The human fibroblast receptor for gp86 of human cytomegalovirus is a phosphorylated glycoprotein

Impact of Annexin A2 on virus life cycles

Due to the limited size of viral genomes, hijacking host machinery by the viruses taking place throughout the virus life cycle is inevitable for the survival and proliferation of the virus in the infected hosts. Recent reports indicated that Annexin A2 (AnxA2), a calcium- and lipid-binding cellular protein, plays an important role as a critical regulator in various steps of the virus life cycle. The multifarious AnxA2 functions in cells, such as adhesion, adsorption, endocytosis, exocytosis, cell proliferation and division, inflammation, cancer metastasis, angiogenesis, etc., are intimately related to the various clinical courses of viral infection. Ubiquitous expression of AnxA2 across multiple cell types indicates the broad range of susceptibility of diverse species of the virus to induce disparate viral disease in various tissues, and intracellular expression of AnxA2 in the cytoplasmic membrane, cytosol, and nucleus suggests the involvement of AnxA2 in the regulation of the different stages of various virus life cycles within host cells. However, it is yet unclear as to the molecular processes on how AnxA2 and the infected virus interplay to regulate virus life cycles and thereby the virus-associated disease courses, and hence elucidation of the molecular mechanisms on AnxA2-mediated virus life cycle will provide essential clues to develop therapeutics deterring viral disease.

HCMV spread and cell tropism are determined by distinct virus populations

Human cytomegalovirus (HCMV) can infect many different cell types in vivo. Two gH/gL complexes are used for entry into cells. gH/gL/pUL(128,130,131A) shows no selectivity for its host cell, whereas formation of a gH/gL/gO complex only restricts the tropism mainly to fibroblasts. Here, we describe that depending on the cell type in which virus replication takes place, virus carrying the gH/gL/pUL(128,130,131A) complex is either released or retained cell-associated. We observed that virus spread in fibroblast cultures was predominantly supernatant-driven, whereas spread in endothelial cell (EC) cultures was predominantly focal. This was due to properties of virus released from fibroblasts and EC. Fibroblasts released virus which could infect both fibroblasts and EC. In contrast, EC released virus which readily infected fibroblasts, but was barely able to infect EC. The EC infection capacities of virus released from fibroblasts or EC correlated with respectively high or low amounts of gH/gL/pUL(128,130,131A) in virus particles. Moreover, we found that focal spread in EC cultures could be attributed to EC-tropic virus tightly associated with EC and not released into the supernatant. Preincubation of fibroblast-derived virus progeny with EC or beads coated with pUL131A-specific antibodies depleted the fraction that could infect EC, and left a fraction that could predominantly infect fibroblasts. These data strongly suggest that HCMV progeny is composed of distinct virus populations. EC specifically retain the EC-tropic population, whereas fibroblasts release EC-tropic and non EC-tropic virus. Our findings offer completely new views on how HCMV spread may be controlled by its host cells.

gH/gL complexes of herpesviruses are supposed to promote fusion of the viral envelope with cellular membranes. The gH/gL core complex associates with additional proteins which define the tropism for certain cell types by promoting binding to specific receptors. Two alternative gH/gL complexes of human cytomegalovirus (HCMV) define the cell tropism, the entry pathway and the spread of virus. Formation of a gH/gL/gO complex during infection determines release of infectious virus into the supernatant. The gH/gL/pUL(128,130,131A) complex determines the tropism for endothelial cells (EC) and promotes focal spread. Here, we could show that HCMV-infected cells produce EC-tropic and non EC-tropic virus populations. While fibroblasts release both populations into the supernatant, EC predominantly release the non EC-tropic population. Different host cells of HCMV thus may direct the distribution of virus progeny.

Heterogeneous pathways of maternal-fetal transmission of human viruses (review)

Several viruses can pass the maternal-fetal barrier, and cause diseases of the fetus or the newborn. Recently, however, it became obvious, that viruses may invade fetal cells and organs through different routes without acute consequences. Spermatozoa, seminal fluid and lymphocytes in the sperm may transfer viruses into the human zygotes. Viruses were shown to be integrated into human chromosomes and transferred into fetal tissues. The regular maternal-fetal transport of maternal cells has also been discovered. This transport might implicate that lymphotropic viruses can be released into the fetal organs following cellular invasion. It has been shown that many viruses may replicate in human trophoblasts and syncytiotrophoblast cells thus passing the barrier of the maternal-fetal interface. The transport of viral immunocomplexes had also been suggested, and the possibility has been put forward that even anti-idiotypes mimicking viral epitopes might be transferred by natural mechanisms into the fetal plasma, in spite of the selective mechanisms of apical to basolateral transcytosis in syncytiotrophoblast and basolateral to apical transcytosis in fetal capillary endothelium. The mechanisms of maternal-fetal transcytosis seem to be different of those observed in differentiated cells and tissue cultures. Membrane fusion and lipid rafts of high cholesterol content are probably the main requirements of fetal transcytosis. The long term presence of viruses in fetal tissues and their interactions with the fetal immune system might result in post partum consequences as far as increased risk of the development of malignancies and chronic pathologic conditions are discussed.

Characterization of human cytomegalovirus glycoprotein-induced cell-cell fusion

Human cytomegalovirus (CMV) infection is dependent on the functions of structural glycoproteins at multiple stages of the viral life cycle. These proteins mediate the initial attachment and fusion events that occur between the viral envelope and a host cell membrane, as well as virion-independent cell-cell spread of the infection. Here we have utilized a cell-based fusion assay to identify the fusogenic glycoproteins of CMV. To deliver the glycoprotein genes to various cell lines, we constructed recombinant retroviruses encoding gB, gH, gL, and gO. Cells expressing individual CMV glycoproteins did not form multinucleated syncytia. Conversely, cells expressing gH/gL showed pronounced syncytium formation, although expression of gH or gL alone had no effect. Anti-gH neutralizing antibodies prevented syncytium formation. Coexpression of gB and/or gO with gH/gL did not yield detectably increased numbers of syncytia. For verification, these results were recapitulated in several cell lines. Additionally, we found that fusion was cell line dependent, as nonimmortalized fibroblast strains did not fuse under any conditions. Thus, the CMV gH/gL complex has inherent fusogenic activity that can be measured in certain cell lines; however, fusion in fibroblast strains may involve a more complex mechanism involving additional viral and/or cellular factors.

Integrin alphavbeta3 is a coreceptor for human cytomegalovirus

Human cytomegalovirus (HCMV) is a widespread opportunistic pathogen that causes birth defects in newborns and severe disease in immunocompromised individuals. The broad tropism of HCMV infection suggests that it uses multiple receptors. We recently showed that the epidermal growth factor receptor (EGFR) serves as a receptor for HCMV. Here we show that HCMV also uses integrin alphavbeta3 as a coreceptor. Upon infection, HCMV glycoproteins gB and gH independently bind to EGFR and alphavbeta3, respectively, to initiate viral entry and signaling. Alphavbeta3 then translocates to lipid rafts where it interacts with EGFR to induce coordinated signaling. The coordination between EGFR and alphavbeta3 is essential for the early events of HCMV infection, including viral entry, RhoA downregulation, stress-fiber disassembly and viral nuclear trafficking. Our findings support a model in which EGFR and alphavbeta3 work together as coreceptors for HCMV entry and signaling. This discovery is fundamental to understanding HCMV pathogenesis and developing treatment strategies targeted to viral receptors.

Cloning and epitope mapping of a functional partial fusion receptor for human cytomegalovirus gH

A cDNA clone encoding a partial putative human cytomegalovirus (HCMV) gH fusion receptor (CMVFR) was previously identified. In this report, the cDNA sequence of CMVFR was determined and the role of this CMVFR in HCMV/cell fusion was confirmed by rendering fusion-incompetent MOLT-4 cells susceptible to fusion following transfection with receptor cDNA. Blocking experiments using recombinant gH or either of two MAbs (against recombinant gH or purified viral gH:gL) provided additional evidence for the role of gH binding to this protein in virus fusion. An HCMV-binding domain of 12 aa in the middle hydrophilic region of CMVFR was identified by fusion blocking studies using synthetic receptor peptides. The 1368 bp cDNA of CMVFR contained a predicted ORF of 345 aa with two potential membrane-spanning domains and several possible nuclear localization signals. A search of sequence databases indicated that CMVFR is a novel protein. Further characterization of this cell membrane protein that confers susceptibility to fusion with the viral envelope should provide important information about the mechanism by which HCMV infects cells.

Free thiol groups are essential for infectivity of human cytomegalovirus

The membrane-impermeable thiol blocker 5'5-dithiobis 2- nitrobenzoic acid (DTNB) blocked infectivity of human cytomegalovirus (CMV) although the virus still bound to cells. DTNB-treated CMV regained 65% of its infectivity after incubation with the disulfide bond-reducing agent dithiothreitol. These observations suggest that free thiol groups on CMV are required for infectivity and may participate in disulfide bond formation during virus entry.

Annexin II enhances cytomegalovirus binding and fusion to phospholipid membranes

A number of studies have suggested that the anionic phospholipid (anPL)-binding protein annexin II may play a role in cytomegalovirus (CMV) infection. Since annexin II has been shown to mediate aggregation and fusion of certain membranes, we investigated whether these properties could be exploited by CMV directly. The experiments showed that purified annexin II, but not the homologous protein annexin V (AnV), can mediate the binding of 35S-CMV (strain AD169) to anPL-coated microtiter wells. This association required Ca2+, could be titrated by varying either annexin II (apparent Kd = 4 x 10(-)8 M) or 35S-CMV, was inhibited by unlabeled CMV, and was observed for the heterotetrameric or monomeric form of annexin II. In experiments utilizing the fluorescence dequenching of octadecyl rhodamine incorporated into the CMV envelope, annexin II was furthermore found to enhance the rate of virus-anPL vesicle fusion. The observed fusion was dependent on the concentration of annexin II, Ca2+, and anPL and was mediated principally by the heterotetramer. Interestingly, AnV was observed to inhibit the effects of annexin II on CMV fusion but not binding to anPL, which indicates that annexin II enhances these processes by distinct mechanisms. The results presented here provide the first direct evidence that annexin II has the capacity to bridge CMV to a phospholipid membrane and to enhance virus-membrane fusion. These observations furthermore suggest that AnV may regulate the fusogenic function of annexin II.

Receptor-binding properties of a soluble form of human cytomegalovirus glycoprotein B

The human cytomegalovirus (HCMV) glycoprotein B (gB) (also known as gpUL55) homolog is an important mediator of virus entry and cell-to-cell dissemination of infection. To examine the potential ligand-binding properties of gB, a soluble form of gB (gB-S) was radiolabeled, purified, and tested in cell-binding experiments. Binding of gB-S to human fibroblast cells was found to occur in a dose-dependent, saturable, and specific manner. Scatchard analysis demonstrated a biphasic plot with the following estimated dissociation constants (Kd): Kd1, 4.96 x 10(-6) M; Kd2, 3.07 x 10(-7) M. Cell surface heparan sulfate proteoglycans (HSPGs) were determined to serve as one class of receptors able to facilitate gB-S binding. Both HSPG-deficient Chinese hamster ovary (CHO) cells and fibroblast cells with enzymatically removed HSPGs had 40% reductions in gB-S binding, whereas removal of chondroitin sulfate had no effect. However, a significant proportion of gB-S was able to associate with the cell surface in the absence of HSPGs via an undefined nonheparin component. Binding affinity analysis of gB-S binding to wild-type CHO-K1 cells demonstrated biphasic binding kinetics (Kd1, 9.85 x 10(-6) M; Kd2, 4.03 x 10(-8) M), whereas gB-S binding to HSPG-deficient CHO-677 cells exhibited single-component binding kinetics (Kd, 7.46 x 10(-6) M). Together, these data suggest that gB-S associates with two classes of cellular receptors. The interaction of gB with its receptors is physiologically relevant, as evidenced by an inhibitory effect on HCMV entry when cells were pretreated with purified gB-S. This inhibition was determined to be manifested at the level of virus attachment. We conclude that gB is a ligand for HCMV that mediates an interaction with a cellular receptor(s) during HCMV infection.

Direct interaction between human cytomegalovirus glycoprotein B and cellular annexin II

Cellular annexin II has been shown to specifically bind human cytomegalovirus (HCMV) and be a component of highly purified virions. In this report, we characterize the interaction of annexin II with HCMV. We found that the binding of annexin II to the HCMV envelope occurs partially through the calcium-dependent phospholipid-binding ability of annexin II since some annexin II was dissociated from virions with chelating agents. However, a substantial proportion of virion-associated annexin II was resistant to chelation, which suggested a calcium-independent interaction between annexin II and an HCMV envelope component. The search for a nonphospholipid component to account for this binding led to the discovery that HCMV glycoprotein B (gpUL55) (gB) can physically interact with annexin II. We present three lines of evidence to support the conclusion that HCMV gB can bind host cell annexin II.

The human cytomegalovirus UL55 (gB) and UL75 (gH) glycoprotein ligands initiate the rapid activation of Sp1 and NF-kappaB during infection

The cellular transcription factors Sp1 and NF-kappaB were upregulated shortly after the binding of purified live or UV-inactivated human cytomegalovirus (HCMV) to the cell surface. The rapid time frame of transcription factor induction is similar to that seen in other systems in which cellular factors are induced following receptor-ligand engagement. This similarity suggested that a cellular receptor-viral ligand interaction might be involved in Sp1 and NF-kappaB activation during the earliest stages of HCMV infection. To focus on the possible role viral ligands play in initiating cellular events following infection, we first used purified viral membrane extracts to demonstrate that constituents on the membrane are responsible for cellular activation. Additionally, these studies showed, through the use of neutralizing antibodies, that the viral membrane mediators of this activation are the major envelope glycoproteins gB (UL55) and gH (UL75). To confirm these results, neutralizing anti-gB and -gH antibodies were used to block the interactions of these glycoproteins on whole purified virus with their cell surface receptors. In so doing, we found that Sp1 and NF-kappaB induction was inhibited. Lastly, through the use of purified viral gB protein and an anti-idiotypic antibody that mimics the image of the viral gH protein, it was found that the engagement of individual viral ligands with their appropriate cell surface receptors was sufficient to activate cellular Sp1 and NF-kappaB. These results support our hypothesis that HCMV glycoproteins mediate an initial signal transduction pathway which leads to the upregulation of host cell transcription factors and suggests a model wherein the orderly sequence of virus-mediated changes in cellular activation initiates with viral binding via envelope glycoproteins to the cognate cellular receptor(s).

Evidence that neomycin inhibits human cytomegalovirus infection of fibroblasts

The effect of phosphoinositide-binding aminoglycosides, such as neomycin, gentamicin and streptomycin, on human cytomegalovirus (HCMV) infection of human fibroblasts MRC-5 was studied. The inhibition of HCMV infection was obtained with all of these molecules but neomycin was more effective than the others. We showed that the inoculation of the cells with cell-free viral suspension in presence of neomycin concentrations above 5 mM at 37 degrees C, inhibited more than 98% the HCMV infection. However, the preincubation of the fibroblasts with neomycin at 4 degrees C, before the removal of the drug and the inoculation of the cells, induced only a 30% decrease in the number of infected cells. Addition of neomycin after the HCMV-binding at 4 degrees C or the infection of the cells was less efficient to inhibit HCMV infection than the standard incubation of neomycin during inoculation of the fibroblasts. Indeed, 1 hour after the inoculation of the cells at 37 degrees C, neomycin still inhibited HCMV infection, but 4 hours after the inoculation, this drug had no effect on HCMV infection. Our findings demonstrated that neomycin must be present at the time of infection in order to exert a full inhibiting effect. The effect of neomycin on the HCMV infection was almost immediate upon the addition of the drug (binding and/or internalization) and after the virus internalization (inhibition of immediate-early events). We suggest that neomycin and other aminoglycoside antibiotics may interact with HCMV glycoproteins for binding to similar structural features of cell surface heparan sulfate proteoglycans and may inhibit HCMV infection in fibroblasts by disrupting phosphoinositide-mediated events in the cells.

Human cytomegalovirus carries serine/threonine protein phosphatases PP1 and a host-cell derived PP2A

Human cytomegalovirus (CMV), a herpesvirus, is an important cause of morbidity and mortality in immunocompromised patients. When studying hyper-immediate-early events after contact between CMV virions and the cell membrane, we observed a hypophosphorylation of cellular proteins within 10 min. This can be explained in part by our finding that purified CMV contains serine/threonine protein phosphatase activities. Biochemical analyses indicate that this protein phosphatase activity has all characteristics of type 1 and 2A protein phosphatases (PP1 and PP2A). Specifically, PP1 accounts for approximately 30% and PP2A accounts for the remaining 70% of the phosphorylase phosphatase activity found. CMV produced in astrocytoma cells stably expressing an amino-terminally tagged PP2A catalytic subunit contained tagged enzyme, thus demonstrating the cellular origin of CMV-associated PP2A. PP2A is specifically found inside the virus, associated with the nucleocapsid fraction. Western blot (immunoblot) analysis of purified virus revealed the presence of the catalytic subunits of PP2A and PP1. Furthermore, the catalytic subunit of PP2A appears to be complexed to the regulatory subunits PR65 and PR55, which is also the most abundant configuration of this enzyme found in the host cells. Incubation of virus with okadaic acid before contact of CMV with cells prevented hypophosphorylation of cellular proteins, thus demonstrating the role of CMV-associated phosphatases in this phenomenon. CMV can thus transport an active enzyme from one cell to another.

Cell membrane bound N-acetylneuraminic acid is involved in the infection of fibroblasts and phorbol-ester differentiated monocyte-like cells with human cytomegalovirus (HCMV)

We focused on the role of membrane bound sugar residues in the infection of fibroblasts and monocyte-like cells with human cytomegalovirus (HCMV). Treatment of phorbol 12-myristate 13-acetate (PMA) differentiated monocyte-like cells THP-1 or human fibroblasts MRC-5 with lectins specific for N-acetylneuraminic acid (NeuAc) blocked infection with HCMV. HCMV failed to infect sialidase-treated differentiated THP-1 cells or MRC-5 cells. By using NeuAc, N-glycolylneuraminic acid (NeuGl) and alpha 2-3, but not alpha 2-6, sialyl-oligosaccharide, the infection of cells was less efficient. NeuAc was more potent inhibitor than NeuGl. These observations suggest that the sialic acid specificity and the nature of the interglycosidic linkage at the end of the complex carbohydrates may play an important role. Analogous experiments indicated that HCMV binds to N-acetylglucosamine (GlcNAc) in addition to NeuAc. Human cytomegalovirus infection in differentiated THP-1 cells and in human fibroblasts was inhibited by incubation of the virus with 20 micrograms/ml of heparin before and during the adsorption period. Treatment of the cells with heparinase or heparitinase inhibited infection with HCMV. We emphasized the role of NeuAc and GlcNAc and heparan sulfate proteoglycans at the surface of the cells, in the early steps of infection of both human fibroblasts and PMA differentiated monocyte-like cells with HCMV.

Host cellular annexin II is associated with cytomegalovirus particles isolated from cultured human fibroblasts

A significant amount of host cellular annexin II was found to be associated with human cytomegalovirus isolated from cultured human fibroblasts (approximately 1,160 molecules per virion). This composition was established by four different analytical approaches that included (i) Western blot (immunoblot) analysis of gradient-purified virions with a monoclonal antibody specific for annexin II, (ii) peptide mapping and sequence analysis of virus-associated proteins and proteins dissociated from virus following EDTA treatment, (iii) electron microscopic immunocytochemistry of gradient-purified virions, and (iv) labeling of virus-associated proteins by lactoperoxidase-catalyzed radioiodination. These results indicated that annexin II was primarily localized to the viral surface, where it bound in a divalent cation-dependent manner. In functional experiments, a rabbit antiserum raised against annexin II inhibited cytomegalovirus plaque formation in human foreskin fibroblast monolayers in a concentration-dependent manner. Cumulatively, these studies demonstrate an association of host annexin II with cytomegalovirus particles and provide evidence for the involvement of this cellular protein in virus infectivity.

Increases in [Ca2+]i mediated by the 92.5-kDa putative cell membrane receptor for HCMV gp86

We determined that changes in intracellular Ca2+ concentration ([Ca2+]i) occur in human fibroblasts within the first hour of human cytomegalovirus (HCMV) infection when viral adsorption and fusion take place, and we investigated whether such changes also occur in response to monoclonal anti-idiotype antibodies (MAb2) that mimic HCMV gp86 and bind to a 92.5-kDa putative cell membrane receptor for gp86. Digitized image analysis of fura 2-loaded human embryonic lung fibroblasts indicated specific transient increases in [Ca2+]i beginning in some cells within the first 5 min of incubation with cross-linked MAb2 (70-750 nM, P < 0.01), which were similar in timing and intracellular distribution to those induced by HCMV. A primary source of Ca2+ appeared to be intracellular Ca2+ stores, since prior depletion of these stores with 30 nM thapsigargin inhibited the response (91.7 +/- 8.6%, P < 0.01); influx of Ca2+ from the extracellular medium was apparently necessary to maintain the intracellular Ca2+ stores. Transient increases in inositol trisphosphate (IP3) occurred in response to MAb2 (up to 3,329 +/- 84%, P < 0.001) or HCMV (92.8 +/- 19%, P < 0.01) during this same time period. These data suggest that the 92.5-kDa receptor for HCMV gp86 mediates an increase in IP3 and subsequent release of Ca2+ from intracellular stores.