Role of endocytosis and low pH in murine hepatitis virus strain A59 cell entry

Decay of RNA and infectious SARS-CoV-2 and murine hepatitis virus in wastewater

Wastewater-based epidemiology (WBE) has been an important tool for population surveillance during the COVID-19 pandemic and continues to play a key role in monitoring SARS-CoV-2 infection levels following reductions in national clinical testing schemes. Studies measuring decay profiles of SARS-CoV-2 in wastewater have underscored the value of WBE, however investigations have been hampered by high biosafety requirements for SARS-CoV-2 infection studies. Therefore, surrogate viruses with lower biosafety standards have been used for SARS-CoV-2 decay studies, such as murine hepatitis virus (MHV), but few studies have directly compared decay rates of both viruses. We compared the persistence of SARS-CoV-2 and MHV in wastewater, using 50 % tissue culture infectious dose (TCID50) and reverse transcription quantitative polymerase chain reaction (RT-qPCR) assays to assess infectious virus titre and viral gene markers, respectively. Infectious SARS-CoV-2 and MHV indicate similar endpoints, however observed early decay characteristics differed, with infectious SARS-CoV-2 decaying more rapidly than MHV. We find that MHV is an appropriate infectious virus surrogate for viable SARS-CoV-2, however inconsistencies exist in viral RNA decay parameters, indicating MHV may not be a suitable nucleic acid surrogate across certain temperature regimes. This study highlights the importance of sample preparation and the potential for decay rate overestimation in wastewater surveillance for SARS-CoV-2 and other pathogens.

Broad spectrum post-entry inhibitors of coronavirus replication: Cardiotonic steroids and monensin

A small molecule screen identified several cardiotonic steroids (digitoxin and ouabain) and the ionophore monensin as potent inhibitors of HCoV-229E, HCoV-OC43, and SARS-CoV-2 replication with EC50s in the low nM range. Subsequent tests confirmed antiviral activity in primary cell models including human nasal epithelial cells and lung organoids. Addition of digitoxin, ouabain, or monensin strongly reduced viral gene expression as measured by both viral protein and RNA accumulation. Furthermore, the compounds acted post virus entry. While the antiviral activity of digitoxin was dependent upon activation of the MEK and JNK signaling pathways but not signaling through GPCRs, the antiviral effect of monensin was reversed upon inhibition of several signaling pathways. Together, the data demonstrates the potent anti-coronavirus properties of two classes of FDA approved drugs that function by altering the properties of the infected cell, rendering it unable to support virus replication.

Coronaviruses, Lysosomes, and Secondary Bacterial Infections: Coronaviruses Outsmart the Host

Lysosomes are key organelles that contribute to homeostatic functions such as autophagy-mediated recycling of cellular components and innate immune response through phagocytosis-mediated pathogen killing during infections. Viruses such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19), has developed unique adaptation to not only avoid lysosome-mediated destruction but also actively utilize lysosomal machinery to both enter and exit cells. To survive the highly hostile lysosomal environment, coronaviruses deacidify the lysosomes, potentially by manipulating H+ ion exchange across the lysosomal lumen, ensuring coronavirus survival. At the same time, this deacidification not only impairs cellular homeostatic functions such as autophagy but also renders the host susceptible to secondary bacterial infections. Furthermore, lysosomal enzymes promote extensive cell death and tissue damage during secondary bacterial infections. Thus, targeting lysosomal pathways provide a great opportunity to limit both viral replication and subsequent negative impact on host immunity against secondary bacterial infections.

Known Cellular and Receptor Interactions of Animal and Human Coronaviruses: A Review

This article aims to review all currently known interactions between animal and human coronaviruses and their cellular receptors. Over the past 20 years, three novel coronaviruses have emerged that have caused severe disease in humans, including SARS-CoV-2 (severe acute respiratory syndrome virus 2); therefore, a deeper understanding of coronavirus host-cell interactions is essential. Receptor-binding is the first stage in coronavirus entry prior to replication and can be altered by minor changes within the spike protein-the coronavirus surface glycoprotein responsible for the recognition of cell-surface receptors. The recognition of receptors by coronaviruses is also a major determinant in infection, tropism, and pathogenesis and acts as a key target for host-immune surveillance and other potential intervention strategies. We aim to highlight the need for a continued in-depth understanding of this subject area following on from the SARS-CoV-2 pandemic, with the possibility for more zoonotic transmission events. We also acknowledge the need for more targeted research towards glycan-coronavirus interactions as zoonotic spillover events from animals to humans, following an alteration in glycan-binding capability, have been well-documented for other viruses such as Influenza A.

Aminopeptidase N Is an Entry Co-factor Triggering Porcine Deltacoronavirus Entry via an Endocytotic Pathway

Porcine deltacoronavirus (PDCoV) is a recently discovered coronavirus that poses a potential threat to the global swine industry. Although we know that aminopeptidase N (APN) is important for PDCoV replication, it is unclear whether it is the primary functional receptor, and the mechanism by which it promotes viral replication is not fully understood. Here, we systematically investigated the roles of porcine APN (pAPN) during PDCoV infection of nonsusceptible cells, including in viral attachment and internalization. Using a viral entry assay, we found that PDCoV can enter nonsusceptible cells but then fails to initiate efficient replication. pAPN and PDCoV virions clearly colocalized with the endocytotic markers RAB5, RAB7, and LAMP1, suggesting that pAPN mediates PDCoV entry by an endocytotic pathway. Most importantly, our study shows that regardless of which receptor PDCoV engages, only entry by an endocytotic route ultimately leads to efficient viral replication. This knowledge should contribute to the development of efficient antiviral treatments, which are especially useful in preventing cross-species transmission. IMPORTANCE PDCoV is a pathogen with the potential for transmission across diverse species, although the mechanism of such host-switching events (from swine to other species) is poorly understood. Here, we show that PDCoV enters nonsusceptible cells but without efficient replication. We also investigated the key role played by aminopeptidase N in mediating PDCoV entry via an endocytotic pathway. Our results demonstrate that viral entry via endocytosis is a major determinant of efficient PDCoV replication. This knowledge provides a basis for future studies of the cross-species transmissibility of PDCoV and the development of appropriate antiviral drugs.

Spike Glycoprotein Is Central to Coronavirus Pathogenesis-Parallel Between m-CoV and SARS-CoV-2

Coronaviruses (CoVs) are single-stranded, polyadenylated, enveloped RNA of positive polarity with a unique potential to alter host tropism. This has been exceptionally demonstrated by the emergence of deadly virus outbreaks of the past: Severe Acute Respiratory Syndrome (SARS-CoV) in 2003 and Middle East Respiratory Syndrome (MERS-CoV) in 2012. The 2019 outbreak by the new cross-species transmission of SARS-CoV-2 has put the world on alert. CoV infection is triggered by receptor recognition, membrane fusion, and successive viral entry mediated by the surface Spike (S) glycoprotein. S protein is one of the major antigenic determinants and the target for neutralizing antibodies. It is a valuable target in antiviral therapies because of its central role in cell-cell fusion, viral antigen spread, and host immune responses leading to immunopathogenesis. The receptor-binding domain of S protein has received greater attention as it initiates host attachment and contains major antigenic determinants. However, investigating the therapeutic potential of fusion peptide as a part of the fusion core complex assembled by the heptad repeats 1 and 2 (HR1 and HR2) is also warranted. Along with receptor attachment and entry, fusion mechanisms should also be explored for designing inhibitors as a therapeutic intervention. In this article, we review the S protein function and its role in mediating membrane fusion, spread, tropism, and its associated pathogenesis with notable therapeutic strategies focusing on results obtained from studies on a murine β-Coronavirus (m-CoV) and its associated disease process.

Viral Interactions with Adaptor-Protein Complexes: A Ubiquitous Trait among Viral Species

Numerous viruses hijack cellular protein trafficking pathways to mediate cell entry or to rearrange membrane structures thereby promoting viral replication and antagonizing the immune response. Adaptor protein complexes (AP), which mediate protein sorting in endocytic and secretory transport pathways, are one of the conserved viral targets with many viruses possessing AP-interacting motifs. We present here different mechanisms of viral interference with AP complexes and the functional consequences that allow for efficient viral propagation and evasion of host immune defense. The ubiquity of this phenomenon is evidenced by the fact that there are representatives for AP interference in all major viral families, covered in this review. The best described examples are interactions of human immunodeficiency virus and human herpesviruses with AP complexes. Several other viruses, like Ebola, Nipah, and SARS-CoV-2, are pointed out as high priority disease-causative agents supporting the need for deeper understanding of virus-AP interplay which can be exploited in the design of novel antiviral therapies.

Mechanistic insight into anti-COVID-19 drugs: recent trends and advancements

The Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2) has been established now to be a deadly disease afflicting the whole world with worst consequences on healthcare, economy and day-to-day life activities. Being a communicable disease, which is highly pathogenic in humans, causing cough, throat infection, breathing problems, high fever, muscle pain, and may lead to death in some cases especially those having other comorbid conditions such as heart or kidney problems, and diabetes. Finding an appropriate drug and vaccine candidate against coronavirus disease (COVID-19) remains an ultimate and immediate goal for the global scientific community. Based on previous studies in the literature on SARS-CoV infection, there are a number of drugs that may inhibit the replication of SARS-CoV-2 and its infection. Such drugs comprise of inhibitors of Angiotensin-Converting Enzyme 2 (ACE2), transmembrane Serine Protease 2 (TMPRSS2), nonstructural protein 3C-like protease, nonstructural RNA-dependent RNA polymerase (RdRp) and many more. The antiviral drugs such as chloroquine and hydroxychloroquine, lopinavir and ritonavir as inhibitors for HIV protease, nucleotide analogue remdesivir, and broad-spectrum antiviral drugs are available to treat the SARS-CoV-2-infected patients. Therefore, this review article is planned to gain insight into the mechanism for blocking the entry of SARS-CoV-2, its validation, other inhibition mechanisms, and development of therapeutic drugs and vaccines against SARS-CoV-2.

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.

The molecular virology of coronaviruses

Few human pathogens have been the focus of as much concentrated worldwide attention as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of COVID-19. Its emergence into the human population and ensuing pandemic came on the heels of severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV), two other highly pathogenic coronavirus spillovers, which collectively have reshaped our view of a virus family previously associated primarily with the common cold. It has placed intense pressure on the collective scientific community to develop therapeutics and vaccines, whose engineering relies on a detailed understanding of coronavirus biology. Here, we present the molecular virology of coronavirus infection, including its entry into cells, its remarkably sophisticated gene expression and replication mechanisms, its extensive remodeling of the intracellular environment, and its multifaceted immune evasion strategies. We highlight aspects of the viral life cycle that may be amenable to antiviral targeting as well as key features of its biology that await discovery.

The recent outbreaks of human coronaviruses: A medicinal chemistry perspective

Coronaviruses (CoVs) infect both humans and animals. In humans, CoVs can cause respiratory, kidney, heart, brain, and intestinal infections that can range from mild to lethal. Since the start of the 21st century, three β‐coronaviruses have crossed the species barrier to infect humans: severe‐acute respiratory syndrome (SARS)‐CoV‐1, Middle East respiratory syndrome (MERS)‐CoV, and SARS‐CoV‐2 (2019‐nCoV). These viruses are dangerous and can easily be transmitted from human to human. Therefore, the development of anticoronaviral therapies is urgently needed. However, to date, no approved vaccines or drugs against CoV infections are available. In this review, we focus on the medicinal chemistry efforts toward the development of antiviral agents against SARS‐CoV‐1, MERS‐CoV, SARS‐CoV‐2, targeting biochemical events important for viral replication and its life cycle. These targets include the spike glycoprotein and its host‐receptors for viral entry, proteases that are essential for cleaving polyproteins to produce functional proteins, and RNA‐dependent RNA polymerase for viral RNA replication.

Managing the COVID-19 Pandemic: Research Strategies Based on the Evolutionary and Molecular Characteristics of Coronaviruses

Coronavirus disease 2019 (COVID-19), an ongoing global health emergency, is a highly transmittable and pathogenic viral infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Emerging in Wuhan, China, in December 2019, it spread widely across the world causing panic-worst ever economic depression is visibly predictable. Coronaviruses (CoVs) have emerged as a major public health concern having caused three zoonotic outbreaks; severe acute respiratory syndrome-CoV (SARS-CoV) in 2002-2003, Middle East respiratory syndrome-CoV (MERS-CoV) in 2012, and currently this devastating COVID-19. Research strategies focused on understanding the evolutionary origin, transmission, and molecular basis of SARS-CoV-2 and its pathogenesis need to be urgently formulated to manage the current and possible future coronaviral outbreaks. Current response to the COVID-19 outbreak has been largely limited to monitoring/containment. Although frantic global efforts for developing safe and effective prophylactic and therapeutic agents are on, no licensed antiviral treatment or vaccine exists till date. In this review, research strategies for coping with COVID-19 based on evolutionary and molecular aspects of coronaviruses have been proposed.

SARS-CoV-2 pathophysiology and assessment of coronaviruses in CNS diseases with a focus on therapeutic targets

The novel Coronavirus disease of 2019 (nCOV-19) is a viral outbreak noted first in Wuhan, China. This disease is caused by Severe Acute Respiratory Syndrome (SARS) Coronavirus (CoV)-2. In the past, other members of the coronavirus family, such as SARS and Middle East Respiratory Syndrome (MERS), have made an impact in China and the Arabian peninsula respectively. Both SARS and COVID-19 share similar symptoms such as fever, cough, and difficulty in breathing that can become fatal in later stages. However, SARS and MERS infections were epidemic diseases constrained to limited regions. By March 2020 the SARS-CoV-2 had spread across the globe and on March 11th, 2020 the World Health Organization (WHO) declared COVID-19 as pandemic disease. In severe SARS-CoV-2 infection, many patients succumbed to pneumonia. Higher rates of deaths were seen in older patients who had co-morbidities such as diabetes mellitus, hypertension, cardiovascular disease (CVD), and dementia. In this review paper, we discuss the effect of SARS-CoV-2 on CNS diseases, such as Alzheimer's-like dementia, and diabetes mellitus. We also focus on the virus genome, pathophysiology, theranostics, and autophagy mechanisms. We will assess the multiorgan failure reported in advanced stages of SARS-CoV-2 infection. Our paper will provide mechanistic clues and therapeutic targets for physicians and investigators to combat COVID-19.

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SARS-CoV-2 and Dementia, Multiple Sclerosis, Encephalitis, and Parkinson's disease

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SARS-CoV-2 and Diabetes Mellitus Coronaviruses and Autophagy

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Protease Inhibitors and Repurposed Drugs for SARS-CoV-2 USFDA approved drugs-HCQ and Remdesivir.

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SARS-CoV-2 diagnostics

Targeting the Endocytic Pathway and Autophagy Process as a Novel Therapeutic Strategy in COVID-19

Coronaviruses (CoVs) are a group of enveloped, single-stranded positive genomic RNA viruses and some of them are known to cause severe respiratory diseases in human, including Severe Acute Respiratory Syndrome (SARS), Middle East Respiratory Syndrome (MERS) and the ongoing coronavirus disease-19 (COVID-19). One key element in viral infection is the process of viral entry into the host cells. In the last two decades, there is increasing understanding on the importance of the endocytic pathway and the autophagy process in viral entry and replication. As a result, the endocytic pathway including endosome and lysosome has become important targets for development of therapeutic strategies in combating diseases caused by CoVs. In this mini-review, we will focus on the importance of the endocytic pathway as well as the autophagy process in viral infection of several pathogenic CoVs inclusive of SARS-CoV, MERS-CoV and the new CoV named as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and discuss the development of therapeutic agents by targeting these processes. Such knowledge will provide important clues for control of the ongoing epidemic of SARS-CoV-2 infection and treatment of COVID-19.

PEDV enters cells through clathrin-, caveolae-, and lipid raft-mediated endocytosis and traffics via the endo-/lysosome pathway

With the emergence of highly pathogenic variant strains, porcine epidemic diarrhea virus (PEDV) has led to significant economic loss in the global swine industry. Many studies have described how coronaviruses enter cells, but information on PEDV invasion strategies remains insufficient. Given that the differences in gene sequences and pathogenicity between classical and mutant strains of PEDV may lead to diverse invasion mechanisms, this study focused on the cellular entry pathways and cellular transport of the PEDV GI and GII subtype strains in Vero cells and IPEC-J2 cells. We first characterized the kinetics of PEDV entry into cells and found that the highest invasion rate of PEDV was approximately 33% in the IPEC-J2 cells and approximately 100% in the Vero cells. To clarify the specific endocytic pathways, systematic research methods were used and showed that PEDV enters cells via the clathrin- and caveolae-mediated endocytosis pathways, in which dynamin II, clathrin heavy chain, Eps15, cholesterol, and caveolin-1 were indispensably involved. In addition, lipid raft extraction assay showed that PEDV can also enter cells through lipid raft-mediated endocytosis. To investigate the trafficking of internalized PEDV, we found that PEDV entry into cells relied on low pH and internalized virions reached lysosomes through the early endosome-late endosome-lysosome pathway. The results concretely revealed the entry mechanisms of PEDV and provided an insightful theoretical basis for the further understanding of PEDV pathogenesis and guidance for new targets of antiviral drugs.

Structural insights into coronavirus entry

Coronaviruses (CoVs) have caused outbreaks of deadly pneumonia in humans since the beginning of the 21st century. The severe acute respiratory syndrome coronavirus (SARS-CoV) emerged in 2002 and was responsible for an epidemic that spread to five continents with a fatality rate of 10% before being contained in 2003 (with additional cases reported in 2004). The Middle-East respiratory syndrome coronavirus (MERS-CoV) emerged in the Arabian Peninsula in 2012 and has caused recurrent outbreaks in humans with a fatality rate of 35%. SARS-CoV and MERS-CoV are zoonotic viruses that crossed the species barrier using bats/palm civets and dromedary camels, respectively. No specific treatments or vaccines have been approved against any of the six human coronaviruses, highlighting the need to investigate the principles governing viral entry and cross-species transmission as well as to prepare for zoonotic outbreaks which are likely to occur due to the large reservoir of CoVs found in mammals and birds. Here, we review our understanding of the infection mechanism used by coronaviruses derived from recent structural and biochemical studies.

A proline insertion-deletion in the spike glycoprotein fusion peptide of mouse hepatitis virus strongly alters neuropathology

Fusion peptides (FPs) in spike proteins are key players mediating early events in cell-to-cell fusion, vital for intercellular viral spread. A proline residue located at the central FP region has often been suggested to have a distinctive role in this fusion event. The spike glycoprotein from strain RSA59 (PP) of mouse hepatitis virus (MHV) contains two central, consecutive prolines in the FP. Here, we report that deletion of one of these proline residues, resulting in RSA59 (P), significantly affected neural cell syncytia formation and viral titers postinfection in vitro Transcranial inoculation of C57Bl/6 mice with RSA59 (PP) or RSA59 (P) yielded similar degrees of necrotizing hepatitis and meningitis, but only RSA59 (PP) produced widespread encephalitis that extended deeply into the brain parenchyma. By day 6 postinfection, both virus variants were mostly cleared from the brain. Interestingly, inoculation with the RSA59 (P)-carrying MHV significantly reduced demyelination at the chronic stage. We also found that the presence of two consecutive prolines in FP promotes a more ordered, compact, and rigid structure in the spike protein. These effects on FP structure were due to proline's unique stereochemical properties intrinsic to its secondary amino acid structure, revealed by molecular dynamics and NMR experiments. We therefore propose that the differences in the severity of encephalitis and demyelination between RSA59 (PP) and RSA59 (P) arise from the presence or absence, respectively, of the two consecutive prolines in FP. Our studies define a structural determinant of MHV entry in the brain parenchyma important for altered neuropathogenesis.

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

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

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Intact lipid rafts is involved in IBV entry.

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Low pH in intracyplasmic vesicles is required for IBV entry.

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IBV penetrates cells via clathrin mediated endocytosis.

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

Canine respiratory coronavirus employs caveolin-1-mediated pathway for internalization to HRT-18G cells

Canine respiratory coronavirus (CRCoV), identified in 2003, is a member of the Coronaviridae family. The virus is a betacoronavirus and a close relative of human coronavirus OC43 and bovine coronavirus. Here, we examined entry of CRCoV into human rectal tumor cells (HRT-18G cell line) by analyzing co-localization of single virus particles with cellular markers in the presence or absence of chemical inhibitors of pathways potentially involved in virus entry. We also targeted these pathways using siRNA. The results show that the virus hijacks caveolin-dependent endocytosis to enter cells via endocytic internalization.

Identification of H209 as Essential for pH 8-Triggered Receptor-Independent Syncytium Formation by S Protein of Mouse Hepatitis Virus A59

The spike glycoprotein (S) of murine coronavirus mouse hepatitis virus (MHV) strain A59 uses murine carcinoembryonic antigen-related cell adhesion molecule 1a as its receptor for cell entry, but S protein can also be triggered in the absence of receptor by pH 8.0 alone at 37°C. The mechanism by which conformational changes of this S glycoprotein can be triggered by pH 8.0 has not yet been determined. Here, we show that MHV-A59 S protein is triggered by pH 8.0 at 37°C to induce receptor-independent syncytium (RIS) formation on 293T cells, and that the conformational changes in S proteins triggered by pH 8.0 are very similar to those triggered by receptor binding. We systemically mutated each of 15 histidine residues in S protein and found that H209 is essential for pH 8.0-triggered RIS formation, while H179, H441, H643, and H759 also play important roles in this process. Replacement of H209 with Ala had no effect on receptor binding, but in murine 17Cl.1 cells mutant H209A MHV-A59 showed delayed growth kinetics and was readily outcompeted by wild-type virus when mixed together, indicating that the H209A mutation caused a defect in virus fitness. Finally, the H209A mutation significantly increased the thermostability of S protein in its prefusion conformation, which may raise the energy barrier for conformational change of S protein required for membrane fusion and lead to a decrease in virus fitness in cell culture. Thus, MHV-A59 may have evolved to lower the stability of its S protein in order to increase virus fitness.IMPORTANCE Enveloped viruses enter cells through fusion of viral and cellular membranes, and the process is mediated by interactions between viral envelope proteins and their host receptors. In the prefusion conformation, viral envelope proteins are metastable, and activation to the fusion conformation is tightly regulated, since premature activation would lead to loss of viral infectivity. The stability of viral envelope proteins greatly influences their activation and virus fitness. Here, we report that, similar to the A82V mutation in Ebola glycoprotein, in the S glycoprotein of murine coronavirus MHV-A59, the histidine residue at position of 209 significantly affects the thermal stability of the S protein, determines whether S protein can be activated at 37°C by either pH 8.0 alone or by receptor binding, and affects viral fitness in cell culture. Thus, the spike glycoprotein of MHV-A59 has evolved to retain histidine at position 209 to optimize virus fitness.

Structure, Function, and Evolution of Coronavirus Spike Proteins

The coronavirus spike protein is a multifunctional molecular machine that mediates coronavirus entry into host cells. It first binds to a receptor on the host cell surface through its S1 subunit and then fuses viral and host membranes through its S2 subunit. Two domains in S1 from different coronaviruses recognize a variety of host receptors, leading to viral attachment. The spike protein exists in two structurally distinct conformations, prefusion and postfusion. The transition from prefusion to postfusion conformation of the spike protein must be triggered, leading to membrane fusion. This article reviews current knowledge about the structures and functions of coronavirus spike proteins, illustrating how the two S1 domains recognize different receptors and how the spike proteins are regulated to undergo conformational transitions. I further discuss the evolution of these two critical functions of coronavirus spike proteins, receptor recognition and membrane fusion, in the context of the corresponding functions from other viruses and host cells.

Coronavirus Spike Protein and Tropism Changes

Coronaviruses (CoVs) have a remarkable potential to change tropism. This is particularly illustrated over the last 15 years by the emergence of two zoonotic CoVs, the severe acute respiratory syndrome (SARS)- and Middle East respiratory syndrome (MERS)-CoV. Due to their inherent genetic variability, it is inevitable that new cross-species transmission events of these enveloped, positive-stranded RNA viruses will occur. Research into these medical and veterinary important pathogens—sparked by the SARS and MERS outbreaks—revealed important principles of inter- and intraspecies tropism changes. The primary determinant of CoV tropism is the viral spike (S) entry protein. Trimers of the S glycoproteins on the virion surface accommodate binding to a cell surface receptor and fusion of the viral and cellular membrane. Recently, high-resolution structures of two CoV S proteins have been elucidated by single-particle cryo-electron microscopy. Using this new structural insight, we review the changes in the S protein that relate to changes in virus tropism. Different concepts underlie these tropism changes at the cellular, tissue, and host species level, including the promiscuity or adaptability of S proteins to orthologous receptors, alterations in the proteolytic cleavage activation as well as changes in the S protein metastability. A thorough understanding of the key role of the S protein in CoV entry is critical to further our understanding of virus cross-species transmission and pathogenesis and for development of intervention strategies.

Brain Invasion by Mouse Hepatitis Virus Depends on Impairment of Tight Junctions and Beta Interferon Production in Brain Microvascular Endothelial Cells

Coronaviruses (CoVs) have shown neuroinvasive properties in humans and animals secondary to replication in peripheral organs, but the mechanism of neuroinvasion is unknown. The major aim of our work was to evaluate the ability of CoVs to enter the central nervous system (CNS) through the blood-brain barrier (BBB). Using the highly hepatotropic mouse hepatitis virus type 3 (MHV3), its attenuated variant, 51.6-MHV3, which shows low tropism for endothelial cells, and the weakly hepatotropic MHV-A59 strain from the murine coronavirus group, we investigated the virus-induced dysfunctions of BBB in vivo and in brain microvascular endothelial cells (BMECs) in vitro. We report here a MHV strain-specific ability to cross the BBB during acute infection according to their virulence for liver. Brain invasion was observed only in MHV3-infected mice and correlated with enhanced BBB permeability associated with decreased expression of zona occludens protein 1 (ZO-1), VE-cadherin, and occludin, but not claudin-5, in the brain or in cultured BMECs. BBB breakdown in MHV3 infection was not related to production of barrier-dysregulating inflammatory cytokines or chemokines by infected BMECs but rather to a downregulation of barrier protective beta interferon (IFN-β) production. Our findings highlight the importance of IFN-β production by infected BMECs in preserving BBB function and preventing access of blood-borne infectious viruses to the brain.

IMPORTANCE

Coronaviruses (CoVs) infect several mammals, including humans, and are associated with respiratory, gastrointestinal, and/or neurological diseases. There is some evidence that suggest that human respiratory CoVs may show neuroinvasive properties. Indeed, the severe acute respiratory syndrome coronavirus (SARS-CoV), causing severe acute respiratory syndrome, and the CoVs OC43 and 229E were found in the brains of SARS patients and multiple sclerosis patients, respectively. These findings suggest that hematogenously spread CoVs may gain access to the CNS at the BBB level. Herein we report for the first time that CoVs exhibit the ability to cross the BBB according to strain virulence. BBB invasion by CoVs correlates with virus-induced disruption of tight junctions on BMECs, leading to BBB dysfunction and enhanced permeability. We provide evidence that production of IFN-β by BMECs during CoV infection may prevent BBB breakdown and brain viral invasion.

ATP1A1-mediated Src signaling inhibits coronavirus entry into host cells

In addition to transporting ions, the multisubunit Na(+),K(+)-ATPase also functions by relaying cardiotonic steroid (CTS)-binding-induced signals into cells. In this study, we analyzed the role of Na(+),K(+)-ATPase and, in particular, of its ATP1A1 α subunit during coronavirus (CoV) infection. As controls, the vesicular stomatitis virus (VSV) and influenza A virus (IAV) were included. Using gene silencing, the ATP1A1 protein was shown to be critical for infection of cells with murine hepatitis virus (MHV), feline infectious peritonitis virus (FIPV), and VSV but not with IAV. Lack of ATP1A1 did not affect virus binding to host cells but resulted in inhibited entry of MHV and VSV. Consistently, nanomolar concentrations of the cardiotonic steroids ouabain and bufalin, which are known not to affect the transport function of Na(+),K(+)-ATPase, inhibited infection of cells with MHV, FIPV, Middle East respiratory syndrome (MERS)-CoV, and VSV, but not IAV, when the compounds were present during virus inoculation. Cardiotonic steroids were shown to inhibit entry of MHV at an early stage, resulting in accumulation of virions close to the cell surface and, as a consequence, in reduced fusion. In agreement with an early block in infection, the inhibition of VSV by CTSs could be bypassed by low-pH shock. Viral RNA replication was not affected when these compounds were added after virus entry. The antiviral effect of ouabain could be relieved by the addition of different Src kinase inhibitors, indicating that Src signaling mediated via ATP1A1 plays a crucial role in the inhibition of CoV and VSV infections.

IMPORTANCE

Coronaviruses (CoVs) are important pathogens of animals and humans, as demonstrated by the recent emergence of new human CoVs of zoonotic origin. Antiviral drugs targeting CoV infections are lacking. In the present study, we show that the ATP1A1 subunit of Na(+),K(+)-ATPase, an ion transporter and signaling transducer, supports CoV infection. Targeting ATP1A1 either by gene silencing or by low concentrations of the ATP1A1-binding cardiotonic steroids ouabain and bufalin resulted in inhibition of infection with murine, feline, and MERS-CoVs at an early entry stage. Infection with the control virus VSV was also inhibited. Src signaling mediated by ATP1A1 was shown to play a crucial role in the inhibition of virus entry by ouabain and bufalin. These results suggest that targeting the Na(+),K(+)-ATPase using cardiotonic steroids, several of which are FDA-approved compounds, may be an attractive therapeutic approach against CoV and VSV infections.

Coronavirus cell entry occurs through the endo-/lysosomal pathway in a proteolysis-dependent manner

Enveloped viruses need to fuse with a host cell membrane in order to deliver their genome into the host cell. While some viruses fuse with the plasma membrane, many viruses are endocytosed prior to fusion. Specific cues in the endosomal microenvironment induce conformational changes in the viral fusion proteins leading to viral and host membrane fusion. In the present study we investigated the entry of coronaviruses (CoVs). Using siRNA gene silencing, we found that proteins known to be important for late endosomal maturation and endosome-lysosome fusion profoundly promote infection of cells with mouse hepatitis coronavirus (MHV). Using recombinant MHVs expressing reporter genes as well as a novel, replication-independent fusion assay we confirmed the importance of clathrin-mediated endocytosis and demonstrated that trafficking of MHV to lysosomes is required for fusion and productive entry to occur. Nevertheless, MHV was shown to be less sensitive to perturbation of endosomal pH than vesicular stomatitis virus and influenza A virus, which fuse in early and late endosomes, respectively. Our results indicate that entry of MHV depends on proteolytic processing of its fusion protein S by lysosomal proteases. Fusion of MHV was severely inhibited by a pan-lysosomal protease inhibitor, while trafficking of MHV to lysosomes and processing by lysosomal proteases was no longer required when a furin cleavage site was introduced in the S protein immediately upstream of the fusion peptide. Also entry of feline CoV was shown to depend on trafficking to lysosomes and processing by lysosomal proteases. In contrast, MERS-CoV, which contains a minimal furin cleavage site just upstream of the fusion peptide, was negatively affected by inhibition of furin, but not of lysosomal proteases. We conclude that a proteolytic cleavage site in the CoV S protein directly upstream of the fusion peptide is an essential determinant of the intracellular site of fusion.

Enveloped viruses need to fuse with a host cell membrane in order to deliver their genome into the host cell. In the present study we investigated the entry of coronaviruses (CoVs). CoVs are important pathogens of animals and man with high zoonotic potential as demonstrated by the emergence of SARS- and MERS-CoVs. Previous studies resulted in apparently conflicting results with respect to CoV cell entry, particularly regarding the fusion-activating requirements of the CoV S protein. By combining cell-biological, infection, and fusion assays we demonstrated that murine hepatitis virus (MHV), a prototypic member of the CoV family, enters cells via clathrin-mediated endocytosis. Moreover, although MHV does not depend on a low pH for fusion, the virus was shown to rely on trafficking to lysosomes for proteolytic cleavage of its spike (S) protein and membrane fusion to occur. Based on these results we predicted and subsequently demonstrated that MERS- and feline CoV require cleavage by different proteases and escape the endo/lysosomal system from different compartments. In conclusion, we elucidated the MHV entry pathway in detail and demonstrate that a proteolytic cleavage site in the S protein of different CoVs is an essential determinant of the intracellular site of fusion.

Identification and characterization of a proteolytically primed form of the murine coronavirus spike proteins after fusion with the target cell

Enveloped viruses carry highly specialized glycoproteins that catalyze membrane fusion under strict spatial and temporal control. To prevent premature activation after biosynthesis, viral class I fusion proteins adopt a locked conformation and require proteolytic cleavage to render them fusion-ready. This priming step may occur during virus exit from the infected cell, in the extracellular milieu or during entry at or in the next target cell. Proteolytic processing of coronavirus spike (S) fusion proteins during virus entry has been suggested but not yet formally demonstrated, while the nature and functionality of the resulting subunit is still unclear. We used a prototype coronavirus--mouse hepatitis virus (MHV)--to develop a conditional biotinylation assay that enables the specific identification and biochemical characterization of viral S proteins on virions that mediated membrane fusion with the target cell. We demonstrate that MHV S proteins are indeed cleaved upon virus endocytosis, and we identify a novel processing product S2* with characteristics of a fusion-active subunit. The precise cleavage site and the enzymes involved remain to be elucidated.

IMPORTANCE

Virus entry determines the tropism and is a crucial step in the virus life cycle. We developed an approach to characterize structural components of virus particles after entering new target cells. A prototype coronavirus was used to illustrate how the virus fusion machinery can be controlled.

Mechanisms of coronavirus cell entry mediated by the viral spike protein

Coronaviruses are enveloped positive-stranded RNA viruses that replicate in the cytoplasm. To deliver their nucleocapsid into the host cell, they rely on the fusion of their envelope with the host cell membrane. The spike glycoprotein (S) mediates virus entry and is a primary determinant of cell tropism and pathogenesis. It is classified as a class I fusion protein, and is responsible for binding to the receptor on the host cell as well as mediating the fusion of host and viral membranes—A process driven by major conformational changes of the S protein. This review discusses coronavirus entry mechanisms focusing on the different triggers used by coronaviruses to initiate the conformational change of the S protein: receptor binding, low pH exposure and proteolytic activation. We also highlight commonalities between coronavirus S proteins and other class I viral fusion proteins, as well as distinctive features that confer distinct tropism, pathogenicity and host interspecies transmission characteristics to coronaviruses.

Coronavirus pathogenesis

Coronaviruses infect many species of animals including humans, causing acute and chronic diseases. This review focuses primarily on the pathogenesis of murine coronavirus mouse hepatitis virus (MHV) and severe acute respiratory coronavirus (SARS-CoV). MHV is a collection of strains, which provide models systems for the study of viral tropism and pathogenesis in several organs systems, including the central nervous system, the liver, and the lung, and has been cited as providing one of the few animal models for the study of chronic demyelinating diseases such as multiple sclerosis. SARS-CoV emerged in the human population in China in 2002, causing a worldwide epidemic with severe morbidity and high mortality rates, particularly in older individuals. We review the pathogenesis of both viruses and the several reverse genetics systems that made much of these studies possible. We also review the functions of coronavirus proteins, structural, enzymatic, and accessory, with an emphasis on roles in pathogenesis. Structural proteins in addition to their roles in virion structure and morphogenesis also contribute significantly to viral spread in vivo and in antagonizing host cell responses. Nonstructural proteins include the small accessory proteins that are not at all conserved between MHV and SARS-CoV and the 16 conserved proteins encoded in the replicase locus, many of which have enzymatic activities in RNA metabolism or protein processing in addition to functions in antagonizing host response.

Unconventional use of LC3 by coronaviruses through the alleged subversion of the ERAD tuning pathway

Pathogens of bacterial and viral origin hijack pathways operating in eukaryotic cells in many ways in order to gain access into the host, to establish themselves and to eventually produce their progeny. The detailed molecular characterization of the subversion mechanisms devised by pathogens to infect host cells is crucial to generate targets for therapeutic intervention. Here we review recent data indicating that coronaviruses probably co-opt membranous carriers derived from the endoplasmic reticulum, which contain proteins that regulate disposal of misfolded polypeptides, for their replication. In addition, we also present models describing potential mechanisms that coronaviruses could employ for this hijacking.

Different host cell proteases activate the SARS-coronavirus spike-protein for cell-cell and virus-cell fusion

Severe acute respiratory syndrome coronavirus (SARS-CoV) poses a considerable threat to human health. Activation of the viral spike (S)-protein by host cell proteases is essential for viral infectivity. However, the cleavage sites in SARS-S and the protease(s) activating SARS-S are incompletely defined. We found that R667 was dispensable for SARS-S-driven virus–cell fusion and for SARS-S-activation by trypsin and cathepsin L in a virus–virus fusion assay. Mutation T760R, which optimizes the minimal furin consensus motif 758-RXXR-762, and furin overexpression augmented SARS-S activity, but did not result in detectable SARS-S cleavage. Finally, SARS-S-driven cell–cell fusion was independent of cathepsin L, a protease essential for virus–cell fusion. Instead, a so far unknown leupeptin-sensitive host cell protease activated cellular SARS-S for fusion with target cells expressing high levels of ACE2. Thus, different host cell proteases activate SARS-S for virus–cell and cell–cell fusion and SARS-S cleavage at R667 and 758-RXXR-762 can be dispensable for SARS-S activation.

Coronavirus pathogenesis

Coronaviruses infect many species of animals including humans, causing acute and chronic diseases. This review focuses primarily on the pathogenesis of murine coronavirus mouse hepatitis virus (MHV) and severe acute respiratory coronavirus (SARS-CoV). MHV is a collection of strains, which provide models systems for the study of viral tropism and pathogenesis in several organs systems, including the central nervous system, the liver, and the lung, and has been cited as providing one of the few animal models for the study of chronic demyelinating diseases such as multiple sclerosis. SARS-CoV emerged in the human population in China in 2002, causing a worldwide epidemic with severe morbidity and high mortality rates, particularly in older individuals. We review the pathogenesis of both viruses and the several reverse genetics systems that made much of these studies possible. We also review the functions of coronavirus proteins, structural, enzymatic, and accessory, with an emphasis on roles in pathogenesis. Structural proteins in addition to their roles in virion structure and morphogenesis also contribute significantly to viral spread in vivo and in antagonizing host cell responses. Nonstructural proteins include the small accessory proteins that are not at all conserved between MHV and SARS-CoV and the 16 conserved proteins encoded in the replicase locus, many of which have enzymatic activities in RNA metabolism or protein processing in addition to functions in antagonizing host response.

Population context determines cell-to-cell variability in endocytosis and virus infection

Single-cell heterogeneity in cell populations arises from a combination of intrinsic and extrinsic factors. This heterogeneity has been measured for gene transcription, phosphorylation, cell morphology and drug perturbations, and used to explain various aspects of cellular physiology. In all cases, however, the causes of heterogeneity were not studied. Here we analyse, for the first time, the heterogeneous patterns of related cellular activities, namely virus infection, endocytosis and membrane lipid composition in adherent human cells. We reveal correlations with specific cellular states that are defined by the population context of a cell, and we derive probabilistic models that can explain and predict most cellular heterogeneity of these activities, solely on the basis of each cell's population context. We find that accounting for population-determined heterogeneity is essential for interpreting differences between the activity levels of cell populations. Finally, we reveal that synergy between two molecular components, focal adhesion kinase and the sphingolipid GM1, enhances the population-determined pattern of simian virus 40 (SV40) infection. Our findings provide an explanation for the origin of heterogeneity patterns of cellular activities in adherent cell populations.

Proteolytic activation of the spike protein at a novel RRRR/S motif is implicated in furin-dependent entry, syncytium formation, and infectivity of coronavirus infectious bronchitis virus in cultured cells

The spike (S) protein of the coronavirus (CoV) infectious bronchitis virus (IBV) is cleaved into S1 and S2 subunits at the furin consensus motif RRFRR(537)/S in virus-infected cells. In this study, we observe that the S2 subunit of the IBV Beaudette strain is additionally cleaved at the second furin site (RRRR(690)/S) in cells expressing S constructs and in virus-infected cells. Detailed time course experiments showed that a peptide furin inhibitor, decanoyl-Arg-Val-Lys-Arg-chloromethylketone, blocked both viral entry and syncytium formation. Site-directed mutagenesis studies revealed that the S1/S2 cleavage by furin was not necessary for, but could promote, syncytium formation by and infectivity of IBV in Vero cells. In contrast, the second site is involved in the furin dependence of viral entry and syncytium formation. Mutations of the second site from furin-cleavable RRRR/S to non-furin-cleavable PRRRS and AAARS, respectively, abrogated the furin dependence of IBV entry. Instead, a yet-to-be-identified serine protease(s) was involved, as revealed by protease inhibitor studies. Furthermore, sequence analysis of CoV S proteins by multiple alignments showed conservation of an XXXR/S motif, cleavable by either furin or other trypsin-like proteases, at a position equivalent to the second IBV furin site. Taken together, these results suggest that proteolysis at a novel XXXR/S motif in the S2 subunit might be a common mechanism for the entry of CoV into cells.

Protease-mediated entry via the endosome of human coronavirus 229E

Human coronavirus 229E, classified as a group I coronavirus, utilizes human aminopeptidase N (APN) as a receptor; however, its entry mechanism has not yet been fully elucidated. We found that HeLa cells infected with 229E via APN formed syncytia when treated with trypsin or other proteases but not in a low-pH environment, a finding consistent with syncytium formation by severe acute respiratory syndrome coronavirus (SARS-CoV). In addition, trypsin induced cleavage of the 229E S protein. By using infectious viruses and pseudotyped viruses bearing the 229E S protein, we found that its infection was profoundly blocked by lysosomotropic agents as well as by protease inhibitors that also prevented infection with SARS-CoV but not that caused by murine coronavirus mouse hepatitis virus strain JHMV, which enters cells directly from the cell surface. We found that cathepsin L (CPL) inhibitors blocked 229E infection the most remarkably among a variety of protease inhibitors tested. Furthermore, 229E infection was inhibited in CPL knockdown cells by small interfering RNA, compared with what was seen for a normal counterpart producing CPL. However, its inhibition was not so remarkable as that found with SARS-CoV infection, which seems to indicate that while CPL is involved in the fusogenic activation of 229E S protein in endosomal infection, not-yet-identified proteases could also play a part in that activity. We also found 229E virion S protein to be cleaved by CPL. Furthermore, as with SARS-CoV, 229E entered cells directly from the cell surface when cell-attached viruses were treated with trypsin. These findings suggest that 229E takes an endosomal pathway for cell entry and that proteases like CPL are involved in this mode of entry.

Clathrin- and caveolae-independent entry of feline infectious peritonitis virus in monocytes depends on dynamin

Feline infectious peritonitis virus (FIPV), a coronavirus that causes a lethal chronic disease in cats, enters feline monocytes via endocytosis. In this study, the pathway of internalization is characterized by evaluating the effect of chemical inhibitors and/or expression of dominant-negative (DN) proteins on the percentage of internalized virions per cell and infection. Further, co-localization studies were performed to determine the involvement of certain cellular internalization proteins. FIPV is not internalized through a clathrin-mediated pathway, as chlorpromazine, amantadine and DN eps15 did not influence virus uptake and FIPV did not co-localize with clathrin. The caveolae-mediated pathway could be excluded based on the inability of genistein and DN caveolin-1 to inhibit virus uptake and lack of co-localization between FIPV and caveolin-1. Dynamin inhibitory peptide and DN dynamin effectively inhibited virus internalization. The inhibitor strongly reduced uptake to 20.3+/-1.1% of uptake in untreated cells. In the presence of DN dynamin, uptake was 58.7+/-3.9% relative to uptake in untransduced cells. Internalization of FIPV was slightly reduced to 85.0+/-1.4 and 87.4+/-6.1% of internalization in control cells by the sterol-binding drugs nystatin and methyl-beta-cyclodextrin, respectively. Rho GTPases were inhibited by Clostridium difficile toxin B, but no effect was observed. These results were confirmed with infection studies showing that infection was not influenced by chlorpromazine, amantadine and genistein, but was significantly reduced by dynamin inhibition and nystatin. In conclusion, these results indicate that FIPV enters monocytes through a clathrin- and caveolae-independent pathway that strongly depends on dynamin and is slightly sensitive to cholesterol depletion.

Equine arteritis virus is delivered to an acidic compartment of host cells via clathrin-dependent endocytosis

Equine arteritis virus (EAV) is an enveloped, positive-stranded RNA virus belonging to the family Arteriviridae. Infection by EAV requires the release of the viral genome by fusion with the respective target membrane of the host cell. We have investigated the entry pathway of EAV into Baby Hamster Kindey cells (BHK). Infection of cells assessed by the plaque reduction assay was strongly inhibited by substances which interfere with clathrin-dependent endocytosis and by lysosomotropic compounds. Furthermore, infection of BHK cells was suppressed when clathrin-dependent endocytosis was inhibited by expression of antisense RNA of the clathrin-heavy chain before infection. These results strongly suggest that EAV is taken up via clathrin-dependent endocytosis and is delivered to acidic endosomal compartments.

Entry from the cell surface of severe acute respiratory syndrome coronavirus with cleaved S protein as revealed by pseudotype virus bearing cleaved S protein

Severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV) is known to take an endosomal pathway for cell entry; however, it is thought to enter directly from the cell surface when a receptor-bound virion spike (S) protein is affected by trypsin, which induces cleavage of the S protein and activates its fusion potential. This suggests that SARS-CoV bearing a cleaved form of the S protein can enter cells directly from the cell surface without trypsin treatment. To explore this possibility, we introduced a furin-like cleavage sequence in the S protein at amino acids 798 to 801 and found that the mutated S protein was cleaved and induced cell fusion without trypsin treatment when expressed on the cell surface. Furthermore, a pseudotype virus bearing a cleaved S protein was revealed to infect cells in the presence of a lysosomotropic agent as well as a protease inhibitor, both of which are known to block SARS-CoV infection via an endosome, whereas the infection of pseudotypes with an uncleaved, wild-type S protein was blocked by these agents. A heptad repeat peptide, derived from a SARS-CoV S protein that is known to efficiently block infections from the cell surface, blocked the infection by a pseudotype with a cleaved S protein but not that with an uncleaved S protein. Those results indicate that SARS-CoV with a cleaved S protein is able to enter cells directly from the cell surface and agree with the previous observation of the protease-mediated cell surface entry of SARS-CoV.

Structures and mechanisms of viral membrane fusion proteins: multiple variations on a common theme

Recent work has identified three distinct classes of viral membrane fusion proteins based on structural criteria. In addition, there are at least four distinct mechanisms by which viral fusion proteins can be triggered to undergo fusion-inducing conformational changes. Viral fusion proteins also contain different types of fusion peptides and vary in their reliance on accessory proteins. These differing features combine to yield a rich diversity of fusion proteins. Yet despite this staggering diversity, all characterized viral fusion proteins convert from a fusion-competent state (dimers or trimers, depending on the class) to a membrane-embedded homotrimeric prehairpin, and then to a trimer-of-hairpins that brings the fusion peptide, attached to the target membrane, and the transmembrane domain, attached to the viral membrane, into close proximity thereby facilitating the union of viral and target membranes. During these conformational conversions, the fusion proteins induce membranes to progress through stages of close apposition, hemifusion, and then the formation of small, and finally large, fusion pores. Clearly, highly divergent proteins have converged on the same overall strategy to mediate fusion, an essential step in the life cycle of every enveloped virus.

Mouse hepatitis virus type 2 enters cells through a clathrin-mediated endocytic pathway independent of Eps15

It has recently been shown that cell entry of mouse hepatitis virus type 2 (MHV-2) is mediated through endocytosis (Z. Qiu et al., J. Virol. 80:5768-5776, 2006). However, the molecular mechanism underlying MHV-2 entry is not known. Here we employed multiple chemical and molecular approaches to determine the molecular pathways for MHV-2 entry. Our results showed that MHV-2 gene expression and infectivity were significantly inhibited when cells were treated with chemical and physiologic blockers of the clathrin-mediated pathway, such as chlorpromazine and hypertonic sucrose medium. Furthermore, viral gene expression was significantly inhibited when cells were transfected with a small interfering RNA specific to the clathrin heavy chain. However, these treatments did not affect the infectivity and gene expression of MHV-A59, demonstrating the specificity of the inhibitions. In addition, overexpression of a dominant-negative mutant of caveolin 1 did not have any effect on MHV-2 infection, while it significantly blocked the caveolin-dependent uptake of cholera toxin subunit B. These results demonstrate that MHV-2 utilizes the clathrin- but not caveolin-mediated endocytic pathway for entry. Interestingly, when the cells transiently overexpressed a dominant-negative form (DIII) of Eps15, which is thought to be an essential component of the clathrin pathway, viral gene expression and infectivity were unaffected, although DIII expression blocked transferrin uptake and vesicular stomatitis virus infection, which are dependent on clathrin-mediated endocytosis. Thus, MHV-2 entry is mediated through clathrin-dependent but Eps15-independent endocytosis.

Differential role for low pH and cathepsin-mediated cleavage of the viral spike protein during entry of serotype II feline coronaviruses

Feline infectious peritonitis (FIP) is a terminal disease of cats caused by systemic infection with a feline coronavirus (FCoV). FCoV biotypes that cause FIP are designated feline infectious peritonitis virus (FIPV), and are distinguished by their ability to infect macrophages and monocytes. Antigenically similar to their virulent counterparts are FCoV biotypes designated feline enteric coronavirus (FECV), which usually cause only mild enteritis and are unable to efficiently infect macrophages and monocytes. The FCoV spike protein mediates viral entry into the host cell and has previously been shown to determine the distinct tropism exhibited by certain isolates of FIPV and FECV, however, the molecular mechanism underlying viral pathogenesis has yet to be determined. Here we show that the FECV strain WSU 79-1683 (FECV-1683) is highly dependent on host cell cathepsin B and cathepsin L activity for entry into the host cell, as well as on the low pH of endocytic compartments. In addition, both cathepsin B and cathepsin L are able to induce a specific cleavage event in the FECV-1683 spike protein. In contrast, host cell entry by the FIPV strains WSU 79-1146 (FIPV-1146) and FIPV-DF2 proceeds independently of cathepsin L activity and low pH, but is still highly dependent on cathepsin B activity. In the case of FIPV-1146 and FIPV-DF2, infection of primary feline monocytes was also dependent on host cell cathepsin B activity, indicating that host cell cathepsins may play a role in the distinct tropisms displayed by different feline coronavirus biotypes.