A novel severe acute respiratory syndrome (SARS)-like coronavirus (SARS-CoV-2) is causing the global coronavirus disease 2019 (COVID-19) pandemic. Understanding how SARS-CoV-2 enters human cells is a high priority for deciphering its mystery and curbing its spread. A virus surface spike protein mediates SARS-CoV-2 entry into cells. To fulfill its function, SARS-CoV-2 spike binds to its receptor human ACE2 (hACE2) through its receptor-binding domain (RBD) and is proteolytically activated by human proteases. Here we investigated receptor binding and protease activation of SARS-CoV-2 spike using biochemical and pseudovirus entry assays. Our findings have identified key cell entry mechanisms of SARS-CoV-2. First, SARS-CoV-2 RBD has higher hACE2 binding affinity than SARS-CoV RBD, supporting efficient cell entry. Second, paradoxically, the hACE2 binding affinity of the entire SARS-CoV-2 spike is comparable to or lower than that of SARS-CoV spike, suggesting that SARS-CoV-2 RBD, albeit more potent, is less exposed than SARS-CoV RBD. Third, unlike SARS-CoV, cell entry of SARS-CoV-2 is preactivated by proprotein convertase furin, reducing its dependence on target cell proteases for entry. The high hACE2 binding affinity of the RBD, furin preactivation of the spike, and hidden RBD in the spike potentially allow SARS-CoV-2 to maintain efficient cell entry while evading immune surveillance. These features may contribute to the wide spread of the virus. Successful intervention strategies must target both the potency of SARS-CoV-2 and its evasiveness.

https://www.pnas.org/content/pnas/117/21/11727.full.pdf

Cell entry mechanisms of SARS-CoV-2.

The ORF6, ORF8 and nucleocapsid proteins of SARS-CoV-2 inhibit type I interferon signaling pathway.

Feline coronavirus (FCoV) is a complex viral agent that causes a variety of clinical manifestations in cats, commonly known as feline infectious peritonitis (FIP). It is recognized that FCoV can occur in two different serotypes. However, differences in the S protein are much more than serological or antigenic variants, resulting in the effective presence of two distinct viruses. Here, we review the distinct differences in the S proteins of these viruses, which are likely to translate into distinct biological outcomes. We introduce a new concept related to the non-taxonomical classification and differentiation among FCoVs by analyzing and comparing the genetic, structural, and functional characteristics of FCoV and the FCoV S protein among the two serotypes and FCoV biotypes. Based on our analysis, we suggest that our understanding of FIP needs to consider whether the presence of these two distinct viruses has implications in clinical settings.

https://www.mdpi.com/1999-4915/12/1/83/pdf

A Tale of Two Viruses: The Distinct Spike Glycoproteins of Feline Coronaviruses.

enzymes. Also, the problems of local circulation and capillary distribution were considered in relation to vulnerability of certain areas, i.e., Ammon's horn. The remaining three papers presented material concerned directly with pharmacological studies. These were: studies of the electrical activity of the brain in relation to anesthesia by Mary A. B. Brazier, the ascending reticular system and anesthesia by Horace W. Magoun, and observations on new central nervous system convulsants by Carl C. Pfeiffer. The first two papers reviewed mainly the electrical aspects of the effects of anesthesia. Brazier presented in detail the various electroencephalographic changes that occur under anesthesia especially with barbiturates, showed some of the differences between surface and depth recording, and the electrical responses to sensory stimulation, especially photic. Magoun emphasized the ascending reticular formation in the brain stem concerned with the electroencephalographic arousal reaction as the pathway involved in general anesthesia rather than the classical somatic afferent sensory pathways. The \"reticular route\" to the cortex is regarded as quite susceptible to anesthetic agents because of its multisynaptic organization. The report on new CNS convulsants by Pfeiffer included a presentation of two interesting new drugs. One, acridone, is fluorescent and its distribution in the nervous system may be studied after administration. The other, one of a group of convulsant hydrazides has a well-defined action, being convulsant by virtue of inducing a pyridoxine deficiency. It is hoped that further studies of these drugs might develop information concerning the metabolic basis of neuronal excitability. In the discussion of this material there was brief reference to the effects of hormones on central nervous system function and the psychological effects of lysergic acid or LSD 25. It should be noted that table 16 is presented quite out of place between pages 80 and 81, belonging rather between pages 182 and 183. This conference, therefore, primarily considered certain introductory, basic aspects in the field of neuropharmacology and only two special drug effects, the anesthetic and the convulsant. This field is becoming increasingly wider and more significant, and it is hoped that in further conferences other problems will be considered, especially those concerning anti-convulsants, drugs involved in neuromuscular transmission, drugs affecting the autonomic nervous system, and finally the drugs and other substances such as hormones which produce definite psychological effects such as the adrenal steroids, mescalin, LSD 25, the Rauwolfia substances, and chlorpromazine. This present volume is recommended as an introduction and initial critique and it is hoped that the future conferences will uphold this first standard.

Advances in Virus Research

Cytoplasmic stress granules (SGs) are generally triggered by stress-induced translation arrest for storing mRNAs. Recently, it has been shown that SGs exert anti-viral functions due to their involvement in protein synthesis shut off and recruitment of innate immune signaling intermediates. The largest RNA viruses, coronaviruses, impose great threat to public safety and animal health; however, the significance of SGs in coronavirus infection is largely unknown. Infectious Bronchitis Virus (IBV) is the first identified coronavirus in 1930s and has been prevalent in poultry farm for many years. In this study, we provided evidence that IBV overcomes the host antiviral response by inhibiting SGs formation via the virus-encoded endoribonuclease nsp15. By immunofluorescence analysis, we observed that IBV infection not only did not trigger SGs formation in approximately 80% of the infected cells, but also impaired the formation of SGs triggered by heat shock, sodium arsenite, or NaCl stimuli. We further demonstrated that the intrinsic endoribonuclease activity of nsp15 was responsible for the interference of SGs formation. In fact, nsp15-defective recombinant IBV (rIBV-nsp15-H238A) greatly induced the formation of SGs, along with accumulation of dsRNA and activation of PKR, whereas wild type IBV failed to do so. Consequently, infection with rIBV-nsp15-H238A strongly triggered transcription of IFN-β which in turn greatly affected rIBV-nsp15-H238A replication. Further analysis showed that SGs function as an antiviral hub, as demonstrated by the attenuated IRF3-IFN response and increased production of IBV in SG-defective cells. Additional evidence includes the aggregation of pattern recognition receptors (PRRs) and signaling intermediates to the IBV-induced SGs. Collectively, our data demonstrate that the endoribonuclease nsp15 of IBV interferes with the formation of antiviral hub SGs by regulating the accumulation of viral dsRNA and by antagonizing the activation of PKR, eventually ensuring productive virus replication. We further demonstrated that nsp15s from PEDV, TGEV, SARS-CoV, and SARS-CoV-2 harbor the conserved function to interfere with the formation of chemically-induced SGs. Thus, we speculate that coronaviruses employ similar nsp15-mediated mechanisms to antagonize the host anti-viral SGs formation to ensure efficient virus replication.

/pdf/inhibition-of-anti-viral-stress-granule-formation-by-49jvpigmb1.pdf

Inhibition of anti-viral stress granule formation by coronavirus endoribonuclease nsp15 ensures efficient virus replication.

Coronaviruses (CoVs) encode multiple interferon (IFN) antagonists that modulate the host response to virus replication. Here, we evaluated the host transcriptional response to infection with murine coronaviruses encoding independent mutations in one of two different viral antagonists, the deubiquitinase (DUB) within nonstructural protein 3 or the endoribonuclease (EndoU) within nonstructural protein 15. We used transcriptomics approaches to compare the scope and kinetics of the host response to the wild-type (WT), DUBmut, and EndoUmut viruses in infected macrophages. We found that the EndoUmut virus activates a focused response that predominantly involves type I interferons and interferon-related genes, whereas the WT and DUBmut viruses more broadly stimulate upregulation of over 2,800 genes, including networks associated with activating the unfolded protein response (UPR) and the proinflammatory response associated with viral pathogenesis. This study highlights the role of viral interferon antagonists in shaping the kinetics and magnitude of the host response during virus infection and demonstrates that inactivating a dominant viral antagonist, the coronavirus endoribonuclease, dramatically alters the host response in macrophages.IMPORTANCE Macrophages are an important cell type during coronavirus infections because they "notice" the infection and respond by inducing type I interferons, which limits virus replication. In turn, coronaviruses encode proteins that mitigate the cell's ability to signal an interferon response. Here, we evaluated the host macrophage response to two independent mutant coronaviruses, one with reduced deubiquitinating activity (DUBmut) and the other containing an inactivated endoribonuclease (EndoUmut). We observed a rapid, robust, and focused response to the EndoUmut virus, which was characterized by enhanced expression of interferon and interferon-related genes. In contrast, wild-type virus and the DUBmut virus elicited a more limited interferon response and ultimately activated over 2,800 genes, including players in the unfolded protein response and proinflammatory pathways associated with progression of significant disease. This study reveals that EndoU activity substantially contributes to the ability of coronaviruses to evade the host innate response and to replicate in macrophages.

/pdf/coronavirus-endoribonuclease-and-deubiquitinating-interferon-2e8gciyu5e.pdf

Coronavirus Endoribonuclease and Deubiquitinating Interferon Antagonists Differentially Modulate the Host Response during Replication in Macrophages.

Characterizing replication kinetics and plaque production of type I feline infectious peritonitis virus in three feline cell lines

Coronaviruses encode multiple interferon antagonists that modulate the host response to virus replication. Here, we evaluated pathogenesis and host transcription in response to infection with murine coronaviruses encoding independent mutations in two different viral antagonists: the deubiquitinase (DUB) within nonstructural protein 3 and the endoribonuclease (EndoU) within nonstructural protein 15. The virus with reduced ability to deubiquitinate proteins, herein termed the DUBmut virus, was engineered via X-ray structure-guided mutagenesis and activates an earlier interferon response than the wild type virus. However, the replication kinetics of DUBmut in cultured cells are similar to wild type virus and pathogenesis in mice is also similar to what was observed during infection with wild type virus. On the other hand, we previously reported that an EndoUmut virus containing an inactivated endoribonuclease activity elicited rapid and robust activation of type I interferon, which limited virus replication and pathogenesis. Here, using a transcriptomics approach, we compared the scope and kinetics of the host response to the wild type, DUBmut, and EndoUmut viruses in infected macrophages. We found that the EndoUmut virus activates a focused response, predominantly involving type I interferons and a subset of interferon-responsive genes, within 12 hours after infection. In contrast, the wild type and DUBmut viruses stimulate upregulation of over 2,800 genes, including activation of unfolded protein response (UPR) pathways and a proinflammatory profile associated with viral pathogenesis. This study highlights the role of viral interferon antagonists in shaping the kinetics and magnitude of the host response during virus infection and demonstrates that inactivation of a dominant viral antagonist, the coronavirus endoribonuclease, dramatically alters the host response in macrophages and the disease process. Author Summary Macrophages are an important cell type during coronavirus infections because they “notice” the infection and respond by activating type I interferons, which then act to establish antiviral defenses and limit virus replication. In turn, coronaviruses encode proteins that mitigate the cell’s ability to detect virus replication or amplify the interferon response. Here, we evaluated the host macrophage response to two independent mutant coronaviruses: one with a reduced deubiquitinating activity (DUBmut), and the other containing an inactivated endoribonuclease (EndoUmut). We observed a rapid, robust, and focused response to the EndoUmut virus, which was characterized by enhanced expression of interferon and interferon-stimulated genes. These results indicate that coronaviruses utilize EndoU activity for preventing early activation of interferon in macrophages, thereby allowing for viral replication. In contrast, DUBmut elicited a transient interferon response and ultimately activated over 2,800 genes, including many well-known players in pro-inflammatory pathways and the unfolded protein response. These DUBmut-induced pathways are associated with development and progression of significant disease, similar to what is observed during wild type virus infection. This study demonstrates the distinct consequences of mutating different viral interferon antagonists and reveals that intact coronaviral EndoU activity substantially contributes to the ability of coronaviruses to replicate in macrophages.

/pdf/coronavirus-interferon-antagonists-differentially-modulate-4asrp19kmf.pdf

Coronavirus Interferon Antagonists Differentially Modulate the Host Response during Replication in Macrophages

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Coronavirus Endoribonuclease and Deubiquitinating Interferon Antagonists Differentially Modulate the Host Response during Replication in Macrophages.

Characterizing replication kinetics and plaque production of type I feline infectious peritonitis virus in three feline cell lines

Coronavirus Interferon Antagonists Differentially Modulate the Host Response during Replication in Macrophages