Mutational analysis of the fusion peptide of Moloney murine leukemia virus transmembrane protein p15E

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.

Sequence variation of koala retrovirus transmembrane protein p15E among koalas from different geographic regions

The koala retrovirus (KoRV), which is transitioning from an exogenous to an endogenous form, has been associated with high mortality in koalas. For other retroviruses, the envelope protein p15E has been considered a candidate for vaccine development. We therefore examined proviral sequence variation of KoRV p15E in a captive Queensland and three wild southern Australian koalas. We generated 163 sequences with intact open reading frames, which grouped into 39 distinct haplotypes. Sixteen distinct haplotypes comprising 139 of the sequences (85%) coded for the same polypeptide. Among the remaining 23 haplotypes, 22 were detected only once among the sequences, and each had 1 or 2 non-synonymous differences from the majority sequence. Several analyses suggested that p15E was under purifying selection. Important epitopes and domains were highly conserved across the p15E sequences and in previously reported exogenous KoRVs. Overall, these results support the potential use of p15E for KoRV vaccine development.

Cooperative cleavage of the R peptide in the Env trimer of Moloney murine leukemia virus facilitates its maturation for fusion competence

The spike protein of murine leukemia virus, MLV, is made as a trimer of the Env precursor. This is primed for receptor-induced activation of its membrane fusion function first by cellular furin cleavage in the ectodomain and then by viral protease cleavage in the endodomain. The first cleavage separates the peripheral surface (SU) subunit from the transmembrane (TM) subunit, and the latter releases a 16-residue-long peptide (R) from the TM endodomain. Here, we have studied the distribution of R peptide cleavages in the spike TM subunits of Moloney MLV preparations with partially R-peptide-processed spikes. The spikes were solubilized as trimers and separated with an R peptide antibody. This showed that the spikes were either uncleaved or cleaved in all of its TM subunits. Further studies showed that R peptide cleavage-inhibited Env mutants, L(649)V and L(649)I, were rescued by wild-type (wt) Env in heterotrimeric spikes. These findings suggested that the R peptide cleavages in the spike are facilitated through positive allosteric cooperativity; i.e., the cleavage of the TM subunit in one Env promoted the cleavages of the TMs in the other Envs. The mechanism ensures that protease cleavage in newly released virus will generate R-peptide-cleaved homotrimers rather than heterotrimeric intermediates. However, using a cleavage site Env mutant, L(649)R, which was not rescued by wt Env, it was possible to produce virus with heterotrimers. These were shown to be less fusion active than the R-peptide-cleaved homotrimers. Therefore, the cooperative cleavage will speed up the maturation of released virus for fusion competence.

The presence of a single N-terminal histidine residue enhances the fusogenic properties of a Membranotropic peptide derived from herpes simplex virus type 1 glycoprotein H

Herpes simplex virus type 1 (HSV-1)-induced membrane fusion remains one of the most elusive mechanisms to be deciphered in viral entry. The structure resolution of glycoprotein gB has revealed the presence of fusogenic domains in this protein and pointed out the key role of gB in the entry mechanism of HSV-1. A second putative fusogenic glycoprotein is represented by the heterodimer comprising the membrane-anchored glycoprotein H (gH) and the small secreted glycoprotein L, which remains on the viral envelope in virtue of its non-covalent interaction with gH. Different domains scattered on the ectodomain of HSV-1 gH have been demonstrated to display membranotropic characteristics. The segment from amino acid 626 to 644 represents the most fusogenic region identified by studies with synthetic peptides and model membranes. Herein we have identified the minimal fusogenic sequence present on gH. An enlongation at the N terminus of a single histidine (His) has proved to profoundly increase the fusogenic activity of the original sequence. Nuclear magnetic resonance (NMR) studies have shown that the addition of the N-terminal His contributes to the formation and stabilization of an alpha-helical domain with high fusion propensity.

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.

Membrane activity of an amphiphilic alpha-helical membrane-proximal cytoplasmic domain of the MoMuLV envelope glycoprotein

In the Moloney murine leukemia virus (MoMuLV) envelope glycoprotein (Env) we identified a membrane-proximal cytoplasmic domain (residues 598-616) that facilitates the Env incorporation into virions and Env-mediated fusion [Rozenberg, Y., Conner, J., Aguilar-Carreno, H., Chakraborti, S., Dimiter, D.S., Anderson, W.F., 2008. Viral entry: membrane-proximal cytoplasmic domain of MoMuLV envelope tail facilitates fusion. In the same issue. (accompanying paper)]. By biophysical methods (CD, EPR) a corresponding peptide (membrane-proximal peptide, 598-616) was demonstrated to form a membrane-parallel amphiphilic alpha-helix in the presence of membranes. Electrophysiological studies with planar bilayers and liposomes indicate that the membrane-proximal peptide is membrane destabilizing. This peptide and the fusion peptide from the MoMuLV transmembrane (TM) ectodomain were tested for their effect on the bilayer for hexagonal phase transition temperature of dipalmitoleoylphosphatidylethanolamine (T(H)). Importantly, the external fusion peptide and the internal membrane-proximal peptides of MoMuLV env exert opposite effects on membrane curvature. The fusion peptide lowers T(H) while the membrane proximal peptide raises it. These effects on T(H) correlate with the ability of these peptides to induce lipid mixing in large unilamellar vesicles composed of dioleoylphosphatidylethanolamine: dioleoylphosphatidylcholine:cholesterol (1:1:1 mol). When added externally to preformed liposomes, the N-terminal fusion peptide promotes lipid mixing while the cytoplasmic membrane-proximal peptide inhibits this effect. These finding indicate a possible mechanism by which the membrane-proximal domain in MoMuLV Env may affect the formation of membrane fusion intermediates.

TR1.3 viral pathogenesis and syncytium formation are linked to Env-Gag cooperation

Infection with murine leukemia virus (MLV) TR1.3 or the related molecular construct W102G causes severe neuropathology in vivo. Infection is causally linked to the development of extensive syncytia in brain capillary endothelial cells (BCEC). These viruses also induce cell fusion of murine cell lines, such as SC-1 and NIH 3T3, which are otherwise resistant to MLV-induced syncytium formation. Although the virulence of these viruses maps within the env gene, the mechanism of fusion enhancement is not fully determined. To this end, we examined the capacity of the syncytium-inducing (SI) TR1.3 and W102G MLVs to overcome the fusion inhibitory activity inherent in the full-length Env cytoplasmic tail. These studies showed that the TR1.3 and W102G Envs did not induce premature cleavage of p2E, nor did they override p2E fusion inhibition. Indeed, in the presence of mutations that disrupt p2E function, the TR1.3 and W102G Envs significantly increased the extent of cell fusion compared to that with the non-syncytium-inducing MLV FB29. Surprisingly, we also observed that TR1.3 and W102G Envs failed to elicit syncytium formation in these in vitro assays. Coexpression of gag-pol with env restored syncytium formation, and accordingly, mutations within gag-pol were used to examine the minimal functional requirements for the SI phenotype. The results indicate that both gag-dependent particle budding and cleavage of p2E are required to activate the SI phenotype of TR1.3 and W102G viruses. Collectively, these data suggest that the TR1.3 and W102G viruses induce cell fusion by the fusion-from-without pathway.

Increase in infectivity of targeted Moloney murine leukemia virus-based gene-delivery vectors through lowering the threshold for fusion

Many research groups have developed targeted vectors for gene therapy based on Moloney murine leukemia virus (MoMLV). Despite proper binding of the targeted vector to the target molecule, little or no infectivity of human cells expressing the target molecule has been achieved in most studies. One of the reasons for this lack of infectivity may be steric hindrance within the targeted envelope glycoprotein (Env), impeding the conformational changes required for fusion and infection. Here, attempts were made to solve this problem by mutating key residues within Env of two targeted MoMLV-based vectors, MoMLV-E-Sel and MoMLV-FBP. Selection of key residues was based on an Env with reduced threshold for fusion, that of the CD4-independent human immunodeficiency virus type 2 isolate ROD/B. It was shown here that vectors bearing MoMLV-FBP Env with a V512M substitution had higher titres and faster kinetics of entry than vectors bearing parental targeted Env proteins. This could be due to the partial release of steric constraints that result in an Env with a reduced threshold for fusion.

The role of human endogenous retroviruses in trophoblast differentiation and placental development

A major portion of the human genome appears to be of retroviral origin. These endogenous retroviral elements are expressed in a variety of normal tissues and during disease states, such as autoimmune and malignant conditions. Recently, potential roles have been described for endogenous retroviral envelope proteins in normal differentiation of human villous cytotrophoblast into syncytiotrophoblast. This article provides a brief critical review of the current state of knowledge concerning the expression of the env regions of three endogenous retroviral elements: ERV-3, HERV-W, and HERV-FRD. A testable model of villous cytotrophoblast differentiation is constructed, in which a complementary expression of endogenous retroviral envelope proteins initiates hCG production, decreased cell proliferation, and intercellular fusion.

Conserved glycine residues in the fusion peptide of the paramyxovirus fusion protein regulate activation of the native state

Hydrophobic fusion peptides (FPs) are the most highly conserved regions of class I viral fusion-mediating glycoproteins (vFGPs). FPs often contain conserved glycine residues thought to be critical for forming structures that destabilize target membranes. Unexpectedly, a mutation of glycine residues in the FP of the fusion (F) protein from the paramyxovirus simian parainfluenza virus 5 (SV5) resulted in mutant F proteins with hyperactive fusion phenotypes (C. M. Horvath and R. A. Lamb, J. Virol. 66:2443-2455, 1992). Here, we constructed G3A and G7A mutations into the F proteins of SV5 (W3A and WR isolates), Newcastle disease virus (NDV), and human parainfluenza virus type 3 (HPIV3). All of the mutant F proteins, except NDV G7A, caused increased cell-cell fusion despite having slight to moderate reductions in cell surface expression compared to those of wild-type F proteins. The G3A and G7A mutations cause SV5 WR F, but not NDV F or HPIV3 F, to be triggered to cause fusion in the absence of coexpression of its homotypic receptor-binding protein hemagglutinin-neuraminidase (HN), suggesting that NDV and HPIV3 F have stricter requirements for homotypic HN for fusion activation. Dye transfer assays show that the G3A and G7A mutations decrease the energy required to activate F at a step in the fusion cascade preceding prehairpin intermediate formation and hemifusion. Conserved glycine residues in the FP of paramyxovirus F appear to have a primary role in regulating the activation of the metastable native form of F. Glycine residues in the FPs of other class I vFGPs may also regulate fusion activation.

Rapid and sensitive detection of retrovirus entry by using a novel luciferase-based content-mixing assay

We describe a novel assay that permits measurement of entry of murine leukemia virus and pseudotypes with greater sensitivity and more rapidly than previously possible. To achieve this, we encapsulated a sensitive reporter enzyme, luciferase, directly into fully infectious, intact viral particles. The enzyme is specifically targeted to the viral lumen, as a C-terminal fusion on the viral envelope protein. Only when the incorporated luciferase is released from the viral lumen and gains access to its substrates is light emitted and readily detected. When cells are perfused with luciferin, quantitative measurements of entry can be made in real time on live cells. Uniquely, the amount of cell-bound virus can be determined in the same assay by addition of detergent to expose the luciferase. We demonstrate that virus carrying a mutation in the fusion peptide binds normally to cells but is unable to infect them and gives no entry signal. Using this assay, we show that inhibitors of endosomal acidification inhibit signal from vesicular stomatitis virus pseudotypes but not murine leukemia virus, consistent with a pH-independent mode of entry for the latter virus. Additionally, the fusion kinetics are rapid, with a half-life of 25 min after a delay of 10 to 15 min. The future use of this assay will permit a detailed examination of the entry mechanism of viruses and provide a convenient platform to discover novel entry inhibitors. The design also permits packaging of potential therapeutic protein cargoes into functional virus particles and their specific delivery to cellular targets.

Promotion of retroviral entry in the absence of envelope protein by chlorpromazine

Retrovirus packaging cell lines that express the Moloney murine leukemia virus gag, pol, and env genes and a retroviral vector genome can produce virus particles that are capable of transducing cells. Normally if the packaging cell line does not produce a functional viral fusion glycoprotein, such as the retroviral envelope protein or a foreign viral glycoprotein, then the viruses will be incapable of transducing cells. We have found that incubating envelope protein-deficient virus particles bound to cells with chlorpromazine leads to transduction. Chlorpromazine (CPZ) is a membrane-active reagent that is commonly used to induce the hemifusion to fusion transition when membrane fusion is mediated by partially defective viral glycoproteins. The concentration and pH dependence of the promotion of transduction by CPZ is consistent with a role for CPZ micelle formation in viral entry. These data indicate that caution is warranted when experiments concerning membrane fusion completion promoted by CPZ are analyzed.

Identification of conformational and cold-sensitive mutations in the MuLV envelope protein

The structure and function of the C-terminal domain of the murine leukemia virus Surface protein (MuLV SU) is not well defined. Passage of chimeric ecotropic-amphotropic MuLV viruses with junctions within the SU C-terminus results in the selection of specific point mutations which improve virus viability and Env function. Point mutations were characterized that alter the conformation of the SU/TM heterodimers on the viral particles. Mutation of position E311 within the Moloney MuLV SU protein alters the conformation of the TM protein and its recognition by antibody 42-114 in immunoprecipitation reactions. Mutation of either G541R in the amphotropic 4070A TM, V421M in the 4070A SU, or deletion of S39 and P40 at the N-terminus of the M-MuLV SU results in an irreversible cold-sensitive phenotype at 4 degrees C. This loss of viral titer can be restored by incorporating V421M plus G541R or del S39 P40 plus G541R in cis within the SU/TM.

Structural criteria for regulation of membrane fusion and virion incorporation by the murine leukemia virus TM cytoplasmic domain

The cytoplasmic domains of viral glycoproteins influence the trafficking and subcellular localization of the glycoproteins and their incorporation into virions. They also promote correct virus morphology and viral budding. The cytoplasmic domains of murine-leukemia-virus envelope-protein TM subunits regulate membrane fusion. During virion maturation the carboxy-terminal 16 amino acid residues of the TM protein are removed by the retroviral protease. Deletion of these residues activates envelope-protein-mediated membrane fusion. Our quantitative analysis of the effects of Moloney murine leukemia virus TM mutations on envelope-protein function support the proposition that a trimeric coiled coil in the TM cytoplasmic domain inhibits fusion. The data demonstrate that cleavage of the TM cytoplasmic domain is not required for viral entry and provide evidence for a model in which fusogenic and nonfusogenic conformations of the envelope protein exists in an equilibrium that is regulated by the cytoplasmic domain. In addition, a conserved tyrosine residue in the TM cytoplasmic domain was shown to play an important role in envelope-protein incorporation into retroviral particles.

Characterization of a cytolytic strain of equine infectious anemia virus

A novel strain of equine infectious anemia virus (EIAV) called vMA-1c that rapidly and specifically killed infected equine fibroblasts (ED cells) but not other infectible cell lines was established. This strain was generated from an avirulent, noncytopathic strain of EIAV, MA-1. Studies with this new cytolytic strain of virus have permitted us to define viral parameters associated with EIAV-induced cell killing and begin to explore the mechanism. vMA-1c infection resulted in induction of rapid cell death, enhanced fusogenic activity, and increased rates of spread in equine fibroblasts compared to other strains of EIAV. The highly cytolytic nature of vMA-1c suggested that this strain might be superinfecting equine fibroblasts. Receptor interference studies demonstrated that prior infection of equine fibroblasts with EIAV did not alter the ability of vMA-1c to infect and kill these cells. In similar studies in a canine fibroblast cell line, receptor interference did occur. vMA-1c infection of equine fibroblasts was also associated with large quantities of unintegrated viral DNA, a well-established hallmark of retroviral superinfection. Cloning of the vMA-1c genome identified nucleotide changes that would result in at least one amino acid change in all viral proteins. A chimeric infectious molecular clone containing the vMA-1c tat, S2, and env open reading frames recapitulated most of the characteristics of vMA-1c, including superinfection, fibroblast killing, and fusogenic activity. In summary, in vitro selection for a strain of EIAV that rapidly killed cells resulted in the generation of a virus that was able to superinfect these cells, presumably by the use of a novel mechanism of cell entry. This phenotype mapped to the 3' half of the genome.

Comprehensive mutational analysis of the Moloney murine leukemia virus envelope protein

The envelope (Env) protein of Moloney murine leukemia virus is the primary mediator of viral entry. We constructed a large pool of insertion mutations in the env gene and analyzed the fitness of each mutant in completing two critical steps in the virus life cycle: (i) the expression and delivery of the Env protein to the cell surface during virion assembly and (ii) the infectivity of virions displaying the mutant proteins. The majority of the mutants were poorly expressed at the producer cell surface, suggesting folding defects due to the presence of the inserted residues. The mutants with residual infectivity had insertions either in the amino-terminal signal sequence region, two disulfide-bonded loops in the receptor binding domain, discrete regions of the carboxy-terminal region of the surface subunit (SU), or the cytoplasmic tail. Insertions that allowed the mutants to reach the cell surface but not to mediate detectable infection were located within the amino-terminal sequence of the mature Env, within the SU carboxy-terminal region, near putative receptor binding residues, and throughout the fusion peptide. Independent analysis of select mutants in this group allowed more precise identification of the defect in Env function. Mapping of mutant phenotypes to a structural model of the receptor-binding domain provides insights into the protein's functional organization. The high-resolution functional map reported here will be valuable for the engineering of the Env protein for a variety of uses, including gene therapy.

Fv-4: identification of the defect in Env and the mechanism of resistance to ecotropic murine leukemia virus

Mice expressing the Fv-4 gene are resistant to infection by ecotropic murine leukemia viruses (MuLVs). The Fv-4 gene encodes an envelope (Env) protein whose putative receptor-binding domain resembles that of ecotropic MuLV Env protein. Resistance to ecotropic MuLVs appears to result from viral interference involving binding of the endogenously expressed Fv-4 env-encoded protein to the ecotropic receptor, although the immune system also plays a role in resistance. The Fv-4 env-encoded protein is processed normally and can be incorporated into virus particles but is unable to promote viral entry. Among the many sequence variations between the transmembrane (TM) subunit of the Fv-4 env-encoded protein and the TM subunits of other MuLV Env proteins, there is a substitution of an arginine residue in the Fv-4 env-encoded protein for a glycine residue (gly-491 in Moloney MuLV Env) that is otherwise conserved in all of the other MuLVs. This residue is present in the MuLV TM fusion peptide sequence. In this study, gly-491 of Moloney MuLV Env has been replaced with other residues and a mutant Env bearing a substitution for gly-487 was also created. G491R recapitulates the Fv-4 Env phenotype in cell culture, indicating that this substitution is sufficient for creation of an Env protein that can establish the interference-mediated resistance to ecotropic viruses produced by the Fv-4 gene. Analysis of the mutant MuLV Env proteins also has implications for an understanding of the role of conserved glycine residues in fusion peptides and for the engineering of organismal resistance to retroviruses.

Lesion-targeted injectable vectors for vascular restenosis

Pathologic lesions caused by catheter-based revascularization procedures for occlusive artery disease include disruption of the endothelium, exposure of extracellular matrix (ECM) proteins, and proliferation of vascular smooth muscle cells, which lead to neointima formation and restenosis. We have developed matrix-collagen-targeted retroviral vectors that are able to accumulate at sites of vascular injury (Hall et al., Hum. Gene Ther. 1997;8:2183-2192; Hall et al., Hum. Gene Ther. 2000;11:983-993). The primary tissue-targeting motif, adapted from the physiological surveillance sequence found in von Willebrand factor, served to localize and concentrate the vector within vascular lesions. In the present study, we evaluated the efficiency of this vector-targeting system in rats with nonligated balloon-injured carotid arteries. Both intraarterial (by retrograde femoral artery catheterization) and intravenous (via femoral vein) injection of a matrix-targeted vector enhanced transduction of neointimal cells ( approximately 20%) at severely denuded areas when compared with the nontargeted vector (<1%). Further, intraarterial instillation of a matrix-targeted, but not a nontargeted, vector bearing an antisense cyclin G1 construct inhibited neointima lesion formation in the injured carotid arteries. Taken together, these data indicate that strategic targeting of retroviral vectors to vascular lesions would have therapeutic potential in the management of vascular restenosis and many other disorders of uncontrolled proliferation where endothelial disruption, ECM remodeling, and collagen deposition form the nexus for preferential vector localization and concentration in vivo.

Activation of membrane fusion by murine leukemia viruses is controlled in cis or in trans by interactions between the receptor-binding domain and a conserved disulfide loop of the carboxy terminus of the surface glycoprotein

Cell entry of retroviruses is initiated by the recognition of cellular receptors and the subsequent membrane fusion between viral and cellular membranes. These two steps are mediated by the surface (SU) and transmembrane (TM) subunits of the retroviral envelope glycoprotein (Env), respectively. Determinants regulating membrane fusion have been described throughout SU and TM, but the processes coupling receptor recognition to fusion are still elusive. Here we establish that a critical interaction is formed between the receptor-binding domain (RBD) and the major disulfide loop of the carboxy-terminal domain (C domain) of the murine leukemia virus SU. Receptor binding causes an alteration of this interaction and, in turn, promotes further events of Env fusion activation. We characterize mutations which, by lowering this interaction and reducing the compatibility between the RBD and C domains of Env glycoprotein chimeras, affect both Env fusogenicity and sensitivity to receptor interference. Additionally, we demonstrate that suboptimal interactions in such mutant Env proteins can be compensated in trans by soluble RBDs in a manner that depends on their compatibility with the C domain. Our results therefore indicate that RBD/C domain interactions may occur in cis, via the proper RBD of the viral Env itself, or in trans, via a distinct RBD expressed by virion-free Env glycoproteins expressed endogenously by the infected cells or provided by neighboring Env trimers.

Substitution of Asp114 or Arg116 in the fingers domain of moloney murine leukemia virus reverse transcriptase affects interactions with the template-primer resulting in decreased processivity

Reverse transcriptase, an essential retroviral DNA polymerase, replicates the single-stranded RNA genome of the retrovirus, producing a double-stranded DNA copy, which is subsequently integrated into the host's genome. Substitution of Ala for either Asp114 or Arg116, two highly conserved residues in the fingers domain of Moloney murine leukemia virus reverse transcriptase, results in enzymes (D114A or R116A) with significant defects in their abilities to processively synthesize DNA using RNA or DNA as a template. D114A and R116A enzymes also bind more weakly to template-primer in the presence of added deoxyribonucleotides, as seen by gel-shift analysis, but retain the ability to strand transfer and accumulate smaller RNase H cleavage products when compared to the wild-type enzyme. In addition, mutant proviruses, including D114A and R116A substitutions in Moloney murine leukemia virus reverse transcriptase, are not viable despite the presence of processed reverse transcriptase in the viral particles. A potential mechanistic role in processive synthesis for D114 and R116 is discussed in the context of our results, related studies on HIV-1 reverse transcriptase, and previous structural studies.

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.

Incorporation of tumor vasculature targeting motifs into moloney murine leukemia virus env escort proteins enhances retrovirus binding and transduction of human endothelial cells

Adhesion receptors expressed on the surfaces of tumor-activated endothelial cells provide an advantageous locus for targeting gene therapy vectors to angiogenic tissues and/or tumor vasculature. In this study, we engineered a series of Asn-Gly-Arg (NGR)-containing congeners of the presumptive cell binding motif contained within the ninth type III repeat of fibronectin and displayed these tumor vasculature targeting motifs (TVTMs) within the context of Moloney murine leukemia envelope "escort" proteins. Comparative studies of envelope incorporation into viral particles and evaluation of the cell binding properties of the targeted vectors revealed critical structural features, thus identifying a subset of optimal TVTMs. Utilizing a modified ELISA to evaluate viral binding to target cells, we observed a significant down-regulation of TVTM-virion binding to human endothelial cells following sustained (48-h) exposure to VEGF. Normalized for equivalent titers (10(6) CFU/ml), as assayed on NIH 3T3 cells, vectors displaying TVTM escort proteins significantly enhanced the transduction efficiency from 12.2 to 37.4% in human KSY-1 endothelial cell cultures (P < 0.001) and from 0.4 to 4.1% in human umbilical vein endothelial cell (HUVEC) cultures (P < 0.001). In summary, these studies utilized an engineering approach to identify a subset of TVTMs that are stably incorporated as envelope "escort" proteins into retroviral vectors and that, by functioning to improve the binding efficiency and transduction of both HUVEC and KSY1 endothelial cells, may have therapeutic potential for targeting gene delivery to the tumor-associated vasculature.

Molecular engineering of matrix-targeted retroviral vectors incorporating a surveillance function inherent in von Willebrand factor

A major obstacle that limits the potential of human gene therapy is the inefficiency of gene delivery to appropriate sites in vivo. Previous studies demonstrated that the physiological surveillance function performed by von Willebrand factor (vWF) could be incorporated into retroviral vectors by molecular engineering of the MuLV ecotropic envelope (Env) protein. To advance the application of vWF targeting technology beyond laboratory animals, we prepared an extensive series of Env proteins bearing modified vWF-derived matrix-binding sequences and assembled these chimeric proteins into targeted vectors that are capable of transducing human cells. Initially, a dual envelope configuration was utilized, which required coexpression of a wild-type amphotropic Env. Subsequently, streamlined "escort" Env proteins were constructed wherein the inoperative receptor-binding domain of the targeting partner was replaced by the vWF-derived collagen-binding motif. Ultimately, an optimal construct was developed that exhibited properties of both extracellular matrix (ECM)-targeting and near wild-type amphotropic infectivity, and could be arrayed as a single envelope on a retroviral particle. On intraarterial instillation, enhanced focal transduction of neointimal cells (approximately 20%) was demonstrated in a rat model of balloon angioplasty. Moreover, transduction of tumor foci (approximately 1-3%) was detected after portal vein infusion of a matrix-targeted vector in a nude mouse model of liver metastasis. We conclude that the unique properties of these targeted injectable retroviral vectors would be suitable for improving therapeutic gene delivery in numerous clinical applications, including vascular restenosis, laser and other surgical procedures, orthopedic injuries, wound healing, ischemia, arthritis, inflammatory disease, and metastatic cancer.

Mutational analysis of the subgroup A avian sarcoma and leukosis virus putative fusion peptide domain

Short hydrophobic regions referred to as fusion peptide domains (FPDs) at or near the amino terminus of the membrane-anchoring subunit of viral glycoproteins are believed to insert into the host membrane during the initial stage of enveloped viral entry. Avian sarcoma and leukosis viruses (ASLV) are unusual among retroviruses in that the region in the envelope glycoprotein (EnvA) proposed to be the FPD is internal and contains a centrally located proline residue. To begin analyzing the function of this region of EnvA, 20 substitution mutations were introduced into the putative FPD. The mutant envelope glycoproteins were evaluated for effects on virion incorporation, receptor binding, and infection. Interestingly, most of the single-substitution mutations had little effect on any of these processes. In contrast, a bulky hydrophobic substitution for the central proline reduced viral titers 15-fold without affecting virion incorporation or receptor binding, whereas substitution of glycine for the proline had only a nominal effect on EnvA function. Similar to other viral FPDs, the putative ASLV FPD has been modeled as an amphipathic helix where most of the bulky hydrophobic residues form a patch on one face of the helix. A series of alanine insertion mutations designed to interrupt the hydrophobic patch on the helix had differential effects on infectivity, and the results of that analysis together with the results observed with the substitution mutations suggest no correlation between maintenance of the hydrophobic patch and glycoprotein function.

Activation of a cell entry pathway common to type C mammalian retroviruses by soluble envelope fragments

Mutations that negatively or positively affect the fusion properties of murine leukemia viruses (MLVs) have been found within all subdomains of their SU (surface) and TM (transmembrane) envelope units. Yet, the interrelations between these different regions of the envelope complex during the cell entry process are still elusive. Deletion of the histidine residue of the conserved PHQV motif at the amino terminus of the amphotropic or the ecotropic MLV SU resulted in the AdelH or the MOdelH fusion-defective mutant envelope, respectively. These delH mutant envelopes are incorporated on retroviral particles at normal densities and normally mediate virion binding to cells expressing the retroviral receptors. However, both their cell-cell and virus-cell fusogenicities were fully prevented at an early postbinding stage. We show here that the fusion defect of AdelH or MOdelH envelopes was also almost completely reverted by providing either soluble SU or a polypeptide encompassing the receptor-binding domain (RBD) to the target cells, provided that the integrity of the amino-terminal end of either polypeptide was preserved. Restoration of delH envelope fusogenicity was caused by activation of the target cells via specific interaction of the latter polypeptides with the retrovirus receptor rather than by their association with the delH envelope complexes. Moreover crossactivation of the target cells, leading to fusion activation of AdelH or MOdelH envelopes, was achieved by polypeptides containing various type C mammalian retrovirus RBDs, irrespective of the type of entry-defective glycoprotein that was used for infection. Our results indicate that although they recognize different receptors for binding to the cell surface, type C mammalian retroviruses use a common entry pathway which is activated by a conserved feature of their envelope glycoproteins.

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.

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.

A proline-rich motif downstream of the receptor binding domain modulates conformation and fusogenicity of murine retroviral envelopes

The entry of retroviruses into cells depends on receptor recognition by the viral envelope surface subunit SU followed by membrane fusion, which is thought to be mediated by a fusion peptide located at the amino terminus of the envelope transmembrane subunit TM. Several fusion determinants have been previously identified in murine leukemia virus (MLV) envelopes, but their functional interrelationships as well as the processes involved in fusion activation upon retroviral receptor recognition remain unelucidated. Despite both structural and functional similarities of their envelope glycoproteins, ecotropic and amphotropic MLVs display two different postbinding properties: (i) while amphotropic MLVs fuse the cells at neutral pH, penetration of ecotropic MLVs is relatively acid pH dependent and (ii) ecotropic envelopes are more efficient than amphotropic envelopes in inducing cell-to-cell fusion and syncytium formation. By exploiting the latter characteristic in the analysis of chimeras of ecotropic and amphotropic MLV envelopes, we show here that substitution of the ecotropic MLV proline-rich region (PRR), located in the SU between the amino-terminal receptor binding domain and the TM-interacting SU carboxy-terminal domains, is sufficient to revert the amphotropic low-fusogenic phenotype into a high-fusogenic one. Furthermore, we have identified potential beta-turns in the PRR that control the stability of SU-TM associations as well as the thresholds required to trigger either cell-to-cell or virus-to-cell fusion. These data, demonstrating that the PRR functions as a signal which induces envelope conformational changes leading to fusion, have enabled us to derive envelopes which can infect cells harboring low levels of available amphotropic receptors.

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.