Fusion activity of virions of murine leukemia virus

Acquisition and Exaptation of Endogenous Retroviruses in Mammalian Placenta

Endogenous retroviruses (ERVs) are retrovirus-like sequences that were previously integrated into the host genome. Although most ERVs are inactivated by mutations, deletions, or epigenetic regulation, some remain transcriptionally active and impact host physiology. Several ERV-encoded proteins, such as Syncytins and Suppressyn, contribute to placenta acquisition, a crucial adaptation in mammals that protects the fetus from external threats and other risks while enabling the maternal supply of oxygen, nutrients, and antibodies. In primates, Syncytin-1 and Syncytin-2 facilitate cell-cell fusion for placental formation. Suppressyn is the first ERV-derived protein that inhibits cell fusion by binding to ASCT2, the receptor for Syncytin-1. Furthermore, Syncytin-2 likely inserted into the genome of the common ancestor of Anthropoidea, whereas Syncytin-1 and Suppressyn likely inserted into the ancestor of catarrhines; however, they were inactivated in some lineages, suggesting that multiple exaptation events had occurred. This review discusses the role of ERV-encoded proteins, particularly Syncytins and Suppressyn, in placental development and function, focusing on the integration of ERVs into the host genome and their contribution to the genetic mechanisms underlying placentogenesis. This review provides valuable insights into the molecular and genetic aspects of placentation, potentially shedding light on broader evolutionary and physiological processes in mammals.

Kinetic analyses of the surface-transmembrane disulfide bond isomerization-controlled fusion activation pathway in Moloney murine leukemia virus

The surface (SU) and transmembrane (TM) subunits of Moloney murine leukemia virus (Mo-MLV) Env are disulfide linked. The linking cysteine in SU is part of a conserved CXXC motif in which the other cysteine carries a free thiol. Recently, we showed that receptor binding activates its free thiol to isomerize the intersubunit disulfide bond into a disulfide within the motif instead (M. Wallin, M. Ekström and H. Garoff, EMBO J. 23:54-65, 2004). This facilitated SU dissociation and activation of TM for membrane fusion. The evidence was mainly based on the finding that alkylation of the CXXC-thiol prevented isomerization. This arrested membrane fusion, but the activity could be rescued by cleaving the intersubunit disulfide bond with dithiothreitol (DTT). Here, we demonstrate directly that receptor binding causes SU-TM disulfide bond isomerization in a subfraction of the viral Envs. The kinetics of the isomerization followed that of virus-cell membrane fusion. Arresting the fusion with lysophosphatidylcholine did not arrest isomerization, suggesting that isomerization precedes the hemifusion stage of fusion. Our earlier finding that native Env was not possible to alkylate but required isomerization induction by receptor binding intimated that alkylation trapped an intermediate form of Env. To further clarify this possibility, we analyzed the kinetics by which the alkylation-sensitive Env was generated during fusion. We found that it followed the fusion kinetics. In contrast, the release of fusion from alkylated, isomerization-blocked virus by DTT reduction of the SU-TM disulfide bond was much faster. These results suggest that the alkylation-sensitive form of Env is a true intermediate in the fusion activation pathway of Env.

Requirements for different components of the host cell cytoskeleton distinguish ecotropic murine leukemia virus entry via endocytosis from entry via surface fusion

Murine ecotropic leukemia viruses use a common receptor for entry into host cells; however, the site of virus fusion appears to differ with the host cell. Entry in mouse NIH 3T3 fibroblasts is by endocytosis, whereas entry in rat XC sarcoma cells is by surface fusion. We report here the identification of a step common to both entry pathways, as well as of a step unique to the endocytic pathway. Recent demonstration of the clustering of the virus receptor on rat cells suggested a possible interaction of the receptor with the cellular cytoskeleton (M. H. Woodard, W. A. Dunn, R. O. Laine, M. Malandro, R. McMahon, O. Simell, E. R. Block, and M. S. Kilberg, Am. J. Physiol. 266:E817-E824, 1994). We tested the hypothesis that such an interaction might influence receptor function. We found that entry into NIH 3T3 and XC cells was greatly diminished by the disruption of the actin network before but not shortly after virus internalization, suggesting the actin network plays a critical role in an early step common to both entry pathways. Disruption of microtubules before and shortly after virus internalization markedly reduced entry in NIH 3T3 cells, while entry into XC cells remained efficient. These data suggest that intact microtubules are required in a postpenetration step unique to efficient virus entry via endocytosis. The physiological function of the receptor was not affected by disruption of either the actin network or the microtubules, as the uptake of cationic amino acids in NIH 3T3 and XC cells was comparable to that in control cells even when the cytoskeleton remained disrupted for as long as 3 h.

A domain of murine retrovirus surface protein gp70 mediates cell fusion, as shown in a novel SC-1 cell fusion system

Virus-induced cell fusion of the fusion-from-without type was observed in SC-1 cells infected with Moloney murine leukemia virus when grown in NIH 3T3 cells. Replication-competent virus mutants with altered surface protein gp70 were examined. Fusion mutations were found in the proline-rich region of gp70. They acted on a step after binding and before or during endocytosis. The fusion mutants had an altered gp70 isomer pattern, presumably caused by different glycosylation. Other mutants with deleted glycans were analyzed, and some which also showed defective fusion were found. The interrelationship of the proline-rich region, glycosylation, and fusion is discussed.

Cell fusion induced by the murine leukemia virus envelope glycoprotein

To determine whether ecotropic murine leukemia virus (MuLV) envelope glycoproteins are sufficient to cause cell-to-cell fusion when expressed in the absence of virus production, we used an ecotropic MuLV, AKV, to construct env expression vectors that lack the gag and pol genes. The rat cell line XC, which undergoes cell-to-cell fusion upon infection with ecotropic MuLV, was transfected with wild-type env expression vectors, and high levels of syncytium formation resulted. Transfection of the murine cell line NIH 3T3 with expression vectors containing the wild-type or mutated env region did not result in syncytium formation. Immunoprecipitation analysis of the envelope glycoproteins expressed in NIH 3T3 and XC cells showed that the mature surface glycoprotein expressed in XC cells was of a much lower apparent molecular weight than that expressed in NIH 3T3 cells. Further characterization showed that most if not all of this difference was the result of differences in glycosylation. Finally, site-directed mutagenesis was used to introduce several conservative and nonconservative changes into the amino-terminal region of the transmembrane protein. Analysis of the effect of these mutations confirmed that this region is a fusion domain.

Cationic liposomes (Lipofectin) mediate retroviral infection in the absence of specific receptors

We have used cationic liposomes (Lipofectin) to facilitate retrovirus infection of cells lacking the homologous viral receptor. Ecotropic murine leukemia virus and packaged retroviral vectors were shown to infect mink cells, and amphotropic packaged retroviral vectors were shown to infect hamster cells in the presence of Lipofectin but not in the presence of Polybrene. Lipofectin-mediated infection of cells lacking the homologous receptor results in a titer approximately 0.1% of the titer in cells with the homologous receptor, using the standard Polybrene protocol. The use of Lipofectin may provide a simple means to experimentally infect a wide variety of cells with viruses not normally infectious for the species, tissue, or cell type of interest.

Antibody-mediated binding of a murine ecotropic Moloney retroviral vector to human cells allows internalization but not the establishment of the proviral state

We have attempted to introduce a neomycin-resistance gene, recombined in a murine (ecotropic) Moloney retrovirus, into HEp2 human cells which are resistant to infection by this retrovirus. To allow the binding of the retroviral vector on HEp2 cells, we have tested several antibody constructions between two monoclonal antibodies, one (mAb 5E9) directed against the human transferrin receptor and the other (mAb 615) directed against the gp70 envelope viral protein. The crosslinking of mAbs 5E9 and 615 by a sheep anti-murine kappa light chain antibody allowed the binding of virus on HEp2 cells. After incubation at 37 degrees, virus was internalized by HEp2 cells. However, no neomycin-resistant colonies of HEp2 cells have been obtained under various conditions. Our results suggest that binding and internalization of a retrovirus are not sufficient for establishment of the proviral state in cells which do not express a specific receptor.

Cell surface effects of human immunodeficiency virus

Cell killing by human immunodeficiency virus (HIV) is thought to contribute to many of the defects of the acquired immunodeficiency syndrome (AIDS). Two types of cytopathology are observed in HIV-infected cultured cells: cell-cell fusion and killing of single cells. Both killing processes appear to involve cell surface effects of HIV. A model is proposed for the HIV-mediated cell surface processes which could result in cell-cell fusion and single cell killing. The purpose of this model is to define the potential roles of individual viral envelope and cell surface molecules in cell killing processes and to identify alternative routes to the establishment of persistently-infected cells. Elucidation of HIV-induced cell surface effects may provide the basis for a rational approach to the design of antiviral agents which are selective for HIV-infected cells.

Ecotropic murine leukemia virus-induced fusion of murine cells

Extensive fusion occurs upon cocultivation of murine fibroblasts producing ecotropic murine leukemia viruses (MuLVs) with a large variety of murine cell lines in the presence of the polyene antibiotic amphotericin B, the active component of the antifungal agent Fungizone. The resulting polykaryocytes contain nuclei from both infected and uninfected cells, as evidenced by autoradiographic labeling experiments in which one or the other parent cell type was separately labeled with [3H]thymidine and fused with an unlabeled parent. This cell fusion specifically requires the presence of an ecotropic MuLV-producing parent and is not observed for cells producing xenotropic, amphotropic, or dualtropic viruses. Mouse cells infected with nonecotropic viruses retain their sensitivity toward fusion, whereas infection with ecotropic viruses abrogates the fusion of these cells upon cocultivation with other ecotropic MuLV-producing cells. Nonmurine cells lacking the ecotropic gp70 receptor are not fused under similar conditions. Fusion is effectively inhibited by monospecific antisera to gp70, but not by antisera to p15(E), and studies with monoclonal antibodies identify distinct amino- and carboxy-terminal gp70 regions which play a role in the fusion reaction. The enhanced fusion which occurs in the presence of amphotericin B provides a rapid and sensitive assay for the expression of ecotropic MuLVs and should facilitate further mechanistic studies of MuLV-induced fusion of murine cells.

Infectious entry of murine retroviruses into mouse cells: evidence of a postadsorption step inhibited by acidic pH

The entry into cells by many enveloped RNA viruses is accomplished by endocytosis and subsequent penetration of the endosomal membrane by an acidic pH-dependent fusion event. In the current study, we examined early events in the infectious entry of mouse retroviruses, using as a framework the observation that infection of a mouse tail skin cell line by the ecotropic virus Friend murine leukemia virus was inhibited at mildly acidic pH (pH 6). This inhibition operated on a postadsorption step, since binding of virus was unaffected at this pH. The rate of penetration of preadsorbed virus, which displayed first-order kinetics, was markedly affected by changes in the pH of the medium. The half-time for disappearance of infectious cell surface virus at 37 degrees C was approximately 10 min at pH 7.6. At pH 6.0, however, greater than 98% of the adsorbed infectivity remained at the cell surface after 45 min. This cell surface virus, though not infecting the cell at pH 6.0, retained its capacity to enter and infect the cell when the pH of the medium was raised. Acidic pH had little effect on the rate of fluid uptake by the cells, as measured by internalization of [3H]sucrose, indicating that global inhibition of endocytosis had not occurred. In contrast, cell fusion induced by Friend murine leukemia virus was optimal at pH 7.6 but markedly inhibited at a pH of less than 6.4. This inhibitory effect of acidic pH on membrane fusion is unique among the enveloped viruses which have been studied and would preclude entry of Friend murine leukemia virus from within acidified endocytic vesicles. Entry of other members of the ecotropic, mink cell focus-forming, and xenotropic host range groups displayed similar pH sensitivity. However, one xenotropic virus was relatively resistant to the effect of acidic pH, suggesting that differences might exist in the requirements for entry of different retroviruses.

Two base changes restore infectivity to a noninfectious molecular clone of Moloney murine leukemia virus (pMLV-1)

The complete nucleotide sequence of a molecular clone of Moloney murine leukemia virus (pMLV-1) has previously been reported (Shinnick et al., Nature [London] 293:543-548, 1981). However, pMLV-1 does not generate infectious virus after transfection into cells (Berns et al., J. Virol. 36:254-263, 1980). The lesion in pMLV-1 has been localized by determining the biological activity of recombinants containing DNA from an infectious clone of Moloney murine leukemia virus (pMLV-48) and pMLV-1. Replacement of a 1.0-kilobase pair region which spans the gag-pol junction of pMLV-1 with the corresponding DNA fragment from the infectious clone restores its infectivity. Nucleotide sequence analysis of this fragment obtained from the infectious clone (pMLV-48) and pMLV-1 reveals two single base pair changes, one in the p30gag gene and the other in the 5' end of the pol gene. The mutation in the pol gene does not affect the production of infectious virus but renders them XC negative, whereas the mutation in the gag gene appears to be lethal. The complete nucleotide sequence of an infectious clone of Moloney murine leukemia virus can now be deduced.

Molecular properties of a gag- pol- env+ murine leukemia virus from cultured AKR lymphoma cells

We have described the isolation of a replication-defective murine leukemia virus from a culture of AKR lymphoma cells [Rein et al., Nature (London) 282:753-754, 1979]. To facilitate the characterization of this murine leukemia virus, we transmitted it to mink cells and analyzed its genome by restriction mapping of the mink cellular DNA. This genome resembled the Akv genome quite closely, but it had an additional KpnI cleavage site at 1.3 kilobase pairs from the 5' end of the provirus and a small (approximately 50-base-pair) deletion between 1.8 and 3.0 kilobase pairs from the 5' end. When we tested these mink cells by immune precipitation or by competition radioimmunoassay, we found that they synthesized gPr82env, but contained no detectable gag or pol proteins. It seems likely that the KpnI cleavage site at 1.3 kilobase pairs reflects an abnormal sequence at or near the beginning of the gag gene, which prevents gag or pol translation by introducing a frameshift or termination codon into this region.

Early syncytium formation by bovine leukemia virus

Bovine leukemia virus (BLV) from either persistently infected bat cells or fetal lamb kidney cells induced rapid syncytium formation in F81 indicator cells. Distinct syncytia were seen within 2 h after inoculation of cells with highly concentrated (500-fold) cell-free BLV preparations and within 4 to 8 h when unconcentrated cell-free BLV preparations were used. Indicator cell densities of 1 x 10(5) to 2 x 10(5) were optimal for rapid and maximal syncytium formation. Pretreatment of BLV with reference BLV leukemic serum and antiserum prepared against purified BLV significantly inhibited (95%) syncytium formation. Reference bovine viral diarrhea virus serum, foamy-like bovine syncytial virus serum, and control serum had little effect (17% inhibition). Antiserum to BLV gp51 inhibited syncytium formation by greater than 96%, whereas antiserum to BLV p24 reduced syncytium activity to a much lesser extent (38% inhibition). Treatment of BLV with beta-propiolactone (0.005 to 0.05%) had little or no effect upon syncytium-forming activity, whereas UV irradiation (15 ergs/mm(2) per s for 30 min) reduced, but did not completely destroy, the fusion activity. However, both beta-propiolactone and UV irradiation drastically reduced the replication potential of BLV, as demonstrated by the lack of p24 expression in the inoculated cells. Concentrations of cycloheximide, cytosine arabinoside, tunicamycin, and 2-deoxy-D-glucose which effectively blocked cellular macromolecular synthesis did not significantly inhibit syncytium formation. These latter results suggested that de novo protein and DNA synthesis as well as protein glycosylation were not required for early syncytium formation. Thus, these experiments demonstrated that replication of BLV by the indicator cells was not essential for cell fusion.

Glycopeptides of murine leukemia viruses. II. Comparison of xenotropic and dual-tropic viruses

The glycosylation patterns of the gp70 glycoproteins of xenotropic and dualtropic murine leukemia virus (MuLV) were compared with those of ecotropic viruses. Ecotropic viruses contain a large glycopeptide size class designated G1 (molecular weight, approximately 5100), and such glycopeptides were not detected in xenotropic viruses grown in mink cells nor in dual-tropic viruses grown in mouse or mink lung cells. Both xenotropic and dual-tropic MuLV had glycopeptide size classes designated G2, G3, and G4 (molecular weights, approximately 2900, 2,200, and 1,500, respectively). G2 glycopeptides of xenotropic and dual-tropic MuLV were shown to be resistant to endo-beta-N-acetylglucosaminidase H, whereas G3 and G4 glycopeptides were susceptible. The relative abudance of glycopeptide G3 was increased in xenotropic and dual-tropic viruses as compared with ecotropic viruses, whereas the relative amount of G4 was decreased in xenotropic viruses. The similarity in the glycosylation patterns of a number of xenotropic and dual-tropic viruses suggests that glycosylation sites are highly conserved within the env gene products of each of these classes of viruses.