Recombinant Macaca mulatta (Rhesus macaque) C-C chemokine receptor type 2 (CCR2)

What is the molecular structure of recombinant rhesus CCR2?

Recombinant full-length Macaca mulatta CCR2 is a 360 amino acid protein that functions as a G protein-coupled receptor. The protein's amino acid sequence includes crucial structural domains typical of chemokine receptors, with transmembrane regions and extracellular loops involved in ligand binding. When expressed with an N-terminal His tag in E. coli systems, the protein maintains its structural integrity for experimental applications . The rhesus CCR2B receptor shares significant homology with human CCR2B, making it valuable for comparative studies and as a model system for human receptor research .

How is recombinant rhesus CCR2 typically expressed and purified?

Recombinant rhesus CCR2B has been successfully expressed using two primary systems:

• Mammalian expression system: The CCR2B coding region can be obtained by PCR from genomic rhesus DNA and expressed as stable transfectants in Chinese Hamster Ovary (CHO) cells. This approach allows proper folding and post-translational modifications essential for functional studies .

• Bacterial expression system: For structural and biochemical studies, rhesus CCR2 can be expressed in E. coli with an N-terminal His tag, typically resulting in preparations with greater than 90% purity as determined by SDS-PAGE .

Purified recombinant protein is commonly stored as a lyophilized powder and reconstituted in deionized sterile water to a concentration of 0.1-1.0 mg/mL. For long-term storage, adding 5-50% glycerol and aliquoting for storage at -20°C/-80°C is recommended, with repeated freeze-thaw cycles being avoided to maintain protein integrity .

What are the ligand binding properties of rhesus CCR2?

Rhesus CCR2B exhibits selective high-affinity binding to the monocyte chemoattractant protein (MCP) family of chemokines, with notably similar binding profiles for both rhesus and human ligands:

Most other CC family chemokines demonstrate little affinity for this receptor. Interestingly, eotaxin (primarily a CCR3 ligand) binds to rhesus CCR2B with relatively high affinity but shows no functional activity as either an agonist or antagonist in downstream assays . This binding profile makes rhesus CCR2 a valuable model for studying chemokine-receptor interactions.

How can researchers assess the functional activity of recombinant rhesus CCR2?

Multiple complementary approaches can be employed to assess functional activity:

• Surface expression verification: Flow cytometry using commercially available monoclonal anti-hCCR2B antibodies that cross-react with rhesus CCR2B can detect expression on recombinant systems or on monocytes in peripheral blood mononuclear cell preparations from rhesus whole blood .

• Ligand binding assays: Competition binding experiments using radiolabeled ligands like 125I-rhMCP-1 or 125I-hMCP-1 help determine binding affinities and selectivity profiles .

• Calcium mobilization assays: Measuring intracellular calcium release in response to chemokines provides functional readouts of receptor activation. The relative potencies of the MCP family chemokines in these assays typically correlate with their binding affinities .

• Antagonist studies: CCR2 antagonists like TAK-799 can be used to probe receptor function. TAK-799 demonstrates high affinity for rhesus CCR2B (Ki = 0.5 nM) and potently blocks MCP-1-induced calcium mobilization .

What is known about the dual function of rhesus CCR2 in signaling and scavenging?

CCR2 demonstrates remarkable dual functionality that distinguishes it from typical GPCRs:

• Signaling function: Like other G protein-coupled chemokine receptors, CCR2 promotes monocyte infiltration into tissues in response to CCL2, activating downstream signaling cascades through G protein coupling .

• Scavenging function: Similar to atypical chemokine receptors (ACKRs), CCR2 removes chemokine from the extracellular environment through internalization, thereby limiting CCL2 signaling .

This dual function allows CCR2 to both initiate and regulate chemokine responses, creating a self-limiting system. The mechanisms underlying these functions have been investigated using CRISPR knockout cell lines, revealing that CCR2 scavenges by constitutively internalizing to remove CCL2 from the extracellular space and recycling back to the cell surface for further rounds of ligand sequestration .

What molecular pathways regulate CCR2 internalization and recycling?

Research has revealed distinct pathways for constitutive versus ligand-induced internalization of CCR2:

Studies using CRISPR knockout cell lines showed that constitutive internalization continues unabated even in cells lacking all G proteins, GRKs, or β-arrestins, while ligand-induced internalization is severely compromised in these cells . This mechanistic separation allows CCR2 to maintain its homeostatic scavenging function independently of its signaling-induced trafficking.

How does CCR2-tropic SIVrcm differ from CCR5-tropic SIVs in infection patterns?

SIVrcm (from red-capped mangabeys) uniquely uses CCR2 as its primary coreceptor, unlike most SIV strains that primarily use CCR5. This distinctive feature results in several key differences in infection patterns:

• Viral dynamics: SIVrcm infects rhesus macaques with peak viremia occurring around 2 weeks post-infection, but viral loads typically decrease to undetectable levels within one month after inoculation .

• Target cell specificity: Unlike CCR5-tropic SIVs that deplete memory CD4+CCR5+ T cells in gut-associated lymphoid tissue, SIVrcm causes some decrease in CD4+ T cells in the gut during early infection, but proportions of memory CD4+CCR5+ T cells remain largely unaffected .

• Pathogenicity: SIVrcm appears nonpathogenic in rhesus macaques, and remarkably, serial passage through 9 rhesus macaques did not increase its virulence .

• Cross-protection potential: Prior infection with SIVrcm provides partial protection against subsequent challenge with pathogenic SIVmac251, with 100-fold lower levels of SIVmac251 in plasma compared to naive animals .

Why does SIVrcm utilize CCR2 instead of CCR5, and what are the evolutionary implications?

The preferential use of CCR2 by SIVrcm is hypothesized to be an evolutionary adaptation to host genetic factors:

• CCR5 deletion prevalence: Red-capped mangabeys (the natural host of SIVrcm) have a high frequency of a 24 base pair deletion in the CCR5 gene, analogous to the delta 32 mutation in humans .

• Selection pressure: This CCR5 deletion renders cells uninfectable by viruses that rely on CCR5 for entry, creating a strong selection pressure for the virus to adapt to alternative coreceptors .

• Viral adaptation: The shift to CCR2 usage represents a successful adaptation that allowed SIVrcm to persist in its natural host population despite the prevalence of CCR5 deletions .

This example of viral adaptation to host genetic barriers provides valuable insights for understanding potential pathways of viral evolution in human populations with CCR5 mutations and for developing strategies to prevent viral escape from entry inhibitors targeting specific coreceptors.

What makes rhesus CCR2 a useful model for human CCR2 in drug development?

Several properties make rhesus CCR2 particularly valuable for translational research and drug development:

• Structural and functional conservation: Rhesus CCR2B exhibits remarkably similar binding properties to human CCR2B for both natural ligands and synthetic antagonists, suggesting highly conserved binding pockets and activation mechanisms .

• Cross-species pharmacology: The dual CCR2/CCR5 antagonist TAK-799 demonstrates comparable high affinity for rhesus CCR2B (Ki = 0.5 nM) and effectively blocks MCP-1-induced calcium mobilization, indicating that rhesus models can predict human responses to CCR2-targeted therapeutics .

• Preclinical model validity: These similarities address a key concern in drug development - species selectivity of chemokine receptor antagonists - by confirming that rhesus monkey disease models can support preclinical assessments of compounds targeting CCR2 .

• Dual functionality conservation: The dual signaling and scavenging functions observed in human CCR2 are preserved in the rhesus receptor, allowing for comprehensive evaluation of compounds that might differentially affect these functions .

How do phosphorylation patterns regulate different aspects of CCR2 function?

Phosphorylation plays a critical role in regulating CCR2 functions, particularly through the action of G protein-coupled receptor kinases (GRKs). Research examining CCR2 mutants with altered phosphorylation sites has revealed:

• Phosphorylation targets: Key serine and threonine residues, particularly those located in the C-terminal region of CCR2, serve as targets for GRK-mediated phosphorylation .

• Functional specificity: Mutation of nine serine/threonine residues to alanine (CCR2-ST/9A) causes:

  • Only minor loss of CCL2 scavenging capability

  • Significant reduction in β-arrestin recruitment

  • Substantial decrease in CCL2-dependent internalization

  • No effect on constitutive internalization

• Pathway separation: The S356 residue at the extreme C-terminus of CCR2 is particularly important as it forms part of a putative class II PDZ-binding motif, which may be required for receptor trafficking or recycling .

These findings demonstrate that phosphorylation primarily regulates ligand-induced processes while having minimal impact on constitutive functions, suggesting distinct molecular mechanisms for these different aspects of receptor biology.

What methods are most effective for monitoring CCR2 trafficking in real-time?

Several complementary techniques have proven valuable for studying different aspects of CCR2 trafficking:

• BRET-based methods: Bioluminescence Resonance Energy Transfer using CCR2-RlucII and rGFP-CAAX BRET pairs effectively monitors ligand-induced receptor internalization. This approach can also be adapted using the early endosome marker rGFP-FYVE to specifically track endosomal trafficking .

• Limitations of BRET: While valuable for ligand-induced changes, BRET methods cannot detect constitutive internalization processes that occur continuously at equilibrium, as these create a stable baseline BRET state .

• "Pre-label" flow cytometry: This specialized technique was developed to monitor constitutive receptor trafficking:

  • Cell surface receptors are labeled at 4°C (preventing internalization)

  • Cells are warmed to 37°C to resume trafficking

  • Surface-remaining receptor is quantified with fluorescent secondary antibody

  • Decrease in fluorescence compared to 4°C controls indicates constitutive internalization

• Fluorescent ligand tracking: Fluorescently labeled chemokines (e.g., CCL2-mCherry) allow visualization of ligand uptake and intracellular trafficking in real-time, providing insights into scavenging mechanisms .

How can researchers distinguish between the signaling and scavenging functions of CCR2?

Separating these dual functions experimentally requires specialized approaches:

• Scavenging-specific assays: Measuring the disappearance of chemokine from culture media over time quantifies scavenging independently of signaling. This can be accomplished through ELISA measurements of remaining CCL2 or by tracking fluorescently labeled CCL2 .

• Signaling-specific readouts: Calcium flux assays provide immediate readouts of receptor signaling activity without being influenced by subsequent scavenging processes .

• Genetic separation of functions:

  • β-arrestin1/2 knockout cells show complete loss of CCL2-induced receptor internalization but maintain about 80% of CCL2 scavenging capability

  • GRK2/3/5/6 deletion similarly separates these functions

  • CCR2-ST/9A phosphorylation-deficient mutants maintain scavenging while losing much of their signaling capacity

• Pharmacological manipulation: G protein inhibition using Pertussis Toxin (PTx) blocks signaling while minimally affecting scavenging, providing a tool to isolate these functions .

This functional separation demonstrates that while these processes are typically linked in wild-type receptors, they rely on distinct cellular machinery and can be experimentally or pharmacologically uncoupled.

What are the optimal storage and handling conditions for recombinant rhesus CCR2?

Proper handling of recombinant CCR2 is critical for maintaining its functional integrity:

• Storage temperature: Store lyophilized powder at -20°C/-80°C upon receipt. For working aliquots, storage at 4°C for up to one week is acceptable .

• Reconstitution protocol: Briefly centrifuge vials before opening to bring contents to the bottom. Reconstitute in deionized sterile water to a concentration of 0.1-1.0 mg/mL .

• Long-term preservation: Add 5-50% glycerol (final concentration) and aliquot for long-term storage at -20°C/-80°C to prevent protein degradation from repeated freeze-thaw cycles .

• Buffer composition: Recombinant protein is typically provided in Tris/PBS-based buffer with 6% Trehalose at pH 8.0, which helps maintain stability .

• Quality control: Purity should be greater than 90% as determined by SDS-PAGE for reliable experimental results .

What controls should be included in experiments using recombinant rhesus CCR2?

Robust experimental design for CCR2 studies should include several key controls:

• Expression controls:

  • Untransfected parental cell lines to establish baseline

  • Flow cytometry verification of surface expression using anti-CCR2 antibodies

  • Western blot confirmation of protein expression at expected molecular weight

• Functional controls:

  • Known CCR2 ligands (MCP-1) as positive controls for binding and signaling

  • Unrelated chemokines as negative controls for specificity

  • Calcium flux responses to serial dilutions of agonists to establish dose-response relationships

• Specificity controls:

  • CCR2 antagonists like TAK-799 to confirm receptor-specific responses

  • Cells expressing related chemokine receptors (CCR5, CCR3) to verify selectivity

• Trafficking controls:

  • 4°C incubation conditions to block endocytosis as control for internalization studies

  • Dynamin inhibitors to distinguish clathrin-dependent from independent pathways

How can cross-species differences be addressed when using rhesus CCR2 as a model for human CCR2?

While rhesus and human CCR2 share significant homology, addressing potential species differences requires careful consideration:

How might understanding CCR2's dual function inform therapeutic development?

The dual signaling-scavenging function of CCR2 offers unique therapeutic opportunities:

• Function-selective targeting: Compounds that preferentially inhibit signaling while preserving scavenging could reduce inflammatory responses while maintaining homeostatic chemokine clearance .

• Pathway-specific modulators: Targeting specific GRKs or β-arrestin recruitment could selectively modulate CCR2 functions based on their differential dependence on these pathways .

• Constitutive activity modulation: Agents that enhance constitutive internalization without triggering signaling could promote chemokine clearance for anti-inflammatory effects .

• Trafficking machinery targeting: Compounds affecting the cellular machinery involved in CCR2 recycling could indirectly modulate receptor availability and function .

Understanding the molecular details separating these functions could lead to more precise pharmacological targeting with fewer off-target effects than current approaches.

What insights from CCR2-tropic SIV studies might be relevant to HIV research?

The unique properties of CCR2-tropic SIVrcm provide several valuable insights for HIV research:

• Alternative coreceptor usage: SIVrcm demonstrates how lentiviruses can adapt to use alternative coreceptors when primary coreceptors are genetically restricted, informing predictions about potential HIV evolution in populations with CCR5 mutations or under CCR5 inhibitor treatment .

• Pathogenicity determinants: The nonpathogenic nature of CCR2-tropic SIVrcm in rhesus macaques, despite successful infection, suggests that coreceptor usage alone does not determine pathogenic potential .

• Cross-protection mechanisms: The observation that prior SIVrcm infection provides partial protection against pathogenic SIVmac251 challenge (100-fold lower viral loads) suggests potential immune or cellular mechanisms that might be exploited for preventive or therapeutic approaches .

• Viral adaptation limitations: The inability of SIVrcm to increase its virulence despite serial passage through 9 rhesus macaques indicates inherent constraints on viral adaptation that might inform therapeutic resistance barriers .

What are the key technical challenges in studying the constitutive trafficking of CCR2?

Several technical challenges complicate the study of constitutive receptor trafficking:

• Detection limitations: Traditional BRET-based methods cannot detect constitutive processes at equilibrium, necessitating development of specialized techniques like the "pre-label" flow cytometry approach .

• Baseline stability: Constitutive processes create a stable baseline state, making it difficult to distinguish from technical background without appropriate controls and time-course measurements .

• Pathway overlap: The molecular machinery involved in constitutive trafficking may partially overlap with ligand-induced pathways, requiring careful genetic or pharmacological dissection .

• Real-time visualization: Tracking receptor molecules through constitutive cycles requires specialized microscopy approaches with high temporal and spatial resolution.

• Quantification challenges: Measuring the rate and efficiency of constitutive trafficking requires distinguishing newly synthesized receptors from recycled ones, often necessitating pulse-chase approaches or photoactivatable tags.

Overcoming these challenges will be essential for developing a complete understanding of CCR2 biology and for designing therapeutics that differentially target its various functions.