T-tropic human immunodeficiency virus type 1 (HIV-1)-derived V3 loop peptides directly bind to CXCR-4 and inhibit T-tropic HIV-1 infection

Novel small synthetic HIV-1 V3 crown variants: CCR5 targeting ligands

The CC chemokine receptor 5 (CCR5) antagonism represents a promising pharmacological strategy for therapeutic intervention as it plays a significant role in reducing the severity and progression of a wide range of pathological conditions. Here we designed and generated peptide ligands targeting the chemokine receptor, CCR5, that were derived from the critical interaction sites of the V3 crown domain of envelope protein glycoprotein gp120 (TRKSIHIGPGRAFYTTGEI) of HIV-1 using computational biology approach and the peptide sequence corresponding to this region was taken as the template peptide, designated as TMP-1. The peptide variants were synthesized by employing Fmoc chemistry using polymer support and were labelled with rhodamine B to study their interaction with the CCR5 receptor expressed on various cells. TMP-1 and TMP-2 were selected as the high-affinity ligands from in vitro receptor-binding assays. Specific receptor-binding experiments in activated peripheral blood mononuclear cells and HOS.CCR5 cells indicated that TMP-1 and TMP-2 had significant CCR5 specificity. Further, the functional analysis of TMP peptides using chemotactic migration assay showed that both peptides did not mediate the migration of responsive cells. Thus, template TMP-1 and TMP-2 represent promising CCR5 targeting peptide candidates.

Biparatopic Protein Nanoparticles for the Precision Therapy of CXCR4+ Cancers

The accumulated molecular knowledge about human cancer enables the identification of multiple cell surface markers as highly specific therapeutic targets. A proper tumor targeting could significantly avoid drug exposure of healthy cells, minimizing side effects, but it is also expected to increase the therapeutic index. Specifically, colorectal cancer has a particularly poor prognosis in late stages, being drug targeting an appropriate strategy to substantially improve the therapeutic efficacy. In this study, we have explored the potential of the human albumin-derived peptide, EPI-X4, as a suitable ligand to target colorectal cancer via the cell surface protein CXCR4, a chemokine receptor overexpressed in cancer stem cells. To explore the potential use of this ligand, self-assembling protein nanoparticles have been generated displaying an engineered EPI-X4 version, which conferred a modest CXCR4 targeting and fast and high level of cell apoptosis in tumor CXCR4+ cells, in vitro and in vivo. In addition, when EPI-X4-based building blocks are combined with biologically inert polypeptides containing the CXCR4 ligand T22, the resulting biparatopic nanoparticles show a dramatically improved biodistribution in mouse models of CXCR4+ human cancer, faster cell internalization and enhanced target cell death when compared to the version based on a single ligand. The generation of biparatopic materials opens exciting possibilities in oncotherapies based on high precision drug delivery based on the receptor CXCR4.

The CXCL12-CXCR4 Signaling Axis Plays a Key Role in Cancer Metastasis and is a Potential Target for Developing Novel Therapeutics against Metastatic Cancer

Metastasis is the main cause of death in cancer patients; there is currently no effective treatment for cancer metastasis. This is primarily due to our insufficient understanding of the metastatic mechanisms in cancer. An increasing number of studies have shown that the C-X-C motif chemokine Ligand 12 (CXCL12) is overexpressed in various tissues and organs. It is a key niche factor that nurtures the pre-metastatic niches (tumorigenic soil) and recruits tumor cells (oncogenic "seeds") to these niches, thereby fostering cancer cell aggression and metastatic capabilities. However, the C-X-C motif chemokine Receptor 4 (CXCR4) is aberrantly overexpressed in various cancer stem/progenitor cells and functions as a CXCL12 receptor. CXCL12 activates CXCR4 as well as multiple downstream multiple tumorigenic signaling pathways, promoting the expression of various oncogenes. Activation of the CXCL12-CXCR4 signaling axis promotes Epithelial-Mesenchymal Transition (EMT) and mobilization of cancer stem/progenitor cells to pre-metastatic niches. It also nurtures cancer cells with high motility, invasion, and dissemination phenotypes, thereby escalating multiple proximal or distal cancer metastasis; this results in poor patient prognosis. Based on this evidence, recent studies have explored either CXCL12- or CXCR4-targeted anti-cancer therapeutics and have achieved promising results in the preclinical trials. Further exploration of this new strategy and its potent therapeutics effect against metastatic cancer through the targeting of the CXCL12- CXCR4 signaling axis may lead to a novel therapy that can clean up the tumor microenvironment ("soil") and kill the cancer cells, particularly the cancer stem/progenitor cells ("seeds"), in cancer patients. Ultimately, this approach has the potential to effectively treat metastatic cancer.

Gp41-targeted antibodies restore infectivity of a fusion-deficient HIV-1 envelope glycoprotein

The HIV-1 envelope glycoprotein (Env) mediates viral entry via conformational changes associated with binding the cell surface receptor (CD4) and coreceptor (CCR5/CXCR4), resulting in subsequent fusion of the viral and cellular membranes. While the gp120 Env surface subunit has been extensively studied for its role in viral entry and evasion of the host immune response, the gp41 transmembrane glycoprotein and its role in natural infection are less well characterized. Here, we identified a primary HIV-1 Env variant that consistently supports >300% increased viral infectivity in the presence of autologous or heterologous HIV-positive plasma. However, in the absence of HIV-positive plasma, viruses with this Env exhibited reduced infectivity that was not due to decreased CD4 binding. Using Env chimeras and sequence analysis, we mapped this phenotype to a change Q563R, in the gp41 heptad repeat 1 (HR1) region. We demonstrate that Q563R reduces viral infection by disrupting formation of the gp41 six-helix bundle required for virus-cell membrane fusion. Intriguingly, antibodies that bind cluster I epitopes on gp41 overcome this inhibitory effect, restoring infectivity to wild-type levels. We further demonstrate that the Q563R change increases HIV-1 sensitivity to broadly neutralizing antibodies (bNAbs) targeting the gp41 membrane-proximal external region (MPER). In summary, we identify an HIV-1 Env variant with impaired infectivity whose Env functionality is restored through the binding of host antibodies. These data contribute to our understanding of gp41 residues involved in membrane fusion and identify a mechanism by which host factors can alleviate a viral defect.

Development of peptide inhibitors of HIV transmission

Treatment of HIV has long faced the challenge of high mutation rates leading to rapid development of resistance, with ongoing need to develop new methods to effectively fight the infection. Traditionally, early HIV medications were designed to inhibit RNA replication and protein production through small molecular drugs. Peptide based therapeutics are a versatile, promising field in HIV therapy, which continues to develop as we expand our understanding of key protein-protein interactions that occur in HIV replication and infection. This review begins with an introduction to HIV, followed by the biological basis of disease, current clinical management of the disease, therapeutics on the market, and finally potential avenues for improved drug development.

Negatively charged glyconanoparticles modulate and stabilize the secondary structures of a gp120 V3 loop peptide: toward fully synthetic HIV vaccine candidates

The third variable region (V3 peptide) of the HIV-1 gp120 is a major immunogenic domain of HIV-1. Controlling the formation of the immunologically active conformation is a crucial step to the rational design of fully synthetic candidate vaccines. Herein, we present the modulation and stabilization of either the α-helix or β-strand conformation of the V3 peptide by conjugation to negatively charged gold glyconanoparticles (GNPs). The formation of the secondary structure can be triggered by the variation of the buffer concentration and/or pH as indicated by circular dichoism. The peptide on the GNPs shows increased stability toward peptidase degradation as compared to the free peptide. Moreover, only the V3β-GNPs bind to the anti-V3 human broadly neutralizing mAb 447-52D as demonstrated by surface plasmon resonance (SPR). The strong binding of V3β-GNPs to the 447-52D mAb was the starting point to address its study as immunogen. V3β-GNPs elicit antibodies in rabbits that recognize a recombinant gp120 and the serum displayed low but consistent neutralizing activity. These results open up the way for the design of new fully synthetic HIV vaccine candidates.

Sialic acid-binding Ig-like lectin-7 interacts with HIV-1 gp120 and facilitates infection of CD4pos T cells and macrophages

Background

Sialic acid-binding Ig-like lectin-7 (Siglec-7) expression is strongly reduced on natural killer (NK) cells from HIV-1 infected viremic patients. To investigate the mechanism(s) underlying this phenomenon, we hypothesized that Siglec-7 could contribute to the infection of CD4^pos target cells following its interaction with HIV-1 envelope (Env) glycoprotein 120 (gp120).

Results

The ability of Siglec-7 to bind gp120 Env in a sialic acid-dependent manner facilitates the infection of both T cells and monocyte-derived macrophages (MDMs). Indeed, pre-incubation of HIV-1 with soluble Siglec-7 (sSiglec-7) increases the infection rate of CD4^pos T cells, which do not constitutively express Siglec-7. Conversely, selective blockade of Siglec-7 markedly reduces the degree of HIV-1 infection in Siglec-7^pos MDMs. Finally, the sSiglec-7 amount is increased in the serum of AIDS patients with high levels of HIV-1 viremia and inversely correlates with CD4^pos T cell counts.

Conclusions

Our results show that Siglec-7 binds HIV-1 and contributes to enhance the susceptibility to infection of CD4^pos T cells and MDMs. This phenomenon plays a role in HIV-1 pathogenesis and in disease progression, as suggested by the inverse correlation between high serum level of sSiglec-7 and the low CD4^pos T cell count observed in AIDS patients in the presence of chronic viral replication.

Novel peptides based on HIV-1 gp120 sequence with homology to chemokines inhibit HIV infection in cell culture

The sequential interaction of the envelope glycoprotein of the human immunodeficiency virus type 1 (HIV-1) with CD4 and certain chemokine coreceptors initiates host cell entry of the virus. The appropriate chemokines have been shown to inhibit viral replication by blocking interaction of the gp120 envelope protein with the coreceptors. We considered the possibility that this interaction involves a motif of the gp120 that may be structurally homologous to the chemokines. In the amino acid sequences of most chemokines there is a Trp residue located at the beginning of the C-terminal α-helix, which is separated by six residues from the fourth Cys residue. The gp120 of all HIV-1 isolates have a similar motif, which includes the C-terminal part of a variable loop 3 (V3) and N-terminal part of a conserved region 3 (C3). Two synthetic peptides, derived from the relevant gp120 sequence inhibited HIV-1 replication in macrophages and T lymphocytes in sequence-dependent manner. The peptides also prevented binding of anti-CXCR4 antibodies to CXCR4, and inhibited the intracellular Ca²⁺ influx in response to CXCL12/SDF-1α. Thus these peptides can be used to dissect gp120 interactions with chemokine receptors and could serve as leads for the design of new inhibitors of HIV-1.

Constrained use of CCR5 on CD4+ lymphocytes by R5X4 HIV-1: efficiency of Env-CCR5 interactions and low CCR5 expression determine a range of restricted CCR5-mediated entry

R5X4 HIV-1 has impaired utilization of CCR5 on primary CD4+ lymphocytes but the mechanisms responsible are not well defined. Using a panel of diverse R5X4 Envs we identified a spectrum of CCR5 use on CD4+ lymphocytes. Greater lymphocyte CCR5 use correlated with relative resistance to CCR5 mAbs and small molecule antagonists. Increasing CCR5 expression on lymphocytes increased the proportion of entry mediated by CCR5 for all R5X4 isolates except 89.6. In cell lines with regulated CCR5 expression, strains with greater lymphocyte CCR5 use better exploited limiting levels of CCR5. Introduction of an R306S mutation in the 89.6 V3 domain enhanced its utilization of CCR5 at low levels and switched its preference to CCR5 for lymphocyte entry. Thus, the degree to which R5X4 HIV-1 use primary lymphocyte CCR5 is determined by low CCR5 expression coupled with variations in the efficiency of Env-CCR5 interactions, which is in part governed by V3 sequences.

The human Pk histo-blood group antigen provides protection against HIV-1 infection

Several human histo-blood groups are glycosphingolipids, including P/P1/Pk. Glycosphingolipids are implicated in HIV-host-cell-fusion and some bind to HIV-gp120 in vitro. Based on our previous studies on Fabry disease, where Pk accumulates and reduces infection, and a soluble Pk analog that inhibits infection, we investigated cell surface–expressed Pk in HIV infection. HIV-1 infection of peripheral blood–derived mononuclear cells (PBMCs) from otherwise healthy persons, with blood group P1k, where Pk is overexpressed, or blood group p, that completely lacks Pk, were compared with draw date–matched controls. Fluorescence-activated cell sorter analysis and/or thin layer chromatography were used to verify Pk levels. P1k PBMCs were highly resistant to R5 and X4 HIV-1 infection. In contrast, p PBMCs showed 10- to 1000-fold increased susceptibility to HIV-1 infection. Surface and total cell expression of Pk, but not CD4 or chemokine coreceptor expression, correlated with infection. Pk liposome–fused cells and CD4+ HeLa cells manipulated to express high or low Pk levels confirmed a protective effect of Pk. We conclude that Pk expression strongly influences susceptibility to HIV-1 infection, which implicates Pk as a new endogenous cell-surface factor that may provide protection against HIV-1 infection.

CXCR4, inhibitors and mechanisms of action

In this review, the author discusses recent advances in anti-HIV inhibitors, targeting CXCR4, including natural and modified chemokines, peptides and organic compounds, their mechanisms of action, and the molecular process of virus invasion of immune cells. Peptides with strong anti-HIV activity exhibit several common features, such as electrostatic charges, cyclization, beta-turns and dimerization induced by a sulphide bond. Organic compounds, such as cyclams, display a unique metal-mediated mechanism in the binding process to its target CXCR4. Understanding of their mechanisms of action may be useful for the design of more effective drugs. Consecutive interactions of viral glycoprotein gp120 with CD4 and the co-receptor, CXCR4 or another co-receptor CCR5 on the cell surface leads to virus invasion into host cells. The molecular details of the binding between HIV glycoproteins and the co-receptors also provide a basis for anti-HIV therapy.

Mapping of the CXCR4 binding site within variable region 3 of the feline immunodeficiency virus surface glycoprotein

Feline immunodeficiency virus (FIV) shares with T-cell tropic strains of human immunodeficiency virus type 1 (HIV-1) the use of the chemokine receptor CXCR4 for cellular entry. In order to map the interaction of the FIV envelope surface unit (SU) with CXCR4, full-length FIV SU-Fc as well as constructs with deletions of extended loop L2, V3, V4, or V5 were produced in stable CHO cell lines. Binding studies were performed using these proteins on 3201 cells (CXCR4(hi) CD134(-)), with or without the CXCR4 inhibitor AMD3100. The findings established that SU binding to CXCR4 specifically requires the V3 region of SU. Synthetic peptides spanning the V3 region as well as a panel of monoclonal antibodies (MAbs) to SU were used to further map the site of CXCR4 interaction. Both the SU V3-specific antibodies and the full-length V3 peptide potently blocked binding of SU to CXCR4 and virus entry. By using a set of nested peptides overlapping a region of SU specifically recognized by CD134-dependent neutralizing V3 MAbs, we showed that the neutralizing epitope and the region required for CXCR4 binding are within the same contiguous nine-amino-acid sequence of V3. Site-directed mutagenesis was used to reveal that serine 393 and tryptophan 394 at the predicted tip of V3 are required to facilitate entry into the target cell via CXCR4. Although the amino acid sequences are not identical between FIV and HIV, the ability of FIV to bind and utilize both feline and human CXCR4 makes the feline model an attractive venue for development of broad-based entry antagonists.

Bile salt-dependent lipase interacts with platelet CXCR4 and modulates thrombus formation in mice and humans

Bile salt-dependent lipase (BSDL) is an enzyme involved in the duodenal hydrolysis and absorption of cholesteryl esters. Although some BSDL is transported to blood, the role of circulating BSDL is unknown. Here, we demonstrate that BSDL is stored in platelets and released upon platelet activation. Because BSDL contains a region that is structurally homologous to the V3 loop of HIV-1, which binds to CXC chemokine receptor 4 (CXCR4), we hypothesized that BSDL might bind to CXCR4 present on platelets. In human platelets in vitro, both BSDL and a peptide corresponding to its V3-like loop induced calcium mobilization and enhanced thrombin-mediated platelet aggregation, spreading, and activated alpha(IIb)beta(3) levels. These effects were abolished by CXCR4 inhibition. BSDL also increased the production of prostacyclin by human endothelial cells. In a mouse thrombosis model, BSDL accumulated at sites of vessel wall injury. When CXCR4 was antagonized, the accumulation of BSDL was inhibited and thrombus size was reduced. In BSDL(-/-) mice, calcium mobilization in platelets and thrombus formation were attenuated and tail bleeding times were increased in comparison with those of wild-type mice. We conclude that BSDL plays a role in optimal platelet activation and thrombus formation by interacting with CXCR4 on platelets.

[HIV entry into the cells--mechanisms and therapeutic possibilities]

Human immunodeficiency virus (HIV) entry into cells is the first step in the viral replication cycle, which has been explored as a new therapeutic target. A better knowledge of the mechanisms involved in the entry process has led to the development of agents, which may inhibit each of the different steps of the viral entry process: attachment of the gp120 to the CD4 cell receptor; binding of the gp120 to CCR5 or CXCR4 coreceptors; and the fusion of viral and cell membranes. Entry inhibitors are the latest family of antiretroviral compounds, being enfuvirtide, a fusion inhibitor, the first approved. Several other entry inhibitors are currently in clinical development and hopefully soon will be part of the therapeutic armamentarium against HIV.

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.

Solid-state NMR yields structural constraints on the V3 loop from HIV-1 Gp120 bound to the 447-52D antibody Fv fragment

Solid-state NMR measurements were performed on the complex of an 18-residue peptide derived from the V3 loop sequence of the gp120 envelope glycoprotein of the HIV-1 MN strain with Fv fragments of the human anti-gp120 monoclonal antibody 447-52D in a frozen glycerol/water solution. The peptide was uniformly (15)N- and (13)C-labeled in a 7-residue segment containing the conserved GPGR motif in the epitope. (15)N and (13)C NMR chemical shift assignments for the labeled segment were obtained from two-dimensional (13)C-(13)C and (15)N-(13)C magic-angle spinning NMR spectra. Reductions in (13)C NMR line widths and changes in chemical shifts upon complex formation indicate the adoption of a well-defined, antibody-dependent structure. Intramolecular (13)C-(13)C distances in the complex, which constrain the peptide backbone and side chain conformations in the GPGR motif, were determined from an analysis of rotational resonance (RR) data. Structural constraints from chemical shifts and RR measurements are in good agreement with recent solution NMR and crystallographic studies of this system, although differences regarding structural ordering of certain peptide side chains are noted. These experiments explore and help delineate the utility of solid state NMR techniques as structural probes of peptide/protein complexes in general, potentially including membrane-associated hormone/receptor complexes.

Atomic insight into the CD4 binding-induced conformational changes in HIV-1 gp120

The entry of HIV-1 into a target cell requires gp120 and receptor CD4 as well as coreceptor CCR5/CXCR4 recognition events associated with conformational changes of the involved proteins. The binding of CD4 to gp120 is the initiation step of the whole process involving structural rearrangements that are crucial for subsequent pathways. Despite the wealth of knowledge about the gp120/CD4 interactions, details of the conformational changes occurring at this stage remain elusive. We have performed molecular dynamics simulations in explicit solvent based on the gp120/CD4/CD4i crystal structure in conjunction with modeled V3 and V4 loops to gain insight into the dynamics of the binding process. Three differentiated interaction modes between CD4 and gp120 were found, which involve electrostatics, hydrogen bond and van der Waals networks. A "binding funnel" model is proposed based on the dynamical nature of the binding interface together with a CD4-attraction gradient centered in gp120 at the CD4-Phe43-binding cavity. Distinct dynamical behaviors of free and CD4-bound gp120 were monitored, which likely represent the ground and pre-fusogenic states, respectively. The transition between these states revealed concerted motions in gp120 leading to: i) loop contractions around the CD4-Phe43-insertion cavity; ii) stabilization of the four-stranded "bridging sheet" structure; and iii) translocation and clustering of the V3 loop and the bridging sheet leading to the formation of the coreceptor binding site. Our results provide new insight into the dynamic of the underlying molecular recognition mechanism that complements the biochemical and structural studies.

Successful identification of novel agents to control infectious diseases from screening mixture-based peptide combinatorial libraries in complex cell-based bioassays

Mixture-based peptide synthetic combinatorial libraries (SCLs) represent a valuable source for the development of novel agents to control infectious diseases. Indeed, a number of studies have now proven the ability of identifying active peptides from libraries composed of thousands to millions of peptides in cell-based biosystems of varying complexity. Furthermore, progressing knowledge on the importance of endogenous peptides in various immune responses lead to a regain in importance for peptides as potential therapeutic agents. This article is aimed at providing recent studies in our laboratory for the development of antimicrobial or antiviral peptides derived from mixture-based SCLs using cell-based assays, as well as a short review of the importance of such peptides in the control of infectious diseases. Furthermore, the use of positional scanning (PS) SCL-based biometrical analyses for the identification of native optimal epitopes specific to HIV-1 proteins is also presented.

Comparison of SIVmac239(352-382) and SIVsmmPBj41(360-390) enterotoxic synthetic peptides

To characterize the active domain of the simian immunodeficiency virus (SIV) surface unit (SU) enterotoxin, peptides corresponding to the V3 loop of SIVmac239 (SIVmac) and SIVsmmPBj41 (SIVpbj) were synthesized and examined for enterotoxic activity, alpha-helical structure, and interaction(s) with model membranes. SIVmac and SIVpbj induced a dose-dependent diarrhea in 6-8-day-old mouse pups similar to full-length SU. The peptides mobilized [Ca(2+)](i) in HT-29 cells with distinct oscillations and elevated inositol triphosphate levels. Circular dichroism analyses showed the peptides were predominantly random coil in buffer, but increased in alpha-helical content when placed in a hydrophobic environment or with cholesterol-containing membrane vesicles that are rich in anionic phospholipids. None of the peptides underwent significant secondary structural changes in the presence of neutral vesicles indicating ionic interactions were important. These data show that the SIV SU enterotoxic domain localizes in part to the V3 loop region and interacts with anionic membrane domains on the host cell surface.

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).

Alternative conformations of HIV-1 V3 loops mimic beta hairpins in chemokines, suggesting a mechanism for coreceptor selectivity

The V3 loop of the HIV-1 envelope glycoprotein gp120 is involved in binding to the CCR5 and CXCR4 coreceptors. The structure of an HIV-1(MN) V3 peptide bound to the Fv of the broadly neutralizing human monoclonal antibody 447-52D was solved by NMR and found to be a beta hairpin. This structure of V3(MN) was found to have conformation and sequence similarities to beta hairpins in CD8 and CCR5 ligands MIP-1alpha, MIP-1beta, and RANTES and differed from the beta hairpin of a V3(IIIB) peptide bound to the strain-specific murine anti-gp120(IIIB) antibody 0.5beta. In contrast to the structure of the bound V3(MN) peptide, the V3(IIIB) peptide resembles a beta hairpin in SDF-1, a CXCR4 ligand. These data suggest that the 447-52D-bound V3(MN) and the 0.5beta-bound V3(IIIB) structures represent alternative V3 conformations responsible for selective interactions with CCR5 and CXCR4, respectively.

Infection of cells expressing CXCR4 mutants lacking N-glycosylation at the N-terminal extracellular domain is enhanced for R5X4-dualtropic human immunodeficiency virus type-1

BACKGROUND

Infection with human immunodeficiency virus type-1 (HIV-1) requires binding of the viral envelope gp120 to CD4 and to the CXCR4 coreceptor. Both, gp120 and CXCR4 are subject to N-glycosylation. Here we investigated the influence of the N-linked glycans g1 and g2 present on CXCR4 for HIV-1 infection.

METHODS

The two CXCR4 N-glycosylation sites g1 (NYT) and g2 (NVS) were mutated by changing the first or third amino acids N or T/S to Q and A respectively (g1; N11Q or T13A; g2, N176Q or S178A). Human osteosarcoma cells (GHOST) expressing human CD4 and the various CXCR4 glycosylation mutants were tested for infection using NL4-3-based viruses with X4, R5 or R5X4-tropism differing only in the V3 loop region.

RESULTS

All constructed cell lines expressing the various CXCR4 glycomutants showed similar permissiveness for the X4-monotropic virus and no change in the coreceptor specificity that allows infection of a CCR5-dependent R5-monotropic virus. Interestingly, the removal of glycan g1 significantly enhanced the permissiveness of GHOST cells for the R5X4 dualtropic virus. GHOST cells expressing the CXCR4-g1 or CXCR4-g1/2 mutants were infected at higher rates by the R5X4-dualtropic virus compared to cells expressing CXCR4-wt or CXCR4-g2 coreceptors.

CONCLUSION

Our present observations underscore a role for glycans present on the CXCR4 coreceptor in the entry process of HIV-1. The data will help to better understand the multifaceted mechanism of HIV infection and the selective forces which drive HIV-1 evolution from mono- to dual-tropism.

The N-linked glycan g15 within the V3 loop of the HIV-1 external glycoprotein gp120 affects coreceptor usage, cellular tropism, and neutralization

We have studied infectivity and neutralization of X4, R5, and R5X4 tropic HIV-1 mutants, which are lacking N-linked glycosylation sites for glycans g13, g14, g15, and g17 in the V3 loop region of gp120. X4-tropic NL4-3 mutants lacking combinations of g14/15 or g15/17 showed markedly higher infectivity in CXCR4-specific infection. The role of g15 in CCR5-specific infection was investigated using viruses with high (NL-918, R5-monotropic), medium (NL-991, R5-monotropic), and low (NL-952, R5X4-dualtropic) CCR5-specific infectivity. For NL-991, a reduction of infectivity on GHOST-CCR5 cells was observed for a mutant lacking g15. For NL-952 mutants all lacking g15, a complete loss of CCR5-specificity was observed and NL-952 was shifted from R5X4 to X4 tropism. For all mutants of NL4-3, NL-991, and NL-952, where the lack of g15 markedly influenced infectivity or coreceptor usage, neutralization was enhanced. In contrast, NL-918 mutants with or without g15 showed no difference in neutralization and no difference in GHOST-CCR5 infection rates. Thus, for viruses with a low or medium CCR5-specificity the role of g15 for changing CCR5-usage and sensitivity to neutralization was more significant than for viruses with high infection rates on GHOST-CCR5 cells. Our data demonstrate that V3 glycans play an important role in the usage of CXCR4 and CCR5. The lack of g15 was relevant for a more efficient use of CXCR4, whereas interaction with CCR5 was facilitated in the presence of g15. This study also demonstrates that glycan g15 is involved in blocking of neutralizing antibodies and shifting HIV tropism from R5X4 to X4.

Inactivation of human immunodeficiency virus type 1 by porphyrins

We have evaluated the anti-human immunodeficiency virus (HIV) activity of a series of natural and synthetic porphyrins to identify compounds that could potentially be used as microbicides to provide a defense against infection by sexually transmitted virus. For assays we used an epithelial HeLa-CD4 cell line with an integrated long terminal repeat-beta-galactosidase gene. For structure-activity analysis, we divided the porphyrins tested into three classes: (i) natural porphyrins, (ii) metallo-tetraphenylporphyrin tetrasulfonate (metallo-TPPS4) derivatives, and (iii) sulfonated tetra-arylporphyrin derivatives. None of the natural porphyrins studied reduced infection by more than 80% at a concentration of 5 micro g/ml in these assays. Some metal chelates of TPPS4 were more active, and a number of sulfonated tetra-aryl derivatives showed significantly higher activity. Some of the most active compounds were the sulfonated tetranaphthyl porphyrin (TNapPS), sulfonated tetra-anthracenyl porphyrin (TAnthPS), and sulfonated 2,6-difluoro-meso-tetraphenylporphine [TPP(2,6-F2)S] and its copper chelate [TPP(2,6-F2)S,Cu], which reduced infection by 99, 96, 94, and 96%, respectively. Our observations indicate that at least some of these compounds are virucidal, i.e., that they render the virus noninfectious. The active compounds were found to inhibit binding of the HIV type 1 gp120 to CD4 and also to completely inhibit the ability of Env proteins expressed from recombinant vectors to induce cell fusion with receptor-bearing target cells. These results support the conclusion that modified porphyrins exhibit substantial activity against HIV and that their target is the HIV Env protein.

Activation of gp120 of human immunodeficiency virus by their V3 loop-derived peptides

V3 loop peptides from three different human immunodeficiency virus type 1 (HIV-1) strains were synthesized. BH10, ADA, and 89.6 strains whose infections are dependent on CXCR4, CCR5, and both, respectively, were selected. Co-transfection of luciferase reporter gene and corresponding envelope genes (HXB2, ADA, and 89.6) generate pseudotype viruses (HXB2/Luc, ADA/Luc, and 89.6/Luc). The effects of each peptide on the infection of U87 cells expressing CD4 and one of the coreceptors with all pseudotype viruses were evaluated. V3 loop peptide from BH10 (V3-BH10) alone increased the HXB2/Luc infection by 93% at 10 microM. Both V3-ADA and V3-89.6 enhanced ADA/Luc infection by 38% and by 55% at 10 microM, respectively. For 89.6/Luc infection, only V3-89.6 enhanced the infections on both target cells. V3-BH10 modulated the epitopes of coreceptor binding site and V2 loop of gp120 on HIV-1 IIIB infected H9 cells, indicating that V3 loop peptide activates viral gp120 and enhances infectivity.

Identification of conserved and variable structures in the human immunodeficiency virus gp120 glycoprotein of importance for CXCR4 binding

CD4 and the chemokine receptors, CXCR4 and CCR5, serve as receptors for human immunodeficiency virus type 1 (HIV-1). Binding of the HIV-1 gp120 envelope glycoprotein to the chemokine receptors normally requires prior interaction with CD4. Mapping the determinants on gp120 for the low-affinity interaction with CXCR4 has been difficult due to the nonspecific binding of this viral glycoprotein to cell surfaces. Here we examine the binding of a panel of gp120 mutants to paramagnetic proteoliposomes displaying CXCR4 on their surfaces. We show that the gp120 beta19 strand and third variable (V3) loop contain residues important for CXCR4 interaction. Basic residues from both elements, as well as a conserved hydrophobic residue at the V3 tip, contribute to CXCR4 binding. Removal of the gp120 V1/V2 variable loops allows the envelope glycoprotein to bind CXCR4 in a CD4-independent manner. These results indicate that although some variable gp120 residues contribute to the specific binding to CCR5 or CXCR4, gp120 elements common to CXCR4- or CCR5-using strains are involved in the interaction with both coreceptors.

Neomycin B-arginine conjugate, a novel HIV-1 Tat antagonist: synthesis and anti-HIV activities

HIV-1 transactivating protein Tat is essential for virus replication and progression of HIV disease. HIV-1 Tat stimulates transactivation by binding to HIV-1 transactivator responsive element (TAR) RNA, and while secreted extracellularly, it acts as an immunosuppressor, an activator of quiescent T-cells for productive HIV-1 infection, and by binding to CXC chemokine receptor type 4 (CXCR4) as a chemokine analogue. Here we present a novel HIV-1 Tat antagonist, a neomycin B-hexaarginine conjugate (NeoR), which inhibits Tat transactivation and antagonizes Tat extracellular activities, such as increased viral production, induction of CXCR4 expression, suppression of CD3-activated proliferation of lymphocytes, and upregulation of the CD8 receptor. Moreover, Tat inhibits binding of fluoresceine isothiocyanate (FITC)-labeled NeoR to human peripheral blood mononuclear cells (PBMC), indicating that Tat and NeoR bind to the same cellular target. This is further substantiated by the finding that NeoR competes with the binding of monoclonal Abs to CXCR4. Furthermore, NeoR suppresses HIV-1 binding to cells. Importantly, NeoR accumulates in the cell nuclei and inhibits the replication of M- and T-tropic HIV-1 laboratory isolates (EC(50) = 0.8-5.3 microM). A putative model structure for the TAR-NeoR complex, which complies with available experimental data, is presented. We conclude that NeoR is a multitarget HIV-1 inhibitor; the structure, and molecular modeling and dynamics, suggest its binding to TAR RNA. NeoR inhibits HIV-1 binding to cells, partially by blocking the CXCR4 HIV-1 coreceptor, and it antagonizes Tat functions. NeoR is therefore an attractive lead compound, capable of interfering with different stages of HIV infection and AIDS pathogenesis.

Replacement of the V3 region of gp120 with SDF-1 preserves the infectivity of T-cell line-tropic human immunodeficiency virus type 1

Interaction between the human immunodeficiency virus type 1 (HIV-1) envelope and the relevant chemokine receptors is crucial for subsequent membrane fusion and viral entry. Although the V3 region of gp120 is known to determine the cell tropism as well as the coreceptor usage, the significance of the binding of the V3 region to the chemokine receptor has not been fully understood. To address this issue, we adopted the pseudotyped virus infection assay in which the V3 region of the T-cell line-tropic (T-tropic) NL4-3 envelope was replaced with a portion of stromal cell-derived factor 1 (SDF-1), the ligand of CXCR4. The V3 region of the NL4-3 envelope expression vector was replaced with three different stretches of SDF-1 cDNA. Expression of each chimeric envelope protein was confirmed by immunoprecipitation and Western blotting. Luciferase reporter viruses were prepared by cotransfection of the pNL4-3.Luc.E(-)R(-) vector and each chimeric envelope expression vector, and the infection assay was then carried out. We showed that pseudotyped viruses with one of the chimeric envelopes, NL4-3/SDF1-51, could infect U87.CD4.CXCR4 but not U87.CD4 or U87.CXCR4 cells and that this infection was inhibited by the ligand of CXCR4, SDF-1beta, by anti-human SDF-1 antibody, or by an anti-CD4 antibody, Leu3a, in a dose-dependent manner. Furthermore, chimeric NL4-3/SDF1-51 gp120 significantly inhibited binding of labeled SDF-1 to CXCR4. It was suggested that replacement of the V3 region of the NL4-3 envelope with SDF-1 preserved the CD4-dependent infectivity of T-tropic HIV-1. These results indicate that binding between the V3 region and the relevant coreceptor is important for viral entry, whether its amino acid sequence is indigenous to the virus or not.

Ability of the V3 loop of simian immunodeficiency virus to serve as a target for antibody-mediated neutralization: correlation of neutralization sensitivity, growth in macrophages, and decreased dependence on CD4

To better define the effects of sequence variation and tropism on the ability of the simian immunodeficiency virus SIVmac V3 loop to act as a target of antibody-mediated neutralization, a series of experiments were performed. Three SIV strains, SIVmac239, SIVmac316, and SIVmac155/T3, each with defined differences in env sequence and tropism, were used to construct a panel of viruses chimeric for a portion of envelope that includes the V2 and V3 regions. Peptides with sequences corresponding to the V3 loops of the parental viruses were used to immunize rabbits. The polyclonal rabbit antibodies and plasma from SIVmac239-infected animals were then used to assess the neutralization sensitivity of the parental and chimeric viruses. One of the parental viruses, SIVmac316, which is able to replicate to high titer in alveolar macrophages and can infect cells in a CD4-independent fashion, was highly sensitive to neutralization by plasma from SIVmac-infected rhesus macaques, with average 50% neutralization titers of 1:20,480; this same strain was also sensitive to neutralization by the anti-V3 loop peptide sera. Other parental and chimeric viruses were less sensitive to neutralization with this same panel of antibodies, but as seen with SIVmac316, those viruses that were able to productively replicate in alveolar macrophages were more sensitive to antibody-mediated neutralization. To further define the amino acids involved in increased sensitivity to neutralization, a panel of viruses was constructed by changing envelope residues in SIVmac316 to the corresponding SIVmac239 amino acids. The increased neutralization sensitivity observed for SIVmac316 was mapped principally to three amino acid changes spread throughout gp120. In addition, the increased sensitivity to neutralization by V3-directed antibodies correlated with the ability of the various viruses to replicate to high levels in alveolar macrophage cultures and a CD4-negative cell line, BC7/CCR5. These results demonstrate that the V3 loop of SIVmac Env can act as an efficient target of neutralizing antibodies in a fashion that is highly dependent on sequence context. In addition, these studies suggest a correlation between decreased dependence on CD4 and increased sensitivity to antibody-mediated neutralization.

Direct role of plasma membrane-expressed gp120/41 in toxicity to human astrocytes induced by HIV-1-infected macrophages

OBJECTIVE

To compare astrocyte toxicity induced by plasma membrane-expressed gp120/41 and soluble gp120.

DESIGN

Analysis of morphological alterations and lactate dehydrogenase (LDH) release from astrocytes in culture with monocytes infected with HIV-1, microglia expressing Env of a macrophage-tropic HIV-1 isolate or soluble Env.

METHODS

Primary human embryonic astrocytes were cultured with: monocytes infected with two M-tropic HIV-1 isolates (HIV-1(9533), HIV-1(BX08)); human microglia infected with the defective Semliki Forest virus (SFV) vector coding for the env gene of HIV-1(BX08) isolate (SFVenvBX08); and soluble gp140 purified from baby hamster kidney cells transfected with the env gene of HIV-1(BX08) lacking the intracytoplasmic region of gp41 (SFVdelta envBX08). Gp120 mRNA levels were assessed by quantitative reverse transcriptase-polymerase chain reaction and the protein was detected by immunofluorescence in infected monocytes or microglia.

RESULTS

Contact of HIV-infected monocytes induced morphological changes in astrocytes and a 137% increase in LDH release at day 2 of co-culture compared with controls (uninfected monocytes). Gp120/41(BX08)-expressing microglia induced a 170% increase in LDH release (relative to SFVLacZ-infected microglia). Pretreatment of co-cultures with an anti-gp120 monoclonal antibody (mAb; NEA-9305) directed against the V3 loop inhibited LDH release. Soluble purified gp140 from BX08 isolate induced only a weak LDH release (104%). Finally, cytotoxicity was not blocked by treatment of the co-culture with Bordetella pertussis toxin, an inhibitor of Gi alpha protein-dependent receptors.

CONCLUSION

HIV envelope glycoprotein expressed at the plasma membrane induced astrocyte damage more efficiently than its soluble counterpart. The V3 loop was involved in toxicity induction through a pathway independent of the Gi alpha protein-coupled receptor.

New targets for inhibitors of HIV-1 replication

Despite the success of protease and reverse transcriptase inhibitors, new drugs to suppress HIV-1 replication are still needed. Several other early events in the viral life cycle (stages before the viral genome is inserted into host cell DNA) are susceptible to drugs, including virus attachment to target cells, membrane fusion and post-entry events such as integration, accessory-gene function and assembly of viral particles. Among these, inhibitors of virus-cell fusion and integration are the most promising candidates.

Comparative analysis of amino acid sequences from envelope proteins isolated from different hepatitis C virus variants: possible role of conservative and variable regions

Sequences of the E1 and E2 envelope proteins of hepatitis C virus (HCV) (827 non-identical items) were collected from available sources and aligned. Analysis of the alignment identified regions with different sequence variability. It was found that 33% and 50% of positions within E1 and E2, respectively, were highly conservative. Such conservation can be considered as the minimum for maintaining stability of the three-dimensional structure and function of these proteins. Conserved cysteines in E1 and E2 (eight and 18 residues, respectively) were presumed to form intramolecular disulphide bonds. Both envelope proteins were predicted to contain 14 conservative glycosylation sites. Two additional glycosylation sites were predicted in 58% of E1 and 30% of E2 sequences within the corresponding regions. We describe the positions of six conservative regions in E1 and E2, which have several charged and aromatic residues known to participate frequently in protein-protein recognition. Peculiarities in the amino acid content of conservative fragments and putative differences in glycosylation were considered with regard to antigenic specificity and possible binding to surface structures of target cells. We also analysed the hypervariable region 1 (HVR1), located in the E2 protein. Aligned positions of HVR1 were described in relation to the maintenance of conformational stability and recognition of cell receptors.

Inhibition of HIV-1 infection by an intramolecular antisense peptide to T20 in gp160

Antisense amino acids are amino acids which can be translated from the corresponding anti-codons of a sense amino acid. Antisense peptides encoded by the noncoding DNA strand have a tendency to interact with each other. We have demonstrated that antisense peptide sequences are present intramolecularly, and these may contribute to the folding and maintenance of the tertiary structure of a protein. T20 is a synthetic peptide with an amino acid sequence in the gp41 of HIV-1 and has been demonstrated to be a potent inhibitor of HIV-1 infection. We searched for intramolecular peptide sequences which are antisense to portions of T20. A synthetic peptide (TA-1L) consisting of amino acids 84 to 97 of gp160, which contains an antisense peptide sequence (TA-1) to T20, was shown to inhibit HIV-1(IIIB) infection of MT-4 cells. Interaction of these antisense peptides could be involved in sustaining HIV-1 infectivity. The TA-1L site, which exists in the C1 domain of gp160, is highly homologous among strains of HIV-1, especially at TA-1 and in the amino acids flanking the C terminus. Although the TA-1 sites of 18 out of 30 HIV-1 strains were antisense to the T20 region, those of the remaining 12 strains, including HIV-1(MN), were not. However, TA-1L inhibited infection by HIV-1(MN), which has no antisense peptide in T20 corresponding to TA-1, although the inhibitory effect was weaker. TA-1L may thus also interfere with the gp160 interaction with CD4, which has an antisense sequence to TA-1.

Activation of the chemotactic peptide receptor FPRL1 in monocytes phosphorylates the chemokine receptor CCR5 and attenuates cell responses to selected chemokines

FPRL1 is a seven-transmembrane (STM), G-protein coupled receptor which was originally identified as a low affinity receptor for the bacterial chemotactic formyl peptide and a high affinity receptor for the lipid metabolite lipoxin A4. We recently discovered that a number of peptides, including several synthetic domains of the HIV-1 envelope proteins and the serum amyloid A, use FPRL1 to induce migration and calcium mobilization in human monocytes and neutrophils. In this study, we report that a synthetic peptide domain of the V3 region of the HIV-1 envelope gp120, activates the FPRL1 receptor in monocytes and neutrophils. Furthermore, monocytes prestimulated with V3 peptide showed reduced response to several chemokines that use multiple cell receptors. This is associated with a rapid phosphorylation of the chemokine receptor CCR5 on the serine residues. Our study suggests that FPRL1, as a classical chemoattractant receptor, may play an important role in modulating monocyte activation in the presence of multiple stimuli.

Specific interaction of CCR5 amino-terminal domain peptides containing sulfotyrosines with HIV-1 envelope glycoprotein gp120

The HIV-1 envelope glycoprotein gp120 interacts consecutively with CD4 and the CCR5 coreceptor to mediate the entry of certain HIV-1 strains into target cells. Acidic residues and sulfotyrosines in the amino-terminal domain (Nt) of CCR5 are crucial for viral fusion and entry. We tested the binding of a panel of CCR5 Nt peptides to different soluble gp120/CD4 complexes and anti-CCR5 mAbs. The tyrosine residues in the peptides were sulfated, phosphorylated, or unmodified. None of the gp120/CD4 complexes associated with peptides containing unmodified or phosphorylated tyrosines. The gp120/CD4 complexes containing envelope glycoproteins from isolates that use CCR5 as a coreceptor associated with Nt peptides containing sulfotyrosines but not with peptides containing sulfotyrosines in scrambled Nt sequences. Finally, only peptides containing sulfotyrosines inhibited the entry of an R5 isolate. Our data show that proper posttranslational modification of the CCR5 Nt is required for gp120 binding and viral entry. More importantly, the Nt domain determines the specificity of the interaction between CCR5 and gp120s from isolates that use this coreceptor.

NMR structure of an anti-gp120 antibody complex with a V3 peptide reveals a surface important for co-receptor binding

BACKGROUND

The protein 0.5beta is a potent strain-specific human immunodeficiency virus type 1 (HIV-1) neutralizing antibody raised against the entire envelope glycoprotein (gp120) of the HIV-1(IIIB) strain. The epitope recognized by 0.5beta is located within the third hypervariable region (V3) of gp120. Recently, several HIV-1 V3 residues involved in co-receptor utilization and selection were identified.

RESULTS

Virtually complete sidechain assignment of the variable fragment (Fv) of 0.5beta in complex with the V3(IIIB) peptide P1053 (RKSIRIQRGPGRAFVTIG, in single-letter amino acid code) was accomplished and the combining site structure of 0.5beta Fv complexed with P1053 was solved using multidimensional nuclear magnetic resonance (NMR). Five of the six complementarity determining regions (CDRs) of the antibody adopt standard canonical conformations, whereas CDR3 of the heavy chain assumes an unexpected fold. The epitope recognized by 0.5beta encompasses 14 of the 18 P1053 residues. The bound peptide assumes a beta-hairpin conformation with a QRGPGR loop located at the very center of the binding pocket. The Fv and peptide surface areas buried upon binding are 601 A and 743 A(2), respectively, in the 0.5beta Fv-P1053 mean structure. The surface of P1053 interacting with the antibody is more extensive and the V3 peptide orientation in the binding site is significantly different compared with those derived from the crystal structures of a V3 peptide of the HIV-1 MN strain (V3(MN)) complexed to three different anti-peptide antibodies.

CONCLUSIONS

The surface of P1053 that is in contact with the anti-protein antibody 0.5beta is likely to correspond to a solvent-exposed region in the native gp120 molecule. Some residues of this region of gp120 are involved in co-receptor binding, and in discrimination between different chemokine receptors utilized by the protein. Several highly variable residues in the V3 loop limit the specificity of the 0.5beta antibody, helping the virus to escape from the immune system. The highly conserved GPG sequence might have a role in maintaining the beta-hairpin conformation of the V3 loop despite insertions, deletions and mutations in the flanking regions.

Glycans and proteoglycans are involved in the interactions of human immunodeficiency virus type 1 envelope glycoprotein and of SDF-1alpha with membrane ligands of CD4(+) CXCR4(+) cells

We demonstrate that human immunodeficiency virus HIV-1(LAI) envelope glycoprotein 120 (gp120(LAi)) specifically interacts with several membrane ligands on lymphoid CEM or monocytic U937 cells in addition to its previously identified receptor, CD4, and CXCR4, its coreceptor. In its native state, gp120(LAI) is able to elicit specific multimolecular complexes with these membrane ligands at the surface of the cells; most of the interactions are abolished by mannan or heparin but not by dextran. Similarly, stromal cell-derived factor (SDF)-1alpha interacts not only with CXCR4 expressed by CXCR4(+) CD4(+) U937, CEM, and HOS-CD4(+) CXCR4(+) cells but also with CD4 expressed by intact U937, CEM, and HOS-CD4(+) CXCR4(+/-) cells or electroblotted onto Immobilon. SDF-1alpha binding to CD4(+) CXCR4(+/-) cells, or soluble CD4 electroblotted onto Immobilon, is significantly inhibited by sCD4, whereas truncated sCD4 lacking D3 and D4 domains had no significant effect, which indicates that SDF-1 binds to CD4 but at regions different from the HIV-gp120-binding site. Heparin and mannan also inhibit SDF-1alpha binding to intact CD4(+) CXCR4(+/-) cells, and electroblotted soluble CD4. Heparitinase treatment of such cells reduced SDF-1alpha binding. These data demonstrate that glycans and glycosaminoglycans are directly or indirectly involved in the interactions of HIV-1 gp120(LAI) and of SDF-1alpha with membrane ligands of CD4(+) CXCR4(+) cells and thus could play a role both in HIV-1 infection and in the physiology of SDF-1alpha.

Inhibitory mechanism of the CXCR4 antagonist T22 against human immunodeficiency virus type 1 infection

We recently reported that a cationic peptide, T22 ([Tyr(5,12), Lys(7)]-polyphemusin II), specifically inhibits human immunodeficiency virus type 1 (HIV-1) infection mediated by CXCR4 (T. Murakami et al., J. Exp. Med. 186:1389-1393, 1997). Here we demonstrate that T22 effectively inhibits replication of T-tropic HIV-1, including primary isolates, but not of non-T-tropic strains. By using a panel of chimeric viruses between T- and M-tropic HIV-1 strains, viral determinants for T22 susceptibility were mapped to the V3 loop region of gp120. T22 bound to CXCR4 and interfered with stromal-cell-derived factor-1alpha-CXCR4 interactions in a competitive manner. Blocking of anti-CXCR4 monoclonal antibodies by T22 suggested that the peptide interacts with the N terminus and two of the extracellular loops of CXCR4. Furthermore, the inhibition of cell-cell fusion in cells expressing CXCR4/CXCR2 chimeric receptors suggested that determinants for sensitivity of CXCR4 to T22 include the three extracellular loops of the coreceptor.

Role of the HIV type 1 glycoprotein 120 V3 loop in determining coreceptor usage

Macrophage (M)-tropic HIV-1 isolates use the beta-chemokine receptor CCR5 as a coreceptor for entry, while T cell line-adapted (TCLA) strains use CXCR4 and dual-tropic strains can use either CCR5 or CXCR4. To investigate the viral determinants involved in choice of coreceptor, we used a fusion assay based on the infection of CD4+ HeLa cells that express one or both coreceptors with Semliki Forest virus (SFV) recombinants expressing the native HIV-1 gp160 of a primary M-tropic isolate (HIV-1BX08), a TCLA isolate (HIV-1LAI), or a dual-tropic strain (HIV-1MN). We examined whether the V3 region of these glycoproteins interacts directly with the corresponding coreceptors by assaying coreceptor-dependent cell-to-cell fusion mediated by the different recombinants in the presence of various synthetic linear peptides. Synthetic peptides corresponding to different V3 loop sequences blocked syncytium formation in a coreceptor-specific manner. Synthetic V2 peptides were also inhibitory for syncytium formation, but showed no apparent coreceptor specificity. A BX08 V3 peptide with a D320 --> R substitution retained no inhibitory capacity for BX08 Env-mediated cell-to-cell fusion, but inhibited LAI Env-mediated fusion as efficiently as the homologous LAI V3 peptide. The same mutation engineered in the BX08 env gene rendered it able to form syncytia on CD4+CXCR4+CCR5-HeLa cells and susceptible to inhibition by SDF-1alpha and MIP-1beta. Other substitutions tested (D320 --> Q/D324 --> N or S306 --> R) exhibited intermediate effects on coreceptor usage. These results underscore the importance of the V3 loop in modulating coreceptor choice and show that single amino acid modifications in V3 can dramatically modify coreceptor usage. Moreover, they provide evidence that linear V3 loop peptides can compete with intact cell surface-expressed gp120/gp41 for CCR5 or CXCR4 interaction.