Nef does not affect the efficiency of human immunodeficiency virus type 1 fusion with target cells

Human immunodeficiency virus integrates directly into naive resting CD4+ T cells but enters naive cells less efficiently than memory cells

Resting CD4(+) T cells restrict human immunodeficiency virus (HIV) infection at or before reverse transcription, resulting in slower kinetics of reverse transcription. In a previous study, we showed that, despite this restriction at reverse transcription, HIV integration occurs in resting CD4(+) T cells, albeit with slower kinetics. In that study, the resting T cells were a mixture of memory and naïve cells. Here we asked whether the more quiescent naïve cell subset could be directly infected by HIV and, if so, whether the level of integration in naïve cells was comparable to that in memory cells. We found that HIV integrates in the naïve subset of resting CD4(+) T cells without prior activation of the cells. The level of integration (proviruses/cell) in naïve cells was lower than that in memory cells. This difference between naïve and memory cells was observed whether we inoculated the cells with R5 or X4 HIV and could not be explained solely by differences in coreceptor expression. The presence of endogenous dendritic cells did not change the number of proviruses/cell in memory or naïve cells, and deoxynucleoside pools were equally limiting. Our results instead indicate the existence of a novel restriction point in naïve T cells at viral fusion that results in reduced levels of fusion to naïve CD4(+) T cells. We conclude that HIV can integrate into both naïve and memory cells directly. Our data further support our hypothesis that integrated proviral infection of resting T cells can be established without T-cell activation.

Implications of Nef: host cell interactions in viral persistence and progression to AIDS

The HIV and SIV Nef accessory proteins are potent enhancers of viral persistence and accelerate progression to AIDS in HIV-1-infected patients and non-human primate models. Although relatively small (27-35 kD), Nef can interact with a multitude of cellular factors and induce complex changes in trafficking, signal transduction, and gene expression that together converge to promote viral replication and immune evasion. In particular, Nef recruits several immunologically relevant cellular receptors to the endocytic machinery to reduce the recognition and elimination of virally infected cells by the host immune system, while simultaneously interacting with various kinases to promote T cell activation and viral replication. This review provides an overview on selected Nef interactions with host cell proteins, and discusses their possible relevance for viral spread and pathogenicity.

Human immunodeficiency virus type 1 Nef incorporation into virions does not increase infectivity

The viral protein Nef contributes to the optimal infectivity of human and simian immunodeficiency viruses. The requirement for Nef during viral biogenesis particles suggests that Nef might play a role in this process. Alternatively, because Nef is incorporated into viruses, it might play a role when progeny virions reach target cells. We challenged these hypotheses by manipulating the amounts of Nef incorporated in viruses while keeping its expression level constant in producer cells. This was achieved by forcing the incorporation of Nef into viral particles by fusing a Vpr sequence to the C-terminal end of Nef. A cleavage site for the viral protease was introduced between Nef and Vpr to allow the release of Nef fragments from the fusion protein during virus maturation. We show that the resulting Nef-CS-Vpr fusion partially retains the ability of Nef to downregulate cell surface CD4 and that high amounts of Nef-CS-Vpr are incorporated into viral particles compared with what is seen for wild-type Nef. The fusion protein is processed during virion maturation and releases Nef fragments similar to those found in viruses produced in the presence of wild-type Nef. Unlike viruses produced in the presence of wild-type Nef, viruses produced in the presence of Nef-CS-Vpr do not have an increase in infectivity and are as poorly infectious as viruses produced in the absence of Nef. These findings demonstrate that the presence of Nef in viral particles is not sufficient to increase human immunodeficiency virus type 1 infectivity and suggest that Nef plays a role during the biogenesis of viral particles.

Common principles and intermediates of viral protein-mediated fusion: the HIV-1 paradigm

Enveloped viruses encode specialized fusion proteins which promote the merger of viral and cell membranes, permitting the cytosolic release of the viral cores. Understanding the molecular details of this process is essential for antiviral strategies. Recent structural studies revealed a stunning diversity of viral fusion proteins in their native state. In spite of this diversity, the post-fusion structures of these proteins share a common trimeric hairpin motif in which the amino- and carboxy-terminal hydrophobic domains are positioned at the same end of a rod-shaped molecule. The converging hairpin motif, along with biochemical and functional data, implies that disparate viral proteins promote membrane merger via a universal "cast-and-fold" mechanism. According to this model, fusion proteins first anchor themselves to the target membrane through their hydrophobic segments and then fold back, bringing the viral and cellular membranes together and forcing their merger. However, the pathways of protein refolding and the mechanism by which this refolding is coupled to membrane rearrangements are still not understood. The availability of specific inhibitors targeting distinct steps of HIV-1 entry permitted the identification of key conformational states of its envelope glycoprotein en route to fusion. These studies provided functional evidence for the direct engagement of the target membrane by HIV-1 envelope glycoprotein prior to fusion and revealed the role of partially folded pre-hairpin conformations in promoting the pore formation.

Characterizing the anti-HIV activity of papuamide A

Papuamide A is representative of a class of marine derived cyclic depsipeptides, reported to have cytoprotective activity against HIV-1 in vitro. We show here that papuamide A acts as an entry inhibitor, preventing human immunodeficiency virus infection of host cells and that this inhibition is not specific to R5 or X4 tropic virus. This inhibition of viral entry was determined to not be due to papuamide A binding to CD4 or HIV gp120, the two proteins involved in the cell-virus recognition and binding. Furthermore, papuamide A was able to inhibit HIV pseudotype viruses expressing envelope glycoproteins from vesicular stomatitis virus or amphotropic murine leukemia virus indicating the mechanism of viral entry inhibition is not HIV-1 envelope glycoprotein specific. Time delayed addition studies with the pseudotyped viruses show that papuamide A inhibits viral infection only at the initial stage of the viral life cycle. Additionally, pretreatment studies revealed that the virus, and not the cell, is the target of papuamide A’s action. Together, these results suggest a direct virucidal mechanism of HIV-1 inhibition by papuamide A. We also demonstrate here that the other papuamides (B-D) are able to inhibit viral entry indicating that the free amino moiety of 2,3-diaminobutanoic acid residue is not required for the virucidal activity.

HIV envelope-CXCR4 signaling activates cofilin to overcome cortical actin restriction in resting CD4 T cells

Binding of the HIV envelope to the chemokine coreceptors triggers membrane fusion and signal transduction. The fusion process has been well characterized, yet the role of coreceptor signaling remains elusive. Here, we describe a critical function of the chemokine coreceptor signaling in facilitating HIV infection of resting CD4 T cells. We find that static cortical actin in resting T cells represents a restriction and that HIV utilizes the Galphai-dependent signaling from the chemokine coreceptor CXCR4 to activate a cellular actin-depolymerizing factor, cofilin, to overcome this restriction. HIV envelope-mediated cofilin activation and actin dynamics are important for a postentry process that leads to viral nuclear localization. Inhibition of HIV-mediated actin rearrangement markedly diminishes viral latent infection of resting T cells. Conversely, induction of active cofilin greatly facilitates it. These findings shed light on viral exploitation of cellular machinery in resting T cells, where chemokine receptor signaling becomes obligatory.

Nef can enhance the infectivity of receptor-pseudotyped human immunodeficiency virus type 1 particles

Nef is an accessory protein of human immunodeficiency virus type 1 (HIV-1) that enhances the infectivity of progeny virions when expressed in virus-producing cells. The requirement for Nef for optimal infectivity is, at least in part, determined by the envelope (Env) glycoprotein, because it can be eliminated by pseudotyping HIV-1 particles with pH-dependent Env proteins. To investigate the role of Env in the function of Nef, we have examined the effect of Nef on the infectivity of Env-deficient HIV-1 particles pseudotyped with viral receptors for cells expressing cognate Env proteins. We found that Nef significantly enhances the infectivity of CD4-chemokine receptor pseudotypes for cells expressing HIV-1 Env. Nef also increased the infectivity of HIV-1 particles pseudotyped with Tva, the receptor for subgroup A Rous sarcoma virus (RSV-A), even though Nef had no effect if the pH-dependent Env protein of RSV-A was used for pseudotyping. However, Nef does not always enhance viral infectivity if the normal orientation of the Env-receptor interaction is reversed, because the entry of Env-deficient HIV-1 into cells expressing the vesicular stomatitis virus G protein was unaffected by Nef. Together, our results demonstrate that the presence of a viral Env protein during virus production is not required for the ability of Nef to increase viral infectivity. Furthermore, since the infectivity of Tva pseudotypes was blocked by inhibitors of endosomal acidification, we conclude that low-pH-dependent entry does not always bypass the requirement for Nef.

Nef does not contribute to replication differences between R5 pre-AIDS and AIDS HIV-1 clones from patient ACH142

AIDS-associated, CCR5-tropic (R5) HIV-1 clones, isolated from a patient that never developed CXCR4-tropic HIV-1, replicate to a greater extent and cause greater cytopathic effects than R5 HIV-1 clones isolated before the onset of AIDS. Previously, we showed that HIV-1 Env substantially contributed to the enhanced replication of an AIDS clone. In order to determine if Nef makes a similar contribution, we cloned and phenotypically analyzed nef genes from a series of patient ACH142 derived R5 HIV-1 clones. The AIDS-associated Nef contains a series of residues found in Nef proteins from progressors [1]. In contrast to other reports [1-3], this AIDS-associated Nef downmodulated MHC-I to a greater extent and CD4 less than pre-AIDS Nef proteins. Additionally, all Nef proteins enhanced infectivity similarly in a single round of replication. Combined with our previous study, these data show that evolution of the HIV-1 env gene, but not the nef gene, within patient ACH142 significantly contributed to the enhanced replication and cytopathic effects of the AIDS-associated R5 HIV-1 clone.

Nef enhances HIV-1 infectivity via association with the virus assembly complex

The HIV-1 accessory protein Nef enhances virus infectivity by facilitating an early post-entry step of infection. Nef acts in the virus producer cell, leading to a beneficial modification to HIV-1 particles. Nef itself is incorporated into HIV-1 particles, where it is cleaved by the viral protease during virion maturation. To probe the role of virion-associated Nef in HIV-1 infection, we generated a fusion protein consisting of the host protein cyclophilin A (CypA) linked to the amino terminus of Nef. The resulting CypA-Nef protein enhanced the infectivity of Nef-defective HIV-1 particles and was specifically incorporated into the virions via association with Gag during particle assembly. Pharmacologic or genetic inhibition of CypA-Nef binding to Gag prevented incorporation of CypA-Nef into virions and inhibited infectivity enhancement. Our results indicate that infectivity enhancement by Nef requires its association with a component of the assembling HIV-1 particle.

Rev-dependent indicator T cell line

Measuring virion infectivity is critical for studying and monitoring the process of HIV-1 infection. The easiest and the most common method utilizes reporter cell lines based on the HIV LTR promoter. The early HIV gene product Tat amplifies expression from the LTR; however, there is a background transcriptional activity that is independent of Tat. Furthermore, LTR activity can be influenced by cellular activation states. We have recently constructed a Rev-dependent expression vector, and as a test of this construct's functionality, we have integrated this vector into a continuous T cell line. This novel indicator cell has no measurable background signal, is not affected by elevated metabolic states, and yet responds robustly to the presence of HIV. The line is able to complete TCID50 assays in 3-5 days, and appears sensitive to both CCR5- and CXCR4-utilizing viruses.

Intracellular trafficking of plasmids for gene therapy: mechanisms of cytoplasmic movement and nuclear import

Under physiologically relevant conditions, the levels of non-viral gene transfer are low at best. The reason for this is that many barriers exist for the efficient transfer of genes to cells, even before any gene expression can occur. While many transfection strategies focus on DNA condensation and overcoming the plasma membrane, events associated with the intracellular trafficking of the DNA complexes have not been as extensively studied. Once internalized, plasmids must travel potentially long distances through the cytoplasm to reach their next barrier, the nuclear envelope. This review summarizes the current progress on the cytoplasmic trafficking and nuclear transport of plasmids used for gene therapy applications. Both of these processes utilize specific and defined mechanisms to facilitate movement of DNA complexes through the cell. The continued elucidation and exploitation of these mechanisms will lead to improved strategies for transfection and successful gene therapy.

Role of envelope processing and gp41 membrane spanning domain in the formation of human immunodeficiency virus type 1 (HIV-1) fusion-competent envelope glycoprotein complex

Human immunodeficiency virus type 1 (HIV-1) entry into target cells is directed by the envelope (Env) glycoproteins, which are present on the surface of HIV-1 virion or infected cells in the form of trimers consisting of gp120/gp41 complexes. The surface subunit, gp120, initiates the entry process by interacting sequentially with the CD4 receptor and a co-receptor, thereby inducing a conformational change that allows the transmembrane (TM) gp41 subunit to mediate fusion between viral and target cell membranes. Cleavage of Env into its gp120 and gp41 components is necessary for activation of its fusogenic activity. Here, the gp41 TM glycoprotein was altered by either deleting an isoleucine residue (DeltaI642) in a critical region of its ectodomain or by substituting its membrane spanning domain (MSD) by that of the influenza hemagglutinin (HA) glycoprotein (TM-HA) to examine the contribution of these regions to Env functions. Characterization of these mutant forms of gp41 revealed that they both affected the infectivity of pseudotyped virions, however, through distinct defects in Env functions. While deletion of Ile 642 drastically altered processing of Env, replacement of gp41 MSD by that of HA led to a marked fusion defect even though the TM-HA Env was efficiently processed and incorporated into viral particles. Interestingly, both DeltaI642 and TM-HA Env were found to act as trans dominant-negative mutant of viral infectivity, presumably via their ability to form hetero-oligomers with wild type Env. Together, these results support a previously proposed model whereby all three gp120-gp41 monomers must be cleaved for the Env homo-trimer to function and suggest that the gp41 MSD plays a critical role in the formation of fusion-competent Env trimers.

Selective restriction of Nef-defective human immunodeficiency virus type 1 by a proteasome-dependent mechanism

The Nef protein enhances human immunodeficiency virus type 1 (HIV-1) infectivity by facilitating an early postentry step in the virus life cycle. We report here that the addition of MG132 or lactacystin, each a specific inhibitor of cellular proteasome activity, preferentially enhances cellular permissiveness to infection by Nef-defective versus wild-type HIV-1. Pseudotyping by the glycoprotein of vesicular stomatitis virus rendered Nef-defective HIV-1 particles minimally responsive to the enhancing effects of proteasome inhibitors. These results suggest that Nef enhances the infectivity of HIV-1 particles by reducing their susceptibility to proteasomal degradation in target cells.

The tyrosine kinase inhibitor genistein blocks HIV-1 infection in primary human macrophages

Binding of HIV-1 envelope glycoprotein (Env) to its cellular receptors elicits a variety of signaling events, including the activation of select tyrosine kinases. To evaluate the potential role of such signaling, we examined the effects of the tyrosine kinase inhibitor, genistein, on HIV-1 entry and infection of human macrophages using a variety of assays. Without altering cell viability, cell surface expression of CD4 and CCR5 or their abilities to interact with Env, genistein inhibited infection of macrophages by reporter gene-encoding, beta-lactamase containing, or wild type virions, as well as Env-mediated cell-fusion. The observation that genistein blocked virus infection if applied before, during or immediately after the infection period, but not 24h later; coupled with a more pronounced inhibition of infection in the reporter gene assays as compared to both beta-lactamase and p24 particle entry assays, imply that genistein exerts its inhibitory effects on both entry and early post-entry steps. These findings suggest that other exploitable targets, or steps, of the HIV-1 infection process may exist and could serve as additional opportunities for the development of new therapeutics.

Functional characterization of HIV-1 Nef mutants in the context of viral infection

Nef is an important pathogenesis factor of HIV-1 with a multitude of effector functions. We have designed a broad panel of isogenic viruses encoding defined mutants of HIV-1(SF2) Nef and analyzed their biological activity in the context of productive HIV-1 infection. Analysis of subcellular localization, virion incorporation, downregulation of cell surface CD4 and MHC-I, enhancement of virion infectivity and facilitation of HIV replication in primary human T lymphocytes mostly confirmed the mapping of Nef determinants previously reported upon isolated expression of Nef. However, reduced activity in downregulation of CD4, infectivity enhancement and virion incorporation of a Nef variant (Delta12-39) lacking an amphipatic helix required for binding of a cellular kinase complex and the association of Nef with MHC-I/AP-1 suggested a novel role of this N-terminal motif. The SH3 binding motif of Nef was partially required for infectivity enhancement and replication but not for receptor downmodulation. In contrast to previous results obtained using other Nef alleles, non-myristoylated SF2-Nef was only partly defective when expressed during HIV infection and was present in HIV-1 particles. Importantly, incorporation of Nef into HIV-1 virions was not required for any of the tested Nef activities. Altogether, this study provides a broad characterization and mapping of multiple Nef activities in HIV-infected cells. The results emphasize that multiple activities govern Nef's effects on HIV replication and argue against a role of virion incorporation for Nef's activity as pathogenicity factor.

Human immunodeficiency virus fusion to dendritic cells declines as cells mature

The maturation of dendritic cells (DCs) is associated with a diminished ability to support human immunodeficiency virus (HIV) replication; however, the precise step in the HIV life cycle impaired by DC maturation remains uncertain. Using an HIV virion-based fusion assay, we now show that HIV fusion to monocyte-derived DCs (MDDCs) both decreases and kinetically slows when DCs are induced to mature with poly(I:C) and tumor necrosis factor alpha. Specifically, laboratory-adapted CCR5-tropic 81A virions fused with markedly lower efficiency to mature MDDCs than immature DCs. In contrast, fusion of NL4-3, the isogenic CXCR4-tropic counterpart of 81A, was low in both immature and mature MDDCs. Fusion mediated by primary HIV envelopes, including seven CCR5- and four CXCR4-tropic envelopes, also decreased with DC maturation. The kinetics of virion fusion were also altered by both the state of DC maturation and the coreceptor utilized. Fusion of 81A and NL4-3 virions was delayed in mature compared to immature MDDCs, and NL4-3 fused more slowly than 81A in both mature and immature MDDCs. Surprisingly, primary envelopes with CXCR4 tropism mediated fusion to immature MDDCs with efficiencies similar to those of primary CCR5-tropic envelopes. This result contrasted with the marked preferential fusion of the laboratory-adapted 81A over NL4-3 in immature MDDCs and in ex vivo Langerhans cells, indicating that these laboratory-adapted HIV strains do not fully recapitulate all of the properties of primary HIV isolates. In conclusion, our results demonstrate that the defect in HIV replication observed in mature MDDCs stems at least in part from a decline in viral fusion.

Gene therapy progress and prospects: viral trafficking during infection

The intracellular steps involved in viral infection, namely cytoplasmic trafficking and nuclear import, are critical events in the viral life cycle that have lagged behind other areas of viral research. This review examines recent advances in our understanding of these steps for viruses commonly employed as viral gene delivery vectors. Steps governing the cytoplasmic trafficking and nuclear import of Herpes Simplex virus, Human Immunodeficiency virus and Adenovirus are reviewed in this article.

Intracellular trafficking of retroviral vectors: obstacles and advances

Retroviruses are efficient vehicles for delivering transgenes in vivo. Their ability to integrate into the host genome, providing a permanent imprint of their genes in the host, is a key asset for gene therapy. Furthermore, the lentivirus subset of retroviruses can infect nondividing as well as dividing cells. This expands the cell types capable of gene therapy, driving the development of lentiviral vectors. However, the precise mechanisms used by different retroviruses to efficiently deliver their genes into cell nuclei remains largely unclear. Understanding these molecular mechanisms may reveal features to improve the efficacy of current retroviral vectors. Moreover, this knowledge may expose elements pliable to other gene therapy vehicles to improve their in vivo performance and circumvent the biosafety concerns of using retroviral vectors. Therefore, the mechanisms underlying the early trafficking of retroviral vectors in host cells are reviewed here, as understood from studying the native retroviruses. Events after virus entry up to nuclear delivery of the viral cDNA are discussed. Cellular obstacles faced by these retroviral vectors and how they advance beyond these barriers is emphasized.

A human monoclonal antibody neutralizes diverse HIV-1 isolates by binding a critical gp41 epitope

HIV-1 entry into cells is mediated by the envelope glycoprotein receptor-binding (gp120) and membrane fusion-promoting (gp41) subunits. The gp41 heptad repeat 1 (HR1) domain is the molecular target of the fusion-inhibitor drug enfuvirtide (T20). The HR1 sequence is highly conserved and therefore considered an attractive target for vaccine development, but it is unknown whether antibodies can access HR1. Herein, we use gp41-based peptides to select a human antibody, 5H/I1-BMV-D5 (D5), that binds to HR1 and inhibits the assembly of fusion intermediates in vitro. D5 inhibits the replication of diverse HIV-1 clinical isolates and therefore represents a previously unknown example of a crossneutralizing IgG selected by binding to designed antigens. NMR studies and functional analyses map the D5-binding site to a previously identified hydrophobic pocket situated in the HR1 groove. This hydrophobic pocket was proposed as a drug target and subsequently identified as a common binding site for peptide and peptidomimetic fusion inhibitors. The finding that the D5 fusion-inhibitory antibody shares the same binding site suggests that the hydrophobic pocket is a "hot spot" for fusion inhibition and an ideal target on which to focus a vaccine-elicited antibody response. Our data provide a structural framework for the design of new immunogens and therapeutic antibodies with crossneutralizing potential.

Time-resolved imaging of HIV-1 Env-mediated lipid and content mixing between a single virion and cell membrane

A method has been developed to follow fusion of individual pseudotyped virus expressing HIV-1 Env to cells by time-resolved fluorescence microscopy. Viral envelopes were labeled with a fluorescent lipid dye (DiD) and virus content was rendered visible by incorporating a Gag-GFP chimera. The Gag-GFP is naturally cleaved to the much smaller NC-GFP fragment in the mature virions. NC-GFP was readily released upon permeabilization of the viral envelope, whereas the capsid was retained. The NC-GFP thus provides a relatively small and mobile aqueous marker to follow viral content transfer. In fusion experiments, virions were bound to cells at low temperature, and fusion was synchronously triggered by a temperature jump. DiD transferred from virions to cells without a significant lag after the temperature jump. Some virions released DiD but retained NC-GFP. Surprisingly, the fraction of lipid mixing events yielding NC-GFP transfer was dependent on the type of target cell: of three infectable cell lines, only one permitted NC-GFP transfer within minutes of raising temperature. NC-GFP release did not correlate with the level of CD4 or coreceptor expression in the target cells. The data indicate that fusion pores formed by HIV-1 Env can remain small for a relatively long time before they enlarge.

HIV accessory proteins and surviving the host cell

Human immunodeficiency virus generates the accessory proteins Nef, viral infectivity factor (Vif), viral protein R, and viral protein U or viral protein X during viral replication in host cells. Although the significance of these accessory proteins is often lost in vitro, they are essential for viral pathogenesis in vivo. Therefore, these proteins have much potential as antiviral targets. Recent data reveal Vif perturbs an ill-defined antiviral pathway in host cells allowing HIV replication. These data highlight a common feature among HIV accessory proteins in manipulating the host to aid viral pathogenesis. Therefore, these new insights into Vif and other HIV accessory proteins are reviewed, emphasizing host cell interactions and new targets for therapeutic intervention.

Involvement of clathrin-mediated endocytosis in human immunodeficiency virus type 1 entry

Productive entry of human immunodeficiency virus (HIV) is believed to occur by direct fusion at the plasma membrane. Endocytic uptake of HIV particles has been observed in several studies but is considered to be nonproductive, leading to virus degradation in the lysosome. We show here that endocytosis contributes significantly to productive HIV entry in HeLa cells by using trans dominant-negative mutants of dynamin and Eps15. Inducible expression of a dominant-negative mutant of dynamin in a CD4-positive HeLa cell line reduced HIV infection by 40 to 80%. This effect was independent of the infectious dose and was observed for three different isolates. Analysis of reverse transcription products by real-time PCR and of virus entry by delivery of a virion-associated Vpr-beta-lactamase fusion protein revealed a similar reduction, indicating that the block occurred at the entry stage. A strong reduction of HIV entry was also observed upon transient transfection of a different trans dominant-negative variant of dynamin, and this reduction correlated with the relative inhibition of transferrin endocytosis. Expression of a dominant-negative variant of Eps15, which is specific for clathrin-dependent endocytosis, reduced HIV entry in HeLa cells by ca 95%, confirming the role of endocytosis for productive infection. In contrast, no effect was observed for a dominant-negative variant of caveolin. We conclude that dynamin-dependent, clathrin-mediated endocytosis can lead to productive entry of HIV in HeLa cells, suggesting this pathway as an alternative route of virus entry.

HIV-1 virion fusion assay: uncoating not required and no effect of Nef on fusion

We recently described a sensitive and specific assay that detects the fusion of HIV-1 virions to a broad range of target cells, including primary CD4 cells. This assay involves the use of virions containing beta-lactamase-Vpr (BlaM-Vpr) and the loading of target cells with CCF2, a fluorogenic substrate of beta-lactamase. Since Vpr strongly associates with the viral core, uncoating of the viral particle might be required for effective cleavage of CCF2 by BlaM-Vpr. Here, we show that BlaM-Vpr within mature viral cores effectively cleaves CCF2, indicating that this assay measures virion fusion independently of uncoating. We also show that wildtype and Nef-deficient HIV-1 virions fuse with equivalent efficiency to HeLa-CD4 cells, SupT1 T cells, and primary CD4 T cells. Since Nef enhances cytoplasmic delivery of viral cores and increases viral infectivity, these findings indicate that Nef enhances an early post-fusion event in the multistep process of viral entry. Possible sites of Nef action include enlargement of the fusion pore, enhanced uncoating of viral particles, and more efficient passage of viral cores through the dense cortical actin network located immediately beneath the plasma membrane.

Murine T cells potently restrict human immunodeficiency virus infection

Development of a mouse model for human immunodeficiency virus type 1 (HIV-1) infection has advanced through the progressive identification of host cell factors required for HIV-1 replication. Murine cells lack HIV-1 receptor molecules, do not support efficient viral gene expression, and lack factors necessary for the assembly and release of virions. Many of these blocks have been described using mouse fibroblast cell lines. Here we identify a postentry block to HIV-1 infection in mouse T-cell lines that has not been detected in mouse fibroblasts. While murine fibroblastic lines are comparable to human T-cell lines in permissivity to HIV-1 transduction, infection of murine T cells is 100-fold less efficient. Virus entry occurs efficiently in murine T cells. However, reduced efficiency of the completion of reverse transcription and nuclear transfer of the viral preintegration complex are observed. Although this block has similarities to the restriction of murine retroviruses by Fv1, there is no correlation of HIV-1 susceptibility with cellular Fv1 genotypes. In addition, the block to HIV-1 infection in murine T-cell lines cannot be saturated by a high virus dose. Further studies of this newly identified block may lend insight into the early events of retroviral replication and reveal new targets for antiretroviral interventions.

Low pH is required for avian sarcoma and leukosis virus Env-dependent viral penetration into the cytosol and not for viral uncoating

A novel entry mechanism has been proposed for the avian sarcoma and leukosis virus (ASLV), whereby interaction with specific cell surface receptors activates or primes the viral envelope glycoprotein (Env), rendering it sensitive to subsequent low-pH-dependent fusion triggering in acidic intracellular organelles. However, ASLV fusion seems to proceed to a lipid mixing stage at neutral pH, leading to the suggestion that low pH might instead be required for a later stage of viral entry such as uncoating (L. J. Earp, S. E. Delos, R. C. Netter, P. Bates, and J. M. White. J. Virol. 77:3058-3066, 2003). To address this possibility, hybrid virus particles were generated with the core of human immunodeficiency virus type 1 (HIV-1), a known pH-independent virus, and with subgroups A or B ASLV Env proteins. Infection of cells by these pseudotyped virions was blocked by lysosomotropic agents, as judged by inhibition of HIV-1 DNA synthesis. Furthermore, by using HIV-1 cores that contain a Vpr-beta-lactamase fusion protein (Vpr-BlaM) to monitor viral penetration into the cytosol, we demonstrated that virions bearing ASLV Env, but not HIV-1 Env, enter the cytosol in a low-pH-dependent manner. This effect was independent of the presence of the cytoplasmic tail of ASLV Env. These studies provide strong support for the model, indicating that low pH is required for ASLV Env-dependent viral penetration into the cytosol and not for viral uncoating.

Sequential roles of receptor binding and low pH in forming prehairpin and hairpin conformations of a retroviral envelope glycoprotein

A general model has been proposed for the fusion mechanisms of class I viral fusion proteins. According to this model a metastable trimer, anchored in the viral membrane through its transmembrane domain, transits to a trimeric prehairpin intermediate, anchored at its opposite end in the target membrane through its fusion peptide. A subsequent refolding event creates a trimer of hairpins (often termed a six-helix bundle) in which the previously well-separated transmembrane domain and fusion peptide (and their attached membranes) are brought together, thereby driving membrane fusion. While there is ample biochemical and structural information on the trimer-of-hairpins conformation of class I viral fusion proteins, less is known about intermediate states between native metastable trimers and the final trimer of hairpins. In this study we analyzed conformational states of the transmembrane subunit (TM), the fusion subunit, of the Env glycoprotein of the subtype A avian sarcoma and leukosis virus (ASLV-A). By analyzing forms of EnvA TM on mildly denaturing sodium dodecyl sulfate gels we identified five conformational states of EnvA TM. Following interaction of virions with a soluble form of the ASLV-A receptor at 37 degrees C, the metastable form of EnvA TM (which migrates at 37 kDa) transits to a 70-kDa and then to a 150-kDa species. Following subsequent exposure to a low pH (or an elevated temperature or the fusion promoting agent chlorpromazine), an additional set of bands at >150 kDa, and then a final band at 100 kDa, forms. Both an EnvA C-helix peptide (which inhibits virus fusion and infectivity) and the fusion-inhibitory agent lysophosphatidylcholine inhibit the formation of the >150- and 100-kDa bands. Our data are consistent with the 70- and 150-kDa bands representing precursor and fully formed prehairpin conformations of EnvA TM. Our data are also consistent with the >150-kDa bands representing higher-order oligomers of EnvA TM and with the 100-kDa band representing the fully formed six-helix bundle. In addition to resolving fusion-relevant conformational intermediates of EnvA TM, our data are compatible with a model in which the EnvA protein is activated by its receptor (at neutral pH and a temperature greater than or equal to room temperature) to form prehairpin conformations of EnvA TM, and in which subsequent exposure to a low pH is required to stabilize the final six-helix bundle, which drives a later stage of fusion.

Disruption of the actin cytoskeleton can complement the ability of Nef to enhance human immunodeficiency virus type 1 infectivity

The human immunodeficiency virus (HIV) protein Nef has been shown to increase the infectivity of HIV at an early point during infection. Since Nef is known to interact with proteins involved in actin cytoskeleton rearrangements, we tested the possibility that Nef may enhance HIV infectivity via a mechanism that involves the actin cytoskeleton. We find that disruption of the actin cytoskeleton complements the Nef infectivity defect. The ability of disruption of the actin cytoskeleton to complement the Nef defect was specific to envelopes that fuse at the cell surface, including a variety of HIV envelopes and the murine leukemia virus amphotropic envelope. In contrast, the infectivity of HIV virions pseudotyped to enter cells via endocytosis, which is known to complement the HIV Nef infectivity defect and can naturally penetrate the cortical actin barrier, was not altered by actin cytoskeleton disruption. The results presented here suggest that Nef functions to allow the HIV genome to penetrate the cortical actin network, a known barrier for intracellular parasitic organisms.

The effects of HIV-1 Nef on CD4 surface expression and viral infectivity in lymphoid cells are independent of rafts

The HIV-1 Nef protein is a critical virulence factor that exerts multiple effects during viral replication. Nef modulates surface expression of various cellular proteins including CD4 and MHC-I, enhances viral infectivity, and affects signal transduction pathways. Nef has been shown to partially associate with rafts, where it can prime T cells for activation. The contribution of rafts during Nef-induced CD4 down-regulation and enhancement of viral replication remains poorly understood. We show here that Nef does not modify the palmitoylation state of CD4 or its partition within rafts. Moreover, CD4 mutants lacking palmitoylation or unable to associate with rafts are efficiently down-regulated by Nef. In HIV-infected cells, viral assembly and budding occurs from rafts, and Nef has been suggested to increase this process. However, using T cells acutely infected with wild-type or nef-deleted HIV, we did not observe any impact of Nef on raft segregation of viral structural proteins. We have also designed a palmitoylated mutant of Nef (NefG3C), which significantly accumulates in rafts. Interestingly, the efficiency of NefG3C to down-regulate CD4 and MHC-I, and to promote viral replication was not increased when compared with the wild-type protein. Altogether, these results strongly suggest that rafts are not a key element involved in the effects of Nef on trafficking of cellular proteins and on viral replication.

HIV-1 Nef Enhances Both Membrane Expression and Virion Incorporation of Env Products

The expression of human immunodeficiency virus Nef increases the viral infectivity through mechanisms still not fully elucidated. Here we report that wild-type (wt) human immunodeficiency virus, type 1 (HIV-1), particles were neutralized by higher concentrations of either anti-Env glycoprotein (gp) 41 antibodies or recombinant soluble human CD4 compared with Deltanef HIV-1. This appeared to be the result of a Nef-induced increase of virion incorporation of both gp41 (transmembrane (TM)) and surface gp120 Env products likely originating from enhanced steady-state levels of cell membrane-associated Env products. This, in turn, seemed to be the consequence of a reduced retention of the Env precursor. Most interesting, we found that both the Nef-directed increase of Env membrane expression and the Nef-induced enhancement of HIV-1 infectivity relied on the presence of the intracytoplasmic domain of TM, supporting the hypothesis of a functional correlation between these effects. Mutagenesis studies allowed us to establish that the two leucine residues at the TM C terminus, which are part of a sorting motif involved in the control of Env membrane expression, and the 181-210-residue Nef C-terminal region were critically involved in the Nef/Env functional interaction. In conclusion, we propose that Nef increases the infectivity of HIV-1 at least in part by enhancing the amounts of Env products incorporated into virus particles.

Coupling of human immunodeficiency virus type 1 fusion to virion maturation: a novel role of the gp41 cytoplasmic tail

Retrovirus particles are not infectious until they undergo proteolytic maturation to form a functional core. Here we report a link between human immunodeficiency virus type 1 (HIV-1) core maturation and the ability of the virus to fuse with target cells. Using a recently developed reporter assay of HIV-1 virus-cell fusion, we show that immature HIV-1 particles are 5- to 10-fold less active for fusion with target cells than are mature virions. The fusion of mature and immature virions was rendered equivalent by truncating the gp41 cytoplasmic domain or by pseudotyping viruses with the glycoprotein of vesicular stomatitis virus. An analysis of a panel of mutants containing mutated cleavage sites indicated that HIV-1 fusion competence is activated by the cleavage of Gag at any site between the MA and NC segments and not as an indirect consequence of an altered core structure. These results suggest a mechanism by which binding of the gp41 cytoplasmic tail to Gag within immature HIV-1 particles inhibits Env conformational changes on the surface of the virion that are required for membrane fusion. This "inside-out" regulation of HIV-1 fusion could play an important role in the virus life cycle by preventing the entry of immature, noninfectious particles.

The membrane-proximal tyrosine-based sorting signal of human immunodeficiency virus type 1 gp41 is required for optimal viral infectivity

The membrane-proximal tyrosine-based sorting motif in the cytoplasmic domain of the human immunodeficiency virus type 1 Env glycoprotein is important for endocytosis from the plasma membrane, basolateral targeting of viral budding in polarized epithelial cells, and polarized budding from a localized region of the lymphocyte plasma membrane. To study the role of the Env sorting motif (Y712XXL) in infectivity, the incorporation of Env into virions, and viral entry, we disrupted the motif with a tyrosine-to-alanine substitution. To investigate the relationship between the Env sorting motif and the enhancement of infectivity by Nef, the EnvY712A substitution was made in both Nef-positive and Nef-negative backgrounds. In spreading infections, including those using primary lymphocytes, the growth of the Y712A mutant was as impaired as Nef-negative virus, and the EnvY712A/Delta-Nef combination mutant was almost completely defective. In single-round infections using CD4-positive HeLa cells, the EnvY712A mutation impaired infectivity, and Nef retained the ability to enhance the infectivity in the context of EnvY712A. EnvY712 and Nef were required for the optimal infectivity of virions produced from either HEK293T or MT4 cells, but these sequences were required for the optimal incorporation of Env only when virions were produced from MT4 cells. Despite the wild-type levels of Env in viruses produced from 293T cells, the entry of the EnvY712A and Delta-Nef mutants into target cells was impaired. We conclude that the membrane-proximal tyrosine-based sorting motif of gp41 Env is, like Nef, important for optimal viral infectivity and, in the case of MT4 T cells, virion incorporation of Env. Nef does not require the Y712XXL motif to enhance viral infectivity. The finding that EnvY712 and Nef each affect the efficiency of viral entry independently of the Env content of virions suggests that both viral proteins are involved in trafficking events that influence morphogenesis to produce maximally fusogenic virus.