Influence of N-linked glycans on intracellular transport of hepatitis C virus E1 chimeric glycoprotein and its role in pseudotype virus infectivity

The RSV F p27 peptide: current knowledge, important questions

Respiratory syncytial virus (RSV) remains a leading cause of hospitalizations and death for young children and adults over 65. The worldwide impact of RSV has prioritized the search for an RSV vaccine, with most targeting the critical fusion (F) protein. However, questions remain about the mechanism of RSV entry and RSV F triggering and fusion promotion. This review highlights these questions, specifically those surrounding a cleaved 27 amino acids long peptide within F, p27.

SARS-CoV-2 spike protein receptor-binding domain N-glycans facilitate viral internalization in respiratory epithelial cells

N-glycosylation plays an important role in the pathogenesis of viral infections. However, the role of SARS-CoV-2 RBD N-glycosylation in viral entry remains elusive. In this study, we expressed and purified N331 and N343 N-glycosite mutants of SARS-CoV-2 RBD. We found that de-glycosylation at N331 and N343 drastically reduces the RBD binding to ACE2. More importantly, based on qualitative and quantitative virology research methods, we show that the mutation of RBD N-glycosites interfered with SARS-CoV-2 internalization rather than attachment potentially by decreasing RBD binding to the receptors. Also, the double N-glycosites mutant (N331 + N343) showed significantly increased sensitivity against the designated RBD neutralizing antibodies. Taken together, these results suggest that N-glycosylation of SARS-CoV-2 RBD is not only critical for viral internalization into respiratory epithelial cells but also shields the virus from neutralization. It may provide new insights into the biological process of early-stage SARS-CoV-2 infection with potential therapeutic implications.

Hepatitis C Virus Glycan-Dependent Interactions and the Potential for Novel Preventative Strategies

Chronic hepatitis C virus (HCV) infections continue to be a major contributor to liver disease worldwide. HCV treatment has become highly effective, yet there are still no vaccines or prophylactic strategies available to prevent infection and allow effective management of the global HCV burden. Glycan-dependent interactions are crucial to many aspects of the highly complex HCV entry process, and also modulate immune evasion. This review provides an overview of the roles of viral and cellular glycans in HCV infection and highlights glycan-focused advances in the development of entry inhibitors and vaccines to effectively prevent HCV infection.

Guinea pig cytomegalovirus trimer complex gH/gL/gO uses PDGFRA as universal receptor for cell fusion and entry

Species-specific guinea pig cytomegalovirus (GPCMV) causes congenital CMV and the virus encodes homolog glycoprotein complexes to human CMV, including gH-based trimer (gH/gL/gO) and pentamer-complex (PC). Platelet-derived growth factor receptor alpha (gpPDGFRA), only present on fibroblast cells, was identified via CRISPR as the putative receptor for PC-independent GPCMV infection. Immunoprecipitation assays demonstrated direct interaction of gH/gL/gO with gpPDGFRA but not in absence of gO. Expression of viral gB also resulted in precipitation of gB/gH/gL/gO/gpPDGFRA complex. Cell-cell fusion assays determined that expression of gpPDGFRA and gH/gL/gO in adjacent cells enabled cell fusion, which was not enhanced by gB. N-linked gpPDGFRA glycosylation inhibition had limited effect and blocking tyrosine kinase (TK) transduction had no impact on infection. Ectopically expressed gpPDGFRA or TK-domain mutant in trophoblast or epithelial cells previously non-susceptible to GPCMV(PC-) enabled viral infection. In contrast, transient human PDGFRA expression did not complement GPCMV(PC-) infection, a potential basis for viral species specificity.

Blocking cellular N-glycosylation suppresses human cytomegalovirus entry in human fibroblasts

N-glycosylation plays an important role in the pathogenesis of viral infections. However, the role of host cell N-glycosylation in human cytomegalovirus (hCMV) infection remains to be elucidated. In this study, we found that blocking or removal of cellular N-glycosylation by tunicamycin, peptide-N-glycosidase F (PNGase F) treatment, or N-acetylglucosaminyltransferase I (MGAT1) knockdown resulted in suppression of hCMV infection in human fibroblasts. This suppression was reversed following N-glycosylation restoration. Immunofluorescence and flow cytometry analysis showed that blockade of cellular N-glycosylation interfered with hCMV entry rather than binding. Removal of N-glycosylation on epidermal growth factor (EGFR) and integrin β3, two proposed hCMV receptors, blocked their interaction with hCMV glycoproteins B and H. It also suppressed activation of these receptors and downstream integrin β3/Src signaling. Taken together, these results suggest that N-glycosylation of host cell glycoproteins including two proposed hCMV receptors is critical for hCMV entry rather than attachment. They provide novel insights into the biological process important for the early stage of hCMV infection with potential therapeutic implications.

Glycosylation of viral surface proteins probed by mass spectrometry

Glycosylation is a common and biologically significant post-translational modification that is found on numerous virus surface proteins (VSPs). Many of these glycans affect virulence through modulating virus receptor binding, masking antigenic sites, or by stimulating the host immune response. Mass spectrometry (MS) has arisen as a pivotal technique for the characterization of VSP glycosylation. This review will cover how MS-based analyses, such as released glycan profiles, glycan site localization, site-occupancy, and site-specific heterogeneity, are being utilized to map VSP glycosylation. Furthermore, this review will provide information on how MS glycoprofiling data are being used in conjunction with molecular and structural experiments to provide a better understanding of the role of specific glycans in VSP function.

Characterization of the role of N-glycosylation sites in the respiratory syncytial virus fusion protein in virus replication, syncytium formation and antigenicity

Respiratory syncytial virus (RSV) is a leading cause of infant hospitalization worldwide each year and there is presently no licensed vaccine to prevent severe RSV infections. Two major RSV glycoproteins, attachment (G) and fusion (F) protein, regulate viral replication and both proteins contain potential glycosylation sites which are highly variable for the G protein and conserved for the F protein among virus isolates. The RSV F sequence possesses five N-glycosylation sites located in the F2 subunit (N27 and N70), the p27 peptide (N116 and N126) and the F1 subunit (N500). The importance of RSV F N-glycosylation in virus replication and immunogenicity is not yet fully understood, and a better understanding may provide new insights for vaccine development. By using a BAC-based reverse genetics system, recombinant viruses expressing F proteins with loss of N-glycosylation sites were made. Mutant viruses with single N-glycosylation sites removed could be recovered, while this was not possible with the mutant with all N-glycosylation sites removed. Although the individual RSV F N-glycosylation sites were shown not to be essential for viral replication, they do contribute to the efficiency of in vitro and in vivo viral infection. To evaluate the role of N-glycosylation sites on RSV F antigenicity, serum antibody titers were determined after infection of BALB/c mice with RSV expressing the glycomutant F proteins. Infection with recombinant virus lacking the N-glycosylation site at position N116 (RSV F N116Q) resulted in significant higher neutralizing antibody titers compared to RSV F WT infection, which is surprising since this N-glycan is present in the p27 peptide which is assumed to be absent from the mature F protein in virions. Thus, single or combined RSV F glycomutations which affect virus replication and fusogenicity, and which may induce enhanced antibody responses upon immunization could have the potential to improve the efficacy of RSV LAV approaches.

Removal of the N-Glycosylation Sequon at Position N116 Located in p27 of the Respiratory Syncytial Virus Fusion Protein Elicits Enhanced Antibody Responses after DNA Immunization

Prevention of severe lower respiratory tract infections in infants caused by the human respiratory syncytial virus (hRSV) remains a major public health priority. Currently, the major focus of vaccine development relies on the RSV fusion (F) protein since it is the main target protein for neutralizing antibodies induced by natural infection. The protein conserves 5 N-glycosylation sites, two of which are located in the F2 subunit (N27 and N70), one in the F1 subunit (N500) and two in the p27 peptide (N116 and N126). To study the influence of the loss of one or more N-glycosylation sites on RSV F immunogenicity, BALB/c mice were immunized with plasmids encoding RSV F glycomutants. In comparison with F WT DNA immunized mice, higher neutralizing titres were observed following immunization with F N116Q. Moreover, RSV A2-K-line19F challenge of mice that had been immunized with mutant F N116Q DNA was associated with lower RSV RNA levels compared with those in challenged WT F DNA immunized animals. Since p27 is assumed to be post-translationally released after cleavage and thus not present on the mature RSV F protein, it remains to be elucidated how deletion of this glycan can contribute to enhanced antibody responses and protection upon challenge. These findings provide new insights to improve the immunogenicity of RSV F in potential vaccine candidates.

Lectin microarray and mass spectrometric analysis of hepatitis C proteins reveals N-linked glycosylation

We used lectin microarray and mass spectrometric analysis to identify the N-linked glycosylation patterns of hepatitis C virus (HCV) particles. HCV J6/JFH-1 chimeric cell culture (HCVcc) in the culture supernatant was concentrated and purified by ultrafiltration and sucrose gradient ultracentrifugation. Twelve fractions were collected from the top and analyzed for viral infectivity and HCV RNA content after sucrose gradient separation. HCV RNA and proteins were separated by ultracentrifugation in a continuous 10% to 60% sucrose gradient to purify viral particles based on their sedimentation velocities. HCVcc particles were found mainly in fractions 6 to 8, as determined by quantitative polymerase chain reaction (qPCR) analysis for HCV RNA and ELISA of the HCV core protein. The N-glycans on HCV proteins were analyzed by lectin microarray and mass spectrometry. We identified that 32 of 37 lectins displayed the positive binding signals and 16 types of N-glycoforms of which the major HCV glycoforms were high mannose-type N-linked oligosaccharides, hybrid N-glycans, and fucosylated N-glycans. Our study provided new detailed information regarding the majority of the glycan-protein profile, complementing to previous findings of glycan-HCV protein interactions.

Myeloid C-Type Lectin Receptors in Viral Recognition and Antiviral Immunity

Recognition of viral glycans by pattern recognition receptors (PRRs) in innate immunity contributes to antiviral immune responses. C-type lectin receptors (CLRs) are PRRs capable of sensing glycans present in viral pathogens to activate antiviral immune responses such as phagocytosis, antigen processing and presentation, and subsequent T cell activation. The ability of CLRs to elicit and shape adaptive immunity plays a critical role in the inhibition of viral spread within the host. However, certain viruses exploit CLRs for viral entry into host cells to avoid immune recognition. To block CLR interactions with viral glycoproteins, antiviral strategies may involve the use of multivalent glycan carrier systems. In this review, we describe the role of CLRs in antiviral immunity and we highlight their dual function in viral clearance and exploitation by viral pathogens.

Monitoring of S protein maturation in the endoplasmic reticulum by calnexin is important for the infectivity of severe acute respiratory syndrome coronavirus

Severe acute respiratory syndrome coronavirus (SARS-CoV) is the etiological agent of SARS, a fatal pulmonary disorder with no effective treatment. We found that SARS-CoV spike glycoprotein (S protein), a key molecule for viral entry, binds to calnexin, a molecular chaperone in the endoplasmic reticulum (ER), but not to calreticulin, a homolog of calnexin. Calnexin bound to most truncated mutants of S protein, and S protein bound to all mutants of calnexin. Pseudotyped virus carrying S protein (S-pseudovirus) produced by human cells that were treated with small interfering RNA (siRNA) for calnexin expression (calnexin siRNA-treated cells) showed significantly lower infectivity than S-pseudoviruses produced by untreated and control siRNA-treated cells. S-pseudovirus produced by calnexin siRNA-treated cells contained S protein modified with N-glycan side chains differently from other two S proteins and consisted of two kinds of viral particles: those of normal density with little S protein and those of high density with abundant S protein. Treatment with peptide-N-glycosidase F (PNGase F), which removes all types of N-glycan side chains from glycoproteins, eliminated the infectivity of S-pseudovirus. S-pseudovirus and SARS-CoV produced in the presence of α-glucosidase inhibitors, which disrupt the interaction between calnexin and its substrates, showed significantly lower infectivity than each virus produced in the absence of those compounds. In S-pseudovirus, the incorporation of S protein into viral particles was obviously inhibited. In SARS-CoV, viral production was obviously inhibited. These findings demonstrated that calnexin strictly monitors the maturation of S protein by its direct binding, resulting in conferring infectivity on SARS-CoV.

Progress toward development of a hepatitis C vaccine with broad shoulders

Hepatitis C virus (HCV) infection in humans can cause progressive and end-stage liver disease. As such, preventive measures against HCV, including vaccine development, are a priority among researchers in the field. The report from Garrone et al. describes the development of a vaccine platform to generate HCV-neutralizing antibodies that are based on retrovirus-derived virus-like particles (VLPs) pseudotyped with heterologous viral envelope proteins. Immunization with these VLPs induced neutralizing antibodies in mouse and macaque models. These results, when considered in the context of an earlier clinical trial that used recombinant HCV E1/E2 purified protein as a subunit vaccine and additional findings from the VLP strategy, may lead to a new HCV vaccine that induces a neutralizing antibody response.

A new model to produce infectious hepatitis C virus without the replication requirement

Numerous constraints significantly hamper the experimental study of hepatitis C virus (HCV). Robust replication in cell culture occurs with only a few strains, and is invariably accompanied by adaptive mutations that impair in vivo infectivity/replication. This problem complicates the production and study of authentic HCV, including the most prevalent and clinically important genotype 1 (subtypes 1a and 1b). Here we describe a novel cell culture approach to generate infectious HCV virions without the HCV replication requirement and the associated cell-adaptive mutations. The system is based on our finding that the intracellular environment generated by a West-Nile virus (WNV) subgenomic replicon rendered a mammalian cell line permissive for assembly and release of infectious HCV particles, wherein the HCV RNA with correct 5′ and 3′ termini was produced in the cytoplasm by a plasmid-driven dual bacteriophage RNA polymerase-based transcription/amplification system. The released particles preferentially contained the HCV-based RNA compared to the WNV subgenomic RNA. Several variations of this system are described with different HCV-based RNAs: (i) HCV bicistronic particles (HCVbp) containing RNA encoding the HCV structural genes upstream of a cell-adapted subgenomic replicon, (ii) HCV reporter particles (HCVrp) containing RNA encoding the bacteriophage SP6 RNA polymerase in place of HCV nonstructural genes, and (iii) HCV wild-type particles (HCVwt) containing unmodified RNA genomes of diverse genotypes (1a, strain H77; 1b, strain Con1; 2a, strain JFH-1). Infectivity was assessed based on the signals generated by the HCV RNA molecules introduced into the cytoplasm of target cells upon virus entry, i.e. HCV RNA replication and protein production for HCVbp in Huh-7.5 cells as well as for HCVwt in HepG2-CD81 cells and human liver slices, and SP6 RNA polymerase-driven firefly luciferase for HCVrp in target cells displaying candidate HCV surface receptors. HCV infectivity was inhibited by pre-incubation of the particles with anti-HCV antibodies and by a treatment of the target cells with leukocyte interferon plus ribavirin. The production of authentic infectious HCV particles of virtually any genotype without the adaptive mutations associated with in vitro HCV replication represents a new paradigm to decipher the requirements for HCV assembly, release, and entry, amenable to analyses of wild type and genetically modified viruses of the most clinically significant HCV genotypes.

Two decades after its identification, hepatitis C virus (HCV) remains a leading cause of serious liver diseases worldwide. The poor in vitro propagation of patient isolates has impaired their study. Conversely, viral strains of the most prevalent (∼70% of total infections) and clinically problematic (∼45% cured with the standard of care) genotype 1 adapted for in vitro replication display mutations impairing yield and/or in vivo infectivity. We established a new cell culture model for producing infectious HCV in a cell line stably bearing a subgenomic replicon from West Nile virus (a flavivirus belonging to the same family as HCV) that circumvents the requirement for HCV RNA replication. To study viral infectivity in vitro, we devised several HCV genome-based constructs. This system produced wild type HCV particles of subtypes 1a, 1b, 2a and a 1b/2a chimera. All specifically infected permissive target cells, and HCV particles containing wild type genomes known to be infectious in vivo infected human liver slices ex vivo. The production of authentic HCV particles independent of HCV RNA replication represents a new paradigm to decipher requirements for HCV assembly, release, and entry, amenable to analyses of wild type and genetically modified viruses of the most clinically significant genotypes.

Pathobiology of virus glycosylation: implications to disease and prospects for treatment

Changes to the overall glycosylation profile of viral glycoproteins have been shown to be advantageous to virus survival and virulence. Many human viral pathogens rely on specific oligosaccharides to evade detection by the host immune system. Viruses such as HIV, Hendra, SARS-CoV, influenza, respiratory syncytial virus, hepatitis and West Nile virus rely on N-linked and O-Linked glycosylation for critical functions such as entry into host cells, proteolytic processing and protein trafficking. Recent findings demonstrate the importance of glycosylation to viral virulence, infectivity and immune evasion in several virus families impacting on human health. This review considers the role of glycosylation in viral infection and will detail several potential therapies for these important human pathogens and emerging infections.

Deletion of N-glycosylation sites of hepatitis C virus envelope protein E1 enhances specific cellular and humoral immune responses

N-linked glycosylations of viral proteins have been implicated in immunogenicity. In this study, the effects of the N-linked glycosylation of the hepatitis C virus (HCV) E1 protein, a naturally poor immunogen, on the induction of specific immune response were examined. We constructed the plasmids containing the genes encoding both wild type and mutant E1 proteins in which N-linked glycosylation sites are mutated individually or in combination by site-directed mutagenesis. The immunogenicity of wild type E1 and six mutated E1 proteins was analyzed in BALB/C mice using a DNA-based vaccination approach. We found that E1-M2 mutant (at site of N209SS) significantly enhanced E1-specific CD8(+)T cells cytotoxic T lymphocytes (CTL) activities, expression of IFN-gamma producing T cells, and suppression of tumor growth. While E1-M4 mutant (at site of N305CS) induced the highest specific antibody response among all groups. Moreover, E1 wild-type vaccinated mice developed a mixture of IgG1 and Ig2a, but E1-M2 mutant induced only IgG2a isotype, and E1-M4 mutant dominantly developed IgG1 isotype. Our data showed that N-linked glycosylation can limit both cellular and antibody response to the HCV E1 protein and deletion of the N-glycosylation sites at N209SS and N305CS of hepatitis C virus envelope protein E1 provided potential applications for the development of DNA vaccine with enhanced immunogenicity.

Virus glycosylation: role in virulence and immune interactions

The study of N-linked glycosylation as it relates to virus biology has become an area of intense interest in recent years due to its ability to impart various advantages to virus survival and virulence. HIV and influenza, two clear threats to human health, have been shown to rely on expression of specific oligosaccharides to evade detection by the host immune system. Additionally, other viruses such as Hendra, SARS-CoV, influenza, hepatitis and West Nile rely on N-linked glycosylation for crucial functions such as entry into host cells, proteolytic processing and protein trafficking. This review focuses on recent findings on the importance of glycosylation to viral virulence and immune evasion for several prominent human pathogens.

Sulfated homologues of heparin inhibit hepatitis C virus entry into mammalian cells

The mechanism of entry of hepatitis C virus (HCV) through interactions between the envelope glycoproteins and specific cell surface receptors remains unclear at this time. We have previously shown with the vesicular stomatitis virus (VSV)/HCV pseudotype model that the hypervariable region 1 of the HCV E2 envelope glycoprotein helps in binding with glycosaminoglycans present on the cell surface. In this study, we have examined the binding of HCV envelope glycoproteins with chemically modified derivatives of heparin. Furthermore, we have determined the functional relevance of the interaction of heparin derivatives with HCV envelope glycoproteins for infectivity by using a human immunodeficiency virus (HIV)/HCV pseudotype, a VSV/HCV pseudotype, and cell culture-grown HCV genotype 1a. Taken together, our results suggest that the HCV envelope glycoproteins rely upon O-sulfated esters of a heparin homologue to facilitate entry into mammalian cells.

Factors that influence VSV-G pseudotyping and transduction efficiency of lentiviral vectors—in vitro andin vivo implications

Pseudotyping viral vectors with vesicular stomatitis virus glycoprotein (VSV-G) enables the transduction of an extensive range of cell types from different species. We have discovered two important parameters of the VSV-G-pseudotyping phenomenon that relate directly to the transduction potential of lentiviral vectors: (1) the glycosylation status of VSV-G, and (2) the quantity of glycoprotein associated with virions. We measured production-cell and virion-associated quantities of two isoform variants of VSV-G, which differ in their glycosylation status, VSV-G1 and VSV-G2, and assessed the impact of this difference on the efficiency of mammalian cell transduction by lentiviral vectors. The glycosylation of VSV-G at N336 allowed greater maximal expression of VSV-G in HEK293T cells, thus facilitating vector pseudotyping. The transduction of primate cell lines was substantially affected (up to 50-fold) by the degree of VSV-G1 or VSV-G2 incorporation, whereas other cell lines, such as D17 (canine), were less sensitive to virion-associated VSV-G1/2 quantities. These data indicate that the minimum required concentration of virion-associated VSV-G differs substantially between cell species/types. The implications of these data with regard to VSV-G-pseudotyped vector production, titration, and use in host-cell restriction studies, are discussed.

Use of influenza C virus glycoprotein HEF for generation of vesicular stomatitis virus pseudotypes

Influenza C virus contains two envelope glycoproteins: CM2, a putative ion channel protein; and HEF, a unique multifunctional protein that performs receptor-binding, receptor-destroying and fusion activities. Here, it is demonstrated that expression of HEF is sufficient to pseudotype replication-incompetent vesicular stomatitis virus (VSV) that lacks the VSV glycoprotein (G) gene. The pseudotyped virus showed characteristic features of influenza C virus with respect to proteolytic activation, receptor usage and cell tropism. Chimeric glycoproteins composed of HEF ectodomain and VSV-G C-terminal domains were efficiently incorporated into VSV particles and showed receptor-binding and receptor-destroying activities but, unlike authentic HEF, did not mediate efficient infection, probably because of impaired fusion activity. HEF-pseudotyped VSV efficiently infected polarized Madin-Darby canine kidney cells via the apical plasma membrane, whereas entry of VSV-G-complemented virus was restricted to the basolateral membrane. These findings suggest that pseudotyping of viral vectors with HEF might be useful for efficient apical gene transfer into polarized epithelial cells and for targeting cells that express 9-O-acetylated sialic acids.

Coexpression of hepatitis C virus E1 and E2 chimeric envelope glycoproteins displays separable ligand sensitivity and increases pseudotype infectious titer

We have previously reported that a pseudotype virus generated by reconstitution of hepatitis C virus (HCV) chimeric envelope glycoprotein E1-G or E2-G on the surface of a temperature-sensitive mutant of vesicular stomatitis virus (VSVts045) interacts independently with mammalian cells to initiate infection. Here, we examined whether coexpression of both of the envelope glycoproteins on pseudotype particles would augment virus infectivity and/or alter the functional properties of the individual subunits. Stable transfectants of baby hamster kidney (BHK) epithelial cells expressing either one or both of the chimeric envelope glycoproteins of HCV on the cell surface were generated. The infectious titer of the VSV pseudotype, derived from a stable cell line incorporating both of the chimeric glycoproteins of HCV, was approximately 4- to 5-fold higher than that of a pseudotype bearing E1-G alone or approximately 25- to 30-fold higher than that of E2-G alone when assayed with a number of mammalian cell lines. Further studies suggested that that the E1-G/E2-G or E2-G pseudotype was more sensitive to the inhibitory effect of heparin than the E1-G pseudotype. Treatment of the E1-G/E2-G pseudotype with a negatively charged sulfated sialyl lipid (NMSO3) displayed a approximately 4-fold-higher sensitivity to neutralization than pseudotypes with either of the two individual glycoproteins. In contrast, VSVts045, used as a backbone for the generation of pseudotypes, displayed at least 20-fold-higher sensitivity to NMSO3-mediated inhibition of virus plaque formation. The effect of low-density lipoprotein on the E1-G pseudotype was greater than that apparent for the E1-G/E2-G pseudotype. The treatment of cells with monoclonal antibodies to CD81 displayed an inhibitory effect upon the pseudotype with E1-G/E2-G or with E2-G alone. Taken together, our results indicate that the HCV E1 and E2 glycoproteins have separable functional properties and that the presence of these two envelope glycoproteins on VSV/HCV pseudotype particles increases infectious titer.