Biophysical and structural investigation of bacterially expressed and engineered CCR5, a G protein-coupled receptor

The chemokine receptor CCR5 belongs to the class of G protein-coupled receptors. Besides its role in leukocyte trafficking, it is also the major HIV-1 coreceptor and hence a target for HIV-1 entry inhibitors. Here, we report Escherichia coli expression and a broad range of biophysical studies on E. coli-produced CCR5. After systematic screening and optimization, we obtained 10 mg of purified, detergent-solubilized, folded CCR5 from 1L culture in a triply isotope-labeled ((2)H/(15)N/(13)C) minimal medium. Thus the material is suitable for NMR spectroscopic studies. The expected α-helical secondary structure content is confirmed by circular dichroism spectroscopy. The solubilized CCR5 is monodisperse and homogeneous as judged by transmission electron microscopy. Interactions of CCR5 with its ligands, RANTES and MIP-1β were assessed by surface plasmon resonance yielding K(D) values in the nanomolar range. Using size exclusion chromatography, stable monomeric CCR5 could be isolated. We show that cysteine residues affect both the yield and oligomer distribution of CCR5. HSQC spectra suggest that the transmembrane domains of CCR5 are in equilibrium between several conformations. In addition we present a model of CCR5 based on the crystal structure of CXCR4 as a starting point for protein engineering.

High-level production, solubilization and purification of synthetic human GPCR chemokine receptors CCR5, CCR3, CXCR4 and CX3CR1

Chemokine receptors belong to a class of integral membrane G-protein coupled receptors (GPCRs) and are responsible for transmitting signals from the extracellular environment. However, the structural changes in the receptor, connecting ligand binding to G-protein activation, remain elusive for most GPCRs due to the difficulty to produce them for structural and functional studies. We here report high-level production in E.coli of 4 human GPCRs, namely chemokine receptors (hCRs) CCR5, CCR3, CXCR4 and CX3CR1 that are directly involved in HIV-1 infection, asthma and cancer metastasis. The synthetic genes of CCR5, CCR3, CXCR4 and CX3CR1 were synthesized using a two-step assembly/amplification PCR method and inserted into two different kinds of expression systems. After systematic screening of growth conditions and host strains, TB medium was selected for expression of pEXP-hCRs. The low copy number pBAD-DEST49 plasmid, with a moderately strong promoter tightly regulated by L-arabinose, proved helpful for reducing toxicity of expressed membrane proteins. The synthetic Trx-hCR fusion genes in the pBAD-DEST49 vector were expressed at high levels in the Top10 strain. After a systematic screen of 96 detergents, the zwitterionic detergents of the Fos-choline series (FC9-FC16) emerged as the most effective for isolation of the hCRs. The FC14 was selected both for solubilization from bacterial lysates and for stabilization of the Trx-hCRs during purification. Thus, the FC-14 solubilized Trx-hCRs could be purified using size exclusion chromatography as monomers and dimers with the correct apparent MW and their alpha-helical content determined by circular dichroism. The identity of two of the expressed hCRs (CCR3 and CCR5) was confirmed using immunoblots using specific monoclonal antibodies. After optimization of expression systems and detergent-mediated purification procedures, we achieved large-scale, high-level production of 4 human GPCR chemokine receptor in a two-step purification, yielding milligram quantities of CCR5, CCR3, CXCR4 and CX3CR1 for biochemical, biophysical and structural analysis.

A biosensor-based approach toward purification and crystallization of G protein-coupled receptors

Biacore technology was used to develop an affinity purification method and screen cocrystallization conditions for the chemokine receptor CCR5. We characterized the binding of nine HIV gp120 variants and identified a truncated construct (YU2DV1V2) that bound CCR5 independent of CD4. This construct was used in an affinity purification step to improve the activity of detergent-solubilized receptor by approximately 300%. The biosensor was also used to screen receptor binding activity automatically under 50 different crystallization conditions. We found that high-molecular-weight polyethylene glycols (PEGs 4,000 and 8,000 Da) most often stabilized the receptor and improved complex formation with potential cocrystallization partners such as conformationally sensitive monoclonal antibodies and gp120. Our results show how biosensors can provide unique insights into receptor purification methods and reveal the effects of crystallization conditions on complex formation. Importantly, these methods can be readily applied to other systems.

Construction, purification, and immunogenicity of recombinant cystein-cystein type chemokine receptor 5 vaccine

Cystein-Cystein type chemokine receptor 5 (CCR5) is a seven-transmembrane, G-protein coupled receptor. It is a major coreceptor with CD4 glycoprotein mediating cellular entry of CCR5 strains of HIV-1. A lack of cell-surface expression of CCR5 found in the homozygous Delta32 CCR5 mutation, upregulation of CC chemokines and antibodies to CCR5 are associated with resistance to HIV infection. In addition, CCR5 can be blocked by three CC chemokines and antibodies to three extracellular domains of CCR5. Consequently, CCR5 is considered an attractive therapeutic target against HIV infection. In the current study, we constructed a recombinant vaccine by coupling a T helper epitope AKFVAAWTLKAA (PADRE) to the N terminus of CCR5 extracellular domains (PADRE-CCR5) and expressed this protein in Escherichia coli. We have developed an inexpensive and scalable purification process for the fusion protein from inclusion bodies and the final yields of 6mg purified fusion protein per gram of cell paste was obtained. The immunogenicity of the recombinant vaccine generated was examined in BALB/c mice. Sera from the vaccinated mice demonstrated high-titer specific antibodies to the recombinant vaccine, suggesting that PADRE-rCCR5 may be used as a candidate of active CCR5 vaccine.

Solubilization, stabilization, and purification of chemokine receptors using biosensor technology

Establishing solubilization conditions for membrane-associated receptors is often a tedious empirical process. Here we describe a novel application of SPR biosensor technology to screen solubilization conditions automatically and to assess receptor activity directly. We focus on two chemokine receptors, CXCR4 and CCR5, which are important in HIV cell invasion. The autosampler in Biacore 3000 permitted whole cells expressing C-terminally tagged receptors to be automatically lysed under a given solubilization condition and the lysates to be injected over an antibody surface. The total amount of solubilized receptor could be quantitated from the antibody capture level, whereas the amount of active receptor could be quantitated using a subsequent injection of conformationally sensitive antibody or protein. Using this approach, we identified detergent/lipid/buffer combinations that enhanced and maintained receptor activity. We also used the biosensor to demonstrate CD4-dependent binding of gp120 to solubilized CCR5 and to develop affinity chromatography-based purification methods that increased receptor activity more than 300%. Together, these results illustrate the benefits of using the biosensor as a tool for isolating functional membrane receptors and for analyzing ligand/receptor interactions.

Specific reactions between purified HIV-1 particles and CD4+ cell membrane fragments in a cell-free system of virus fusion or entry

The initial step of human immunodeficiency virus type 1 (HIV-1) infection has been studied by Env-mediated fusion or entry assays with appropriate cells expressing CD4 or CXCR4/CCR5 receptors in cultures, where many factors underlying cellular activities likely regulate the fusion/entry efficiency. Here we attempted to develop a more simplified in vitro cell-free fusion/entry reaction that mimics HIV-1 infection in cultures. Membrane fragments of target cells and intact infectious HIV-1 particles were purified, mixed and incubated. The core p24 protein was released from the purified virions and detected by ELISA without detergents in the supernatant of the reaction mixtures. This release reaction proceeded temperature-dependently and in a dose-dependent manner between the virion and membrane fractions, and was specific for HIV-1 Env and CD4. Env-deleted or VSV-G-pseudotyped HIV-1 released little p24, if any. Pretreatment of the membrane fragments with anti-CD4 antibodies inhibited the p24 induction from both X4-tropic and R5-tropic HIV-1. Furthermore, X4 but not R5 HIV-1 reacted with the membrane prepared from intrinsically CXCR4-positive HeLa-CD4 cells, whereas both viruses reacted with that prepared from CCR5-transduced HeLa-CD4 cells, indicating that this cell-free reaction mimics coreceptor usage of HIV-1 infection. Therefore, a potent entry inhibitor of X4 HIV-1, SDF-1alpha, blocked the release from X4 but not R5 HIV-1. Inversely, a weak entry inhibitor of R5 HIV-1, MIP-1beta, partially affected only the release from R5 HIV-1. These results suggest that this cell-free reaction system provides a useful tool to study biochemical fusion/entry mechanisms of HIV-1 and its inhibitors.

Methadone enhances human immunodeficiency virus infection of human immune cells

Opiate abuse has been postulated to be a cofactor in the immunopathogenesis of acquired immunodeficiency syndrome (AIDS). This study evaluated whether methadone, a drug widely prescribed for the treatment of drug abusers with opioid dependence, affects human immunodeficiency virus (HIV) infection of human immune cells. When added to human fetal microglia and blood monocyte-derived macrophage cultures, methadone significantly enhanced HIV infection of these cells. This enhancement was associated with the up-regulation of expression of CCR5, a primary coreceptor for macrophage-tropic HIV entry into macrophages. Most importantly, the addition of methadone to the cultures of latently infected peripheral blood mononuclear cells from HIV-infected patients enhanced viral activation and replication. Although the in vivo relevance of these findings remains to be determined, the data underscore the necessity of further studies to define the role of opioids, including methadone, in the immunopathogenesis of HIV infection and AIDS.

Enhanced expression, native purification, and characterization of CCR5, a principal HIV-1 coreceptor

Seven-transmembrane segment, G protein-coupled receptors (GPCRs) play important roles in many biological processes in which pharmaceutical intervention may be useful. High level expression and native purification of GPCRs are important steps in the biochemical and structural characterization of these molecules. Here, we describe enhanced mammalian cell expression and purification of a codon-optimized variant of the chemokine receptor CCR5, a GPCR that plays a central role in the entry of the human immunodeficiency virus-1 (HIV-1) into immune cells. CCR5 could be solubilized in its native state as determined by its ability to be precipitated by 2D7, a conformation-dependent anti-CCR5 antibody, and by the HIV-1 gp120 envelope glycoprotein. The 2D7 antibody recognized immature and mature forms of CCR5 equally, whereas gp120 preferentially recognized the mature form, a result that underscores a role for posttranslational modification of CCR5 in its HIV-1 coreceptor function. The methods described herein contribute to the analysis of CCR5 and are likely to be applicable to many other GPCRs.

Lack of protection from HIV infection by the mutant HIV coreceptor CCR5 in intravenously HIV infected hemophilia patients

The CCR5 chemokine receptor is an important coreceptor for macrophage-tropic HIV strains. Homozygous carriers of the mutated CCR5 receptor with a 32 bp deletion (delta 32-CCR5) are highly protected against HIV infection. A protective effect has also been described for heterozygous individuals carrying both mutated and wildtype CCR5 receptors. We compared the frequency of the mutated delta 32-CCR5 HIV coreceptor in HIV positive patients infected by sexual contact (N = 160) with intravenously HIV infected hemophilic patients (N = 84) and HIV negative individuals (N = 421). We found no protective effect of delta 32-CCR5 HIV coreceptor in hemophilic patients (p = 0.0134). If proteins of plasma concentrates would be responsible for facilitating the entry of HIV macrophages by upregulation of the CCR5 wildtype receptor it would be of therapeutical interest to identify the responsible plasma proteins.

Gene organization and promoter function for CC chemokine receptor 5 (CCR5)

CC chemokine receptor 5 (CCR5) functions physiologically as a receptor for the leukocyte chemoattractants macrophage inflammatory protein-1alpha, macrophage inflammatory protein-1beta, and RANTES, and functions pathologically as a key cell entry coreceptor for HIV-1. The factors that regulate CCR5 expression may be useful therapeutic targets for HIV-1 infection. To identify nuclear regulatory factors, we have located and functionally characterized the CCR5 gene promoter. The gene consists of two exons separated by a 1.9-kb intron. Exon 1 contains 43 bp of the 5'-untranslated region; exon 2 contains 11 bp of the 5'-untranslated region and the complete open reading frame. Primer extension analysis identified two adjacent transcriptional start points (tsp) that map to the first 2 bp found in the longest known CCR5 cDNA sequence. A TATA box is present 31 bp upstream from the first tsp. CCR5 mRNA was detected constitutively in both primary human myeloid and lymphoid cells by Northern blot hybridization. Consistent with this, transcription of a chloramphenicol acetyltransferase reporter gene was constitutively activated in both transiently transfected myeloid and lymphoid cell lines by the 80-bp gene fragment located immediately upstream of the tsp. Deletion analysis located a strong silencer element between nucleotides -244 and -80, and a strong enhancer element between -486 and -244. These results suggest that the gene region between -486 and -1 may regulate the expression of CCR5 in monocyte/macrophages and T lymphocytes.