Isolation and characterization of a mouse cell line containing a defective Moloney murine leukemia virus genome

Truncated gag products encoded by Gv-1-responsive endogenous retrovirus loci

The conversion of endogenous or exogenous murine retroviruses to a leukemogenic phenotype involves recombination with retroviral sequences present in host genomic DNA. In the 129 Gix+ inbred strain, these endogenous sequences are replication defective but still express retroviral proteins under the apparent transcriptional control of the Gv-1 regulatory locus. To study the protein-coding potential of Gv-1-regulated endogenous retroviral loci, we used oligonucleotide probes directed to env deletion breakpoints identified in previously characterized cDNA clones. Four endogenous retroviral loci were isolated from a library of 129 Gix+ genomic DNA with these probes. Three loci cloned with the env deletion probe del env-1 had virtually identical proviral inserts by restriction analysis. A unique locus was identified and cloned with the del env-2 probe, which must therefore represent a Gv-1-responsive element. Restriction enzyme and nucleotide sequence analyses indicated that the del env-1 and del env-2 loci represented members of the polytropic and modified polytropic classes of endogenous retrovirus, respectively. Despite this divergence, members of both classes contained identical deletions of 19 nucleotides within p30gag and of 1,474 nucleotides from p10gag into the reverse transcriptase-coding region of pol, suggesting that a recombination event had occurred between these proviral sequences prior to insertion within the genome. The del env-1 and del env-2 loci retained coding capacity for truncated gag polyproteins, confirmed by in vitro translation and immunoprecipitation of the protein products. Nucleotide sequence comparison of the untranslated leader (L) regions of the del env-1 and del env-2 loci to a replication-competent ecotropic virus indicated regions that might be important to dispersion of these endogenous retroviral elements throughout the host genome.

Mutants of murine leukemia viruses and retroviral replication

The analysis of retroviral mutants has played a critical role in the development of our understanding of the complex viral life cycle. The most fundamental result of that analysis has been the definition of the replication functions encoded by the viruses. From a biochemical examination of a particular step in the life cycle it is difficult to determine, for example, whether that step is catalyzed by a viral or a host enzyme; but the isolation of a viral mutant defective in that step can firmly establish that a viral function is involved. In this way many facts about the viruses have been established. We know that reverse transcriptase is encoded by the virus; that RNAase H and DNA polymerase activities reside on the same gene product; that processing of many precursor proteins is mediated by a viral proteinase; and that establishment of the integrated provirus requires a viral protein. The list of functions mediated by viral enzymes has largely been defined by the mutants isolated and studied in various laboratories. The second significant result of the studies of viral mutants has been the assignation of the replication functions to particular viral genes, and then more specifically to particular domains of these genes. Mutants and viral variants have been essential in the determination, for example, that the gag protein is the critical gene product for the assembly of a virion particle; that the env protein is the determinant of species specificity of infection; or that the LTR is a major determinant of tissue tropism and leukemogenicity. The subdivisions of functions within a given gene have similarly hinged on mutants. Genetic mapping was needed to establish that P30 is the most important region for assembly; that the proteinase and integrase functions reside, respectively, in the 5' and 3' portions of the pol gene; and that the glycosylated gag protein is dispensable for replication. A third important area of knowledge has depended heavily on viral mutants: the determination of host functions and proteins that interact with viral proteins. Variant viruses with altered or restricted host ranges serve to define differences between pairs of different host cells, and the mapping of the viral mutations serves to define the viral protein important in that interaction with the host. These studies are only in their infancy, but it is clear that substantial efforts will be made to further analyze these host functions.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Different murine cell lines manifest unique patterns of interference to superinfection by murine leukemia viruses

Interference to superinfection by murine leukemia viruses (MuLV) was analyzed in cells chronically infected with other MuLVs. A new sensitive focal immunofluorescence assay employing monoclonal antibodies was used to detect foci of virus infection in live cell monolayers. Monoclonal antibodies were chosen which reacted with the challenge virus but not with the interfering virus. The results obtained confirmed some of the findings of previous workers using Moloney sarcoma virus pseudotypes as challenge viruses on mouse and nonmouse cells. In addition, SC-1 mouse cells nonproductively infected with defective spleen focus-forming virus were found to be resistant to superinfection by recombinant dual-tropic viruses. Furthermore, results indicated that interference patterns between some pairs of viruses differed in different cell types. Thus, xenotropic MuLV blocked superinfection by recombinant dual-tropic viruses in SC-1 feral mouse cells, but not in two lines of NZB mouse cells. Also, in a Mus dunii tail fibroblast cell line some unique patterns of interference were observed. One ecotropic MuLV blocked infection by two xenotropic viruses and three recombinant dual-tropic viruses. Two other ecotropic viruses blocked infection by only one of the two xenotropic viruses tested. These two ecotropic viruses also differed from each other in their ability to block the three recombinant viruses. In addition, two strains of amphotropic MuLV also differed in their interference capacity. As expected, strain 1504A did not block any viruses tested, whereas strain 4070A surprisingly blocked one xenotropic and one ecotropic MuLV. The lack of homogeneity in interference patterns seen in the Mus dunii cells suggested either that a large number of heterogeneous virus receptors were present on this cell line or that interference in these cells might operate through a mechanism other than blocking of virus receptors by the envelope protein of the interfering virus.

The Parodi-Irgens feline sarcoma virus and simian sarcoma virus have homologous oncogenes, but in different contexts of the viral genomes

We have identified the oncogene and the putative transforming protein of the Parodi-Irgens feline sarcoma virus (PI-FeSV). The PI-FeSV is defective and needs a helper virus for its replication. The v-onc sequences in the PI-FeSV were found to be related to the v-sis sequences of the simian sarcoma virus (SSV). PI-FeSV nonproducer cells express two viral RNAs, a 6.8-and a 3.3-kilobase RNA. The 6.8-kilobase RNA contains gag, sis, and env sequences but lacks the pol gene. The 3.3-kilobase RNA, on the other hand, contains only env sequences. We have detected one feline leukemia virus-related protein product in these cells, namely, a 76-kilodalton protein which contains determinants of the feline leukemia virus gag proteins p15 and p30. The v-sis sequences in the PI-FeSV have been located near the 5' end of the viral genome. Taken together, these results imply that the p76 protein contains both feline leukemia virus gag and sis sequences and probably is the transforming protein of this virus. In contrast, in SSV the sis sequences are located towards the 3' end of the viral genome, and the sis protein is thought to be expressed via a subgenomic RNA. PI-FeSV and SSV therefore use different schemes to express their onc-related sequences. The v-sis sequences in the PI-FeSV contain restriction sites which reflect the different origin of the v-sis sequences in the PI-FeSV and SSV. The homologous oncogenes of the PI-FeSV and SSV thus were transduced by two different retroviruses, feline leukemia virus and the simian sarcoma-associated virus, apparently from the genomes of different species.

New class of leukemogenic ecotropic recombinant murine leukemia virus isolated from radiation-induced thymomas of C57BL/6 mice

We previously reported the establishment of several lymphoid cell lines from X-ray-induced thymomas of C57BL/Ka mice, and all, except one, produce retroviruses (P. Sankar-Mistry and P. Jolicoeur, J. Virol.35:270-275, 1980). Biological characterization of five of these new primary radiation leukemia viruses (RadLVs) indicated that they had a B-tropic, fibrotropic, and ecotropic host range and were leukemogenic when reinjected into C57BL/Ka newborn mice. The leukemogenic potential of one isolate (G(6)T(2)) was further assessed and shown to be retained after prolonged passaging on fibroblasts in vitro. Restriction endonuclease analysis of the DNA of four of our new RadLV isolates (G(6)T(2), Ti-7, Ti-8, and Ti-9) revealed that G(6)T(2) and Ti-7 murine leukemia virus (MuLV) genomes had identical restriction maps, whereas Ti-8 and Ti-9 genomes were different from each other and from the G(6)T(2) and Ti-7 genomes. The physical maps of these genomes were similar to that of known ecotropic MuLV genomes (including the C57BL/Ka endogenous ecotropic MuLV) within their long terminal repeats, env, the right portion of pol, and the left portion of gag. However, a region covering the end of gag and the beginning of pol was different and showed several similarities with xenotropic MuLV genomes of BALB/c, AKR, and C58 mice previously mapped. Our results suggest that these primary RadLV genomes are recombinants between the parental ecotropic MuLV genome and a nonecotropic (xenotropic) sequence. This nonecotropic gag-pol region might be important in conferring the leukemogenic potential to these isolates. Therefore, these RadLVs appear to form a new class of leukemogenic recombinant MuLVs recovered from leukemic tissues of mice. They appear to be distinct from the recombinant AKR mink cell focus-inducing MuLVs which have a dual-tropic host range and harbor xenotropic env sequences. To further study the leukemogenic potential of these RadLVs, the genome of one of them (G(6)T(2)) was cloned in Charon 21A as an infectious molecule.

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.

Four Moloney murine leukemia virus-infected rat cell clones producing replication-defective particles: protein and nucleic acid analyses

Four cloned rat cell lines (NX-1 to -4) infected with Moloney murine leukemia virus and defective in virus replication were found to be all different by viral protein and nucleic acid analyses. All four clones produced noninfectious particles and, except for NX-2, at about the same level as wild type. Compared with wild-type virions these defective particles contained larger amounts of gag precursor proteins and very little or no p30 or p15. Analysis of intracellular precursor proteins revealed that NX-2 to -4 synthesized normal Pr65gag, whereas NX-1 produced a slightly smaller precursor. Both NX-1 and NX-4 synthesized an intracellular polyprotein with a size similar to that of wild-type Pr180 gag-pol. Restriction endonuclease analysis of NX-1 to -4 cellular DNA showed that each clone contained a single integrated provirus which possessed large terminal repeat sequences at both the 5' and 3' ends. The proviruses of NX-1 to -3 appeared normal by restriction endonuclease analysis, but NX-4 provirus had a deletion of 1,700 base pairs comprising part of the polymerase region. The noninfectious particles produced by all four clones packaged Moloney viral RNAs and rat RNAs of two different sizes.

Generation of novel, biologically active Harvey sarcoma viruses via apparent illegitimate recombination

NIH 3T3 cells transfected with Harvey sarcoma virus (HSV) DNA may acquire deleted proviruses (Goldfarb and Weinberg, J. Virol. 38:125-135, 1981). Such proviruses lack the right end of the wild-type HSV DNA genome corresponding to the 3'-proximal portion of the viral RNA. As expected, the RNA transcripts of these deleted HSV (delHSV) proviruses lacked sequences normally found at the 3' end of wild-type HSV RNA. Since frequently these delHSV RNA transcripts were longer than wild-type HSV RNA, we suggest that transcription proceeded through the deleted provirus and continued into flanking nonviral sequences. When delHSV-transformed cells were infected with Moloney murine leukemia virus (M-MLV), delHSV RNA was pseudotyped into new virus particles, demonstrating that the 3'-proximal sequences of wild-type HSV RNA are not essential for virion RNA encapsidation. Cells which carried a delHSV genome and were infected with M-MLV helper released very low titers of highly transmissible sarcoma virus. The inability to rescue high titers of sarcoma virus from these cells reflected the necessary presence of the deleted 3'-terminal sequences for normal efficient transmission of the sarcoma virus genome (Goldfarb and Weinberg, J. Virol. 38:125-135, 1981). The small amount of highly transmissible sarcoma virus rescuable from delHSV-transformed cells originated via genetic recombination between del HSV and the M-MLV helper used for the sarcoma virus rescue. The recombinant sarcoma virus genomes reacquired a competent 3' genomic end from the parental M-MLV genome, which restored efficient transmissibility. The locations of sites for recombination between the delHSV and M-MLV genomes appeared to be nonrandom. These sites were in genomic regions where the parental genomes bore no detectable sequence homology. Structural mapping of these recombinant sarcoma virus genomes indicated that the HSV transformation gene lies within 2.0 kilobases of the RNA 5' end. Based upon our genetic recombination studies, we suggest a model to explain how leukemia viruses can recombine with cellular sequences to generate novel defective viruses.

Replication-defective Friend murine leukemia virus particles containing uncleaved gag polyproteins and decreased levels of envelope glycoprotein

An erythroleukemia cell clone, 7C, which failed to produce reverse transcriptase-containing virions or infectious virus, was found to produce noninfectious virus particles by gradient banding of [3H]leucine- and [3H]uridine-labeled virions. The RNA from the 7C virus was shown to consist of the normal 70S size component, which converted to 35S upon heat denaturation. In contrast, the 7C virion proteins showed multiple defects. Analysis of the virion proteins by gel electrophoresis demonstrated that the pr65 gag precursor was incorporated into the 7C virus and that the processing of this precursor was severely diminished. Polymerase proteins pr180gag-pol and pr120pol were also detected in virions, and a third possible polymerase protein, p70, was reduced in size compared to its normal counterpart, p80. Incorporation of the viral gp70 glycoprotein into particles was also reduced 10-fold, despite synthesis and incorporation of gp70 into the 7C cell membrane in normal amounts. Pulse-chase analysis of the synthesis of the viral gag and env proteins in 7C cells showed greatly reduced amounts of pr180gag-pol, pr65gag, p80gag, and p42gag, whereas pr90env, gp70, and spleen focus-forming virus-specific gp55 were synthesized and processed normally. These results suggested that at least one defect in 7C virus was impaired cleavage of gag or pol proteins or both, most likely due to a lack of the appropriate viral protease, and that this lack of cleavage might affect incorporation of gp70 into virus particles.

Characterization of a unique defective type C virus associated with a Moloney leukemia virus-induced splenic T-cell lymphoma cell line

Moloney leukemia virus (MoLV) induces lymphomas in BALB/c mice which either involve an immature thymic T-cell subpopulation or a splenic mature T-cell subpopulation. To investigate further the possible virological and immunological differences in these lymphomas, several lymphoma cell lines were derived. Although the majority of these cell lines expressed only the parental MoLV, one lymphoma cell line (5F4) was found which expressed only a defective virus. 5F4 virions lacked detectable reverse transcriptase activity and by immunoprecipitation lacked a serologically detectable reverse transcriptase. The lack of reverse transcriptase did not appear to be due to a deletion in the viral genome. Intracellularly 5F4 cells synthesized normal gag gene precursors but had little, if any, detectable Pr180gag-pol or an altered precursor. These results suggest that the defect of the 5F4 virus is associated with the inability to translate the appropriate precursor for reverse transcriptase. The possible origin of the detective 5F4 virus was also examined by competition radioimmunoassays. These results demonstrate that the type-specific proteins, gp71 and p12, are serologically identical to those of the endogenous ecotropic virus and distinct from the MoLV proteins. Competition assays of 5F4 cell extracts further demonstrated the lack of any detectable MoLV type-specific proteins, although the tumor was presumably induced by MoLV. The significance of these observations to leukemogenesis is discussed.

Mutant of B-tropic murine leukemia virus synthesizing an altered polymerase molecule

A nonconditional mutant of B-tropic murine leukemia virus (MuLV), defective in polymerase, has been isolated by cloning chronically infected cells. The cell clone containing the mutant produced virus particles which were noninfectious. However, superinfection of the cells by replication-competent XC-negative viruses resulted in the rescue of virus capable of forming plaques in a modified XC test, termed the "complementation plaque assay" (A. Rein and R. H. Bassin, J. Virol. 28:656-660, 1978). Analysis of the noninfectious virions produced without superinfection demonstrated that they contained only 2 to 5% of the wild-type level of reverse transcriptase activity. Purification of this activity indicated that it was associated with a smaller molecule than that produced by wild-type virus. Cells producing the mutant virions did not contain the gag-pol precursor, Pr180gag-pol; however the cells contained proteins of 147K and 114K daltons precipitable with anti-pol serum. All of the normal structural proteins as well as 70S genomic RNA could be detected in the mutant particles. An interference test indicated that a functional ecotropic glycoprotein was synthesized by the mutant. These results indicate that the mutant has a unique defect in the pol gene.

Virus-like 30S RNA in mouse cells

Uninfected JLS-V9 mouse cells are known to express high levels of viral sequences that hybridize to complementary DNA made by the BrdU-induced virus of JLS-V9 cells. The genome in the BrdU-induced virus has been found to consist mainly of an RNA species that migrates as 30S RNA material during electrophoresis through agarose gels. This virus-like 30S RNA, designated VL30 RNA, apparently represents a new class of endogenous defective retroviruses that are not generally evident because of their defectiveness and lack of biological function. Fingerprint analysis and hybridization studies show that VL30 RNA does not have homology with the standard nondefective murine leukemia viruses. Upon superinfection with a nondefective murine leukemia virus, or upon induction of endogenous virus with BrdU, VL30 RNA is rescued into virions by phenotypic mixing. When VL30 RNA is rescued by BrdU induction, the VL30 RNA is mainly organized as a 50S complex, but when VL30 is rescued by superinfection, VL30 is also found in 70S RNA. Rescued VL30 RNA sequences can be reverse transcribed by the virion-associated DNA polymerase in an endogenous reaction. Many mouse cells express the sequences, whereas heterologous cells such as rat or rabbit cells do not contain them. By using hybridization of a complementary DNA probe to cellular RNA immobilized on paper, no subgenomic RNA related to the VL30 RNA could be found in cells expressing the VL30 sequences. From 20 to 50 copies of these sequences were found to be contained in the mouse genome. VL30 RNA is probably present in most stocks of leukemia and sarcoma viruses made in mouse cells.