Purification and biochemical characterization of the D6 chemokine receptor

Tuning inflammation and immunity by chemokine sequestration: decoys and more

A set of chemokine receptors are structurally unable to elicit migration or conventional signalling responses after ligand engagement. These 'silent' (non-signalling) chemokine receptors regulate inflammatory and immune reactions in different ways, including by acting as decoys and scavengers. Chemokine decoy receptors recognize distinct and complementary sets of ligands and are strategically expressed in different cellular contexts. Importantly, viruses and parasites have evolved multiple strategies to elude chemokines, including the expression of decoy receptors. So, decoy receptors for chemokines represent a general strategy to tune, shape and temper innate and adaptive immunity.

Chemokine scavenging by D6: a movable feast?

The atypical chemokine receptor, D6, is efficient at sequestering and scavenging inflammatory CC chemokines. The absence of D6 blocks the successful resolution of immune responses in models of inflammation, suggesting that CC-chemokine scavenging by D6 is an important component of the resolution phase of in vivo inflammatory responses. Most studies have suggested that lymphatic endothelial cells are the main vehicles for D6 function in vivo. Here, we propose that leukocytes, which also express D6, could be more-effective vehicles for D6 scavenging function. Thus, leukocytes might be the primary cell type that removes inflammatory chemokines from inflamed tissues. We also propose that lymphatic endothelial cell-expressed D6 might have a distinct but complementary role in restricting inflammatory leukocyte access to the lymphatic vasculature.

In vitro reconstitution and preparative purification of complexes between the chemokine receptor CXCR4 and its ligands SDF-1alpha, gp120-CD4 and AMD3100

CXCR4 belongs to the family of G protein-coupled receptors and mediates the various developmental and regulatory effects of the chemokine SDF-1alpha. In addition, CXCR4 acts as a co-receptor along with CD4 for the HIV-1 viral glycoprotein gp120. Recently, there has also been a small molecule described that antagonizes both SDF-1 and gp120 binding to CXCR4. The structural and mechanistic basis for this dual recognition ability of CXCR4 is unknown largely due to the technical challenges of biochemically producing the components of the various complexes. We expressed the human CXCR4 receptor using a modified baculovirus expression vector that facilitates a single step antibody affinity purification of CXCR4 to >80% purity from Hi5 cells. The recombinant receptor undergoes N-linked glycosylation, tyrosine sulfation and is recognized by the 12G5 conformation specific antibody against human CXCR4. We are able to purify CXCR4 alone as well as complexed with its endogenous ligand SDF-1, its viral ligand gp120, and a small molecule antagonist AMD3100 by ion-exchange chromatography. We anticipate that the expression and purification scheme described in this paper will facilitate structure-function studies aimed at elucidating the molecular basis for CXCR4 recognition of its endogenous chemokine and viral ligands.

Large-scale purification and characterization of human parathyroid hormone-1 receptor stably expressed in HEK293S GnTI− cells

Human parathyroid hormone-1 receptor (hPTHR1) belongs to class II of the G protein-coupled receptor (GPCR) family, whose members all contain a seven-transmembrane helix domain. The receptor regulates bone metabolism through interactions with its ligand, human parathyroid hormone (hPTH). For structural studies of the hPTHR1/hPTH complex, we constructed a mammalian cell line to stably express recombinant hPTHR1 in large-scale. The receptor was solubilized with dodecyl maltoside and purified with affinity chromatography. The purified receptor displayed restricted N-glycosylation as expected. Functionality was demonstrated: the hPTHR1 retained affinity for bPTH-(1-34) and specifically cross-linked to a radioiodinated bPTH-(1-34) analog. This work describes an approach for preparing milligram-scale quantities of receptor for elucidation of the structural biology of this seven-transmembrane GPCR.

Solubilization, purification, and mass spectrometry analysis of the human mu-opioid receptor expressed in Pichia pastoris

The human mu-opioid receptor was expressed in Pichia pastoris with or without EGFP at the N-terminal end. Expression yields of the recombinant proteins reached several tens of milligram of receptor per liter of culture medium in shacked flasks. Pharmacological studies using specific ligands demonstrated a typical opioid profile for the HuMOR-c-myc-his-tag construct, whereas the GFP-HuMOR-c-myc-his-tag receptor was unable to bind opioid drugs. The hexahistidine epitope-tagged receptors were purified by immobilized-nickel affinity chromatography. The identity of the purified mu-opioid receptor proteins was confirmed by Western blot and mass spectrometry analysis. In conclusion, the expression, solubilization, and purification strategies described herein allow to isolate very high quantities of purified receptor, up to 12 mg/L.

Angiogenesis and rhodopsin-like receptors: a role for N-terminal acidic residues?

Numerous rhodopsin-like G-protein coupling receptors induce or inhibit angiogenesis. The active human receptors include several chemokine receptors, apelin APJ receptor, neuropeptide Y Y2 receptor, Duffy antigen, and herpes virus-8 receptor. A common and striking feature of these receptors is the large fraction (up to 42%) of residues with anionic sidechains (Asp, Glu, and benzene anions Tyr, Trp, and Phe) in the N-terminal extracellular domain. These residues (which are frequently clustered) can assist the binding of ligand peptides, but should also support interactions that help tubular arraying of cells, e.g., via cationic bridges and/or hydrogen bonding with cell-connecting receptors such as integrins, or with proteins of the extracellular matrix.

Chemokine receptor internalization and intracellular trafficking

The internalization and intracellular trafficking of chemokine receptors have important implications for the cellular responses elicited by chemokine receptors. The major pathway by which chemokine receptors internalize is the clathrin-mediated pathway, but some receptors may utilize lipid rafts/caveolae-dependent internalization routes. This review discusses the current knowledge and controversies regarding these two different routes of endocytosis. The functional consequences of internalization and the regulation of chemokine receptor recycling will also be addressed. Modifications of chemokine receptors, such as palmitoylation, ubiquitination, glycosylation, and sulfation, may also impact trafficking, chemotaxis and signaling. Finally, this review will cover the internalization and trafficking of viral and decoy chemokine receptors.

Investigation of Ligand-Receptor Systems by High-Resolution Solid-State NMR: Recent Progress and Perspectives

Solid-state Nuclear Magnetic Resonance (NMR) provides a general method to study molecular structure and dynamics in a non-crystalline and insoluble environment. We discuss the latest methodological progress to construct 3D molecular structures from solid-state NMR data obtained under magic-angle-spinning conditions. As shown for the neurotensin/NTS-1 system, these methods can be readily applied to the investigation of ligand-binding to G-protein coupled receptors.

The chemokine receptor D6 limits the inflammatory response in vivo

How the inflammatory response is initiated has been well defined but relatively little is known about how such responses are resolved. Here we show that the D6 chemokine receptor is involved in the post-inflammatory clearance of beta-chemokines from cutaneous sites. After induction of inflammation by phorbol esters, wild-type mice showed a transient inflammatory response. However, in D6-deficient mice, an excess concentration of residual chemokines caused a notable inflammatory pathology with similarities to human psoriasis. These results suggest that D6 is involved in the resolution of the cutaneous inflammatory response.

Post-translational control of chemokines: a role for decoy receptors?

It is well-established that chemokines play a critical role in the orchestration of inflammation and immunity. Interactions between chemokines and their receptors are essential for the homing of specific subsets of leukocytes to their functional microenvironments. They also influence other diverse biological processes such as development, leukocyte activation, Th1/Th2 polarisation, tumour metastasis, angiogenesis, and HIV pathogenesis. However, despite their importance, only now are we beginning to understand the complex regulation brought to bear on these molecules. In this review, we discuss a number of these key chemokine regulators that exert their influence once these proteins have been synthesised. We examine (i) chemokine storage, release, and presentation, (ii) protease regulation, (iii) viral manipulation of host chemokines, and (iv) natural mammalian receptor antagonists. Principally, the growing evidence for a role for decoy receptors in the chemokine system is discussed. In particular, the potential decoy function of the 'silent' pro-inflammatory chemokine receptor D6 is described alongside two other candidate decoy receptor molecules, DARC, and CCX-CKR. Dissecting the biological and pathological function of these chemokine controllers will lead to a deeper understanding of chemokine regulation, and may reveal novel strategies to therapeutically modify the chemokine system.

The chemokine receptor D6 constitutively traffics to and from the cell surface to internalize and degrade chemokines

The D6 heptahelical membrane protein, expressed by lymphatic endothelial cells, is able to bind with high affinity to multiple proinflammatory CC chemokines. However, this binding does not allow D6 to couple to the signaling pathways activated by typical chemokine receptors such as CC-chemokine receptor-5 (CCR5). Here, we show that D6, like CCR5, can rapidly internalize chemokines. However, D6-internalized chemokines are more effectively retained intracellularly because they more readily dissociate from the receptor during vesicle acidification. These chemokines are then degraded while the receptor recycles to the cell surface. Interestingly, D6-mediated chemokine internalization occurs without bringing about a reduction in cell surface D6 levels. This is possible because unlike CCR5, D6 is predominantly localized in recycling endosomes capable of trafficking to and from the cell surface in the absence of ligand. When chemokine is present, it can enter the cells associated with D6 already destined for internalization. By this mechanism, D6 can target chemokines for degradation without the necessity for cell signaling, and without desensitizing the cell to subsequent chemokine exposure.