dominant PKC isoform in DCs. Genetic studies demonstrated the functional role of PKCδ in DC macropinocytosis of antigens, their subsequent maturation, and secretion of various T-cell stimulatory cytokines, including IL-1α, TNF-α and IFN-β. Additional mechanistic studies identified NADPH oxidase 2 (Nox2) and intracellular superoxide anion as important players in DC macropinocytosis of antigens downstream of PKCδ activation.

Conclusion

The findings of the present study demonstrate a novel mechanism by which PKCδ activation via stimulation of Nox2 activity and downstream redox signaling promotes DC macropinocytosis of antigens. PKCδ/Nox2-mediated antigen macropinocytosis stimulates maturation of DCs and secretion of T-cell stimulatory cytokines. These findings may contribute to a better understanding of the regulatory mechanisms in DC macropinocytosis and downstream regulation of T-cell-mediated responses.

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.

A Conserved Leucine Zipper Motif in Gammaherpesvirus ORF52 Is Critical for Distinct Microtubule Rearrangements

Productive viral infection often depends on the manipulation of the cytoskeleton. Herpesviruses, including rhesus monkey rhadinovirus (RRV) and its close homolog, the oncogenic human gammaherpesvirus Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 (KSHV/HHV8), exploit microtubule (MT)-based retrograde transport to deliver their genomes to the nucleus. Subsequently, during the lytic phase of the life cycle, the maturing viral particles undergo orchestrated translocation to specialized regions within the cytoplasm, leading to tegumentation, secondary envelopment, and then egress. As a result, we hypothesized that RRV might induce changes in the cytoskeleton at both early and late stages of infection. Using confocal imaging, we found that RRV infection led to the thickening and acetylation of MTs emanating from the MT-organizing center (MTOC) shortly after viral entry and more pronounced and diffuse MT reorganization during peak stages of lytic gene expression and virion production. We subsequently identified open reading frame 52 (ORF52), a multifunctional and abundant tegument protein, as being the only virally encoded component responsible for these cytoskeletal changes. Mutational and modeling analyses indicated that an evolutionarily conserved, truncated leucine zipper motif near the N terminus as well as a strictly conserved arginine residue toward the C terminus of ORF52 play critical roles in its ability to rearrange the architecture of the MT cytoskeleton. Taken together, our findings combined with data from previous studies describing diverse roles for ORF52 suggest that it likely binds to different cellular components, thereby allowing context-dependent modulation of function.

IMPORTANCE A thorough understanding of the processes governing viral infection includes knowledge of how viruses manipulate their intracellular milieu, including the cytoskeleton. Altering the dynamics of actin or MT polymerization, for example, is a common strategy employed by viruses to ensure efficient entry, maturation, and egress as well as the avoidance of antiviral defenses through the sequestration of key cellular factors. We found that infection with RRV, a homolog of the human pathogen KSHV, led to perinuclear wrapping by acetylated MT bundles and identified ORF52 as the viral protein underlying these changes. Remarkably, incoming virions were able to supply sufficient ORF52 to induce MT thickening and acetylation near the MTOC, potentially aiding in the delivery viral genomes to the nucleus. Although the function of MT alterations during late stages of infection requires further study, ORF52 shares functional and structural similarities with alphaherpesvirus VP22, underscoring the evolutionary importance of MT cytoskeletal manipulations for this virus family.

Self-assembled α-Tocopherol Transfer Protein Nanoparticles Promote Vitamin E Delivery Across an Endothelial Barrier

Vitamin E is one of the most important natural antioxidants, protecting polyunsaturated fatty acids in the membranes of cells. Among different chemical isoforms assimilated from dietary regimes, RRR-α-tocopherol is the only one retained in higher animals. This is possible thanks to α-Tocopherol Transfer Protein (α-TTP), which extracts α-tocopherol from endosomal compartments in liver cells, facilitating its distribution into the body. Here we show that, upon binding to its substrate, α-TTP acquires tendency to aggregation into thermodynamically stable high molecular weight oligomers. Determination of the structure of such aggregates by X-ray crystallography revealed a spheroidal particle formed by 24 protein monomers. Oligomerization is triggered by refolding of the N-terminus. Experiments with cultured cell monolayers demonstrate that the same oligomers are efficiently transported through an endothelial barrier (HUVEC) and not through an epithelial one (Caco-2). Discovery of a human endogenous transport protein with intrinsic capability of crossing endothelial tissues opens to new ways of drug delivery into the brain or other tissues protected by endothelial barriers.

A preliminary study on the interaction between Asn-Gly-Arg (NGR)-modified multifunctional nanoparticles and vascular epithelial cells

Previously developed Asn-Gly-Arg (NGR) peptide-modified multifunctional poly(ethyleneimine)–poly(ethylene glycol) (PEI–PEG)-based nanoparticles (TPIC) have been considered to be promising carriers for the co-delivery of DNA and doxorubicin (DOX). As a continued effort, the aim of the present study was to further evaluate the interaction between TPIC and human umbilical vein endothelial cells (HUVEC) to better understand the cellular entry mechanism. In the present investigation, experiments relevant to co-localization, endocytosis inhibitors and factors influencing the internalization were performed. Without any treatment, there was no co-localization between aminopeptidase N/CD13 (APN/CD13) and caveolin 1 (CAV1). However, co-localization between CD13 and CAV1 was observed when cells were incubated with an anti-CD13 antibody or TPIC. As compared with antibody treatment, TPIC accelerated the speed and enhanced the degree of co-localization. TPIC entered HUVEC not only together with CD13 but also together with CAV1. However, this internalization was not dependent on the enzyme activity of CD13 but could be inhibited by methyl-β-eyclodextfin (MβCD), further identifying the involvement of caveolae-mediated endocytosis (CvME). This conclusion was also verified by endocytosis inhibitor experiments.

Insight into the role of dual-ligand modification in low molecular weight heparin based nanocarrier for targeted delivery of doxorubicin

Low molecular weight heparin nanoparticles (LMWH) modified by glycyrrhetinic acid (GA) (LMWH-GA) and further decorated by lactobionic acid (LA) (LA-LMWH-GA) were reported as novel hepatocellular carcinoma (HPC)-targeted carriers to overcome multidrug resistance (MDR) of doxorubicin (DOX). The drug-loaded nanoparticles had negative charge of around -25mV and average size range of 70-170nm. These nanoparticles performed sustained drug release in vitro and prolonged DOX residence time in blood circulation in vivo. Compared to free DOX, DOX-loaded nanoparticles demonstrated increased DOX accumulation in drug-resistance HepG2/ADR cells and enhanced in vitro therapeutic efficacy. However, DOX/LA-LMWH-GA with dual ligands didn't show higher cellular uptake and cytotoxicity than single GA modified DOX/LMWH-GA, although both GA-mediated and LA-mediated endocytosis were involved in their cell internalization. Uptake pathway inhibition study revealed the less efficacy of DOX/LA-LMWH-GA in cellular level could be attributed to the reduced effect of micropinocytosis and caveolae-mediated endocytosis in cellular uptake. Interestingly, the DOX-loaded nanoparticles developed from lower drug/carrier feeding ratio possessed higher performance in cell internalization and in vitro efficacy compared to those developed from higher drug/carrier feeding ratio, which could highlight the role of carrier in drug delivery process.

Lysosome-Dependent Activation of Human Dendritic Cells by the Vaccine Adjuvant QS-21

The adjuvant properties of the saponin QS-21 have been known for decades. It is a component of the Adjuvant System AS01 that is used in several vaccine candidates. QS-21 strongly potentiates both cellular and humoral immune responses to purified antigens, yet how it activates immune cells is largely unknown. Here, we report that QS-21 directly activated human monocyte-derived dendritic cells (moDCs) and promoted a pro-inflammatory transcriptional program. Cholesterol-dependent QS-21 endocytosis followed by lysosomal destabilization and Syk kinase activation were prerequisites for this response. Cathepsin B, a lysosomal cysteine protease, was essential for moDC activation in vitro and contributed to the adjuvant effects of QS-21 in vivo. Collectively, these findings provide new insights into the pathways involved in the direct activation of antigen-presenting cells by a clinically relevant QS-21 formulation.

Herpes simplex virus Membrane Fusion

Herpes simplex virus mediates multiple distinct fusion events during infection. HSV entry is initiated by fusion of the viral envelope with either the limiting membrane of a host cell endocytic compartment or the plasma membrane. In the infected cell during viral assembly, immature, enveloped HSV particles in the perinuclear space fuse with the outer nuclear membrane in a process termed de-envelopment. A cell infected with some strains of HSV with defined mutations spread to neighboring cells by a fusion event called syncytium formation. Two experimental methods, the transient cell-cell fusion approach and fusion from without, are useful surrogate assays of HSV fusion. These five fusion processes are considered in terms of their requirements, mechanism, and regulation. The execution and modulation of these events require distinct yet often overlapping sets of viral proteins and host cell factors. The core machinery of HSV gB, gD, and the heterodimer gH/gL is required for most if not all of the HSV fusion mechanisms.

Kaposi's Sarcoma-Associated Herpesvirus: Epidemiology and Molecular Biology

Kaposi's sarcoma-associated herpesvirus (KSHV), also known as Human herpesvirus 8 (HHV-8), is a member of the lymphotropic gammaherpesvirus subfamily and a human oncogenic virus. Since its discovery in AIDS-associated KS tissues by Drs. Yuan Chang and Patrick Moore, much progress has been made in the past two decades. There are four types of KS including classic KS, endemic KS, immunosuppressive therapy-related KS, and AIDS-associated KS. In addition to KS, KSHV is also involved in the development of primary effusion lymphoma (PEL) and certain types of multicentric Castleman's disease. KSHV manipulates numerous viral proteins to promote the progression of angiogenesis and tumorigenesis. In this chapter, we review the epidemiology and molecular biology of KSHV and the mechanisms underlying KSHV-induced diseases.

Mechanisms of Filovirus Entry

Filovirus entry into cells is complex, perhaps as complex as any viral entry mechanism identified to date. However, over the past 10 years, the important events required for filoviruses to enter into the endosomal compartment and fuse with vesicular membranes have been elucidated (Fig. 1). Here, we highlight the important steps that are required for productive entry of filoviruses into mammalian cells.

KSHV Entry and Trafficking in Target Cells-Hijacking of Cell Signal Pathways, Actin and Membrane Dynamics

Kaposi's sarcoma associated herpesvirus (KSHV) is etiologically associated with human endothelial cell hyperplastic Kaposi's sarcoma and B-cell primary effusion lymphoma. KSHV infection of adherent endothelial and fibroblast cells are used as in vitro models for infection and KSHV enters these cells by host membrane bleb and actin mediated macropinocytosis or clathrin endocytosis pathways, respectively. Infection in endothelial and fibroblast cells is initiated by the interactions between multiple viral envelope glycoproteins and cell surface associated heparan sulfate (HS), integrins (α3β1, αVβ3 and αVβ5), and EphA2 receptor tyrosine kinase (EphA2R). This review summarizes the accumulated studies demonstrating that KSHV manipulates the host signal pathways to enter and traffic in the cytoplasm of the target cells, to deliver the viral genome into the nucleus, and initiate viral gene expression. KSHV interactions with the cell surface receptors is the key platform for the manipulations of host signal pathways which results in the simultaneous induction of FAK, Src, PI3-K, Rho-GTPase, ROS, Dia-2, PKC ζ, c-Cbl, CIB1, Crk, p130Cas and GEF-C3G signal and adaptor molecules that play critical roles in the modulation of membrane and actin dynamics, and in the various steps of the early stages of infection such as entry and trafficking towards the nucleus. The Endosomal Sorting Complexes Required for Transport (ESCRT) proteins are also recruited to assist in viral entry and trafficking. In addition, KSHV interactions with the cell surface receptors also induces the host transcription factors NF-κB, ERK1/2, and Nrf2 early during infection to initiate and modulate viral and host gene expression. Nuclear delivery of the viral dsDNA genome is immediately followed by the host innate responses such as the DNA damage response (DDR), inflammasome and interferon responses. Overall, these studies form the initial framework for further studies of simultaneous targeting of KSHV glycoproteins, host receptor, signal molecules and trafficking machinery that would lead into novel therapeutic methods to prevent KSHV infection of target cells and consequently the associated malignancies.

Multiple Roles of the Cytoplasmic Domain of Herpes Simplex Virus 1 Envelope Glycoprotein D in Infected Cells

Herpes simplex virus 1 (HSV-1) envelope glycoprotein D (gD) plays an essential role in viral entry. The functional regions of gD responsible for viral entry have been mapped to its extracellular domain, whereas the gD cytoplasmic domain plays no obvious role in viral entry. Thus far, the role(s) of the gD cytoplasmic domain in HSV-1 replication has remained to be elucidated. In this study, we show that ectopic expression of gD induces microvillus-like tubular structures at the plasma membrane which resemble the reported projection structures of the plasma membrane induced in HSV-1-infected cells. Mutations in the arginine cluster (residues 365 to 367) in the gD cytoplasmic domain greatly reduced gD-induced plasma membrane remodeling. In agreement with this, the mutations in the arginine cluster in the gD cytoplasmic domain reduced the number of microvillus-like tubular structures at the plasma membrane in HSV-1-infected cells. In addition, the mutations produced an accumulation of unenveloped nucleocapsids in the cytoplasm and reduced viral replication and cell-cell spread. These results suggest that the arginine cluster in the gD cytoplasmic domain is required for the efficient induction of plasma membrane projections and viral final envelopment, and these functions of the gD domain may lead to efficient viral replication and cell-cell spread.

IMPORTANCE

The cytoplasmic domain of HSV-1 gD, an envelope glycoprotein essential for viral entry, was reported to promote viral replication and cell-cell spread, but the role(s) of the domain during HSV-1 infection has remained unknown. In this study, we clarify two functions of the arginine cluster in the HSV-1 gD cytoplasmic domain, both of which require host cell membrane remodeling, i.e., the formation of microvillus-like projections at the plasma membrane and viral final envelopment in HSV-1-infected cells. We also show that the gD arginine cluster is required for efficient HSV-1 replication and cell-cell spread. This is the first report clarifying not only the functions of the gD cytoplasmic domain but also identifying the gD arginine cluster to be the HSV-1 factor responsible for the induction of plasma membrane projections in HSV-1-infected cells. Our results elucidate some of the functions of this multifunctional envelope glycoprotein during HSV-1 infection.

Endocytic Pathways Used by Andes Virus to Enter Primary Human Lung Endothelial Cells

Andes virus (ANDV) is the major cause of hantavirus pulmonary syndrome (HPS) in South America. Despite a high fatality rate (up to 40%), no vaccines or antiviral therapies are approved to treat ANDV infection. To understand the role of endocytic pathways in ANDV infection, we used 3 complementary approaches to identify cellular factors required for ANDV entry into human lung microvascular endothelial cells. We screened an siRNA library targeting 140 genes involved in membrane trafficking, and identified 55 genes required for ANDV infection. These genes control the major endocytic pathways, endosomal transport, cell signaling, and cytoskeleton rearrangement. We then used infectious ANDV and retroviral pseudovirions to further characterize the possible involvement of 9 of these genes in the early steps of ANDV entry. In addition, we used markers of cellular endocytosis along with chemical inhibitors of known endocytic pathways to show that ANDV uses multiple routes of entry to infect target cells. These entry mechanisms are mainly clathrin-, dynamin-, and cholesterol-dependent, but can also occur via a clathrin-independent manner.

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.

ESCRT-I Protein Tsg101 Plays a Role in the Post-macropinocytic Trafficking and Infection of Endothelial Cells by Kaposi's Sarcoma-Associated Herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) binding to the endothelial cell surface heparan sulfate is followed by sequential interactions with α3β1, αVβ3 and αVβ5 integrins and Ephrin A2 receptor tyrosine kinase (EphA2R). These interactions activate host cell pre-existing FAK, Src, PI3-K and RhoGTPase signaling cascades, c-Cbl mediated ubiquitination of receptors, recruitment of CIB1, p130Cas and Crk adaptor molecules, and membrane bleb formation leading to lipid raft dependent macropinocytosis of KSHV into human microvascular dermal endothelial (HMVEC-d) cells. The Endosomal Sorting Complexes Required for Transport (ESCRT) proteins, ESCRT-0, -I, -II, and-III, play a central role in clathrin-mediated internalized ubiquitinated receptor endosomal trafficking and sorting. ESCRT proteins have also been shown to play roles in viral egress. We have recently shown that ESCRT-0 component Hrs protein associates with the plasma membrane during macropinocytosis and mediates KSHV entry via ROCK1 mediated phosphorylation of NHE1 and local membrane pH change. Here, we demonstrate that the ESCRT-I complex Tsg101 protein also participates in the macropinocytosis of KSHV and plays a role in KSHV trafficking. Knockdown of Tsg101 did not affect virus entry in HMVEC-d and human umbilical vein endothelial (HUVEC) cells but significantly inhibited the KSHV genome entry into the nucleus and consequently viral gene expression in these cells. Double and triple immunofluorescence, proximity ligation immunofluorescence and co-immuoprecipitation studies revealed the association of Tsg101 with the KSHV containing macropinosomes, and increased levels of Tsg101 association/interactions with EphA2R, c-Cbl, p130Cas and Crk signal molecules, as well as with upstream and downstream ESCRT components such as Hrs (ESCRT-0), EAP45 (ESCRT-II), CHMP6 (ESCRT-III) and CHMP5 (ESCRT-III) in the KSHV infected cells. Tsg101 was also associated with early (Rab5) and late endosomal (Rab7) stages of KSHV intracellular trafficking, and CHMP5 (ESCRT-III) was also associated with Rab 5 and Rab 7. Knockdown of Tsg101 significantly inhibited the transition of virus from early to late endosomes. Collectively, our studies reveal that Tsg101 plays a role in the trafficking of macropinocytosed KSHV in the endothelial cells which is essential for the successful viral genome delivery into the nucleus, viral gene expression and infection. Thus, ESCRT molecules could serve as therapeutic targets to combat KSHV infection.

Cell Surface THY-1 Contributes to Human Cytomegalovirus Entry via a Macropinocytosis-Like Process

Previously we showed that THY-1 has a critical role in the initial stage of infection of certain cell types with human cytomegalovirus (HCMV) and that THY-1 is important for HCMV-mediated activation of phosphatidylinositol 3-kinase (PI3K)/Akt during virus entry. THY-1 is known to interact with integrins and is a major cargo protein of clathrin-independent endocytic vesicles. Since macropinocytosis involves integrin signaling, is PI3K/Akt dependent, and is a clathrin-independent endocytic process, we determined whether THY-1 has a role in HCMV entry by macropinocytosis. Using electron microscopy in two cell lines that support HCMV infection in a THY-1-dependent manner, we found that HCMV enters these cells by a macropinocytosis-like process. THY-1 associated with HCMV virions on the cell surface and colocalized with virus inside macropinosomes. 5-(N-Ethyl-N-isopropyl)amiloride (EIPA) and soluble THY-1 blocked HCMV infection in the cell lines by ≥80% and 60%, respectively. HCMV entry into the cells triggered increased influx of extracellular fluid, a marker of macropinocytosis, and this increased fluid uptake was inhibited by EIPA and by soluble THY-1. Blocking actin depolymerization, Na⁺/H⁺ exchange, PI3K, and Pak1 kinase, which are critical for macropinocytosis, impaired HCMV infection. Neither internalized HCMV virions nor THY-1 in virus-infected cells colocalized with transferrin as determined by confocal microscopy, indicating that clathrin-mediated endocytosis was not involved in THY-1-associated virus entry. These results suggest that HCMV has adapted to utilize THY-1, a cargo protein of clathrin-independent endocytotic vesicles, to facilitate efficient entry into certain cell types by a macropinocytosis-like process.

IMPORTANCE

Human cytomegalovirus (HCMV) infects over half of the population and is the most common infectious cause of birth defects. The virus is the most important infection occurring in transplant recipients. The mechanism of how HCMV enters cells is controversial. In this study, we show that THY-1, a cell surface protein that is critical for the early stage of entry of HCMV into certain cell types, contributes to virus entry by macropinocytosis. Our findings suggest that HCMV has adapted to utilize THY-1 to facilitate entry of HCMV into macropinosomes in certain cell types. Further knowledge about the mechanism of HCMV entry into cells may facilitate the development of novel inhibitors of virus infection.

Herpesvirus Entry into Host Cells Mediated by Endosomal Low pH

Herpesviral pathogenesis stems from infection of multiple cell types including the site of latency and cells that support lytic replication. Herpesviruses utilize distinct cellular pathways, including low pH endocytic pathways, to enter different pathophysiologically relevant target cells. This review details the impact of the mildly acidic milieu of endosomes on the entry of herpesviruses, with particular emphasis on herpes simplex virus 1 (HSV-1). Epithelial cells, the portal of primary HSV-1 infection, support entry via low pH endocytosis mechanisms. Mildly acidic pH triggers reversible conformational changes in the HSV-1 class III fusion protein glycoprotein B (gB). In vitro treatment of herpes simplex virions with a similar pH range inactivates infectivity, likely by prematurely activating the viral entry machinery in the absence of a target membrane. How a given herpesvirus mediates both low pH and pH-independent entry events is a key unresolved question.

Integrins as Herpesvirus Receptors and Mediators of the Host Signalosome

The repertoire of herpesvirus receptors consists of nonintegrin and integrin molecules. Integrins interact with the conserved glycoproteins gH/gL or gB. This interaction is a conserved biology across the Herpesviridae family, likely directed to promote virus entry and endocytosis. Herpesviruses exploit this interaction to execute a range of critical functions that include (a) relocation of nonintegrin receptors (e.g., herpes simplex virus nectin1 and Kaposi's sarcoma-associated herpesvirus EphA2), or association with nonintegrin receptors (i.e., human cytomegalovirus EGFR), to dictate species-specific entry pathways; (b) activation of multiple signaling pathways (e.g., Ca²⁺ release, c-Src, FAK, MAPK, and PI3K); and (c) association with Rho GTPases, tyrosine kinase receptors, Toll-like receptors, which result in cytoskeletal remodeling, differential cell type targeting, and innate responses. In turn, integrins can be modulated by viral proteins (e.g., Epstein-Barr virus LMPs) to favor spread of transformed cells. We propose that herpesviruses evolved a multipartite entry system to allow interaction with multiple receptors, including integrins, required for their sophisticated life cycle.

Suppression of Kaposi's Sarcoma-Associated Herpesvirus Infection and Replication by 5'-AMP-Activated Protein Kinase

The host intracellular antiviral restriction factors inhibit viral infection and replication. The 5'-AMP-activated protein kinase (AMPK) is a cellular energy sensor regulating metabolic homeostasis. Activated AMPK inhibits the replication of numerous RNA viruses but enhances the entry of vaccinia virus. However, the role of AMPK in herpesvirus infection is unclear. In this study, we showed that the constitutive AMPK activity restricted Kaposi's sarcoma-associated herpesvirus (KSHV) lytic replication in primary human umbilical vein endothelial cells while KSHV infection did not markedly affect the endogenous AMPK activity. Knockdown of the AMPKα1 considerably enhanced the expression of viral lytic genes and the production of infectious virions, while overexpression of a constitutively active AMPK had the opposite effects. Accordingly, an AMPK inhibitor, compound C, augmented viral lytic gene expressions and virion productions but an AMPK agonist, 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR), suppressed both. Furthermore, a common diabetes drug, metformin, which carries an AMPK-agonistic activity, drastically inhibited the expression of viral lytic genes and the production of infectious virions, suggesting the use of metformin as a therapeutic agent for KSHV infection and replication. Together, these results identify the host AMPK as a KSHV restriction factor that can serve as a potential therapeutic target.

IMPORTANCE

Host cells encode specific proteins to restrict viral infection and replication. Kaposi's sarcoma-associated herpesvirus (KSHV) is a human tumor virus associated with several cancers. In this study, we have identified 5'-AMP-activated protein kinase (AMPK), a cellular energy sensor, as a restriction factor of KSHV lytic replication during primary infection. Activation of AMPK suppresses, while inhibition of AMPK enhances, KSHV lytic replication by regulating the expression of viral genes. AICAR and metformin, both of which are AMPK agonists currently used in clinics for the treatment of conditions associated with metabolic disorders, inhibit KSHV lytic replication. Thus, our work has identified AMPK as a potential therapeutic target and AICAR and metformin as potential therapeutic agents for KSHV-associated cancers.

Cell Walls and the Convergent Evolution of the Viral Envelope

Why some viruses are enveloped while others lack an outer lipid bilayer is a major question in viral evolution but one that has received relatively little attention. The viral envelope serves several functions, including protecting the RNA or DNA molecule(s), evading recognition by the immune system, and facilitating virus entry. Despite these commonalities, viral envelopes come in a wide variety of shapes and configurations. The evolution of the viral envelope is made more puzzling by the fact that nonenveloped viruses are able to infect a diverse range of hosts across the tree of life. We reviewed the entry, transmission, and exit pathways of all (101) viral families on the 2013 International Committee on Taxonomy of Viruses (ICTV) list. By doing this, we revealed a strong association between the lack of a viral envelope and the presence of a cell wall in the hosts these viruses infect. We were able to propose a new hypothesis for the existence of enveloped and nonenveloped viruses, in which the latter represent an adaptation to cells surrounded by a cell wall, while the former are an adaptation to animal cells where cell walls are absent. In particular, cell walls inhibit viral entry and exit, as well as viral transport within an organism, all of which are critical waypoints for successful infection and spread. Finally, we discuss how this new model for the origin of the viral envelope impacts our overall understanding of virus evolution.

ESCRT-0 Component Hrs Promotes Macropinocytosis of Kaposi's Sarcoma-Associated Herpesvirus in Human Dermal Microvascular Endothelial Cells

Kaposi's sarcoma-associated herpesvirus (KSHV) enters human dermal microvascular endothelial cells (HMVEC-d), its naturalin vivotarget cells, by lipid raft-dependent macropinocytosis. The internalized viral envelope fuses with the macropinocytic membrane, and released capsid is transported to the nuclear vicinity, resulting in the nuclear entry of viral DNA. The endosomal sorting complexes required for transport (ESCRT) proteins, which include ESCRT-0, -I, -II, and -III, play a central role in endosomal trafficking and sorting of internalized and ubiquitinated receptors. Here, we examined the role of ESCRT-0 component Hrs (hepatocyte growth factor-regulated tyrosine kinase substrate) in KSHV entry into HMVEC-d by macropinocytosis. Knockdown of Hrs by short hairpin RNA (shRNA) transduction resulted in significant decreases in KSHV entry and viral gene expression. Immunofluorescence analysis (IFA) and plasma membrane isolation and proximity ligation assay (PLA) demonstrated the translocation of Hrs from the cytosol to the plasma membrane of infected cells and association with α-actinin-4. In addition, infection induced the plasma membrane translocation and activation of the serine/threonine kinase ROCK1, a downstream target of the RhoA GTPase. Hrs knockdown reduced these associations, suggesting that the recruitment of ROCK1 is an Hrs-mediated event. Interaction between Hrs and ROCK1 is essential for the ROCK1-induced phosphorylation of NHE1 (Na(+)/H(+)exchanger 1), which is involved in the regulation of intracellular pH. Thus, our studies demonstrate the plasma membrane association of ESCRT protein Hrs during macropinocytosis and suggest that KSHV entry requires both Hrs- and ROCK1-dependent mechanisms and that ROCK1-mediated phosphorylation of NHE1 and pH change is an essential event required for the macropinocytosis of KSHV.

IMPORTANCE

Macropinocytosis is the major entry pathway of KSHV in human dermal microvascular endothelial cells, the natural target cells of KSHV. Although the role of ESCRT protein Hrs has been extensively studied with respect to endosomal movement and sorting of ubiquitinated proteins into lysosomes, its function in macropinocytosis is not known. In the present study, we demonstrate for the first time that upon KSHV infection, the endogenous Hrs localizes to the plasma membrane and the membrane-associated Hrs facilitates assembly of signaling molecules, macropinocytosis, and virus entry. Hrs recruits ROCK1 to the membrane, which is required for the activation of NHE1 and an increase in submembranous intracellular pH occurring during macropinocytosis. These studies demonstrate that the localization of Hrs from the cytosol to the plasma membrane is important for coupling membrane dynamics to the cytosolic signaling events during macropinocytosis of KSHV.