[Show abstract][Hide abstract] ABSTRACT: Chemokine receptor CXCR4 is one of two principal coreceptors for the entry of HIV-1 into target cells. CXCR4 is known to form homodimers. We previously demonstrated that the amino terminus of viral macrophage protein II (vMIP-II) is the major determinant for CXCR4 recognition, and that V1 peptide derived from the N-terminus of vMIP-II (1-21 residues) showed significant CXCR4 binding. Interestingly, an all-d-amino acid analogue of V1 peptide, DV1 peptide, displayed an even higher binding affinity and strong antiviral activity in inhibiting the replication of CXCR4-dependent HIV-1 strains. In this study, we synthetically linked two DV1 peptides with the formation of a disulfide bond between the two cysteine residues present in the peptide sequence to generate a dimeric molecule potentially capable of interacting with two CXCR4 receptors. DV1 dimer exhibited enhanced binding affinity and antiviral activity compared with those of DV1 monomer. Ligand binding site mapping experiments showed that DV1 dimer overlaps with HIV-1 gp120 on CXCR4 binding sites, including several transmembrane (TM) residues located close to the extracellular side and the N-terminus of CXCR4. This finding was supported by the molecular modeling of CXCR4 dimer-DV1 dimer interaction based on the crystal structure of CXCR4, which showed that DV1 dimer is capable of interacting with the CXCR4 dimeric structure by allowing the N-terminus of each DV1 monomer to reach into the binding pocket of CXCR4 monomer. The development of this bivalent ligand provides a tool for further probing the functions of CXCR4 dimerization and studying CXCR4 heterodimerization with other receptors.
[Show abstract][Hide abstract] ABSTRACT: The replication of human immunodeficiency virus type 1 (HIV-1) can be profoundly inhibited by the natural ligands of two major HIV-1 coreceptors, CXCR4 and CCR5. Stromal cell-derived factor-1α (SDF-1α) is a natural ligand of CXCR4. We have recently developed a synthetic biology approach of using synthetically and modularly modified (SMM)-chemokines to dissect various aspects of the structure-function relationship of chemokines and their receptors. Here, we used this approach to design novel SMM-SDF-1α analogues containing unnatural N-methylated residues in the amino terminus to investigate whether the polypeptide main chain amide bonds in the N-terminus of SDF-1α play a role in SDF-1α signaling via CXCR4 and/or receptor internalization. The results show that SDF-1α analogues with a modified N-methylated main chain at position 2, 3, or 5 retain significant CXCR4 binding and yet completely lose signaling activities. Furthermore, a representative N-methylated analogue has been shown to be incapable of causing CXCR4 internalization. These results suggest that the ability of SDF-1α to activate CXCR4 signaling and internalization is dependent upon the main chain amide bonds in the N-terminus of SDF-1α. This study demonstrates the feasibility and value of applying a synthetic biology approach to chemically engineer natural proteins and peptide ligands as probes of important biological functions that are not addressed by other biological techniques.
[Show abstract][Hide abstract] ABSTRACT: The natural ligands of two major human immunodeficiency virus type 1 (HIV-1) co-receptors, CXCR4 and CCR5, can profoundly inhibit the replication of HIV-1 that uses these co-receptors for entry into the target cells. It has been postulated that these natural chemokines inhibit HIV-1 infection by blocking common binding sites on CXCR4 or CCR5 that are required for HIV-1 envelope glycoprotein gp120 interaction with its co-receptor and/or by inducing receptor internalization. To investigate whether receptor internalization caused by stromal cell-derived factor (SDF)-1α, a natural ligand of CXCR4, plays a role in its anti-HIV activity, we applied the SMM (synthetically and modularly modified)-chemokine approach to generate a functional probe of SDF-1α that retains significant CXCR4 binding but does not induce CXCR4 internalization. The antiviral study of this functional probe analog versus wild-type SDF-1α showed that, despite the significant CXCR4 binding activity, this probe analog displayed a complete loss of effect in causing CXCR4 internalization and greatly diminished antiviral activity. Interestingly, this new analog also showed a decreased number of overlapping binding sites with HIV-1 on CXCR4 transmembrane and extracellular domains. The correlation of the decrease in the anti-HIV activity with the loss of CXCR4 internalization observed with this probe molecule suggests that receptor internalization may play an important role in the anti-HIV activity of SDF-1α and possibly other natural chemokines. This further implies that any modifications in SDF-1α that result in a reduction or loss of internalization activity may result in analogs that are not suitable as effective HIV-1 inhibitors that target CXCR4, unless such modifications also result in improved CXCR4 interaction with increased number of overlapping binding sites with HIV-1, thus leading to more effective steric hindrance against HIV-1.
Experimental Biology and Medicine 11/2011; 236(12):1413-9. DOI:10.1258/ebm.2011.011260 · 2.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Chemokines and their receptors play important roles in normal physiological functions and the pathogeneses of a wide range of human diseases, including the entry of human immunodeficiency virus type 1 (HIV-1). However, the use of natural chemokines to probe receptor biology or to develop therapeutic drugs is limited by their lack of selectivity and the poor understanding of mechanisms in ligand-receptor recognition. We addressed these issues by combining chemical and structural biology in research into molecular recognition and inhibitor design. Specifically, the concepts of chemical biology were used to develop synthetically and modularly modified (SMM) chemokines that are unnatural and yet have properties improved over those of natural chemokines in terms of receptor selectivity, affinity, and the ability to explore receptor functions. This was followed by using structural biology to determine the structural basis for synthetically perturbed ligand-receptor selectivity. As a proof-of-principle for this combined chemical and structural-biology approach, we report a novel D-amino acid-containing SMM-chemokine designed based on the natural chemokine called viral macrophage inflammatory protein II (vMIP-II). The incorporation of unnatural D-amino acids enhanced the affinity of this molecule for CXCR4 but significantly diminished that for CCR5 or CCR2, thus yielding much more selective recognition of CXCR4 than wild-type vMIP-II. This D-amino acid-containing chemokine also showed more potent and specific inhibitory activity against HIV-1 entry via CXCR4 than natural chemokines. Furthermore, the high-resolution crystal structure of this D-amino acid-containing chemokine and a molecular-modeling study of its complex with CXCR4 provided the structure-based mechanism for the selective interaction between the ligand and chemokine receptors and the potent anti-HIV activity of D-amino acid-containing chemokines.
Journal of Virology 09/2007; 81(20):11489-11498. DOI:10.1128/JVI.02845-06 · 4.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: As the main coreceptors for human immunodeficiency virus type 1 (HIV-1) entry, CXCR4 and CCR5 play important roles in HIV-associated dementia (HAD). HIV-1 glycoprotein gp120 contributes to HAD by causing neuronal damage and death, either directly by triggering apoptotic pathways or indirectly by stimulating glial cells to release neurotoxins. Here, to understand the mechanism of CXCR4 or CCR5 signaling in neuronal apoptosis associated with HAD, we have applied synthetically and modularly modified (SMM)-chemokine analogs derived from natural stromal cell-derived factor-1alpha or viral macrophage inflammatory protein-II as chemical probes of the mechanism(s) whereby these SMM-chemokines prevent or promote neuronal apoptosis. We show that inherently neurotoxic natural ligands of CXCR4, such as stromal cell-derived factor-1alpha or viral macrophage inflammatory protein-II, can be modified to protect neurons from apoptosis induced by CXCR4-preferring gp120(IIIB), and that the inhibition of CCR5 by antagonist SMM-chemokines, unlike neuroprotective CCR5 natural ligands, leads to neurotoxicity by activating a p38 mitogen-activated protein kinase (MAPK)-dependent pathway. Furthermore, we discover distinct signaling pathways activated by different chemokine ligands that are either natural agonists or synthetic antagonists, thus demonstrating a chemical biology strategy of using chemically engineered inhibitors of chemokine receptors to study the signaling mechanism of neuronal apoptosis and survival.
[Show abstract][Hide abstract] ABSTRACT: Chemokines and their receptors play important roles in numerous physiological and pathological processes. To develop natural chemokines into receptor probes and inhibitors of pathological processes, the lack of chemokine-receptor selectivity must be overcome. Here, we apply chemical synthesis and the concept of modular modifications to generate unnatural synthetically and modularly modified (SMM)-chemokines that have high receptor selectivity and affinity, and reduced toxicity. A proof of the concept was shown by transforming the nonselective viral macrophage inflammatory protein-II into new analogs with enhanced selectivity and potency for CXCR4 or CCR5, two principal coreceptors for human immunodeficiency virus (HIV)-1 entry. These new analogs provided insights into receptor binding and signaling mechanisms and acted as potent HIV-1 inhibitors. These results support the concept of SMM-chemokines for studying and controlling the function of other chemokine receptors.
[Show abstract][Hide abstract] ABSTRACT: The chemokine receptor CXCR4 plays an important role as the receptor for the normal physiological function of stromal cell-derived factor 1alpha (SDF-1alpha) and the coreceptor for the entry of human immunodeficiency virus type 1 (HIV-1) into the cell. In a recent work (S. Tian et al., J. Virol. 79:12667-12673, 2005), we found that many residues throughout CXCR4 transmembrane (TM) and extracellular loop 2 domains are specifically involved in interaction with HIV-1 gp120, as most of these sites did not play a role in either SDF-1alpha binding or signaling. These results provided direct experimental evidence for the distinct functional sites on CXCR4 for HIV-1 and the normal ligand SDF-1alpha. To further understand the CXCR4-ligand interaction and to develop new CXCR4 inhibitors to block HIV-1 entry, we have recently generated a new family of unnatural chemokines, termed synthetically and modularly modified (SMM) chemokines, derived from the native sequence of SDF-1alpha or viral macrophage inflammatory protein II (vMIP-II). These SMM chemokines contain various de novo-designed sequence replacements and substitutions by d-amino acids and display more enhanced CXCR4 selectivity, binding affinities, and/or anti-HIV activities than natural chemokines. Using these novel CXCR4-targeting SMM chemokines as receptor probes, we conducted ligand binding site mapping experiments on a panel of site-directed mutants of CXCR4. Here, we provide the first experimental evidence demonstrating that SMM chemokines interact with many residues on CXCR4 TM and extracellular domains that are important for HIV-1 entry, but not SDF-1alpha binding or signaling. The preferential overlapping in the CXCR4 binding residues of SMM chemokines with HIV-1 over SDF-1alpha illustrates a mechanism for the potent HIV-1 inhibition by these SMM chemokines. The discovery of distinct functional sites or conformational states influenced by these receptor sites mediating different functions of the natural ligand versus the viral or synthetic ligands has important implications for drug discovery, since the sites shared by SMM chemokines and HIV-1 but not by SDF-1alpha can be targeted for the development of selective HIV-1 inhibitors devoid of interference with normal SDF-1alpha function.
Journal of Virology 01/2006; 79(24):15398-404. DOI:10.1128/JVI.79.24.15398-15404.2005 · 4.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Human immunodeficiency virus type 1 (HIV-1) uses a chemokine receptor, usually CXCR4 or CCR5, for entry into the target cells. Here, we used a chemical biology approach to demonstrate that binding and signaling domains in CXCR4 are possibly distinct and separate, as the new analogue, D(1-10)-vMIP-II-(9-68)-SDF-1 alpha (RCP222), could not activate CXCR4 despite the fact that its binding activity was comparable to that of stromal cell-derived factor (SDF)-1 alpha, the only natural ligand of CXCR4.
[Show abstract][Hide abstract] ABSTRACT: Chemokine receptor CXCR4 plays an important role in the immune system and the cellular entry of human immunodeficiency virus type 1 (HIV-1). To probe the stereospecificity of the CXCR4-ligand interface, d-amino acid peptides derived from natural chemokines, viral macrophage inflammatory protein II (vMIP-II) and stromal cell-derived factor-1alpha (SDF-1alpha), were synthesized and found to compete with (125)I-SDF-1alpha and monoclonal antibody 12G5 binding to CXCR4 with potency and selectivity comparable with or higher than their l-peptide counterparts. This was surprising because of the profoundly different side chain topologies between d- and l-enantiomers, which circular dichroism spectroscopy showed adopt mirror image conformations. Further direct binding experiments using d-peptide labeled with fluorescein (designated as FAM-DV1) demonstrated that d- and l-peptides shared similar or at least overlapping binding site(s) on the CXCR4 receptor. Structure-activity analyses of related peptide analogs of mixed chiralities or containing alanine replacements revealed specific residues at the N-terminal half of the peptides as key binding determinants. Acting as CXCR4 antagonists and with much higher biological stability than l-counterparts, the d-peptides showed significant activity in inhibiting the replication of CXCR4-dependent HIV-1 strains. These results show the remarkable stereochemical flexibility of the CXCR4-peptide interface. Further studies to understand the mechanism of this unusual feature of the CXCR4 binding surface might aid the development of novel CXCR4-binding molecules like the d-peptides that have high affinity and stability.