Hepatitis C virus entry depends on clathrin-mediated endocytosis

Proximity labeling of host factor ANXA3 in HCV infection reveals a novel LARP1 function in viral entry

Hepatitis C virus (HCV) infection is tightly connected to the lipid metabolism with lipid droplets (LDs) serving as assembly sites for progeny virions. A previous LD proteome analysis identified annexin A3 (ANXA3) as an important HCV host factor that is enriched at LDs in infected cells and required for HCV morphogenesis. To further characterize ANXA3 function in HCV, we performed proximity labeling using ANXA3-BioID2 as bait in HCV-infected cells. Two of the top proteins identified proximal to ANXA3 during HCV infection were the La-related protein 1 (LARP1) and the ADP ribosylation factor-like protein 8B (ARL8B), both of which have been previously described to act in HCV particle production. In follow-up experiments, ARL8B functioned as a pro-viral HCV host factor without localizing to LDs and thus likely independent of ANXA3. In contrast, LARP1 interacts with HCV core protein in an RNA-dependent manner and is translocated to LDs by core protein. Knockdown of LARP1 decreased HCV spreading without altering HCV RNA replication or viral titers. Unexpectedly, entry of HCV particles and E1/E2-pseudotyped lentiviral particles was reduced by LARP1 depletion, whereas particle production was not altered. Using a recombinant vesicular stomatitis virus (VSV)ΔG entry assay, we showed that LARP1 depletion also decreased entry of VSV with VSV, MERS, and CHIKV glycoproteins. Therefore, our data expand the role of LARP1 as an HCV host factor that is most prominently involved in the early steps of infection, likely contributing to endocytosis of viral particles through the pleiotropic effect LARP1 has on the cellular translatome.

Biomarkers in Detection of Hepatitis C Virus Infection

The hepatitis C virus (HCV) infection affects 58 million people worldwide. In the United States, the incidence rate of acute hepatitis C has doubled since 2014; during 2021, this increased to 5% from 2020. Acute hepatitis C is defined by any symptom of acute viral hepatitis plus either jaundice or elevated serum alanine aminotransferase (ALT) activity with the detection of HCV RNA, the anti-HCV antibody, or hepatitis C virus antigen(s). However, most patients with acute infection are asymptomatic. In addition, ALT activity and HCV RNA levels can fluctuate, and a delayed detection of the anti-HCV antibody can occur among some immunocompromised persons with HCV infection. The detection of specific biomarkers can be of great value in the early detection of HCV infection at an asymptomatic stage. The high rate of HCV replication (which is approximately 1010 to 1012 virions per day) and the lack of proofreading by the viral RNA polymerase leads to enormous genetic diversity, creating a major challenge for the host immune response. This broad genetic diversity contributes to the likelihood of developing chronic infection, thus leading to the development of cirrhosis and liver cancer. Direct-acting antiviral (DAA) therapies for HCV infection are highly effective with a cure rate of up to 99%. At the same time, many patients with HCV infection are unaware of their infection status because of the mostly asymptomatic nature of hepatitis C, so they remain undiagnosed until the liver damage has advanced. Molecular mechanisms induced by HCV have been intensely investigated to find biomarkers for diagnosing the acute and chronic phases of the infection. However, there are no clinically verified biomarkers for patients with hepatitis C. In this review, we discuss the biomarkers that can differentiate acute from chronic hepatitis C, and we summarize the current state of the literature on the useful biomarkers that are detectable during acute and chronic HCV infection, liver fibrosis/cirrhosis, and hepatocellular carcinoma (HCC).

Packaging Quantum Dots in Viral Particles via a Strep-tag II/Streptavidin System for Single-Virus Tracking

Single-virus tracking provides a powerful tool for studying virus infection with high spatiotemporal resolution. Quantum dots (QDs) are used to label and track viral particles due to their brightness and photostability. However, labeling viral particles with QDs is not easy. We developed a new method for labeling viral particles with QDs by using the Strep-tag II/streptavidin system. In this method, QDs were site-specifically ligated to viral proteins in live cells and then packaged into viral-like particles (VLPs) of tick-borne encephalitis virus (TBEV) and Ebola virus during viral assembly. With TBEV VLP-QDs, we tracked the clathrin-mediated endocytic entry of TBEV and studied its intracellular dynamics at the single-particle level. Our Strep-tag II/streptavidin labeling procedure eliminates the need for BirA protein expression or biotin addition, providing a simple and general method for site-specifically labeling viral particles with QDs for single-virus tracking.

HCV-induced autophagy and innate immunity

The interplay between autophagy and host innate immunity has been of great interest. Hepatitis C virus (HCV) impedes signaling pathways initiated by pattern-recognition receptors (PRRs) that recognize pathogens-associated molecular patterns (PAMPs). Autophagy, a cellular catabolic process, delivers damaged organelles and protein aggregates to lysosomes for degradation and recycling. Autophagy is also an innate immune response of cells to trap pathogens in membrane vesicles for removal. However, HCV controls the autophagic pathway and uses autophagic membranes to enhance its replication. Mitophagy, a selective autophagy targeting mitochondria, alters the dynamics and metabolism of mitochondria, which play important roles in host antiviral responses. HCV also alters mitochondrial dynamics and promotes mitophagy to prevent premature cell death and attenuate the interferon (IFN) response. In addition, the dysregulation of the inflammasomal response by HCV leads to IFN resistance and immune tolerance. These immune evasion properties of HCV allow HCV to successfully replicate and persist in its host cells. In this article, we discuss HCV-induced autophagy/mitophagy and its associated immunological responses and provide a review of our current understanding of how these processes are regulated in HCV-infected cells.

Roles of epidermal growth factor receptor, claudin-1 and occludin in multi-step entry of hepatitis C virus into polarized hepatoma spheroids

The multi-step process of hepatitis C virus (HCV) entry is facilitated by various host factors, including epidermal growth factor receptor (EGFR) and the tight junction proteins claudin-1 (CLDN1) and occludin (OCLN), which are thought to function at later stages of the HCV entry process. Using single particle imaging of HCV infection of polarized hepatoma spheroids, we observed that EGFR performs multiple functions in HCV entry, both phosphorylation-dependent and -independent. We previously observed, and in this study confirmed, that EGFR is not required for HCV migration to the tight junction. EGFR is required for the recruitment of clathrin to HCV in a phosphorylation-independent manner. EGFR phosphorylation is required for virion internalization at a stage following the recruitment of clathrin. HCV entry activates the RAF-MEK-ERK signaling pathway downstream of EGFR phosphorylation. This signaling pathway regulates the sorting and maturation of internalized HCV into APPL1- and EEA1-associated early endosomes, which form the site of virion uncoating. The tight junction proteins, CLDN1 and OCLN, function at two distinct stages of HCV entry. Despite its appreciated function as a "late receptor" in HCV entry, CLDN1 is required for efficient HCV virion accumulation at the tight junction. Huh-7.5 cells lacking CLDN1 accumulate HCV virions primarily at the initial basolateral surface. OCLN is required for the late stages of virion internalization. This study produced further insight into the unusually complex HCV endocytic process.

Modulation of the replication of positive-sense RNA viruses by the natural plant metabolite xanthohumol and its derivatives

The COVID-19 pandemic has highlighted the importance of identifying new potent antiviral agents. Nutrients as well as plant-derived substances are promising candidates because they are usually well tolerated by the human body and readily available in nature, and consequently mostly cheap to produce. A variety of antiviral effects have recently been described for the hop chalcone xanthohumol (XN), and to a lesser extent for its derivatives, making these hop compounds particularly attractive for further investigation. Noteworthy, mounting evidence indicated that XN can suppress a wide range of viruses belonging to several virus families, all of which share a common reproductive cycle. As a result, the purpose of this review is to summarize the most recent research on the antiviral properties of XN and its derivatives, with a particular emphasis on the positive-sense RNA viruses human hepatitis C virus (HCV), porcine reproductive and respiratory syndrome virus (PRRSV), and severe acute respiratory syndrome corona virus (SARS-CoV-2).

CRISPR-Cas system to discover host-virus interactions in Flaviviridae

The Flaviviridae virus family members cause severe human diseases and are responsible for considerable mortality and morbidity worldwide. Therefore, researchers have conducted genetic screens to enhance insight into viral dependency and develop potential anti-viral strategies to treat and prevent these infections. The host factors identified by the clustered regularly interspaced short palindromic repeats (CRISPR) system can be potential targets for drug development. Meanwhile, CRISPR technology can be efficiently used to treat viral diseases as it targets both DNA and RNA. This paper discusses the host factors related to the life cycle of viruses of this family that were recently discovered using the CRISPR system. It also explores the role of immune factors and recent advances in gene editing in treating flavivirus-related diseases. The ever-increasing advancements of this technology may promise new therapeutic approaches with unique capabilities, surpassing the traditional methods of drug production and treatment.

Genetic and pharmacological perturbation of hepatitis-C virus entry

Hepatitis-C virus (HCV) chronically infects 58 million individuals worldwide with variable disease outcome. While a subfraction of individuals exposed to the virus clear the infection, the majority develop chronic infection if untreated. Another subfraction of chronically ill proceeds to severe liver disease. The underlying causes of this interindividual variability include genetic polymorphisms in interferon genes. Here, we review available data on the influence of genetic or pharmacological perturbation of HCV host dependency factors on the clinically observed interindividual differences in disease outcome. We focus on host factors mediating virus entry into human liver cells. We assess available data on genetic variants of the major entry factors scavenger receptor class-B type I, CD81, claudin-1, and occludin as well as pharmacological perturbation of these entry factors. We review cell culture experimental and clinical cohort study data and conclude that entry factor perturbation may contribute to disease outcome of hepatitis C.

Advancement in Cellular Topographic and Nanoparticle Capture Imaging by High Resolution Microscopy Incorporating a Freeze-Drying and Gaseous Nitrogen-based Approach

Scanning electron microscopy (SEM) offers an unparalleled view of the membrane topography of mammalian cells by using a conventional osmium (OsO4) and ethanol-based tissue preparation. However, conventional SEM methods limit optimal resolution due to ethanol and lipid interactions and interfere with visualization of fluorescent reporter proteins. Therefore, SEM correlative light and electron microscopy (CLEM) has been hindered by the adverse effects of ethanol and OsO4 on retention of fluorescence signals. To overcome this technological gap in achieving high-resolution SEM and retain fluorescent reporter signals, we developed a freeze-drying method with gaseous nitrogen (FDGN). We demonstrate that FDGN preserves cyto-architecture to allow visualization of detailed membrane topography while retaining fluorescent signals and that FDGN processing can be used in conjunction with a variety of high-resolution imaging systems to enable collection and validation of unique, high-quality data from these approaches. In particular, we show that FDGN coupled with high resolution microscopy provided detailed insight into viral or tumor-derived extracellular vesicle (TEV)-host cell interactions and may aid in designing new approaches to intervene during viral infection or to harness TEVs as therapeutic agents.

Host Cell Targets for Unconventional Antivirals against RNA Viruses

The recent COVID-19 crisis has highlighted the importance of RNA-based viruses. The most prominent members of this group are SARS-CoV-2 (coronavirus), HIV (human immunodeficiency virus), EBOV (Ebola virus), DENV (dengue virus), HCV (hepatitis C virus), ZIKV (Zika virus), CHIKV (chikungunya virus), and influenza A virus. With the exception of retroviruses which produce reverse transcriptase, the majority of RNA viruses encode RNA-dependent RNA polymerases which do not include molecular proofreading tools, underlying the high mutation capacity of these viruses as they multiply in the host cells. Together with their ability to manipulate the immune system of the host in different ways, their high mutation frequency poses a challenge to develop effective and durable vaccination and/or treatments. Consequently, the use of antiviral targeting agents, while an important part of the therapeutic strategy against infection, may lead to the selection of drug-resistant variants. The crucial role of the host cell replicative and processing machinery is essential for the replicative cycle of the viruses and has driven attention to the potential use of drugs directed to the host machinery as therapeutic alternatives to treat viral infections. In this review, we discuss small molecules with antiviral effects that target cellular factors in different steps of the infectious cycle of many RNA viruses. We emphasize the repurposing of FDA-approved drugs with broad-spectrum antiviral activity. Finally, we postulate that the ferruginol analog (18-(phthalimide-2-yl) ferruginol) is a potential host-targeted antiviral.

Astrocytes in the pathophysiology of neuroinfection

Key homeostasis providing cells in the central nervous system (CNS) are astrocytes, which belong to the class of cells known as atroglia, a highly heterogeneous type of neuroglia and a prominent element of the brain defence. Diseases evolve due to altered homeostatic state, associated with pathology-induced astroglia remodelling represented by reactive astrocytes, astroglial atrophy and astrodegeneration. These features are hallmarks of most infectious insults, mediated by bacteria, protozoa and viruses; they are also prominent in the systemic infection. The COVID-19 pandemic revived the focus into neurotropic viruses such as SARS-CoV2 (Coronaviridae) but also the Flaviviridae viruses including tick-borne encephalitis (TBEV) and Zika virus (ZIKV) causing the epidemic in South America prior to COVID-19. Astrocytes provide a key response to neurotropic infections in the CNS. Astrocytes form a parenchymal part of the blood-brain barrier, the site of virus entry into the CNS. Astrocytes exhibit aerobic glycolysis, a form of metabolism characteristic of highly morphologically plastic cells, like cancer cells, hence a suitable milieu for multiplication of infectious agent, including viral particles. However, why the protection afforded by astrocytes fails in some circumstances is an open question to be studied in the future.

The Role of Cytosolic Lipid Droplets in Hepatitis C Virus Replication, Assembly, and Release

The hepatitis C virus (HCV) causes chronic hepatitis by establishing a persistent infection. Patients with chronic hepatitis frequently develop hepatic cirrhosis, which can lead to liver cancer-the progressive liver damage results from the host's immune response to the unresolved infection. The HCV replication process, including the entry, replication, assembly, and release stages, while the virus circulates in the bloodstream, it is intricately linked to the host's lipid metabolism, including the dynamic of the cytosolic lipid droplets (cLDs). This review article depicts how this interaction regulates viral cell tropism and aids immune evasion by coining viral particle characteristics. cLDs are intracellular organelles that store most of the cytoplasmic components of neutral lipids and are assumed to play an increasingly important role in the pathophysiology of lipid metabolism and host-virus interactions. cLDs are involved in the replication of several clinically significant viruses, where viruses alter the lipidomic profiles of host cells to improve viral life cycles. cLDs are involved in almost every phase of the HCV life cycle. Indeed, pharmacological modulators of cholesterol synthesis and intracellular trafficking, lipoprotein maturation, and lipid signaling molecules inhibit the assembly of HCV virions. Likewise, small-molecule inhibitors of cLD-regulating proteins inhibit HCV replication. Thus, addressing the molecular architecture of HCV replication will aid in elucidating its pathogenesis and devising preventive interventions that impede persistent infection and prevent disease progression. This is possible via repurposing the available therapeutic agents that alter cLDs metabolism. This review highlights the role of cLD in HCV replication.

A review on hepatitis C virus: role of viral and host-cellular factors in replication and existing therapeutic strategies

Background

Hepatitis C virus, a member of Flaviviridae is a single-stranded positive-sense RNA virus infecting 62–79 million people around the globe. This blood-borne virus is one of the leading causes of liver diseases worldwide. This review aims to identify novel potential genes linked to cellular host factors, as well as revise the roles of each gene in hepatitis C Virus infection. This review also aims to provide a comprehensive insight into therapeutic advancements against HCV.

Methods

For this review article, 190 articles were searched via PubMed Central, Bio-One, National Academy of Science, Google Scholar, and Worldwide Science. 0ut of these 190 studies, 55 articles were selected for this review. The inclusion of articles was done on the criteria of high citation and Q1 ranking.

Results

The information gathered from previously published articles highlighted a critical link between host-cellular factors that are important for HCV infection.

Conclusion

Although many advancements in HCV treatment have been made like DAAs and HTAs, the development of a completely effective HCV therapy is still a challenge. Further research on combinations of DAAs and HTAs can help in developing a better therapeutic alternative. Keywords: Hepatitis C virus, Replication cycle, Non-structural proteins, Host-cellular factors, Treatment strategies

Avian Reovirus σB Interacts with Caveolin-1 in Lipid Rafts during Dynamin-Dependent Caveolae-Mediated Endocytosis

Caveolin-1 (Cav-1) is the basic component of caveolae, a specialized form of lipid raft that plays an essential role in endocytic viral entry. However, the evidence of direct involvement of caveolae and Cav-1 in avian reovirus (ARV) entry remains insufficient. In this study, the membrane lipid rafts were isolated as detergent-resistant microdomains (DRMs) by sucrose gradient centrifugation, and the capsid protein σB of ARV was found to associate with Cav-1 in DRMs fractions. Additionally, the interaction between ARV σB protein and Cav-1 was demonstrated by immunofluorescence co-localization and co-immunoprecipitation assays. Furthermore, we found that the internalization of ARV is sensitive to caveolae and dynamin inhibitors, while it is insensitive to clathrin inhibitors. In conclusion, these results indicate that the ARV σB protein interacts with Cav-1 during dynamin-dependent caveolae-mediated endocytosis for the entry of ARV.

Occludin stalls HCV particle dynamics apart from hepatocyte tight junctions, promoting virion internalization

BACKGROUND AND AIMS

Numerous HCV entry factors have been identified, and yet information regarding their spatiotemporal dynamics is still limited. Specifically, one of the main entry factors of HCV is occludin (OCLN), a protein clustered at tight junctions (TJs), away from the HCV landing site. Thus, whether HCV particles slide toward TJs or, conversely, OCLN is recruited away from TJs remain debated.

APPROACH AND RESULTS

Here, we generated CRISPR/CRISPR-associated protein 9 edited Huh7.5.1 cells expressing endogenous levels of enhanced green fluorescent protein/OCLN and showed that incoming HCV particles recruit OCLN outside TJs, independently of claudin 1 (CLDN1) expression, another important HCV entry factor located at TJs. Using ex vivo organotypic culture of hepatic slices obtained from human liver explants, a physiologically relevant model that preserves the overall tissue architecture, we confirmed that HCV associates with OCLN away from TJs. Furthermore, we showed, by live cell imaging, that increased OCLN recruitment beneath HCV particles correlated with lower HCV motility. To decipher the mechanism underlying virus slow-down upon OCLN recruitment, we performed CRISPR knockout (KO) of CLDN1, an HCV entry factor proposed to act upstream of OCLN. Although CLDN1 KO potently inhibits HCV infection, OCLN kept accumulating underneath the particle, indicating that OCLN recruitment is CLDN1 independent. Moreover, inhibition of the phosphorylation of Ezrin, a protein involved in HCV entry that links receptors to the actin cytoskeleton, increased OCLN accumulation and correlated with more efficient HCV internalization.

CONCLUSIONS

Together, our data provide robust evidence that HCV particles interact with OCLN away from TJs and shed mechanistic insights regarding the manipulation of transmembrane receptor localization by extracellular virus particles.

Senescent endothelial cells are predisposed to SARS-CoV-2 infection and subsequent endothelial dysfunction

The coronavirus disease 2019 (COVID-19), caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), remains to spread worldwide. COVID-19 is characterized by the striking high mortality in elderly; however, its mechanistic insights remain unclear. Systemic thrombosis has been highlighted in the pathogenesis of COVID-19, and lung microangiopathy in association with endothelial cells (ECs) injury has been reported by post-mortem analysis of the lungs. Here, we experimentally investigated the SARS-CoV-2 infection in cultured human ECs, and performed a comparative analysis for post-infection molecular events using early passage and replicative senescent ECs. We found that; (1) SARS-CoV-2 infects ECs but does not replicate and disappears in 72 hours without causing severe cell damage, (2) Senescent ECs are highly susceptible to SARS-CoV-2 infection, (3) SARS-CoV-2 infection alters various genes expression, which could cause EC dysfunctions, (4) More genes expression is affected in senescent ECs by SARS-CoV-2 infection than in early passage ECs, which might causes further exacerbated dysfunction in senescent ECs. These data suggest that sustained EC dysfunctions due to SARS-CoV-2 infection may contribute to the microangiopathy in the lungs, leading to deteriorated inflammation and thrombosis in COVID-19. Our data also suggest a possible causative role of EC senescence in the aggravated disease in elder COVID-19 patients.

Generation of fluorescent HCV pseudoparticles to study early viral entry events- involvement of Rab1a in HCV entry

Understanding the early events in viral biology holds the key to the development of potent preventives. In this study, fluorescent hepatitis C virus pseudoparticles (HCVpp) have been generated where the envelope glycoprotein of Hepatitis C virus (HCV) has an EGFP tag. Using these pseudoparticles, entry assays were conducted where their entry was tracked via confocal microscopy. Using this system, fusion of host and viral membranes is predicted to occur within 15 min of HCV entry. Using cells with a knockdown for Rab1a, HCV trafficking was observed to be altered, indicating a role of Rab1a in HCV trafficking. In conclusion, this study reports the generation and use of fluorescent HCVpp which may be used to understand the early events of viral entry. This system may be adapted for the study of other enveloped viruses as well.

An entropic safety catch controls hepatitis C virus entry and antibody resistance

E1 and E2 (E1E2), the fusion proteins of Hepatitis C Virus (HCV), are unlike that of any other virus yet described, and the detailed molecular mechanisms of HCV entry/fusion remain unknown. Hypervariable region-1 (HVR-1) of E2 is a putative intrinsically disordered protein tail. Here, we demonstrate that HVR-1 has an autoinhibitory function that suppresses the activity of E1E2 on free virions; this is dependent on its conformational entropy. Thus, HVR-1 is akin to a safety catch that prevents premature triggering of E1E2 activity. Crucially, this mechanism is turned off by host receptor interactions at the cell surface to allow entry. Mutations that reduce conformational entropy in HVR-1, or genetic deletion of HVR-1, turn off the safety catch to generate hyper-reactive HCV that exhibits enhanced virus entry but is thermally unstable and acutely sensitive to neutralising antibodies. Therefore, the HVR-1 safety catch controls the efficiency of virus entry and maintains resistance to neutralising antibodies. This discovery provides an explanation for the ability of HCV to persist in the face of continual immune assault and represents a novel regulatory mechanism that is likely to be found in other viral fusion machinery.

Insights Into the Coinfections of Human Immunodeficiency Virus-Hepatitis B Virus, Human Immunodeficiency Virus-Hepatitis C Virus, and Hepatitis B Virus-Hepatitis C Virus: Prevalence, Risk Factors, Pathogenesis, Diagnosis, and Treatment

Human immunodeficiency virus, hepatitis B virus, and hepatitis C virus are three blood-borne viruses that can cause major global health issues by increasing severe morbidity. There is a high risk of coinfection with these viruses in individuals because of their same transmission routes through blood using shared needles, syringes, other injection equipment, sexual transmission, or even vertical transmission. Coinfection can cause various liver-related illnesses, non-hepatic organ dysfunction, followed by death compared to any of these single infections. The treatment of coinfected patients is complicated due to the side effects of antiviral medication, resulting in drug resistance, hepatotoxicity, and a lack of required responses. On the other hand, coinfected individuals must be treated with multiple drugs simultaneously, such as for HIV either along with HBV or HCV and HBV and HCV. Therefore, diagnosing, treating, and controlling dual infections with HIV, HBV, or HCV is complicated and needs further investigation. This review focuses on the current prevalence, risk factors, and pathogenesis of dual infections with HIV, HBV, and HCV. We also briefly overviewed the diagnosis and treatment of coinfections of these three blood-borne viruses.

CD46 isoforms influence the mode of entry by human herpesvirus 6A/B in T cells

CD46 is a receptor for human herpesvirus 6A (HHV-6A) and is in some cells also important for infection with HHV-6B. CD46 has several isoforms of which the most commonly expressed can be distinguished by expression of a BC domain or a C domain in a serine-threonine-proline rich (STP) extracellular region. Using a SupT1 CD46 CRISPR-Cas9 knockout model system reconstituted with specific CD46 isoforms, we demonstrated that HHV-6A infection was more efficient when BC-isoforms were expressed as opposed to C-isoforms, measured by higher levels of intracellular viral transcripts and recovery of more progeny virus. Although the B domain contains several O-glycosylations, mutations of Ser and Thr residues did not prevent infection with HHV-6A. The HHV-6A infection was blocked by inhibitors of clathrin-mediated endocytosis. In contrast, infection with HHV-6B was preferentially promoted by C-isoforms mediating fusion-from-without, and this infection was less affected by inhibitors of clathrin-mediated endocytosis. Taken together, HHV-6A preferred BC isoforms, mediating endocytosis, whereas HHV-6B preferred C isoforms, mediating fusion-from-without. This demonstrates that the STP region of CD46 is important for regulating the mode of infection in SupT1 cells and suggests an epigenetic regulation of the host susceptibility to HHV-6A and HHV-6B infection. Importance CD46 is the receptor used by human herpesvirus 6A (HHV-6A) during infection of T cells, but it is also involved in infection of certain T cells by HHV-6B. The gene for CD46 allows expression of several variants of CD46, known as isoforms, but whether the isoforms matter for infection of T cells is unknown. We used a genetic approach to delete CD46 from T cells and reconstituted them with separate isoforms to study these individually. We expressed the isoforms known as BC and C, which are distinguished by the potential inclusion of a B domain in the CD46 molecule. We demonstrate that HHV-6A prefers the BC isoform to infect T cells, and this occurs predominantly by clathrin-mediated endocytosis. In contrast, HHV-6B prefers the C isoform and infects predominantly by fusion-from-without. Thus, CD46 isoforms may affect susceptibility of T cells to infection with HHV-6A and HHV-6B.

Detection methods targeting the positive- and negative-sense RNA transcripts from plus-stranded RNA viruses

The largest group of viruses in the Baltimore classification system comprises viruses with a positive-sense, single-stranded RNA genome. Once the viral genome is released into the cytoplasm of a specific host cell following virus entry, it functions directly as a mRNA and the virus-encoded proteins that are essential for genome replication, are produced by the translation apparatus of the host cell. The positive-sense genome is replicated in two stages, initially the positive strand is copied to make a negative-sense RNA, which then functions as the template for transcription of many new positive-sense genomes. Virus infections can be detected at different stages throughout the infection cycle for diagnostic and scientific purposes. Here, the advantages and disadvantages of some of the relevant methods for genome detection will be briefly reviewed with special emphasis on techniques allowing strand-specific RNA detection. Furthermore, tools of the future are considered.

Host Defence RNases as Antiviral Agents against Enveloped Single Stranded RNA Viruses

Owing to the recent outbreak of Coronavirus Disease of 2019 (COVID-19), it is urgent to develop effective and safe drugs to treat the present pandemic and prevent other viral infections that might come in the future. Proteins from our own innate immune system can serve as ideal sources of novel drug candidates thanks to their safety and immune regulation versatility. Some host defense RNases equipped with antiviral activity have been reported over time. Here, we try to summarize the currently available information on human RNases that can target viral pathogens, with special focus on enveloped single-stranded RNA (ssRNA) viruses. Overall, host RNases can fight viruses by a combined multifaceted strategy, including the enzymatic target of the viral genome, recognition of virus unique patterns, immune modulation, control of stress granule formation, and induction of autophagy/apoptosis pathways. The review also includes a detailed description of representative enveloped ssRNA viruses and their strategies to interact with the host and evade immune recognition. For comparative purposes, we also provide an exhaustive revision of the currently approved or experimental antiviral drugs. Finally, we sum up the current perspectives of drug development to achieve successful eradication of viral infections.

Algal and Cyanobacterial Lectins and Their Antimicrobial Properties

Lectins are proteins with a remarkably high affinity and specificity for carbohydrates. Many organisms naturally produce them, including animals, plants, fungi, protists, bacteria, archaea, and viruses. The present report focuses on lectins produced by marine or freshwater organisms, in particular algae and cyanobacteria. We explore their structure, function, classification, and antimicrobial properties. Furthermore, we look at the expression of lectins in heterologous systems and the current research on the preclinical and clinical evaluation of these fascinating molecules. The further development of these molecules might positively impact human health, particularly the prevention or treatment of diseases caused by pathogens such as human immunodeficiency virus, influenza, and severe acute respiratory coronaviruses, among others.

Involvement of adaptor proteins in clathrin-mediated endocytosis of virus entry

The first step in the initiation of effective viral infection is breaking through the cytomembrane to enter the cell. Clathrin-mediated endocytosis is a key vesicular trafficking process in which a variety of cargo molecules are transported from the outside to the inside of the cell. This process is hijacked by numerous families of enveloped or non-enveloped viruses, which use it to enter host cells, followed by trafficking to their replicating sites. Various adaptor proteins that assist in cargo selection, coat assembly, and clathrin-coated bud maturation are important in this process. Research data documented on the involvement of adaptor proteins, such as AP-2, Eps-15, Epsin1, and AP180/CALM, in the invasion of viruses via the clathrin-mediated endocytosis have provided novel insights into understanding the viral life cycle and have led to the development of novel therapeutics. Here, we summarize the latest discoveries on the role of these adaptor proteins in clathrin-mediated endocytosis of virus entry and also discuss the future trends in this field.

Aptamers in Virology-A Consolidated Review of the Most Recent Advancements in Diagnosis and Therapy

The use of short oligonucleotide or peptide molecules as target-specific aptamers has recently garnered substantial attention in the field of the detection and treatment of viral infections. Based on their high affinity and high specificity to desired targets, their use is on the rise to replace antibodies for the detection of viruses and viral antigens. Furthermore, aptamers inhibit intracellular viral transcription and translation, in addition to restricting viral entry into host cells. This has opened up a plethora of new targets for the research and development of novel vaccines against viruses. Here, we discuss the advances made in aptamer technology for viral diagnosis and therapy in the past decade.

Phenothiazines as dual inhibitors of SARS-CoV-2 main protease and COVID-19 inflammation

COVID-19, caused by the severe acute respiratory coronavirus 2 (SARS-CoV-2), currently has no treatment for acute infection. The main protease (Mpro) of SARS-CoV-2 is an essential enzyme for viral replication and an attractive target for disease intervention. The phenothiazine moiety has demonstrated drug versatility for biological systems, including inhibition of butyrylcholinesterase, a property important in the cholinesterase anti-inflammatory cascade. Nineteen phenothiazine drugs were investigated using in silico modelling techniques to predict binding energies and inhibition constants (Ki values) with SARS-CoV-2 Mpro. Because most side-effects of phenothiazines are due to interactions with various neurotransmitter receptors and transporters, phenothiazines with few such interactions were also investigated. All compounds were found to bind to the active site of SARS-CoV-2 Mpro and showed Ki values ranging from 1.30 to 52.4 µM in a rigid active site. Nine phenothiazines showed inhibition constants <10 µM. The compounds with limited interactions with neurotransmitter receptors and transporters showed micromolar (µM) Ki values. Docking results were compared with remdesivir and showed similar interactions with key residues Glu-166 and Gln-189 in the active site. This work has identified several phenothiazines with limited neurotransmitter receptor and transporter interactions and that may provide the dual action of inhibiting SARS-CoV-2 Mpro to prevent viral replication and promote the release of anti-inflammatory cytokines to curb viral-induced inflammation. These compounds are promising candidates for further investigation against SARS-CoV-2.

Small-Molecule Inhibition of Viral Fusion Glycoproteins

Viral fusion glycoproteins catalyze membrane fusion during viral entry. Unlike most enzymes, however, they lack a conventional active site in which formation or scission of a specific covalent bond is catalyzed. Instead, they drive the membrane fusion reaction by cojoining highly regulated changes in conformation to membrane deformation. Despite the challenges in applying inhibitor design approaches to these proteins, recent advances in knowledge of the structures and mechanisms of viral fusogens have enabled the development of small-molecule inhibitors of both class I and class II viral fusion proteins. Here, we review well-validated inhibitors, including their discovery, targets, and mechanism(s) of action, while highlighting mechanistic similarities and differences. Together, these examples make a compelling case for small-molecule inhibitors as tools for probing the mechanisms of viral glycoprotein-mediated fusion and for viral glycoproteins as druggable targets.

Challenges and Prospects of Plant-Derived Oral Vaccines against Hepatitis B and C Viruses

Hepatitis B and C viruses chronically affect approximately 3.5% of the global population, causing more than 800,000 deaths yearly due to severe liver pathogenesis. Current HBV vaccines have significantly contributed to the reduction of chronic HBV infections, supporting the notion that virus eradication is a feasible public health objective in the near future. In contrast to HBV, a prophylactic vaccine against HCV infection is not available yet; however, intense research efforts within the last decade have significantly advanced the field and several vaccine candidates are shortlisted for clinical trials. A successful vaccine against an infectious disease of global importance must not only be efficient and safe, but also easy to produce, distribute, administer, and economically affordable to ensure appropriate coverage. Some of these requirements could be fulfilled by oral vaccines that could complement traditional immunization strategies. In this review, we discuss the potential of edible plant-based oral vaccines in assisting the worldwide fight against hepatitis B and C infections. We highlight the latest research efforts to reveal the potential of oral vaccines, discuss novel antigen designs and delivery strategies, as well as the limitations and controversies of oral administration that remain to be addressed to make this approach successful.

The degree of polymerization and sulfation patterns in heparan sulfate are critical determinants of cytomegalovirus entry into host cells

Several enveloped viruses, including herpesviruses attach to host cells by initially interacting with cell surface heparan sulfate (HS) proteoglycans followed by specific coreceptor engagement which culminates in virus-host membrane fusion and virus entry. Interfering with HS-herpesvirus interactions has long been known to result in significant reduction in virus infectivity indicating that HS play important roles in initiating virus entry. In this study, we provide a series of evidence to prove that specific sulfations as well as the degree of polymerization (dp) of HS govern human cytomegalovirus (CMV) binding and infection. First, purified CMV extracellular virions preferentially bind to sulfated longer chain HS on a glycoarray compared to a variety of unsulfated glycosaminoglycans including unsulfated shorter chain HS. Second, the fraction of glycosaminoglycans (GAG) displaying higher dp and sulfation has a larger impact on CMV titers compared to other fractions. Third, cell lines deficient in specific glucosaminyl sulfotransferases produce significantly reduced CMV titers compared to wild-type cells and virus entry is compromised in these mutant cells. Finally, purified glycoprotein B shows strong binding to heparin, and desulfated heparin analogs compete poorly with heparin for gB binding. Taken together, these results highlight the significance of HS chain length and sulfation patterns in CMV attachment and infectivity.

Epigenetic Mechanisms Involved in HCV-Induced Hepatocellular Carcinoma (HCC)

Hepatocellular carcinoma (HCC), is the third leading cause of cancer-related deaths, which is largely caused by virus infection. About 80% of the virus-infected people develop a chronic infection that eventually leads to liver cirrhosis and hepatocellular carcinoma (HCC). With approximately 71 million HCV chronic infected patients worldwide, they still have a high risk of HCC in the near future. However, the mechanisms of carcinogenesis in chronic HCV infection have not been still fully understood, which involve a complex epigenetic regulation and cellular signaling pathways. Here, we summarize 18 specific gene targets and different signaling pathways involved in recent findings. With these epigenetic alterations requiring histone modifications and DNA hyper or hypo-methylation of these specific genes, the dysregulation of gene expression is also associated with different signaling pathways for the HCV life cycle and HCC. These findings provide a novel insight into a correlation between HCV infection and HCC tumorigenesis, as well as potentially preventable approaches. Hepatitis C virus (HCV) infection largely causes hepatocellular carcinoma (HCC) worldwide with 3 to 4 million newly infected cases diagnosed each year. It is urgent to explore its underlying molecular mechanisms for therapeutic treatment and biomarker discovery. However, the mechanisms of carcinogenesis in chronic HCV infection have not been still fully understood, which involve a complex epigenetic regulation and cellular signaling pathways. Here, we summarize 18 specific gene targets and different signaling pathways involved in recent findings. With these epigenetic alterations requiring histone modifications and DNA hyper or hypo-methylation of these specific genes, the dysregulation of gene expression is also associated with different signaling pathways for the HCV life cycle and HCC. These findings provide a novel insight into a correlation between HCV infection and HCC tumorigenesis, as well as potentially preventable approaches.

Combating Human Viral Diseases: Will Plant-Based Vaccines Be the Answer?

Molecular pharming or the technology of application of plants and plant cell culture to manufacture high-value recombinant proteins has progressed a long way over the last three decades. Whether generated in transgenic plants by stable expression or in plant virus-based transient expression systems, biopharmaceuticals have been produced to combat several human viral diseases that have impacted the world in pandemic proportions. Plants have been variously employed in expressing a host of viral antigens as well as monoclonal antibodies. Many of these biopharmaceuticals have shown great promise in animal models and several of them have performed successfully in clinical trials. The current review elaborates the strategies and successes achieved in generating plant-derived vaccines to target several virus-induced health concerns including highly communicable infectious viral diseases. Importantly, plant-made biopharmaceuticals against hepatitis B virus (HBV), hepatitis C virus (HCV), the cancer-causing virus human papillomavirus (HPV), human immunodeficiency virus (HIV), influenza virus, zika virus, and the emerging respiratory virus, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) have been discussed. The use of plant virus-derived nanoparticles (VNPs) and virus-like particles (VLPs) in generating plant-based vaccines are extensively addressed. The review closes with a critical look at the caveats of plant-based molecular pharming and future prospects towards further advancements in this technology. The use of biopharmed viral vaccines in human medicine and as part of emergency response vaccines and therapeutics in humans looks promising for the near future.

Hepatitis C virus genotype 4: A poorly characterized endemic genotype

Globally, 13% of all hepatitis C virus (HCV) infections are caused by genotype 4 (GT4), which consists of 17 subtypes with various levels of susceptibility to anti-HCV therapy. This genotype is endemic in the Middle East and Africa and has considerably spread to Europe lately. The molecular features of HCV-GT4 infection, as well as its appropriate therapeutics, are poorly characterized as it has not been the subject of widespread basic research. As such, in this review, we aim to gather the current state of knowledge of this genotype with a particular emphasis on its heterogeneity, sequence signatures, resistance-associated substitutions, and available in vivo and in vitro models used for its study. We urge developing more cell-culture models based on different GT4 subtypes to better understand the virology and therapeutic response of this particular genotype. This review may raise more awareness about this genotype and trigger more basic research work to develop its research tools. This will be critical to design better therapeutics and help to provide adequate guidelines for physicians working with HCV-GT4 patients.

NPC1-regulated dynamic of clathrin-coated pits is essential for viral entry

Viruses utilize cellular lipids and manipulate host lipid metabolism to ensure their replication and spread. Therefore, the identification of lipids and metabolic pathways that are suitable targets for antiviral development is crucial. Using a library of compounds targeting host lipid metabolic factors and testing them for their ability to block pseudorabies virus (PRV) and vesicular stomatitis virus (VSV) infection, we found that U18666A, a specific inhibitor of Niemann-Pick C1 (NPC1), is highly potent in suppressing the entry of diverse viruses including pseudotyped severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). NPC1 deficiency markedly attenuates viral growth by decreasing cholesterol abundance in the plasma membrane, thereby inhibiting the dynamics of clathrin-coated pits (CCPs), which are indispensable for clathrin-mediated endocytosis. Significantly, exogenous cholesterol can complement the dynamics of CCPs, leading to efficient viral entry and infectivity. Administration of U18666A improves the survival and pathology of PRV- and influenza A virus-infected mice. Thus, our studies demonstrate a unique mechanism by which NPC1 inhibition achieves broad antiviral activity, indicating a potential new therapeutic strategy against SARS-CoV-2, as well as other emerging viruses.

PmAP2-β depletion enhanced activation of the Toll signaling pathway during yellow head virus infection in the black tiger shrimp Penaeus monodon

Yellow head virus (YHV) is a pathogen which causes high mortality in penaeid shrimp. Previous studies suggested that YHV enters shrimp cells via clathrin-mediated endocytosis. This research investigated the roles of clathrin adaptor protein 2 subunit β (AP-2β) from Penaeus monodon during YHV infection. PmAP2-β was continuously up-regulated more than twofold during 6–36 hpi. Suppression of PmAP2-β significantly reduced YHV copy numbers and delayed shrimp mortality. Quantitative RT-PCR revealed that knockdown of PmAP2-β significantly enhanced the expression level of PmSpätzle, a signaling ligand in the Toll pathway, by 30-fold at 6 and 12 hpi. Moreover, the expression levels of gene components in the Imd and JAK/STAT signaling pathways under the suppression of PmAP2-β during YHV infection were also investigated. Interestingly, anti-lipopolysaccharide factor isoform 3 (ALFPm3) was up-regulated by 40-fold in PmAP2-β knockdown shrimp upon YHV infection. In addition, silencing of PmAP2-β dramatically enhanced crustinPm1 expression in YHV-infected shrimp. Knockdown of ALFPm3 and crustinPm1 significantly reduced shrimp survival rate. Taken together, this work suggested that PmAP2-β-deficiency promoted the Toll pathway signalings, resulting in elevated levels of ALFPm3 and crustinPm1, the crucial antimicrobial peptides in defence against YHV.

Strategy, Progress, and Challenges of Drug Repurposing for Efficient Antiviral Discovery

Emerging or re-emerging viruses are still major threats to public health. Prophylactic vaccines represent the most effective way to prevent virus infection; however, antivirals are more promising for those viruses against which vaccines are not effective enough or contemporarily unavailable. Because of the slow pace of novel antiviral discovery, the high disuse rates, and the substantial cost, repurposing of the well-characterized therapeutics, either approved or under investigation, is becoming an attractive strategy to identify the new directions to treat virus infections. In this review, we described recent progress in identifying broad-spectrum antivirals through drug repurposing. We defined the two major categories of the repurposed antivirals, direct-acting repurposed antivirals (DARA) and host-targeting repurposed antivirals (HTRA). Under each category, we summarized repurposed antivirals with potential broad-spectrum activity against a variety of viruses and discussed the possible mechanisms of action. Finally, we proposed the potential investigative directions of drug repurposing.

Drug repurposing approach to combating coronavirus: Potential drugs and drug targets

In the past two decades, three highly pathogenic human coronaviruses severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus, and, recently, SARS-CoV-2, have caused pandemics of severe acute respiratory diseases with alarming morbidity and mortality. Due to the lack of specific anti-CoV therapies, the ongoing pandemic of coronavirus disease 2019 (COVID-19) poses a great challenge to clinical management and highlights an urgent need for effective interventions. Drug repurposing is a rapid and feasible strategy to identify effective drugs for combating this deadly infection. In this review, we summarize the therapeutic CoV targets, focus on the existing small molecule drugs that have the potential to be repurposed for existing and emerging CoV infections of the future, and discuss the clinical progress of developing small molecule drugs for COVID-19.

Chloroquine, an Anti-Malaria Drug as Effective Prevention for Hantavirus Infections

We investigated whether chloroquine can prevent hantavirus infection and disease in vitro and in vivo, using the Hantaan virus newborn C57BL/6 mice model and the Syrian hamster model for Andes virus. In vitro antiviral experiments were performed using Vero E6 cells, and Old World and New World hantavirus species. Hantavirus RNA was detected using quantitative RT-PCR. For all hantavirus species tested, results indicate that the IC₅₀ of chloroquine (mean 10.2 ± 1.43 μM) is significantly lower than the CC₅₀ (mean 260 ± 2.52 μM) yielding an overall selectivity index of 25.5. We also investigated the potential of chloroquine to prevent death in newborn mice after Hantaan virus infection and its antiviral effect in the hantavirus Syrian hamster model. For this purpose, C57Bl/6 mother mice were treated subcutaneously with daily doses of chloroquine. Subsequently, 1-day-old suckling mice were inoculated intracerebrally with 5 x 10² Hantaan virus particles. In litters of untreated mothers, none of the pups survived challenge. The highest survival rate (72.7% of pups) was found when mother mice were administered a concentration of 10 mg/kg chloroquine. Survival rates declined in a dose-dependent manner, with 47.6% survival when treated with 5 mg/kg chloroquine, and 4.2% when treated with 1 mg/kg chloroquine. Assessing the antiviral therapeutic and prophylactic effect of chloroquine in the Syrian hamster model was done using two different administration routes (intraperitoneally and subcutaneously using an osmotic pump system). Evaluating the prophylactic effect, a delay in onset of disease was noted and for the osmotic pump, 60% survival was observed. Our results show that chloroquine can be highly effective against Hantaan virus infection in newborn mice and against Andes virus in Syrian hamsters.

Inhibition of the replication of SARS-CoV-2 in human cells by the FDA-approved drug chlorpromazine

INTRODUCTION

Urgent action is needed to fight the ongoing coronavirus disease 2019 (COVID-19) pandemic by reducing the number of infected cases, contagiousness and severity. Chlorpromazine (CPZ), an antipsychotic from the phenothiazine group, is known to inhibit clathrin-mediated endocytosis and has antiviral activity against severe acute respiratory syndrome coronavirus-1 (SARS-CoV-1) and Middle East respiratory syndrome coronavirus. The aim of this in-vitro study was to test CPZ against SARS-CoV-2 in monkey and human cells.

MATERIALS AND METHODS

Monkey VeroE6 cells and human alveolar basal epithelial A549-ACE2 cells were infected with SARS-CoV-2 in the presence of various concentrations of CPZ. Supernatants were harvested at day 2 and analysed by quantitative reverse transcription polymerase chain reaction (RT-qPCR) for the presence of SARS-CoV-2 RNA. Cell viability was assessed in non-infected cells.

RESULTS

CPZ was found to have antiviral activity against SARS-CoV-2 in monkey VeroE6 cells, with a half maximal inhibitory concentration (IC50) of 8.2 µM, half maximal cytotoxic concentration (CC50) of 13.5 µM, and selectivity index (SI) of 1.65. In human A549-ACE2 cells, CPZ was also found to have anti-SARS-CoV-2 activity, with IC50 of 11.3 µM, CC50 of 23.1 µM and SI of 2.04.

DISCUSSION

Although the measured SI values are low, the IC50 values measured in vitro may translate to CPZ dosages used in routine clinical practice because of the high biodistribution of CPZ in lungs and saliva. Also, the distribution of CPZ in brain could be of interest for treating or preventing neurological and psychiatric forms of COVID-19.

CONCLUSIONS

These preclinical findings support clinical investigation of the repurposing of CPZ, a drug with mild side effects, in the treatment of patients with COVID-19.

Membrane Rafts: Portals for Viral Entry

Membrane rafts are dynamic, small (10-200 nm) domains enriched with cholesterol and sphingolipids that compartmentalize cellular processes. Rafts participate in roles essential to the lifecycle of different viral families including virus entry, assembly and/or budding events. Rafts seem to participate in virus attachment and recruitment to the cell surface, as well as the endocytic and non-endocytic mechanisms some viruses use to enter host cells. In this review, we will introduce the specific role of rafts in viral entry and define cellular factors implied in the choice of one entry pathway over the others. Finally, we will summarize the most relevant information about raft participation in the entry process of enveloped and non-enveloped viruses.

Rab18 binds to classical swine fever virus NS5A and mediates viral replication and assembly in swine umbilical vein endothelial cells

Classical swine fever virus (CSFV), a positive-sense RNA virus, hijacks cell host proteins for its own replication. Rab18, a small Rab GTPase, regulates intracellular membrane-trafficking events between various compartments in cells and is involved in the life cycle of multiple viruses. However, the effect of Rab18 on the production of CSFV remains uncertain. In this study, we showed that knockdown of Rab18 by lentiviruses inhibited CSFV production, while overexpression of Rab18 by lentiviruses enhanced CSFV production. Subsequent experiments revealed that the negative-mutant Rab18-S22 N inhibited CSFV infection, while the positive-mutant Rab18-Q67 L enhanced CSFV infection. Furthermore, we showed that CSFV RNA replication and virion assembly, measured by real-time fluorescence quantitative PCR (RT-qPCR), indirect immunofluorescence assay (IFA), and confocal microscopy, were reduced in cells lacking Rab18 expression. In addition, co-immunoprecipitation, GST-pulldown, and confocal microscopy assays revealed that Rab18 bound to the viral protein NS5A. Further, NS5A was shown to be redistributed in Rab18 knockdown cells. Taken together, these findings demonstrate Rab18 as a novel host factor required for CSFV RNA replication and particle assembly by interaction with the viral protein NS5A.

Structural similarity-based prediction of host factors associated with SARS-CoV-2 infection and pathogenesis

The current pandemic resulted from SARS-CoV-2 still remains as the major public health concern globally. The precise mechanism of viral pathogenesis is not fully understood, which remains a major hurdle for medical intervention. Here we generated an interactome profile of protein-protein interactions based on host and viral protein structural similarities information. Further computational biological study combined with Gene enrichment analysis predicted key enriched pathways associated with viral pathogenesis. The results show that axon guidance, membrane trafficking, vesicle-mediated transport, apoptosis, clathrin-mediated endocytosis, Vpu mediated degradation of CD4 T cell, and interferon-gamma signaling are key events associated in SARS-CoV-2 life cycle. Further, degree centrality analysis reveals that IRF1/9/7, TP53, and CASP3, UBA52, and UBC are vital proteins for IFN-γ-mediated signaling, apoptosis, and proteasomal degradation of CD4, respectively. We crafted chronological events of the virus life cycle. The SARS-CoV-2 enters through clathrin-mediated endocytosis, and the genome is trafficked to the early endosomes in a RAB5-dependent manner. It is predicted to replicate in a double-membrane vesicle (DMV) composed of the endoplasmic reticulum, autophagosome, and ERAD machinery. The SARS-CoV-2 down-regulates host translational machinery by interacting with protein kinase R, PKR-like endoplasmic reticulum kinase, and heme-regulated inhibitor and can phosphorylate eIF2a. The virion assembly occurs in the ER-Golgi intermediate compartment (ERGIC) organized by the spike and matrix protein. Collectively, we have established a spatial link between viral entry, RNA synthesis, assembly, pathogenesis, and their associated diverse host factors, those could pave the way for therapeutic intervention.

Virus-Mediated Cell-Cell Fusion

Cell-cell fusion between eukaryotic cells is a general process involved in many physiological and pathological conditions, including infections by bacteria, parasites, and viruses. As obligate intracellular pathogens, viruses use intracellular machineries and pathways for efficient replication in their host target cells. Interestingly, certain viruses, and, more especially, enveloped viruses belonging to different viral families and including human pathogens, can mediate cell-cell fusion between infected cells and neighboring non-infected cells. Depending of the cellular environment and tissue organization, this virus-mediated cell-cell fusion leads to the merge of membrane and cytoplasm contents and formation of multinucleated cells, also called syncytia, that can express high amount of viral antigens in tissues and organs of infected hosts. This ability of some viruses to trigger cell-cell fusion between infected cells as virus-donor cells and surrounding non-infected target cells is mainly related to virus-encoded fusion proteins, known as viral fusogens displaying high fusogenic properties, and expressed at the cell surface of the virus-donor cells. Virus-induced cell-cell fusion is then mediated by interactions of these viral fusion proteins with surface molecules or receptors involved in virus entry and expressed on neighboring non-infected cells. Thus, the goal of this review is to give an overview of the different animal virus families, with a more special focus on human pathogens, that can trigger cell-cell fusion.

Identification of an Antiviral Compound from the Pandemic Response Box that Efficiently Inhibits SARS-CoV-2 Infection In Vitro

With over 50 million currently confirmed cases worldwide, including more than 1.3 million deaths, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has a major impact on the economy and health care system. Currently, limited prophylactic or therapeutic intervention options are available against SARS-CoV-2. In this study, 400 compounds from the antimicrobial “pandemic response box” library were screened for inhibiting properties against SARS-CoV-2. An initial screen on Vero E6 cells identified five compounds that inhibited SARS-CoV-2 replication. However, validation of the selected hits in a human lung cell line highlighted that only a single compound, namely Retro-2.1, efficiently inhibited SARS-CoV-2 replication. Additional analysis revealed that the antiviral activity of Retro-2.1 occurs at a post-entry stage of the viral replication cycle. Combined, these data demonstrate that stringent in vitro screening of preselected compounds in multiple cell lines refines the rapid identification of new potential antiviral candidate drugs targeting SARS-CoV-2.

Inhibitors of SARS-CoV-2 Entry: Current and Future Opportunities

Recently, a novel coronavirus initially designated 2019-nCoV but now termed SARS-CoV-2 has emerged and raised global concerns due to its virulence. SARS-CoV-2 is the etiological agent of "coronavirus disease 2019", abbreviated to COVID-19, which despite only being identified at the very end of 2019, has now been classified as a pandemic by the World Health Organization (WHO). At this time, no specific prophylactic or postexposure therapy for COVID-19 are currently available. Viral entry is the first step in the SARS-CoV-2 lifecycle and is mediated by the trimeric spike protein. Being the first stage in infection, entry of SARS-CoV-2 into host cells is an extremely attractive therapeutic intervention point. Within this review, we highlight therapeutic intervention strategies for anti-SARS-CoV, MERS-CoV, and other coronaviruses and speculate upon future directions for SARS-CoV-2 entry inhibitor designs.

The Kv1.3 ion channel acts as a host factor restricting viral entry

Virus entry into cells is the initial stage of infection and involves multiple steps, and interfering viral entry represents potential antiviral approaches. Ion channels are pore-forming membrane proteins controlling cellular ion homeostasis and regulating many physiological processes, but their roles during viral infection have rarely been explored. Here, the functional Kv1.3 ion channel was found to be expressed in human hepatic cells and tissues. The Kv1.3 was then revealed to restrict HCV entry via inhibiting endosome acidification-mediated viral membrane fusion. The Kv1.3 was also demonstrated to inhibit DENV and ZIKV with an endosome acidification-dependent entry, but have no effect on SeV with a neutral pH penetration. A Kv1.3 antagonist PAP-1 treatment accelerated animal death in ZIKV-infected Ifnar1^-/- mice. Moreover, Kv1.3-deletion was found to promote weight loss and reduce survival rate in ZIKV-infected Kv1.3^-/- mice. Altogether, the Kv1.3 ion channel behaves as a host factor restricting viral entry. These findings broaden understanding about ion channel biology.

Importance of Endocytosis for the Biological Activity of Cedar Virus Fusion Protein

Endocytosis plays a particular role in the proteolytic activation of highly pathogenic henipaviruses Hendra (HeV) and Nipah virus (NiV) fusion (F) protein precursors. These proteins require endocytic uptake from the cell surface to be cleaved by cellular proteases within the endosomal compartment, followed by recycling to the plasma membrane for incorporation into budding virions or mediation of cell-cell fusion. This internalization largely depends on a tyrosine-based consensus motif for endocytosis present in the cytoplasmic tail of HeV and NiV F. Given the large number of tyrosine residues present in the F protein cytoplasmic domain of Cedar virus (CedV), a closely related but low pathogenic henipavirus, we aimed to investigate whether CedV F protein undergoes signal-mediated endocytosis from the cell surface controlled by tyrosine-based motifs present in its cytoplasmic tail and whether endocytosis is relevant for its biological activity. Therefore, tyrosine-based signals were mutated, and mutations were assessed for their effect on F cell surface expression, endocytosis, and biological activity. A membrane-proximal YXXΦ motif and a C-terminal di-tyrosine motif are of particular importance for cell surface expression and endocytosis rate. Furthermore, our data strongly indicate the pivotal role of endocytosis for the biological activity of the CedV F protein.

Single-nucleotide variants in human CD81 influence hepatitis C virus infection of hepatoma cells

An estimated number of 71 million people are living with chronic hepatitis C virus (HCV) infection worldwide and 400,000 annual deaths are related to the infection. HCV entry into the hepatocytes is complex and involves several host factors. The tetraspanin human CD81 (hCD81) is one of the four essential entry factors and is composed of one large extracellular loop, one small extracellular loop, four transmembrane domains, one intracellular loop and two intracellular tails. The large extracellular loop interacts with the E2 glycoprotein of HCV. Regions outside the large extracellular loop (backbone) of hCD81 have a critical role in post-binding entry steps and determine susceptibility of hepatocytes to HCV. Here, we investigated the effect of five non-synonymous single-nucleotide variants in the backbone of hCD81 on HCV susceptibility. We generated cell lines that stably express the hCD81 variants and infected the cells using HCV pseudoparticles and cell culture-derived HCV. Our results show that all the tested hCD81 variants support HCV pseudoparticle entry with similar efficiency as wild-type hCD81. In contrast, variants A54V, V211M and M220I are less supportive to cell culture-derived HCV infection. This altered susceptibility is HCV genotype dependent and specifically affected the cell entry step. Our findings identify three hCD81 genetic variants that are impaired in their function as HCV host factors for specific viral genotypes. This study provides additional evidence that genetic host variation contributes to inter-individual differences in HCV infection and outcome.