Role of a membrane glycoprotein in Friend virus-induced erythroleukemia: studies of mutant and revertant viruses

Unique Structure and Distinctive Properties of the Ancient and Ubiquitous Gamma-Type Envelope Glycoprotein

After the onset of the AIDS pandemic, HIV-1 (genus Lentivirus) became the predominant model for studying retrovirus Env glycoproteins and their role in entry. However, HIV Env is an inadequate model for understanding entry of viruses in the Alpharetrovirus, Gammaretrovirus and Deltaretrovirus genera. For example, oncogenic model system viruses such as Rous sarcoma virus (RSV, Alpharetrovirus), murine leukemia virus (MLV, Gammaretrovirus) and human T-cell leukemia viruses (HTLV-I and HTLV-II, Deltaretrovirus) encode Envs that are structurally and functionally distinct from HIV Env. We refer to these as Gamma-type Envs. Gamma-type Envs are probably the most widespread retroviral Envs in nature. They are found in exogenous and endogenous retroviruses representing a broad spectrum of vertebrate hosts including amphibians, birds, reptiles, mammals and fish. In endogenous form, gamma-type Envs have been evolutionarily coopted numerous times, most notably as placental syncytins (e.g., human SYNC1 and SYNC2). Remarkably, gamma-type Envs are also found outside of the Retroviridae. Gp2 proteins of filoviruses (e.g., Ebolavirus) and snake arenaviruses in the genus Reptarenavirus are gamma-type Env homologs, products of ancient recombination events involving viruses of different Baltimore classes. Distinctive hallmarks of gamma-type Envs include a labile disulfide bond linking the surface and transmembrane subunits, a multi-stage attachment and fusion mechanism, a highly conserved (but poorly understood) "immunosuppressive domain", and activation by the viral protease during virion maturation. Here, we synthesize work from diverse retrovirus model systems to illustrate these distinctive properties and to highlight avenues for further exploration of gamma-type Env structure and function.

Host-range control of a retroviral disease: Friend erythroleukemia

Since its discovery in 1957, Friend viral erythroleukemia has been the major model for understanding host genetic barriers to retroviral diseases and has facilitated the discovery of many polymorphic leukemia-control genes of mice. Some of these genes limit helper-virus replication, target-cell (erythroblast) pools or immune responses. At least one host gene appears to block the viral oncoprotein.

Deletions in one domain of the Friend virus-encoded membrane glycoprotein overcome host range restrictions for erythroleukemia

Although the Friend virus-encoded membrane glycoprotein (gp55) activates erythropoietin receptors (EpoR) to cause erythroblastosis only in certain inbred strains of mice but not in other species, mutant viruses can overcome aspects of mouse resistance. Thus, mice homozygous for the resistance allele of the Fv-2 gene are unaffected by gp55 but are susceptible to mutant glycoproteins that have partial deletions in their ecotropic domains. These and other results have suggested that proteins coded for by polymorphic Fv-2 alleles might directly or indirectly interact with EpoR and that changes in gp55 can overcome this defense. A new viral mutant with an exceptionally large deletion in its ecotropic domain is now also shown to overcome Fv-2rr resistance. In all cases, the glycoproteins that activate EpoR are processed to cell surfaces as disulfide-bonded dimers. To initiate analysis of nonmurine resistances, we expressed human EpoR and mouse EpoR in the interleukin 3-dependent mouse cell line BaF3 and compared the abilities of Friend virus-encoded glycoproteins to convert these cells to growth factor independence. Human EpoR was activated in these cells by erythropoietin but was resistant to gp55. However, human EpoR was efficiently activated in these cells by the same viral mutants that overcome Fv-2rr resistance in mice. By construction and analysis of human-mouse EpoR chimeras, we obtained evidence that the cytosolic domain of human EpoR contributes to its resistance to gp55 and that this resistance is mediated by accessory cellular factors. Aspects of host resistance in both murine and nonmurine species are targeted specifically against the ecotropic domain of gp55.

A Friend virus mutant encodes a small glycoprotein that causes erythroleukemia

The Pvu delta mutant of Friend spleen focus-forming virus encodes the smallest env glycoprotein (apparent M(r), 41,000) known to activate erythropoietin receptors. In vivo, Pvu delta causes erythroblastosis and the development of erythroleukemia. We isolated two leukemic cell lines that contain Pvu delta; both synthesize hemoglobin in response to dimethyl sulfoxide. The Pvu delta env gene contains a 204-base deletion in the ecotropic-specific region, suggesting that this domain of the glycoprotein is not essential for viral pathogenesis.

Glycosylation of glycoprotein 55 encoded by the anaemia-inducing strain of Friend spleen focus-forming virus

Normal rat kidney cells, non-productively infected with the anaemia-inducing variant of Friend spleen focus-forming virus (F-SFFVA), were metabolically labelled with [2-3H]mannose. The primary translation product of the viral envelope gene (env), representing a glycoprotein with an apparent molecular M(r) of 55,000 (gp55), was isolated from cell lysates by immunoaffinity chromatography and purified by preparative SDS/PAGE. Radiolabelled oligosaccharides, released from tryptic glycopeptides by treatment with endo-beta-N-acetylglucosaminidase H, were characterized chromatographically, by enzymic digestion and by acetolysis. The results revealed that F-SFFVA gp55 obtained from this source carried predominantly oligomannose type sugar chains with five to nine mannoses. As a characteristic feature, glycans with seven to nine mannoses contained, in part, an additional glucose residue. Although the amount of glucosylated species found was higher in F-SFFVA gp55 (about 25% of total endo-H-sensitive oligosaccharides) than in gp55 of the corresponding polycythaemia-inducing variant (F-SFFVP, 16.3%), the overall glycosylation pattern of the F-SFFVA env product closely resembled that of F-SFFVP gp55 [Strube et al. (1988) J Biol Chem 263:3762-71]. Hence, our results demonstrate that the different intracellular processing and transport of the primary F-SFFVA env product cannot be attributed to aberrant trimming of its oligomannose type glycans.

A Friend virus mutant that overcomes Fv-2rr host resistance encodes a small glycoprotein that dimerizes, is processed to cell surfaces, and specifically activates erythropoietin receptors

The env gene of Friend spleen focus-forming virus (SFFV) encodes a membrane glycoprotein (gp55) that is inefficiently (3 to 5%) processed from the rough endoplasmic reticulum to form a larger dimeric plasma membrane derivative (gp55p). Moreover, the SFFV env glycoprotein associates with erythropoietin receptors (EpoR) to cause proliferation of infected erythroblasts [J.-P. Li, A. D. D'Andrea, H. F. Lodish, and D. Baltimore, Nature (London) 343:762-764, 1990]. Interestingly, the mitogenic effect of SFFV is blocked in mice homozygous for the Fv-2r resistance gene, but mutant SFFVs can overcome this resistance. Recent evidence suggested that these mutants contain partial env deletions that truncate the membrane-proximal extracellular domain of the encoded glycoproteins (M. H. Majumdar, C.-L. Cho, M. T. Fox, K. L. Eckner, S. Kozak, D. Kabat, and R. W. Geib, J. Virol. 66:3652-3660, 1992). Mutant BB6, which encodes a gp42 glycoprotein that has a large deletion in this domain, causes erythroblastosis in DBA/2 (Fv-2s) as well as in congenic D2.R (Fv-2r) mice. Analogous to gp55, gp42 is processed inefficiently as a disulfide-bonded dimer to form cell surface gp42p. Retroviral vectors with SFFV and BB6 env genes have no effect on interleukin 3-dependent BaF3 hematopoietic cells, but they cause growth factor independency of BaF3/EpoR cells, a derivative that contains recombinant EpoR. After binding 125I-Epo to surface EpoR on these factor-independent cells and adding the covalent cross-linking reagent disuccinimidyl suberate, complexes that had immunological properties and sizes demonstrating that they consisted of 125I-Epo-gp55p and 125I-Epo-gp42p were isolated from cell lysates. Contrary to a previous report, SFFV or BB6 env glycoproteins did not promiscuously activate other members of the EpoR superfamily. Although the related env glycoproteins encoded by dualtropic murine leukemia viruses formed detectable complexes with EpoR, strong mitogenic signalling did not ensue. Our results indicate that the SFFV and BB6 env glycoproteins specifically activate EpoR; they help to define the glycoprotein properties important for its functions; and they strongly suggest that the Fv-2 leukemia control gene encodes an EpoR-associated regulatory factor.

Erythropoietin receptor (EpoR)-dependent mitogenicity of spleen focus-forming virus correlates with viral pathogenicity and processing of env protein but not with formation of gp52-EpoR complexes in the endoplasmic reticulum

Recent evidence suggests that interactions between spleen focus-forming virus (SFFV) env products and the erythropoietin receptor (EpoR) are responsible for viral pathogenicity. Infection of factor-dependent cell lines expressing epoR (the cloned gene for EpoR) with SFFVP is mitogenic, generating cell lines that are no longer dependent on added growth factor, and an immunoprecipitable complex between EpoR and immature env protein in the endoplasmic reticulum has been identified. The dependence of these in vitro activities on env protein processing and their relationship to pathogenicity of SFFV were explored by using glycosylation site mutants of SFFV env. Mutants carrying Asn-->Asp mutations at each of the two consensus signals for N-linked glycosylation in the N-terminal domain of SFFVAP-L env (gs1 and gs2), the gs1-2- double mutant, and the gs0 quadruple mutant (mutated at all four signals utilized for N-linked glycosylation in SFFVAP-L env) were made. The primary translation products (gp52) of single-site mutant envs were processed into more highly glycosylated forms, and the corresponding viruses induced splenomegaly in susceptible mice, whereas the gs1-2- and gs0 proteins were not processed, and these viruses were not pathogenic. Unprocessed env proteins of both pathogenic and nonpathogenic mutants coprecipitated with EpoR. In the BaF3 cell assay for epoR-dependent mitogenicity, the pathogenic single mutants induced factor-independent growth efficiently whereas the nonpathogenic gs1-2- and gs0 mutants did not. These data demonstrate that the ability of gp52 to form complexes with EpoR in the endoplasmic reticulum is not sufficient for either mitogenicity in cell culture or induction of splenomegaly in mice while supporting the hypothesis that pathogenicity and mitogenicity of SFFV both result from an interaction between EpoR and SFFV env protein.

Receptor choice determinants in the envelope glycoproteins of amphotropic, xenotropic, and polytropic murine leukemia viruses

The envelope glycoproteins (SU) of mammalian type C retroviruses possess an amino-terminal domain of about 200 residues, which is involved in binding a cell surface receptor. In this domain, highly conserved amino acid sequences are interrupted by two segments of variable length and sequence, VRA and VRB. We have studied the role of these variable regions in receptor recognition and binding by constructing chimeric molecules in which portions of the amino-terminal domains from amphotropic (4070A), xenotropic (NZB), and polytropic (MCF 247) murine leukemia virus SU proteins were permuted. These chimeras, which exchanged either one or two variable regions, were expressed at the surface of replication-defective viral particles by a pseudotyping assay. Wild-type or recombinant env genes were transfected into a cell line producing Moloney murine leukemia virus particles devoid of envelope glycoproteins in which a retrovirus vector genome carrying an Escherichia coli lacZ gene was packaged. The host range and sensitivity to interference of pseudotyped virions were assayed, and we observed which permutations resulted in receptor switch or loss of function. Our results indicate that the determinants of receptor choice are found within the just 120 amino acids of SU proteins. Downstream sequences contribute to the stabilization of the receptor-specific structure.

An amino-terminal fragment of the Friend murine leukemia virus envelope glycoprotein binds the ecotropic receptor

Retrovirus entry into cells is mediated by specific binding of the envelope glycoprotein to a cell membrane receptor. Constitutive envelope gene expression prevents infection by interfering with the binding of viruses which recognize the same receptor. We have used this property to investigate the receptor binding capacities of deleted or truncated murine leukemia virus ecotropic envelope glycoproteins. Friend murine leukemia virus envelope glycoproteins bearing internal amino-terminal deletions, or a soluble 245-amino-acid gp70 amino-terminal fragment, were expressed in NIH 3T3 cells. The susceptibility of these cells to ecotropic and amphotropic virus infection was determined. We observed that both membrane-bound and soluble forms of the gp70 245-amino-acid amino-terminal domain induced resistance to ecotropic virus, indicating that this fragment binds the ecotropic receptor. Binding occurs both at the cell surface and in the endoplasmic reticulum, as shown by the use of soluble envelope fragments either secreted in the culture supernatants or retained in the endoplasmic reticulum lumen by a KDEL sequence. These results suggest that the gp70 amino-terminal domain folds into a structure which recognizes the ecotropic receptor regardless of the carboxy-terminal part of the molecule.

Characterization and significance of delayed processing of the feline leukemia virus FeLV-FAIDS envelope glycoprotein

FeLV-FAIDS, an immunodeficiency-inducing isolate of feline leukemia virus, is composed of a pathogenic but replication-defective genome (molecular clone 61C) and a replication-competent but non-immunodeficiency-inducing variant genome (molecular clone 61E). The chimeric virus EECC, composed of the 5' gag-pol of 61E fused to the env-3' LTR of 61C, also induces immunodeficiency. The 61C (or EECC) gp80 can be distinguished from that of 61E on the basis of antigenic recognition, size, and rate of posttranslational processing. We found that the nascent precursor polypeptides of the two viruses were the same size; however, the 61E gp80 rapidly shifted to a smaller size and was subsequently cleaved to gp70, whereas EECC gp80 maintained its nascent size and was cleaved to gp70 only after a prolonged time. Endo-beta-N-acetyl glucosaminidase H and N-glycanase digestions of newly formed glycoproteins resulted in a similar banding pattern for both viruses, indicating that both contained the same number of oligosaccharide side chains and that all of these were high mannose sugars. The metabolic inhibitors of glycosylation, castanospermine or N-methyldeoxynojirimycin, prevented both the rapid trimming of 61E gp80 and its cleavage to gp70. Treatment with mannosidase inhibitors, however, did not affect 61E gp80 processing or size, suggesting that retention of glucose residues on EECC was responsible for these distinguishing properties of the glycoprotein. The pathological consequence of aberrant viral glycoprotein processing was evaluated in feline 3201 T lymphocytes, which are infectable by both 61E and EECC but are killed only by EECC. As in fibroblasts, the EECC glycoprotein produced in lymphocytes was larger, antigenically distinct, and processed more slowly than was the glycoprotein of 61E. Castanospermine treatment of 61E-infected 3201 T cells, however, not only abrogated the antigenic differences between the 61E and EECC glycoproteins but also resulted in a cytopathic effect. Our results suggest that (i) intracellular accumulation of EECC envelope glycoprotein may occur consequent to retention of glucose residues on carbohydrate side chains and (ii) a strong correlation exists between delayed glycoprotein processing and cytopathicity in FeLV-FAIDS-infected T lymphocytes.

Biochemical characterization of cell-associated and extracellular products of the Friend spleen focus-forming virus env gene

The mature product of the env gene of Friend spleen focus-forming viruses (F-SFFV) is efficiently released from both leukemia cells and infected fibroblasts. Analyses of the kinetics of env protein synthesis and secretion in NRK cells infected with the Lilly-Steeves strain of SFFVp indicated that this product, gp65, was formed rapidly and remained stably associated with cells for up to 4 hr, at which point it was first detected in supernatant medium. By 12 hr after synthesis, greater than 95% of gp65 was found extracellularly. The release of this component was effectively blocked by 10 mM 1-deoxynojirimycin, an inhibitor of oligosaccharide processing, demonstrating a requirement for processing of high mannose precursor oligosaccharides in the secretion of gp65. Similar oligosaccharide substituents were found on cell-associated and extracellular forms of gp65. Enzymatic deglycosylation experiments demonstrated that in addition to the predicted four N-linked oligosaccharides, gp65 contains O-linked carbohydrates which are resistant to the action of peptide N-Glycanase F, but sensitive to neuraminidase and O-Glycanase. These structures may be related to O-linked oligosaccharides previously found on the env gene products of murine leukemia viruses. Comparison of the sizes of the deglycosylated forms of cell-associated and supernatant gp65 demonstrated that the extracellular molecules are approximately 3 kDa smaller than the cell-associated components. These data suggest the involvement of proteolysis at a C-terminal site in the release of gp65 from the plasma membrane.

RB virus: a strain of Friend virus that produces a 'Friend virus-like' disease in Fv-2rr mice

RB virus is a newly derived strain of Friend virus that was adapted to produce a 'Friend-like' disease in mice that are genetically resistant to wild-type Friend virus. RB virus was produced by passing high titers of the wild-type Friend virus (Lilly-Steeves polycythemia-producing strain) through adult Fv-2rr mice. Titration of the defective spleen focus-forming virus indicated RB virus infected similar numbers of Fv-2ss or Fv-2rr target cells. Analysis of the spleens from mice infected with RB virus indicated that RB induced the early stage of Friend disease (erythroid proliferation) in both Fv-2rr and Fv-2ss mice. Fv-2ss mice infected with RB virus developed the classical Friend disease within 3 weeks. In contrast, the percentage of Fv-2rr mice developing the 'Friend-like' disease after infection with RB virus never exceeded 60%. The latency period of RBV in Fv-2rr mice was strain dependent. D2.R16 (Fv-2rr) developed the syndrome more rapidly than C57BL/6 (Fv-2rr). RB virus retained the capacity to transform erythroprogenitor cells from both Fv-2ss and Fv-2rr animals. Cells infected with RB virus consistently produced a modified SFFV envelope protein, gp48.

Leukemogenic membrane glycoprotein encoded by Friend spleen focus-forming virus: transport to cell surfaces and shedding are controlled by disulfide-bonded dimerization and by cleavage of a hydrophobic membrane anchor

The leukemogenic glycoprotein (gp55) encoded by Friend spleen focus-forming virus is predominantly retained in the rough endoplasmic reticulum (RER). However, a small proportion (ca. 5%) is processed to form a derivative that occurs on plasma membranes and causes mitosis of infected erythroblasts. We have now found that gp55 folds heterogeneously in the RER to form components with different disulfide bonds and that this difference may determine their processing fates. RER gp55 consists predominantly of monomers with intrachain disulfide bonds. In contrast, the processed molecules are disulfide-bonded dimers. These dimers are extensively modified in transit to cell surfaces by conversion of four N-linked high-mannose oligosaccharides to complex derivatives and by attachment of a sialylated O-linked oligosaccharide. The plasma membrane dimers are then slowly shed into the medium by a mechanism that involves proteolytic cleavage of approximately 25 membrane-anchoring hydrophobic amino acids from the carboxyl termini of the glycoproteins. Consequently, shed molecules have shorter polypeptide chains than cell-associated gp55. We conclude that gp55 folds into different disulfide-bonded components that do not substantially isomerize, and that only one specific dimer is competent for export from the RER. Mitogenic activity of gp55 could be caused by the cell surface dimers, by the shed derivative, or by the carboxyl-terminal hydrophobic anchors that remain in the membranes after the shedding reaction.

Glycosylation of glycoproteins 52 and 65 encoded by the polycythemia-inducing strain of Friend spleen focus-forming virus. Isolation of glycopeptides containing individual glycosylation sites

The primary envelope gene product of the polycythemia-inducing strain of Friend spleen focus-forming virus, glycoprotein 52 (gp52), as well as its processed form, glycoprotein 65 (gp65), were isolated from virus-infected normal rat kidney cells metabolically labeled with [2-3H]mannose. Following digestion with trypsin, glycopeptides containing individual N-glycosylation sites were obtained by gel filtration and subsequent reversed-phase high-performance liquid chromatography. N-terminal amino acid sequencing of the glycopeptides demonstrated that only asparagine residues 11 and 26, located in the N-terminal domains of gp52 and gp65, carry carbohydrate substituents, while the potential N-glycosylation sites in the C-terminal portions of the molecules are not used. Carbohydrates attached were liberated by treatment with endo-beta-N-acetylglucosaminidase H or peptide: N-glycosidase F and characterized by high-performance liquid chromatography. The results demonstrated that gp52 carries similar patterns of oligomannosidic glycans in both positions. In gp65, however, asparagine residue 11 is almost exclusively substituted by complete, fucosylated N-acetyllactosaminic oligosaccharides, whereas asparagine residue 26 carries oligomannosidic or truncated N-acetyllactosaminic glycans.

Carbohydrate structure of glycoprotein 65 encoded by the polycythemia-inducing strain of Friend spleen focus-forming virus

The secondary envelope-gene product, glycoprotein 65 (gp65), of the polycythemia-inducing variant of Friend spleen focus-forming virus (F-SFFVp) was isolated from F-SFFVp-infected normal rat kidney cells cultivated in the presence or absence (-Glc) of glucose. Oligosaccharide side chains present were sequentially liberated by treatment of tryptic glycopeptides with endo-beta-N-acetylglucosaminidase H and peptide N-glycosidase F and fractionated by high-performance liquid chromatography. The glycans were characterized by digestion with exoglycosidases, by chromatographic comparison with oligosaccharide standards and by methylation analysis. The results demonstrate that gp65 contains oligomannosidic, hybrid and N-acetyllactosaminic glycans. The oligomannosidic glycans represent the same partially glucosylated species with six to nine mannose residues present in F-SFFVp gp52, the biosynthetic precursor of gp65 [Strube, K.-H. Schott, H.-H. and Geyer, R. (1988) J. Biol. Chem. 263, 3762-3771]. Oligosaccharides of the hybrid type were found to comprise one sialylated lactosamine unit and three or four alpha-linked mannose residues. Analysis of the N-acetyllactosaminic glycans revealed that gp65 carries fucosylated, partially sialylated bi-antennary, tri-antennary and tetra-antennary oligosaccharides, in addition to incomplete species. The glycosylation of gp65(-Glc) is characterized by the presence of oligomannosidic glycans with five to nine mannose residues, similar hybrid-type species and by increased amounts of incomplete N-acetyllactosaminic oligosaccharides, a decrease in sialylation and the lack of tetra-antennary species.

Molecular biology of Friend viral erythroleukemia

Friend virus clearly provides an important model for understanding the molecular biology of cancer. Moreover, the most important aspects of the erythroleukemia can be caused by a single SFFV infection in the absence of any helper virus. The SFFV env gene encodes a membrane glycoprotein, gp55. This glycoprotein, when expressed on erythroblast surfaces, causes a constitutive mitogenesis. However, SFFV infections only rarely increase the cell's self-renewal capability or abrogate its commitment to differentiate. Therefore, the consequence of infection is initially a polyclonal erythroblastosis. This polyclonal proliferation usually leads to cell differentiation and to recovery unless helper virus is present to cause continuing infection of new erythroblasts. Extremely rare SFFV proviral integrations, however, result in abrogation of the cell's commitment to differentiate and in the concomitant acquisition of cell immortality. These immortalizing proviral integrations occur at only a small number of sites in the mouse genome. Therefore, the mitogenic and immortalizing stages of erythroleukemia are now known to be caused by discrete genetic events--the first involving the SFFV env gene and the second involving the rare proviral integration sites. In early investigations of Friend virus, the first stage always preceded the second stage by at least several weeks. Now it is known that this delay in onset of the second stage is caused solely by statistics. Every SFFV-infected erythroblast is mitogenically activated, yet only rarely does the SFFV proviral integration produce immortality. Both steps in leukemogenesis can be caused simultaneously in an erythroblast by a rare single SFFV proviral integration. There has been an explosion of interest in retroviral env gene-mediated pathogenesis. Such pathogenesis has been recently associated with most of the naturally transmitted retroviral diseases including AIDS. Such pathogenesis involves in different viruses immunosuppression, anemia, neuropathy, and leukemia (Mathes et al. 1978; Simon et al. 1984, 1987; Weiss et al. 1985; Lifson et al. 1986; Riedel et al. 1986; Sitbon et al. 1986; Sodroski et al. 1986; Mitani et al. 1987; Schmidt et al. 1987; Klase et al. 1988; Overbaugh et al. 1988a, b). The shuffling and dynamic env gene rearrangements that have been associated with murine retroviral leukemogenesis have also now been seen in FeLV-FAIDS and HIV (Fisher et al. 1988; Overbaugh et al. 1 t88b; Saag et al. 1988; Tersmette et al. 1988). Friend virus provides an important established example of such env gene pathogenesis. Although we still do not understand precisely how gp55 causes erythroblast mitosis, workers in this field have discovered important clues that may lead to answers.(ABSTRACT TRUNCATED AT 400 WORDS)

Posttranslational modifications distinguish the envelope glycoprotein of the immunodeficiency disease-inducing feline leukemia virus retrovirus

The envelope glycoprotein (gp70) of a molecularly cloned, replication-defective feline leukemia virus (FeLV-FAIDS clone 61C) carries determinants for induction of fatal immunodeficiency disease, whereas the gp70 of its companion replication-competent, probably parent virus (clone 61E) does not. Immunoprecipitation analysis of the extracellular glycoproteins of 61E and EECC, a replication-competent viral construct composed of the 61C env and 3' long terminal repeat fused to the 61E gag-pol genes, demonstrated that the gp70 of EECC could be distinguished from that of 61E by both feline immune serum and a murine monoclonal antibody. Molecular weights of both the envelope precursor polyprotein (gp80) and the mature extracellular glycoprotein (gp70) of 61E were smaller than the corresponding proteins from the pathogenic EECC. Both the molecular weight disparity and monoclonal antibody discrimination of the two gp80s were abolished by inhibition of envelope protein glycosylation with tunicamycin, whereas the apparent gp70 size differences were resolved by enzymatic removal of N-linked oligosaccharides. Pulse-chase studies in EECC-infected cells demonstrated that processing of gp80 to gp70 was delayed and that this retardation of envelope glycoprotein processing could be simulated in 61E-infected cells by treatment with the glucosidase inhibitor N-methyldeoxynojirimycin, a compound that causes retention of oligosaccharides in the high-mannose form. The resultant 61E gp70 then could be recognized by sera from EECC-immunized cats. The presence of a higher content of sialic acid on the apathogenic 61E gp70 indicated that oligosaccharides of 61E and EECC gp70 were processed differently. These data suggested that the unique biochemical properties which distinguish the envelope glycoproteins of the FeLV-FAIDS variant from its companion apathogenic parent virus were responsible for T-cell cytopathicity and induction of immunodeficiency disease. Further biochemical characterization of these glycoproteins should be useful in understanding the pathogenic mechanisms of immunodeficiency disease induced by retroviruses.

An enhancer sequence instability that diversifies the cell repertoire for expression of a murine leukemia virus

Studies of recombinants between murine leukemia viruses (MuLVs) that cause thymic or erythroid leukemias have shown that enhancer sequences in the long-terminal repeats (LTRs) can determine the target tissues for pathogenesis. It has been inferred that the enhancers may specifically target viral expression into the cells that then become neoplastic. However, the neoplasms in those studies formed after latencies and contained ultimate viruses (called MCFs) that differed from the injected viruses in their enhancer sequences and envelope (env) genes. Transcriptional activities of LTRs from these proximal and ultimate viruses have not been thoroughly analyzed in different hematopoietic lineages. We present evidence that the enhancer of Friend spleen focus-forming virus (SFFV), an ultimate erythroleukemogenic retrovirus, contains an unstable 42-nucleotide direct repeat. Other ultimate erythroleukemogenic MuLVs (Friend MCFs) contain an enhancer nearly identical to that of SFFV both in its sequence and in its specific instability. The instability occurs in sequences that contain inverted repeats and we propose that it occurs by a simple reverse transcriptase hop mechanism. We constructed plasmids that contain the two forms of the SFFV LTR linked to the bacterial chloramphenicol acetyltransferase (CAT) gene, and we compared these in transient transfection assays with LTR-CAT plasmids constructed from Friend and Moloney MuLVs. The assays employed erythroleukemia cells, thymic lymphoma cells, and fibroblasts. The tropisms of expression correlated only weakly with tissue specificities of pathogenesis and each LTR was active in all cells. The SFFV 42-nucleotide duplication reduced expression in erythroid cells and increased expression in fibroblasts. We conclude that retroviral enhancers do not stringently direct gene expression into specific cell lineages, but on the contrary they are leaky and contain replicative instabilities that also may facilitate viral entrenchment throughout the host. These results have important implications for understanding murine retroviral evolution and the multi-step process of leukemogenesis.

The membrane glycoprotein of Friend spleen focus-forming virus: evidence that the cell surface component is required for pathogenesis and that it binds to a receptor

The leukemogenic membrane glycoprotein of Friend spleen focus-forming virus (SFFV) has an apparent Mr of 55,000 (gp55), is encoded by a recombinant env gene, and occurs on cell surfaces and in intracellular organelles. There is evidence that the amino-terminal region of gp55 forms a dualtropic-specific domain that is connected to the remainder of the glycoprotein by a proline-rich linker (C. Machida, R. Bestwick, B. Boswell, and D. Kabat, Virology 144:158-172, 1985). Using the colinear form of a cloned polycythemic strain of SFFV proviral DNA, we constructed seven in-phase env mutants by insertion of linkers and by a deletion. The mutagenized SFFVs were transfected into fibroblasts and were rescued by superinfection with a helper murine leukemia virus. Four of the mutants cause erythroblastosis. These include one with a 6-base-pair (bp) insert in the ecotropic-related sequence near the 3' end of the gene, two with a 12- or 18-bp insert in the region that encodes the proline-rich linker, and one with a 6-bp insert in the dualtropic-specific region. The other mutants (RI, Sm1, and Sm2) are nonpathogenic and contain lesions in dualtropic-specific region. The other mutants (RI, Sm1, and Sm2) are nonpathogenic and contain lesions in dualtropic-specific sequences that are highly conserved among strains of SFFV. A pathogenic revertant (RI-rev) was isolated from one mouse that developed erythroblastosis 3 weeks after infection with RI. RI-rev contains a second-site env mutation that affects the same domain as the primary mutation does and that increases the size of the encoded glycoprotein. All pathogenic SFFVs encode glycoproteins that are expressed on cell surfaces, whereas the nonpathogenic glycoproteins are exclusively intracellular. The pathogenic SFFVs also specifically cause a weak interference to superinfection by dualtropic MuLVs. These results are compatible with the multidomain model for the structure of gp55 and suggest that processing of gp55 to plasma membranes is required for pathogenesis. The amino-terminal region of gp55 binds to dualtropic murine leukemia virus receptors, and this interaction is preserved in the SFFV mutants that cause erythroblastosis.

The four classes of endogenous murine leukemia virus: structural relationships and potential for recombination

The process by which leukemogenic viruses are generated during the lifetime of certain strains of mice is poorly understood. We have therefore set out to define all the murine leukemia virus-related endogenous proviruses of HRS/J mice. We have cloned 34 different proviral fragments and their flanking cellular sequences. These have been characterized by restriction enzyme analysis, by fingerprinting in vitro-synthesized RNA, and by DNA sequencing. We conclude that all the proviruses can be assigned into one of four different classes: the previously characterized ecotropic, xenotropic, and polytropic viruses, as well as a new class we have termed modified polytropic viruses. The xenotropic, polytropic, and modified polytropic classes are closely related to one another, but as a group they differ considerably from the ecotropic class. Sequence analyses show that both polytropic and modified polytropic sequences can contribute env sequences to recombinant viruses.

Natural killer cell suppression of Friend virus-induced preleukemic hemopoietic stem cells

To determine whether hemopoietic cells infected with Friend polycythemia-inducing spleen focus-forming virus (SFFVp) are conserved or suppressed via natural surveillance in leukemia-resistant adult mice, we engrafted C57BL/6 recipients with isologous transgenic (donor origin marker) or natural killer (NK) cell-deficient B6 beige marrow cells exposed to SFFVp in vitro. Both groups of primary recipients were viremic and nonleukemic. Spleen cells from primary SFFVp-infected chimeras were engrafted into irradiated leukemia-susceptible secondary recipients to reveal dormant leukemia and grew as tumors of donor origin in 8 of 38 (21%) and 33 of 47 (70%) instances, respectively. Treatment of marrow donors and recipients with anti-asialo GM1 serum resulted in the depression of NK cell activity and the rapid development of dormant leukemia. We conclude that NK cells are an effective surveillance mechanism able to suppress SFFVp-induced preleukemic stem cells.

Characterization of cell lines derived from enlarged spleens induced in C57BL/6 mice by the variant BSB strain of Friend erythroleukemia virus

BSB is a variant strain of Friend virus selected for pathogenicity in C57BL/6 mice that are resistant to parental Friend virus strains by virtue of their homozygosity for the recessive Fv-2r allele (Steeves et al., 1970, Int. J. Cancer 5, 346-356). Lines and clones of erythroleukemia cells could readily be established in culture from the enlarged spleens of BSB-infected Fv-2r homozygotes. All lines expressed viral gene products and could be induced to express hemoglobin. Some lines produced infectious virus. In addition to the viral envelope-related proteins (gPr85, gp70, and gp52) detected by precipitation with goat anti-Rauscher gp70 antiserum from tumor cell lines induced by parental Friend virus strains, BSB-induced cell lines also expressed gp80, p52, and gp45 products precipitable with the same antiserum. A rat monoclonal antibody that recognizes an epitope of an amino-terminal region of gp52 (Wolff et al., 1982, J. Virol. 43, 472-481) also precipitated the gp80 and gp45 viral proteins. The data indicate that the BSB strain of Friend virus is oncogenic in Fv-2r homozygotes. Transformation is correlated with the expression of an altered SFFV env-gene product.

Domains of virus glycoproteins

This chapter reviews current information about the structure and function of virus glycoproteins. There are few virus glycoproteins that provide prototypes for illustrating important relationships between the functions and glycoprotein structure. The discussion presented in the chapter concentrates on those viral glycoproteins that (1) span the lipid bilayer once, (2) are oriented such that the carboxy terminus comprises the cytoplasmic domain, and (3) contain asparagine-linked oligosaccharides. There are also viral glycoproteins with extensive O-linked glycosylation, some of which are also presented in the discussion. The chapter also focuses on the studies involving directed mutagenesis and construction of chimeric proteins. The effects of altering specific amino acid sequences, of swapping domains, and of adding a new domain to a protein serve to define the functions of a domain and to show that a domain can be independently associated with a specific function. The experiments described have been carried out by inserting the genes of particular viral glycoproteins—such as cDNAs—into expression vectors and transcribing the cDNAs from the promoter provided by the expression vector. This approach established that localization and functions such as the fusogenic activity are properties of the viral glycoprotein per se and do not require other viral-coded components.

Role of a membrane glycoprotein in Friend virus erythroleukemia: nucleotide sequences of nonleukemogenic mutant and spontaneous revertant viruses

We previously isolated spontaneous env gene mutants of Friend spleen focus-forming virus that are nonleukemogenic in adult mice but form leukemogenic revertants in newborns; we found that the revertants contain secondary env mutations. To identify sites in the encoded membrane glycoprotein that are important for its pathogenic function, we molecularly cloned and partially sequenced the env genes of two mutant viruses (clone 63 and clone 4) and one revertant (clone 4REV). Clone 63 contained three noncontiguous point mutations that caused nonconservative amino acid substitutions of Gly-119----Arg-119, Cys-180----Tyr-180, and Gly-203----Arg-203 in the xenotropic-related domain of the env glycoprotein. These substitutions were presumably responsible for the altered electrophoretic and pathogenic properties of the mutant glycoprotein. The presence of these and several other G-A nucleotide substitutions at different sites in one spontaneous mutant provided striking evidence that error-rich proviruses can form during retroviral replication. Clone 4 contained a point mutation that generated a premature termination condon at amino acid residue 304 (Gln-304----Ochre-304). This termination codon was located immediately after the proposed xenotropic-ecotropic recombination site and eliminated the ecotropic-related domain, including the putative membrane anchor of the glycoprotein. Clone 4REV was a true revertant derived from clone 4 in which the premature termination codon had back-mutated to re-form the wild-type sequence. These results confirm an essential role for the env gene in Friend spleen focus-forming virus pathogenesis and suggest that the encoded membrane glycoprotein contains different domains that contribute to its pathogenic function.

Roles of helper and defective retroviral genomes in murine erythroleukemia: studies of spleen focus-forming virus in the absence of helper

Retroviruses that cause acute oncogenesis are generally complexes of a replication-competent helper virus and a replication-defective component. However, the pure defective components have not been previously available. We prepared the defective spleen focus-forming virus component of Rauscher erythroleukemia virus (R-SFFV) by transfecting a colinear R-SFFV DNA clone into a retroviral packaging cell line (psi 2 cells). The transfected cells released virus (psi 2/SFFV) that was free of helper virus and that induced erythropoietin-dependent erythroid burst formation in bone marrow cultures. When injected into normal adult NIH/Swiss mice in moderate doses, psi 2/SFFV caused a rapid splenic erythroblastosis that regressed. Extensive erythroblastosis could be maintained by repeated injections of psi 2/SFFV into anemic mice or by the addition of a helper virus. We conclude that R-SFFV alone causes proliferation but not immortalization of a population of erythroblasts that is normally replenished from a precursor stem cell pool. Because these precursor cells are inefficiently infected, a single moderate inoculum of psi 2/SFFV causes a wave of erythroblastosis. The properties of the proliferating erythroblasts are substantially determined by the R-SFFV viral component.