Biosynthesis, structure, and biological activities of envelope protein gp65 of murine coronavirus

Vaccines and Therapeutics for COVID-19 - How Can Understanding SARS-CoV-2 Glycosylation Lead to Pharmaceutical Advances?

The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has infected more than 160 million people worldwide. Researchers have targeted the SARS-CoV-2 structural proteins to better combat the pandemic. Of the four structural proteins, spike (S), membrane (M), envelope (E) and nucleocapsid (N), the S, M and E proteins are glycosylated whereas the N protein is phosphorylated. The glycosylation of the S protein has been reported previously by multiple research groups, and this knowledge has assisted the pharmaceutical industry in developing vaccines and treatment options. In the United States, there are currently three approved COVID-19 vaccines. All three of these vaccines use the S protein to teach host cells how to react when SARS-CoV-2 particles are present. Treatment options utilizing antivirals and immunosuppressants are being developed in addition to vaccines. Different treatment approaches are needed based on the severity of COVID-19 infection. The therapeutic options currently available are not derived through the direct knowledge on SARS-CoV-2 glycosylation. However, more research on the glycosylation of the structural proteins and how this effects SARS-CoV-2 and host cell binding could lead to new and more effective therapeutics. Herein we outline the current vaccine and therapeutic options against COVID-19 available to the public, as well as those still in development.

Novel human coronavirus (SARS-CoV-2): A lesson from animal coronaviruses

The recent pandemic caused by the novel human coronavirus, referrred to as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), not only is having a great impact on the health care systems and economies in all continents but it is also causing radical changes of common habits and life styles. The novel coronavirus (CoV) recognises, with high probability, a zoonotic origin but the role of animals in the SARS-CoV-2 epidemiology is still largely unknown. However, CoVs have been known in animals since several decades, so that veterinary coronavirologists have a great expertise on how to face CoV infections in animals, which could represent a model for SARS-CoV-2 infection in humans. In the present paper, we provide an up-to-date review of the literature currently available on animal CoVs, focusing on the molecular mechanisms that are responsible for the emergence of novel CoV strains with different antigenic, biologic and/or pathogenetic features. A full comprehension of the mechanisms driving the evolution of animal CoVs will help better understand the emergence, spreading, and evolution of SARS-CoV-2.

Post-translational modifications of coronavirus proteins: roles and function

Post-translational modifications (PTMs) refer to the covalent modifications of polypeptides after they are synthesized, adding temporal and spatial regulation to modulate protein functions. Being obligate intracellular parasites, viruses rely on the protein synthesis machinery of host cells to support replication, and not surprisingly, many viral proteins are subjected to PTMs. Coronavirus (CoV) is a group of enveloped RNA viruses causing diseases in both human and animals. Many CoV proteins are modified by PTMs, including glycosylation and palmitoylation of the spike and envelope protein, N- or O-linked glycosylation of the membrane protein, phosphorylation and ADP-ribosylation of the nucleocapsid protein, and other PTMs on nonstructural and accessory proteins. In this review, we summarize the current knowledge on PTMs of CoV proteins, with an emphasis on their impact on viral replication and pathogenesis. The ability of some CoV proteins to interfere with PTMs of host proteins will also be discussed.

Sialic Acid Receptors of Viruses

Sialic acid linked to glycoproteins and gangliosides is used by many viruses as a receptor for cell entry. These viruses include important human and animal pathogens, such as influenza, parainfluenza, mumps, corona, noro, rota, and DNA tumor viruses. Attachment to sialic acid is mediated by receptor binding proteins that are constituents of viral envelopes or exposed at the surface of non-enveloped viruses. Some of these viruses are also equipped with a neuraminidase or a sialyl-O-acetyl-esterase. These receptor-destroying enzymes promote virus release from infected cells and neutralize sialic acid-containing soluble proteins interfering with cell surface binding of the virus. Variations in the receptor specificity are important determinants for host range, tissue tropism, pathogenicity, and transmissibility of these viruses.

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.

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.

Attachment of mouse hepatitis virus to O-acetylated sialic acid is mediated by hemagglutinin-esterase and not by the spike protein

The members of Betacoronavirus phylocluster A possess two types of surface projections, one comprised of the spike protein (S) and the other of hemagglutinin-esterase (HE). Purportedly, these viruses bind to O-acetylated sialic acids (O-Ac-Sias) primarily through S, with HE serving merely as receptor-destroying enzyme. Here, we show that, in apparent contrast to human and ungulate host range variants of Betacoronavirus-1, murine coronaviruses actually bind to O-Ac-Sias via HE exclusively. Apparently, expansion of group A betacoronaviruses into new hosts and niches was accompanied by changes in HE ligand and substrate preference and in the roles of HE and S in Sia receptor usage.

Pathogenesis of murine coronavirus in the central nervous system

Murine coronavirus (mouse hepatitis virus, MHV) is a collection of strains that induce disease in several organ systems of mice. Infection with neurotropic strains JHM and A59 causes acute encephalitis, and in survivors, chronic demyelination, the latter of which serves as an animal model for multiple sclerosis. The MHV receptor is a carcinoembryonic antigen-related cell adhesion molecule, CEACAM1a; paradoxically, CEACAM1a is poorly expressed in the central nervous system (CNS), leading to speculation of an additional receptor. Comparison of highly neurovirulent JHM isolates with less virulent variants and the weakly neurovirulent A59 strain, combined with the use of reverse genetics, has allowed mapping of pathogenic properties to individual viral genes. The spike protein, responsible for viral entry, is a major determinant of tropism and virulence. Other viral proteins, both structural and nonstructural, also contribute to pathogenesis in the CNS. Studies of host responses to MHV indicate that both innate and adaptive responses are crucial to antiviral defense. Type I interferon is essential to prevent very early mortality after infection. CD8 T cells, with the help of CD4 T cells, are crucial for viral clearance during acute disease and persist in the CNS during chronic disease. B cells are necessary to prevent reactivation of virus in the CNS following clearance of acute infection. Despite advances in understanding of coronavirus pathogenesis, questions remain regarding the mechanisms of viral entry and spread in cell types expressing low levels of receptor, as well as the unique interplay between virus and the host immune system during acute and chronic disease.

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.

Sialic acids as receptor determinants for coronaviruses

Among coronaviruses, several members are able to interact with sialic acids. For bovine coronavirus (BCoV) and related viruses, binding to cell surface components containing N-acetyl-9- O-acetylneuraminic acid is essential for initiation of an infection. These viruses resemble influenza C viruses because they share not only the receptor determinant, but also the presence of an acetylesterase that releases the 9- O-acetyl group from sialic acid and thus abolishes the ability of the respective sialoglycoconjugate to function as a receptor for BCoV. As in the case of influenza viruses, the receptor-destroying enzyme of BCoV is believed to facilitate the spread of virus infection by removing receptor determinants from the surface of infected cells and by preventing the formation of virus aggregates. Another coronavirus, porcine transmissible gastroenteritis virus (TGEV) preferentially recognizes N-glycolylneuraminic acid. TGEV does not contain a receptor-destroying enzyme and does not depend on the sialic acid binding activity for infection of cultured cells. However, binding to sialic acids is required for the enteropathogenicity of TGEV. Interaction with sialoglycoconjugates may help the virus to pass through the sialic acid-rich mucus layer that covers the viral target cells in the epithelium of the small intestine. We discuss that the BCoV group of viruses may have evolved from a TGEV-like ancestor by acquiring an acetylesterase gene through heterologous recombination.

Suppression of coronavirus replication by inhibition of the MEK signaling pathway

We previously demonstrated that infection of cultured cells with murine coronavirus mouse hepatitis virus (MHV) resulted in activation of the mitogen-activated protein kinase (Raf/MEK/ERK) signal transduction pathway (Y. Cai et al., Virology 355:152-163, 2006). Here we show that inhibition of the Raf/MEK/ERK signaling pathway by the MEK inhibitor UO126 significantly impaired MHV progeny production (a reduction of 95 to 99% in virus titer), which correlated with the phosphorylation status of ERK1/2. Moreover, knockdown of MEK1/2 and ERK1/2 by small interfering RNAs suppressed MHV replication. The inhibitory effect of UO126 on MHV production appeared to be a general phenomenon since the effect was consistently observed in all six different MHV strains and in three different cell types tested; it was likely exerted at the postentry steps of the virus life cycle because the virus titers were similarly inhibited from infected cells treated at 1 h prior to, during, or after infection. Furthermore, the treatment did not affect the virus entry, as revealed by the virus internalization assay. Metabolic labeling and reporter gene assays demonstrated that translation of cellular and viral mRNAs appeared unaffected by UO126 treatment. However, synthesis of viral genomic and subgenomic RNAs was severely suppressed by UO126 treatment, as demonstrated by a reduced incorporation of [3H]uridine and a decrease in chloramphenicol acetyltransferase (CAT) activity in a defective-interfering RNA-CAT reporter assay. These findings indicate that the Raf/MEK/ERK signaling pathway is involved in MHV RNA synthesis.

The molecular biology of coronaviruses

Coronaviruses are large, enveloped RNA viruses of both medical and veterinary importance. Interest in this viral family has intensified in the past few years as a result of the identification of a newly emerged coronavirus as the causative agent of severe acute respiratory syndrome (SARS). At the molecular level, coronaviruses employ a variety of unusual strategies to accomplish a complex program of gene expression. Coronavirus replication entails ribosome frameshifting during genome translation, the synthesis of both genomic and multiple subgenomic RNA species, and the assembly of progeny virions by a pathway that is unique among enveloped RNA viruses. Progress in the investigation of these processes has been enhanced by the development of reverse genetic systems, an advance that was heretofore obstructed by the enormous size of the coronavirus genome. This review summarizes both classical and contemporary discoveries in the study of the molecular biology of these infectious agents, with particular emphasis on the nature and recognition of viral receptors, viral RNA synthesis, and the molecular interactions governing virion assembly.

Luxury at a cost? Recombinant mouse hepatitis viruses expressing the accessory hemagglutinin esterase protein display reduced fitness in vitro

Group 2 coronaviruses encode an accessory envelope glycoprotein species, the hemagglutinin esterase (HE), which possesses sialate-O-acetylesterase activity and which, presumably, promotes virus spread and entry in vivo by facilitating reversible virion attachment to O-acetylated sialic acids. While HE may provide a strong selective advantage during natural infection, many laboratory strains of mouse hepatitis virus (MHV) fail to produce the protein. Apparently, their HE genes were inactivated during cell culture adaptation. For this report, we have studied the molecular basis of this phenomenon. By using targeted RNA recombination, we generated isogenic recombinant MHVs which differ exclusively in their expression of HE and produce either the wild-type protein (HE+), an enzymatically inactive HE protein (HE0), or no HE at all. HE expression or the lack thereof did not lead to gross differences in in vitro growth properties. Yet the expression of HE was rapidly lost during serial cell culture passaging. Competition experiments with mixed infections revealed that this was not due to the enzymatic activity: MHVs expressing HE+ or HE0 propagated with equal efficiencies. During the propagation of recombinant MHV-HE+, two types of spontaneous mutants accumulated. One produced an anchorless HE, while the other had a Gly-to-Trp substitution at the predicted C-terminal residue of the HE signal peptide. Neither mutant incorporated HE into virion particles, suggesting that wild-type HE reduces the in vitro propagation efficiency, either at the assembly stage or at a postassembly level. Our findings demonstrate that the expression of "luxury" proteins may come at a fitness penalty. Apparently, under natural conditions the costs of maintaining HE are outweighed by the benefits.

Coronavirus pathogenesis and the emerging pathogen severe acute respiratory syndrome coronavirus

Coronaviruses are a family of enveloped, single-stranded, positive-strand RNA viruses classified within the Nidovirales order. This coronavirus family consists of pathogens of many animal species and of humans, including the recently isolated severe acute respiratory syndrome coronavirus (SARS-CoV). This review is divided into two main parts; the first concerns the animal coronaviruses and their pathogenesis, with an emphasis on the functions of individual viral genes, and the second discusses the newly described human emerging pathogen, SARS-CoV. The coronavirus part covers (i) a description of a group of coronaviruses and the diseases they cause, including the prototype coronavirus, murine hepatitis virus, which is one of the recognized animal models for multiple sclerosis, as well as viruses of veterinary importance that infect the pig, chicken, and cat and a summary of the human viruses; (ii) a short summary of the replication cycle of coronaviruses in cell culture; (iii) the development and application of reverse genetics systems; and (iv) the roles of individual coronavirus proteins in replication and pathogenesis. The SARS-CoV part covers the pathogenesis of SARS, the developing animal models for infection, and the progress in vaccine development and antiviral therapies. The data gathered on the animal coronaviruses continue to be helpful in understanding SARS-CoV.

Structure, function and evolution of the hemagglutinin-esterase proteins of corona- and toroviruses

Virus attachment to host cells is mediated by dedicated virion proteins, which specifically recognize one or, at most, a limited number of cell surface molecules. Receptor binding often involves protein-protein interactions, but carbohydrates may serve as receptor determinants as well. In fact, many different viruses use members of the sialic acid family either as their main receptor or as an initial attachment factor. Sialic acids (Sias) are 9-carbon negatively-charged monosaccharides commonly occurring as terminal residues of glycoconjugates. They come in a large variety and are differentially expressed in cells and tissues. By targeting specific Sia subtypes, viruses achieve host cell selectivity, but only to a certain extent. The Sia of choice might still be abundantly present on non-cell associated molecules, on non-target cells (including cells already infected) and even on virus particles themselves. This poses a hazard, as high-affinity virion binding to any of such "false'' receptors would result in loss of infectivity. Some enveloped RNA viruses deal with this problem by encoding virion-associated receptor-destroying enzymes (RDEs). These enzymes make the attachment to Sia reversible, thus providing the virus with an escape ticket. RDEs occur in two types: neuraminidases and sialate-O-acetylesterases. The latter, originally discovered in influenza C virus, are also found in certain nidoviruses, namely in group 2 coronaviruses and in toroviruses, as well as in infectious salmon anemia virus, an orthomyxovirus of teleosts. Here, the structure, function and evolution of viral sialate-O-acetylesterases is reviewed with main focus on the hemagglutinin-esterases of nidoviruses.

Coronavirus pathogenesis and the emerging pathogen severe acute respiratory syndrome coronavirus

Coronaviruses are a family of enveloped, single-stranded, positive-strand RNA viruses classified within the Nidovirales order. This coronavirus family consists of pathogens of many animal species and of humans, including the recently isolated severe acute respiratory syndrome coronavirus (SARS-CoV). This review is divided into two main parts; the first concerns the animal coronaviruses and their pathogenesis, with an emphasis on the functions of individual viral genes, and the second discusses the newly described human emerging pathogen, SARS-CoV. The coronavirus part covers (i) a description of a group of coronaviruses and the diseases they cause, including the prototype coronavirus, murine hepatitis virus, which is one of the recognized animal models for multiple sclerosis, as well as viruses of veterinary importance that infect the pig, chicken, and cat and a summary of the human viruses; (ii) a short summary of the replication cycle of coronaviruses in cell culture; (iii) the development and application of reverse genetics systems; and (iv) the roles of individual coronavirus proteins in replication and pathogenesis. The SARS-CoV part covers the pathogenesis of SARS, the developing animal models for infection, and the progress in vaccine development and antiviral therapies. The data gathered on the animal coronaviruses continue to be helpful in understanding SARS-CoV.

Molecular interactions in the assembly of coronaviruses

This chapter describes the interactions between the different structural components of the viruses and discusses their relevance for the process of virion formation. Two key factors determine the efficiency of the assembly process: intracellular transport and molecular interactions. Many viruses have evolved elaborate strategies to ensure the swift and accurate delivery of the virion components to the cellular compartment(s) where they must meet and form (sub) structures. Assembly of viruses starts in the nucleus by the encapsidation of viral DNA, using cytoplasmically synthesized capsid proteins; nucleocapsids then migrate to the cytosol, by budding at the inner nuclear membrane followed by deenvelopment, to pick up the tegument proteins.

Induction of caspase-dependent apoptosis in cultured rat oligodendrocytes by murine coronavirus is mediated during cell entry and does not require virus replication

Murine coronavirus mouse hepatitis virus (MHV) causes demyelination of the central nervous system (CNS) in rats and mice. Apoptotic oligodendrocytes have been detected in the vicinity of the CNS demyelinating lesions in these animals. However, whether MHV can directly induce oligodendrocyte apoptosis has not been documented. Here, we established a rat oligodendrocyte culture that is morphologically and phenotypically indistinguishable from the primary rat oligodendrocytes. Using this culture, we showed that mature rat oligodendrocytes were permissive to MHV infection but did not support productive virus replication. Significantly, oligodendrocytes infected with both live and ultraviolet light-inactivated viruses underwent apoptosis to a similar extent, which was readily detectable at 24 h postinfection as revealed by apoptotic bodies and DNA fragmentation, indicating that MHV-induced apoptosis is mediated during the early stages of the virus life cycle and does not require virus replication. Prior treatment of cells with the lysosomotropic agents NH(4)Cl and chloroquine as well as the vacuolar proton pump-ATPase inhibitor bafilomycin A1, all of which block the acidification of the endosome, prevented oligodendrocytes from succumbing to apoptosis induced by MHV mutant OBLV60, which enters cells via endocytosis, indicating that fusion between the viral envelope and cell membranes triggers the apoptotic cascade. Treatment with the pan-caspase inhibitor Z-VAD-fmk blocked MHV-induced apoptosis, suggesting an involvement of the caspase-dependent pathway. Our results, thus, for the first time provide unequivocal evidence that infection of oligodendrocytes with MHV directly results in apoptosis. This finding provides an explanation for the destruction of oligodendrocytes and the damage of myelin sheath in MHV-infected CNS and suggests that oligodendrocyte apoptosis may be one of the underlying mechanisms for the pathogenesis of MHV-induced demyelinating diseases in animals.

Down-regulation of transcription of the proapoptotic gene BNip3 in cultured astrocytes by murine coronavirus infection

Murine coronavirus mouse hepatitis virus (MHV) causes encephalitis and demyelination in the central nervous system of susceptible rodents. Astrocytes are the major target for MHV persistence. However, the mechanisms by which astrocytes survive MHV infection and permit viral persistence are not known. Here we performed DNA microarray analysis on differential gene expression in astrocyte DBT cells by MHV infection and found that the mRNA of the proapoptotic gene BNip3 was significantly decreased following MHV infection. This finding was further confirmed by quantitative reverse transcription-polymerase chain reaction, Western blot analysis, and BNip3-promoter-luciferase reporter system. Interestingly, infection with live and ultraviolet light-inactivated viruses equally repressed BNip3 expression, indicating that the down-regulation of BNip3 expression does not require virus replication and is mediated during cell entry. Furthermore, treatment of cells with chloroquine, which blocks the acidification of endosomes, significantly inhibited the repression of the BNip3 promoter activity induced by the acidic pH-dependent MHV mutant OBLV60, which enters cells via endocytosis, indicating that the down-regulation of BNip3 expression is mediated by fusion between viral envelope and cell membranes during entry. Deletion analysis showed that the sequence between nucleotides 262 and 550 of the 588-base-pair BNip3 promoter is necessary and sufficient for driving the BNip3 expression and that it contains signals that are responsible for MHV-induced down-regulation of BNip3 expression in DBT cells. These results may provide insights into the mechanisms by which MHV evades host antiviral defense and promotes cell survival, thereby allowing its persistence in the host astrocytes.

Induction of caspase-dependent apoptosis in cultured rat oligodendrocytes by murine coronavirus is mediated during cell entry and does not require virus replication

Murine coronavirus mouse hepatitis virus (MHV) causes demyelination of the central nervous system (CNS) in rats and mice. Apoptotic oligodendrocytes have been detected in the vicinity of the CNS demyelinating lesions in these animals. However, whether MHV can directly induce oligodendrocyte apoptosis has not been documented. Here, we established a rat oligodendrocyte culture that is morphologically and phenotypically indistinguishable from the primary rat oligodendrocytes. Using this culture, we showed that mature rat oligodendrocytes were permissive to MHV infection but did not support productive virus replication. Significantly, oligodendrocytes infected with both live and ultraviolet light-inactivated viruses underwent apoptosis to a similar extent, which was readily detectable at 24 h postinfection as revealed by apoptotic bodies and DNA fragmentation, indicating that MHV-induced apoptosis is mediated during the early stages of the virus life cycle and does not require virus replication. Prior treatment of cells with the lysosomotropic agents NH(4)Cl and chloroquine as well as the vacuolar proton pump-ATPase inhibitor bafilomycin A1, all of which block the acidification of the endosome, prevented oligodendrocytes from succumbing to apoptosis induced by MHV mutant OBLV60, which enters cells via endocytosis, indicating that fusion between the viral envelope and cell membranes triggers the apoptotic cascade. Treatment with the pan-caspase inhibitor Z-VAD-fmk blocked MHV-induced apoptosis, suggesting an involvement of the caspase-dependent pathway. Our results, thus, for the first time provide unequivocal evidence that infection of oligodendrocytes with MHV directly results in apoptosis. This finding provides an explanation for the destruction of oligodendrocytes and the damage of myelin sheath in MHV-infected CNS and suggests that oligodendrocyte apoptosis may be one of the underlying mechanisms for the pathogenesis of MHV-induced demyelinating diseases in animals.

The spike but not the hemagglutinin/esterase protein of bovine coronavirus is necessary and sufficient for viral infection

The spike (S) and hemagglutinin/esterase (HE) of bovine coronavirus (BCV) are the two envelope proteins that recognize the same receptor-determinant of 9-O-acetylneuraminic acid on host cells. However, the precise and relative roles of the two proteins in BCV infectivity remain elusive. To unequivocally determine their roles in viral cytopathogenicity, we developed a system in which phenotypically chimeric viruses were generated by infecting a closely related mouse hepatitis virus (MHV) in cells that stably express an individual BCV protein (S or HE). The chimeric viruses were then used to infect human rectal tumor (HRT)-18 cells that are permissive to BCV but are nonsusceptible to MHV. Using this approach, we found that the chimeric virus containing the BCV S protein on the virion surface entered and replicated in HRT-18 cells; this was specifically blocked by prior treatment of the virus with a neutralizing antibody specific to the BCV S protein, indicating that the BCV S protein is responsible for initiating chimeric virus infection. In contrast, chimeric viruses that contain biologically active and functional BCV HE protein on the surface failed to enter HRT-18 cells, indicating that the BCV HE protein alone is not sufficient for BCV infection. Taken together, these results demonstrate that the S protein but not the HE protein of BCV is necessary and sufficient for infection of the chimeric viruses in HRT-18 cells, suggesting that BCV likely uses the S protein as a primary vehicle to infect permissive cells.

Membrane topology of coronavirus E protein

Coronavirus small envelope protein E has two known biological functions: it plays a pivotal role in virus envelope formation, and the murine coronavirus E protein induces apoptosis in E protein-expressing cultured cells. The E protein is an integral membrane protein. Its C-terminal region extends cytoplasmically in the infected cell and in the virion toward the interior. The N-terminal two-thirds of the E protein is hydrophobic and lies buried within the membrane, but its orientation in the lipid membrane is not known. Immunofluorescent analyses of cells expressing biologically active murine coronavirus E protein with a hydrophilic short epitope tag at the N-terminus showed that the epitope tag was exposed cytoplasmically. Immunoprecipitation analyses of the purified microsomal membrane vesicles that contain the same tagged E protein revealed the N-terminal epitope tag outside the microsomal membrane vesicles. These analyses demonstrated that the epitope tag at the N-terminus of the E protein was exposed cytoplasmically. Our data were consistent with an E protein topology model, in which the N-terminal two-thirds of the transmembrane domain spans the lipid bilayer twice, exposing the C-terminal region to the cytoplasm or virion interior.

The hemagglutinin-esterase of mouse hepatitis virus strain S is a sialate-4-O-acetylesterase

By comparative analysis of the hemagglutinin-esterase (HE) protein of mouse hepatitis virus strain S (MHV-S) and the HE protein of influenza C virus, we found major differences in substrate specificities. In striking contrast to the influenza C virus enzyme, the MHV-S esterase was unable to release acetate from bovine submandibulary gland mucin. Furthermore, MHV-S could not remove influenza C virus receptors from erythrocytes. Analysis with free sialic acid derivatives revealed that the MHV-S HE protein specifically de-O-acetylates 5-N-acetyl-4-O-acetyl sialic acid (Neu4, 5Ac2) but not 5-N-acetyl-9-O-acetyl sialic acid (Neu5,9Ac2), which is the major substrate for esterases of influenza C virus and bovine coronaviruses. In addition, the MHV-S esterase converted glycosidically bound Neu4,5Ac2 of guinea pig serum glycoproteins to Neu5Ac. By expression of the MHV esterase with recombinant vaccinia virus and incubation with guinea pig serum, we demonstrated that the viral HE possesses sialate-4-O-acetylesterase activity. In addition to observed enzymatic activity, MHV-S exhibited affinity to guinea pig and horse serum glycoproteins. Binding required sialate-4-O-acetyl groups and was abolished by chemical de-O-acetylation. Since Neu4,5Ac2 has not been identified in mice, the nature of potential substrates and/or secondary receptors for MHV-S in the natural host remains to be determined. The esterase of MHV-S is the first example of a viral enzyme with high specificity and affinity toward 4-O-acetylated sialic acids.

Identification of a coronavirus hemagglutinin-esterase with a substrate specificity different from those of influenza C virus and bovine coronavirus

We have characterized the hemagglutinin-esterase (HE) of puffinosis virus (PV), a coronavirus closely related to mouse hepatitis virus (MHV). Analysis of the cloned gene revealed approximately 85% sequence identity to HE proteins of MHV and approximately 60% identity to the corresponding esterase of bovine coronavirus. The HE protein exhibited acetylesterase activity with synthetic substrates p-nitrophenyl acetate, alpha-naphthyl acetate, and 4-methylumbelliferyl acetate. In contrast to other viral esterases, no activity was detectable with natural substrates containing 9-O-acetylated sialic acids. Furthermore, PV esterase was unable to remove influenza C virus receptors from human erythrocytes, indicating a substrate specificity different from HEs of influenza C virus and bovine coronavirus. Solid-phase binding assays revealed that purified PV was unable to bind to sialic acid-containing glycoconjugates like bovine submaxillary mucin, mouse alpha1 macroglobulin or bovine brain extract. Because of the close relationship to MHV, possible implications on the substrate specificity of MHV esterases are suggested.

Using a defective-interfering RNA system to express the HE protein of mouse hepatitis virus for studying viral pathogenesis

We have developed a defective-interfering (DI) RNA of mouse hepatitis virus (MHV) as a vector for expressing a variety of cellular and viral genes including the chloramphenicol acetyltransferase (CAT), hemagglutinin' esterase (HE), and gamma interferon. Here, we used the HE-expressing DI RNA for examining the role of HE protein in viral pathogenesis. The pseudorecombinant virus containing an expressed HE protein was generated by infecting cells with MHV-A59, which does not express, HE, and transfecting the in vitro-transcribed DI RNA containing the HE gene. These pseudorecombinant viruses (DE-HE A59) were then inoculated intracerebrally into mice. Viruses recovered from cells infected with A59 and transfected with DI RNA expressing the CAT gene (DE-CAT A59) were used as a control. At various time points after inoculation, mice were observed for clinical symptoms. Tissues (brains and livers) were obtained for determining the replication of DI RNA by RT-PCR, virus replication by plaque assay, antigen expression by immunohistochemistry, and pathological changes. Results showed that all mice infected with DE-CAT A59 succumbed to infection by 9 days postinfection (d p.i). These data are identical to the pathogenesis of the parental A59 virus, demonstrating that inclusion of the DI RNA did not by itself alter pathogenesis. In contrast, only 40% of mice infected with DE-HE A59 succumbed to infection. The subgenomic mRNAs transcribed from the DI vector were detected at 1 and 2 d p.i. but not at subsequent time points, indicating that the genes in the DI vector were expressed only at an early stage of viral infection. No significant difference in virus replication in the brains was detected between these two groups of mice, suggesting that virus replication in brains was not affected by the expression of the HE. Histopathological examination showed only a small increase in the extent of inflammatory cell infiltration and reduced viral antigen in the mice infected with DE-HE A59. There was no difference in virus replication in the livers at 2 and 4 d p.i., but a 3 log10 reduction was detected in the livers of mice infected with DE-HE A59 at 6 d p.i. Histological examination showed a significant reduction in viral antigen, inflammation and necrosis in mice infected with DE-HE A59. These results indicate that the expression of HE from the DI vector altered the viral pathogenesis. This study thus demonstrates the usefulness of this system in studying the role of viral or cellular genes expressed locally at the sites of viral infection in viral pathogenesis.

Expression of hemagglutinin/esterase by a mouse hepatitis virus coronavirus defective-interfering RNA alters viral pathogenesis

A defective-interfering (DI) RNA of mouse hepatitis virus (MHV) was developed as a vector for expressing MHV hemagglutinin/esterase (HE) protein. The virus containing an expressed HE protein (A59-DE-HE) was generated by infecting cells with MHV-A59, which does not express HE, and transfecting the in vitro-transcribed DI RNA containing the HE gene. A similar virus (A59-DE-CAT) expressing the chloramphenicol acetyltransferase (CAT) was used as a control. These viruses were inoculated intracerebrally into mice, and the role of the HE protein in viral pathogenesis was evaluated. Results showed that all mice infected with parental A59 or A59-DE-CAT succumbed to infection by 9 days postinfection (p.i.), demonstrating that inclusion of the DI did not by itself alter pathogenesis. In contrast, 60% of mice infected with A59-DE-HE survived infection. HE- or CAT-specific subgenomic mRNAs were detected in the brains at days 1 and 2 p.i. but not later, indicating that the genes in the DI vector were expressed only in the early stage of viral infection. No significant difference in virus titer or viral antigen expression in brains was observed between A59-DE-HE- and A59-DE-CAT-infected mice, suggesting that virus replication in brain was not affected by the expression of HE. However, at day 3 p.i. there was a slight increase in the extent of inflammatory cell infiltration in the brains of the A59-DE-HE-infected mice. Surprisingly, virus titers in the livers of A59-DE-HE-infected mice were 3 log10 lower than that of the A59-DE-CAT-infected mice at day 6 p.i. Also, substantially less necrosis and viral antigen were detected in the livers of the A59-DE-HE-infected mice. This may account for the reduced mortality of these mice. The possible contribution of the host immune system to this difference in pathogenesis was analyzed by comparing the expression of four cytokines. Results showed that both tumor necrosis factor-alpha and interleukin-6 mRNAs increased in the brains of the A59-DE-HE-infected mice at day 2 p.i., whereas interferon-gamma and interleukin-1 alpha mRNAs were similar between A59-DE-HE- and A59-DE-CAT-infected mice. These data suggest that the transient expression of HE protein enhances an early innate immune response, possibly contributing to the eventual clearance of virus from the liver. This study indicates the feasibility of the DI expression system for studying roles of viral proteins during MHV infection.

The molecular biology of coronaviruses

This chapter discusses the manipulation of clones of coronavirus and of complementary DNAs (cDNAs) of defective-interfering (DI) RNAs to study coronavirus RNA replication, transcription, recombination, processing and transport of proteins, virion assembly, identification of cell receptors for coronaviruses, and processing of the polymerase. The nature of the coronavirus genome is nonsegmented, single-stranded, and positive-sense RNA. Its size ranges from 27 to 32 kb, which is significantly larger when compared with other RNA viruses. The gene encoding the large surface glycoprotein is up to 4.4 kb, encoding an imposing trimeric, highly glycosylated protein. This soars some 20 nm above the virion envelope, giving the virus the appearance-with a little imagination-of a crown or coronet. Coronavirus research has contributed to the understanding of many aspects of molecular biology in general, such as the mechanism of RNA synthesis, translational control, and protein transport and processing. It remains a treasure capable of generating unexpected insights.

The molecular biology of coronaviruses

This chapter discusses the manipulation of clones of coronavirus and of complementary DNAs (cDNAs) of defective-interfering (DI) RNAs to study coronavirus RNA replication, transcription, recombination, processing and transport of proteins, virion assembly, identification of cell receptors for coronaviruses, and processing of the polymerase. The nature of the coronavirus genome is nonsegmented, single-stranded, and positive-sense RNA. Its size ranges from 27 to 32 kb, which is significantly larger when compared with other RNA viruses. The gene encoding the large surface glycoprotein is up to 4.4 kb, encoding an imposing trimeric, highly glycosylated protein. This soars some 20 nm above the virion envelope, giving the virus the appearance-with a little imagination-of a crown or coronet. Coronavirus research has contributed to the understanding of many aspects of molecular biology in general, such as the mechanism of RNA synthesis, translational control, and protein transport and processing. It remains a treasure capable of generating unexpected insights.

JHM virus-specific cytotoxic T cells derived from the central nervous system

Spleen cells cultured from Balb/c mice immunized with the JHM strain of mouse hepatitis virus (JHMV) have CD8+ cytotoxic T cells (CTL) specific for both the S and N proteins, but not the M or HE proteins. T cell lines were established from the brains of Balb/c mice infected with JHMV. The majority of the lines (20 of 22) were specific for JHMV. Analysis of the viral structural proteins which served as target structures indicate that most (15 of 20) were specific for the N protein. One line was specific for the S protein and four lines were specific for JHMV but the protein recognized could not be determined. These data suggest that early during infection there is a preferential recruitment of N protein specific CTL into the CNS of infected mice.

Deletion mapping of a mouse hepatitis virus defective interfering RNA reveals the requirement of an internal and discontiguous sequence for replication

All of the defective interfering (DI) RNAs of mouse hepatitis virus (MHV) contain both the 5' and 3' ends of the viral genomic RNA, which presumably include the cis sequences required for RNA replication. To define the replication signal of MHV RNA, we have used a vaccinia virus-T7 polymerase-transcribed MHV DI RNA to study the effects of sequence deletion on DI RNA replication. Following infection of susceptible cells with a recombinant vaccinia virus expressing T7 RNA polymerase, various cDNA clones derived from a DI RNA (DIssF) of the JHM strain of MHV, which is a 3.5-kb naturally occurring DI RNA, behind a T7 promoter were transfected. On superinfection with a helper MHV, the ability of various DI RNAs to replicate was determined. Serial deletions from the middle of the RNA toward both the 5' and 3' ends demonstrated that 859 nucleotides from the 5' end and 436 nucleotides from the 3' end of the MHV RNA genome were necessary for RNA replication. Surprisingly, an additional stretch of 135 nucleotides located at 3.1 to 3.3 kb from the 5' end of the genome was also required. This stretch is discontiguous from the 5'-end cis replication signal and is present in all of the naturally occurring DI RNAs studied so far. The requirement for a long stretch of 5'- and 3'-end sequences predicts that the subgenomic MHV mRNAs cannot replicate. The efficiency of RNA replication varied with different cDNA constructs, suggesting possible interaction between different regions of DI RNA. The identification of MHV RNA replication signals allowed the construction of an MHV DI-based expression vector, which can express foreign genes, such as the chloramphenicol acetyltransferase gene.

The receptor for mouse hepatitis virus in the resistant mouse strain SJL is functional: implications for the requirement of a second factor for viral infection

The SJL mouse strain is resistant to infection by some strains of the murine coronavirus mouse hepatitis virus (MHV), such as JHM and A59. The block to virus infection has been variously attributed to defects in virus receptors or virus spread. Since the cellular receptors for MHV, mmCGM1 and mmCGM2, have recently been identified as members of the carcinoembryonic antigen family, we reexamined the possible defectiveness of the MHV receptors in SJL mouse strain. Cloning and sequencing of the cDNAs of both mmCGMs RNAs from SJL mice revealed that they were identical in size to those of the susceptible C57BL/6 (B6) mouse. There was some sequence divergence in the N terminus of the mmCGM molecules between the two mouse strains, resulting in a different number of potential glycosylation sites. This was confirmed by in vitro translation of the mmCGM RNAs, which showed that the glycosylated mmCGM2 of SJL was smaller than that of B6 mice. However, transfection of either mmCGM1 or mmCGM2 from SJL mice into MHV-resistant Cos 7 cells rendered the cells susceptible to MHV infection. The ability of the SJL mmCGM molecules to serve as MHV receptors was comparable to that of those from B6. These molecules are expressed in SJL mouse brain and liver in a similar ratio and in amounts equivalent to those in the B6 mouse. Furthermore, we demonstrated that an SJL-derived cell line was susceptible to A59 but resistant to JHM infection. We concluded that the MHV receptor molecules in the SJL mouse are functional and that the resistance of SJL mice to infection by some MHV strains most likely results from some other factor(s) required for virus entry or some other step(s) in virus replication.

Cell receptor-independent infection by a neurotropic murine coronavirus

The cellular receptors for a coronavirus, mouse hepatitis virus (MHV), have been recently identified as one or more members of the carcinoembryonic antigen (CEA) family. The neurotropic JHM strain of MHV (MHV-JHM) possesses a highly fusogenic surface (S) glycoprotein. This protein is now shown to promote the spread of MHV into cells lacking the specific CEA-related MHV receptor. Resistant cells are recruited into MHV-induced syncytium with consequent production of progeny virus. Cell-to-cell spread of virus via membrane fusion without the requirement for specific cell surface receptor offers a novel way for virus to spread within infected hosts.

Evidence for variable rates of recombination in the MHV genome

Mouse hepatitis virus has been shown to undergo RNA recombination at high frequency during mixed infection. Temperature-sensitive mutants were isolated using 5-fluorouracil and 5-azacytidine as mutagen. Six RNA+ mutants that reside within a single complementation group mapping within the S glycoprotein gene of MHV-A59 were isolated which did not cause syncytium at the restrictive temperature. Using standard genetic techniques, a recombination map was established that indicated that these mutants mapped into two distinct domains designated F1 and F2. These genetic domains may correspond to mutations mapping within the S1 and S2 glycoproteins, respectively, and suggest that both the S1 and S2 domains are important in eliciting the fusogenic activity of the S glycoprotein gene. In addition, assuming that most distal ts alleles map roughly 4.0 kb apart, a recombination frequency of 1% per 575-676 bp was predicted through the S glycoprotein gene. Interestingly, this represents a threefold increase in the recombination frequency as compared to rates predicted through the polymerase region. The increase in the recombination rate was probably not due to recombination events resulting in large deletions or insertions (greater than 50 bp), but rather was probably due to a combination of homologous and nonhomologous recombination. A variety of explanations could account for the increased rates of recombination in the S gene.

Hemagglutinin-esterase-specific monoclonal antibodies alter the neuropathogenicity of mouse hepatitis virus

Some of mouse hepatitis virus strains contain an optional envelope glycoprotein, hemagglutinin-esterase (HE) protein. To understand the functional significance of this protein, monoclonal antibodies (MAbs) specific for this protein were generated and used for passive immunization of mice. None of these MAbs showed any virus-neutralizing activity in vitro; however, mice passively immunized with the purified MAbs were protected from lethal infection by the JHM strain of mouse hepatitis virus. Passive immunization altered the pathogenicity such that the virus caused subacute and chronic demyelination instead of acute lethal encephalitis. Virus titers in the brains of the immunized mice were significantly lower than those for the nonimmunized control mice, suggesting that the virus replication or spread was inhibited. In addition, histopathological analysis indicated that the spread of virus in the brain and spinal cord was significantly inhibited in the immunized mice. Furthermore, the mononuclear cell infiltration in the immunized mice appeared earlier than in the nonimmunized mice, suggesting that the exogenous antibody might have activated host immune responses, and thus facilitated clearance of the virus or virus-infected cells. The same protective effects were observed for both JHM(2) and JHM(3) viruses, which expressed different amounts of the HE protein. In contrast, mice infected with At11f, a variant of JHM which does not express the HE protein, were not protected by these MAbs, suggesting that protection was mediated by the specific interaction between the MAb and the HE protein. Thus, the mechanism of protection by the exogenous HE-specific MAbs may represent the early activation of innate immune mechanisms in response to the interaction between the MAbs and the HE protein.

RNA recombination in animal and plant viruses

An increasing number of animal and plant viruses have been shown to undergo RNA-RNA recombination, which is defined as the exchange of genetic information between nonsegmented RNAs. Only some of these viruses have been shown to undergo recombination in experimental infection of tissue culture, animals, and plants. However, a survey of viral RNA structure and sequences suggests that many RNA viruses were derived form homologous or nonhomologous recombination between viruses or between viruses and cellular genes during natural viral evolution. The high frequency and widespread nature of RNA recombination indicate that this phenomenon plays a more significant role in the biology of RNA viruses than was previously recognized. Three types of RNA recombination are defined: homologous recombination; aberrant homologous recombination, which results in sequence duplication, insertion, or deletion during recombination; and nonhomologous (illegitimate) recombination, which does not involve sequence homology. RNA recombination has been shown to occur by a copy choice mechanism in some viruses. A model for this recombination mechanism is presented.

The hemagglutinin/esterase gene of human coronavirus strain OC43: phylogenetic relationships to bovine and murine coronaviruses and influenza C virus

The complete nucleotide sequences of the hemagglutinin/esterase (HE) genes of human coronavirus (HCV) strain OC43 and bovine respiratory coronavirus (BRCV) strain G95 were determined from single-stranded cDNA fragments generated by reverse transcription of virus-specific mRNAs and amplified by polymerase chain reaction. An open reading frame of 1272 nucleotides was identified as the putative HE gene by homology to the bovine coronavirus HE gene. This open reading frame encodes a protein of 424 amino acids with an estimated molecular weight of 47.7 kDa. Ten potential N-linked glycosylation sites were predicted in the HE protein of HCV-OC43 while nine of them were present in BRCV-G95. Fourteen cysteine residues were conserved in the HE proteins of both viruses. Two hydrophobic sequences at the N-terminus and the C-terminus may serve as signal peptide and transmembrane anchoring domain, respectively. The predicted HE protein of HCV-OC43 was 95% identical to the HEs of BRCV-G95 and other bovine coronaviruses, and 60% identical to the HEs of mouse hepatitis viruses. Phylogenetic analysis suggests that the HE genes of coronaviruses and influenza C virus have a common ancestral origin, and that bovine coronaviruses and HCV-OC43 are closely related.

Comparison of hemagglutinating, receptor-destroying, and acetylesterase activities of avirulent and virulent bovine coronavirus strains

Hemagglutinating and acetylesterase functions as well as the 124 kDa glycoprotein were present in the highly cell-culture adapted, avirulent bovine coronavirus strain BCV-L9, in the Norden vaccine strain derived from it, and in 5 wild-type, virulent strains that multiplied in HRT-18 cells but were restricted in several types of cultured bovine cells. The BCV-L9 and the wild-type strain BCV-LY-138 agglutinated chicken and mouse erythrocytes. The acetylesterase facilitated break-down of the BCV-erythrocyte complex with chicken but only to a minimal extent with mouse erythrocytes in the receptor-destroying enzyme test. Purified preparations of the vaccine and the wild-type strains agglutinated chicken erythrocytes at low titers and mouse erythrocytes at 128 to 256 times higher titers whereas receptor destroying enzyme activity was detectable only with chicken erythrocytes. When wild-type strains were propagated in HRT cells at low passage levels, they produced 5 x 10(5) to 4.5 x 10(6) plaque forming units per 50 microliters which agglutinated erythrocytes from mice but not from chickens. Diisopropylfluoro-phosphate moderately increased the hemagglutination titers, but completely inhibited the receptor destroying enzyme of purified virus of all strains. It had virtually no influence on the plaque-forming infectivity of the different BCV strains. The acetylesterase of strain BCV-L9 reacting in the receptor-destroying enzyme test was stable for 3 h at 37 and 42 degrees C. It was inactivated within 30 min at 56 degrees C while the hemagglutinin function of this strain was stable for 3 h at 37, 42, and 56 degrees C, but it was inactivated at 65 degrees C within 1 h.

The hemagglutinin/esterase glycoprotein of bovine coronaviruses: sequence and functional comparisons between virulent and avirulent strains

The entire nucleotide sequences of the hemagglutinin/esterase (HE) genes specified by the highly virulent strain LY138 and the avirulent strain L9 of bovine coronavirus (BCV) were determined. These sequences were compared with recently published sequences of the HE genes of the Quebec and Mebus strains. A large open reading frame of 1272 nt encoding a protein of 424 amino acid residues was predicted. The putative esterase active site was conserved in the virulent and avirulent BCV strains, indicating that this domain is probably not a determinant for BCV virulence. Four amino acid substitutions occurred between the HE proteins of BCV-L9 and BCV-LY138 (leu to Pro at 5, Leu to Val at 103, Ser to Pro at 367, and Thr to Asn at 379). Monoclonal antibodies specific for the HE glycoprotein inhibited the hemagglutination and acetylesterase activities of BCV-L9, but showed no inhibitory effect on the acetylesterase activity of BCV-LY138. These results suggest that at least one epitope is located proximal to one of the three strain-specific amino acids. Four S-specific monoclonal antibodies inhibited hemagglutination but not acetylesterase activity of BCV-L9, implying that the S glycoprotein can promote hemagglutination of chicken erythrocytes in addition to the HE glycoprotein.

A system for study of coronavirus mRNA synthesis: a regulated, expressed subgenomic defective interfering RNA results from intergenic site insertion

A system that exploits defective interfering (DI) RNAs of mouse hepatitis virus (MHV) for deciphering the mechanisms of coronavirus mRNA transcription was developed. A complete cDNA clone of MHV DI RNA containing an inserted intergenic region, derived from the area of genomic RNA between genes 6 and 7, was constructed. After transfection of the in vitro-synthesized DI RNA into MHV-infected cells, replication of genomic DI RNA as well as transcription of the subgenomic DI RNA was observed. S1 nuclease protection experiments, sequence analysis, and Northern (RNA) blotting analysis revealed that the subgenomic DI RNA contained the leader sequence at its 5' end and that the body of the subgenomic DI RNA started from the inserted intergenic sequence. Two subgenomic DI RNAs were synthesized after inserting two intergenic sites into the MHV DI RNA. Metabolic labeling of virus-specific protein in DI RNA replicating cells demonstrated that a protein was translated from the subgenomic DI RNA, which can therefore be considered a functional mRNA. Transfection study of gel-purified genomic DI RNA and subgenomic DI RNA revealed that the introduction of the genomic DI RNA, but not subgenomic DI RNA, into MHV-infected cells was required for synthesis of the subgenomic DI RNA. A series of deletion mutations in the intergenic site demonstrated that the sequence flanking the consensus sequence of UCUAAAC affected the efficiency of subgenomic DI RNA transcription and that the consensus sequence was necessary but not sufficient for the synthesis of the subgenomic DI RNA.

Mouse hepatitis virus S RNA sequence reveals that nonstructural proteins ns4 and ns5a are not essential for murine coronavirus replication

Genes 4 and 5 of mouse hepatitis virus (MHV) are known to encode nonstructural proteins ns4, ns5a, and ns5b, whose function is unknown. In this study, we demonstrated that one of the MHV strains, MHV-S, did not synthesize mRNA 4 and made a smaller mRNA 5. Sequence analysis showed that the transcription initiation site for gene 4 of MHV-S was mutated from the consensus UCUAAAC to UUUAAAC, consistent with the idea that mutations in this region abolish mRNA synthesis. Furthermore, within gene 5 there were deletions totaling 307 nucleotides which deleted almost all of open reading frame 5a, but preserved open reading frame 5b of gene 5. Comparison of the growth of MHV-S with other MHV strains in DBT cells revealed no significant growth defect in MHV-S. These results suggest that ns4 and ns5a are not essential for viral replication in tissue culture cells, and thus join gene 2 and the hemagglutinin-esterase (HE) gene as nonessential viral genes in MHV.

Heterogeneity of gene expression of the hemagglutinin-esterase (HE) protein of murine coronaviruses

The hemagglutinin-esterase (HE) membrane glycoprotein is present only in some members of the coronavirus family, including some strains of mouse hepatitis virus (MHV). In the JHM strain of MHV, expression of the HE gene is variable and corresponds to the number of copies of a UCUAA pentanucleotide sequence present at the 3'-end of the leader RNA. This copy number varies among MHV strains, depending on their passage history. The JHM isolates with two copies of UCUAA in their leader RNA showed a high level of HE expression, whereas the JHM isolate with three copies had a low-level expression. In this study, the analysis of HE gene expression was extended to other MHV strains. The synthesis of HE mRNA in these viruses also correlates with the copy number of UCUAA in the leader RNA and the particular intergenic sequence preceding the HE gene. In one MHV strain, MHV-1, no detectable HE mRNA was synthesized, despite the presence of a proper transcription initiation signal. This lack of HE mRNA expression was consistent with a leader RNA containing three UCUAA copies. However, mutations and deletions within the coding region of the MHV-1 HE gene have generated a stretch of sequence which resembled the transcriptional initiation motif, and was shown to initiate the synthesis of a novel smaller mRNA. These findings strengthened the theory that interactions between leader RNA and transcriptional initiation sequences regulate MHV subgenomic mRNA transcription. Sequence analysis revealed that most MHV strains, through extensive mutations, deletions, or insertions, have lost the complete HE open reading frame, thus turning HE into a pseudogene. This high degree of variation is unusual as the other three structural proteins (spike, membrane, and nucleocapsid) are well-maintained. In contrast to bovine coronavirus, which apparently requires HE for viral replication, the HE protein in MHV may be only an accessory protein which is not necessary for viral replication. JHM and MHV-S, however, have preserved the expression of HE protein.

Localization of extensive deletions in the structural genes of two neurotropic variants of murine coronavirus JHM

The intracellular RNA of two neurotropic variants of the JHM strain of mouse hepatitis virus (MHV) independently isolated from the brain and spinal cord of an infected Wistar Furth rat were compared with that of the parental virus. The mRNAs corresponding to the genes encoding the peplomer (S) and the hemagglutinin-esterase (HE) proteins of the variant viruses were found to be smaller in size. The possible sequence changes were studied by oligonucleotide fingerprinting and direct RNA sequencing. Both variants have a large deletion of 246 amino acids in the carboxy-terminal end of the HE protein. However, this truncated protein was not detected in the infected cells, suggesting either a translational regulation or rapid degradation of the truncated protein in these cells. The variant virus isolated from the spinal cord has a second deletion of 147 amino acids in the amino-terminal half of the S protein. This deletion site corresponds to a hypervariable region where deletions have been frequently noted among MHV variants with different biological properties. These findings suggest that the changes in pathogenic properties of the two neural isolates are associated with drastic alterations of the viral structural glycoproteins.

Structure and function of the HEF glycoprotein of influenza C virus

Soon after the first isolation of an influenza C virus from a patient, it became obvious that this virus differs from other myxoviruses in several aspects. Pronounced differences have been observed in the interactions between the virus and cell surfaces, suggesting that influenza C virus attaches to the receptors different from those recognized by other myxoviruses. While influenza A and B viruses agglutinate erythrocytes from many species, including humans, the spectrum of erythrocytes agglutinated by influenza C virus is much more restricted. Erythrocytes from rats, mice, and adult chickens are suitable for hemagglutination and hemadsorption tests; cells from other species, however, react not at all or only poorly with influenza C virus. Differences are also observed so far as hemagglutination inhibitors are concerned. A variety of glycoproteins have been shown to prevent influenza A and B viruses from agglutinating erythrocytes. In the case of influenza C virus, rat serum was for a long time the only known hemagglutination inhibitor. A difference in the receptors for influenza C virus and other myxo-viruses was also suggested by studies on the receptor-destroying enzyme. The ability of influenza C virus to inactivate its own receptors was reported soon after the first isolation of this virus from a patient. However, the influenza C enzyme did not affect the receptors of other myxoviruses and, conversely, the receptor-destroying enzyme of either of the latter viruses was unable to inactivate the receptors for influenza C virus on erythrocytes. While the enzyme of influenza A and B virus was characterized as a neuraminidase in the 1950s, even with refined methodology no such activity was detectable with influenza C virus. It is now known that both the receptor-binding and receptor-destroying activities, as well as the fusion activity of influenza C virus are mediated by the only glycoprotein present on the surface of the virus particle. The structure and functions of this protein, which is designated as HEF, are reviewed in this chapter.

Isolated HE-protein from hemagglutinating encephalomyelitis virus and bovine coronavirus has receptor-destroying and receptor-binding activity

Bovine coronavirus (BCV) and hemagglutinating encephalomyelitis virus (HEV) from swine were found to grow to high titers in MDCK I cells, a subline of Madin Darby canine kidney cells. Virus grown in these cells was used to isolate and purify the HE-protein. This protein has been shown recently to have acetylesterase activity and to function as the receptor-destroying enzyme of BCV. Here we show that HEV contains this enzyme, too. The glycoproteins were solubilized by treatment of virions with octylglucoside. Following centrifugation through a sucrose gradient the surface proteins S and HE (hemagglutinin-esterase) were obtained in purified form. After removal of the detergent by dialysis, HE formed rosettes as shown by electron microscopy. The purified HE protein retained acetylesterase activity and was able to function as a receptor-destroying enzyme rendering red blood cells resistant against agglutination by both coronaviruses. HE protein released from the viral membrane failed to agglutinate red blood cells. However, it was found to recognize glycoconjugates containing N-acetyl-9-O-acetylneuraminic acid as indicated by a binding assay with rat serum proteins blotted to nitrocellulose and by its ability to inhibit the hemagglutinating activity of BCV, HEV, and influenza C virus. The purified enzyme provides a useful tool for analyzing the cellular receptors for coronaviruses.

Analysis of efficiently packaged defective interfering RNAs of murine coronavirus: localization of a possible RNA-packaging signal

We have previously shown that most of the defective interfering (DI) RNA of mouse hepatitis virus (MHV) are not packaged into virions. We have now identified, after 21 serial undiluted passages of MHV, a small DI RNA, DIssF, which is efficiently packaged into virions. The DIssF RNA replicated at a high efficiency on its transfection into the helper virus-infected cells. The virus released from the transfected cells interfered strongly with mRNA synthesis and growth of helper virus. cDNA cloning and sequence analysis of DIssF RNA revealed that it is 3.6 kb and consists of sequences derived from five discontinuous regions of the genome of the nondefective virus. The first four regions (domains I to IV) from the 5' end are derived from gene 1, which presumably encodes the RNA polymerase of the nondefective virus. The entire domain I (859 nucleotides) and the first 750 nucleotides of domain II are also present in a previously characterized DI RNA, DIssE, which is not efficiently packaged into virions. Furthermore, the junction between these two domains is identical between the two DI RNAs. The remaining 77 nucleotides at the 3' end of domain II and all of domains III (655 nucleotides) and IV (770 nucleotides) are not present in DIssE RNA. These four domains are derived from gene 1. In contrast, the 3'-most domain (domain V, 447 nucleotides) is derived from the 3' end of the genomic RNA and is also present in DIssE. The comparison of primary sequences and packaging properties between DIsse and DIssF RNAs suggested that domains III and IV and part of the 3' end of domain II contain the packaging signal for MHV RNA. This conclusion was confirmed by inserting these DIssF-unique sequences into a DIssE cDNA construct; the in vitro-transcribed RNA from this hybrid construct was efficiently packaged into virion particles. DIssF RNA also contains an open reading frame, which begins from domain I and ends at the 5'-end 20 bases of domain III. In vitro translation of DIssF RNA and metabolic labeling of the virus-infected cells showed that this open reading frame is indeed translated into a 75-kDa protein. The structures of both DIssE and DIssF RNAs suggest that a protein-encoding capability is a common characteristic of MHV DI RNA.

Genetics of mouse hepatitis virus transcription: identification of cistrons which may function in positive and negative strand RNA synthesis

A panel of 26 temperature-sensitive mutants of MHV-A59 were selected by mutagenesis with either 5-fluorouracil or 5-azacytidine. Complementation analysis revealed the presence of one RNA+ and five RNA- complementation groups. None of the RNA- complementation groups transcribed detectable levels of positive- or negative-stranded RNA at the restrictive temperature. Temperature shift experiments after the onset of mRNA synthesis revealed at least two classes of RNA- mutants. RNA- complementation groups A, B, D, and E were blocked in the ability to release infectious virus and transcribe mRNA and genome, while group C mutants continued to release infectious virus and transcribe both mRNA and genome. Temperature shift experiments at different times postinfection suggest that the group C mutants encode a function required early in viral transcription which affects the overall rate of positive strand synthesis. Analysis of steady state levels of negative strand RNA after the shift indicate that the group C mutants were probably blocked in the ability to synthesize additional minus strand RNA under conditions in which the group E mutants continued low levels of minus strand synthesis. These data suggest that at least four cistrons may be required for positive strand synthesis while the group C cistron functions during minus strand synthesis.