Identification of gp120 binding sites on CXCR4 by using CD4-independent human immunodeficiency virus type 2 Env proteins

Peptide-Induced Fusion of Dynamic Membrane Nanodomains: Implications in a Viral Entry

Enveloped viruses infect host cells via protein-mediated membrane fusion. However, insights into the microscopic rearrangement induced by the viral proteins and peptides have not yet emerged. Here, we report a new methodology to extract viral fusion peptide (FP)-mediated biomembrane dynamical nanodomain fusion parameter, λ, based on stimulated emission depletion microscopy coupled with fluorescence correlation spectroscopy. We also define another dynamical parameter membrane gradient, defined in terms of the ratio of average lipid diffusion coefficients across dynamic crossover length scales, ξ. Significantly, we observe that λ as well as these mobility gradients are larger in the stiffer liquid-ordered (Lo) phase compared to the liquid-disordered phase and are more effective at the smaller nanodomain interfaces, which are only present in the Lo phase. The results could possibly help to resolve a long-standing puzzle about the enhanced fusogenicity of FP in the Lo phase. Results obtained from the diffusion results have been correlated with the human immunodeficiency virus gp41 FP-induced membrane fusion.

Integrating deep mutational scanning and low-throughput mutagenesis data to predict the impact of amino acid variants

BACKGROUND

Evaluating the impact of amino acid variants has been a critical challenge for studying protein function and interpreting genomic data. High-throughput experimental methods like deep mutational scanning (DMS) can measure the effect of large numbers of variants in a target protein, but because DMS studies have not been performed on all proteins, researchers also model DMS data computationally to estimate variant impacts by predictors.

RESULTS

In this study, we extended a linear regression-based predictor to explore whether incorporating data from alanine scanning (AS), a widely used low-throughput mutagenesis method, would improve prediction results. To evaluate our model, we collected 146 AS datasets, mapping to 54 DMS datasets across 22 distinct proteins.

CONCLUSIONS

We show that improved model performance depends on the compatibility of the DMS and AS assays, and the scale of improvement is closely related to the correlation between DMS and AS results.

Combination of Oleic Acid and the gp41 Fusion Peptide Switches the Phosphatidylethanolamine-Induced Membrane Fusion Mechanism from a Nonclassical to a Classical Stalk Model

Membrane fusion is considered to be one of the crucial processes for the existence of eukaryotes and the entry of enveloped viruses into host cells. The fusion mechanism depends on the lipid composition of the membrane as well as the properties of fusion proteins or peptides. The gp41 fusion peptide from the human immunodeficiency virus (HIV) is known to catalyze membrane fusion by altering the physical properties of the membrane. Earlier, we demonstrated that a membrane containing 30 mol % phosphatidylethanolamine (PE) circumvents the classical stalk model because of its intrinsic negative curvature. In this work, we demonstrated how the gp41 fusion peptide influences the fusion mechanism of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dioleoyl-sn-glycero-3-phos-pho¬ethanolamine (DOPE) (70/30 mol %) membranes. We further evaluated the effect of the same peptide on the mechanism of fusion for membranes containing 30 mol % PE and a fatty acid with an intrinsic positive curvature (oleic acid (OA)). Our results show that gp41 switches the fusion mechanism from a nonclassical to a classical stalk model when membranes contain OA, but fails to do so for DOPC/DOPE membranes. This could be due to the extreme influence of the intrinsic negative curvature of PE, which is partially downregulated in the presence of OA.

CXCL12-PLGA/Pluronic Nanoparticle Internalization Abrogates CXCR4-Mediated Cell Migration

Chemokine-induced chemotaxis mediates physiological and pathological immune cell trafficking, as well as several processes involving cell migration. Among them, the role of CXCL12/CXCR4 signaling in cancer and metastasis is well known, and CXCR4 has been often targeted with small molecule-antagonists or short CXCL12-derived peptides to limit the pathological processes of cell migration and invasion. To reduce CXCR4-mediated chemotaxis, we adopted a different approach. We manufactured poly(lactic acid-co-glycolic acid) (PLGA)/Pluronic F127 nanoparticles through microfluidics-assisted nanoprecipitation and functionalized them with streptavidin to docking a biotinylated CXCL12 to be exposed on the nanoparticle surface. Our results show that CXCL12-decorated nanoparticles are non-toxic and do not induce inflammatory cytokine release in THP-1 monocytes cultured in fetal bovine and human serum-supplemented media. The cell internalization of our chemokine receptor-targeting particles increases in accordance with CXCR4 expression in FBS/medium. We demonstrated that CXCL12-decorated nanoparticles do not induce cell migration on their own, but their pre-incubation with THP-1 significantly decreases CXCR4⁺-cell migration, thereby antagonizing the chemotactic action of CXCL12. The use of biodegradable and immune-compatible chemokine-mimetic nanoparticles to reduce cell migration opens the way to novel antagonists with potential application in cancer treatments and inflammation.

Genetic variations in the host dependency factors ALCAM and TPST2 impact HIV-1 disease progression

OBJECTIVES

Recently, the activated leukocyte cell adhesion molecule (ALCAM) and tyrosylprotein sulfotransferase 2 (TPST2) have been identified as important host dependency factors (HDFs) for in-vitro HIV-1 replication. To determine whether these genes play a role in HIV-1 pathogenesis, we analysed whether naturally occurring genetic variations were associated with the clinical course of infection.

DESIGN/METHODS

Single nucleotide polymorphisms (SNPs) in ALCAM and TPST2 were analysed in a cohort of 304 HIV-1-infected men who have sex with men and survival analysis was used to determine their effect on the outcome of untreated HIV-1 infection. Flowcytometry was used to determine the effect of SNPs on CD4 T-cell activation prior to HIV-1 infection and 1 and 5 years after infection. In-vitro HIV-1 infections were performed to analyse the effect of the SNPs on HIV-1 replication.

RESULTS

We observed that the minor allele of rs1344861 in ALCAM was associated with accelerated disease progression, whereas the minor allele of rs9613199 in TPST2 was associated with delayed disease progression. In-vitro infection assays did not demonstrate any differences in HIV-1 replication associated with rs9613199. However, the increase in CD4 T-cell immune activation levels during HIV-1 infection was less pronounced in infected individuals homozygous for rs9613199, which is in agreement with delayed disease progression.

CONCLUSION

Our data demonstrate that ALCAM and TPST2 play a role in HIV-1 pathogenesis. SNPs in these genes, without known functional implications, had a major effect on disease progression, and therefore, these HDFs may be attractive and effective targets for new treatment strategies.

HIV long-term non-progressors share similar features with simian immunodeficiency virus infection of chimpanzees

HIV-1 infection in human beings has been an outcome of cross-species transmission event of simian immunodeficiency virus from chimpanzees (SIVcpz). Present study reveals differential features of envelope genes representing different categories of HIV-1 disease progression in human beings, namely, rapid progressors (RP), slow progressors (SP) and long-term non-progressors (LTNP) with respect to SIVcpz, based on their amino acid usage patterns. It was evident that SP, LTNP and SIVcpz envelope genes displayed similar patterns of amino acid usage which strongly contrasted with the features exhibited by the envelope genes representing RP category. Robust analysis revealed that selection constraint of human host on SP and LTNP associated envelope genes and chimpanzee host on SIVcpz envelope genes were more severe compared to selection pressure operational on RP associated envelope genes. Evolutionary forces of selection appeared to be comparatively more relaxed on the RP envelope genes in contrast to SP, LTNP and SIVcpz types. Better binding of RP envelope glycoprotein 120 (gp120) compared to envelope gp120 representing SP, LTNP and SIVcpz with host cellular receptor CD4, as inferred employing molecular docking approaches, promises to confer meaningful insights into the event of speedy progression of HIV in rapid progressors. It was interesting to note that envelope glycoprotein exhibited a tendency of hindering proper interaction of host (human/chimpanzee) CD4 and major histocompatibility complex II (MHC II), with a better efficacy in rapid progressors, thus, facilitating highest degrees of immune suppression. Proper identification of the contrasting features might confer a scope to modulate rapid progression of HIV to a long-term non-progressive controlled case, as observed in LTNP and SIVcpz infection, simultaneously aiding therapeutic research against AIDS targeted at drug and vaccine development.Communicated by Ramaswamy H. Sarma.

Delivery of gene therapy to resting immune cells for an HIV cure

PURPOSE OF REVIEW

The utilization of genetically modified T cells to therapeutically target to various previously incurable diseases such, as cancer, has expanded exponentially in recent years. This success now provides the motivating force in applying the same technology for incurable infectious diseases including HIV. The common bottleneck in gene therapy continues to be at the level of gene delivery. Although present approaches adapt the cell to the delivery technology, emerging techniques now focus on leaving cells in their phenotypically resting state. In doing so, engraftment and proliferation potential are retained and in turn increase the efficacy of this approach at a lowered cost. This review will outline the main efforts of gene delivery using viral vectors or nonviral vectors and challenges moving forward not only in resting T cells, but also in other resting immune cells including hematopoietic stem cells.

RECENT FINDINGS

In focusing on HIV cure efforts using gene therapy, progress on solving the challenges of gene delivery will be described for both viral and nonviral vectors. Advances in the basic virology of lentiviruses have led to the proposal of many next generation lentiviral vector platforms for resting immune cells. Moreover, we will also highlight the progress made in nonviral approaches using nanotechnology as alternatives and/or synergistic technologies to be used alongside lentiviral platforms.

SUMMARY

The innovative approaches described in these recent studies, particularly those using the natural mechanisms employed by HIV to enhance for example virus entry or virus latency, will enable future optimization of gene delivery platforms and therapeutics, which will importantly, provide a pathway toward translation into clinical practice.

Cholesterol Modulates Membrane Properties and the Interaction of gp41 Fusion Peptide To Promote Membrane Fusion

An envelope glycoprotein, gp41, is crucial for the entry of human immunodeficiency virus (HIV) into the host cell. The 20-23 N-terminal amino acid sequence of gp41 plays an important role in promoting fusion between viral and host cells. Interestingly, the structure and function of the fusion peptide are extremely sensitive to the characteristics of the lipid environment. In this present work, we have extensively utilized steady-state and time-resolved fluorescence spectroscopy in tandem with molecular dynamics simulation to elucidate peptide binding and peptide-induced perturbation to the membrane. We have used two depth-dependent fluorescence probes, 1,6-diphenyl-1,3,5-hexatriene (DPH) and its trimethylammonium derivative (TMA-DPH), to monitor the effect of peptide binding along the bilayer normal and have reconciled the experimental observation with the insights from the simulated molecular events. We have further monitored the effect of membrane cholesterol on peptide-induced membrane perturbation. The molecular dynamics simulation data show that the peptide alters the membrane properties in the vicinity of the peptide and it penetrates to a larger extent into the bilayer when the membrane contains cholesterol. Our results clearly elucidate that cholesterol alters the membrane physical properties in favor of membrane fusion and interaction pattern of the fusion peptide with the membrane in a concentration-dependent fashion. The role of cholesterol is specifically important as the host eukaryotic cells contain a decent amount of cholesterol that might be critical for the entry of HIV into the host cells.

Molecular Mechanism of HIV-1 Entry

HIV-1 envelope glycoprotein [Env; trimeric (gp160)₃ cleaved to (gp120/gp41)₃] attaches the virion to a susceptible cell and induces fusion of viral and cell membranes to initiate infection. It interacts with the primary receptor CD4 and coreceptor (e.g., chemokine receptor CCR5 or CXCR4) to allow viral entry by triggering large structural rearrangements and unleashing the fusogenic potential of gp41 to induce membrane fusion. Recent advances in structural biology of HIV-1 Env and its complexes with the cellular receptors have revealed molecular details of HIV-1 entry and yielded new mechanistic insights. In this review, I summarize our latest understanding of the HIV-1 membrane fusion process and discuss possible pathways for productive viral entry.

Functional mimetic of the G-protein coupled receptor CXCR4 on a soluble antibody scaffold

G-protein coupled receptors (GPCRs) constitute major drug targets due to their involvement in critical biological functions and pathophysiological disorders. The leading challenge in their structural and functional characterization has been the need for a lipid environment to accommodate their hydrophobic cores. Here, we report an antibody scaffold mimetic (ASM) platform where we have recapitulated the extracellular functional domains of the GPCR, C-X-C chemokine receptor 4 (CXCR4) on a soluble antibody framework. The engineered ASM molecule can accommodate the N-terminal loop and all three extracellular loops of CXCR4. These extracellular features are important players in ligand recruitment and interaction for allostery and signal transduction. Our study shows that ASM^CXCR4 can be recognized by the anti-CXCR4 antibodies, MEDI3185, 2B11, and 12G5, and that ASM^CXCR4 can bind the HIV-1 glycoprotein ligand gp120, and the natural chemokine ligand SDF-1α. Further, we show that ASM^CXCR4 can competitively inhibit the SDF-1α signaling pathway, and be used as an immunogen to generate CXCR4-specific antibodies. This platform will be useful in the study of GPCR biology in a soluble receptor context for evaluating its extracellular ligand interactions.

Effects of HIV-1 gp120 and TAT-derived microvesicles on endothelial cell function

The aims of this study were twofold. The first was to determine if human immunodeficiency virus (HIV)-1 glycoprotein (gp) 120 and transactivator of transcription (Tat) stimulate the release of endothelial microvesicles (EMVs). The second was to determine whether viral protein-induced EMVs are deleterious to endothelial cell function (inducing endothelial cell inflammation, oxidative stress, senescence and increasing apoptotic susceptibility). Human aortic endothelial cells (HAECs) were treated with recombinant HIV-1 proteins Bal gp120 (R5), Lav gp120 (X4), or Tat. EMVs released in response to each viral protein were isolated and quantified. Fresh HAECs were treated with EMVs generated under control conditions and from each of the viral protein conditions for 24 h. EMV release was higher (P < 0.05) in HAECs treated with R5 (141 ± 21 MV/µl)_, X4 (132 ± 20 MV/µl), and Tat (130 ± 20 MV/µl) compared with control (61 ± 13 MV/µl). Viral protein EMVs induced significantly higher endothelial cell release of proinflammatory cytokines and expression of cell adhesion molecules than control. Reactive oxygen species production was more pronounced (P < 0.05) in the R5-, X4- and Tat-EMV-treated cells. In addition, viral protein-stimulated EMVs significantly augmented endothelial cell senescence and apoptotic susceptibility. Concomitant with these functional changes, viral protein-stimulated EMVs disrupted cell expression of micro-RNAs 34a, 126, 146a, 181b, 221, and miR-Let-7a (P < 0.05). These results demonstrate that HIV-1 gp120 and Tat stimulate microvesicle release from endothelial cells, and these microvesicles confer pathological effects on endothelial cells by inducing inflammation, oxidative stress, and senescence as well as enhancing susceptibility to apoptosis. Viral protein-generated EMVs may contribute to the increased risk of vascular disease in patients with HIV-1. NEW & NOTEWORTHY Human immunodeficiency virus (HIV)-1-related proteins glycoprotein (gp) 120 and transactivator of transcription (Tat)-mediated endothelial damage and dysfunction are poorly understood. Endothelial microvesicles (EMVs) serve as indicators and potent mediators of endothelial dysfunction. In the present study we determined if HIV-1 R5- and X4-tropic gp120 and Tat stimulate EMV release in vitro and if viral protein-induced EMVs are deleterious to endothelial cell function. gp120 and Tat induced a marked increase in EMV release. Viral protein-induced EMVs significantly increased endothelial cell inflammation, oxidative stress, senescence, and apoptotic susceptibility in vitro. gp120- and Tat-derived EMVs promote a proinflammatory, pro-oxidative, prosenescent, and proapoptotic endothelial phenotype and may contribute to the endothelial damage and dysfunction associated with gp120 and Tat.

Structural basis of coreceptor recognition by HIV-1 envelope spike

HIV-1 envelope glycoprotein (Env), which consists of trimeric (gp160)₃ cleaved to (gp120 and gp41)₃, interacts with the primary receptor CD4 and a coreceptor (such as chemokine receptor CCR5) to fuse viral and target-cell membranes. The gp120-coreceptor interaction has previously been proposed as the most crucial trigger for unleashing the fusogenic potential of gp41. Here we report a cryo-electron microscopy structure of a full-length gp120 in complex with soluble CD4 and unmodified human CCR5, at 3.9 Å resolution. The V3 loop of gp120 inserts into the chemokine-binding pocket formed by seven transmembrane helices of CCR5, and the N terminus of CCR5 contacts the CD4-induced bridging sheet of gp120. CCR5 induces no obvious allosteric changes in gp120 that can propagate to gp41; it does bring the Env trimer close to the target membrane. The N terminus of gp120, which is gripped by gp41 in the pre-fusion or CD4-bound Env, flips back in the CCR5-bound conformation and may irreversibly destabilize gp41 to initiate fusion. The coreceptor probably functions by stabilizing and anchoring the CD4-induced conformation of Env near the cell membrane. These results advance our understanding of HIV-1 entry into host cells and may guide the development of vaccines and therapeutic agents.

Convective forces increase CXCR4-dependent glioblastoma cell invasion in GL261 murine model

Glioblastoma is the most common and malignant form of brain cancer. Its invasive nature limits treatment efficacy and promotes inevitable recurrence. Previous in vitro studies showed that interstitial fluid flow, a factor characteristically increased in cancer, increases glioma cell invasion through CXCR4-CXCL12 signaling. It is currently unknown if these effects translate in vivo. We used the therapeutic technique of convection enhanced delivery (CED) to test if convective flow alters glioma invasion in a syngeneic GL261 mouse model of glioblastoma. The GL261 cell line was flow responsive in vitro, dependent upon CXCR4 and CXCL12. Additionally, transplanting GL261 intracranially increased the populations of CXCR4+ and double positive cells versus 3D culture. We showed that inducing convective flow within implanted tumors indeed increased invasion over untreated controls, and administering the CXCR4 antagonist AMD3100 (5 mg/kg) effectively eliminated this response. These data confirm that glioma invasion is stimulated by convective flow in vivo and depends on CXCR4 signaling. We also showed that expression of CXCR4 and CXCL12 is increased in patients having received standard therapy, when CED might be elected. Hence, targeting flow-stimulated invasion may prove beneficial as a second line of therapy, particularly in patients chosen to receive treatment by convection enhanced delivery.

Antiviral Activity of HIV gp120-Targeting Bispecific T Cell Engager Antibody Constructs

Today's gold standard in HIV therapy is combined antiretroviral therapy (cART). It requires strict adherence by patients and lifelong medication, which can lower the viral load below detection limits and prevent HIV-associated immunodeficiency but cannot cure patients. The bispecific T cell-engaging (BiTE) antibody technology has demonstrated long-term relapse-free outcomes in patients with relapsed and refractory acute lymphocytic leukemia. Here, we generated BiTE antibody constructs that target the HIV-1 envelope protein gp120 (HIV gp120) using either the scFv B12 or VRC01, the first two extracellular domains (1 + 2) of human CD4 alone or joined to the single chain variable fragment (scFv) of the antibody 17b fused to an anti-human CD3ε scFv. These engineered human BiTE antibody constructs showed engagement of T cells for redirected lysis of HIV gp120-transfected CHO cells. Furthermore, they substantially inhibited HIV-1 replication in peripheral blood mononuclear cells (PBMCs) as well as in macrophages cocultured with autologous CD8⁺ T cells, the most potent being the human CD4(1 + 2) BiTE [termed CD(1 + 2) h BiTE] antibody construct and the CD4(1 + 2)L17b BiTE antibody construct. The CD4(1 + 2) h BiTE antibody construct promoted HIV infection of human CD4^-/CD8⁺ T cells. In contrast, the neutralizing B12 and the VRC01 BiTE antibody constructs, as well as the CD4(1 + 2)L17b BiTE antibody construct, did not. Thus, BiTE antibody constructs targeting HIV gp120 are very promising for constraining HIV and warrant further development as novel antiviral therapy with curative potential.IMPORTANCE HIV is a chronic infection well controlled with the current cART. However, we lack a cure for HIV, and the HIV pandemic goes on. Here, we showed in vitro and ex vivo that a BiTE antibody construct targeting HIV gp120 resulted in substantially reduced HIV replication. In addition, these BiTE antibody constructs display efficient killing of gp120-expressing cells and inhibited replication in ex vivo HIV-infected PBMCs or macrophages. We believe that BiTE antibody constructs recognizing HIV gp120 could be a very valuable strategy for a cure of HIV in combination with cART and compounds which reverse latency.

A genome-wide CRISPR screen identifies a restricted set of HIV host dependency factors

Host proteins are essential for HIV entry and replication and can be important nonviral therapeutic targets. Large-scale RNA interference (RNAi)-based screens have identified nearly a thousand candidate host factors, but there is little agreement among studies and few factors have been validated. Here we demonstrate that a genome-wide CRISPR-based screen identifies host factors in a physiologically relevant cell system. We identify five factors, including the HIV co-receptors CD4 and CCR5, that are required for HIV infection yet are dispensable for cellular proliferation and viability. Tyrosylprotein sulfotransferase 2 (TPST2) and solute carrier family 35 member B2 (SLC35B2) function in a common pathway to sulfate CCR5 on extracellular tyrosine residues, facilitating CCR5 recognition by the HIV envelope. Activated leukocyte cell adhesion molecule (ALCAM) mediates cell aggregation, which is required for cell-to-cell HIV transmission. We validated these pathways in primary human CD4⁺ T cells through Cas9-mediated knockout and antibody blockade. Our findings indicate that HIV infection and replication rely on a limited set of host-dispensable genes and suggest that these pathways can be studied for therapeutic intervention.

[Comparative analysis of genetic variants of the HIV-1 circulating in the Irkutsk region in 1999 and 2012]

The pol and env genome regions of the HIV-1 genetic variants circulating in the irkutsk region of russia in 1999 and 2012 were compared. The results of this work showed the dominance of the HIV-1 subtype a IDU-A genetic variant (100%) in this region. No primary resistance mutations in the pol gene in the treatment-naive patients were found. The heterogeneity of the viral population was found to be significantly increased based on the pol and env analysis among HIV-variants isolated in 2012 (12.88% and 2.16%) from the intravenous drug users as compared to HIV-variants that caused the outbreak of the HIV infection in 1999 (1.64% and 0.47%). In addition, the comparison of genetic distances of the pol and env gene sequences in the viruses isolated in 2012 from the HIV-positive persons infected through heterosexual intercourse and intravenous drug use demonstrated that the transmission route influenced the variability of the virus population. Among the viruses of IDU-A variant circulating in the area in 2012 the prevalence of X4-tropic variants was 24.7%.

Replacement of the V3 domain in the surface subunit of the feline immunodeficiency virus envelope glycoprotein with the equivalent region of a T cell-tropic human immunodeficiency virus type 1 results in a chimeric surface protein that efficiently binds to CXCR4

Feline immunodeficiency virus (FIV) and the T cell-tropic strains of human immunodeficiency virus type 1 (HIV-1) share the use of the chemokine receptor CXCR4 for cell entry. To study this process further we developed a cell surface binding assay based on the expression of a soluble version of the FIV SU C-terminally tagged with the influenza virus hemagglutinin epitope (HA). The specificity of the assay was demonstrated by the following evidence: (1) the SU-HA protein bound to HeLa cells that express CXCR4 but not to MDCK cells that lack this chemokine receptor; and (2) binding of the SU-HA to HeLa cells was blocked by incubation with the CXCR4 antagonist AMD3100 as well as with the anti-CXCR4 monoclonal antibody (MAb) 12G5. Deletion of the V3 region from the FIV SU glycoprotein abolished its ability to bind CXCR4-expressing cells. Remarkably, substitution of the V3 domain of the FIV SU by the equivalent region of the HIV-1 NL4-3 isolate resulted in efficient cell surface binding of the chimeric SU protein to CXCR4. Moreover, transfection of MDCK cells with a plasmid encoding human CXCR4 allowed the association of the chimeric SU-HA glycoprotein to the transfected cells. Interestingly, while cell binding of the chimeric FIV-HIV SU was inhibited by an anti-HIV-1 V3 MAb, its association with CXCR4 was found to be resistant to AMD3100. Of note, the chimeric FIV-HIV Env glycoprotein was capable of promoting CXCR4-dependent cell-to-cell fusion.

The transmembrane domain and acidic lipid flip-flop regulates voltage-dependent fusion mediated by class II and III viral proteins

Voltage dependence of fusion induced by class II and class III viral fusion proteins was investigated. Class II proteins from Ross River and Sindbus virus and a mutant class III protein from Epstein Barr virus were found to induce cell-cell fusion that is voltage dependent. Combined with previous studies, in all, four class II and two class III protein have now been shown to exhibit voltage-dependent fusion, demonstrating that this is probably a general phenomenon for these two classes of viral fusion proteins. In the present study, monitoring fusion of pseudovirus expressing Vesicular Stomatitis virus (VSV) G within endosomes shows that here, too, fusion is voltage dependent. This supports the claim that voltage dependence of fusion is biologically relevant and that cell-cell fusion reliably models the voltage dependence. Fusion induced by class I viral proteins is independent of voltage; chimeras expressing the ectodomain of a class I fusion protein and the transmembrane domain of VSV G could therefore be used to explore the location within the protein responsible for voltage dependence. Results showed that the transmembrane domain is the region associated with voltage dependence. Experiments in which cells were enriched with acidic lipids led to the conclusion that it is the flip-flop of acidic lipids that carries the charge responsible for the observed voltage dependence of fusion. This flip-flop occurred downstream of hemifusion, in accord with previous findings that the voltage dependent steps of fusion occur at a stage subsequent to hemifusion.

Mimicking Protein-Protein Interactions through Peptide-Peptide Interactions: HIV-1 gp120 and CXCR4

We have recently designed a soluble synthetic peptide that functionally mimics the HIV-1 coreceptor CXCR4, which is a chemokine receptor that belongs to the family of seven-transmembrane GPCRs. This CXCR4 mimetic peptide, termed CX4-M1, presents the three extracellular loops (ECLs) of the receptor. In binding assays involving recombinant proteins, as well as in cellular infection assays, CX4-M1 was found to selectively recognize gp120 from HIV-1 strains that use CXCR4 for cell entry (X4 tropic HIV-1). Furthermore, anti-HIV-1 antibodies modulate this interaction in a molecular mechanism related to that of their impact on the gp120-CXCR4 interaction. We could now show that the selectivity of CX4-M1 pertains not only to gp120 from X4 tropic HIV-1, but also to synthetic peptides presenting the V3 loops of these gp120 proteins. The V3 loop is thought to be an essential part of the coreceptor binding site of gp120 that contacts the second ECL of the coreceptor. We were able to experimentally confirm this notion in binding assays using substitution analogs of CX4-M1 and the V3 loop peptides, respectively, as well as in cellular infection assays. These results indicate that interactions of the HIV-1 Env with coreceptors can be mimicked by synthetic peptides, which may be useful to explore these interactions at the molecular level in more detail.

Entry inhibitors and their use in the treatment of HIV-1 infection

Entry of HIV into target cells is a complex, multi-stage process involving sequential attachment and CD4 binding, coreceptor binding, and membrane fusion. HIV entry inhibitors are a complex group of drugs with multiple mechanisms of action depending on the stage of the viral entry process they target. Two entry inhibitors are currently approved for the treatment of HIV-infected patients. Maraviroc, a CCR5 antagonist, blocks interactions between the viral envelope proteins and the CCR5 coreceptor. Enfuvirtide, a fusion inhibitor, disrupts conformational changes in gp41 that drive membrane fusion. A wide array of additional agents are in various stages of development. This review covers the entry inhibitors and their use in the treatment of HIV-infected patients.

Cytotoxic effect of HIV-1 gp120 on primary cultured human retinal capillary endothelial cells

PURPOSE

This research was conducted to make a primary culture of human retinal capillary endothelial cells (HRCEC) and to study the cytotoxic effect of human immunodeficiency virus-1 (envelope) glycoprotein 120 (HIV-1 gp120) on cultured HRCEC.

METHODS

HRCEC were isolated and primarily cultured as dissociated single cell cultures. Immunohistochemistry and immunofluorescence were used to identify specific markers of HRCEC and to reveal HIV-1 gp120 related receptors (cluster of differentiation 4 [CD4], C-X-C chemokine receptor type 4 [CXCR4], and C-C chemokine receptor type 5 [CCR5]). The 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) assay was used to demonstrate the effect of HIV-1 gp120 on cell viability at seven different concentrations (0.01-0.15 mg/l) for 24 h or at a fixed concentration of 0.08 mg/l for varying time intervals (4-72 h). After 0.08, 0.1, 0.12, and 0.15 mg/l HIV-1 gp120 were applied to HRCEC for 24 h, cell apoptotic rates and the mitochondrial membrane potential were measured with flow cytometry; pro-caspase-9 and cleaved caspase-9 were evaluated with immunoblotting. Under each research condition, 0.15 mg/l of HIV-1 gp120 mutated proteins (423 I/P) were used as controls.

RESULTS

Primary cultures of pure HRCEC were established, and the cells were characterized with their specific markers. HIV-1 gp120 receptors CXCR4 and CCR5 were found on the cell surface of HRCEC; however, CD4 was negative. Treatment of HRCEC with HIV-1 gp120 at concentrations <0.08 mg/l did not influence cell viability. However, a concentration- and time-dependent increase of HIV-1 gp120-induced cell inhibition was demonstrated with MTT, when the concentration of HIV-1 gp120 was more than 0.08 mg/l (r=-0.763, p<0.01). With increasing concentrations of HIV-1 gp120, the numbers of apoptotic cells and expression of cleaved caspase-9 protein increased, but Rho123 staining mitochondrial membrane potential decreased.

CONCLUSIONS

HIV-1 gp120 assistant receptors CXCR4 and CCR5 are expressed on the cell surface of HRCEC, and HIV-1 gp120 can inhibit cell viability and induce apoptosis of HRCEC. The mitochondrial pathway is probably involved in HIV-1 gp120-induced apoptosis of HRCEC, but the specific mechanisms remain to be uncovered.

Effects of stabilization of the gp41 cytoplasmic domain on fusion activity and infectivity of SIVmac239

We investigated the effects of introducing specific sequences that are predicted to affect trimer stability into the CT domain of the SIV Env protein. Two constructs, 3HBai and 3HBaa, with additional GCN4-related sequences in the CT domain (45 aa) had enhanced infectivity, and differed in their fusion activity and trimer stability. Another construct, 3HBii, exhibited a very stable trimeric structure. Pseudotyped virions containing 3HBii retained infectivity despite the lack of syncytia formation. In contrast, 3HBai and 3HBaa, which caused extensive syncytia formation, had a less stable trimeric structure. We observed an inverse correlation between trimer stability and fusion activity but no correlation between syncytia formation activity and infectivity. Quantitative cell-cell fusion assays, analysis of Env incorporation, measurement of ectodomain conformation by CD4 binding, and CCR5 blocking assays indicated differential effects on fusion activity and infectivity of the viruses with Env CT modifications. Differences in interaction with CD4 were not affected by trimer stability and were not related to fusion activity or infectivity. The results indicate that changes in the stability of the CT domain can have significant effects on functional activities of the Env external domain and can impact viral biological properties.

Entry inhibitors in the treatment of HIV-1 infection

Infection of target cells by HIV is a complex, multi-stage process involving attachment to host cells and CD4 binding, coreceptor binding, and membrane fusion. Drugs that block HIV entry are collectively known as entry inhibitors, but comprise a complex group of drugs with multiple mechanisms of action depending on the stage of the entry process at which they act. Two entry inhibitors, maraviroc and enfuvirtide, have been approved for the treatment of HIV-1 infection, and a number of agents are in development. This review covers the entry inhibitors and their use in the management of HIV-1 infection. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol 85, issue 1, 2010.

The role of the N-terminal segment of CCR5 in HIV-1 Env-mediated membrane fusion and the mechanism of virus adaptation to CCR5 lacking this segment

Background

HIV-1 envelope glycoprotein (Env) induces membrane fusion as a result of sequential binding to CD4 and chemokine receptors (CCR5 or CXCR4). The critical determinants of CCR5 coreceptor function are the N-terminal domain (Nt) and the second extracellular loop. However, mutations in gp120 adapt HIV-1 to grow on cells expressing the N-terminally truncated CCR5(Δ18) (Platt et al., J. Virol. 2005, 79: 4357–68).

Results

We have functionally characterized the adapted Env (designated Env(NYP)) using a quantitative cell-cell fusion assay. The rate of fusion with target cells expressing wild-type CCR5 and the resistance to fusion inhibitors was virtually identical for wild-type Env and Env(NYP), implying that the coreceptor affinity had not increased as a result of adaptation. In contrast, Env(NYP)-induced fusion with cells expressing CCR5(Δ18) occurred at a slower rate and was extremely sensitive to the CCR5 binding inhibitor, Sch-C. Resistance to Sch-C drastically increased after pre-incubation of Env(NYP)- and CCR5(Δ18)-expressing cells at a temperature that was not permissive to fusion. This indicates that ternary Env(NYP)-CD4-CCR5(Δ18) complexes accumulate at sub-threshold temperature and that low-affinity interactions with the truncated coreceptor are sufficient for triggering conformational changes in the gp41 of Env(NYP) but not in wild-type Env. We also demonstrated that the ability of CCR5(Δ18) to support fusion and infection mediated by wild-type Env can be partially reconstituted in the presence of a synthetic sulfated peptide corresponding to the CCR5 Nt. Pre-incubation of wild-type Env- and CCR5(Δ18)-expressing cells with the sulfated peptide at sub-threshold temperature markedly increased the efficiency of fusion.

Conclusion

We propose that, upon binding the Nt region of CCR5, wild-type Env acquires the ability to productively engage the extracellular loop(s) of CCR5 – an event that triggers gp41 refolding and membrane merger. The adaptive mutations in Env(NYP) enable it to more readily release its hold on gp41, even when it interacts weakly with a severely damaged coreceptor in the absence of the sulfopeptide.

Replication-competent variants of human immunodeficiency virus type 2 lacking the V3 loop exhibit resistance to chemokine receptor antagonists

Entry of human immunodeficiency virus type 1 (HIV-1) and HIV-2 requires interactions between the envelope glycoprotein (Env) on the virus and CD4 and a chemokine receptor, either CCR5 or CXCR4, on the cell surface. The V3 loop of the HIV gp120 glycoprotein plays a critical role in this process, determining tropism for CCR5- or CXCR4-expressing cells, but details of how V3 interacts with these receptors have not been defined. Using an iterative process of deletion mutagenesis and in vitro adaptation of infectious viruses, variants of HIV-2 were derived that could replicate without V3, either with or without a deletion of the V1/V2 variable loops. The generation of these functional but markedly minimized Envs required adaptive changes on the gp120 core and gp41 transmembrane glycoprotein. V3-deleted Envs exhibited tropism for both CCR5- and CXCR4-expressing cells, suggesting that domains on the gp120 core were mediating interactions with determinants shared by both coreceptors. Remarkably, HIV-2 Envs with V3 deletions became resistant to small-molecule inhibitors of CCR5 and CXCR4, suggesting that these drugs inhibit wild-type viruses by disrupting a specific V3 interaction with the coreceptor. This study represents a proof of concept that HIV Envs lacking V3 alone or in combination with V1/V2 that retain functional domains required for viral entry can be derived. Such minimized Envs may be useful in understanding Env function, screening for new inhibitors of gp120 core interactions with chemokine receptors, and designing novel immunogens for vaccines.

Prediction of HIV-1 entry inhibitors neomycin-arginine conjugates interaction with the CD4-gp120 binding site by molecular modeling and multistep docking procedure

Developing of multi-target HIV-1 entry inhibitors represents an important avenue of drug therapy. Two such inhibitors are hexa-arginine-neomycin-conjugate (NeoR6) and nona-d-arginine-neomycin-conjugate (Neo-r9). Our findings that NeoR6-resistant mutations appear in the gp120 constant regions; and NeoR6 is not CCR5 antagonist, but inhibits CXCR4 and CCR5 HIV-1 using isolates, led us to suggest that NeoR6 may inhibit HIV-1 entry by interfering with the CD4-gp120 binding. To support this notion, we constructed a homology model of unliganded HIV-1(IIIB) gp120 and docked NeoR6 and Neo-r9 to it, using a multistep docking procedure: geometric-electrostatic docking by MolFit; flexible ligand docking by Autodock3 and final refinement of the obtained complexes by Discover3. Binding free energies were calculated by MM-PBSA methodology. The model predicts competitive inhibition of CD4-gp120 binding by NeoR6 and Neo-r9. We determined plausible binding sites between constructed CD4-bound gp120 trimer and homology modeled membranal CXCR4, and tested NeoR6 and Neo-r9 interfering with this interaction. These models support our notion that another mechanism of anti-HIV-1 activity of NeoR6 is inhibition of gp120-CXCR4 binding. These structural models and interaction of NeoR6 and Neo-r9 with gp120 and CXCR4 provide a powerful approach for structural based drug design for selective targeting of HIV-1 entry and/or for inhibition of other retroviruses with similar mechanism of entry.

Structure and function of the HIV envelope glycoprotein as entry mediator, vaccine immunogen, and target for inhibitors

This chapter discusses the advances of the envelope glycoprotein (Env) structure as related to the interactions of conserved Env structures with receptor molecules and antibodies with implications for the design of vaccine immunogens and inhibitors. The human immunodeficiency virus (HIV) Env binds to cell surface–associated receptor (CD4) and coreceptor (CCR5 or CXCR4) by one of its two non-covalently associated subunits, gp120. The induced conformational changes activate the other subunit (gp41), which causes the fusion of the viral with the plasma cell membranes resulting in the delivery of the viral genome into the cell and the initiation of the infection cycle. As the only HIV protein exposed to the environment, the Env is also a major immunogen to which neutralizing antibodies are directed and a target that is relatively easy to access by inhibitors. A fundamental problem in the development of effective vaccines and inhibitors against HIV is the rapid generation of alterations at high levels of expression during long chronic infection and the resulting significant heterogeneity of the Env. The preservation of the Env function as an entry mediator and limitations on size and expression impose restrictions on its variability and lead to the existence of conserved structures.

Kinetic studies of HIV-1 and HIV-2 envelope glycoprotein-mediated fusion

Background

HIV envelope glycoprotein (Env)-mediated fusion is driven by the concerted coalescence of the HIV gp41 N-helical and C-helical regions, which results in the formation of 6 helix bundles. Kinetics of HIV Env-mediated fusion is an important determinant of sensitivity to entry inhibitors and antibodies. However, the parameters that govern the HIV Env fusion cascade have yet to be fully elucidated. We address this issue by comparing the kinetics HIV-1_IIIB Env with those mediated by HIV-2 from two strains with different affinities for CD4 and CXCR4.

Results

HIV-1 and HIV-2 Env-mediated cell fusion occurred with half times of about 60 and 30 min, respectively. Binding experiments of soluble HIV gp120 proteins to CD4 and co-receptor did not correlate with the differences in kinetics of fusion mediated by the three different HIV Envs. However, escape from inhibition by reagents that block gp120-CD4 binding, CD4-induced CXCR4 binding and 6-helix bundle formation, respectively, indicated large difference between HIV-1 and HIV-2 envelope glycoproteins in their CD4-induced rates of engagement with CXCR4.

Conclusion

The HIV-2 Env proteins studied here exhibited a significantly reduced window of time between the engagement of gp120 with CD4 and exposure of the CXCR4 binding site on gp120 as compared with HIV-1_IIIB Env. The efficiency with which HIV-2 Env undergoes this CD4-induced conformational change is the major cause of the relatively rapid rate of HIV-2 Env mediated-fusion.

Simian immunodeficiency virus envelope compartmentalizes in brain regions independent of neuropathology

Simian immunodeficiency virus (SIV) and human immunodeficiency virus (HIV) gp160s obtained from the brain are often genetically distinct from those isolated from other organs, suggesting the presence of brain-specific selective pressures or founder effects that result in the compartmentalization of viral quasi-species. Whereas HIV has also been found to compartmentalize within different regions of the brain, the extent of brain-regional compartmentalization of SIV in rhesus macaques has not been characterized. Furthermore, much is still unknown about whether phenotypic differences exist in envelopes from different brain regions. To address these questions, env DNA sequences were amplified from four SIVmac239-infected macaques and subjected to phylogenetic and phenetic analysis. The authors demonstrated that sequences from different areas of the brain form distinct clades, and that the long-term progressing macaques demonstrated a greater degree of regional compartmentalization compared to the rapidly progressing macaques. In addition, regional compartmentalization occurred regardless of the presence of giant-cell encephalitis. Nucleotide substitution rates at synonymous and nonsynonymous sites (ds:dn rates) indicated that positive selection varied among envelopes from different brain regions. In one macaque, envelopes from some but not all brain regions acquired changes in a conserved CD4-binding motif GGGDPE at amino acids 382 to 387. Furthermore, gp160s with the mutation G383E were able to mediate cell-to-cell fusion in a CD4-independent manner and were more susceptible to fusion inhibition by pooled plasma from infected macaques. Reversion of this mutation by site-directed mutagenesis resulted in reduction of CD4-independence and resistance to fusion inhibition in cell fusion assays. These studies demonstrate that SIV evolution within the brain results in a heterogeneous viral population with different phenotypes among different regions.

Antigenic properties of a transport-competent influenza HA/HIV Env chimeric protein

The transmembrane subunit (gp41) of the HIV Env glycoprotein contains conserved neutralizing epitopes which are not well-exposed in wild-type HIV Env proteins. To enhance the exposure of these epitopes, a chimeric protein, HA/gp41, in which the gp41 of HIV-1 89.6 envelope protein was fused to the C-terminus of the HA1 subunit of the influenza HA protein, was constructed. Characterization of protein expression showed that the HA/gp41 chimeric proteins were expressed on cell surfaces and formed trimeric oligomers, as found in the HIV Env as well as influenza HA proteins. In addition, the HA/gp41 chimeric protein expressed on the cell surface can also be cleaved into 2 subunits by trypsin treatment, similar to the influenza HA. Moreover, the HA/gp41 chimeric protein was found to maintain a pre-fusion conformation. Interestingly, the HA/gp41 chimeric proteins on cell surfaces exhibited increased reactivity to monoclonal antibodies against the HIV Env gp41 subunit compared with the HIV-1 envelope protein, including the two broadly neutralizing monoclonal antibodies 2F5 and 4E10. Immunization of mice with a DNA vaccine expressing the HA/gp41 chimeric protein induced antibodies against the HIV gp41 protein and these antibodies exhibit neutralizing activity against infection by an HIV SF162 pseudovirus. These results demonstrate that the construction of such chimeric proteins can provide enhanced exposure of conserved epitopes in the HIV Env gp41 and may represent a novel vaccine design strategy for inducing broadly neutralizing antibodies against HIV.

A single amino acid change and truncated TM are sufficient for simian immunodeficiency virus to enter cells using CCR5 in a CD4-independent pathway

Entry of HIV and SIV into susceptible cells is mediated by CD4 and chemokine receptors, which act as coreceptors. To study cell entry of SIV, we constructed a cell line, xKLuSIV, derived from non-susceptible human K562 cells, that express the firefly luciferase reporter gene under control of a minimal SIV long terminal repeat (LTR). Using these susceptible cells, we studied the entry of a well-characterized molecularly cloned macrophage-tropic SIV. xKLuSIV cells that express rhesus macaque CD4 and/or the rhesus chemokine receptor CCR5 are susceptible to infection with the macrophage-tropic, neurovirulent strain SIV/17E-Fr, but only xKLuSIV cells expressing both CCR5 and CD4 were susceptible to infection by the macrophage-tropic, non-neurovirulent strain SIV/17E-Cl. CCR5-dependent, CD4-independent infection by SIV/17E-Fr was abrogated by pre-incubation of the cells with AOP-RANTES, a ligand for CCR5. In addition to viral entry occurring by a CD4-independent mechanism, neutralization of SIV/17E-Fr with rhesus mAbs from 3 different neutralization groups blocked entry into x KLuSIV cells by both CD4-dependent and -independent mechanisms. Triggering the env glycoprotein of SIV-17 EFr with soluble CD4 had no significant effect in infectivity, but triggering of the same glycoprotein of SIV/17E-Cl allowed it to enter cells in a CD4-independent fashion. Using mutant molecular clones, we studied the determinants for CD4 independence, all of which are confined to the env gene. We report here that truncation of the TM at amino acid 764 and changing a single amino acid (R751G) in the SIV envelope transmembrane protein (TM) conferred the observed CD4-independent phenotype. Our data suggest that the envelope from the neurovirulent SIV/17E-Fr interacts with CCR5 in a CD4-independent manner, and changes in the TM protein of this virus are important components that contribute to neurovirulence in SIV.

HIV chemokine receptor inhibitors as novel anti-HIV drugs

The chemokine receptors CXCR4 and CCR5 are the main coreceptors used by the T-cell-tropic (CXCR4-using, X4) and macrophage-tropic (CCR5-using, R5) HIV-1 strains, respectively, for entering their CD4+ target cells. In this review, we focus on the function of these chemokine receptors in HIV infection and their role as novel targets for viral inhibition. Besides some modified chemokines with antiviral activity, several low-molecular weight CCR5 and CXCR4 antagonistic compounds have been described with potent antiviral activity. The best CXCR4 antagonists described are the bicyclam derivatives, which consistently block X4 but also R5/X4 viral replication in PBMCs. We believe that chemokine receptor antagonists will become important new antiviral drugs to combat AIDS. Both CXCR4 and CCR5 chemokine receptor inhibitors will be needed in combination and even in combinations of antiviral drugs that also target other aspects of the HIV replication cycle to obtain optimum antiviral therapeutic effects.

V3: HIV's switch-hitter

The third variable region, V3, of the gp120 surface envelope glycoprotein is an approximately 35-residue-long, frequently glycosylated, highly variable, disulfide-bonded structure that has a major influence on HIV-1 tropism. Thus the sequence of V3, directly or indirectly, can determine which coreceptor (CCR5 or CXCR4) is used to trigger the fusion potential of the Env complex, and hence which cells the virus can infect. V3 also influences HIV-1's sensitivity to, and ability to escape from, entry inhibitors that are being developed as antiviral drugs. For some strains, V3 is a prominent target for HIV-1 neutralizing antibodies (NAbs); indeed, for many years it was considered to be the "principal neutralization determinant" (PND). Some efforts to use V3 as a vaccine target continue to this day, despite disappointing progress over more than a decade. Recent findings on the structure, function, antigenicity, and immunogenicity of V3 cast new doubts on the value of this vaccine approach. Here, we review recent advances in the understanding of V3 as a determinant of viral tropism, and discuss how this new knowledge may inform the development of HIV-1 drugs and vaccines.

Disparate regions of envelope protein regulate syncytium formation versus spongiform encephalopathy in neurological disease induced by murine leukemia virus TR

The murine leukemia virus (MLV) TR1.3 provides an excellent model to study the wide range of retrovirus-induced central nervous system (CNS) pathology and disease. TR1.3 rapidly induces thrombotic events in brain microvessels and causes cell-specific syncytium formation of brain capillary endothelial cells (BCEC). A single amino acid substitution, W102G, in the MLV envelope protein (Env) regulates the pathogenic effects. The role of Env in determining this disease phenotype compared to the induction of spongiform encephalomyelitis with a longer latency, as seen in several other MLV and in human retroviruses, was determined by studying in vitro-attenuated TR1.3. Virus cloned from this selection, termed TRM, induced progressive neurological disease characterized by ataxia and paralysis and the appearance of spongiform neurodegeneration throughout the brain stem and spinal cord. This disease was associated with virus replication in both BCEC and highly ramified glial cells. TRM did not induce syncytium formation, either in vivo or in vitro. Sequence and mutational analyses demonstrated that TRM contained a reversion of Env G102W but that neurological disease mapped to the single amino acid substitution Env S159P. The results demonstrate that single nucleotide changes within disparate regions of Env control dramatically different CNS disease patterns.

Impact of mutations in the coreceptor binding site on human immunodeficiency virus type 1 fusion, infection, and entry inhibitor sensitivity

An increasingly large number of antiviral agents that prevent entry of human immunodeficiency virus (HIV) into cells are in preclinical and clinical development. The envelope (Env) protein of HIV is the major viral determinant that affects sensitivity to these compounds. To understand how changes in Env can impact entry inhibitor sensitivity, we introduced six mutations into the conserved coreceptor binding site of the R5 HIV-1 strain YU-2 and measured the effect of these changes on CD4 and coreceptor binding, membrane fusion levels and rates, virus infection, and sensitivity to the fusion inhibitors enfuvirtide (T-20) and T-1249, the CCR5 inhibitor TAK-779, and an antibody to CD4. The mutations had little effect on CD4 binding but reduced CCR5 binding to various extents. In general, reductions in coreceptor binding efficiency resulted in slower fusion kinetics and increased sensitivity to TAK-779 and enfuvirtide. In addition, low CCR5 binding usually reduced overall fusion and infection levels. However, one mutation adjacent to the bridging sheet beta21 strand, P438A, had little effect on fusion activity, fusion rate, infectivity, or sensitivity to enfuvirtide or T-1249 despite causing a marked reduction in CCR5 binding and a significant increase in TAK-779 sensitivity. Thus, our findings indicate that changes in the coreceptor binding site of Env can modulate its fusion activity, infectivity, and entry inhibitor sensitivity by multiple mechanisms and suggest that reductions in coreceptor binding do not always result in prolonged fusion kinetics and increased sensitivity to enfuvirtide.

The N-terminal of the V3 loop in HIV type 1 gp120 is responsible for its conformation-dependent interaction with cell surface molecules

The V3 loop of HIV-1 gp120 plays an important role in the interaction of the viral envelope with cellular coreceptors and/or with other cell surface molecules. To clarify this interaction we used a panel of monoclonal antibodies (MAbs) against V3 loop and synthetic looped V3 peptides V3-BH10, V3-ADA, and V3-89.6, derived from the V3 regions of the BH10 clone of IIIB (X4-tropic), ADA (R5-tropic), and 89.6 (R5X4-tropic), respectively. A linear mutant peptide, V3-BH10/CA, was also synthesized as a control. Biotinylated V3-BH10, -BH10/CA, and-ADA were also made. The binding abilities of the biotinylated and nonbiotinylated peptides to various types of cells were investigated by using flow cytometry. Subsequently, the principal region of the V3 loop involved in cell surface binding was analyzed by using MAbs against the tip (447-52D and 694-98D), N-termini (IIIB-V3-21) or C-termini (IIIB-V3-01) of the V3 loop in flow cytometry and enzyme-linked immunoabsorbent assay. We demonstrate that looped V3 peptides of both X4 and R5X4 HIV (V3-BH10 and V3-89.6) can bind to various types of cells irrespective of their CD4 and/or coreceptor expression in a conformation-dependent manner. In contrast, the V3 loop of R5 HIV (V3-ADA) can scarcely bind to the cells. Using MAbs whose epitopes cover the entire V3 loop we found that MAb IIIB-V3-21 can react with platebound but not cell-bound peptides, and the MAb blocked biotin-V3-BH10 binding suggesting that the N-terminal of the V3 loop interacts directly with cell surface molecule(s).

Differential N-linked glycosylation of human immunodeficiency virus and Ebola virus envelope glycoproteins modulates interactions with DC-SIGN and DC-SIGNR

The C-type lectins DC-SIGN and DC-SIGNR [collectively referred to as DC-SIGN(R)] bind and transmit human immunodeficiency virus (HIV) and simian immunodeficiency virus to T cells via the viral envelope glycoprotein (Env). Other viruses containing heavily glycosylated glycoproteins (GPs) fail to interact with DC-SIGN(R), suggesting some degree of specificity in this interaction. We show here that DC-SIGN(R) selectively interact with HIV Env and Ebola virus GPs containing more high-mannose than complex carbohydrate structures. Modulation of N-glycans on Env or GP through production of viruses in different primary cells or in the presence of the mannosidase I inhibitor deoxymannojirimycin dramatically affected DC-SIGN(R) infectivity enhancement. Further, murine leukemia virus, which typically does not interact efficiently with DC-SIGN(R), could do so when produced in the presence of deoxymannojirimycin. We predict that other viruses containing GPs with a large proportion of high-mannose N-glycans will efficiently interact with DC-SIGN(R), whereas those with solely complex N-glycans will not. Thus, the virus-producing cell type is an important factor in dictating both N-glycan status and virus interactions with DC-SIGN(R), which may impact virus tropism and transmissibility in vivo.