Identification of envelope protein residues required for the expanded host range of 10A1 murine leukemia virus

Construction of replication-competent oncolytic retroviral vectors expressing R peptide-truncated 10A1 envelope glycoprotein

Replication-deficient retroviral (RDR) vectors have been generally used for gene therapy, but clinically beneficial transduction efficiency is difficult to achieve with these vectors. In recent times, attention has been focused on the use of murine leukemia virus (MLV)-based replication-competent retroviral (RCR) vectors. RCR vectors have been shown to achieve efficient tumor reduction in a wide variety of cancer models. Most RCR vectors have been developed from amphotropic 4070 A MLV env, which is broadly applied in basic research. In this study, we generated RCR vectors based on Moloney MLV by replacing the native env gene in a full-length viral genome with the 10A1 env gene. 10A1 MLV can infect a wide variety of cells. Unlike amphotropic MLV, the 10A1 MLV can use amphotropic MLV receptor Pit2 or gibbon ape leukemia virus (GaLV) receptor Pit1. The resulting construct MoMLV-10A1-EGFP was able to replicate in 293 T, NIH3T3, and Mus dunni cells. To evaluate the potential of MoMLV-10A1 vector as a therapeutic agent, we incorporated the yeast cytosine deaminase (CD) suicide gene into vectors. The resulting vector MoMLV-10A1-CD could inhibit the growth of human 293T cells upon 5-fluorocytosine (5-FC) administration. In addition, to lyse tumor cells by syncytium, MoMLV-10A1-R(-)-EGFP was generated by replacing wild-type 10A1 env with the 16-amino acid R peptide-truncated 10A1 env gene. Syncytium formation was observed in the TE671 human tumor cells, 293 T and PG13 cells upon transfection of the MoMLV-10A1-R(-)-EGFP vector. This result suggests that replication of this vector could be oncolytic in itself. We also found that syncytium could contribute to enhance cell-to-cell transmission of the retroviral vectors. Our results thus show that the MoMLV-10A1 vectors can be potentially useful for cancer gene therapy.

Library screening and receptor-directed targeting of gammaretroviral vectors

Gene- and cell-based therapies hold great potential for the advancement of the personalized medicine movement. Gene therapy vectors have made dramatic leaps forward since their inception. Retroviral-based vectors were the first to gain clinical attention and still offer the best hope for the long-term correction of many disorders. The fear of nonspecific transduction makes targeting a necessary feature for most clinical applications. However, this remains a difficult feature to optimize, with specificity often coming at the expense of efficiency. The aim of this article is to discuss the various methods employed to retarget retroviral entry. Our focus will lie on the modification of gammaretroviral envelope proteins with an in-depth discussion of the creation and screening of envelope libraries.

Mus spicilegus endogenous retrovirus HEMV uses murine sodium-dependent myo-inositol transporter 1 as a receptor

We sought to determine the relationship between two recent additions to the murine leukemia virus (MLV) ecotropic subgroup: Mus cervicolor isolate M813 and Mus spicilegus endogenous retrovirus HEMV. Though divergent in sequence, the two viruses share an Env protein with similarly curtailed VRA and VRB regions, and infection by both is restricted to mouse cells. HEMV and M813 displayed reciprocal receptor interference, suggesting that they share a receptor. Expression of the M813 receptor murine sodium-dependent myo-inositol transporter 1 (mSMIT1) allowed previously nonpermissive cells to be infected by HEMV, indicating that mSMIT1 also serves as a receptor for HEMV. Our findings add HEMV as a second member to the MLV subgroup that uses mSMIT1 to gain entry into cells.

Glycosylation modulates arenavirus glycoprotein expression and function

The glycoprotein of lymphocytic choriomeningitis virus (LCMV) contains nine potential N-linked glycosylation sites. We investigated the function of these N-glycosylations by using alanine-scanning mutagenesis. All the available sites were occupied on GP1 and two of three on GP2. N-linked glycan mutations at positions 87 and 97 on GP1 resulted in reduction of expression and absence of cleavage and were necessary for downstream functions, as confirmed by the loss of GP-mediated fusion activity with T87A and S97A mutants. In contrast, T234A and E379N/A381T mutants impaired GP-mediated cell fusion without altered expression or processing. Infectivity via virus-like particles required glycans and a cleaved glycoprotein. Glycosylation at the first site within GP2, not normally utilized by LCMV, exhibited increased VLP infectivity. We also confirmed the role of the N-linked glycan at position 173 in the masking of the neutralizing epitope GP-1D. Taken together, our results indicated a strong relationship between fusion and infectivity.

Retroviral vectors encoding ADA regulatory locus control region provide enhanced T-cell-specific transgene expression

Background

Murine retroviral vectors have been used in several hundred gene therapy clinical trials, but have fallen out of favor for a number of reasons. One issue is that gene expression from viral or internal promoters is highly variable and essentially unregulated. Moreover, with retroviral vectors, gene expression is usually silenced over time. Mammalian genes, in contrast, are characterized by highly regulated, precise levels of expression in both a temporal and a cell-specific manner. To ascertain if recapitulation of endogenous adenosine deaminase (ADA) expression can be achieved in a vector construct we created a new series of Moloney murine leukemia virus (MuLV) based retroviral vector that carry human regulatory elements including combinations of the ADA promoter, the ADA locus control region (LCR), ADA introns and human polyadenylation sequences in a self-inactivating vector backbone.

Methods

A MuLV-based retroviral vector with a self-inactivating (SIN) backbone, the phosphoglycerate kinase promoter (PGK) and the enhanced green fluorescent protein (eGFP), as a reporter gene, was generated. Subsequent vectors were constructed from this basic vector by deletion or addition of certain elements. The added elements that were assessed are the human ADA promoter, human ADA locus control region (LCR), introns 7, 8, and 11 from the human ADA gene, and human growth hormone polyadenylation signal. Retroviral vector particles were produced by transient three-plasmid transfection of 293T cells. Retroviral vectors encoding eGFP were titered by transducing 293A cells, and then the proportion of GFP-positive cells was determined using fluorescence-activated cell sorting (FACS). Non T-cell and T-cell lines were transduced at a multiplicity of infection (MOI) of 0.1 and the yield of eGFP transgene expression was evaluated by FACS analysis using mean fluorescent intensity (MFI) detection.

Results

Vectors that contained the ADA LCR were preferentially expressed in T-cell lines. Further improvements in T-cell specific gene expression were observed with the incorporation of additional cis-regulatory elements, such as a human polyadenylation signal and intron 7 from the human ADA gene.

Conclusion

These studies suggest that the combination of an authentically regulated ADA gene in a murine retroviral vector, together with additional locus-specific regulatory refinements, will yield a vector with a safer profile and greater efficacy in terms of high-level, therapeutic, regulated gene expression for the treatment of ADA-deficient severe combined immunodeficiency.

Recombinant porcine endogenous retroviruses (PERV-A/C): a new risk for xenotransplantation?

PERVs are integrated in the genome of all pigs. Some of them infect human cells and represent therefore a potential risk for xenotransplantation using pig cells or organs. Three replication-competent subtypes have been described, PERV-A, PERV-B and PERV-C. Whereas PERV-A and PERV-B are polytropic viruses and infect, among others, human cells, PERV-C is an ecotropic virus, infecting only pig cells. Recombinant PERV-A/C are able to infect human cells, they are characterised by high-titre replication and their proviruses have been found de novo integrated in the genome of somatic pig cells, but not in the germ line. This review compares recombinant PERVs with other recombinant retroviruses in order to evaluate their potential pathogenicity.

The interferon response inhibits HIV particle production by induction of TRIM22

Treatment of human cells with Type 1 interferons restricts HIV replication. Here we report that the tripartite motif protein TRIM22 is a key mediator. We used transcriptional profiling to identify cellular genes that were induced by interferon treatment and identified TRIM22 as one of the most strongly up-regulated genes. We confirmed, as in previous studies, that TRIM22 over-expression inhibited HIV replication. To assess the role of TRIM22 expressed under natural inducing conditions, we compared the effects of interferon in cells depleted for TRIM22 using RNAi and found that HIV particle release was significantly increased in the knockdown, implying that TRIM22 acts as a natural antiviral effector. Further studies showed that TRIM22 inhibited budding of virus-like particles containing Gag only, indicating that Gag was the target of TRIM22. TRIM22 did not block the release of MLV or EIAV Gag particles. Inhibition was associated with diffuse cytoplasmic staining of HIV Gag rather than accumulation at the plasma membrane, suggesting TRIM22 disrupts proper trafficking. Mutational analyses of TRIM22 showed that the catalytic amino acids Cys15 and Cys18 of the RING domain are required for TRIM22 antiviral activity. These data disclose a pathway by which Type 1 interferons obstruct HIV replication.

Interferons are produced by cells in response to challenge by foreign pathogens such as viruses. The molecular mechanisms by which Type I interferons (e.g., IFNβ) inhibit the replication of HIV-1 are not fully clarified. We identified a gene called TRIM22 that belongs to the tripartite motif (TRIM) family that was strongly induced by IFNβ. Using RNA interference to reduce the expression of TRIM22, we showed that TRIM22 is a key mediator of the IFNβ response when expressed at natural levels. We demonstrate that TRIM22 blocks the intracellular trafficking of the viral structural protein Gag to the surface of the cell, and that the antiviral activity of TRIM22 is dependent on two cysteine residues (Cys15 and Cys18) that are critical for the E3 ligase activity of RING-containing proteins. This report describes a mechanism by which Type I interferons block HIV-1 replication.

Preparation and quantification of pseudotyped retroviral vector

Retroviral vectors have been widely used for research and clinical trials in gene therapy because of their high transduction efficiency. Retroviruses interact with target cells through their surface molecules (i.e., envelope proteins) and cellular receptors, which limit the susceptibility of target cells to retroviral vectors. Murine leukemia retrovirus (MuLV) pseudotyped with vesicular stomatitis virus G glycoprotein (VSV-G) overcomes the species barrier and is more resistant to mechanical and biochemical inactivation. A cell line producing VSV-G pseudotyped MuLV vector can be established by transfecting 293T cells expressing Gag, Pol, and VSV-G (293 GPG cell line) with a retroviral vector plasmid. Transduction potency of the resulting VSV-G pseudotyped MuLV retroviral supernatant can be quantified by titration, electron microscopy (EM), and the reverse transcriptase (RT) assay. These protocols provide methods to prepare and quantify a pseudotyped retroviral vector with high transduction rates for most types of target cells.

New world clade B arenaviruses can use transferrin receptor 1 (TfR1)-dependent and -independent entry pathways, and glycoproteins from human pathogenic strains are associated with the use of TfR1

Arenaviruses are rodent-borne viruses, with five members of the family capable of causing severe hemorrhagic fevers if transmitted to humans. To date, two distinct cellular receptors have been identified that are used by different pathogenic viruses, alpha-dystroglycan by Lassa fever virus and transferrin receptor 1 (TfR1) by certain New World clade B viruses. Our previous studies have suggested that other, as-yet-unknown receptors are involved in arenavirus entry. In the present study, we examined the use of TfR1 by the glycoproteins (GPs) from a panel of New World clade B arenaviruses comprising three pathogenic and two nonpathogenic strains. Interestingly, we found that TfR1 was only used by the GPs from the pathogenic viruses, with entry of the nonpathogenic strains being TfR1 independent. The pathogenic GPs could also direct entry into cells by TfR1-independent pathways, albeit less efficiently. A comparison of the abilities of TfR1 orthologs from different species to support arenavirus entry found that the human and feline receptors were able to enhance entry of the pathogenic strains, but that neither the murine or canine forms were functional. Since the ability to use TfR1 is a characteristic feature of the human pathogens, this interaction may represent an important target in the treatment of New World hemorrhagic fevers. In addition, the ability to use TfR1 may be a useful tool to predict the likelihood that any existing or newly discovered viruses in this family could infect humans.

Molecular and phylogenetic analyses of a new amphotropic murine leukemia virus (MuLV-1313)

BACKGROUND

The amphotropic murine leukemia viruses (MuLV-A's) are naturally occurring, exogenously acquired gammaretroviruses that are indigenous to the Southern California wild mice. These viruses replicate in a wide range of cell types including human cells in vitro and they can cause both hematological and neurological disorders in feral as well as in the inbred laboratory mice. Since MuLV-A's also exhibit discrete interference and neutralization properties, the envelope proteins of these viruses have been extremely useful for studying virus-host cell interactions and as vehicles for transfer of foreign genes into a variety of hosts including human cells. However, the genomic structure of any of the several known MuLV-A's has not been established and the evolutionary relationship of amphotropic retroviruses to the numerous exogenous or endogenous MuLV strains remains elusive. Herein we present a complete genetic structure of a novel amphotropic virus designated MuLV-1313 and demonstrate that this retrovirus together with other MuLV-A's belongs to a distinct molecular, biological and phylogenetic class among the MuLV strains isolated from a large number of the laboratory inbred or feral mice.

RESULTS

The host range of MuLV-1313 is similar to the previously isolated MuLV-A's except that this virus replicates efficiently in mammalian as well as in chicken cells. Compared to ENV proteins of other MuLV-A's (4070A, 1504A and 10A-1), the gp70 protein of MuLV-1313 exhibits differences in its signal peptides and the proline-rich hinge regions. However, the MuLV-1313 envelope protein is totally unrelated to those present in a broad range of murine retroviruses that have been isolated from various inbred and feral mice globally. Genetic analysis of the entire MuLV-1313 genome by dot plot analyses, which compares each nucleotide of one genome with the corresponding nucleotide of another, revealed that the genome of this virus, with the exception of the env gene, is more closely related to the biologically distinct wild mouse ecotropic retrovirus (Cas-Br-E) isolated from another region of the Southern California, than to any of the 15 MuLV strains whose full-length sequences are present in the GenBank. This finding was corroborated by phylogenetic analyses and hierarchical clustering of the entire genomic sequence of MuLV-1313, which also placed all MULV-A's in a genetically distinct category among the large family of retroviruses isolated from numerous mouse strains globally. Likewise, construction of separate dendrograms for each of the Gag, Pol and Env proteins of MuLV-1313 demonstrated that the amphotropic retroviruses belong to a phylogenetically exclusive group of gammaretroviruses compared to all known MuLV strains.

CONCLUSION

The molecular, biological and phylogenetic properties of amphotropic retroviruses including MuLV-1313 are distinct compared to a large family of exogenously- or endogenously-transmitted ecotropic, polytropic and xenotropic MuLV strains of the laboratory and feral mice. Further, both the naturally occurring amphotropic and a biologically discrete ecotropic retrovirus of the Southern California wild mice are more closely related to each other on the evolutionary tree than any other mammalian gammaretrovirus indicating a common origin of these viruses. This is the first report of a complete genomic analysis of a unique group of phylogenetically distinct amphotropic virus.

Identification of a PAX-FKHR gene expression signature that defines molecular classes and determines the prognosis of alveolar rhabdomyosarcomas

Alveolar rhabdomyosarcomas (ARMS) are aggressive soft-tissue sarcomas affecting children and young adults. Most ARMS tumors express the PAX3-FKHR or PAX7-FKHR (PAX-FKHR) fusion genes resulting from the t(2;13) or t(1;13) chromosomal translocations, respectively. However, up to 25% of ARMS tumors are fusion negative, making it unclear whether ARMS represent a single disease or multiple clinical and biological entities with a common phenotype. To test to what extent PAX-FKHR determine class and behavior of ARMS, we used oligonucleotide microarray expression profiling on 139 primary rhabdomyosarcoma tumors and an in vitro model. We found that ARMS tumors expressing either PAX-FKHR gene share a common expression profile distinct from fusion-negative ARMS and from the other rhabdomyosarcoma variants. We also observed that PAX-FKHR expression above a minimum level is necessary for the detection of this expression profile. Using an ectopic PAX3-FKHR and PAX7-FKHR expression model, we identified an expression signature regulated by PAX-FKHR that is specific to PAX-FKHR-positive ARMS tumors. Data mining for functional annotations of signature genes suggested a role for PAX-FKHR in regulating ARMS proliferation and differentiation. Cox regression modeling identified a subset of genes within the PAX-FKHR expression signature that segregated ARMS patients into three risk groups with 5-year overall survival estimates of 7%, 48%, and 93%. These prognostic classes were independent of conventional clinical risk factors. Our results show that PAX-FKHR dictate a specific expression signature that helps define the molecular phenotype of PAX-FKHR-positive ARMS tumors and, because it is linked with disease outcome in ARMS patients, determine tumor behavior.

Selection of feline leukemia virus envelope proteins from a library by functional association with a murine leukemia virus envelope

Libraries of feline leukemia virus subgroup A (FeLV-A)-derived envelope (Env) proteins with random peptides incorporated into the cell-targeting region were screened for productive gene delivery to the PC-3 human prostate cell line. In order to increase the efficiency of recovering and testing functional clones, the screen was performed in the presence of a replication-competent 4070A Env-expressing virus under conditions of viral interference. The Env proteins resulting from this library screen were able to mediate gene delivery to 4070A-infected human PC-3, DU145 prostate and TE671 rhabdomyosarcoma cells in the presence, but not absence, of 4070A helper virus. FeLV-A, FeLV-B and Moloney murine leukemia virus (Mo-MuLV) Env proteins were unable to substitute for 4070A Env. Flow cytometry and Western blot analyses indicated increased cell-surface expression and virion incorporation of library-derived Env proteins in the presence of 4070A Env. Interference assays on cells infected with both 4070A and FeLV-B are consistent with the combination of library-derived and 4070A Env proteins utilizing the Pit1 receptor.

The infectivity and host range of the ecotropic porcine endogenous retrovirus, PERV-C, is modulated by residues in the C-terminal region of its surface envelope protein

Endogenous retroviral genetic material serves as a reservoir for the generation of retroviral pathogens by recombination between activated endogenous or exogenous infectious agents. Some porcine tissues actively express infectious porcine endogenous retroviruses (PERVs). Of the three classes of PERV characterized to date, two, PERV-A and B, are capable of infecting human cells in vitro, whereas PERV-C cannot. Here, we demonstrate that the PERV-C envelope surface protein (SU) when disassociated from its C-terminus binds human cells. Further, we show that PERV-C binding to human cells is not inhibited in 293 cells productively infected with PERV-A, confirming that the molecule PERV-C interacts with on human cells is distinct from that used by PERV-A. Moreover, we demonstrate that the envelope region encompassing the proline-rich region is required for binding to cells in addition to the putative variable region A (VRA) and B (VRB). The region in the C-terminus of the SU that alters the binding and infectivity properties of PERV-C differs by only nine residues from the analogous region of PERV-A. Caution may be warranted even when a xenotransplantation product is from source pigs that do not express human-tropic viruses, as minimal mutations within PERV-C combined with selection in a human recipient could render PERV-C infectious in humans.

Mutations of a residue within the polyproline-rich region of Env alter the replication rate and level of cytopathic effects in chimeric avian retroviral vectors

Previous attempts to extend the host range of the avian sarcoma/leukosis virus (ASLV)-based RCASBP vectors produced two viral vectors, RCASBP M2C (4070A) and RCASBP M2C (797-8), which replicate using the amphotropic murine leukemia virus 4070A Env protein (2). Both viruses were adapted to replicate efficiently in the avian cell line DF-1, but RCASBP M2C (4070A) caused extensive cytopathic effects (CPE) in DF-1 cells whereas RCASBP M2C (797-8) induced low levels of CPE. The two viruses differed only at amino acid 242 of the polyproline-rich region in the surface (SU) subunit of the Env protein. In RCASBP M2C (4070A), an isoleucine replaced the wild-type proline residue, whereas a threonine residue was found in RCASBP M2C (797-8). In the present study, we show that other amino acid substitutions at position 242 strongly influence the CPE and replication rate of the chimeric viruses. There was a correlation between the amount of unintegrated linear retroviral DNA present in infected DF-1 cells and the level of CPE. This suggests that there may be a role for superinfection in the CPE. The treatment of RCASBP M2C (4070A)-infected cells with dantrolene, which inhibits the release of calcium from the endoplasmic reticulum (ER), reduced the amount of CPE seen during infection with the highly cytotoxic virus. Dantrolene treatment did not appear to affect virus production, suggesting that Ca2+ release from the ER had a role in the CPE caused by these viruses.

Functional domains within the human immunodeficiency virus type 2 envelope protein required to enhance virus production

Primate lentiviruses code for a protein that stimulates virus production. In human immunodeficiency virus type 1 (HIV-1), the activity is provided by the accessory protein, Vpu, while in HIV-2 and simian immunodeficiency virus it is a property of the envelope (Env) glycoprotein. Using a group of diverse retroviruses and cell types, we have confirmed the functional equivalence of the two proteins. However, despite these similarities, the two proteins have markedly different functional domains. While the Vpu activity is associated primarily with its membrane-spanning region, we have determined that the HIV-2 Env activity requires both the cytoplasmic tail and ectodomain of the protein, with the membrane-spanning domain being less important. Within the Env cytoplasmic tail, we further defined the necessary sequence as a membrane-proximal tyrosine-based motif. Providing the two Env regions separately as distinct CD8 chimeric proteins did not increase virus release. This suggests that the two domains must be either contained within a single protein or closely associated within a multiprotein oligomer, such as the Env trimer, in order to function. Finally, we observed that wild-type levels of incorporation of the HIV-2 Env into budding viruses were not required for this activity.

The central half of Pit2 is not required for its function as a retroviral receptor

The type III sodium-dependent phosphate (NaPi) cotransporter, Pit2, is a receptor for amphotropic murine leukemia virus (A-MuLV) and 10A1 MuLV. In order to determine what is sufficient for Pit2 receptor function, a deletion mutant lacking about the middle half of the protein was made. The mutant supported entry for both viruses, unequivocally narrowing down the identification of the sequence that is sufficient to specify the receptor functions of Pit2 to its N-terminal 182 amino acids and C-terminal 170 amino acids.

Cytoplasmic tail of Moloney murine leukemia virus envelope protein influences the conformation of the extracellular domain: implications for mechanism of action of the R Peptide

The envelope (Env) protein of Moloney murine leukemia virus (MoMuLV) is a homotrimeric complex whose monomers consist of linked surface (SU) and transmembrane (TM) proteins cleaved from a precursor protein by a cellular protease. In addition, a significant fraction of virion-associated TM is further processed by the viral protease to remove the C-terminal 16 amino acids of the cytoplasmic domain, the R peptide. This cleavage greatly enhances the fusogenicity of the protein and is necessary for the formation of a fully functional Env protein complex. We have previously proposed that R peptide cleavage enhances fusogenicity by altering the conformation of the ectodomain of the protein (Y. Zhao et al., J. Virol. 72:5392-5398, 1998). Using a series of truncation and point mutants of MoMuLV Env, we now provide direct biochemical and immunological evidence that the cytoplasmic tail and the membrane-spanning region of Env can influence the overall structure of the ectodomain of the protein and alter the strength of the SU-TM interaction. The R-peptide-truncated form of the protein, in particular, exhibits a markedly different conformation than the full-length protein.

G100R mutation within 4070A murine leukemia virus Env increases virus receptor binding, kinetics of entry, and viral transduction efficiency

Passage of 4070A murine leukemia virus (MuLV) in D17 cells resulted in a G-to-R change at position 100 within the VRA of the envelope protein (Env). Compared with 4070A MuLV, virus with the G100R Env displayed enhanced binding on target cells, internalized the virus more rapidly, and increased the overall viral titer in multiple cell types. This provides a direct correlation between binding strength and efficiency of viral entry. Deletion of a His residue at the SU N terminus eliminated the transduction efficiency by the G100R virus, suggesting that the G100R virus maintains the regulatory characteristics of 4070A viral entry. The improved transduction efficiency of G100R Env would be an asset for gene delivery systems.

Fusion-defective gibbon ape leukemia virus vectors can be rescued by homologous but not heterologous soluble envelope proteins

Murine leukemia virus (MLV)-derived envelope proteins containing alterations in or adjacent to the highly conserved PHQ motif present at the N terminus of the envelope surface subunit (SU) are incorporated into vector particles but are not infectious due to a postbinding block to viral entry. These mutants can be rendered infectious by the addition of soluble receptor-binding domain (RBD) proteins in the culture medium. The RBD proteins that rescue the infectivity of these defective MLV vectors can be derived from the same MLV or from other MLVs that use distinct receptors to mediate entry. We have now constructed functional immunologically reactive gibbon ape leukemia virus (GALV) envelope proteins, tagged with a feline leukemia virus (FeLV)-derived epitope tag, which are efficiently incorporated into infectious particles. Tagged GALV envelope proteins bind specifically to cells expressing the phosphate transporter protein Pit1, demonstrating for the first time that Pit1 is the binding receptor for GALV and not a coreceptor or another type of GALV entry factor. We have also determined that GALV particles bearing SU proteins with an insertion C-terminal to the PHQ motif (GALV I(10)) bind Pit1 but fail to infect cells. Incubation with soluble GALV RBD renders GALV I(10) particles infectious, whereas incubation with soluble RBDs from MLV or FeLV-B does not. This finding is consistent with the results obtained by Lauring et al. using FeLV-T, a virus that employs Pit1 as a receptor but requires soluble FeLV RBD for entry. MLV and GALV RBDs are not able to render FeLV-T infectious (A. S. Lauring, M. M. Anderson, and J. Overbaugh, J. Virol. 75:8888-8898, 2001). Together, these results suggest that fusion-defective FeLV-T and GALV are restricted to homologous RBD rescue of infectivity.

Altering retroviral tropism using a random-display envelope library

Tissue-specific gene delivery is an important aspect of many gene therapy applications. The experiments reported here constitute the first successful demonstration that cell-specific entry can be obtained by screening a random library of retroviral envelope proteins produced from a mammalian cell system. The library consisted of 10(6) different subgroup A feline leukemia virus envelope protein variants with 10 randomly substituted amino acids in the receptor-determining region. Selecting the library for fully functional envelope proteins able to mediate stable gene transfer resulted in the identification of a single envelope protein variant (EF). Subsequent examination of the host range of EF revealed that it was highly specific for D17 canine osteosarcoma cells. This was in contrast to the host ranges of the parental subgroup A and closely related subgroup C envelope proteins. Interference assays on D17 cells further indicated that receptor usage by EF was also altered compared with the A and C envelope proteins. The EF envelope protein thus isolated should be useful for studying gene therapy treatments of osteosarcoma in a large-animal model.

Receptors and entry cofactors for retroviruses include single and multiple transmembrane-spanning proteins as well as newly described glycophosphatidylinositol-anchored and secreted proteins

In the past few years, many retrovirus receptors, coreceptors, and cofactors have been identified. These molecules are important for some aspects of viral entry, although in some cases it remains to be determined whether they are required for binding or postbinding stages in entry, such as fusion. There are certain common features to the molecules that many retroviruses use to gain entry into the cell. For example, the receptors for most mammalian oncoretroviruses are multiple membrane-spanning transport proteins. However, avian retroviruses use single-pass membrane proteins, and a sheep retrovirus uses a glycosylphosphatidylinositol-anchored molecule as its receptor. For some retroviruses, particularly the lentiviruses, two cell surface molecules are required for efficient entry. More recently, a soluble protein that is required for viral entry has been identified for a feline oncoretrovirus. In this review, we will focus on the various strategies used by mammalian retroviruses to gain entry into the cell. The choice of receptors will also be discussed in light of pressures that drive viral evolution and persistence.

Identification of envelope determinants of feline leukemia virus subgroup B that permit infection and gene transfer to cells expressing human Pit1 or Pit2

The retroviral vector systems that are in common use for gene therapy are designed to infect cells expressing either of two widely expressed phosphate transporter proteins, Pit1 or Pit2. Subgroup B feline leukemia viruses (FeLV-Bs) use the gibbon ape leukemia virus receptor, Pit1, as a receptor for entry. Our previous studies showed that some chimeric envelope proteins encoding portions of FeLV-B could also enter cells by using a related receptor protein, Pit2, which serves as the amphotropic murine leukemia virus receptor (S. Boomer, M. Eiden, C. C. Burns, and J. Overbaugh, J. Virol. 71:8116--8123, 1997). Here we show that an arginine at position 73 within variable region A (VRA) of the FeLV-B envelope surface unit (SU) is necessary for viral entry into cells via the human Pit2 receptor. However, C-terminal SU sequences have a dominant effect in determining human Pit2 entry, even though this portion of the protein is outside known receptor binding domains. This suggests that a combination of specific VRA sequences and C-terminal sequences may influence interactions between FeLV-B SU and the human Pit2 receptor. Binding studies suggest that the C-terminal sequences may affect a postbinding step in viral entry via the Pit2 receptor, although in all cases, binding of FeLV-B SU to human Pit2 was weak. In contrast, neither the arginine 73 nor specific C-terminal sequences are required for efficient binding or infection with Pit1. Taken together, these data suggest that different residues in SU may interact with these two receptors. The specific FeLV-Bs described here, which can enter cells using either human Pit receptor, may be useful as envelope pseudotypes for viruses used in gene therapy.

Role of the non-homologous DNA end joining pathway in the early steps of retroviral infection

Early after infection, the retroviral RNA genome is reverse transcribed to generate a linear cDNA copy, then that copy is integrated into a chromosome of the host cell. We report that unintegrated viral cDNA is a substrate for the host cell non-homologous DNA end joining (NHEJ) pathway, which normally repairs cellular double-strand breaks by end ligation. NHEJ activity was found to be required for an end-ligation reaction that circularizes a portion of the unintegrated viral cDNA in infected cells. Consistent with this, the NHEJ proteins Ku70 and Ku80 were found to be bound to purified retroviral replication intermediates. Cells defective in NHEJ are known to undergo apoptosis in response to retroviral infection, a response that we show requires reverse transcription to form the cDNA genome but not subsequent integration. We propose that the double-strand ends present in unintegrated cDNA promote apoptosis, as is known to be the case for chromosomal double-strand breaks, and cDNA circularization removes the pro-apoptotic signal.

Sequences in the cytoplasmic tail of the gibbon ape leukemia virus envelope protein that prevent its incorporation into lentivirus vectors

Pseudotyping retrovirus and lentivirus vectors with different viral fusion proteins is a useful strategy to alter the host range of the vectors. Although lentivirus vectors are efficiently pseudotyped by Env proteins from several different subtypes of murine leukemia virus (MuLV), the related protein from gibbon ape leukemia virus (GaLV) does not form functional pseudotypes. We have determined that this arises because of an inability of GaLV Env to be incorporated into lentivirus vector particles. By exploiting the homology between the GaLV and MuLV Env proteins, we have mapped the determinants of incompatibility in the GaLV Env. Three modifications that allowed GaLV Env to pseudotype human immunodeficiency virus type 1 particles were identified: removal of the R peptide (C-terminal half of the cytoplasmic domain), replacement of the whole cytoplasmic tail with the corresponding MuLV region, and mutation of two residues upstream of the R peptide cleavage site. In addition, we have previously proposed that removal of the R peptide from MuLV Env proteins enhances their fusogenicity by transmitting a conformational change to the ectodomain of the protein (Y. Zhao et al., J. Virol. 72:5392-5398, 1998). Our analysis of chimeric MuLV/GaLV Env proteins provides further evidence in support of this model and suggests that proper Env function involves both interactions within the cytoplasmic tail and more long-range interactions between the cytoplasmic tail, the membrane-spanning region, and the ectodomain of the protein.

Detection and characterization of porcine endogenous retrovirus in porcine plasma and porcine factor VIII

The pig genome contains porcine endogenous retroviruses (PERVs) capable of infecting human cells. Detection of infectious retrovirus in porcine peripheral blood mononuclear cells and endothelial cells suggested to us that pig plasma is likely to contain PERV. Both PERV env sequences and viral reverse transcriptase (RT) activity were detected in all plasma samples isolated from four NIH minipigs. To detect infectious virus from plasma, we performed a culture assay using three cell lines of feline, swine, and human origin that had previously been shown to be permissive for PERV. Infectious virus was successfully cultured from all four NIH minipig plasmas on the swine cell line ST-IOWA. Using RT-PCR with env-specific primers, we could detect expression of PERV class C envelope in the supernatant of ST-IOWA cells that had been exposed to each pig plasma. We next examined a pig plasma derivative, Hyate:C (porcine factor VIII), and found evidence of PERV particles, since all six lots examined were positive for PERV RNA and RT activity. However, infectious virus could not be detected in clinical lots of Hyate:C, suggesting that the manufacturing process might reduce the load of infectious virus to levels below detectable limits of the assay. Detection of infectious virus in porcine plasma confirms and extends the previous findings that certain porcine cells express PERV when manipulated in vitro and clearly demonstrates that there are porcine cells that express infectious PERV constitutively in vivo.

Issues in the manufacture and transplantation of genetically modified hematopoietic stem cells

The advent of safe and practical means to correct, enhance or protect blood cells at the genetic level offers tantalizing therapeutic perspectives. At present, gene delivery using a replication-defective retrovirus is the most efficient method to stably transduce hematopoietic cells. The successful adaptation of retroviral infection to hematopoietic stem cells requires optimized transduction conditions that maximize gene transfer while preserving the cells' potential for engraftment and longterm hematopoiesis. The successful establishment of effective transduction protocols hinges on retrovirus biology as well as stem cell and transplantation biology. Interestingly, the genetic approach could permit novel strategies to promote host repopulation by transplanted stem cells. However, regulated and predictable expression of any transgene integrated at random chromosomal locations cannot be taken for granted. Investigation of the control of transgene expression and prevention of vector silencing will become increasingly important.

Efficient cell infection by Moloney murine leukemia virus-derived particles requires minimal amounts of envelope glycoprotein

Retrovirus entry into cells is mediated by specific interactions between the retrovirally encoded Env envelope glycoprotein and a host cell surface receptor. Though a number of peptide motifs responsible for the structure as well as for the binding and fusion activities of Env have been identified, only a few quantitative data concerning the infection process are available. Using an inducible expression system, we have expressed various amounts of ecotropic and amphotropic Env at the surfaces of Moloney murine leukemia virus-derived vectors and assayed for the infectivity of viral particles. Contrary to the current view that numerous noncooperative Env-viral receptor interactions are required for cell infection, we report here that very small amounts of Env are sufficient for optimal infection. However, increasing Env density clearly accelerates the rate at which infectious attachment to cells occurs. Moreover, our data also show that a surprisingly small number of Env molecules are sufficient to drive infection, albeit at a reduced efficiency, and that, under conditions of low expression, Env molecules act cooperatively. These observations have important consequences for our understanding of natural retroviral infection as well as for the design of cell-targeted infection techniques involving retroviral vectors.

Superior transduction of mouse hematopoietic stem cells with 10A1 and VSV-G pseudotyped retrovirus vectors

The inefficient transduction of human hematopoietic stem cells (HSC) with amphotropic retroviral vectors has been an obstacle to gene therapy for hematopoietic diseases. We have previously reported low levels of amphotropic retrovirus receptor (Pit-2) mRNA and higher levels of gibbon ape leukemia virus (GALV) or 10A1 retrovirus receptor (Pit-1) mRNA in mouse and human HSC. The vesicular stomatitis virus (VSV-G) uses an abundant membrane phospholipid as a receptor. We hypothesized that transduction of HSC requires relatively high levels of retrovirus receptor molecules. Because mouse HSC can be efficiently transduced by ecotropic virus through the abundant ecotropic receptor, the mouse is an ideal model to compare receptor levels and transduction. We have developed a cotransduction assay where ecotropic retrovirus transduction is a positive internal control for downstream steps in retrovirus transduction. A comparison of mouse HSC transduction with amphotropic, 10A1, and VSV-G envelopes showed that the level of amphotropic and 10A1 receptor mRNA in HSC correlated with the frequency of transduction. Transduction with VSV-G vectors was similar to that with 10A1 vectors. We conclude that the level of retrovirus receptor on HSC is critical for HSC transduction and that GALV or VSV-G vectors would be better for human HSC transduction.

Identification of regions in the Moloney murine leukemia virus SU protein that tolerate the insertion of an integrin-binding peptide

Targeting of retroviral vectors to specific cells has been attempted through engineering of the surface (SU) protein of the murine leukemia viruses (MuLVs), but in many cases this has adversely affected protein function and targeted delivery has been difficult to achieve. In this study, we have inserted a 15-mer peptide that binds specifically to the alpha(v)beta(3) integrin into the Moloney MuLV SU protein, including regions that are surface exposed in the crystal structure of the ecotropic receptor-binding domain. We have concentrated in particular on the variable regions VRA, VRB, and VRC, which are responsible for the use of distinct cellular receptors by different MuLV subtypes and therefore may be more likely to accommodate a heterologous binding moiety. Despite these considerations, only 8 of 26 insertion sites were tolerated, including two separate regions in VRA, a cluster of sites in VRC, and previously identified sites at the N-terminus of the protein and in the proline-rich region immediately downstream of the receptor-binding domain. When expressed on retroviral vector particles, all of the viable proteins retained the ability to bind to and transduce murine cells, although the VRC mutants and an insertion in VRA gave reduced binding and titer. Finally, although all of the viable chimeras could bind to alpha(v)beta(3) in a solid-phase binding assay, we were unable to demonstrate expanded tropism for alpha(v)beta(3)-expressing human cells. This study highlights the difficulty of engineering the Moloney MuLV SU protein, even when structural information is available, and provides guidelines for the insertion of peptide ligands into the SU protein.

Second-site changes affect viability of amphotropic/ecotropic chimeric enveloped murine leukemia viruses

Chimeras were previously generated between the ecotropic (Moloney-MuLV) and amphotropic (4070A) SU and TM proteins of murine leukemia virus (MuLV). After passage in D17 cells, three chimeras with junctions in the C terminus of SU (AE5, AE6, and AE7), showed improved kinetics of viral spreading, suggesting that they had adapted. Sequencing of the viruses derived from the D17 cell lines revealed second-site changes within the env gene. Changes were detected in the receptor binding domain, the proline-rich region, the C terminus of SU, and the ectodomain of TM. Second-site changes were subcloned into the parental DNA, singly and in combination, and tested for viability. All viruses had maintained their original cloned mutations and junctions. Reconstruction and passage of AE7 or AE6 virus with single point mutations recovered the additional second-site changes identified in the parental population. The AE5 isolate required changes in the VRA, the VRC, the VRB-hinge region, and the C terminus of SU for efficient infection. Passage of virus, including the parental 4070A, in D17 cells resulted in a predominant G100R mutation within the receptor binding domain. Viruses were subjected to titer determination in three cell types, NIH 3T3, canine D17, and 293T. AE6 viruses with changes in the proline-rich region initially adapted for growth on D17 cells could infect all cell types tested. AE6-based chimeras with additional mutations in the C terminus of SU could infect D17 and 293T cells. Infection of NIH 3T3 cells was dependent on the proline-rich mutation. AE7-based chimeras encoding L538Q and G100R were impaired in infecting NIH 3T3 and 293T cells.

A comprehensive approach to mapping the interacting surfaces of murine amphotropic and feline subgroup B leukemia viruses with their cell surface receptors

Because mutations in envelope glycoproteins of retroviruses or in their cell surface receptors can eliminate function by multiple mechanisms, it has been difficult to unambiguously identify sites for their interactions by site-directed mutagenesis. Recently, we developed a gain-of-function approach to overcome this problem. Our strategy relies on the fact that feline leukemia virus subgroup B (FeLV-B) and amphotropic murine leukemia virus (A-MLV) have closely related gp70 surface envelope glycoproteins and use related Na(+)-dependent phosphate symporters, Pit1 and Pit2, respectively, as their receptors. We previously observed that FeLV-B/A-MLV envelope glycoprotein chimeras spliced between the variable regions VRA and VRB were unable to use Pit1 or Pit2 as a receptor but could efficiently use specific Pit1/Pit2 chimeras. The latter study suggested that the VRA of A-MLV and FeLV-B functionally interact with the presumptive extracellular loops 4 and 5 (ECL4 and -5) of their respective receptors, whereas VRB interacts with ECL2. We also found that FeLV-B gp70 residues F60 and P61 and A-MLV residues Y60 and V61 in the first disulfide-bonded loop of VRA were important for functional interaction with the receptor's ECL4 or -5. We have now extended this approach to identify additional VRA and VRB residues that are involved in receptor recognition. Our studies imply that FeLV-B VRA residues F60 and P61 interact with the Pit1 ECL5 region, whereas VRA residues 66 to 78 interact with Pit1 ECL4. Correspondingly, A-MLV VRA residues Y60 and V61 interact with the Pit2 ECL5 region, whereas residues 66 to 78 interact with Pit2 ECL4. Similar studies that focused on the gp70 VRB implicated residues 129 to 139 as contributing to specific interactions with the receptor ECL2. These results identify three regions of gp70 that interact in a specific manner with distinct portions of their receptors, thereby providing a map of the functionally interacting surfaces.

High efficiency in vitro gene transfer into vascular tissues using a pseudotyped retroviral vector without pseudotransduction

Murine leukemia virus (MuLV)-derived retroviral vectors have had limited application in vascular gene therapy because of low transduction efficiency of vascular tissues, both in vitro and in vivo. In this study, we compared the gene transfer efficiency of two retroviral vectors: amphotropic MuLV and a MuLV vector pseudotyped with the vesicular stomatitis virus G glycoprotein (VSV-G) envelope. Target vascular tissues included human endothelial cells (EC), smooth muscle cells (SMC) and saphenous veins (SV). Transduction efficiency of human EC and SMC was significantly higher for VSV-G pseudotyped MuLV vector (90%) than for Amphotropic MuLV (20%). Luminal surface en face analysis of transduced cultured SV showed a six- to 10-fold greater transduction efficiency with VSV-G pseudotyped MuLV. The tissue plasminogen activator (tPA) gene was transduced into EC using each vector. Four days following transduction, a 12-fold higher tPA antigen concentration and a 38-fold higher tPA enzymatic activity was measured from cells transduced with the VSV-G pseudotyped vectors as compared with the amphotropic MuLV. There was no detectable pseudotransduction (protein transfer) associated with the VSV-G MuLV vector. Both AZT inhibition of reverse transcriptase and cell division arrest by gamma irradiation inhibited transduction, indicating that viral transduction correlated with RNA reverse transcription and cell proliferation. MuLV pseudotyped with the VSV-G envelope glycoprotein is an effective retroviral vector for vascular gene therapy.

Retroviral vector-mediated transfer and expression of human tissue plasminogen activator cDNA in bovine brain endothelial cells

OBJECTIVES

Gene transfer of thrombolytic enzymes to vascular endothelial cells may influence the kinetics of intravascular thrombosis. This study defines the potential for gene transfer of tissue plasminogen activator (tPA) into bovine brain endothelial cells (BBEC).

METHODS

The retroviral vectors derived from murine leukemia virus (MuLV) were used to transfer human tPA cDNA to BBEC. The tPA activity, tPA antigen and tPA inhibitor 1 (PAI-1) antigen were determined in the supernatant of transduced (BBEC/tPA) cell cultures by an immunoassay.

RESULTS

The tPA antigen and enzymatic activity in cell culture supernatants of BBEC/tPA transduced cells were 75 ng/ml and 14 IU/ml after 4 days, that was 25 and 28-fold higher compared to the respective values in control cells. The PAI-1 antigen was not affected by tPA cDNA transfer. The Western blot assay of cell lysates confirmed that the majority of tPA in BBEC/tPA transduced cells was in the form of free tPA. While the maximal transduction efficiency of BBEC with an amphotropic MuLV vector was about 15%, a MuLV pseudotyped with vesicular stomatitis virus G glycoprotein envelope achieved high > 90% maximal transduction efficiency.

CONCLUSIONS

The fibrinolytic activity of brain endothelial cells can be enhanced by transferring human tPA cDNA. These findings provide an initial step in implementation of future studies that investigate the use of this technology as an adjunctive treatment for cerebrovascular disease.

Receptor-mediated Moloney murine leukemia virus entry can occur independently of the clathrin-coated-pit-mediated endocytic pathway

To investigate receptor-mediated Moloney murine leukemia virus (MoMuLV) entry, the green fluorescent protein (GFP)-tagged ecotropic receptor designated murine cationic amino acid transporter (MCAT-1) (MCAT-1-GFP) was constructed and expressed in 293 cells (293/MCAT-1-GFP). 293/MCAT-1-GFP cells displayed green fluorescence primarily at the cell membrane and supported wild-type levels of MoMuLV vector binding and transduction. Using immunofluorescence labeling and confocal microscopy, it was demonstrated that the surface envelope protein (SU) gp70 of MoMuLV virions began to appear inside cells 5 min after virus binding and was colocalized with MCAT-1-GFP. However, clathrin was not colocalized with MCAT-1-GFP, suggesting that MoMuLV entry, mediated by MCAT-1, does not involve clathrin. Double immunofluorescence labeling of SU and clathrin in 293 cells expressing untagged receptor (293/MCAT-1) gave the same results, i.e., SU and clathrin did not colocalize. In addition, we examined the transduction ability of MoMuLV vector on HeLa cells overexpressing the dominant-negative GTPase mutant of dynamin (K44A). HeLa cells overexpressing mutant dynamin have a severe block in endocytosis by the clathrin-coated-pit pathway. No significant titer difference was observed when MoMuLV vector was tranduced into HeLa cells overexpressing either wild-type or mutant dynamin, while the transduction ability of vesicular stomatitis virus glycoprotein pseudotyped vector into HeLa cells overexpressing mutant dynamin was decreased significantly. Taken together, these data suggest that MoMuLV entry does not occur through the clathrin-coated-pit-mediated endocytic pathway.

Genetic engineering of stent grafts with a highly efficient pseudotyped retroviral vector

PURPOSE

The purpose of this study was first to compare the gene transfer efficiency of amphotrophic murine leukemia viral vector (ampho-MuLV) with the efficiency of MuLV pseudotyped with the vesicular stomatitis virus G glycoprotein (VSVG-MuLV) in tissue of vascular origin. The second purpose of this study was to determine cell retention after the implantation of genetically engineered stent grafts.

METHODS

Gene transfer efficiency was ascertained with the b-galactosidase assay. The target tissues included endothelial cells (ECs), smooth muscle cells (SMCs), and human saphenous veins (HSVs). Polyurethane stent grafts were suffused with lac Z-transduced ECs and SMCs that were harvested from porcine jugular vein. The grafts were implanted into the iliac artery of each pig whose jugular vein had been harvested. Cell retention was analyzed at 1 and 4 weeks with X-Gal staining.

RESULTS

VSVG-MuLV transduction efficiency exceeded that of ampho-MuLV in human ECs (VSVG-MuLV, n = 24, 89% +/- 6%; ampho-MuLV, n = 18, 14% +/- 6%; P <. 001), human SMCs (VSVG-MuLV, n = 5, 92% +/- 3%; ampho-MuLV, n = 4, 17% +/- 2%; P <.001), pig ECs (VSVG-MuLV, n = 4, 81% +/- 2%; ampho-MuLV, n = 4, 13% +/- 3%; P <.001), and pig SMCs (VSVG-MuLV, n = 5, 89% +/- 3%; ampho-MuLV, n = 4, 16% +/- 1%; P <.001). As much as a 10-fold higher transduction efficiency was observed with VSVG-MuLV in HSVs. After the stent graft implantation, the engineered cells were retained and proliferated on the stent membrane, with ingrowth into the underlying intima.

CONCLUSION

VSVG-MuLV significantly increased the gene transfer efficiency in vascular SMCs and ECs and in organ-cultured HSVs. The cells were retained and proliferated on stent grafts for the short term in the pig.

Identification of the block in targeted retroviral-mediated gene transfer

A chimeric retroviral vector (33E67) containing a CD33-specific single-chain antibody was generated in an attempt to target cells displaying the CD33 surface antigen. The chimeric envelope protein was translated, processed, and incorporated into viral particles as efficiently as wild-type envelope protein. The viral particles carrying the 33E67 envelope protein could bind efficiently to the CD33 receptor on target cells and were internalized, but no gene transfer occurred. A unique experimental approach was used to examine the basis for this postbinding block. Our data indicate that the chimeric envelope protein itself cannot participate in the fusion process, the most reasonable explanation being that this chimeric protein cannot undergo the appropriate conformational change that is thought to be triggered by receptor binding, a suggested prerequisite to subsequent fusion and core entry. These results indicate that the block to gene transfer in this system, and probably in most of the current chimeric retroviral vectors to date, is the inability of the chimeric envelope protein to undergo this obligatory conformational change.

Targeting retroviral vectors to CD34-expressing cells: binding to CD34 does not catalyze virus-cell fusion

We have attempted to engineer murine leukemia virus (MuLV)-based retroviral vectors to specifically transduce cells expressing human CD34, an antigen present on the surface of undifferentiated hematopoietic stem cells. A number of chimeric ecotropic MuLV envelope (Env) proteins were constructed that contained anti-CD34 single-chain antibody variable fragments (scFvs). The scFv-Env proteins were generated either by replacing the receptor-binding domain of Env with the scFv or by inserting the scFv into the N terminus of the Env protein. Only chimeric Env proteins with scFv insertions between amino acids 6 and 7 were incorporated into viral particles, and coexpression of native MuLV Env did not rescue incorporation-defective proteins. In addition, the efficiency of incorporation varied with the specific anti-CD34 scFv that was used. Retroviral vectors containing the scFv-Env proteins bound to CD34+ cells and transduced NIH 3T3 cells expressing human CD34 (3T3-CD34 cells) at approximately twice the efficiency of the parental NIH 3T3 cells. However, the introduction of the mutation D84K, which prevents binding to the ecotropic MuLV receptor mcat-1, prevented transduction of both NIH 3T3 and 3T3-CD34 cells. Complementation cell-cell fusion assays [Zhao et al. (1997). J. Virol. 71, 6967-6972] in 3T3-CD34 cells revealed that although the scFv-Env proteins could contribute postbinding entry functions when bound to mcat-1, they were unable to do so when bound to CD34. Taken together, these data suggest that although the interaction with CD34 effectively increased the concentration of virus on 3T3-CD34 cells, entry could occur only through an interaction with mcat-1; CD34 alone was not capable of triggering the appropriate postbinding changes that lead to viral entry.

Simian sarcoma-associated virus fails to infect Chinese hamster cells despite the presence of functional gibbon ape leukemia virus receptors

We have sequenced the envelope genes from each of the five members of the gibbon ape leukemia virus (GALV) family of type C retroviruses. Four of the GALVs, including GALV strain SEATO (GALV-S), were originally isolated from gibbon apes, whereas the fifth member of this family, simian sarcoma-associated virus (SSAV), was isolated from a woolly monkey and shares 78% amino acid identity with GALV-S. To determine whether these viruses have identical host ranges, we evaluated the susceptibility of several cell lines to either GALV-S or SSAV infection. GALV-S and SSAV have the same host range with the exception of Chinese hamster lung E36 cells, which are susceptible to GALV-S but not SSAV. We used retroviral vectors that differ only in their envelope composition (e.g., they contain either SSAV or GALV-S envelope protein) to show that the envelope of SSAV restricts entry into E36 cells. Although unable to infect E36 cells, SSAV infects GALV-resistant murine cells expressing the E36-derived viral receptor, HaPit2. These results suggest that the receptors present on E36 cells function for SSAV. We have constructed several vectors containing GALV-S/SSAV chimeric envelope proteins to map the region of the SSAV envelope that blocks infection of E36 cells. Vectors bearing chimeric envelopes comprised of the N-terminal region of the GALV-S SU protein and the C-terminal region of SSAV infect E36 cells, whereas vectors containing the N-terminal portion of the SSAV SU protein and C-terminal portion of GALV-S fail to infect E36 cells. This finding indicates that the region of the SSAV envelope protein responsible for restricting SSAV infection of E36 cells lies within its amino-terminal region.

Role of variable regions A and B in receptor binding domain of amphotropic murine leukemia virus envelope protein

For the amphotropic murine leukemia virus (MuLV), a 208-amino-acid amino-terminal fragment of the surface unit (SU) of the envelope glycoprotein is sufficient to bind to its receptor, Pit2. Within this binding domain, two hypervariable regions, VRA and VRB, have been proposed to be important for receptor recognition. In order to specifically locate residues that are important for the interaction with Pit2, we generated a number of site-specific mutations in both VRA and VRB and analyzed the resulting envelope proteins when expressed on retroviral vectors. Concurrently, we substituted portions of the amphotropic SU with homologous regions from the polytropic MuLV envelope protein. The amphotropic SU was unaffected by most of the point mutations we introduced. In addition, the deletion of eight residues in a region of VRA that was previously suggested to be essential for Pit2 utilization only decreased titer on NIH 3T3 cells by 1 order of magnitude. Although the replacement of the amino-terminal two-thirds of VRA with the polytropic sequence abolished receptor binding, smaller nonoverlapping substitutions did not affect the function of the protein. We were not able to identify a single critical receptor contact point within VRA, and we suggest that the amphotropic receptor binding domain probably makes multiple contacts with the receptor and that the loss of some of these contacts can be tolerated.

Characterization of the proline-rich region of murine leukemia virus envelope protein

Mammalian type C retroviral envelope proteins contain a variable proline-rich region (PRR), located between the N-terminal receptor-binding domain and the more highly conserved C-terminal portion of the surface (SU) subunit. We have investigated the role of the PRR in the function of murine leukemia virus (MuLV) envelope protein. In the MuLVs, the PRR contains a highly conserved N-terminal sequence and a hypervariable C-terminal sequence. Despite this variability, the amphotropic PRR could functionally substitute for the ecotropic PRR. The hypervariable region of the PRR was not absolutely required for envelope protein function. However, truncations in this region resulted in decreased levels of both the SU and TM proteins in viral particles and increased amounts of the uncleaved precursor protein, Pr85. In contrast, the N-terminal conserved region was essential for viral infectivity. Deletion of this region prevented the stable incorporation of envelope proteins into viral particles in spite of normal envelope protein processing, wild-type levels of cell surface expression, and a wild-type ability to induce syncytia in an XC cell cocultivation assay. However, higher levels of the SU protein were shed into the supernatant, suggesting a defect in SU-TM interactions. Our data are most consistent with a role for the PRR in stabilizing the overall structure of the protein, thereby affecting the proper processing of Pr85, SU-TM interactions, and the stable incorporation of envelope proteins into viral particles. In addition, we have demonstrated that the PRR can tolerate the insertion of a peptide-binding domain, making this a potentially useful site for constructing targetable retroviral vectors.

Functional domains in the retroviral transmembrane protein

The envelope glycoproteins of the mammalian type C retroviruses consist of two subunits, a surface (SU) protein and a transmembrane (TM) protein. SU binds to the viral receptor and is thought to trigger conformational changes in the associated TM protein that ultimately lead to the fusion of viral and host cell membranes. For Moloney murine leukemia virus (MoMuLV), the envelope protein probably exists as a trimer. We have previously demonstrated that the coexpression of envelope proteins that are individually defective in either the SU or TM subunits can lead to functional complementation (Y. Zhao et al., J. Virol. 71:6967-6972, 1997). We have now extended these studies to investigate the abilities of a panel of fusion-defective TM mutants to complement each other. This analysis identified distinct complementation groups within TM, with implications for interactions between different regions of TM in the fusion process. In viral particles, the C-terminal 16 amino acids of the MoMuLV TM (the R peptide) are cleaved by the viral protease, resulting in an increased fusogenicity of the envelope protein. We have examined the consequences of R peptide cleavage for the different TM fusion mutants and have found that this enhancement of fusogenicity can only occur in cis to certain of the TM mutants. These results suggest that R peptide cleavage enhances the fusogenicity of the envelope protein by influencing the interaction of two distinct regions in the TM ectodomain.