Validation of chemokine receptors as drug targets

The number of known potential drug targets within the chemokine system has expanded in the last decade with the characterization of new chemokines and chemokine receptors. However, at present, limited information exists on the potential of these receptors as drug targets due to the lack of small molecule chemokine receptor antagonists. This review focuses on current information relating to these receptors obtained through genetic knockout experiments, as well as experiments performed with peptide antagonists and receptor antibodies.

Selective elimination of high constitutive activity or chemokine binding in the human herpesvirus 8 encoded seven transmembrane oncogene ORF74

Open reading frame 74 (ORF74) encoded by human herpesvirus 8 is a highly constitutively active seven transmembrane (7TM) receptor stimulated by angiogenic chemokines, e.g. growth-related oncogene-alpha, and inhibited by angiostatic chemokines e.g. interferon-gamma-inducible protein. Transgenic mice expressing ORF74 under control of the CD2 promoter develop highly vascularized Kaposi's sarcoma-like tumors. Through targeted mutagenesis we here create three distinct phenotypes of ORF74: a receptor with normal, high constitutive signaling through the phospholipase C pathway but deprived of binding and action of chemokines obtained through deletion of 22 amino acids from the N-terminal extension; an ORF74 with high constitutive activity but with selective elimination of stimulatory regulation by angiogenic chemokines obtained through substitution of basic residues at the extracellular ends of TM-V or TM-VI; and an ORF74 lacking constitutive activity but with preserved ability to be stimulated by agonist chemokines obtained through introduction of an Asp residue on the hydrophobic, presumed membrane-exposed face of TM-II. It is concluded that careful molecular dissection can selectively eliminate either agonist or inverse agonist modulation as well as high constitutive activity of the virally encoded oncogene ORF74 and that these mutant forms presumably can be used in transgenic animals to identify the molecular mechanism of its transforming activity.

Partial agonism through a zinc-Ion switch constructed between transmembrane domains III and VII in the tachykinin NK(1) receptor

Partly due to lack of detailed knowledge of the molecular recognition of ligands the structural basis for partial versus full agonism is not known. In the beta(2)-adrenergic receptor the agonist binding site has previously been structurally and functionally exchanged with an activating metal-ion site located between AspIII:08-or a His residue introduced at this position in transmembrane domain (TM)-III-and a Cys residue substituted for AsnVII:06 in TM-VII. Here, this interhelical, bidentate metal-ion site is without loss of Zn(2+) affinity transferred to the tachykinin NK(1) receptor. In contrast to the similarly mutated beta(2)-adrenergic receptor, signal transduction-i.e., inositol phosphate turnover-could be stimulated by both Zn(2+) and by the natural agonist, Substance P in the mutated NK(1) receptor. The metal-ion acted as a 25% partial agonist through binding to the bidentate zinc switch located exactly one helical turn below the two previously identified interaction points for Substance P in, respectively, TM-III and -VII. The metal-ion chelator, phenantroline, which in the beta(2)-adrenergic receptor increased both the potency and the agonistic efficacy of Zn(2+) or Cu(2+) in complex with the chelator, also bound to the metal-ion site-engineered NK(1) receptor, but here the metal-ion chelator complex instead acted as a pure antagonist. It is concluded that signaling of even distantly related rhodopsin-like 7TM receptors can be activated through Zn(2+) coordination between metal-ion binding residues located at positions III:08 and VII:06. It is suggested that only partial agonism is obtained through this simple well defined metal-ion coordination due to lack of proper interactions with residues also in TM-VI.

Binding of norbinaltorphimine (norBNI) congeners to wild-type and mutant mu and kappa opioid receptors: molecular recognition loci for the pharmacophore and address components of kappa antagonists

Molecular modifications of both the kappa opioid antagonist norbinaltorphimine (norBNI, 1) and the kappa receptor have provided evidence that the selectivity of this ligand is conferred through ionic interaction if its N17' protonated amine group (an "address") with a nonconserved acidic residue (Glu297) on the kappa receptor. In the present study, we have examined the effect of structural modifications on the affinity of norBNI analogues for wild-type and mutant kappa and mu opioid receptors expressed in COS-7 cells. Compounds 2, 3, and 7, which have an antagonist pharmacophore and basic N17' group in common with norBNI, retained high affinity for the wild-type kappa but exhibited greatly reduced affinity for mutant kappa receptors (E297K and E297A). Modification of the phenolic or N-substituent groups of the antagonist pharmacophore (4 and 5) or removal of basicity at the address N17' center (6) led to greatly reduced affinity for the wild-type and mutant receptors. The reduced affinity upon modification of the kappa receptor is consistent with the ionic interaction of the protonated N17' group of kappa antagonists (1-3, 7) with the carboxylate group of E297 at the top of TM6. This was supported by the greatly enhanced affinity of compounds 1-3 for the mutant mu receptor (K303E), as compared to the wild-type mu receptor, given that residue K303 occupies a position equivalent to that of E297 in the kappa receptor. In view of the high degree of homology of the seven TM domains of the kappa and mu opioid receptors, it is suggested that the antagonist pharmacophore is bound within this highly conserved region of the kappa or mutant mu receptor and that an anionic residue at the top of TM6 (E297 or K303E, respectively) provides additional binding affinity.

Potency of ligands correlates with affinity measured against agonist and inverse agonists but not against neutral ligand in constitutively active chemokine receptor

ORF-74, a 7TM receptor oncogene encoded by human herpes virus 8, shows 50% constitutive activity in stimulating phosphatidylinositol turnover and binds a large variety of CXC chemokines. These endogenous ligands cover a full spectrum of pharmacological properties with growth-related oncogene (GRO)-alpha and -gamma functioning as full agonists; GRObeta as a partial agonist; interleukin (IL)-8, neutrophil-activating peptide (NAP)-2, and epithelial cell-derived activating peptide (ENA)-78 as neutral ligands; granulocyte colony-stimulating factor (GCP)-2 as a partial inverse agonist; and interferon-gamma inducible protein (IP)-10 and stromal cell-derived factor (SDF)-1alpha as full inverse agonists. The affinity for the agonists was independent of whether it was determined in competition binding against the agonist (125)I-GROalpha, against the inverse agonist (125)I-IP-10, or against the neutral ligand (125)I-IL-8. Similarly, the affinities of the inverse agonists were within 1 order of magnitude independent of the choice of radioligand. In contrast, the neutral ligands IL-8, NAP-2, and ENA-78, which all displaced (125)I-IL-8 with single-digit nanomolar affinity showed up to 1000-fold lower affinity against both the radioactive agonist and against the radioactive inverse agonist. A close correlation was observed between the EC(50) values for the ligands and their IC(50) values measured against either radioactive agonist or radioactive inverse agonist, but a poor correlation was found to the IC(50) value measured against the neutral ligand. It is concluded that in ORF-74, ligands compete for binding more according to pharmacological property than to structural homology and that both agonists and inverse agonists, in contrast to neutral ligands, apparently bind with high affinity either to a common conformation of the receptor or to readily interconvertible states, not available for the neutral ligands.

Disulfide bridge engineering in the tachykinin NK1 receptor

As in most other seven-transmembrane receptors, the central disulfide bridge from the extracellular end of TM-III to the middle of the second extracellular loop was essential for ligand binding in the NK1 receptor. However, introduction of "extra", single Cys residues in the second extracellular loop, at positions where disease-associated Cys substitutions impair receptor function in the vasopressin V2 receptor and in rhodopsin, did not cause mispairing with the Cys residues involved in this central disulfide bridge. Cys residues were introduced in the N-terminal extension and in the third extracellular loop, respectively, in such a way that disulfide bridge formation could be monitored by loss of substance P binding and breakage of the bridge could be monitored by gain of ligand binding. This disulfide bridge formed spontaneously in the whole population of receptors and could be titrated with low concentrations of reducing agent, dithiothreitol. Another putative disulfide bridge "switch" was constructed at the extracellular ends of TM-V and -VI, i.e., at positions where a high-affinity zinc site previously had been constructed with His substitutions. Disulfide bridge formation at this position, monitored by loss of binding of the nonpeptide antagonist [3H]LY303.870, occurred spontaneously only in a small fraction of the receptors. It is concluded that disulfide bridges form readily between Cys residues introduced appropriately in the N-terminal extension and the third extracellular loop, whereas they form with more difficulty between Cys residues placed at the extracellular ends of the transmembrane segments even at positions where high-affinity metal ion sites can be constructed with His residues.

Conversion of agonist site to metal-ion chelator site in the beta(2)-adrenergic receptor

Previously metal-ion sites have been used as structural and functional probes in seven transmembrane receptors (7TM), but as yet all the engineered sites have been inactivating. Based on presumed agonist interaction points in transmembrane III (TM-III) and -VII of the beta(2)-adrenergic receptor, in this paper we construct an activating metal-ion site between the amine-binding Asp-113 in TM-III-or a His residue introduced at this position-and a Cys residue substituted for Asn-312 in TM-VII. No increase in constitutive activity was observed in the mutant receptors. Signal transduction was activated in the mutant receptors not by normal catecholamine ligands but instead either by free zinc ions or by zinc or copper ions in complex with small hydrophobic metal-ion chelators. Chelation of the metal ions by small hydrophobic chelators such as phenanthroline or bipyridine protected the cells from the toxic effect of, for example Cu(2+), and in several cases increased the affinity of the ions for the agonistic site. Wash-out experiments and structure-activity analysis indicated, that the high-affinity chelators and the metal ions bind and activate the mutant receptor as metal ion guided ligand complexes. Because of the well-understood binding geometry of the small metal ions, an important distance constraint has here been imposed between TM-III and -VII in the active, signaling conformation of 7TM receptors. It is suggested that atoxic metal-ion chelator complexes could possibly in the future be used as generic, pharmacologic tools to switch 7TM receptors with engineered metal-ion sites on or off at will.

CD4-Chemokine receptor hybrids in human immunodeficiency virus type 1 infection

Most human immunodeficiency virus (HIV) strains require both CD4 and a chemokine receptor for entry into a host cell. In order to analyze how the HIV-1 envelope glycoprotein interacts with these cellular molecules, we constructed single-molecule hybrids of CD4 and chemokine receptors and expressed these constructs in the mink cell line Mv-1-lu. The two N-terminal (2D) or all four (4D) extracellular domains of CD4 were linked to the N terminus of the chemokine receptor CXCR4. The CD4(2D)CXCR4 hybrid mediated infection by HIV-1(LAI) to nearly the same extent as the wild-type molecules, whereas CD4(4D)CXCR4 was less efficient. Recombinant SU(LAI) protein competed more efficiently with the CXCR4-specific monoclonal antibody 12G5 for binding to CD4(2D)CXCR4 than for binding to CD4(4D)CXCR4. Stromal cell-derived factor 1 (SDF-1) blocked HIV-1(LAI) infection of cells expressing CD4(2D)CXCR4 less efficiently than for cells expressing wild-type CXCR4 and CD4, whereas down-modulation of CXCR4 by SDF-1 was similar for hybrids and wild-type CXCR4. In contrast, the bicyclam AMD3100, a nonpeptide CXCR4 ligand that did not down-modulate the hybrids, blocked hybrid-mediated infection at least as potently as for wild-type CXCR4. Thus SDF-1, but not the smaller molecule AMD3100, may interfere at multiple points with the binding of the surface unit (SU)-CD4 complex to CXCR4, a mechanism that the covalent linkage of CD4 to CXCR4 impedes. Although the CD4-CXCR4 hybrids yielded enhanced SU interactions with the chemokine receptor moiety, this did not overcome the specific coreceptor requirement of different HIV-1 strains: the X4 virus HIV-1(LAI) and the X4R5 virus HIV-1(89. 6), unlike the R5 strain HIV-1(SF162), infected Mv-1-lu cells expressing the CD4(2D)CXCR4 hybrid, but none could use hybrids of CD4 and the chemokine receptor CCR2b, CCR5, or CXCR2. Thus single-molecule hybrid constructs that mimic receptor-coreceptor complexes can be used to dissect coreceptor function and its inhibition.

Agonists and inverse agonists for the herpesvirus 8-encoded constitutively active seven-transmembrane oncogene product, ORF-74

A number of CXC chemokines competed with similar, nanomolar affinity against 125I-interleukin-8 (IL-8) binding to ORF-74, a constitutively active seven-transmembrane receptor encoded by human herpesvirus 8. However, in competition against 125I-labeled growth-related oncogene (GRO)-alpha, the ORF-74 receptor was highly selective for GRO peptides, with IL-8 being 10,000-fold less potent. The constitutive stimulating activity of ORF-74 on phosphatidylinositol turnover was not influenced by, for example, IL-8 binding. In contrast, GRO peptides acted as potent agonists in stimulating ORF-74 signaling, whereas IP-10 and stromal cell-derived factor-1alpha surprisingly acted as inverse agonists. These peptides had similar pharmacological properties with regard to enhancing or inhibiting, respectively, the stimulatory effect of ORF-74 on NIH-3T3 cell proliferation. Construction of a high affinity zinc switch through introduction of two His residues at the extracellular end of transmembrane segment V enabled Zn2+ to act as a prototype non-peptide inverse agonist, which eliminated the constitutive signaling. It is concluded that ORF-74, which is believed to be causally involved in the formation of highly vascularized tumors, has been optimized for agonist and inverse agonist modulation by the endogenous angiogenic GRO peptides and angiostatic IP-10 and stromal cell-derived factor-1alpha, respectively. ORF-74 could serve as a target for the development of non-peptide inverse agonist drugs as demonstrated by the effect of Zn2+ on the metal ion site-engineered receptor.

Selective recognition of the membrane-bound CX3C chemokine, fractalkine, by the human cytomegalovirus-encoded broad-spectrum receptor US28

The 7TM receptor, US28, encoded by human cytomegalovirus binds a broad spectrum of endogenous CC chemokines with sub-nanomolar affinity as determined in homologous competition binding assays. We here find that US28 also recognizes the membrane-associated CX3C chemokine, fractalkine, with sub-nanomolar affinity (IC50=0.42+/-0.09 nM). Importantly, although fractalkine could compete with high affinity against the binding of CC chemokines, the secreted CC chemokines were only able to compete for binding against radioactive fractalkine with very low affinity. It is concluded that US28, which is known to enhance cell-cell fusion processes through interaction with an as yet unidentified, human cell-specific factor, has been optimized by cytomegalovirus to selectively recognize the membrane-associated fractalkine. It is suggested that US28 expressed on the surface of infected cells and possibly on the envelope of the virion is involved in transfer of the virus from cell to cell.

Natural agonist enhancing bis-His zinc-site in transmembrane segment V of the tachykinin NK3 receptor

In the wild-type tachykinin NK3A receptor histidyl residues are present at two positions in TM-V, V:01 and V:05, at which Zn2+ functions as an antagonist in NK1 and kappa-opioid receptors with engineered metal-ion sites. Surprisingly, in the NK3A receptor Zn2+ instead increased the binding of the agonist 125I-[MePhe7]neurokinin B to 150%. [MePhe7]neurokinin B bound to the NK3A receptor in a two-component mode of which Zn2+ eliminated the subnanomolar binding mode but induced a higher binding capacity of the nanomolar binding mode. Signal transduction was not induced by ZnCl2 but 10 microM ZnCl2 enhanced the effect of neurokinin B. Ala-substitution of HisV:01 eliminated the enhancing effect of Zn2+ on peptide binding. It is concluded that physiological concentrations of Zn2+ have a positive modulatory effect on the binding and function of neurokinin B on the NK3A receptor through a bis-His site in TM-V.

Differential regulation of CXCR4 and CCR5 endocytosis

The chemokine receptors CCR5 and CXCR4 are major co-receptors/receptors for the CD4-dependent and CD4-independent entry of human and simian immunodeficiency viruses. The chemokines that bind and activate these receptors can inhibit the entry of viruses that use the respective co-receptor molecules. Chemokine-induced co-receptor internalisation is a significant component of the mechanism through which chemokines inhibit virus entry. CXCR4 internalisation is induced by the CXCR4 ligand stromal cell derived factor-1 (SDF-1), phorbol esters and, in T cells, cellular activation. Here we show that CXCR4 endocytosis can be mediated through either one of two distinct internalisation signals. A COOH-terminal serine rich domain is required for ligand- but not phorbol ester- induced CXCR4 internalisation. However, a Ser/IleLeu motif, similar to that required for the endocytosis of CD4 and the T cell receptor/CD3 complex, is required for phorbol ester-induced, but not ligand-induced, CXCR4 endocytosis. By contrast, CCR5 internalisation is induced by the beta-chemokine RANTES but not by phorbol esters. CCR5 lacks the Ser/IleLeu sequence required for phorbol ester-induced uptake of CXCR4. Together these results indicate that distinct mechanisms can regulate CXCR4 and CCR5 endocytosis and trafficking.

The CCR5 receptor acts as an alloantigen in CCR5Delta32 homozygous individuals: identification of chemokineand HIV-1-blocking human antibodies

The chemokine receptor CCR5 is the major coreceptor for infection by macrophage-tropic R5 HIV-1. A 32-bp deletion in the gene coding for CCR5 (CCR5Delta32) occurs with a frequency of 10% in the Caucasian population and results in a receptor protein that is truncated and not expressed at the cell surface. CCR5Delta32 homozygous individuals are apparently normal but resistant to infection with R5 HIV-1. In two individuals homozygous for CCR5Delta32, who had been repeatedly exposed to CCR5-expressing blood cells through sexual activity, we have identified antibodies to CCR5 that bound specifically to the surface of CCR5-expressing cell lines. Serum from these individuals, in contrast to serum from CCR5(+/+) individuals, competed with radiolabeled RANTES for binding to the CCR5 receptor and inhibited infection of peripheral blood mononuclear cells with R5, but not X4, primary isolates of HIV-1. The identified human antibodies to CCR5 define an alloantigen that may cause allograft rejection in a mismatch situation even in individuals with no history of blood transfusions or i.v. drug abuse.

Split-receptors in the tachykinin neurokinin-1 system--mutational analysis of intracellular loop 3

Several membrane proteins have been functionally expressed from non-covalently coupled, contiguous segments especially with the split-site located between natural domains. Experiments using such 'split-proteins' were here performed in the tachykinin neurokinin-1 (NK1) receptor with co-expression of contiguous segments with split-sites positioned in various intracellular and extracellular loops. The construct where the split-site was located in intracellular loop 3 gave a reasonable expression level of substance-P-binding sites, i.e. 12% of wild-type expression. Of the other split-receptors tested, only the one with the split-site located just outside transmembrane (TM) segment-V gave any detectable substance P binding, which however only was 1% of the wild-type expression level. The construct with the split-site located in intracellular loop 3 bound all of the tested peptide agonists and non-peptide antagonists with normal affinity and was able to stimulate inositol phosphate turnover with a normal EC50 for substance P and an Emax according to the expression level. When intracellular loop 3 was either extended with 112 amino acid residues derived from the muscarine M2 receptor or, when major parts of the loop were deleted in the non-split NK1 receptor, the affinity for neither substance P nor for the prototype nonpeptide antagonist, CP96,345 was affected, yet an increase in EC50 for substance P was observed. Also in the split-receptor, most of intracellular loop 3 could be substituted or even deleted without affecting ligand affinity, although a decreased expression level was observed in constructs having major deletions. It is concluded, that the NK1 receptor is preferentially reconstituted by co-expression of a putative A-domain including TM-I-V and a B-domain including TM-VI and -VII. It is suggested that a number of rhodopsin-like 7TM receptors may function as two-domain structures based on the finding that a network of short loops has been highly conserved within each of the putative domains and, that these domains are separated by a relatively long and in respect of length poorly conserved loop, i.e. intracellular loop 3.

Constitutive activity of glucagon receptor mutants

Increased constitutive activity has been observed in the PTH receptor in association with naturally occurring mutations of two residues that are conserved between members of the glucagon/vasoactive intestinal peptide/calcitonin 7TM receptor family. Here, the corresponding residues of the glucagon receptor, His178 and Thr352, were probed by mutagenesis. An elevated level of basal cAMP production was observed after the exchange of His178 into Arg, but not for the exchange into Lys, Ala, or Glu. However, for all of these His178 substitutions, an increased binding affinity for glucagon was observed [dissociation constant (Kd) ranging from 1.1-6.4 nM, wild type: Kd = 12.0 nM]. A further increase in cAMP production was observed for the [H178R] construct upon stimulation with glucagon, albeit the EC50 surprisingly was increased approximately 10-fold relative to the wild-type receptor. Substitution of Thr352, located at the intracellular end of transmembrane segment VI, with Ala led to a slightly elevated basal cAMP level, while the introduction of Pro or Ser at this position affected rather the binding affinity of glucagon or the EC50 for stimulation of cAMP production. The large extracellular segment, which is essential for glucagon binding, was not required for constitutive activation of the glucagon receptor as the introduction of the [H178R] mutation into an N-terminally truncated construct exhibited an elevated basal level of cAMP production. The analog des-His1-[Glu9]glucagon amide, which in vivo is a glucagon antagonist, was an agonist on both the wild-type and the [H178R] receptor and did not display any activity as an inverse agonist. It is concluded that the various phenotypes displayed by the constitutively active glucagon receptor mutants reflect the existence of multiple agonist-preferring receptor conformers, which include functionally active as well as inactive states. This view agrees with a recent multi-state extension of the ternary complex model for 7TM receptor activation.

Steric hindrance mutagenesis versus alanine scan in mapping of ligand binding sites in the tachykinin NK1 receptor

Residues in transmembrane domain (TM)-III, TM-V, TM-VI, and TM-VII believed to be facing the deep part of the presumed main ligand-binding pocket of the NK1 receptor were probed by alanine substitution and introduction of residues with larger and/or chemically distinct side chains. Unaltered or even improved binding affinity for four peptide agonists, substance P, substance P-O-methyl ester, eledoisin, and neurokinin A, as well as normal EC50 values for substance P in stimulating phosphatidylinositol turnover indicated that these mutations did not alter the overall functional integrity of the receptor. The alanine substitutions in general had only minor effects on nonpeptide antagonist binding. However, the introduction of the larger and polar aspartic acid and histidine residues at positions corresponding to the monoamine binding aspartic acid in TM-III of the beta 2-adrenoceptor (ProIII:08, Pro112 in the NK1 receptor) and to the presumed monoamine interacting "two serines" in TM-V (ThrV:09, Thr201; and IleV:12, Ile204) impaired by > 100-fold the binding of a group of nonpeptide antagonists, including CP96,345, CP99,994, RP67,580, RPR100,893, and CAM4092. In contrast, another group of nonpeptide antagonists, LY303,870, FK888, and SR140,333, were little or not at all affected by the space-filling substitutions. Two of these compounds, FK888 and LY303,870, were those most seriously affected (75-89-fold) by alanine substitution of PheVI:20 located in the upper part of the main ligand-binding crevice. Surprisingly, substitution of AlaIII:11 (Ala115), which is located in the middle of TM-III, conceivably pointing toward TM-VII, with a larger valine residue increased the affinity for all 13 ligands tested, presumably by creating a closer interhelical packing. It is concluded that the introduction of larger side chains at positions at which molecular models indicate that this is structurally allowed can be a powerful method of locating ligand-binding sites due to the considerable difference between positive and negative results. Such steric hindrance mutagenesis strongly indicates that one population of nonpeptide antagonists bind in the deep pocket of the main ligand-binding crevice of the NK1 receptor, whereas another group of nonpeptide antagonists, especially SR140,333, was surprisingly resistant to mutational mapping in this pocket.

Plagiarism of the host immune system: lessons about chemokine immunology from viruses

In their attempts to evade the host immune response, mammalian viruses have evolved a wide range of strategies. These include the expression and modification of various host cytokines and receptors. Understanding the mechanism of action of these virally encoded proteins will clearly deepen our insights into immunology. In the past few months several new virally encoded chemokines have been described which can modify both the host immune and antiviral response. Their manipulation of the cytokine structure-function relationship may also be useful in the development of reagents for treating immune and proliferative diseases.

Phorbol esters and SDF-1 induce rapid endocytosis and down modulation of the chemokine receptor CXCR4

The chemokine receptor CXCR4 is required, together with CD4, for entry by some isolates of HIV-1, particularly those that emerge late in infection. The use of CXCR4 by these viruses likely has profound effects on viral host range and correlates with the evolution of immunodeficiency. Stromal cell-derived factor-1 (SDF-1), the ligand for CXCR4, can inhibit infection by CXCR4-dependent viruses. To understand the mechanism of this inhibition, we used a monoclonal antibody that is specific for CXCR4 to analyze the effects of phorbol esters and SDF-1 on surface expression of CXCR4. On human T cell lines SupT1 and BC7, CXCR4 undergoes slow constitutive internalization (1.0% of the cell surface pool/min). Addition of phorbol esters increased this endocytosis rate >6-fold and reduced cell surface CXCR4 expression by 60 to 90% over 120 min. CXCR4 was internalized through coated pits and coated vesicles and subsequently localized in endosomal compartments from where it could recycle to the cell surface after removal of the phorbol ester. SDF-1 also induced the rapid down modulation (half time approximately 5 min) of CXCR4. Using mink lung epithelial cells expressing CXCR4 and a COOH-terminal deletion mutant of CXCR4, we found that an intact cytoplasmic COOH-terminal domain was required for both PMA and ligand-induced CXCR4 endocytosis. However, experiments using inhibitors of protein kinase C indicated that SDF-1 and phorbol esters trigger down modulation through different cellular mechanisms. SDF-1 inhibited HIV-1 infection of mink cells expressing CD4 and CXCR4. The inhibition of infection was less efficient for CXCR4 lacking the COOH-terminal domain, suggesting at least in part that SDF-1 inhibition of virus infection was mediated through ligand-induced internalization of CXCR4. Significantly, ligand induced internalization of CXCR4 but not CD4, suggesting that CXCR4 and CD4 do not normally physically interact on the cell surface. Together these studies indicate that endocytosis can regulate the cell-surface expression of CXCR4 and that SDF-1-mediated down regulation of cell-surface coreceptor expression contributes to chemokine-mediated inhibition of HIV infection.

A broad-spectrum chemokine antagonist encoded by Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus encodes a chemokine called vMIP-II. This protein displayed a broader spectrum of receptor activities than any mammalian chemokine as it bound with high affinity to a number of both CC and CXC chemokine receptors. Binding of vMIP-II, however, was not associated with the normal, rapid mobilization of calcium from intracellular stores; instead, it blocked calcium mobilization induced by endogenous chemokines. In freshly isolated human monocytes the virally encoded vMIP-II acted as a potent and efficient antagonist of chemotaxis induced by chemokines. Because vMIP-II could inhibit cell entry of human immunodeficiency virus (HIV) mediated through CCR3 and CCR5 as well as CXCR4, this protein may serve as a lead for development of broad-spectrum anti-HIV agents.

Characterization of non-peptide antagonist and peptide agonist binding sites of the NK1 receptor with fluorescent ligands

Ligand recognition of the NK1 receptor (substance P receptor) by peptide agonist and non-peptide antagonist has been investigated and compared by the use of fluorescent ligands and spectrofluorometric methods. Analogues of substance P (SP) labeled with the environment-sensitive fluorescent group 5-dimethylaminonaphthalene-1-sulfonyl (dansyl) at either position 3, 8, or 11 or with fluorescein at the Nalpha position were synthesized and characterized. Peptides modified at the alpha-amino group or at positions 3 or 11 conserved a relatively good affinity for NK1 and agonistic properties. Modification at position 8 resulted in an 18, 000-fold decrease in affinity. A fluorescent dansyl analogue of the non-peptide antagonist CP96,345 was prepared and characterized. The quantum yield of fluorescence for dansyl-CP96,345 was much higher than for any of the dansyl-labeled peptides indicating that the micro-environment of the binding site is more hydrophobic for the non-peptide antagonist than for the peptide agonists. Comparison of collisional quenching of fluorescence by the water-soluble hydroxy-Tempo compound showed that dansyl-CP96,345 is buried and virtually inaccessible to aqueous quenchers, whereas dansyl- or fluoresceinyl-labeled peptides were exposed to the solvent. Anisotropy of all fluorescent ligands increased upon binding to NK1 indicating a restricted motional freedom. However, this increase in anisotropy was more pronounced for the dansyl attached to the non-peptide antagonist CP96,345 than for the fluorescent probes attached to different positions of SP. In conclusion, our data indicate that the environment surrounding non-peptide antagonist and peptide agonists are vastly different when bound to the NK1 receptor. These results support recent observations by mutagenesis and cross-linking work suggesting that peptide agonists have their major interaction points in the N-terminal extension and the loops forming the extracellular face of the NK1 receptor. Our data also suggest that neither the C terminus nor the N terminus of SP appears to penetrate deeply below the extracellular surface in the transmembrane domain of the receptor.