Oligomerization and transport of the envelope protein of Moloney murine leukemia virus-TB and of ts1, a neurovirulent temperature-sensitive mutant of MoMuLV-TB

Characterizing the Murine Leukemia Virus Envelope Glycoprotein Membrane-Spanning Domain for Its Roles in Interface Alignment and Fusogenicity

The membrane-proximal region of murine leukemia virus envelope (Env) is a critical modulator of its functionality. We have previously shown that the insertion of one amino acid (+1 leucine) within the membrane-spanning domain (MSD) abolished protein functionality in infectivity assays. However, functionality could be restored to this +1 leucine mutant by either inserting two additional amino acids (+3 leucine) or by deleting the cytoplasmic tail domain (CTD) in the +1 leucine background. We inferred that the ectodomain and CTD have protein interfaces that have to be in alignment for Env to be functional. Here, we made single residue deletions to the Env mutant with the +1 leucine insertion to restore the interface alignment (gain of functionality) and therefore define the boundaries of the two interfaces. We identified the glycine-proline pairs near the N terminus (positions 147 and 148) and the C terminus (positions 159 and 160) of the MSD as being the boundaries of the two interfaces. Deletions between these pairs restored function, but deletions outside of them did not. In addition, the vast majority of the single residue deletions regained function if the CTD was deleted. The exceptions were four hydroxyl-containing amino acid residues (T139, T140, S143, and T144) that reside in the ectodomain interface and the proline at position 148, which were all indispensable for functionality. We hypothesize that the hydroxyl-containing residues at positions T139 and S143 could be a driving force for stabilizing the ectodomain interface through formation of a hydrogen-bonding network.

IMPORTANCE

The membrane-proximal external region (MPER) and membrane-spanning domains (MSDs) of viral glycoproteins have been shown to be critical for regulating glycoprotein fusogenicity. However, the roles of these two domains are poorly understood. We report here that point deletions and insertions within the MPER or MSD result in functionally inactive proteins. However, when the C-terminal tail domain (CTD) is deleted, the majority of the proteins remain functional. The only residues that were found to be critical for function regardless of the CTD were four hydroxyl-containing amino acids located at the C terminus of the MPER (T139 and T140) and at the N terminus of the MSD (S143 and T144) and a proline near the beginning of the MSD (P148). We demonstrate that hydrogen-bonding at positions T139 and S143 is critical for protein function. Our findings provide novel insights into the role of the MPER in regulating fusogenic activity of viral glycoproteins.

Two new protocols to enhance the production and isolation of human induced pluripotent stem cell lines

There are two critical stages in the retroviral reprogramming of somatic cells to produce human induced pluripotent stem cell (hiPSC) lines. One is the production of high titer virus required to reprogram somatic cells; the other is identification of true hiPSC colonies from heterogeneous cell populations, and their isolation and expansion to generate a sustainable, pluripotent stem cell line. Here we describe simple, time-saving methods to address the current difficulties at these two critical junctures. First, we have developed a method to increase the number of infectious viral units 600-fold. Second, we have developed a TRA-1-81-based positive selection column method for isolating "true" hiPSCs from the heterogeneous cell populations, which overcomes the labor-intensive and highly subjective method of manual selection of hiPSC colonies. We have used these techniques to produce 8 hiPSC lines from human fibroblasts and we believe that they are of considerable utility to researchers in the hiPSC field.

Effects of retroviral envelope-protein cleavage upon trafficking, incorporation, and membrane fusion

Retroviral envelope glycoproteins undergo proteolytic processing by cellular subtilisin-like proprotein convertases at a polybasic amino-acid site in order to produce the two functional subunits, SU and TM. Most previous studies have indicated that envelope-protein cleavage is required for rendering the protein competent for promoting membrane fusion and for virus infectivity. We have investigated the role of proteolytic processing of the Moloney murine leukemia virus envelope-protein through site-directed mutagenesis of the residues near the SU-TM cleavage site and have established that uncleaved glycoprotein is unable either to be incorporated into virus particles efficiently or to induce membrane fusion. Additionally, the results suggest that cleavage of the envelope protein plays an important role in intracellular trafficking of protein via the cellular secretory pathway. Based on our results it was concluded that a positively charged residue located at either P2 or P4 along with the arginine at P1 is essential for cleavage.

The drug monosodium luminol (GVT) preserves thymic epithelial cell cytoarchitecture and allows thymocyte survival in mice infected with the T cell-tropic, cytopathic retrovirus ts1

A mutant of MoMuLV, called ts1, causes an AIDS-like syndrome in susceptible strains of mice. In mice infected at birth, thymic atrophy, CD4+ T cell loss, body wasting, and death occur by approximately 30-40 days postinfection (dpi). We have shown previously that the death of ts1-infected cells is not caused by viral replication per se, but by oxidative stress and apoptosis following their accumulation the ts1 viral envelope precursor protein, gPr80(env). In infected mice treated with the antioxidant monosodium alpha-luminol (GVT), T cell loss and thymic atrophy are delayed for many weeks, and body wasting and death do not occur until long after infected, untreated control mice have died. We show here that GVT treatment of ts1-infected mice maintains the thymic epithelial cell (TEC) cytoarchitecture and cytokeratin gradients required for thymocyte differentiation. It also suppresses thymocyte reactive oxygen species (ROS) levels, upregulates and stabilizes levels of the antioxidant-regulating transcription factor Nrf2, and prevents accumulation of gPr80(env) in thymocytes. We conclude that GVT treatment can make ts1 a non-cytopathic virus for thymocytes, although it cannot prevent thymocyte infection. Since oxidative stress also contributes to the loss of T cells in HIV-AIDS, the antioxidant effects of GVT may make it a useful therapeutic adjunct to HAART treatment.

The drug monosodium luminol (GVT) preserves crypt-villus epithelial organization and allows survival of intestinal T cells in mice infected with the ts1 retrovirus

Of the cytopathic retroviruses that affect mammals, including HIV-1, many selectively infect CD4+ T cells and cause immunosuppressive syndromes. These diseases destroy both the thymus and the small and large intestines, after infecting and killing T-lineage cells in both tissues. A mutant of the murine leukemia retrovirus MoMuLV-TB, called ts1, causes this syndrome in susceptible strains of mice. In FVB/N strain mice that are infected at birth, thymic atrophy, CD4+ T cell loss, intestinal collapse, body wasting, and death occur by approximately 30-40 days postinfection (dpi). Apoptosis of ts1-infected T-lineage cells, in the thymus, peripheral lymphoid system and intestines is caused by accumulation of the ts1 mutant viral envelope preprotein gPr80(env), which is inefficiently cleaved into the mature viral proteins gp70 and PrP15E. We show here that ts1 infection in the small intestine is followed by loss of intestinal epithelial cell (IEC) thyroid-stimulating hormone (TSH) and cell cycling gradients (along the crypt-villus axes), accumulation of gPr80(env) in intestinal cells, apoptosis of developing T cells in the lamina propria (LP), and intestinal collapse by approximately 30 dpi. In infected mice treated with the antioxidant drug monosodium luminol (GVT), however, normal intestinal epithelial cell gradients are still in place at 30 dpi, and IECs covering both the crypts and villi contain large amounts of the antioxidant transcription factor Nrf2. In addition, no apoptotic cells are present, and accumulated gpr80(env) is absent from the tissue at this time. We conclude that GVT treatment can make ts1 a noncytopathic virus for intestinal lymphoid cells, as it does for thymocytes [25]. As in the thymus, GVT may protect the intestine by reducing oxidant stress in infected intestinal T cells, perhaps by prevention of gPr80(env) accumulation via Nrf2 upregulation in the IECs. These results identify GVT as a potential therapy for intestinal diseases or inflammatory conditions, including HIV-AIDS, in which oxidative stress is a triggering or exacerbating factor.

Oligodendrocytes are a major target of the toxicity of spongiogenic murine retroviruses

The neurovirulent retroviruses FrCasE and Moloney MLV-ts1 cause noninflammatory spongiform neurodegeneration in mice, manifested clinically by progressive spasticity and paralysis. Neurons have been thought to be the primary target of toxicity of these viruses. However the neurons themselves appear not to be infected, and the possible indirect mechanisms driving the neuronal toxicity have remained enigmatic. Here we have re-examined the cells that are damaged by these viruses, using lineage-specific markers. Surprisingly, these cells expressed the basic helix-loop-helix transcription factor Olig2, placing them in the oligodendrocyte lineage. Olig2+ cells were found to be infected, and many of these cells exhibited focal cytoplasmic vacuolation, suggesting that infection by spongiogenic retroviruses is directly toxic to these cells. As cytoplasmic vacuolation progressed, however, signs of viral protein expression appeared to wane, although residual viral RNA was detectable by in situ hybridization. Cells with the most advanced cytoplasmic effacement expressed the C/EBP-homologous protein (CHOP). This protein is up-regulated as a late event in a cellular response termed the integrated stress response. This observation may link the cellular pathology observed in the brain with cellular stress responses known to be induced by these viruses. The relevance of these observations to oligodendropathy in humans is discussed.

Stoichiometry of murine leukemia virus envelope protein-mediated fusion and its neutralization

Envelope glycoproteins (Envs) of retroviruses form trimers that mediate fusion between viral and cellular membranes and are the targets for neutralizing antibodies. Understanding in detail how Env trimers mediate membrane fusion, and how antibodies interfere with this process, is a fundamental problem in biology with practical implications for the development of antiviral drugs and vaccines. We investigated the stoichiometry of Env-mediated fusion and its inhibition by antibody by inserting an epitope from human immunodeficiency virus for a neutralizing antibody (2F5) into the surface (SU) or transmembrane (TM) protein of murine leukemia virus Env, along with point mutations that abrogate SU and TM function but complement one another. We transfected various combinations of these Env genes and investigated Env-mediated cell fusion and its inhibition by 2F5 antibody. Our results showed that heterotrimers with one functional SU molecule were fusion competent in complementation experiments and that one antibody molecule was sufficient to inactivate the fusion function of a trimer when its epitope was in functional SU or TM. 2F5 antibody could also neutralize trimers with the 2F5 epitope in nonfunctional SU or TM, but less efficiently.

Stoichiometry of envelope glycoprotein trimers in the entry of human immunodeficiency virus type 1

The human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins (Envs) function as a trimer, mediating virus entry by promoting the fusion of the viral and target cell membranes. HIV-1 Env trimers induce membrane fusion through a pH-independent pathway driven by the interaction between an Env trimer and its cellular receptors, CD4 and CCR5/CXCR4. We studied viruses with mixed heterotrimers of wild-type and dominant-negative Envs to determine the number (T) of Env trimers required for HIV-1 entry. To our surprise, we found that a single Env trimer is capable of supporting HIV-1 entry; i.e., T = 1. A similar approach was applied to investigate the entry stoichiometry of envelope glycoproteins from amphotropic murine leukemia virus (A-MLV), avian sarcoma/leukosis virus type A (ASLV-A), and influenza A virus. When pseudotyped on HIV-1 virions, the A-MLV and ASLV-A Envs also exhibit a T = 1 entry stoichiometry. In contrast, eight to nine influenza A virus hemagglutinin trimers function cooperatively to achieve membrane fusion and virus entry, using a pH-dependent pathway. The different entry requirements for cooperativity among Env trimers for retroviruses and influenza A virus may influence viral strategies for replication and evasion of the immune system.

Involvement of the C-terminal disulfide-bonded loop of murine leukemia virus SU protein in a postbinding step critical for viral entry

A role for the C-terminal domain (CTD) of murine leukemia virus (MuLV) Env protein in viral fusion was indicated by the potent inhibition of MuLV-induced fusion, but not receptor binding, by two rat monoclonal antibodies (MAbs) specific for epitopes in the CTD. Although these two MAbs, 35/56 and 83A25, have very different patterns of reactivity with viral isolates, determinants of both epitopes were mapped to the last C-terminal disulfide-bonded loop of SU (loop 10), and residues in this loop responsible for the different specificities of these MAbs were identified. Both MAbs reacted with a minor fraction of a truncated SU fragment terminating four residues after loop 10, indicating that while the deleted C-terminal residues were not part of these epitopes, they promoted their formation. Neither MAb recognized the loop 10 region expressed in isolated form, suggesting that these epitopes were not completely localized within loop 10 but required additional sequences located N terminal to the loop. Direct support for a role for loop 10 in fusion was provided by the demonstration that Env mutants containing an extra serine or threonine residue between the second and third positions of the loop were highly attenuated for infectivity and defective in fusion assays, despite wild-type levels of expression, processing, and receptor binding. Other mutations at positions 1 to 3 of loop 10 inhibited processing of the gPr80 precursor protein or led to increased shedding of SU, suggesting that loop 10 also affects Env folding and the stability of the interaction between SU and TM.

Synthesis, assembly, and processing of the Env ERVWE1/syncytin human endogenous retroviral envelope

Syncytin is a fusogenic protein involved in the formation of the placental syncytiotrophoblast layer. This protein is encoded by the envelope gene of the ERVWE1 proviral locus belonging to the human endogenous retrovirus W (HERV-W) family. The HERV-W infectious ancestor entered the primate lineage 25 to 40 million years ago. Although the syncytin fusion property has been clearly demonstrated, little is known about this cellular protein maturation process with respect to classical infectious retrovirus envelope proteins. Here we show that the cellular syncytin protein is synthesized as a glycosylated gPr73 precursor cleaved into two mature proteins, a gp50 surface subunit (SU) and a gp24 transmembrane subunit (TM). These SU and TM subunits are found associated as homotrimers. The intracytoplasmic tail is critical to the fusogenic phenotype, although its cleavage requirements seem to have diverged from those of classical retroviral maturation.

Inhibition of murine leukemia virus envelope protein (env) processing by intracellular expression of the env N-terminal heptad repeat region

A conserved structural motif in the envelope proteins of several viruses consists of an N-terminal, alpha-helical, trimerization domain and a C-terminal region that refolds during fusion to bind the N-helix trimer. Interaction between the N and C regions is believed to pull viral and target membranes together in a crucial step during membrane fusion. For several viruses with type I fusion proteins, C regions pack as alpha-helices in the grooves between N-helix monomers, and exogenously added N- and C-region peptides block fusion by inhibiting the formation of the six-helix bundle. For other viruses, including influenza virus and murine leukemia virus (MLV), there is no evidence for comparably extended C-region alpha-helices, although a short, non-alpha-helical interaction structure has been reported for influenza virus. We tested candidate N-helix and C-region peptides from MLV for their ability to inhibit cell fusion but found no inhibitory activity. In contrast, intracellular expression of the MLV N-helix inhibited fusion by efficiently blocking proteolytic processing and intracellular transport of the envelope protein. The results highlight another mechanism by which the N-helix peptides can inhibit fusion.

Human immunodeficiency virus type 1 genetic diversity in the nervous system: evolutionary epiphenomenon or disease determinant?

Over the past decade there has been a revolution in the understanding and care of human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS)-associated disease. Much of this progress stems from a broader recognition of the importance of differences in viral types, including receptor preference(s), replication properties, and reservoirs, as contributing factors to immunosuppresion and disease progression. In contrast, there is limited conceptualizatin of viral diversity and turnover in the brain and circulation in relation to neurocognitive impairments. Herein, the authors review current concepts regarding viral molecular diversity and phenotypes together with features of HIV-1 neuroinvasion, neurotropism, neurovirulence and neurosusceptiblity. Viral genetic and antigenic diversity is reduced within the brain compared to blood or other systemic organs within individuals. Conversely, viral molecular heterogeneity is greater in patients with HIV-associated dementia compared to nondemented patients, depending on the viral gene examined. Individual viral proteins exert multiple neuropathogenic effects, although the neurological consequences of different viral polymorphisms remain uncertain. Nonetheless, host genetic polymorphisms clearly influence neurological disease outcomes and likely dictate both acquired and innate immune responses, which in turn shape viral evolution within the host. Emerging issues include widespread antiretroviral therapy resistance and increasing awareness of viral superinfections together with viral recombination, all of which are likely to impact on both HIV genetic variation and neuropathogenesis. With the persisting prevalence of HIV-induced neurocognitive disabilities, despite marked improvements in managing immunosuppression, it remains imperative to fully define and understand the mechanisms by which viral dynamics and diversity contribute to neurological disease, permitting the development of new therapeutic strategies.

Selection of functional human antibodies from retroviral display libraries

Antibody library technology represents a powerful tool for the discovery and design of antibodies with high affinity and specificity for their targets. To extend the technique to the expression and selection of antibody libraries in an eukaryotic environment, we provide here a proof of concept that retroviruses can be engineered for the display and selection of variable single-chain fragment (scFv) libraries. A retroviral library displaying the repertoire obtained after a single round of selection of a human synthetic scFv phage display library on laminin was generated. For selection, antigen-bound virus was efficiently recovered by an overlay with cells permissive for infection. This approach allowed more than 10(3)-fold enrichment of antigen binders in a single selection cycle. After three selection cycles, several scFvs were recovered showing similar laminin-binding activities but improved expression levels in mammalian cells as compared with a laminin-specific scFv selected by the conventional phage display approach. Thus, translational problems that occur when phage-selected antibodies have to be transferred onto mammalian expression systems to exert their therapeutic potential can be avoided by the use of retroviral display libraries.

An improved method for detection of replication-competent retrovirus in retrovirus vector products

Contamination by replication-competent retrovirus (RCR) is one of the most important safety issues of retrovirus vector products for gene therapy clinical research. To improve the sensitivity of RCR detection and to shorten the assay period, we have developed a novel RCR detection method (infectivity RT-PCR method) based on real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) in combination with virus infection and a novel virus concentration method using polyethyleneimine (PEI)-conjugated magnetic beads. In this method, permissive cells were infected with RCR samples, and amplified RCR in the culture supernatants was adsorbed by PEI-beads. Then RCR RNA extracted from PEI-beads was quantified by real-time RT-PCR. We demonstrated that 1 infectious unit (iu) of RCR spiked in 10(6) cfu/ml of vector products could be detected within 3 days, and the sensitivity for viral detection was increased 3- to 10-fold compared with the direct S+L- assay. By this method, the presence of retroviral vector interfered with RCR detection only slightly. In conclusion, infectivity RT-PCR conducted in conjunction with virus concentration using PEI-beads can detect RCR more sensitively and rapidly than the conventional infectivity assay.

Endoplasmic reticulum stress is a determinant of retrovirus-induced spongiform neurodegeneration

FrCas(E) is a mouse retrovirus that causes a fatal noninflammatory spongiform neurodegenerative disease with pathological features strikingly similar to those induced by transmissible spongiform encephalopathy (TSE) agents. Neurovirulence is determined by the sequence of the viral envelope protein, though the specific role of this protein in disease pathogenesis is not known. In the present study, we compared host gene expression in the brain stems of mice infected with either FrCas(E) or the avirulent virus F43, differing from FrCas(E) in the sequence of the envelope gene. Four of the 12 disease-specific transcripts up-regulated during the preclinical period represent responses linked to the accumulation of unfolded proteins in the endoplasmic reticulum (ER). Among these genes was CHOP/GADD153, which is induced in response to conditions that perturb endoplasmic reticulum function. In vitro studies with NIH 3T3 cells revealed up-regulation of CHOP as well as BiP, calreticulin, and Grp58/ERp57 in cells infected with FrCas(E) but not with F43. Immunoblot analysis of infected NIH 3T3 cells demonstrated the accumulation of uncleaved envelope precursor protein in FrCas(E)- but not F43-infected cells, consistent with ER retention. These results suggest that retrovirus-induced spongiform neurodegeneration represents a protein-folding disease and thus may provide a useful tool for exploring the causal link between protein misfolding and the cytopathology that it causes.

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.

Human immunodeficiency virus type 1 envelope-mediated neuropathogenesis: targeted gene delivery by a Sindbis virus expression vector

Sindbis virus (SIN) expression vectors offer the opportunity for studying neuropathogenesis because of their distinct neural cell tropism. Here, we demonstrate that a recombinant SIN vector expressing EGFP (SINrep5-EGFP) infected multiple cell types including neural cells from several species relevant to lentivirus pathogenesis with high levels of transgene expression. Infection of human neurons by a recombinant SIN (SINrep5-JRFL) expressing the full-length envelope from a neurovirulent human immunodeficiency virus type 1 (HIV-1) strain (JRFL) caused increased cytotoxicity compared to infection with SINrep5-EGFP (P < 0.001), while no cytotoxicity was observed among infected human astrocytes or monocytoid cells. Both human monocyte-derived macrophages (MDM) (P < 0.01) and astrocytes (P < 0.001) infected with SINrep5-JRFL released soluble neurotoxins in contrast to SINrep5-EGFP or mock-infected cells, although this was most prominent for the astrocytes. Implantation of SINrep5-JRFL into the brains of SCID/NOD mice induced neuroinflammation, neuronal loss, and neurobehavioral changes characteristic of HIV-1 infection, which were not present in SINrep5-EGFP or mock-infected animals. Thus SIN expression vectors represent novel tools for studying in vitro and in vivo HIV-1 neuropathogenesis because of their high levels of transgene expression in specific cell types within the brain.

Distinct mechanisms of neutralization by monoclonal antibodies specific for sites in the N-terminal or C-terminal domain of murine leukemia virus SU

The epitope specificities and functional activities of monoclonal antibodies (MAbs) specific for the murine leukemia virus (MuLV) SU envelope protein subunit were determined. Neutralizing antibodies were directed towards two distinct sites in MuLV SU: one overlapping the major receptor-binding pocket in the N-terminal domain and the other involving a region that includes the most C-terminal disulfide-bonded loop. Two other groups of MAbs, reactive with distinct sites in the N-terminal domain or in the proline-rich region (PRR), did not neutralize MuLV infectivity. Only the neutralizing MAbs specific for the receptor-binding pocket were able to block binding of purified SU and MuLV virions to cells expressing the ecotropic MuLV receptor, mCAT-1. Whereas the neutralizing MAbs specific for the C-terminal domain did not interfere with the SU-mCAT-1 interaction, they efficiently inhibited cell-to-cell fusion mediated by MuLV Env, indicating that they interfered with a postattachment event necessary for fusion. The C-terminal domain MAbs displayed the highest neutralization titers and binding activities. However, the nonneutralizing PRR-specific MAbs bound to intact virions with affinities similar to those of the neutralizing receptor-binding pocket-specific MAbs, indicating that epitope exposure, while necessary, is not sufficient for viral neutralization by MAbs. These results identify two separate neutralization domains in MuLV SU and suggest a role for the C-terminal domain in a postattachment step necessary for viral fusion.

Chimeric ecotropic MLV envelope proteins that carry EGF receptor-specific ligands and the Pseudomonas exotoxin A translocation domain to target gene transfer to human cancer cells

Redirecting retroviral vector transduction simply by insertion of a ligand into the envelope (Env) protein has met with several obstacles. For example, virions targeted to epidermal growth factor receptor (EGFR), after receptor binding, rapidly traffic to the lysosomes, where they are degraded. Exotoxin A of Pseudomonas aeruginosa has the ability to translocate from endosomes to the cytoplasm by means of a translocation domain (TLD). We generated a series of chimeric Env proteins of Moloney murine leukemia virus containing EGFR ligands, where TLD was inserted into different regions. These chimeric proteins were successfully produced, if the translocation domain was not located at the immediate N-terminus of Env. The ability to transduce murine cells via the ecotropic receptor varied but correlated with the amount of Env proteins incorporated into the virions. Chimeric vector particles could bind to EGFR, demonstrating the functional exposure of the peptide ligand. However, transduction of human cells expressing EGFR but not the ecotropic receptor by virions carrying the chimeric protein was not observed.

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.

Concentration of viral vectors by co-precipitation with calcium phosphate

BACKGROUND

The envelope glycoproteins, surface unit (SU) and transmembrane (TM) of the murine leukemia virus (MLV) are not covalently linked and tend to dissociate upon high-speed centrifugation, leading to loss of vector infectivity. This study describes a gentle and simple method to concentrate MLV vectors or HIV vectors pseudotyped with MLV envelopes. Having a fast and inexpensive method to concentrate large volumes of vector supernatant will facilitate in vivo experiments and clinical trials that require high titer vector stocks.

METHODS

The methods employed in the study were co-precipitation of viral supernatant with calcium phosphate, low-speed centrifugation, dialysis, and infection assays with Lac-Z transducing vectors.

RESULTS

Murine leukemia virus vectors and HIV vectors pseudotyped with vesicular stomatitis virus glycoprotein (VSV.G) or MLV envelopes were concentrated successfully using the calcium phosphate co-precipitation method. Parameters that influence virus yield and the reproducibility of the method were investigated. The optimized protocol involves virus harvest in serum-free media, co-precipitation using 60mM calcium chloride, pelleting at 2,000 g, resuspending the pellet in a small volume of 0.1M EDTA-saline, and dialysis against saline to remove EDTA. Volumes were decreased from 300 ml to 10 ml, with 50-100% recovery, and titers can be concentrated up to 1,000-fold.

CONCLUSIONS

The calcium phosphate co-precipitation method to concentrate virus is applicable to retrovirus and lentivirus preparations. It uses simple techniques and does not require expensive equipment. Multiple rounds of co-precipitation can be carried out if required.

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.

The role of the membrane-spanning domain sequence in glycoprotein-mediated membrane fusion

The role of glycoprotein membrane-spanning domains in the process of membrane fusion is poorly understood. It has been demonstrated that replacing all or part of the membrane-spanning domain of a viral fusion protein with sequences that encode signals for glycosylphosphatidylinositol linkage attachment abrogates membrane fusion activity. It has been suggested, however, that the actual amino acid sequence of the membrane-spanning domain is not critical for the activity of viral fusion proteins. We have examined the function of Moloney murine leukemia virus envelope proteins with substitutions in the membrane-spanning domain. Envelope proteins bearing substitutions for proline 617 are processed and incorporated into virus particles normally and bind to the viral receptor. However, they possess greatly reduced or undetectable capacities for the promotion of membrane fusion and infectious virus particle formation. Our results imply a direct role for the residues in the membrane-spanning domain of the murine leukemia virus envelope protein in membrane fusion and its regulation. They also support the thesis that membrane-spanning domains possess a sequence-dependent function in other protein-mediated membrane fusion events.

Early assembly step of a retroviral envelope glycoprotein: analysis using a dominant negative assay

As for most integral membrane proteins, the intracellular transport of retroviral envelope glycoproteins depends on proper folding and oligomeric assembly in the ER. In this study, we considered the hypothesis that a panel of 22 transport-defective mutants of the human T cell leukemia virus type 1 envelope glycoprotein might be defective in ER assembly. Upon cell cotransfection with wild-type envelope, however, the vast majority of these transport-defective mutants (21 of 22) exerted a specific trans-dominant negative effect. This effect was due to random dimerization of the mutated and wild-type glycoproteins that prevented the intracellular transport of the latter. This unexpected result suggests that association of glycoprotein monomers precedes the completion of folding. The only mutation that impaired this early assembly was located at the NH2 terminus of the protein. COOH-terminally truncated, soluble forms of the glycoprotein were also trans-dominant negative provided that their NH2 terminus was intact. The leucine zipper-like domain, although involved in oligomerization of the envelope glycoproteins at the cell surface, did not contribute to their intracellular assembly. We propose that, at a step subsequent to translation, but preceding complete folding of the monomers, glycoproteins assemble via their NH2-terminal domains, which, in turn, permits their cooperative folding.

Modification of retroviral tropism by display of IGF-I

We have displayed insulin-like growth factor I (IGF-I) as an N-terminal extension of 4070A (amphotropic) retroviral envelope protein. Western blot demonstrated that chimaeric envelope proteins were incorporated into retroviral particles. Interaction between the displayed IGF-I and cell-surface receptors impaired gene delivery. The magnitude of this inhibitory effect was smallest on NIH 3T3 cells, greater on NIH 3T3 cells over-expressing insulin receptor, and greatest on NIH 3T3 cells over-expressing human type-I IGF receptor. Hence, both the number of ligand receptors and their affinity for the displayed ligand influenced the level of gene delivery. The inhibitory effect was abrogated by cleaving the displayed domain from the underlying envelope protein with factor Xa protease, and by the addition of free ligand to the infection. Addition of IGF-I or insulin caused a dose-dependent increase in titre. Possible mechanisms for receptor-mediated inhibition of gene delivery by IGF-displaying vectors are discussed, together with the implications of these results for practical applications of retroviral display and for understanding the mechanism of virus entry.

Mapping of a neurovirulence determinant within the envelope protein of a polytropic murine retrovirus: induction of central nervous system disease by low levels of virus

Murine leukemia virus (MuLV) clone Fr98 is a recombinant polytropic virus that causes neurological disease characterized by ataxia in susceptible mouse strains. The envelope gene of Fr98 has been previously shown to encode at least two separate neurovirulence determinants. In the present study, the determinant encoded within the EcoRI/AvrII fragment of the envelope gene was further defined. In these experiments, neurovirulence was associated with a change from a serine to an arginine at position 195 and a glycine to an alanine at position 198 within the envelope protein. Neurovirulent and nonvirulent virus clones, which differed only at these two amino acid residues, showed no difference in the type or location of cells infected. Furthermore, equivalent levels of viral p30 capsid protein were detected in the brains of mice infected with either the neurovirulent or nonvirulent virus clones. These results were consistent with the interpretation that the envelope protein of the neurovirulent virus differed from that of the nonvirulent virus by having a greater toxic effect on central nervous system function.

Evidence for cooperation between murine leukemia virus Env molecules in mixed oligomers

A retroviral Env molecule consists of a surface glycoprotein (SU) complexed with a transmembrane protein (TM). In turn, these complexes are grouped into oligomers on the surfaces of the cell and of the virion. In the case of murine leukemia viruses (MuLVs), the SU moieties are polymorphic, with SU proteins of different viral isolates directed towards different cell surface receptors. During maturation of the released virus particle, the 16 C-terminal residues of TM (the R peptide or p2E) are removed from the protein by the viral protease; this cleavage is believed to activate the membrane-fusing potential of MuLV Env. We have tested the possibility that different MuLV Env proteins in the same cell can interact with each other, both physically and functionally, in mixed oligomers. We found that coexpressed Env molecules can be precipitated out of cell lysates by antiserum which reacts with only one of them. Furthermore, they can evidently cooperate with each other: if one Env species lacks the R peptide, then it can apparently induce fusion if the SU protein of the other Env species encounters its cognate receptor on the surface of another cell. This functional interaction between different Env molecules has a number of implications with respect to the mechanism of induction of membrane fusion, for the genetic analysis of Env function, and for the design of targeted retroviral vectors for gene therapy.

Inverse Targeting of Retroviral Vectors: Selective Gene Transfer in a Mixed Population of Hematopoietic and Nonhematopoietic Cells

We previously reported that retroviral vectors displaying epidermal growth factor (EGF) as part of a chimeric envelope glycoprotein are sequestered upon binding to EGF receptor (EGFR)-positive target cells, leading to loss of infectivity. In the current study, we have displayed stem cell factor (SCF) on β-galactosidase-transducing ecotropic and amphotropic retroviral vector particles as a factor Xa protease-cleavable N-terminal extension of the envelope glycoprotein. Viral incorporation of the SCF chimeric envelopes was demonstrated by immunoblotting of pelleted virions and their specific attachment to Kit receptors was demonstrated by flow cytometry. Gene transfer studies showed that when SCF was displayed on an amphotropic envelope, the infectivity of the SCF-displaying vectors was selectively inhibited on Kit-expressing cells, but could be restored by adding soluble SCF to block the Kit receptors or by cleaving the displayed SCF domain from the vector particles with factor Xa protease. The host range properties of EGF-displaying and SCF-displaying vectors were then compared in cell mixing experiments. When EGFR-positive cancer cells and Kit-positive hematopoietic cells were mixed and exposed to the different engineered vector particles, the cancer cells were selectively transduced by the SCF-displaying vector and the hematopoietic cells were selectively transduced by the EGF-displaying vector. Retroviral display of polypeptide growth factors can therefore provide the basis for a novel inverse targeting strategy with potential use for selective transduction of hematopoietic or nonhematopoietic cells (eg, cancer cells) in a mixed cell population.

Inverse Targeting of Retroviral Vectors: Selective Gene Transfer in a Mixed Population of Hematopoietic and Nonhematopoietic Cells

We previously reported that retroviral vectors displaying epidermal growth factor (EGF) as part of a chimeric envelope glycoprotein are sequestered upon binding to EGF receptor (EGFR)-positive target cells, leading to loss of infectivity. In the current study, we have displayed stem cell factor (SCF) on β-galactosidase-transducing ecotropic and amphotropic retroviral vector particles as a factor Xa protease-cleavable N-terminal extension of the envelope glycoprotein. Viral incorporation of the SCF chimeric envelopes was demonstrated by immunoblotting of pelleted virions and their specific attachment to Kit receptors was demonstrated by flow cytometry. Gene transfer studies showed that when SCF was displayed on an amphotropic envelope, the infectivity of the SCF-displaying vectors was selectively inhibited on Kit-expressing cells, but could be restored by adding soluble SCF to block the Kit receptors or by cleaving the displayed SCF domain from the vector particles with factor Xa protease. The host range properties of EGF-displaying and SCF-displaying vectors were then compared in cell mixing experiments. When EGFR-positive cancer cells and Kit-positive hematopoietic cells were mixed and exposed to the different engineered vector particles, the cancer cells were selectively transduced by the SCF-displaying vector and the hematopoietic cells were selectively transduced by the EGF-displaying vector. Retroviral display of polypeptide growth factors can therefore provide the basis for a novel inverse targeting strategy with potential use for selective transduction of hematopoietic or nonhematopoietic cells (eg, cancer cells) in a mixed cell population.

Functional interactions between monomers of the retroviral envelope protein complex

Retroviral vectors have been widely used in human gene therapy protocols. Entry into target cells is directed by the retroviral envelope protein, with receptor binding and postbinding fusion functions contributed mainly by the SU and TM subunits, respectively. We have generated mutants of the Moloney murine leukemia virus (MoMuLV) envelope protein with mutations in both the receptor binding domain of SU and throughout the TM subunit that are functionally inactive when expressed individually. However, the coexpression of these two classes of mutants partially restores envelope protein function and allows transduction. Several lines of evidence indicate that this complementation occurs in trans within envelope protein heterooligomers. The finding that the binding and postbinding functions of a retroviral envelope protein can be contributed by two different monomers should assist in the engineering of envelope proteins for tissue-specific gene delivery.

Masking of retroviral envelope functions by oligomerizing polypeptide adaptors

We have constructed chimeric retroviral envelopes displaying N-terminal polypeptides that are known to form homotrimeric associations. The amphotropic receptor (RAM-1) binding domain from the trimeric surface (SU) glycoprotein of 4070A murine leukemia virus (MLV)-inhibited ecotropic receptor (Rec-1) mediated infection by the SU glycoprotein of Moloney MLV when grafted to its N-terminus. The block to Rec-1-mediated infection was reversed when the RAM-1 binding domain was cleaved from the vector particles using an engineered factor Xa protease-sensitive cleavage signal between the envelope glycoprotein and its N-terminal extension. Trimeric leucine zipper peptides and the trimeric C-terminal domain of CD40 ligand were shown to inhibit RAM-1-mediated infection of NIH3T3 cells by the 4070A envelope when fused to its N-terminus, whereas monomeric helical peptides and the monomeric epidermal growth factor domain did not. The block to RAM-1-mediated infection was reversed when the trimeric polypeptides were cleaved from the vector particles by addition of factor Xa protease. Envelope binding assays using cleaved and uncleaved chimeric 4070A envelopes revealed that binding to RAM-1 receptors on mammalian cells was hindered by trimeric, but not by monomeric, N-terminal polypeptides. These results have important implications for the design of protease-activatable vectors for targeted gene delivery.

Dissection of a retrovirus envelope protein reveals structural similarity to influenza hemagglutinin

BACKGROUND

The amino-acid sequences of retroviral envelope proteins contain a '4-3 hydrophobic repeat', with hydrophobic amino acids spaced every four and then every three residues, characteristic of sequences that form coiled coils. The 4-3 hydrophobic repeat is located in the transmembrane subunit (TM) of the retroviral envelope protein, adjacent to the fusion peptide, a region that inserts into the host bilayer during the membrane-fusion process. A 4-3 hydrophobic repeat region in an analogous position of the influenza hemagglutinin protein is recruited to extend a three-stranded coiled coil during the conformational change to the fusion-competent state. To determine the conformation of the retroviral TM subunit and the role of the 4-3 hydrophobic repeat, we constructed soluble peptide models of the envelope protein of Moloney murine leukemia virus (MMLV).

RESULTS

The region of the MMLV TM protein external to the lipid envelope (the ectodomain) contains a stably folded, trimeric, protease-resistant core. As predicted, an alpha-helical segment spans the 4-3 repeat. A cysteine-rich region carboxy-terminal to the 4-3 repeat confers a dramatic increase in stability and displays a unique disulfide bonding pattern.

CONCLUSIONS

Our results demonstrate that the MMLV TM subunit can fold into a stable and distinct species in the absence of the receptor-binding 'surface' co-subunit (SU) of the envelope complex. As the SU subunit is readily shed from the surface of the virus, we conclude that the TM subunit structure forms the core of the MMLV membrane-fusion machinery, and that this structure, like the fusion-active conformation of influenza hemagglutinin, contains a three-stranded coiled coil adjacent to the fusion peptide.

The baculovirus cysteine protease has a cathepsin B-like S2-subsite specificity

Autographa californica nuclear polyhedrosis virus (AcNPV) encodes a functional cysteine protease of the papain family which is expressed after infection in Spodoptera fruglperda Sf9 cells. The protease displays an inhibition profile typical for cysteine proteases and is highly active against synthetic peptide substrates. The pH optimum of the bell-shaped pH-activity curve is between 5.0 and 5.5. The best substrate tested is Z-Arg-Arg-MCA which is specific for cathepsin B. The specificity constant (Kcat/Km) of AcNPV protease for this substrate is approximately two times higher than for human cathepsin B. In contrast to human cathepsins, AcNPV protease does not exhibit a discriminating specificity towards neutral hydrophobic residues in the P2 position. These substrates are hydrolysed at a ten-fold lower rate than the P2 arginine containing substrate. The pH activity profile against the Z-Arg-Arg-MCA substrate reveals a pK of 5.35 which can be assigned to a glutamate residue in the S2 subsite pocket. Like in cathepsin B, this residue facilitates the binding of positively charged P2 residues in the primary binding pocket. In this respect, the AcNPV protease resembles cathepsin B more than cathepsins L and S.

The pathogenesis of murine retroviral infection of the central nervous system

Several decades have lapsed since the original description of retroviral infection of the central nervous system (CNS) appeared. With the recent arrival of the autoimmune deficiency syndrome (AIDS) epidemic and the associated human retroviral encephalitis, interest in murine models has been rekindled. In most of the published studies, susceptible mouse strains are infected as neonates with molecularly cloned type-C retroviruses. In most models, a spongiform encephalopathy follows an early CNS endothelial cell infection. The subsequent pathogenesis of this encephalopathy is unknown. In some models neuronal and glial infection is seen, while in others only non-neuroglial elements are infected. This variation can be traced to differences in strains of mice and viruses in addition to differences in assays. The different models offer fertile experimental ground to decipher the role of direct versus indirect neuroglial damage. Reconciliation of these various models where the final neuropathology appears so similar, may be the key to understanding their pathogenesis.

Correlation of specific virus-astrocyte interactions and cytopathic effects induced by ts1, a neurovirulent mutant of Moloney murine leukemia virus

ts1 is a highly neuropathogenic and lymphocytopathic mutant of Moloney murine leukemia virus TB (MoMuLV-TB). We previously reported that the primary neuropathogenic determinant of ts1 maps to a single amino acid substitution, Val-25-->Ile, in precursor envelope protein gPr80env. This Val-25-->Ile substitution apparently renders gPr80env inefficient for transport from the endoplasmic reticulum to the Golgi apparatus. These findings suggest that the cytopathic effect of ts1 in neural cells might be due to the accumulation of gPr80env in the endoplasmic reticulum. Since endothelial and glial cells are targets of ts1 infection in the central nervous system, we established primary endothelial and astrocyte cultures to investigate the mechanism of cell killing caused by ts1. A continuous cell line, TB, was used as a control. Our results showed that both ts1 and MoMuLV-TB replicated and induced a cytopathic effect in astrocyte cultures, albeit to different degrees; ts1 appeared to be more lethal than MoMuLV-TB. On the other hand, ts1 and MoMuLV-TB infections of endothelial or TB cells were not cytopathic. The cytopathic effect in infected astrocytes correlated with the inefficiency of gPr80env transport and the intracellular accumulation of gPr80env as well as aberrant virus particles.

Studies on compartmentation and turnover of murine retrovirus envelope proteins

Several aspects of turnover and degradation of cell membrane proteins were studied in an NIH 3T3 cell clone expressing the env gene of Moloney murine leukemia virus ts1. Both internalization and shedding of the extracellular domain of the envelope protein gp70 occurred at the cell surface, albeit, in the case of shedding, only a very small fraction of gp70 was shed. The turnover rate of gp70 at the cell surface was similar to that of the same protein in the postendoplasmic reticulum intracellular compartment. In the presence of L-methionine methyl ester, the transmembrane domain of the envelope protein Prp15E was degraded faster than gp70.

A 585-bp deletion found in the spleen focus-forming virus (SFFV) env gene is responsible for the defective intracellular transport of SFFV gp52

The Friend spleen focus-forming virus (F-SFFV) codes for a transport defective, leukemogenic envelope glycoprotein designated as gp52. Gp52 closely resembles the envelope glycoproteins (gp70-p15E) encoded by the mink cell focus-forming viruses (MCFV). The major differences between SFFV and MCFV include a 585-bp deletion and a frame-shift mutation near the 3' end of the SFFV env gene. We have constructed a mutant MCFV env gene, which contains a 585-bp deletion like that found in the SFFV env gene, and expressed this gene using recombinant vaccinia vectors or retroviral vectors. The mutant MCFV env gene expressed a truncated, transport defective glycoprotein (gp57). Only a small proportion of gp57 underwent further oligosaccharide processing. Intracellular gp57 remained predominantly monomeric and only a small proportion of gp57 (and its processed forms) formed disulfide-linked dimers and trimers which resembled those formed by SFFV gp52. Processed forms of gp57 were found on the cell surfaces and in culture fluids. The extracellular forms had a faster electrophoretic mobility than the intracellular-processed forms of gp57. These results indicate that the 585-bp deletion found in SFFV env gene is responsible for the folding, transport, and secretion of gp52. Retroviral vectors carrying the mutant MCFV env gene were nonpathogenic (or weakly pathogenic) in adult mice. The results indicate that the 585-bp deletion, although essential, is not the sole determinant of SFFV-induced disease in adult mice.

ts1, a temperature-sensitive mutant of Moloney murine leukemia virus TB, can infect both CD4+ and CD8+ T cells but requires CD4+ T cells in order to cause paralysis and immunodeficiency

When neonatal FVB/N mice were inoculated with ts1, a temperature-sensitive mutant of Moloney murine leukemia virus TB, they developed a progressive bilateral hindlimb paralysis and immunodeficiency leading to death 4 to 6 weeks after inoculation. T lymphocytes have been shown to be primarily responsible for this ts1-induced syndrome. Here we compare the role played by each subset of T lymphocytes, i.e., CD4+ and CD8+ T cells, in disease development. Mice were depleted of a specific subset for the first 10 days of their lives by using either anti-CD4 or anti-CD8 monoclonal antibodies in vivo. Disease development in these mice was then monitored. Depletion of CD4+ T cells significantly attenuated the ts1-induced syndrome: virus replication was decreased, disease latency was extended, and death was prevented in 60% of the mice. Similar treatment with anti-CD8 antibody had almost no effect on disease progression. However, when depletion was begun 2 weeks after neonatal ts1 inoculation, CD4+ T cell depletion did not affect disease development. ts1 infected CD4+ and CD8+ T lymphocytes equally well in vivo, as shown by flow cytometric analysis, but virus replication was restricted primarily to the CD4+ subset of T cells, as found by in vitro assay. Hence, CD4+ T lymphocytes play an important role in the development of ts1-induced paralysis and immunodeficiency. The mechanism of this CD4+ T-cell-mediated disease production by ts1 is not clear; however, increased replication of ts1 in the CD4+ T cells, especially in the early stages of the disease, seems to play a crucial role.

Glucosidase inhibitors for treatment of HIV-1 infection

The HIV-1 envelope protein is a glycoprotein composed of 120 kD and 41 kD subunits. It contains 30-38 potential asparagine-linked glycosylation sites which have been shown to play a role in CD4 binding, virus uptake, and cytopathogenicity. Several inhibitors of oligosaccharide attachment or modification have been tested. An agent which inhibits glucosidases, N-butyl deoxynojirimycin was found to inhibit HIV-1 and SIVmac infectivity, and is currently in clinical trials.