Mouse hepatitis virus strain A59 and blocking antireceptor monoclonal antibody bind to the N-terminal domain of cellular receptor

Kathryn V. Holmes: A Career of Contributions to the Coronavirus Field

Over the past two years, scientific research has moved at an unprecedented rate in response to the COVID-19 pandemic. The rapid development of effective vaccines and therapeutics would not have been possible without extensive background knowledge on coronaviruses developed over decades by researchers, including Kathryn (Kay) Holmes. Kay’s research team discovered the first coronavirus receptors for mouse hepatitis virus and human coronavirus 229E and contributed a wealth of information on coronaviral spike glycoproteins and receptor interactions that are critical determinants of host and tissue specificity. She collaborated with several research laboratories to contribute knowledge in additional areas, including coronaviral pathogenesis, epidemiology, and evolution. Throughout her career, Kay was an extremely dedicated and thoughtful mentor to numerous graduate students and post-doctoral fellows. This article provides a review of her contributions to the coronavirus field and her exemplary mentoring.

Cell Entry of Animal Coronaviruses

Coronaviruses (CoVs) are a group of enveloped positive-sense RNA viruses and can cause deadly diseases in animals and humans. Cell entry is the first and essential step of successful virus infection and can be divided into two ongoing steps: cell binding and membrane fusion. Over the past two decades, stimulated by the global outbreak of SARS-CoV and pandemic of SARS-CoV-2, numerous efforts have been made in the CoV research. As a result, significant progress has been achieved in our understanding of the cell entry process. Here, we review the current knowledge of this essential process, including the viral and host components involved in cell binding and membrane fusion, molecular mechanisms of their interactions, and the sites of virus entry. We highlight the recent findings of host restriction factors that inhibit CoVs entry. This knowledge not only enhances our understanding of the cell entry process, pathogenesis, tissue tropism, host range, and interspecies-transmission of CoVs but also provides a theoretical basis to design effective preventive and therapeutic strategies to control CoVs infection.

Natural selection supports escape from concerted evolution of a recently duplicated CEACAM1 paralog in the ruminant CEA gene family

Concerted evolution is often observed in multigene families such as the CEA gene family. As a result, sequence similarity of paralogous genes is significantly higher than expected from their evolutionary distance. Gene conversion, a “copy paste” DNA repair mechanism that transfers sequences from one gene to another and homologous recombination are drivers of concerted evolution. Nevertheless, some gene family members escape concerted evolution and acquire sufficient sequence differences that orthologous genes can be assigned in descendant species. Reasons why some gene family members can escape while others are captured by concerted evolution are poorly understood. By analyzing the entire CEA gene family in cattle (Bos taurus) we identified a member (CEACAM32) that was created by gene duplication and cooption of a unique transmembrane domain exon in the most recent ancestor of ruminants. CEACAM32 shows a unique, testis-specific expression pattern. Phylogenetic analysis indicated that CEACAM32 is not involved in concerted evolution of CEACAM1 paralogs in ruminants. However, analysis of gene conversion events revealed that CEACAM32 is subject to gene conversion but remarkably, these events are found in the leader exon and intron sequences but not in exons coding for the Ig-like domains. These findings suggest that natural selection hinders gene conversion affecting protein sequences of the mature protein and thereby support escape of CEACAM32 from concerted evolution.

The short isoform of the CEACAM1 receptor in intestinal T cells regulates mucosal immunity and homeostasis via Tfh cell induction

Carcinoembryonic antigen cell adhesion molecule like I (CEACAM1) is expressed on activated T cells and signals through either a long (L) cytoplasmic tail containing immune receptor tyrosine based inhibitory motifs, which provide inhibitory function, or a short (S) cytoplasmic tail with an unknown role. Previous studies on peripheral T cells show that CEACAM1-L isoforms predominate with little to no detectable CEACAM1-S isoforms in mouse and human. We show here that this was not the case in tissue resident T cells of intestines and gut associated lymphoid tissues, which demonstrated predominant expression of CEACAM1-S isoforms relative to CEACAM1-L isoforms in human and mouse. This tissue resident predominance of CEACAM1-S expression was determined by the intestinal environment where it served a stimulatory function leading to the regulation of T cell subsets associated with the generation of secretory IgA immunity, the regulation of mucosal commensalism, and defense of the barrier against enteropathogens.

CEA-Related CAMs

The carcinoembryonic antigen (CEA) family comprises a large number of cellular surface molecules, the CEA-related cell adhesion molecules (CEACAMs), which belong to the Ig superfamily. CEACAMs exhibit a complex expression pattern in normal and malignant tissues. The majority of the CEACAMs are cellular adhesion molecules that are involved in a great variety of distinct cellular processes, for example in the integration of cellular responses through homo- and heterophilic adhesion and interaction with a broad selection of signal regulatory proteins, i.e., integrins or cytoskeletal components and tyrosine kinases. Moreover, expression of CEACAMs affects tumor growth, angiogenesis, cellular differentiation, immune responses, and they serve as receptors for commensal and pathogenic microbes. Recently, new insights into CEACAM structure and function became available, providing further elucidation of their kaleidoscopic functions.

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.

Mouse Hepatitis Virus Receptor as a Determinant of the Mouse Susceptibility to MHV Infection

In this review, we report that the receptor of mouse hepatitis virus (MHV), carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), is an important determinant of mouse susceptibility to MHV infection. This finding was revealed by using mouse strains with two different allelic forms of the MHV receptor, Ceacam1a and Ceacam1b. Although previous studies indicated that susceptibility is determined by a single gene, Ceacam1, our recent work in gene-replaced mice with chimeric Ceacam1 pointed toward the involvement of other host factors (genes) in the susceptibility. Studies on mouse susceptibility to MHV, as well as the factors involved in their susceptibility, are overviewed.

Two palmitylated cysteine residues of the severe acute respiratory syndrome coronavirus spike (S) protein are critical for S incorporation into virus-like particles, but not for M-S co-localization

The endodomain of several coronavirus (CoV) spike (S) proteins contains palmitylated cysteine residues and enables co-localization and interaction with the CoV membrane (M) protein. Depalmitylation of mouse hepatitis virus S proteins abolished this interaction, resulting in the failure of S incorporation into virions. In contrast, an immunofluorescence assay (IFA) showed that depalmitylated severe acute respiratory syndrome coronavirus (SCoV) S proteins still co-localized with the M protein in the budding site. Here, we determined the ability of depalmitylated SCoV S mutants to incorporate S into virus-like particles (VLPs). IFA confirmed that all SCoV S mutants co-localized with the M protein intracellularly. However, the mutants lacking two cysteine residues (C(1234/1235)) failed to incorporate S into VLPs. This indicated that these palmitylated cysteines are essential for S incorporation, but are not involved in S co-localization mediated by the M protein. Our findings suggest that M-S co-localization and S incorporation occur independently of one another in SCoV virion assembly.

CEACAM1 dampens antitumor immunity by down-regulating NKG2D ligand expression on tumor cells

By retaining NKG2D ligands within tumor cells, carcinoembryonic antigen–related cell adhesion molecule 1 (CEACAM1) facilitates tumor cell escape from NK cell–mediated cytolysis in vitro and in vivo.

Although carcinoembryonic antigen (CEA)–related cell adhesion molecule 1 (CEACAM1) has been viewed as a tumor suppressor, increasing clinical evidence shows that high levels of CEACAM1 expression on tumors correlates with poor prognosis and high risk of metastasis. Here, we examined the consequences of CEACAM1 expression on tumor cells. We show that tumor cell–associated CEACAM1 causes intracellular retention of various NKG2D ligands in mouse and human tumor cells. CEACAM1-silenced tumor cells expressed more cell surface NKG2D ligands and exhibited greater sensitivity to natural killer cell–mediated cytolysis in vitro and rejection in vivo. Our studies reveal a novel mechanism through which CEACAM1-bearing tumor cells may escape immune-surveillance.

Apical expression of human full-length hCEACAM1-4L protein renders the Madin Darby Canine Kidney cells responsive to lipopolysaccharide leading to TLR4-dependent Erk1/2 and p38 MAPK signalling

CEACAM1 expressed by granulocytes and epithelial cells is recognized as a membrane-associated receptor by some Gram-negative pathogens. Here we report a previously unsuspected role of human CEACAM1-4L (hCEACAM1-4L) in polarized epithelial cells. We find that in contrast with non-transfected cells, Madin Darby Canine Kidney strain II (MDCK) engineered for the apical expression of the long cytoplasmic chain protein hCEACAM1-4L showed a serum-independent increase in the phosphorylation of the extracellular signal-regulated kinase 1/2 (Erk1/2) and p38 mitogen-activated protein kinases (MAPKs) after treatment with lipopolysaccharide (LPS) of wild-type, diffusely adhering Afa/Dr Escherichia coli (Afa/Dr DAEC) strain IH11128. Aggregates of FITC-LPS bind the apical domain of MDCK-hCEACAM1-4L cells colocalizing with the apically expressed hCEACAM1-4L protein and do not bind MDCK-pCEP cells, and surface plasmon resonance analysis shows that LPS binds to the extracellular domain of the CEACAM1-4L protein. We showed that cell polarization and lipid rafts positively control the LPS-IH11128-induced phosphorylation of Erk1/2 in MDCK-hCEACAM1-4L cells. Structure-function analysis using mutated hCEACAM1-4L protein shows that the cytoplasmic domain of the protein is needed for LPS-induced MAPK signalling, and that phosphorylation of Tyr-residues is not increased in association with MAPK signalling. The hCEACAM1-4L-dependent Erk1/2 phosphorylation develops in the presence of lipid A and does not develop in the presence of penta-acylated LPS. Finally, small interfering RNA (siRNA) silencing of canine TLR4 abolishes the hCEACAM1-4L-dependent, LPS-induced phosphorylation of Erk1/2. Collectively, our results support the notion that the apically expressed, full-length hCEACAM1-4L protein functions as a novel LPS-conveying molecule at the mucosal surface of polarized epithelial cells for subsequent MD-2/TLR4 receptor-dependent MAPK Erk1/2 and p38 signalling.

Pathogenesis of neurotropic murine coronavirus is multifactorial

Although coronavirus tropism is most often ascribed to receptor availability, increasing evidence suggests that for the neurotropic strains of the murine coronavirus mouse hepatitis virus (MHV), spike–receptor interactions cannot fully explain neurovirulence. The canonical MHV receptor CEACAM1a and its spike-binding site have been extensively characterized. However, CEACAM1a is poorly expressed in neurons, and the extremely neurotropic MHV strain JHM.SD infects ceacam1a^−/− mice and spreads among ceacam1a^−/− neurons. Two proposed alternative MHV receptors, CEACAM2 and PSG16, also fail to account for neuronal spread of JHM.SD in the absence of CEACAM1a. It has been reported that JHM.SD has an unusually labile spike protein, enabling it to perform receptor-independent spread (RIS), but it is not clear if the ability to perform RIS is fully responsible for the extremely neurovirulent phenotype. We propose that the extreme neurovirulence of JHM.SD is multifactorial and might include as yet unidentified neuron-specific spread mechanisms.

Murine coronavirus receptors are differentially expressed in the central nervous system and play virus strain-dependent roles in neuronal spread

Coronavirus infection of the murine central nervous system (CNS) provides a model for studies of viral encephalitis and demyelinating disease. Mouse hepatitis virus (MHV) neurotropism varies by strain: MHV-A59 causes mild encephalomyelitis and demyelination, while the highly neurovirulent strain JHM.SD (MHV-4) causes fatal encephalitis with extensive neuronal spread of virus. In addition, while neurons are the predominant CNS cell type infected in vivo, the canonical receptor for MHV, the carcinoembryonic antigen family member CEACAM1a, has been demonstrated only on endothelial cells and microglia. In order to investigate whether CEACAM1a is also expressed in other cell types, ceacam1a mRNA expression was quantified in murine tissues and primary cells. As expected, among CNS cell types, microglia expressed the highest levels of ceacam1a, but lower levels were also detected in oligodendrocytes, astrocytes, and neurons. Given the low levels of neuronal expression of ceacam1a, primary neurons from wild-type and ceacam1a knockout mice were inoculated with MHV to determine the extent to which CEACAM1a-independent infection might contribute to CNS infection. While both A59 and JHM.SD infected small numbers of ceacam1a knockout neurons, only JHM.SD spread efficiently to adjacent cells in the absence of CEACAM1a. Quantification of mRNA for the ceacam1a-related genes ceacam2 and psg16 (bCEA), which encode proposed alternative MHV receptors, revealed low ceacam2 expression in microglia and oligodendrocytes and psg16 expression exclusively in neurons; however, only CEACAM2 mediated infection in human 293T cells. Therefore, neither CEACAM2 nor PSG16 is likely to be an MHV receptor on neurons, and the mechanism for CEACAM1a-independent neuronal spread of JHM.SD remains unknown.

Role of mouse hepatitis virus (MHV) receptor murine CEACAM1 in the resistance of mice to MHV infection: studies of mice with chimeric mCEACAM1a and mCEACAM1b

Although most inbred mouse strains are highly susceptible to mouse hepatitis virus (MHV) infection, the inbred SJL line of mice is highly resistant to its infection. The principal receptor for MHV is murine CEACAM1 (mCEACAM1). Susceptible strains of mice are homozygous for the 1a allele of mCeacam1, while SJL mice are homozygous for the 1b allele. mCEACAM1a (1a) has a 10- to 100-fold-higher receptor activity than does mCEACAM1b (1b). To explore the hypothesis that MHV susceptibility is due to the different MHV receptor activities of 1a and 1b, we established a chimeric C57BL/6 mouse (cB61ba) in which a part of the N-terminal immunoglobulin (Ig)-like domain of the mCeacam1a (1a) gene, which is responsible for MHV receptor function, is replaced by the corresponding region of mCeacam1b (1b). We compared the MHV susceptibility of these chimeric mice to that of SJL and B6 mice. B6 mice that are homozygous for 1a are highly susceptible to MHV-A59 infection, with a 50% lethal dose (LD(50)) of 10(2.5) PFU, while chimeric cB61ba mice and SJL mice homozygous for 1ba and 1b, respectively, survived following inoculation with 10(5) PFU. Unexpectedly, cB61ba mice were more resistant to MHV-A59 infection than SJL mice as measured by virus replication in target organs, including liver and brain. No infectious virus or viral RNA was detected in the organs of cB61ba mice, while viral RNA and infectious virus were detected in target organs of SJL mice. Furthermore, SJL mice produced antiviral antibodies after MHV-A59 inoculation with 10(5) PFU, but cB61ba mice did not. Thus, cB61ba mice are apparently completely resistant to MHV-A59 infection, while SJL mice permit low levels of MHV-A59 virus replication during self-limited, asymptomatic infection. When expressed on cultured BHK cells, the mCEACAM1b and mCEACAM1ba proteins had similar levels of MHV-A59 receptor activity. These results strongly support the hypothesis that although alleles of mCEACAM1 are the principal determinants of mouse susceptibility to MHV-A59, other as-yet-unidentified murine genes may also play a role in susceptibility to MHV.

CEACAM1 recognition by bacterial pathogens is species-specific

BACKGROUND

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), an immunoglobulin (Ig)-related glycoprotein, serves as cellular receptor for a variety of Gram-negative bacterial pathogens associated with the human mucosa. In particular, Neisseria gonorrhoeae, N. meningitidis, Moraxella catarrhalis, and Haemophilus influenzae possess well-characterized CEACAM1-binding adhesins. CEACAM1 is typically involved in cell-cell attachment, epithelial differentiation, neovascularisation and regulation of T-cell proliferation, and is one of the few CEACAM family members with homologues in different mammalian lineages. However, it is unknown whether bacterial adhesins of human pathogens can recognize CEACAM1 orthologues from other mammals.

RESULTS

Sequence comparisons of the amino-terminal Ig-variable-like domain of CEACAM1 reveal that the highest sequence divergence between human, murine, canine and bovine orthologues is found in the beta-strands comprising the bacteria-binding CC'FG-face of the Ig-fold. Using GFP-tagged, soluble amino-terminal domains of CEACAM1, we demonstrate that bacterial pathogens selectively associate with human, but not other mammalian CEACAM1 orthologues. Whereas full-length human CEACAM1 can mediate internalization of Neisseria gonorrhoeae in transfected cells, murine CEACAM1 fails to support bacterial internalization, demonstrating that the sequence divergence of CEACAM1 orthologues has functional consequences with regard to bacterial recognition and cellular invasion.

CONCLUSIONS

Our results establish the selective interaction of several human-restricted bacterial pathogens with human CEACAM1 and suggest that co-evolution of microbial adhesins with their corresponding receptors on mammalian cells contributes to the limited host range of these highly adapted infectious agents.

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.

Cytopathy of an infiltrating monocyte lineage during the early phase of infection with murinecoronavirus in the brain

Viral spread during the early stages after infection was compared between a highly neurovirulent mouse hepatitis virus (MHV), JHMV cl-2 strain (cl-2), and its low-virulent mutant, soluble-receptor-resistant (srr)7. The infection of cells with srr7 (soluble-receptor-resistant mutant 7) is dependent on a known MHV receptor (MHVR), carcinoembryonic cell adhesion molecule 1a, whereas cl-2 shows MHVR-independent infection. Initial viral antigens were detected between 12 and 24 h post-inoculation (p.i) in the infiltrating cells that appeared in the subarachnoidal space of mouse brains infected with viruses. There were no significant differences in the intensity or spread of viral antigens in the inflammatory cells between the two viruses. However, 48 h after infection with cl-2, viral antigen-positive cells in the grey matter with the shape of neurons, which do not express MHVR, were detected, while srr7 infection was observed primarily in the white matter. Some of the viral antigen-positive inflammatory cells found in the subarachnoidal space during the early phase of infection reacted with anti-F4/80 or anti-CD11b monoclonal antibodies. Syncytial giant cells (SGCs) expressing viral and CD11b antigens were also detected among these inflammatory cells. These antigen-positive cells appeared in the subarachnoidal space prior to viral antigen spread into the brain parenchyma, indicating that viral encephalitis starts with the infection of infiltrating monocytes which express MHVR. Furthermore, the observation indicates that viral infection has cytopathic effects on the monocyte lineage, which plays a critical role in innate immunity, leading to the rapid spread of viruses during the early stage of infection.

Role of Ceacam1 in VEGF induced vasculogenesis of murine embryonic stem cell-derived embryoid bodies in 3D culture

CEACAM1 (carcinoembryonic antigen-related cell adhesion molecule 1), a type I transmembrane glycoprotein involved in cell-cell adhesion has been shown to act as an angiogenic factor for mouse and human endothelial cells. Based on the ability of CEACAM1 to initiate lumen formation in human mammary epithelial cells grown in 3D culture (Matrigel), we hypothesized that murine CEACAM1 may play a similar role in vasculogenesis. In order to test this hypothesis, murine embryonic stem (ES) cells stimulated with VEGF were differentiated into embryoid bodies (EB) for 8 days (-8-0 d) and transferred to Matrigel in the presence or absence of anti-CEACAM1 antibody for an additional 12 days (0-12 d). In the absence of anti-CEACAM1 antibody or in the presence of an isotype control antibody, the EB in Matrigel underwent extensive sprouting, generating lengthy vascular structures with well-defined lumina as demonstrated by confocal microscopy, electron microscopy, and immunohistochemical analysis. Both the length and architecture of the vascular tubes were inhibited by anti-CEACAM1 mAb CC1, a mAb that blocks the cell-cell adhesion functions of CEACAM1, thus demonstrating a critical role for this cell-cell adhesion molecule in generating and maintaining vasculogenesis. QRT-PCR analysis of the VEGF treated ES cells grown under conditions that convert them to EB revealed expression of Ceacam1 as early as -5 to -3 d reaching a maximum at day 0 at which time EBs were transferred to Matrigel, thereafter levels at first declined and then increased over time. Other markers of vasculogenesis including Pecam1, VE-Cad, and Tie-1 were not detected until day 0 when EBs were transferred to Matrigel followed by a steady increase in levels, indicating later roles in vasculogenesis. In contrast, Tie-2 and Flk-1 (VEGFR2) were detected on day five of EB formation reaching a maximum at day 0 on transfer to Matrigel, similar to Ceacam1, but after which Tie-2 declined over time, while Flk-1 increased over time. QRT-PCR analysis of the anti-CEACAM1 treated ES cells revealed a significant decrease in the expression of Ceacam1, Pecam1, Tie-1, and Flk-1, while VE-Cad and Tie-2 expression were unaffected. These results suggest that the expression and signaling of CEACAM1 may affect the expression of other factors known to play critical roles in vasculogenesis. Furthermore this 3D model of vasculogenesis in an environment of extracellular matrix may be a useful model for comparison to existing models of angiogenesis.

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.

Intestinal tumor progression is promoted by decreased apoptosis and dysregulated Wnt signaling in Ceacam1-/- mice

The carcinoembryonic antigen cell adhesion molecule 1 (CEACAM1) is downregulated in colonic and intestinal hyperplastic lesions as well as in other cancers, where it functions as a tumor suppressor. To investigate the functions of CEACAM1 in the normal intestine and in intestinal tumors, we generated a compound knockout mouse model and examined both Ceacam1(-/-) and Apc(1638N/+):Ceacam1(-/-) mice. Ceacam1(-/-) intestinal cells exhibited a significant decrease in apoptosis, with no change in proliferation or migration, however. Compound Apc(1638N/+):Ceacam1(-/-) mice demonstrated an increase in intestinal tumor multiplicity and tumor progression. Increases in intussusceptions and desmoid lesions were also observed. We have shown that CEACAM1-L associates with beta-catenin by co-immunoprecipitation and colocalization in CEACAM1-L-transfected CT26 and CT51 mouse colon carcinoma cells. Ceacam1(-/-) enterocytes displayed decreased glycogen synthase kinase 3-beta activity with corresponding nuclear localization of beta-catenin. Increased T-cell factor/Lef transcriptional activity was observed in CEACAM1-null CT51 colonic cells and in Caco2 colon cancer cells in which CEACAM1 was downregulated. A significant increased expression in c-Myc and cyclin D1 targets of the Wnt signaling pathway was also revealed in the Ceacam1(-/-) intestine. CEACAM1 therefore actively participates in Wnt signaling in intestinal cells and its downregulation in intestinal tissue contributes to malignancy by augmenting tumor multiplicity and progression.

Amino acid substitutions in the S2 subunit of mouse hepatitis virus variant V51 encode determinants of host range expansion

We previously described mouse hepatitis virus (MHV) variant V51 derived from a persistent infection of murine DBT cells with an expanded host range (R. S. Baric, E. Sullivan, L. Hensley, B. Yount, and W. Chen, J. Virol. 73:638-649, 1999). Sequencing of the V51 spike gene, the mediator of virus entry, revealed 13 amino acid substitutions relative to the originating MHV A59 strain. Seven substitutions were located in the amino-terminal S1 cleavage subunit, and six were located in the carboxy-terminal S2 cleavage subunit. Using targeted RNA recombination, we constructed a panel of recombinant viruses to map the mediators of host range to the six substitutions in S2, with a subgroup of four changes of particular interest. This subgroup maps to two previously identified domains within S2, a putative fusion peptide and a heptad repeat, both conserved features of class I fusion proteins. In addition to an altered host range, V51 displayed altered utilization of CEACAM1a, the high-affinity receptor for A59. Interestingly, a recombinant with S1 from A59 and S2 from V51 was severely debilitated in its ability to productively infect cells via CEACAM1a, while the inverse recombinant was not. This result suggests that the S2 substitutions exert powerful effects on the fusion trigger that normally passes from S1 to S2. These novel findings play against the existing data that suggest that MHV host range determinants are located in the S1 subunit, which harbors the receptor binding domain, or involve coordinating changes in both S1 and S2. Mounting evidence also suggests that the class I fusion mechanism may possess some innate plasticity that regulates viral host range.

The spike glycoprotein of murine coronavirus MHV-JHM mediates receptor-independent infection and spread in the central nervous systems of Ceacam1a-/- Mice

The MHV-JHM strain of the murine coronavirus mouse hepatitis virus is much more neurovirulent than the MHV-A59 strain, although both strains use murine CEACAM1a (mCEACAM1a) as the receptor to infect murine cells. We previously showed that Ceacam1a(-/-) mice are completely resistant to MHV-A59 infection (E. Hemmila et al., J. Virol. 78:10156-10165, 2004). In vitro, MHV-JHM, but not MHV-A59, can spread from infected murine cells to cells that lack mCEACAM1a, a phenomenon called receptor-independent spread. To determine whether MHV-JHM could infect and spread in the brain independent of mCEACAM1a, we inoculated Ceacam1a(-/-) mice. Although Ceacam1a(-/-) mice were completely resistant to i.c. inoculation with 10(6) PFU of recombinant wild-type MHV-A59 (RA59) virus, these mice were killed by recombinant MHV-JHM (RJHM) and a chimeric virus containing the spike of MHV-JHM in the MHV-A59 genome (SJHM/RA59). Immunohistochemistry showed that RJHM and SJHM/RA59 infected all neural cell types and induced severe microgliosis in both Ceacam1a(-/-) and wild-type mice. For RJHM, the 50% lethal dose (LD(50)) is <10(1.3) in wild-type mice and 10(3.1) in Ceacam1a(-/-) mice. For SJHM/RA59, the LD(50) is <10(1.3) in wild-type mice and 10(3.6) in Ceacam1a(-/-) mice. This study shows that infection and spread of MHV-JHM in the brain are dependent upon the viral spike glycoprotein. RJHM can initiate infection in the brains of Ceacam1a(-/-) mice, but expression of mCEACAM1a increases susceptibility to infection. The spread of infection in the brain is mCEACAM1a independent. Thus, the ability of the MHV-JHM spike to mediate mCEACAM1a-independent spread in the brain is likely an important factor in the severe neurovirulence of MHV-JHM in wild-type mice.

SHP1 Phosphatase-Dependent T Cell Inhibition by CEACAM1 Adhesion Molecule Isoforms

T cell activation through the T cell receptor (TCR) is subsequently modified by secondary signals that are either stimulatory or inhibitory. We show that CEACAM1 adhesion molecule isoforms containing a long cytoplasmic domain inhibited multiple T cell functions as a consequence of TCR ligation. Overexpression of CEACAM1 resulted in decreased proliferation, allogeneic reactivity, and cytokine production in vitro and delayed type hypersensitivity and inflammatory bowel disease in mouse models in vivo. Conditioned deletion of CEACAM1 in T cells caused increased TCR-CD3 complex signaling. This T cell regulation was dependent upon the presence of immunoreceptor tyrosine-based inhibition motifs (ITIM) within the cytoplasmic domain of CEACAM1 and the Src homology 2 domain-containing protein tyrosine-phosphatase 1 (SHP1) in the T cell. Thus, CEACAM1 overexpression or deletion in T cells resulted in T cell inhibition or activation, respectively, revealing a role for CEACAM1 as a class of inhibitory receptors potentially amenable to therapeutic manipulation.

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.

Receptor-independent infection of murine coronavirus: analysis by spinoculation

A highly neurovirulent murine coronavirus JHMV (wild-type [wt] JHMV) is known to spread from cells infected via the murine coronavirus mouse hepatitis virus receptor (MHVR) to cells without MHVR (MHVR-independent infection), whereas a mutant virus isolated from wt JHMV, srr7, spread only in an MHVR-dependent fashion. These observations were obtained by the overlay of JHMV-infected cells onto receptor-negative cells that are otherwise resistant to wt JHMV infection. MHVR-independent infection is hypothetically thought to be attributed to a naturally occurring fusion activation of the wt JHMV S protein, which did not occur in the case of srr7. Attachment of S protein on cells without MHVR during the S-protein activation process seems to be a key condition. Thus, in the present study, we tried to see whether wt JHMV virions that are attached on MHVR-negative cells are able to infect those cells. In order to make virions attach to the cell surface without MHVR, we have used spinoculation, namely, the centrifugation of cells together with inoculated virus at 3,000 rpm for 2 h. This procedure forces viruses to attach to the cell surface, as revealed by quantitative estimation of attached virions by real-time PCR and also facilitated wt JHMV infection to MHVR-negative cells, but failed to do so for srr7. Virions of both wt and srr7 attached on MHVR-negative cells by spinoculation were facilitated for infection in the presence of a soluble form of MHVR that induces conformational changes of both wt and srr7. It was further revealed that wt JHMV S1, but not srr7, was released from the cell surface when S protein was expressed on cells. These observations support the hypothesis that attachment of the virion to MHVR-negative cells is a critical step and that a unique feature of wt JHMV S1 to be released from S2 in a naturally occurring event is involved in an MHVR-independent infection.

Soluble receptor-mediated targeting of mouse hepatitis coronavirus to the human epidermal growth factor receptor

The mouse hepatitis coronavirus (MHV) infects murine cells by binding of its spike (S) protein to murine CEACAM1a. The N-terminal part of this cellular receptor (soR) is sufficient for S binding and for subsequent induction of the conformational changes required for virus-cell membrane fusion. Here we analyzed whether these characteristics can be used to redirect MHV to human cancer cells. To this end, the soR domain was coupled to single-chain monoclonal antibody 425, which is directed against the human epidermal growth factor receptor (EGFR), resulting in a bispecific adapter protein (soR-425). The soR and soR-425 proteins, both produced with the vaccinia virus system, were able to neutralize MHV infection of murine LR7 cells. However, only soR-425 was able to target MHV to human EGFR-expressing cancer cells. Interestingly, the targeted infections induced syncytium formation. Furthermore, the soR-425-mediated infections were blocked by heptad repeat-mimicking peptides, indicating that virus entry requires the regular S protein fusion process. We conclude that the specific spike-binding property of the CEACAM1a N-terminal fragment can be exploited to direct the virus to selected cells by linking it to a moiety able to bind a receptor on those cells. This approach might be useful in the development of tumor-targeted coronaviruses.

Redirecting coronavirus to a nonnative receptor through a virus-encoded targeting adapter

Murine hepatitis coronavirus (MHV)-A59 infection depends on the interaction of its spike (S) protein with the cellular receptor mCEACAM1a present on murine cells. Human cells lack this receptor and are therefore not susceptible to MHV. Specific alleviation of the tropism barrier by redirecting MHV to a tumor-specific receptor could lead to a virus with appealing properties for tumor therapy. To demonstrate that MHV can be retargeted to a nonnative receptor on human cells, we produced bispecific adapter proteins composed of the N-terminal D1 domain of mCEACAM1a linked to a short targeting peptide, the six-amino-acid His tag. Preincubation of MHV with the adapter proteins and subsequent inoculation of human cells expressing an artificial His receptor resulted in infection of these otherwise nonsusceptible cells and led to subsequent production of progeny virus. To generate a self-targeted virus able to establish multiround infection of the target cells, we subsequently incorporated the gene encoding the bispecific adapter protein as an additional expression cassette into the MHV genome through targeted RNA recombination. When inoculated onto murine LR7 cells, the resulting recombinant virus indeed expressed the adapter protein. Furthermore, inoculation of human target cells with the virus resulted in a His receptor-specific infection that was multiround. Extensive cell-cell fusion and rapid cell killing of infected target cells was observed. Our results show that MHV can be genetically redirected via adapters composed of the S protein binding part of mCEACAM1a and a targeting peptide recognizing a nonnative receptor expressed on human cells, consequently leading to rapid cell death. The results provide interesting leads for further investigations of the use of coronaviruses as antitumor agents.

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.

Receptor-independent spread of a highly neurotropic murine coronavirus JHMV strain from initially infected microglial cells in mixed neural cultures

Although neurovirulent mouse hepatitis virus (MHV) strain JHMV multiplies in a variety of brain cells, expression of its receptor carcinoembryonic antigen cell adhesion molecule 1 (CEACAM 1) (MHVR) is restricted only in microglia. The present study was undertaken to clarify the mechanism of an extensive JHMV infection in the brain by using neural cells isolated from mouse brain. In contrast to wild-type (wt) JHMV, a soluble-receptor-resistant mutant (srr7) infects and spreads solely in an MHVR-dependent fashion (F. Taguchi and S. Matsuyama, J. Virol. 76:950-958, 2002). In mixed neural cell cultures, srr7 infected a limited number of cells and infection did not spread, although wt JHMV induced syncytia in most of the cells. srr7-infected cells were positive for GS-lectin, a microglia marker. Fluorescence-activated cell sorter analysis showed that about 80% of the brain cells stained with anti-MHVR antibody (CC1) were also positive for GS-lectin. Pretreatment of those cells with CC1 prevented virus attachment to the cell surface and also blocked virus infection. These results show that microglia express functional MHVR that mediates JHMV infection. As expected, in microglial cell-enriched cultures, both srr7and wt JHMV produced syncytia in a majority of cells. Treatment with CC1 of mixed neural cell cultures and microglia cultures previously infected with wt virus failed to block the spread of infection, indicating that wt infection spreads in an MHVR-independent fashion. Thus, the present study indicates that microglial cells are the major population of the initial target for MHV infection and that the wt spreads from initially infected microglia to a variety of cells in an MHVR-independent fashion.

Substitutions of conserved amino acids in the receptor-binding domain of the spike glycoprotein affect utilization of murine CEACAM1a by the murine coronavirus MHV-A59

The host range of the murine coronavirus (MHV) is limited to susceptible mice and murine cell lines by interactions of the spike glycoprotein (S) with its receptor, mCEACAM1a. We identified five residues in S (S33, L79, T82, Y162 and K183) that are conserved in the receptor-binding domain of MHV strains, but not in related coronaviruses. We used targeted RNA recombination to generate isogenic viruses that differ from MHV-A59 by amino acid substitutions in S. Viruses with S33R and K183R substitutions had wild type growth, while L79A/T82A viruses formed small plaques. Viruses with S33G, L79M/T82M or K183G substitutions could only be recovered from cells that over-expressed a mutant mCEACAM1a. Viruses with Y162H or Y162Q substitutions were never recovered, while Y162A viruses formed minute plaques. However, viruses with Y162F substitutions had wild type growth, suggesting that Y162 may comprise part of a hydrophobic domain that contacts the MHV-binding site of mCEACAM1a.

Coronavirus Receptors

The major receptor for murine coronavirus, mouse hepatitis virus (MHV), is identified as a protein, cell-adhesion molecule 1 in the carcinoembryonic antigen family (CEACAM1), which is classified in the immunoglobulin superfamily. There are four CEACAM1 isoforms, with either four or two ectodomains, resulting from an alternative splicing mechanism. CEACAM1 is expressed on the epithelium and in endothelial cells of a variety of tissues and hemopoietic cells, and functions as a homophilic and heterophilic adhesion molecule. It is used as a receptor for some bacteria as well. The N terminal domain participates in mediating homophilic adhesion. This domain is also responsible for binding to the MHV spike (S) protein; the CC’ face protruding in this domain interacts with an N terminal region of the S protein composed of 330 amino acids (called S1N330). The binding of CEACAM1 with MHV S protein induces S protein conformational changes and converts fusion-negative S protein to a fusion-positive form. The allelic forms of CEACAM1 found among mouse strains are thought to be an important determinant for mouse susceptibility to MHV.

The N-terminal region of the murine coronavirus spike glycoprotein is associated with the extended host range of viruses from persistently infected murine cells

Although murine coronaviruses naturally infect only mice, several virus variants derived from persistently infected murine cell cultures have an extended host range. The mouse hepatitis virus (MHV) variant MHV/BHK can infect hamster, rat, cat, dog, monkey, and human cell lines but not the swine testis (ST) porcine cell line (J. H. Schickli, B. D. Zelus, D. E. Wentworth, S. G. Sawicki, and K. V. Holmes, J. Virol. 71:9499-9507, 1997). The spike (S) gene of MHV/BHK had 63 point mutations and a 21-bp insert that encoded 56 amino acid substitutions and a 7-amino-acid insert compared to the parental MHV strain A59. Recombinant viruses between MHV-A59 and MHV/BHK were selected in hamster cells. All of the recombinants retained 21 amino acid substitutions and a 7-amino-acid insert found in the N-terminal region of S of MHV/BHK, suggesting that these residues were responsible for the extended host range of MHV/BHK. Flow cytometry showed that MHV-A59 bound only to cells that expressed the murine glycoprotein receptor CEACAM1a. In contrast, MHV/BHK and a recombinant virus, k6c, with the 21 amino acid substitutions and 7-amino-acid insert in S bound to hamster (BHK) and ST cells as well as murine cells. Thus, 21 amino acid substitutions and a 7-amino-acid insert in the N-terminal region of the S glycoprotein of MHV/BHK confer the ability to bind and in some cases infect cells of nonmurine species.

Ceacam1a-/- mice are completely resistant to infection by murine coronavirus mouse hepatitis virus A59

CEACAM1a glycoproteins are members of the immunoglobulin (Ig) superfamily and the carcinoembryonic antigen family. Isoforms expressing either two or four alternatively spliced Ig-like domains in mice have been found in a number of epithelial, endothelial, or hematopoietic tissues. CEACAM1a functions as an intercellular adhesion molecule, an angiogenic factor, and a tumor cell growth inhibitor. Moreover, the mouse and human CEACAM1a proteins are targets of viral or bacterial pathogens, respectively, including the murine coronavirus mouse hepatitis virus (MHV), Haemophilus influenzae, Neisseria gonorrhoeae, and Neisseria meningitidis, as well as Moraxella catarrhalis in humans. We have shown that targeted disruption of the Ceacam1a (MHVR) gene resulting in a partial ablation of the protein in mice (p/p mice) led to reduced susceptibility to MHV-A59 infection of the modified mice in the BALB/c background. We have now engineered and produced a Ceacam1a-/- mouse that exhibits complete ablation of the CEACAM1a protein in every tissue where it is normally expressed. We report that 3-week-old Ceacam1a-/- mice in the C57BL/6 genetic background are fully resistant to MHV-A59 infection by both intranasal and intracerebral routes. Whereas virus-inoculated wild-type +/+ C57BL/6 mice showed profound liver damage and spinal cord demyelination under these conditions, Ceacam1a-/- mice displayed normal livers and spinal cords. Virus was recovered from liver and spinal cord tissues of +/+ mice but not of -/- mice. These results indicate that CEACAM1a is the sole receptor for MHV-A59 in both liver and brain and that its deletion from the mouse renders the mouse completely resistant to infection by this virus.

Amino acid substitutions and an insertion in the spike glycoprotein extend the host range of the murine coronavirus MHV-A59

The murine coronavirus [murine hepatitis virus (MHV)] is limited to infection of susceptible mice and murine cell lines by the specificity of the spike glycoprotein (S) for its receptor, murine carcinoembryonic antigen cell adhesion molecule 1a (mCEACAM1a). We have recently shown that 21 aa substitutions and a 7-aa insert in the N-terminal region of S are associated with the extended host range of a virus variant derived from murine cells persistently infected with the A59 strain of MHV (MHV-A59). We used targeted RNA recombination (TRR) to generate isogenic viruses that differ from MHV-A59 by the 21 aa substitutions or the 7-aa insert in S. Only viruses with both the 21 aa substitutions and the 7-aa insert in S infected hamster, feline, and monkey cells. These viruses also infected murine cells in the presence of blocking anti-mCEACAM1a antibodies. Thus, relatively few changes in the N-terminal region of S1 are sufficient to permit MHV-A59 to interact with alternative receptors on murine and non-murine cells.

Receptor-dependent coronavirus infection of dendritic cells

In several mammalian species, including humans, coronavirus infection can modulate the host immune response. We show a potential role of dendritic cells (DC) in murine coronavirus-induced immune modulation and pathogenesis by demonstrating that the JAW SII DC line and primary DC from BALB/c mice and p/p mice with reduced expression of the murine coronavirus receptor, murine CEACAM1a, are susceptible to murine coronavirus infection by a receptor-dependent pathway.

A combination of mutations in the S1 part of the spike glycoprotein gene of coronavirus MHV-A59 abolishes demyelination

The A59 strain of coronavirus, mouse hepatitis virus (MHV), produces acute hepatitis, meningoencephalitis, and chronic demyelination. The authors have previously shown that the spike (S) glycoprotein gene of MHV contains determinants of virulence, hepatitis, and demyelination. They then identified viruses containing mutations in the S gene that exhibit alterations in viral pathogenesis. In the present study, the authors produced new recombinant viruses with each one of these S gene mutations by site-directed mutagenesis and targeted recombination and studied the effect of each individual mutation on the pathogenesis of the virus. They identified a combination of mutations in the S1 gene (I375M and L652I) that abolishes demyelination. Individual mutation and other combinations of mutations in the S gene only interfere with virulence and hepatitis and only reduce demyelination (I375M), but do not abolish demyelination completely. Thus, demyelination determinants exist within genomic regions on both sides of the hypervariable region, downstream from the receptor-binding domain in the S1 part of the MHV spike glycoprotein gene. The structure and precise function of these regions awaits further investigation.

N-terminal domain of the murine coronavirus receptor CEACAM1 is responsible for fusogenic activation and conformational changes of the spike protein

The mouse hepatitis virus (MHV) receptor (MHVR), CEACAM1, has two different functions for MHV entry into cells: binding to MHV spike protein (S protein) and activation of the S protein to execute virus-cell membrane fusion, the latter of which is accompanied by conformational changes of the S protein. The MHVR comprising the N-terminal and fourth domains [R1(1,4)] displays these two activities, and the N domain is thought to be critical for binding to MHV. In this study, we have addressed whether or not the N domain alone is sufficient for these activities. We examined three types of soluble form MHVR (soMHVR), one consisting of the N domain alone [soR1(1)], one with the N and second domains [soR1(1,2)], and one [soR1(1,4)] expressed by recombinant baculoviruses. We assessed the abilities of these three types of soMHVR to bind to MHV, activate fusogenicity, and induce conformational changes of the S protein. All three types of soMHVR similarly bound to MHV, as examined by a solid-phase binding assay and neutralized MHV infectivity. They also activated S protein fusogenicity and induced its conformational changes with similar levels of efficiency. However, R1(1) expressed on the BHK cell surface failed to serve as a receptor in spite of a sufficient level of expression. The inability of expressed R1(1) to work as a receptor was due to the inaccessibility of virions to R1(1); however, these were accessible using the MHVR-specific monoclonal antibody CC1. These results collectively indicated that the N domain retains all biological activities necessary for receptor function.

Conformational changes in the spike glycoprotein of murine coronavirus are induced at 37 degrees C either by soluble murine CEACAM1 receptors or by pH 8

The spike glycoprotein (S) of the murine coronavirus mouse hepatitis virus (MHV) binds to viral murine CEACAM receptor glycoproteins and causes membrane fusion. On virions, the 180-kDa S glycoprotein of the MHV-A59 strain can be cleaved by trypsin to form the 90-kDa N-terminal receptor-binding subunit (S1) and the 90-kDa membrane-anchored fusion subunit (S2). Incubation of virions with purified, soluble CEACAM1a receptor proteins at 37 degrees C and pH 6.5 neutralizes virus infectivity (B. D. Zelus, D. R. Wessner, R. K. Williams, M. N. Pensiero, F. T. Phibbs, M. deSouza, G. S. Dveksler, and K. V. Holmes, J. Virol. 72:7237-7244, 1998). We used liposome flotation and protease sensitivity assays to investigate the mechanism of receptor-induced, temperature-dependent virus neutralization. After incubation with soluble receptor at 37 degrees C and pH 6.5, virions became hydrophobic and bound to liposomes. Receptor binding induced a profound, apparently irreversible conformational change in S on the viral envelope that allowed S2, but not S1, to be degraded by trypsin at 4 degrees C. Various murine CEACAM proteins triggered conformational changes in S on recombinant MHV strains expressing S glycoproteins of MHV-A59 or MHV-4 (MHV-JHM) with the same specificities as seen for virus neutralization and virus-receptor activities. Increased hydrophobicity of virions and conformational change in S2 of MHV-A59 could also be induced by incubating virions at pH 8 and 37 degrees C, without soluble receptor. Surprisingly, the S protein of recombinant MHV-A59 virions with a mutation, H716D, that precluded cleavage between S1 and S2 could also be triggered to undergo a conformational change at 37 degrees C by soluble receptor at neutral pH or by pH 8 alone. A novel 120-kDa subunit was formed following incubation of the receptor-triggered S(A59)H716D virions with trypsin at 4 degrees C. The data show that unlike class 1 fusion glycoproteins of other enveloped viruses, the murine coronavirus S protein can be triggered to a membrane-binding conformation at 37 degrees C either by soluble receptor at neutral pH or by alkaline pH alone, without requiring previous activation by cleavage between S1 and S2.

The N-terminal domain of the murine coronavirus spike glycoprotein determines the CEACAM1 receptor specificity of the virus strain

Using isogenic recombinant murine coronaviruses expressing wild-type murine hepatitis virus strain 4 (MHV-4) or MHV-A59 spike glycoproteins or chimeric MHV-4/MHV-A59 spike glycoproteins, we have demonstrated the biological functionality of the N-terminus of the spike, encompassing the receptor binding domain (RBD). We have used two assays, one an in vitro liposome binding assay and the other a tissue culture replication assay. The liposome binding assay shows that interaction of the receptor with spikes on virions at 37 degrees C causes a conformational change that makes the virions hydrophobic so that they bind to liposomes (B. D. Zelus, J. H. Schickli, D. M. Blau, S. R. Weiss, and K. V. Holmes, J. Virol. 77: 830-840, 2003). Recombinant viruses with spikes containing the RBD of either MHV-A59 or MHV-4 readily associated with liposomes at 37 degrees C in the presence of soluble mCEACAM1(a), except for S(4)R, which expresses the entire wild-type MHV-4 spike and associated only inefficiently with liposomes following incubation with soluble mCEACAM1(a). In contrast, soluble mCEACAM1(b) allowed viruses with the MHV-A59 RBD to associate with liposomes more efficiently than did viruses with the MHV-4 RBD. In the second assay, which requires virus entry and replication, all recombinant viruses replicated efficiently in BHK cells expressing mCEACAM1(a). In BHK cells expressing mCEACAM1(b), only viruses expressing chimeric spikes with the MHV-A59 RBD could replicate, while replication of viruses expressing chimeric spikes with the MHV-4 RBD was undetectable. Despite having the MHV-4 RBD, S(4)R replicated in BHK cells expressing mCEACAM1(b); this is most probably due to spread via CEACAM1 receptor-independent cell-to-cell fusion, an activity displayed only by S(4)R among the recombinant viruses studied here. These data suggest that the RBD domain and the rest of the spike must coevolve to optimize function in viral entry and spread.

Receptor-induced conformational changes of murine coronavirus spike protein

Although murine coronavirus mouse hepatitis virus (MHV) enters cells by virus-cell membrane fusion triggered by its spike (S) protein, it is not well known how the S protein participates in fusion events. We reported that the soluble form of MHV receptor (soMHVR) transformed a nonfusogenic S protein into a fusogenic one (F. Taguchi and S. Matsuyama, J. Virol. 76:950-958, 2002). In the present study, we demonstrate that soMHVR induces the conformational changes of the S protein, as shown by the proteinase digestion test. A cl-2 mutant, srr7, of the MHV JHM virus (JHMV) was digested with proteinase K after treatment with soMHVR, and the resultant S protein was analyzed by Western blotting using monoclonal antibody (MAb) 10G, specific for the membrane-anchored S2 subunit. A 58-kDa fragment, encompassing the two heptad repeats in S2, was detected when srr7 was digested after soMHVR treatment, while no band was seen when the virus was untreated. The appearance of the proteinase-resistant fragment was dependent on the temperature and time of srr7 incubation with soMHVR and also on the concentration of soMHVR. Coimmunoprecipitation indicated that the direct binding of soMHVR to srr7 S protein induced these conformational changes; this was also suggested by the inhibition of the changes following pretreatment of soMHVR with anti-MHVR MAb CC1. soMHVR induced conformational changes of the S proteins of wild-type (wt) JHMV cl-2, as well as revertants from srr7, srr7A and srr7B; however, a major proportion of these S proteins were resistant to proteinase K even without soMHVR treatment. The implications of this proteinase-resistant fraction are discussed. This is the first report on receptor-induced conformational changes of the membrane-anchored fragment of the coronavirus S protein.

Systematic assembly of a full-length infectious cDNA of mouse hepatitis virus strain A59

A novel method was developed to assemble a full-length infectious cDNA of the group II coronavirus mouse hepatitis virus strain A59 (MHV-A59). Seven contiguous cDNA clones that spanned the 31.5-kb MHV genome were isolated. The ends of the cDNAs were engineered with unique junctions and assembled with only the adjacent cDNA subclones, resulting in an intact MHV-A59 cDNA construct of approximately 31.5 kb in length. The interconnecting restriction site junctions that are located at the ends of each cDNA are systematically removed during the assembly of the complete full-length cDNA product, allowing reassembly without the introduction of nucleotide changes. RNA transcripts derived from the full-length MHV-A59 construct were infectious, although transfection frequencies were enhanced 10- to 15-fold in the presence of transcripts encoding the nucleocapsid protein N. Plaque-purified virus derived from the infectious construct replicated efficiently and displayed similar growth kinetics, plaque morphology, and cytopathology in murine cells as did wild-type MHV-A59. Molecularly cloned viruses recognized the MHV receptor (MHVR) for docking and entry, and pretreatment of cells with monoclonal antibodies against MHVR blocked virus entry and replication. Cells infected with molecularly cloned MHV-A59 virus expressed replicase (gene 1) proteins identical to those of laboratory MHV-A59. Importantly, the molecularly cloned viruses contained three marker mutations that had been derived from the engineered component clones. Full-length infectious constructs of MHV-A59 will permit genetic modifications of the entire coronavirus genome, particularly in the replicase gene. The method has the potential to be used to construct viral, microbial, or eukaryotic genomes approaching several million base pairs in length and used to insert restriction sites at any given nucleotide in a microbial genome.

Quaternary structure of coronavirus spikes in complex with carcinoembryonic antigen-related cell adhesion molecule cellular receptors

Oligomeric spike (S) glycoproteins extend from coronavirus membranes. These integral membrane proteins assemble within the endoplasmic reticulum of infected cells and are subsequently endoproteolyzed in the Golgi, generating noncovalently associated S1 and S2 fragments. Once on the surface of infected cells and virions, peripheral S1 fragments bind carcinoembryonic antigen-related cell adhesion molecule (CEACAM) receptors, and this triggers membrane fusion reactions mediated by integral membrane S2 fragments. We focused on the quaternary structure of S and its interaction with CEACAMs. We discovered that soluble S1 fragments were dimers and that CEACAM binding was entirely dependent on this quaternary structure. However, two differentially tagged CEACAMs could not co-precipitate with the S dimers, suggesting that binding sites were closely juxtaposed in the dimer (steric hindrance) or that a single CEACAM generated global conformational changes that precluded additional interactions (negative cooperativity). CEACAM binding did indeed alter S1 conformations, generating alternative disulfide linkages that were revealed on SDS gels. CEACAM binding also induced separation of S1 and S2. Differentially tagged S2 fragments that were free of S1 dimers were not co-precipitated, suggesting that S1 harbored the primary oligomerization determinants. We discuss the distinctions between the S.CEACAM interaction and other virus-receptor complexes involved in receptor-triggered entry.

Crystal structure of murine sCEACAM1a[1,4]: a coronavirus receptor in the CEA family

CEACAM1 is a member of the carcinoembryonic antigen (CEA) family. Isoforms of murine CEACAM1 serve as receptors for mouse hepatitis virus (MHV), a murine coronavirus. Here we report the crystal structure of soluble murine sCEACAM1a[1,4], which is composed of two Ig-like domains and has MHV neutralizing activity. Its N-terminal domain has a uniquely folded CC' loop that encompasses key virus-binding residues. This is the first atomic structure of any member of the CEA family, and provides a prototypic architecture for functional exploration of CEA family members. We discuss the structural basis of virus receptor activities of murine CEACAM1 proteins, binding of Neisseria to human CEACAM1, and other homophilic and heterophilic interactions of CEA family members.

Soluble receptor potentiates receptor-independent infection by murine coronavirus

Mouse hepatitis virus (MHV) infection spreads from MHV-infected DBT cells, which express the MHV receptor CEACAM1 (MHVR), to BHK cells, which are devoid of the receptor, by intercellular membrane fusion (MHVR-independent fusion). This mode of infection is a property of wild-type (wt) JHMV cl-2 virus but is not seen in cultures infected with the mutant virus JHMV srr7. In this study, we show that soluble MHVR (soMHVR) potentiates MHVR-independent fusion in JHMV srr7-infected cultures. Thus, in the presence of soMHVR, JHMV srr7-infected DBT cells overlaid onto BHK cells induce BHK cell syncytia and the spread of JHMV srr7 infection. This does not occur in the absence of soMHVR. soMHVR also enhanced wt virus MHVR-independent fusion. These effects were dependent on the concentration of soMHVR in the culture and were specifically blocked by the anti-MHVR monoclonal antibody CC1. Together with these observations, direct binding of soMHVR to the virus spike (S) glycoprotein as revealed by coimmunoprecipitation demonstrated that the effect is mediated by the binding of soMHVR to the S protein. Furthermore, fusion of BHK cells expressing the JHMV srr7 S protein was also induced by soMHVR. These results indicated that the binding of soMHVR to the S protein expressed on the DBT cell surface potentiates the fusion of MHV-infected DBT cells with nonpermissive BHK cells. We conclude that the binding of soMHVR to the S protein converts the S protein to a fusion-active form competent to mediate cell-cell fusion, in a fashion similar to the fusion of virus and cell membranes.

Targeted disruption of the Ceacam1 (MHVR) gene leads to reduced susceptibility of mice to mouse hepatitis virus infection

The CEACAM1 glycoproteins (formerly called biliary glycoproteins; BGP, C-CAM, CD66a, or MHVR) are members of the carcinoembryonic antigen family of cell adhesion molecules. In the mouse, splice variants of CEACAM1 have either two or four immunoglobulin (Ig) domains linked through a transmembrane domain to either a short or a long cytoplasmic tail. CEACAM1 has cell adhesion activity and acts as a signaling molecule, and long-tail isoforms inhibit the growth of colon and prostate tumor cells in rodents. CEACAM1 isoforms serve as receptors for several viral and bacterial pathogens, including the murine coronavirus mouse hepatitis virus (MHV) and Haemophilus influenzae, Neisseria gonorrhoeae, and Neisseria meningitidis in humans. To elucidate the mechanisms responsible for the many biological activities of CEACAM1, we modified the expression of the mouse Ceacam1 gene in vivo. Manipulation of the Ceacam1 gene in mouse embryonic stem cells that contained the Ceacam1a allele yielded a partial knockout. We obtained one line of mice in which the insert in the Ceacam1a gene had sustained a recombination event. This resulted in the markedly reduced expression of the two CEACAM1a isoforms with four Ig domains, whereas the expression of the two isoforms with two Ig domains was doubled relative to that in wild-type BALB/c (+/+) mice. Homozygous (p/p) Ceacam1a-targeted mice (Ceacam1aDelta4D) had no gross tissue abnormalities and were viable and fertile; however, they were more resistant to MHV A59 infection and death than normal (+/+) mice. Following intranasal inoculation with MHV A59, p/p mice developed markedly fewer and smaller lesions in the liver than +/+ or heterozygous (+/p) mice. The titers of virus produced in the livers were 50- to 100-fold lower in p/p mice than in +/p or +/+ mice. p/p mice survived a dose 100-fold higher than the lethal dose of virus for +/+ mice. +/p mice were intermediate between +/+ and p/p mice in susceptibility to liver damage, virus growth in liver, and susceptibility to killing by MHV. Ceacam1a-targeted mice provide a new model to study the effects of modulation of receptor expression on susceptibility to MHV infection in vivo.

Variations in disparate regions of the murine coronavirus spike protein impact the initiation of membrane fusion

The prototype JHM strain of murine hepatitis virus (MHV) is an enveloped, RNA-containing coronavirus that has been selected in vivo for extreme neurovirulence. This virus encodes spike (S) glycoproteins that are extraordinarily effective mediators of intercellular membrane fusion, unique in their ability to initiate fusion even without prior interaction with the primary MHV receptor, a murine carcinoembryonic antigen-related cell adhesion molecule (CEACAM). In considering the possible role of this hyperactive membrane fusion activity in neurovirulence, we discovered that the growth of JHM in tissue culture selected for variants that had lost murine CEACAM-independent fusion activity. Among the collection of variants, mutations were identified in regions encoding both the receptor-binding (S1) and fusion-inducing (S2) subunits of the spike protein. Each mutation was separately introduced into cDNA encoding the prototype JHM spike, and the set of cDNAs was expressed using vaccinia virus vectors. The variant spikes were similar to that of JHM in their assembly into oligomers, their proteolysis into S1 and S2 cleavage products, their transport to cell surfaces, and their affinity for a soluble form of murine CEACAM. However, these tissue culture-adapted spikes were significantly stabilized as S1-S2 heteromers, and their entirely CEACAM-dependent fusion activity was delayed or reduced relative to prototype JHM spikes. The mutations that we have identified therefore point to regions of the S protein that specifically regulate the membrane fusion reaction. We suggest that cultured cells, unlike certain in vivo environments, select for S proteins with delayed, CEACAM-dependent fusion activities that may increase the likelihood of virus internalization prior to the irreversible uncoating process.

The CEACAM1-L Ser503 residue is crucial for inhibition of colon cancer cell tumorigenicity

CEACAM1 (also known as biliary glycoprotein, C-CAM or CD66a) is a cell adhesion molecule of the immunoglobulin family behaving as a tumor inhibitory protein in colon, prostate, liver, endometrial and breast cancers. Inhibition of tumor development is dependent upon the presence of the long 71-73 amino acid cytoplasmic domain of the CEACAM1 protein (CEACAM1-L). We have recently defined a number of cis-acting motifs within the long cytoplasmic domain participating in tumor cell growth inhibition. These are Tyr488, corresponding to an Immunoreceptor Tyrosine-based Inhibition Motif, as well as the three terminal lysine residues of the protein. In this study, we provide evidence that treatment with phorbol esters leads to increased phosphorylation of in vivo (32)P-labeled CEACAM1-L in mouse CT51 carcinoma cells, in the mouse 1MEA 7R.1 liver carcinoma cells and in 293 human embryonic kidney cells transfected with the Ceacam1-L cDNA. Basal level Ser phosphorylation was abrogated by treatment with the staurosporine inhibitor, but not by the protein kinase C-specific inhibitor calphostin C or other inhibitors such as H7 or sphingosine. Specific inhibitors of protein kinase A or calmodulin kinase had only minimal effects on the levels of basal or PMA-induced Ser phosphorylation. Furthermore, PMA treatment of the CT51 cells induced cell spreading and cellular relocalization of the CEACAM1-L protein. Since Ser503 has been described as a PMA-induced phosphorylation site in other cell systems, we investigated whether Ser503 was involved in these responses in mouse intestinal cells. No differences were noticed in the basal or the PMA-induced phosphorylation levels, kinase inhibitor sensitivity or the PMA-induced relocalization of the protein between the wild-type and the Ser503Ala mutant CEACAM1-L. However, we provide evidence that Ser503 participates in CEACAM1-L-mediated tumor inhibition as its mutation to an Ala led to in vivo tumor development, contrary to the tumor inhibitory phenotype observed with the wild-type CEACAM1-L protein.