Search

Aggregatibacter actinomycetemcomitans cytolethal distending toxin (Cdt) is capable of intoxicating lymphocytes macrophages, mast cells and epithelial cells. Following Cdt binding to cholesterol, in the region of membrane lipid rafts, the CdtB and CdtC subunits are internalized and traffic to intracellular compartments. These events are dependent upon, cellugyrin, a critical component of synaptic like microvesicles (SLMVCg+). Target cells, such as Jurkat cells, rendered unable to express cellugyrin are resistant to Cdt-induced toxicity. Similar to Cdt, SARS-CoV-2 entry into host cells is initiated by binding to cell surface receptors, ACE-2, also associated with cholesterol-rich lipid rafts; this association leads to fusion and/or endocytosis of viral and host cell membranes and intracellular trafficking. The similarity in internalization pathways for both Cdt and SARS-CoV-2 led us to consider the possibility that cellugyrin was a critical component in both processes. Cellugyrin deficient Calu-3 cells (Calu-3Cg-) were prepared using Lentiviral particles containing shRNA; these cells were resistant to infection by VSV/SARS-CoV-2-spike pseudotype virus and partially resistant to VSV/VSV-G pseudotype virus. Synthetic peptides representing various regions of the cellugyrin protein were prepared and assessed for their ability to bind to Cdt subunits using surface plasmon resonance. Cdt was capable of binding to a region designated the middle outer loop (MOL) which corresponds to a region extending into the cytoplasmic surface of the SLMVCg+. SARS-CoV-2 spike proteins were assessed for their ability to bind to cellugyrin peptides; SARS-CoV-2 full length spike protein preferentially binds to a region within the SLMVCg+ lumen, designated intraluminal loop 1A. SARS-CoV-2-spike protein domain S1, which contains the receptor binding domains, binds to cellugyrin N-terminus which extends out from the cytoplasmic surface of SLMV. Binding specificity was further analyzed using cellugyrin scrambled peptide mutants. We propose that SLMVCg+ represent a component of a common pathway that facilitates pathogen and/or pathogen-derived toxins to gain host cell entry. [ABSTRACT FROM AUTHOR]

Bunyavirales constitute the largest order of enveloped RNA viruses, many members of which cause severe diseases in humans and domestic animals. In recent decades, innovative fluorescence‐based methods have paved the way to visualize and track single fluorescent bunyaviral particles in fixed and live cells. This technological breakthrough has enabled imaging of the early stages of infection and the quantification of every step in the bunyavirus cell entry process. Here, we describe the latest procedures for rendering bunyaviral particles fluorescent and discuss the advantages and disadvantages of each approach in light of the most recent advances in fluorescence detection and monitoring of bunyavirus entry. In this mini‐review, we also illustrate how fluorescent viral particles are a powerful tool for deciphering the cellular entry process of bunyaviruses, the vast majority of which have not yet been analyzed. [ABSTRACT FROM AUTHOR]

To identify host factors that affect Bovine Herpes Virus Type 1 (BoHV-1) infection we previously applied a genome wide CRISPR knockout screen targeting all bovine protein coding genes. By doing so we compiled a list of both pro-viral and anti-viral proteins involved in BoHV-1 replication. Here we provide further analysis of those that are potentially involved in viral entry into the host cell. We first generated single cell knockout clones deficient in some of the candidate genes for validation. We provide evidence that Polio Virus Receptor-related protein (PVRL2) serves as a receptor for BoHV-1, mediating more efficient entry than the previously identified Polio Virus Receptor (PVR). By knocking out two enzymes that catalyze HSPG chain elongation, HST2ST1 and GLCE, we further demonstrate the significance of HSPG in BoHV-1 entry. Another intriguing cluster of candidate genes, COG1, COG2 and COG4-7 encode six subunits of the Conserved Oligomeric Golgi (COG) complex. MDBK cells lacking COG6 produced fewer but bigger plaques compared to control cells, suggesting more efficient release of newly produced virions from these COG6 knockout cells, due to impaired HSPG biosynthesis. We further observed that viruses produced by the COG6 knockout cells consist of protein(s) with reduced N-glycosylation, potentially explaining their lower infectivity. To facilitate candidate validation, we also detailed a one-step multiplex CRISPR interference (CRISPRi) system, an orthogonal method to KO that enables quick and simultaneous deployment of three CRISPRs for efficient gene inactivation. Using CRISPR3i, we verified eight candidates that have been implicated in the synthesis of surface heparan sulfate proteoglycans (HSPGs). In summary, our experiments confirmed the two receptors PVR and PVRL2 for BoHV-1 entry into the host cell and other factors that affect this process, likely through the direct or indirect roles they play during HSPG synthesis and glycosylation of viral proteins. [ABSTRACT FROM AUTHOR]

Aggregatibacter actinomycetemcomitans cytolethal distending toxin (Cdt) is capable of intoxicating lymphocytes macrophages, mast cells and epithelial cells. Following Cdt binding to cholesterol, in the region of membrane lipid rafts, the CdtB and CdtC subunits are internalized and traffic to intracellular compartments. These events are dependent upon, cellugyrin, a critical component of synaptic like microvesicles (SLMVCg+). Target cells, such as Jurkat cells, rendered unable to express cellugyrin are resistant to Cdt-induced toxicity. Similar to Cdt, SARS-CoV-2 entry into host cells is initiated by binding to cell surface receptors, ACE-2, also associated with cholesterol-rich lipid rafts; this association leads to fusion and/or endocytosis of viral and host cell membranes and intracellular trafficking. The similarity in internalization pathways for both Cdt and SARS-CoV-2 led us to consider the possibility that cellugyrin was a critical component in both processes. Cellugyrin deficient Calu-3 cells (Calu-3Cg-) were prepared using Lentiviral particles containing shRNA; these cells were resistant to infection by VSV/SARS-CoV-2-spike pseudotype virus and partially resistant to VSV/VSV-G pseudotype virus. Synthetic peptides representing various regions of the cellugyrin protein were prepared and assessed for their ability to bind to Cdt subunits using surface plasmon resonance. Cdt was capable of binding to a region designated the middle outer loop (MOL) which corresponds to a region extending into the cytoplasmic surface of the SLMVCg+. SARS-CoV-2 spike proteins were assessed for their ability to bind to cellugyrin peptides; SARS-CoV-2 full length spike protein preferentially binds to a region within the SLMVCg+ lumen, designated intraluminal loop 1A. SARS-CoV-2-spike protein domain S1, which contains the receptor binding domains, binds to cellugyrin N-terminus which extends out from the cytoplasmic surface of SLMV. Binding specificity was further analyzed using cellugyrin scrambled peptide mutants. We propose that SLMVCg+ represent a component of a common pathway that facilitates pathogen and/or pathogen-derived toxins to gain host cell entry.

The global outbreak of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) threatened human health and public safety. The development of anti-SARS-CoV-2 therapies have been essential to curb the spread of SARS-CoV-2. Particularly, antivirals targeting viral entry have become an attractive target for the development of anti-SARS-CoV-2 therapies. In this review, we elucidate the mechanism of SARS-CoV-2 viral entry and summarize the development of antiviral inhibitors targeting viral entry. Moreover, we speculate upon future directions toward more potent inhibitors of SARS-CoV-2 entry. This study is expected to provide novel insights for the efficient discovery of promising candidate drugs against the entry of SARS-CoV-2, and contribute to the development of broad-spectrum anti-coronavirus drugs.

To identify host factors that affect Bovine Herpes Virus Type 1 (BoHV-1) infection we previously applied a genome wide CRISPR knockout screen targeting all bovine protein coding genes. By doing so we compiled a list of both pro-viral and anti-viral proteins involved in BoHV-1 replication. Here we provide further analysis of those that are potentially involved in viral entry into the host cell. We first generated single cell knockout clones deficient in some of the candidate genes for validation. We provide evidence that Polio Virus Receptor-related protein (PVRL2) serves as a receptor for BoHV-1, mediating more efficient entry than the previously identified Polio Virus Receptor (PVR). By knocking out two enzymes that catalyze HSPG chain elongation, HST2ST1 and GLCE, we further demonstrate the significance of HSPG in BoHV-1 entry. Another intriguing cluster of candidate genes, COG1, COG2 and COG4-7 encode six subunits of the Conserved Oligomeric Golgi (COG) complex. MDBK cells lacking COG6 produced fewer but bigger plaques compared to control cells, suggesting more efficient release of newly produced virions from these COG6 knockout cells, due to impaired HSPG biosynthesis. We further observed that viruses produced by the COG6 knockout cells consist of protein(s) with reduced N-glycosylation, potentially explaining their lower infectivity. To facilitate candidate validation, we also detailed a one-step multiplex CRISPR interference (CRISPRi) system, an orthogonal method to KO that enables quick and simultaneous deployment of three CRISPRs for efficient gene inactivation. Using CRISPR3i, we verified eight candidates that have been implicated in the synthesis of surface heparan sulfate proteoglycans (HSPGs). In summary, our experiments confirmed the two receptors PVR and PVRL2 for BoHV-1 entry into the host cell and other factors that affect this process, likely through the direct or indirect roles they play during HSPG synthesis and glycosylation of viral proteins.

Emerging infections of mosquito-borne Zika virus (ZIKV) pose an increasing threat to human health, as documented over the recent years in South Pacific islands and the Americas in recent years. To better understand molecular mechanisms underlying the increase in human cases with severe pathologies, we recently demonstrated the functional roles of structural proteins capsid (C), pre-membrane (prM), and envelop (E) of ZIKV epidemic strains with the initiation of viral infection in human cells. Specifically, we found that the C-prM region contributes to permissiveness of human host cells to ZIKV infection and ZIKV-induced cytopathic effects, whereas the E protein is associated with viral attachment and early infection. In the present study, we further characterize ZIKV E proteins by investigating the roles of residues isoleucine 152 (Ile152), threonine 156 (Thr156), and histidine 158 (His158) (i.e., the E-152/156/158 residues), which surround a unique N-glycosylation site (E-154), in permissiveness of human host cells to epidemic ZIKV infection. For comparison purpose, we generated mutant molecular clones of epidemic BeH819015 (BR15) and historical MR766-NIID (MR766) strains that carry each others E-152/156/158 residues, respectively. We observed that the BR15 mutant containing the E-152/156/158 residues from MR766 was less infectious in A549-Dual™ cells than parental virus. In contrast, the MR766 mutant containing E-152/156/158 residues from BR15 displayed increased infectivity. The observed differences in infectivity were, however, not correlated with changes in viral binding onto host-cells or cellular responses to viral infection. Instead, the E-152/156/158 residues from BR15 were associated with an increased efficiency of viral membrane fusion inside infected cells due to conformational changes of E protein that enhance exposure of the fusion loop. Our data highlight an important contribution of E-152/156/158 residues to the early steps of ZIKV infection in human cells.

Adeno-associated virus (AAV) is a small ssDNA satellite virus of high interest (in recombinant form) as a safe and effective gene therapy vector. AAV's human cell entry receptor (AAVR) contains polycystic kidney disease (PKD) domains bound by AAV. Seeking understanding of the spectrum of interactions, goat AAVGo.1 is investigated, because its host is the species most distant from human with reciprocal cross-species cell susceptibility. The structure of AAVGo.1, solved by cryo-EM to 2.9 Å resolution, is most similar to AAV5. Through ELISA (enzyme-linked immunosorbent assay) studies, it is shown that AAVGo.1 binds to human AAVR more strongly than do AAV2 or AAV5, and that it joins AAV5 in a class that binds exclusively to PKD domain 1 (PKD1), in contrast to other AAVs that interact primarily with PKD2. The AAVGo.1 cryo-EM structure of a complex with a PKD12 fragment of AAVR at 2.4 Å resolution shows PKD1 bound with minimal change in virus structure. There are only minor conformational adaptations in AAVR, but there is a near-rigid rotation of PKD1 with maximal displacement of the receptor domain by ;1 Å compared to PKD1 bound to AAV5. AAVGo.1 joins AAV5 as the second member of an emerging class of AAVs whose mode of receptor-binding is completely different from other AAVs, typified by AAV2. [ABSTRACT FROM AUTHOR]

Severe acute respiratory syndrome-related coronavirus-2 (SARS-CoV-2) has caused the pandemic that began late December 2019. The co-expression of SARS-CoV-2 structural proteins in cells could assemble into several types of virus-like particles (VLPs) without a viral RNA genome. VLPs containing S proteins with the structural and functional properties of authentic virions are safe materials to exploit for virus-cell entry and vaccine development. In this study, to generate SARS-CoV-2 VLPs (SCoV2-SEM VLPs) composed of three structural proteins including spike (S), envelop (E) protein and membrane (M) protein, a tri-cistronic vector expression system was established in a cell line co-expressing SARS-CoV-2 S, E and M proteins. The SCoV2-SEM VLPs were harvested from the cultured medium, and three structure proteins were confirmed by Western blot assay. A negative-stain TEM assay demonstrated the size of the SCoV2-SEM VLPs with a diameter of about 90 nm. To further characterize the infectious properties of SCoV2-SEM VLPs, the VLPs (atto647N-SCoV2-SEM VLPs) were fluorescence-labeled by conjugation with atto-647N and visualized under confocal microscopy at a single-particle resolution. The results of the infection assay revealed that atto647N-SCoV2-SEM VLPs attached to the surface of the HEK293T cells at the pre-binding phase in a ACE2-dependent manner. At the post-infection phase, atto647N-SCoV2-SEM VLPs either fused with the cellular membrane or internalized into the cytoplasm with mCherry-rab5-positive early endosomes. Moreover, fusion with the cellular membrane and the internalization with early endosomes could be inhibited by the treatment of camostat (a pharmacological inhibitor of TMPRSS2) and chlorpromazine (an endocytosis inhibitor), respectively. These results elucidated that SCoV2-SEM VLPs behave similarly to the authentic live SARS-CoV-2 virus, suggesting that the development of SCoV2-SEM VLPs provide a realistic and safe experimental model for studying the infectious mechanism of SARS-CoV-2. [ABSTRACT FROM AUTHOR]

ABSTRACT Wild-type canine distemper virus (CDV) is an important pathogen of dogs as well as wildlife that can infect immune and epithelial cells through two known receptors: the signaling lymphocytic activation molecule (SLAM) and nectin-4, respectively. Conversely, the ferret and egg-adapted CDV-Onderstepoort strain (CDV-OP) is employed as an effective vaccine for dogs. CDV-OP also exhibits promising oncolytic properties, such as its abilities to infect and kill multiple cancer cells in vitro. Interestingly, several cancer cells do not express SLAM or nectin-4, suggesting the presence of a yet unknown entry factor for CDV-OP. By conducting a genome-wide CRISPR/Cas9 knockout (KO) screen in CDV-OP-susceptible canine mammary carcinoma P114 cells, which neither express SLAM nor nectin-4, we identified low-density lipoprotein receptor-related protein 6 (LRP6) as a host factor that promotes CDV-OP infectivity. Whereas the genetic ablation of LRP6 rendered cells resistant to infection, ectopic expression in resistant LRP6KO cells restored susceptibility. Furthermore, multiple functional studies revealed that (i) the overexpression of LRP6 leads to increased cell-cell fusion, (ii) a soluble construct of the viral receptor-binding protein (solHOP) interacts with a soluble form of LRP6 (solLRP6), (iii) an H-OP point mutant that prevents interaction with solLRP6 abrogates cell entry in multiple cell lines once transferred into recombinant viral particles, and (iv) vesicular stomatitis virus (VSV) pseudotyped with CDV-OP envelope glycoproteins loses its infectivity in LRP6KO cells. Collectively, our study identified LRP6 as the long sought-after cell entry receptor of CDV-OP in multiple cell lines, which set the molecular bases to refine our understanding of viral-cell adaptation and to further investigate its oncolytic properties. IMPORTANCE Oncolytic viruses (OV) have gathered increasing interest in recent years as an alternative option to treat cancers. The Onderstepoort strain of canine distemper virus (CDV-OP), an enveloped RNA virus belonging to the genus Morbillivirus, is employed as a safe and efficient vaccine for dogs against distemper disease. Importantly, although CDV-OP can infect and kill multiple cancer cell lines, the basic mechanisms of entry remain to be elucidated, as most of those transformed cells do not express natural receptors (i.e., SLAM and nectin-4). In this study, using a genome-wide CRISPR/Cas9 knockout screen, we describe the discovery of LRP6 as a novel functional entry receptor for CDV-OP in various cancer cell lines and thereby uncover a basic mechanism of cell culture adaptation. Since LRP6 is upregulated in various cancer types, our data provide important insights in order to further investigate the oncolytic properties of CDV-OP.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the pandemic coronavirus disease 2019 (COVID-19), which threatens human health and public safety. In the urgent campaign to develop anti-SARS-CoV-2 therapies, the initial entry step is one of the most appealing targets. In this review, we summarize the current understanding of SARS-CoV-2 cell entry, and the development of targeted antiviral strategies. Moreover, we speculate upon future directions toward next-generation of SARS-CoV-2 entry inhibitors during the upcoming post-pandemic era.

The spike (S) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) facilitates viral entry into host cells and is the key target for neutralizing antibodies. The SARS-CoV-2 lineage B.1.620 carries fifteen mutations in the S protein and is spread in Africa, the US and Europe, while lineage R.1 harbors four mutations in S and infections were observed in several countries, particularly Japan and the US. However, the impact of the mutations in B.1.620 and R.1 S proteins on antibody-mediated neutralization and host cell entry are largely unknown. Here, we report that these mutations are compatible with robust ACE2 binding and entry into cell lines, and they markedly reduce neutralization by vaccine-induced antibodies. Our results reveal evasion of neutralizing antibodies by B.1.620 and R.1, which might have contributed to the spread of these lineages. [ABSTRACT FROM AUTHOR]

Rotavirus (RV) is a non–enveloped icosahedral virus with an 11–segment double–stranded RNA genome, belonging to the family of rotaviruses. RV is one of the pathogens causing diarrhea in infants and young animals, and it induces the production of type I interferons (IFNs), which can trigger antiviral function by inducing the production of interferon–stimulated genes (ISGs). Although IFITM3, an ISG localizing to late endosomes, can limit many viral infections, whether or not it restricts the infection of RV is still unknown. Therefore, we attempted to determine whether IFITM3 also restricts RV infection by using over–expression and knockout cell strains. It was found that IFITM3–expressing cell strains were less susceptible to RV infection, as the replication of RV in over–expressing cells was significantly less than in control group cells. Correspondingly, IFITM3–knockout cells were significantly susceptible compared to the normal cells. Furthermore, the IFN–induced antiviral effect was significantly attenuated in the absence of IFITM3, and IFITM3 delayed RV escape from endosomes in the presence of IFITM3, suggesting that endogenous IFITM3 is of great importance in type I IFN–mediated antiviral responses and may restrict infection by affecting the function of the late endosomal compartment. In conclusion, these data provide the first evidence that IFITM3 limits RV infection in vitro and delays RV escape from late endosomes into the cytoplasm. [ABSTRACT FROM AUTHOR]

The envelope attachment (H)-protein of canine distemper virus (CDV) mediates receptor engagement and fusion protein-triggering; two key functions in viral cell entry and spread. Signaling lymphocyte activation molecule (SLAM) and Nectin-4 (N4) act as morbilliviral entry receptors in immune and epithelial cells, respectively, which defines very similar pathogeneses. High incidence of brain disorders is however unique to CDV. The wild-type CDV-A75/17 strain (A75) preferentially infects glial cells and spreads from astrocyte-to-astrocyte without inducing massive fusion events, despite the fact that SLAM and N4 expressions remained below detection levels. To investigate whether an A75 H-microdomain required to interact with SLAM may additionally contribute to promote viral spread between astrocytes, we initially engineered a novel A75 H-protein variant (546-SYT/RNR-548) that lost SLAM-binding property and, consequently, lacked fusion protein-triggering activity specifically in SLAM-expressing cells. Collectively, this approach provides the molecular tool to decipher the role of the selected H-microdomain in supporting A75-spread in glial cells.

Human norovirus (HuNoV) is the common cause of acute gastroenteritis worldwide. Norovirus-like particles (VLPs) have been applied to analyze virus–host cell interactions. This study established a highly quantitative detection system for HuNoV VLP entry into live cells based on the NanoLuc luciferase complementation. First, we generated a HiBiT-tagged VLP and examined VLP-HiBiT entry into Vero cells expressing LgBiT. The entry was saturable and competed by a non-labeled VLP, indicating the VLP-specific entry into the host cells. Upon comparison of the VLP-HiBiT entry among some cell lines, Caco-2 cells were found to be the most susceptible. The VLP-HiBiT entry was unaffected by treatment at an acidic pH value above 3.0 and was significantly reduced by heat treatment at 75°C for more than 2 min. These results reflected those of previous studies, indicating the stability of VLPs and binding to histo-blood group antigens at various pH ranges and temperatures and cellular tropism. • This study established quantitative detection system for the entry of norovirus. • HiBiT-tagged VLP (VLP-HiBiT) entered into LgBiT-expressing live cells. • VLP-HiBiT entry was unaffected by treatment at pH value above 3.0. • VLP-HiBiT entry was significantly reduced by heat treatment at 75°C for 2 min. • Caco-2 cells were the most susceptible among some cell lines examined. [ABSTRACT FROM AUTHOR]

Baculoviruses initiate oral infection in the highly alkaline midgut of insects via a group of envelope proteins called per os infectivity factors (PIFs). To date, no high-resolution structural information has been reported for any PIF. Here, we present the crystal structure of the PIF5 ectodomain (PIF5e) from Autographa californica multiple nucleopolyhedrovirus (AcMNPV) at a 2.2-Å resolution. It revealed an open cavity between the N-terminal E1 domain and the C-terminal E2 domain and a cysteine-rich region with three pairs of disulfide bonds in the E2 domain. Multiple conserved intramolecular interactions within PIF5 are essential for maintaining its tertiary structure. Two conserved arginines (Arg8 and Arg74) play critical roles in E1-E2 interactions, and mutagenesis analysis supported their crucial role in oral infection. Importantly, the reduction in the oral infectivity of the Arg8, Arg74, or cysteine mutant viruses was related to the proteolytic cleavage of PIF5 by the endogenous protease embedded in occlusion bodies during alkaline treatment. This suggested that the structural stability of PIF5 under physiological conditions in the insect midgut is critical for baculoviral oral infectivity. IMPORTANCE Per os infection mediated by PIFs is the highly complex mechanism by which baculoviruses initiate infection in insects. Previous studies revealed that multiple PIF proteins form a large PIF complex on the envelope of virions, while PIF5 functions independently of the PIF complex. Here, we report the crystal structure of AcMNPV PIF5e, which, to our knowledge, is the first atomic structure reported for a PIF protein. The structure revealed the precise locations of three previously proposed disulfide bonds and other conserved intramolecular interactions, which are important for the structural stability of PIF5 and are also essential for oral infectivity. These findings advance our understanding of the molecular mechanism of baculovirus oral infection under alkaline conditions. [ABSTRACT FROM AUTHOR]

Viruses and their hosts have coevolved for a long time. This coevolution places both the pathogen and the human immune system under selective pressure; on the one hand, the immune system has evolved to combat viruses and virally infected cells, while viruses have developed sophisticated mechanisms to escape recognition and destruction by the immune system. SARS-CoV-2, the pathogen that is causing the current COVID-19 pandemic, has shown a remarkable ability to escape antibody neutralization, putting vaccine efficacy at risk. One of the virus's immune evasion strategies is mitochondrial sabotage: by causing reactive oxygen species (ROS) production, mitochondrial physiology is impaired, and the interferon antiviral response is suppressed. Seminal studies have identified an intra-cytoplasmatic pathway for viral infection, which occurs through the construction of tunneling nanotubes (TNTs), hence enhancing infection and avoiding immune surveillance. Another method of evading immune monitoring is the disruption of the antigen presentation. In this scenario, SARS-CoV-2 infection reduces MHC-I molecule expression: SARS-CoV-2's open reading frames (ORF 6 and ORF 8) produce viral proteins that specifically downregulate MHC-I molecules. All of these strategies are also exploited by other viruses to elude immune detection and should be studied in depth to improve the effectiveness of future antiviral treatments. Compared to the Wuhan strain or the Delta variant, Omicron has developed mutations that have impaired its ability to generate syncytia, thus reducing its pathogenicity. Conversely, other mutations have allowed it to escape antibody neutralization and preventing cellular immune recognition, making it the most contagious and evasive variant to date. [ABSTRACT FROM AUTHOR]

(1) Background: Porcine deltacoronavirus (PDCoV) is a newly emerged enteric virus affecting pig breeding industries worldwide, and its pathogenic mechanism remains unclear. (2) Methods: In this study, we preliminarily identified the endocytic pathway of PDCoV in PK-15 cells, using six chemical inhibitors (targeting clathrin-mediated endocytosis, caveolae-mediated endocytosis, macropinocytosis pathway and endosomal acidification), overexpression of dominant-negative (DN) mutants to treat PK-15 cells and proteins knockdown. (3) Results: The results revealed that PDCoV entry was not affected after treatment with chlorpromazine (CPZ), 5-(N-ethyl-N-isopropyl) amiloride (EIPA)or ammonium chloride (NH4Cl), indicating that the entry of PDCoV into PK-15 cells were clathrin-, micropinocytosis-, PH-independent endocytosis. Conversely, PDCoV infection was sensitive to nystatin, dynasore and methyl-β-cyclodextrin (MβCD) with reduced PDCoV internalization, indicating that entry of PDCoV into PK-15 cells was caveolae-mediated endocytosis that required dynamin and cholesterol; indirect immunofluorescence and shRNA interference further validated these results. (4) Conclusions: In conclusion, PDCoV entry into PK-15 cells depends on caveolae-mediated endocytosis, which requires cholesterol and dynamin. Our finding is the first initial identification of the endocytic pathway of PDCoV in PK-15 cells, providing a theoretical basis for an in-depth understanding of the pathogenic mechanism of PDCoV and the design of new antiviral targets. [ABSTRACT FROM AUTHOR]

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the pandemic coronavirus disease 2019 (COVID-19), which threatens human health and public safety. In the urgent campaign to develop anti-SARS-CoV-2 therapies, the initial entry step is one of the most appealing targets. In this review, we summarize the current understanding of SARS-CoV-2 cell entry, and the development of targeted antiviral strategies. Moreover, we speculate upon future directions toward next-generation of SARS-CoV-2 entry inhibitors during the upcoming post-pandemic era. Various antivirals were developed to block SARS-CoV-2 entry by targeting the spike protein, including neutralizing monoclonal antibodies, soluble proteins, peptides, as well as small molecules. [Display omitted] [ABSTRACT FROM AUTHOR]

Ebola virus (EBOV) is one of the most pathogenic viruses in humans which can cause a lethal hemorrhagic fever. Understanding the cellular entry mechanisms of EBOV can promote the development of new therapeutic strategies to control virus replication and spread. It has been known that EBOV virions bind to factors expressed at the host cell surface. Subsequently, the virions are internalized by a macropinocytosis-like process, followed by being trafficked through early and late endosomes. Recent researches indicate that the entry of EBOV into cells requires integrated and functional lipid rafts. Whilst lipid rafts have been hypothesized to play a role in virus entry, there is a current lack of supporting data. One major technical hurdle is the lack of effective approaches for observing viral entry. To provide evidence on the involvement of lipid rafts in the entry process of EBOV, we generated the fluorescently labeled Ebola virus like particles (VLPs), and utilized single-particle tracking (SPT) to visualize the entry of fluorescent Ebola VLPs in live cells and the interaction of Ebola VLPs with lipid rafts. In this study, we demonstrate the compartmentalization of Ebola VLPs in lipid rafts during entry process, and inform the essential function of lipid rafts for the entry of Ebola virus. As such, our study provides evidence to show that the raft integrity is critical for Ebola virus pathogenesis and that lipid rafts can serve as potential targets for the development of novel therapeutic strategies.

Lipid rafts, also known as microdomains, are important components of cell membranes and are enriched in cholesterol, glycophospholipids and receptors. They are involved in various essential cellular processes, including endocytosis, exocytosis and cellular signaling. Receptors are concentrated at lipid rafts, through which cellular signaling can be transmitted. Pathogens exploit these signaling mechanisms to enter cells, proliferate and egress. However, lipid rafts also play an important role in initiating antimicrobial responses by sensing pathogens via clustered pathogen-sensing receptors and triggering downstream signaling events such as programmed cell death or cytokine production for pathogen clearance. In this review, we discuss how both host and pathogens use lipid rafts and associated proteins in an arms race to survive. Special attention is given to the involvement of the major vault protein, the main constituent of a ribonucleoprotein complex, which is enriched in lipid rafts upon infection with vaccinia virus.

Severe acute respiratory syndrome-related coronavirus-2 (SARS-CoV-2) has caused the pandemic that began late December 2019. The co-expression of SARS-CoV-2 structural proteins in cells could assemble into several types of virus-like particles (VLPs) without a viral RNA genome. VLPs containing S proteins with the structural and functional properties of authentic virions are safe materials to exploit for virus-cell entry and vaccine development. In this study, to generate SARS-CoV-2 VLPs (SCoV2-SEM VLPs) composed of three structural proteins including spike (S), envelop (E) protein and membrane (M) protein, a tri-cistronic vector expression system was established in a cell line co-expressing SARS-CoV-2 S, E and M proteins. The SCoV2-SEM VLPs were harvested from the cultured medium, and three structure proteins were confirmed by Western blot assay. A negative-stain TEM assay demonstrated the size of the SCoV2-SEM VLPs with a diameter of about 90 nm. To further characterize the infectious properties of SCoV2-SEM VLPs, the VLPs (atto647N-SCoV2-SEM VLPs) were fluorescence-labeled by conjugation with atto-647N and visualized under confocal microscopy at a single-particle resolution. The results of the infection assay revealed that atto647N-SCoV2-SEM VLPs attached to the surface of the HEK293T cells at the pre-binding phase in a ACE2-dependent manner. At the post-infection phase, atto647N-SCoV2-SEM VLPs either fused with the cellular membrane or internalized into the cytoplasm with mCherry-rab5-positive early endosomes. Moreover, fusion with the cellular membrane and the internalization with early endosomes could be inhibited by the treatment of camostat (a pharmacological inhibitor of TMPRSS2) and chlorpromazine (an endocytosis inhibitor), respectively. These results elucidated that SCoV2-SEM VLPs behave similarly to the authentic live SARS-CoV-2 virus, suggesting that the development of SCoV2-SEM VLPs provide a realistic and safe experimental model for studying the infectious mechanism of SARS-CoV-2.

Francisella tularensis is known to release unusually shaped tubular outer membrane vesicles (OMV) containing a number of previously identified virulence factors and immunomodulatory proteins. In this study, we present that OMV isolated from the F. tularensis subsp. holarctica strain FSC200 enter readily into primary bone marrow-derived macrophages (BMDM) and seem to reside in structures resembling late endosomes in the later intervals. The isolated OMV enter BMDM generally via macropinocytosis and clathrin-dependent endocytosis, with a minor role played by lipid raft-dependent endocytosis. OMVs proved to be non-toxic and had no negative impact on the viability of BMDM. Unlike the parent bacterium itself, isolated OMV induced massive and dose-dependent proinflammatory responses in BMDM. Using transmission electron microscopy, we also evaluated OMV release from the bacterial surface during several stages of the interaction of Francisella with BMDM. During adherence and the early phase of the uptake of bacteria, we observed numerous tubular OMV-like protrusions bulging from the bacteria in close proximity to the macrophage plasma membrane. This suggests a possible role of OMV in the entry of bacteria into host cells. On the contrary, the OMV release from the bacterial surface during its cytosolic phase was negligible. We propose that OMV play some role in the extracellular phase of the interaction of Francisella with the host and that they are involved in the entry mechanism of the bacteria into macrophages. [ABSTRACT FROM AUTHOR]

Chikungunya virus (CHIKV) is currently one of the most relevant arboviruses to public health. It is a member of the Togaviridae family and alphavirus genus and causes an arthritogenic disease known as chikungunya fever (CHIKF). It is characterized by a multifaceted disease, which is distinguished from other arbovirus infections by the intense and debilitating arthralgia that can last for months or years in some individuals. Despite the great social and economic burden caused by CHIKV infection, there is no vaccine or specific antiviral drugs currently available. Recent outbreaks have shown a change in the severity profile of the disease in which atypical and severe manifestation lead to hundreds of deaths, reinforcing the necessity to understand the replication and pathogenesis processes. CHIKF is a complex disease resultant from the infection of a plethora of cell types. Although there are several in vivo models for studying CHIKV infection, none of them reproduces integrally the disease signature observed in humans, which is a challenge for vaccine and drug development. Therefore, understanding the potentials and limitations of the state-of-the-art experimental models is imperative to advance in the field. In this context, the present review outlines the present knowledge on CHIKV epidemiology, replication, pathogenesis, and immunity and also brings a critical perspective on the current in vitro and in vivo state-of-the-art experimental models of CHIKF. [ABSTRACT FROM AUTHOR]

Francisella tularensis is known to release unusually shaped tubular outer membrane vesicles (OMV) containing a number of previously identified virulence factors and immunomodulatory proteins. In this study, we present that OMV isolated from the F. tularensis subsp. holarctica strain FSC200 enter readily into primary bone marrow-derived macrophages (BMDM) and seem to reside in structures resembling late endosomes in the later intervals. The isolated OMV enter BMDM generally via macropinocytosis and clathrin-dependent endocytosis, with a minor role played by lipid raft-dependent endocytosis. OMVs proved to be non-toxic and had no negative impact on the viability of BMDM. Unlike the parent bacterium itself, isolated OMV induced massive and dose-dependent proinflammatory responses in BMDM. Using transmission electron microscopy, we also evaluated OMV release from the bacterial surface during several stages of the interaction of Francisella with BMDM. During adherence and the early phase of the uptake of bacteria, we observed numerous tubular OMV-like protrusions bulging from the bacteria in close proximity to the macrophage plasma membrane. This suggests a possible role of OMV in the entry of bacteria into host cells. On the contrary, the OMV release from the bacterial surface during its cytosolic phase was negligible. We propose that OMV play some role in the extracellular phase of the interaction of Francisella with the host and that they are involved in the entry mechanism of the bacteria into macrophages.

The spike (S) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) facilitates viral entry into host cells and is the key target for neutralizing antibodies. The SARS-CoV-2 lineage B.1.620 carries fifteen mutations in the S protein and is spread in Africa, the US and Europe, while lineage R.1 harbors four mutations in S and infections were observed in several countries, particularly Japan and the US. However, the impact of the mutations in B.1.620 and R.1 S proteins on antibody-mediated neutralization and host cell entry are largely unknown. Here, we report that these mutations are compatible with robust ACE2 binding and entry into cell lines, and they markedly reduce neutralization by vaccine-induced antibodies. Our results reveal evasion of neutralizing antibodies by B.1.620 and R.1, which might have contributed to the spread of these lineages.

Rotavirus (RV) is a non–enveloped icosahedral virus with an 11–segment double–stranded RNA genome, belonging to the family of rotaviruses. RV is one of the pathogens causing diarrhea in infants and young animals, and it induces the production of type I interferons (IFNs), which can trigger antiviral function by inducing the production of interferon–stimulated genes (ISGs). Although IFITM3, an ISG localizing to late endosomes, can limit many viral infections, whether or not it restricts the infection of RV is still unknown. Therefore, we attempted to determine whether IFITM3 also restricts RV infection by using over–expression and knockout cell strains. It was found that IFITM3–expressing cell strains were less susceptible to RV infection, as the replication of RV in over–expressing cells was significantly less than in control group cells. Correspondingly, IFITM3–knockout cells were significantly susceptible compared to the normal cells. Furthermore, the IFN–induced antiviral effect was significantly attenuated in the absence of IFITM3, and IFITM3 delayed RV escape from endosomes in the presence of IFITM3, suggesting that endogenous IFITM3 is of great importance in type I IFN–mediated antiviral responses and may restrict infection by affecting the function of the late endosomal compartment. In conclusion, these data provide the first evidence that IFITM3 limits RV infection in vitro and delays RV escape from late endosomes into the cytoplasm.

Cell entry of enveloped viruses relies on the fusion between the viral and plasma or endosomal membranes, through a mechanism that is triggered by a cellular signal. Here we used a combination of computational and experimental approaches to unravel the main determinants of hepatitis B virus (HBV) membrane fusion process. We discovered that ERp57 is a host factor critically involved in triggering HBV fusion and infection. Then, through modeling approaches, we uncovered a putative allosteric cross-strand disulfide (CSD) bond in the HBV S glycoprotein and we demonstrate that its stabilization could prevent membrane fusion. Finally, we identified and characterized a potential fusion peptide in the preS1 domain of the HBV L glycoprotein. These results underscore a membrane fusion mechanism that could be triggered by ERp57, allowing a thiol/disulfide exchange reaction to occur and regulate isomerization of a critical CSD, which ultimately leads to the exposition of the fusion peptide.

In mice persistently infected since birth with the prototypic arenavirus lymphocytic choriomeningitis viurs, viral antigen and RNA are readily detected in most organs and cell types but remarkably absent in skeletal muscle. Here we report that mouse C2C12 myoblasts that are readily infected by LCMV, become highly refractory to LCMV infection upon their differentiation into myotubes. Myotube's resistance to LCMV was not due to an intracellular restriction of virus replication but rather an impaired cell entry mediated by the LCMV surface glycoprotein. Our findings provide an explanation for the observation that in LCMV carrier mice myotubes, which are constantly exposed to blood-containing virus, remain free of viral antigen and RNA despite myotubes express high levels of the LCMV receptor alpha dystroglycan and do not pose an intracellular blockade to LCMV multiplication.

Viruses and their hosts have coevolved for a long time. This coevolution places both the pathogen and the human immune system under selective pressure; on the one hand, the immune system has evolved to combat viruses and virally infected cells, while viruses have developed sophisticated mechanisms to escape recognition and destruction by the immune system. SARS-CoV-2, the pathogen that is causing the current COVID-19 pandemic, has shown a remarkable ability to escape antibody neutralization, putting vaccine efficacy at risk. One of the virus’s immune evasion strategies is mitochondrial sabotage: by causing reactive oxygen species (ROS) production, mitochondrial physiology is impaired, and the interferon antiviral response is suppressed. Seminal studies have identified an intra-cytoplasmatic pathway for viral infection, which occurs through the construction of tunneling nanotubes (TNTs), hence enhancing infection and avoiding immune surveillance. Another method of evading immune monitoring is the disruption of the antigen presentation. In this scenario, SARS-CoV-2 infection reduces MHC-I molecule expression: SARS-CoV-2’s open reading frames (ORF 6 and ORF 8) produce viral proteins that specifically downregulate MHC-I molecules. All of these strategies are also exploited by other viruses to elude immune detection and should be studied in depth to improve the effectiveness of future antiviral treatments. Compared to the Wuhan strain or the Delta variant, Omicron has developed mutations that have impaired its ability to generate syncytia, thus reducing its pathogenicity. Conversely, other mutations have allowed it to escape antibody neutralization and preventing cellular immune recognition, making it the most contagious and evasive variant to date.

Due to the rapidly spreading of novel coronavirus disease (COVID‐19) worldwide, there is an urgent need to develop efficient vaccines and specific antiviral treatments. Pathways of the viral entry into cells are interesting subjects for targeted therapy of severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2). The present study aims to provide a systematic evaluation of the most recent in vitro and in vivo investigations targeting SARS‐CoV‐2 cell entry. A systematic search was carried out in major medical sources, including MEDLINE (through PubMed), Web of Science, Scopus, and EMBASE. Combinations of the following search terms were used: SARS‐CoV‐2, in vitro, in vivo, preclinical, targeted therapy, and cell entry. A modified version of the Consolidated Standards of Reporting Trials and Systematic Review Centre for Laboratory Animal Experimentation assessment tools were applied for evaluating the risk of bias of in vitro and in vivo studies, respectively. A narrative synthesis was performed as a qualitative method for the data synthesis of each outcome measure. A total of 2,649 articles were identified through searching PubMed, Web of Science, Scopus, EMBASE, Google Scholar, and Biorxiv. Finally, 22 studies (one in vivo study and 21 in vitro studies) were included. The spike (S) glycoprotein of the SARS‐CoV‐2 was the main target of investigation in 19 studies. SARS‐CoV‐2 can enter into the host cells through endocytosis or independently. SARS‐CoV‐2 S protein utilizes angiotensin‐converting enzyme 2 or CD147 as its cell‐surface receptor to attach host cells. It consists of S1 and S2 subunits. The S1 subunit mediates viral attachment to the host cells, while the S2 subunit facilitates virus‐host membrane fusion. The cleavage of the S1–S2 protein, which is required for the conformational changes of the S2 subunit and processing of viral fusion, is regulated by the host proteases, including cathepsin L (during endocytosis) and type II membrane serine protease (independently). Targeted therapy strategies against SARS‐CoV‐2 cell entry mechanisms fall into four main categories: strategies targeting virus receptors on the host, strategies neutralizing SARS‐CoV‐2 spike protein, strategies targeting virus fusion to host cells, and strategies targeting endosomal and non‐endosomal dependent pathways of virus entry. Inhibition of the viral entry by targeting host or virus‐related components remains the most potent strategy to prevent and treat COVID‐19. Further high‐quality investigations are needed to assess the efficacy of the proposed targets and develop specific antivirals against SARS‐CoV‐2. [ABSTRACT FROM AUTHOR]

Enterovirus A71 (EV-A71) is a causative agent of hand, foot and mouth disease and occasionally causes death in children. Its infectivity and pathogenesis, however, remain to be better understood. Three sulfonated azo dyes, including acid red 88 (Ar88), were identified to enhance the infectivity of EV-A71, especially isolates with VP1–98K, 145E (-KE), by mainly promoting viral genome release in vitro. Enzymatic removal of sulfated glycosaminoglycans (GAGs) or knockout of xylosyltransferase II (XT2) responsible for biosynthesis of sulfated GAGs weakened the Ar88 enhanced EV-A71 infection. Ar88 is proposed to prevent the -KE variants from being trapped by sulfated GAGs at acidic pH and to facilitate the viral interaction with uncoating factors for genome release in endosomes. The results suggest dual roles of sulfated GAGs as attachment factors and as decoys during host interaction of EV-A71 and caution that these artificial dyes in our environment can enhance viral infection. • Acid red 88 (Ar88) enhanced the infectivity of EV-A71 VP1-98K,145E (-KE) by mainly promoting the viral genome release. • Sulfated glycosaminoglycans (GAGs) was critical for the Ar88 enhanced EV-A71 infection. • Interaction of EV-A71 with heparin, a sulfated GAG, prevented the viruses from binding to viral uncoating factors. • Heparin binding affinity of the -KE variants dramatically increased in acidic pH of endosome and was reduced by Ar88. • Ar88 is proposed to prevent the -KE variants from being trapped by sulfated GAGs and to facilitate the viral uncoating. [ABSTRACT FROM AUTHOR]

(1) Background: Porcine deltacoronavirus (PDCoV) is a newly emerged enteric virus affecting pig breeding industries worldwide, and its pathogenic mechanism remains unclear. (2) Methods: In this study, we preliminarily identified the endocytic pathway of PDCoV in PK-15 cells, using six chemical inhibitors (targeting clathrin-mediated endocytosis, caveolae-mediated endocytosis, macropinocytosis pathway and endosomal acidification), overexpression of dominant-negative (DN) mutants to treat PK-15 cells and proteins knockdown. (3) Results: The results revealed that PDCoV entry was not affected after treatment with chlorpromazine (CPZ), 5-(N-ethyl-N-isopropyl) amiloride (EIPA)or ammonium chloride (NH4Cl), indicating that the entry of PDCoV into PK-15 cells were clathrin-, micropinocytosis-, PH-independent endocytosis. Conversely, PDCoV infection was sensitive to nystatin, dynasore and methyl-β-cyclodextrin (MβCD) with reduced PDCoV internalization, indicating that entry of PDCoV into PK-15 cells was caveolae-mediated endocytosis that required dynamin and cholesterol; indirect immunofluorescence and shRNA interference further validated these results. (4) Conclusions: In conclusion, PDCoV entry into PK-15 cells depends on caveolae-mediated endocytosis, which requires cholesterol and dynamin. Our finding is the first initial identification of the endocytic pathway of PDCoV in PK-15 cells, providing a theoretical basis for an in-depth understanding of the pathogenic mechanism of PDCoV and the design of new antiviral targets.

ABSTRACT Ebola virus (EBOV) causes highly lethal disease outbreaks against which no FDA-approved countermeasures are available. Although many host factors exploited by EBOV for cell entry have been identified, including host cell surface phosphatidylserine receptors, endosomal cysteine proteases, and the lysosomal cholesterol trafficking protein NPC1, key questions remain. Specifically, late entry steps culminating in viral membrane fusion remain enigmatic. Here, we investigated a set of glycoprotein (GP) mutants previously hypothesized to be entry defective and identified one mutation, R64A, that abolished infection with no apparent impact on GP expression, folding, or viral incorporation. R64A profoundly thermostabilized EBOV GP and rendered it highly resistant to proteolysis in vitro. Forward-genetics and cell entry studies strongly suggested that R64A’s effects on GP thermostability and proteolysis arrest viral entry at least at two distinct steps: the first upstream of NPC1 binding and the second at a late entry step downstream of fusion activation. Concordantly, toremifene, a small-molecule entry inhibitor previously shown to bind and destabilize GP, may selectively enhance the infectivity of viral particles bearing GP(R64A) at subinhibitory concentrations. R64A provides a valuable tool to further define the interplay between GP stability, proteolysis, and viral membrane fusion; to explore the rational design of stability-modulating antivirals; and to spur the development of next-generation Ebola virus vaccines with improved stability. IMPORTANCE Ebola virus is a medically relevant virus responsible for outbreaks of severe disease in western and central Africa, with mortality rates reaching as high as 90%. Despite considerable effort, there are currently no FDA-approved therapeutics or targeted interventions available, highlighting the need of development in this area. Host-cell invasion represents an attractive target for antivirals, and several drug candidates have been identified; however, our limited understanding of the complex viral entry process challenges the development of such entry-targeting drugs. Here, we report on a glycoprotein mutation that abrogates viral entry and provides insights into the final steps of this process. In addition, the hyperstabilized phenotype of this mutant makes it useful as a tool in the discovery and design of stability-modulating antivirals and next-generation vaccines against Ebola virus.

With over 80 members worldwide, Orthobunyavirus is the largest genus in the Peribunyaviridae family. Orthobunyaviruses (OBVs) are arthropod-borne viruses that are structurally simple, with a trisegmented, negative-sense RNA genome and only four structural proteins. OBVs are potential agents of emerging and re-emerging diseases and overall represent a global threat to both public and veterinary health. The focus of this review is on the very first steps of OBV infection in mammalian hosts, from virus binding to penetration and release of the viral genome into the cytosol. Here, we address the most current knowledge and advances regarding OBV receptors, endocytosis, and fusion.

A rapid, high‐throughput, and quantitative method for cell entry route characterization is still lacking in nanomedicine research. Here, we report the application of imaging flow cytometry for quantitatively analyzing cell entry routes of actively targeted nanomedicines. We first engineered ICAM1 antibody‐directed fusogenic nanoliposomes (ICAM1‐FusoNLPs) and ICAM1 antibody‐directed endocytic nanolipogels (ICAM1‐EndoNLGs) featuring highly similar surface properties but different cell entry routes: receptor‐mediated membrane fusion and receptor‐mediated endocytosis, respectively. By using imaging flow cytometry, we characterized their intracellular delivery into human breast cancer MDA‐MB‐231 cells. We found that ICAM1‐FusoNLPs mediated a 2.8‐fold increased cell uptake of fluorescent payload, FITC‐dextran, with a 2.4‐fold increased intracellular distribution area in comparison with ICAM1‐EndoNLGs. We also investigated the effects of incubation time and endocytic inhibitors on the cell entry routes of ICAM1‐FusoNLP and ICAM1‐EndoNLG. Our results indicate that receptor‐mediated membrane fusion is a faster and more efficient cell entry route than receptor‐mediated endocytosis, bringing with it a significant therapeutic benefit in a proof‐of‐principle nanomedicine‐mediated siRNA transfection experiment. Our studies suggest that cell entry route may be an important design parameter to be considered in the development of next‐generation nanomedicines. © 2019 International Society for Advancement of Cytometry [ABSTRACT FROM AUTHOR]

Scavenger receptor class B type 1 (SRB1) is a transmembrane protein belonging to the scavenger receptors (SRs) family and it plays an important role in viral entry. Not much is known on SRB1 in teleost fish. Grass carp reovirus (GCRV) cause huge economic losses in grass carp industry. In this study, rare minnow (Gobiocypris rarus) was used as a model fish to investigate the mechanism of GCRV infection, which is sensitive to GCRV. The structure of SRB1 gene in G. rarus (GrSRB1) was cloned and elucidated. GrSRB1 is composed of 13 exons and 12 introns, and its full-length cDNA is 2296 bp in length, with 1521 bp open reading frame (ORF) that encodes a 506 amino acid protein. The Gr SRB1 protein is predicted to contain a typical CD36 domain and two transmembrane regions. In G. rarus , GrSRB1 is expressed strongly in the liver (L), intestines (I), brain (B) and muscle (M), while it is expressed poorly in the heart (H), middle kidney (MK), head kidney (HK) and gills (G). After infection with GCRV, GrSRB1 expression was up-regulated in main immune tissues during the early infection period. Moreover, co-immunoprecipitation assays revealed that Gr SRB1 could interact with the outer capsid protein of GCRV (VP5 and VP7). These results suggest that Gr SRB1 could be a receptor for GCRV. We have managed to characterize the GrSRB1 gene and provide evidence for its potential functions for GCRV entry into host cells. • SRB1 gene in Gobiocypris rarus (GrSRB1) was cloned and elucidated. • GrSRB1 expression was up-regulated in main immune tissues after GCRV challenge. • Gr SRB1 could interacted with the outer capsid protein of GCRV (VP5 and VP7). • Gr SRB1 might play important role in GCRV entry into host cells. [ABSTRACT FROM AUTHOR]

Human metapneumovirus (hMPV) is a crucial pathogen in children. A cell entry is the first step for infection. Our previous study indicated that there was an endocytosis pathway for hMPV cell entry. Lipid raft is a specific structure at the cell surface and it has been demonstrated to play an important role in endocytosis process of many viruses. In this study, we investigated whether and how lipid raft can take part in the hMPV entry. The confocal microscope was used to detect colocalization of hMPV and lipid raft marker. We demonstrated that colocalizations were increased along with the viral infection and hMPV particles transferred to the perinuclear region with lipid raft. When specific lipid raft inhibitors: methyl‐β cyclodextrin and nystatin were used, hMPV cell entry was inhibited and viral titer decreased dramatically. With the replenishment of exogenous cholesterol, hMPV recovered quickly. These data suggest that lipid raft plays an important role in hMPV endocytosis and maybe one of the pathways for hMPV cell entry. Highlights: This study showed lipid raft, as the specific structure at cell surface, plays an important role in hMPV endocytosis and maybe the one of the pathways for hMPV cell entry.This study gave a better understanding of the mechanisms of hMPV cell entry and a new way to prevent and treat its infection. [ABSTRACT FROM AUTHOR]

Lassa virus (LASV) is a notorious human pathogen in West Africa. Its class I trimeric spike complex displays a distinct architecture, and its cell entry mechanism involves unique attributes not shared by other related viruses. We determined the crystal structure of the GP2 fusion glycoprotein from the spike complex of LASV (GP2 LASV) in its post-fusion conformation. GP2 LASV adopts a canonical helical bundle configuration similarly to other viruses in its family. The core packing of GP2 LASV , however, is more organized compared to GP2 from other viruses reducing the formation of internal hydrophobic cavities. We demonstrate a link between the formation of such unfavorable hydrophobic cavities and the efficiencies of membrane fusion and cell entry. Our study suggests that LASV has evolved a more efficient membrane fusogen compared to other viruses from its family by optimizing the post-fusion configuration of its GP2 module. Unlabelled Image • GP2 of Lassa virus adopts a canonical six helical bundle conformation. • Core packing at the membrane proximal region of GP2 varies between viruses. • Unfavorable hydrophobic cavities influence membrane fusion efficiencies. • Lassa virus evolved an optimized fusogen compared to viral family members. [ABSTRACT FROM AUTHOR]

Résumé: Les phlébovirus sont des virus essentiellement transmis à leurs hôtes mammifères par des vecteurs phlébotomes et tiques, plus rarement, moustiques. Ces virus sont présents sur tous les continents et nombreux sont ceux à l'origine de maladies graves, souvent mortelles, chez les animaux domestiques et l'homme. Le réchauffement climatique, l'expansion géographique des réservoirs d'arthropodes et le nombre croissant d'épidémies alertent sur le fait que les phlébovirus doivent être désormais sérieusement considérés comme agents potentiels de maladies émergentes. Cette revue propose de s'intéresser aux étapes précoces de l'infection par les phlébovirus, au niveau moléculaire et cellulaire. Nous aborderons les connaissances et progrès les plus récents sur l'entrée cellulaire de ces virus chez leur hôte mammifère, depuis leur attachement à la surface de la cellule cible jusqu'à leur pénétration dans le cytosol. Nous détaillerons ainsi les récepteurs, les facteurs cellulaires, les voies endocytaires et les mécanismes de fusion utilisés par les phlébovirus pour pénétrer dans la cellule hôte. Phleboviruses constitute a large group of arthropod-borne viruses (arboviruses), mainly transmitted to their hosts by sandflies and ticks, occasionally by mosquitoes. These viruses have a worldwide distribution and many cause serious diseases – often fatal – in both domestic animals and humans. The global warming, the apparent wide distribution of arthropod reservoirs, and the increasing number of outbreaks show that phleboviruses must be taken seriously as emerging disease agents. This review proposes to focus on the early steps of phlebovirus infection, from virus binding to penetration into the cytosol. We address the most recent knowledge and advances in the entry of these viruses into vertebrate host cells, including virus receptors, cellular factors, endocytic pathways, and fusion. [ABSTRACT FROM AUTHOR]

Highlights • Function of tyrosine-protein kinase Src64B-like in the cell entry of BmCPV was investigated. • Silencing the expression of Src64B-like reduced the infection of BmCPV. • Overexpression of Src64B-like enhanced the infection of BmCPV. • BmCPV didn't interact with Src64B-like protein directly in the infection process. Abstract Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) is a non-enveloped dsRNA virus, which specifically infect the midgut epithelium of B. mori. BmCPV enters permissive cells via clathrin-dependent endocytosis employing β1 integrin mediated internalization. Until now, the cell entry mechanism of BmCPV has not been known clearly. Here, we investigated whether tyrosine-protein kinase Src64B-like is involved in the cell entry of BmCPV. The Src64B-like gene was cloned and expressed in Escherichia coli (E. coli), and the recombinant protein Src64B-like was used to immunize mouse for preparation of anti-Src64B-like polyclonal antibody (pAb). After Src64B-like gene was silenced by RNAi, the infection of BmCPV was reduced by 59.48% ± 2.18% and 92.22% ± 1.12% in vitro and in vivo autonomously. Contrary to it, BmCPV infection could be enhanced by increasing the expression of Src64B-like. In addition, immunofluorescence assay showed that Src64B-like protein did not co-localize with BmCPV in the cultured BmN cells during viral infection. These results indicate that Src64B-like protein participates and plays an important role in the cell entry of BmCPV, but not contacting directly with BmCPV. [ABSTRACT FROM AUTHOR]

Phenuiviridae is a large family of arthropod-borne viruses with over 100 species worldwide. Several cause severe diseases in both humans and livestock. Global warming and the apparent geographical expansion of arthropod vectors are good reasons to seriously consider these viruses potential agents of emerging diseases. With an increasing frequency and number of epidemics, some phenuiviruses represent a global threat to public and veterinary health. This review focuses on the early stage of phenuivirus infection in mammalian host cells. We address current knowledge on each step of the cell entry process, from virus binding to penetration into the cytosol. Virus receptors, endocytosis, and fusion mechanisms are discussed in light of the most recent progress on the entry of banda-, phlebo-, and uukuviruses, which together constitute the three prominent genera in the Phenuiviridae family.

Mammalian orthoreovirus (reovirus), a dsRNA virus with a multilayered capsid, serves as a model system for studying the entry of similar viruses. The outermost layer of this capsid undergoes processing to generate a metastable intermediate. The metastable particle undergoes further remodeling to generate an entry-capable form that delivers the genome-containing inner capsid, or core, into the cytoplasm. In this review, we highlight capsid proteins and the intricacies of their interactions that control the stability of the capsid and consequently impact capsid structural changes that are prerequisites for entry. We also discuss a novel proviral role of host membranes in promoting capsid conformational transitions. Current knowledge gaps in the field that are ripe for future investigation are also outlined.

Enveloped viruses employ a class of proteins known as fusogens to orchestrate the merger of their surrounding envelope and a target cell membrane. Most fusogens accomplish this task alone, by binding cellular receptors and subsequently catalyzing the membrane fusion process. Surprisingly, in herpesviruses, these functions are distributed among multiple proteins: the conserved fusogen gB, the conserved gH/gL heterodimer of poorly defined function, and various non-conserved receptor-binding proteins. We summarize what is currently known about gB from two closely related herpesviruses, HSV-1 and HSV-2, with emphasis on the structure of the largely uncharted membrane interacting regions of this fusogen. We propose that the unusual mechanism of herpesvirus fusion could be linked to the unique architecture of gB.

The cellular actin cytoskeleton presents a barrier that must be overcome by many viruses, and it has become increasingly apparent many viral species have developed a diverse repertoire of mechanisms to hijack cellular actin-regulating signalling pathways as part of their cell entry processes. The Rho family GTPase Cdc42 is appreciated as a key moderator of cellular actin dynamics, and the development of specific Cdc42-inhibiting agents has given us an unprecedented ability to investigate its individual role in signalling pathways. However, investigative use of said agents, and the subsequent characterisation of the role Cdc42 plays in viral entry processes has been lacking. Here, we describe the current literature on the role of Cdc42 in human immunodeficiency virus (HIV)-1 cell entry, which represents the most investigated instance of Cdc42 function in viral cell entry processes, and also review evidence of Cdc42 use in other RNA virus cell entries, demonstrating prime areas for more extensive research using similar techniques.

Viruses utilize host cell signaling to facilitate productive infection. Equine herpesvirus type 1 (EHV-1) has been shown to activate Ca2+ release and phospholipase C upon contact with α4β1 integrins on the cell surface. Signaling molecules, including small GTPases, have been shown to be activated downstream of Ca2+ release, and modulate virus entry, membrane remodeling and intracellular transport. In this study, we show that EHV-1 activates the small GTPases Rac1 and Cdc42 during infection. The activation of Rac1 and Cdc42 is necessary for virus-induced acetylation of tubulin, effective viral transport to the nucleus, and cell-to-cell spread. We also show that inhibitors of Rac1 and Cdc42 did not block virus entry, but inhibited overall virus infection. The Rac1 and Cdc42 signaling is presumably orthogonal to Ca2+ release, since Rac1 and Cdc42 inhibitors affected the infection of both EHV-1 and EHV-4, which do not bind to integrins.

Hepatitis C virus (HCV) infection is a major public health issue leading to chronic liver diseases. HCV particles are unique owing to their particular lipid composition, namely the incorporation of neutral lipids and apolipoproteins. The mechanism of association between HCV virion components and these lipoproteins factors remains poorly understood as well as its impact in subsequent steps of the viral life cycle, such as entry into cells. It was proposed that the lipoprotein biogenesis pathway is involved in HCV morphogenesis; yet, recent evidence indicated that HCV particles can mature and evolve biochemically in the extracellular medium after egress. In addition, several viral, cellular and blood components have been shown to influence and regulate this specific association. Finally, this specific structure and composition of HCV particles was found to influence entry into cells as well as their stability and sensitivity to neutralizing antibodies. Due to its specific particle composition, studying the association of HCV particles with lipoproteins remains an important goal towards the rational design of a protective vaccine.

ABSTRACT Botulinum neurotoxins (BoNTs), the most potent toxins known to humans and the causative agent of botulism, exert their effect by entering motor neurons and cleaving and inactivating SNARE proteins, which are essential for neurotransmitter release. BoNTs are proven, valuable pharmaceuticals used to treat more than 200 neuronal disorders. BoNTs comprise 7 serotypes and more than 40 isoforms (subtypes). BoNT/A1 is the only A-subtype used clinically due to its high potency and long duration of action. While other BoNT/A subtypes have been purified and described, only BoNT/A2 is being investigated as an alternative to BoNT/A1. Here we describe subtype BoNT/A6 with improved pharmacological properties compared to BoNT/A1. It was isolated from Clostridium botulinum CDC41370, which produces both BoNT/B2 and BoNT/A6. The gene encoding BoNT/B2 was genetically inactivated, and A6 was isolated to greater than 95% purity. A6 was highly potent in cultured primary rodent neuronal cultures and in human induced pluripotent stem cell-derived neurons, requiring 20-fold less toxin to cause 50% SNAP-25 cleavage than A1. Second, A6 entered hiPSCs faster and more efficiently than A1 and yet had a long duration of action similar to BoNT/A1. Third, BoNT/A6 had similar LD50 as BoNT/A1 after intraperitoneal injection in mice; however, local intramuscular injection resulted in less systemic toxicity than BoNT/A1 and a higher (i.m.) LD50, indicating its potential as a safer pharmaceutical. These data suggest novel characteristics of BoNT/A6 and its potential as an improved pharmaceutical due to more efficient neuronal cell entry, greater ability to remain localized at the injection site, and a long duration. IMPORTANCE Botulinum neurotoxins (BoNTs) have proved to be an effective treatment for a large number of neuropathic conditions. BoNTs comprise a large family of zinc metalloproteases, but BoNT/A1 is used nearly exclusively for pharmaceutical purposes. The genetic inactivation of a second BoNT gene in the native strain enabled expression and isolation of a single BoNT/A6 from cultures. Its characterization indicated that BoNT/A subtype A6 has a long duration of action comparable to A1, while it enters neurons faster and more efficiently and remains more localized after intramuscular injection. These characteristics of BoNT/A6 are of interest for potential use of BoNT/A6 as a novel BoNT-based therapeutic that is effective and has a fast onset, an improved safety profile, and a long duration of action. Use of BoNT/A6 as a pharmaceutical also has the potential to reveal novel treatment motifs compared to currently used treatments.