Recombinant Mouse C-C chemokine receptor-like 2 (Ccrl2)

What is C-C chemokine receptor-like 2 (Ccrl2) and what is its relationship to other chemokine receptors?

Ccrl2 belongs to the G protein-coupled receptor family and serves as one of three known chemerin receptors. It is structurally related to the chemokine receptors CCR1, CCR2, CCR3, and CCR5, sharing significant sequence homology . Unlike typical chemokine receptors, Ccrl2 is considered a non-signaling atypical chemokine receptor that primarily functions by presenting the ligand chemerin to cells expressing the functional chemerin receptors ChemR23/CMKLR1 and possibly GPR1 . This places Ccrl2 functionally similar to the atypical chemokine receptor (ACKR) family, though it has traditionally been classified separately.

What are the primary ligands for Ccrl2 and how does it interact with them?

Chemerin (encoded by the Rarres2 gene) is the primary ligand known to interact with Ccrl2. Unlike typical chemokine receptors, Ccrl2 does not trigger conventional G protein-coupled signaling cascades upon ligand binding . Instead, it functions by binding and presenting chemerin to cells expressing the signaling-competent receptors CMKLR1 (ChemR23) and potentially GPR1 . This presentation mechanism enables Ccrl2 to concentrate bioactive chemerin in the local microenvironment, thereby enhancing chemerin's effects on nearby CMKLR1-expressing cells without directly transducing signals itself .

What are the most effective methods for studying Ccrl2-chemerin binding interactions?

Studying Ccrl2-chemerin binding interactions effectively requires a multi-faceted approach:

• Molecular Dynamics Simulations: Accelerated molecular dynamics (aMD) provides valuable insights into Ccrl2-chemerin binding. This technique reduces energy barriers between different low-energy states, enhancing the sampling of conformational space . Implementation involves:

  • Constructing homology models of CCRL2 using templates such as US28 (PDB: 5WB1)

  • Model refinement with tools like ModRefiner

  • Embedding the receptor in a phospholipid bilayer (typically 20% cholesterol, 80% POPC)

  • Running simulations followed by principal component analysis (PCA) to identify key binding interactions

• Real-time Chemotaxis Assays: For functional studies, the ACEA RTCA-DP instrument can measure cell migration in response to chemerin in real-time, comparing wild-type and Ccrl2-deficient cells . Key metrics include:

  • Cell index (CI) measurements taken every 5 seconds over 3-4 hours

  • Slope calculation during the first 40 minutes of the assay

  • Maximum CI minus minimum CI values (Max-Min) over the entire experiment

• Flow Cytometry: For quantifying Ccrl2 expression levels on different cell populations, flow cytometry using specific anti-Ccrl2 antibodies remains the gold standard, with particular attention to expression differences under inflammatory conditions .

What are the best experimental models for studying Ccrl2 function in inflammation and cancer?

Several well-established experimental models have proven valuable for investigating Ccrl2 function:

• Acute Peritoneal Inflammation Model:

  • Implementation: Intraperitoneal injection of inflammatory stimuli (e.g., 2% Bio-gel or zymosan) in wild-type and Ccrl2-knockout mice

  • Readouts: Peritoneal lavage to quantify recruited inflammatory cells (neutrophils, monocytes); measurement of cytokine/chemokine levels (particularly CXCL1); blood sampling to assess systemic effects

  • Timeline: Early time points (2-4 hours) are critical for observing Ccrl2-dependent differences in inflammatory responses

• Cancer Models:

  • Two-stage Chemical Skin Carcinogenesis (DMBA/TPA): This model allows assessment of Ccrl2's role in de novo tumor development, with papilloma counts, size, and progression to carcinoma as key endpoints

  • Tumor Cell Graft Models: Subcutaneous injection of B16 melanoma or LLC cells into wild-type and Ccrl2-knockout mice, with tumor growth monitoring over time

  • Genetic Manipulation Approaches:

    • CRISPR-Cas9 knockout of Ccrl2 in tumor cell lines

    • Stable overexpression of codon-optimized Ccrl2 in tumor cells

    • Combined host/tumor cell genetic manipulation

• Combinatorial Genetic Models: Crossing Ccrl2-knockout mice with mice deficient in chemerin, Cmklr1, or Gpr1 enables dissection of the specific contributions of different components of the chemerin/receptor system .

What quality control measures should be implemented when working with recombinant mouse Ccrl2?

When working with recombinant mouse Ccrl2, implement the following quality control measures:

• Expression Verification:

  • PCR validation of gene insertion/deletion

  • Flow cytometry confirmation of protein expression using specific antibodies

  • Western blot analysis for protein size and integrity verification

• Functional Validation:

  • Chemerin binding assays to confirm ligand interaction

  • Chemotaxis assays with CMKLR1-expressing cells to verify CCRL2's ability to present chemerin

• Genetic Validation for Knockout Models:

  • PCR amplification followed by DNA heteroduplex mobility assay

  • Sequencing of cloned PCR products to confirm frameshift mutations

  • Validation across multiple independent clones to rule out off-target effects

• Structural Quality Assessment for Recombinant Protein:

  • Backbone geometry validation using tools like MolProbidity

  • Side chain geometry verification

  • Proper folding confirmation through circular dichroism or other spectroscopic methods

How does Ccrl2 expression by tumor cells versus stromal cells differentially impact tumor progression?

The role of Ccrl2 in tumor development reveals a complex context-dependent pattern:

• Host Ccrl2 Expression Effects:

  • In the DMBA/TPA chemical carcinogenesis model, where all cells (including developing tumor cells) lack Ccrl2, knockout mice develop tumors more rapidly and with greater progression to malignancy than wild-type controls

  • This suggests that global loss of Ccrl2 promotes tumor development in de novo carcinogenesis

• Tumor Cell Ccrl2 Expression Effects:

  • When B16 or LLC tumor cells express Ccrl2 (either naturally in vivo or through overexpression), tumor growth is significantly delayed

  • Knockout of Ccrl2 specifically in tumor cells using CRISPR-Cas9 reverses the growth delay observed in Ccrl2-knockout hosts

  • These findings indicate that Ccrl2 expression by tumor cells has anti-tumorigenic effects

• Mechanistic Integration:

  • Ccrl2 expressed by tumor cells concentrates bioactive chemerin in the tumor microenvironment

  • This concentrated chemerin activates CMKLR1-expressing cells, leading to reduced tumor angiogenesis and potentially enhanced immune surveillance

  • The anti-tumoral effects of tumor cell Ccrl2 expression are largely abrogated in chemerin-knockout or CMKLR1-knockout mice, confirming the dependency on the chemerin/CMKLR1 axis

This differential impact highlights the importance of considering cell-specific expression patterns when studying Ccrl2 in cancer contexts and explains seemingly contradictory experimental findings.

What are the mechanisms by which Ccrl2 regulates acute inflammatory responses?

Ccrl2 regulates acute inflammatory responses through several interconnected mechanisms:

• Chemerin Presentation and Availability:

  • Ccrl2 binds chemerin without triggering internalization, effectively concentrating the ligand on cell surfaces

  • This concentration affects local chemerin gradients and availability for interaction with signaling receptors like CMKLR1

  • In Ccrl2-knockout mice, disrupted chemerin handling leads to altered chemerin levels and distribution

• Modulation of Neutrophil Recruitment:

  • Ccrl2-deficient mice show approximately two-fold higher neutrophil recruitment to sites of acute inflammation at early time points (4 hours)

  • This enhanced recruitment is associated with increased levels of the neutrophil chemoattractant CXCL1 both locally and systemically

  • The effect can be recapitulated in wild-type mice by injection of recombinant chemerin and abrogated in Ccrl2-knockout mice by anti-chemerin antibodies

• Temporal Regulation:

  • Ccrl2's regulatory effects appear most significant during the initial stages of inflammation

  • By later time points, differences in inflammatory cell numbers between wild-type and Ccrl2-knockout mice diminish, suggesting Ccrl2 primarily modulates the onset rather than the resolution of inflammation

These findings position Ccrl2 as a negative regulator of early inflammatory responses, likely through its ability to sequester and present chemerin in ways that limit excessive neutrophil recruitment.

How do the structural features of Ccrl2 compare to conventional chemokine receptors and other atypical chemokine receptors?

Ccrl2 shares structural similarities with conventional chemokine receptors but also possesses distinctive features that reflect its atypical function:

• Membrane Topology and Domain Organization:

  • Like conventional chemokine receptors, Ccrl2 has a seven-transmembrane domain architecture characteristic of G protein-coupled receptors

  • The homology modeling of human CCRL2 using US28 as a template yields structures with good backbone geometry (RMSD 0.776 Å), confirming structural conservation

• Ligand Binding Domain Differences:

  • Molecular dynamics simulations identify specific "hot-spot" residues involved in chemerin binding that differ from conventional chemokine receptors

  • The binding pocket appears optimized for ligand presentation rather than signal transduction

• Structural Basis for Non-signaling Properties:

  • Despite similarities in extracellular domains, Ccrl2 likely has altered intracellular coupling domains that prevent efficient G protein interaction

  • This structural distinction aligns with Ccrl2's functional classification alongside atypical chemokine receptors (ACKRs), which similarly bind chemokines without triggering classical G protein signaling

• Conformational States:

  • Active state modeling suggests Ccrl2 can adopt conformations that enable ligand binding but not the conformational changes necessary for signal transduction

  • Principal component analysis of molecular dynamics trajectories reveals distinct conformational states that likely represent different functional modes of the receptor

These structural comparisons highlight how subtle differences in receptor architecture translate to the distinctive functional properties that define Ccrl2 as a non-signaling, chemerin-presenting receptor.

How can researchers reconcile contradictory findings regarding Ccrl2's role in different disease models?

The apparently conflicting findings regarding Ccrl2's role in different disease models can be reconciled through careful consideration of several factors:

• Cell Type-Specific Expression:

  • The primary source of Ccrl2 expression varies between models

  • In the DMBA/TPA skin carcinogenesis model, Ccrl2 is absent from all cells in knockout mice

  • In tumor graft models, expression by tumor cells versus stromal cells can have opposing effects

• Tissue Microenvironment Context:

  • The inflammatory status of the tissue alters Ccrl2 expression and function

  • Different disease models present distinct inflammatory milieus that may influence how Ccrl2 impacts disease progression

• Temporal Dynamics:

  • Ccrl2's effects may vary between early and late stages of disease

  • In acute inflammation, Ccrl2's impact is most pronounced during early phases (2-4 hours) but diminishes at later time points

  • Similarly, in cancer models, effects may differ between initiation, promotion, and progression phases

• Interaction with Parallel Signaling Systems:

  • The availability and activity of chemerin and expression of CMKLR1/GPR1 must be considered

  • Experiments in mice lacking chemerin, CMKLR1, or GPR1 demonstrate that Ccrl2's effects are largely dependent on a functional chemerin/CMKLR1 axis

For accurate interpretation, researchers should thoroughly characterize Ccrl2 expression patterns, measure chemerin levels, and assess CMKLR1/GPR1 expression in their specific model systems.

What methodological approaches can address data inconsistencies in Ccrl2 research?

To address potential inconsistencies in Ccrl2 research data, consider implementing the following methodological approaches:

• Comprehensive Genetic Controls:

  • Use multiple complementary genetic approaches:

    • Global knockout models

    • Cell type-specific conditional knockouts

    • CRISPR-Cas9 editing in cell lines

    • Overexpression systems

  • Validate findings across multiple independent clones or founder lines

• Multi-level Expression Analysis:

  • Quantify Ccrl2 at both mRNA (qRT-PCR, RNAseq) and protein levels (flow cytometry, immunohistochemistry)

  • Compare expression in vitro versus in vivo (particularly important for tumor cells)

  • Distinguish between expression in different cell populations within heterogeneous tissues

• Functional Validation:

  • Complement genetic approaches with pharmacological interventions:

    • Anti-chemerin neutralizing antibodies

    • Recombinant chemerin administration

  • Use receptor antagonists when available

• Systematic Parameter Variation:

  • Examine temporal dynamics with detailed time-course experiments

  • Vary inflammatory stimuli type and concentration

  • Test across multiple disease models with shared pathophysiological mechanisms

• Data Integration Framework:

  Experimental Approach	Parameters to Standardize	Key Readouts	Potential Confounding Factors
  Inflammation Models	Stimulus type, dose, timing	Cell recruitment metrics, cytokine profiles	Strain background, sex, age
  Tumor Graft Models	Cell line, passage number, injection site	Tumor volume, growth rate, angiogenesis	Tumor cell Ccrl2 expression level
  Molecular Dynamics	Template selection, membrane composition	Binding interactions, conformational states	Force field parameters, simulation length

By implementing these approaches systematically, researchers can better reconcile seemingly contradictory findings and build a more coherent understanding of Ccrl2 biology.

What are the most promising future research directions for understanding Ccrl2 function in immunity and cancer?

Several high-priority research directions show particular promise for advancing our understanding of Ccrl2 biology:

• Targeted Cell-Type Specific Manipulation:

  • Develop conditional Ccrl2 knockout/knockin models to distinguish the roles of Ccrl2 on specific cell populations:

    • Myeloid-specific deletion (using LysM-Cre)

    • Endothelial-specific deletion (using Tie2-Cre)

    • Tumor cell-specific expression systems

  • These approaches would help resolve the cell-specific contributions to observed phenotypes

• Molecular Mechanisms of Chemerin Presentation:

  • Investigate how Ccrl2 structurally presents chemerin to CMKLR1

  • Explore potential direct Ccrl2-CMKLR1 interactions or microdomain organization

  • Develop biosensors to visualize chemerin gradient formation in real-time

• Integration with Other Inflammatory Pathways:

  • Examine cross-talk between the Ccrl2/chemerin/CMKLR1 axis and established inflammatory pathways:

    • TLR signaling networks

    • Cytokine feedback loops

    • Resolution mediator systems

  • Identify potential synergistic or antagonistic interactions

• Therapeutic Targeting Strategies:

  • Develop tools to modulate Ccrl2 function:

    • Small molecule inhibitors based on structure-activity relationships

    • Cell-targeted gene therapy approaches

    • Engineered chemerin variants with altered receptor specificity

  • Evaluate whether enhancing Ccrl2 expression by tumor cells could serve as an anti-cancer strategy

• Translational Studies:

  • Correlate findings from mouse models with human patient samples

  • Assess whether CCRL2 expression patterns in human tumors correlate with prognosis

  • Determine if genetic variants in human CCRL2 are associated with inflammatory disease susceptibility or cancer outcomes

These research directions would not only advance our fundamental understanding of Ccrl2 biology but could potentially open new therapeutic avenues for inflammatory diseases and cancer.

What are the recommended protocols for generating and validating Ccrl2-overexpressing cell lines?

The following protocol outlines the key steps for generating and validating Ccrl2-overexpressing cell lines:

• Vector Construction:

  • Synthesize a codon-optimized version of mouse Ccrl2 for improved expression

  • Clone into an appropriate expression vector with a strong promoter (e.g., CMV) and selection marker

  • Verify the construct by sequencing prior to transfection

• Stable Transfection:

  • Plate target cells (e.g., B16, LLC) at 60-70% confluence

  • Transfect using lipofection or electroporation methods

  • Begin selection with appropriate antibiotic 48 hours post-transfection

  • Isolate and expand individual clones

• Expression Validation:

  • mRNA level: Confirm transcript expression by RT-PCR and quantify using qRT-PCR

  • Protein level: Validate surface expression by flow cytometry using anti-Ccrl2 antibodies

  • Functional validation: Perform chemerin binding assays to confirm ligand interaction capacity

• Clone Selection Criteria:

  • Select multiple independent clones with similar growth rates to control cells

  • Verify that selected clones maintain stable expression over multiple passages

  • Ensure clones have comparable baseline characteristics to parent cell line

• Troubleshooting Common Issues:

  Problem	Possible Cause	Solution
  Low expression level	Poor codon optimization	Use synthesized codon-optimized sequence
  Expression decreases over time	Promoter silencing	Consider using different promoters or lentiviral integration
  High clone-to-clone variability	Integration site effects	Screen multiple clones and pool those with similar expression
  Altered cell growth/behavior	Receptor overexpression toxicity	Titrate expression using inducible systems

By following this protocol and addressing potential issues systematically, researchers can generate reliable Ccrl2-overexpressing cell lines for functional studies.

How should researchers design experiments to investigate the interplay between Ccrl2 and the chemerin/CMKLR1 axis?

Effectively investigating the interplay between Ccrl2 and the chemerin/CMKLR1 axis requires careful experimental design:

• Genetic Approach Matrix:

  • Utilize combinations of the following genetic backgrounds:

    • Wild-type controls

    • Ccrl2-knockout mice/cells

    • Chemerin-knockout mice/cells

    • CMKLR1-knockout mice/cells

    • GPR1-knockout mice/cells

    • Double knockout combinations (e.g., CMKLR1/GPR1)

  • This systematic approach allows determination of which components are essential for observed phenotypes

• Chemerin Concentration and Gradient Analysis:

  • Measure local chemerin concentrations using ELISA

  • Assess bioactivity using reporter cell lines expressing CMKLR1

  • Compare free versus cell-bound chemerin fractions

  • Evaluate chemerin processing/activation state

• Cell Co-culture Systems:

  • Design co-cultures combining:

    • Ccrl2-expressing cells (presenting chemerin)

    • CMKLR1-expressing cells (responding to chemerin)

    • Chemerin-producing cells

  • Analyze how spatial organization affects response magnitudes

• In vivo Visualization:

  • Implement intravital microscopy to visualize cell recruitment dynamics

  • Use fluorescently labeled cell populations to track movement relative to chemerin gradients

  • Correlate cellular behaviors with local expression of receptors

• Response Measurement Framework:

  Experimental Context	Primary Readouts	Secondary Readouts
  Inflammation	Neutrophil/monocyte recruitment, CXCL1 levels	Tissue damage markers, resolution timing
  Tumor Models	Growth kinetics, angiogenesis	Immune infiltration patterns, metastasis
  Cell Migration	Chemotaxis parameters, signaling pathway activation	Adhesion molecule expression, morphological changes

By integrating these design elements, researchers can comprehensively dissect how Ccrl2 modulates chemerin availability and subsequent CMKLR1-mediated responses across different physiological and pathological contexts.