Recombinant Pan troglodytes C5a anaphylatoxin chemotactic receptor (C5AR1)

What is C5AR1 and what is its role in the immune system?

C5AR1 (Complement Component 5a Receptor 1) is a G protein-coupled receptor that serves as the primary receptor for complement component C5a, a potent anaphylatoxin and inflammatory mediator. It contains seven transmembrane domains and is 350 amino acids long in humans. The receptor functions as the lynchpin of communication between complement activation and cellular immune responses by mediating chemotaxis, granule enzyme release, superoxide anion production, and upregulation of adhesion molecules. C5AR1 is primarily expressed on myeloid cells including neutrophils, monocytes, dendritic cells, and myeloid-derived suppressor cells, but expression has also been detected on non-immune cells such as neurons, endothelial cells, and tumor cells .

How does C5a binding activate the C5AR1 receptor?

Recent cryo-electron microscopy studies have revealed a three-site binding mode between C5a and C5AR1, expanding on the previously proposed two-site model. The C5a-C5AR1 interaction involves:

• Site 1: The membrane-proximal N-terminal region of C5AR1 interacts with the positively charged H2-H4 cavity of C5a through electrostatic interactions

• Site 2: The C-terminus of C5a, which adopts a hook shape, penetrates into the transmembrane helical bundle of C5AR1

• Site 3: The extracellular loop 2 (ECL2) region of C5AR1 occupies the C5a cavity

This complex binding mechanism triggers conformational changes in C5AR1's intracellular domains, enabling G protein coupling primarily to Gαi2, and subsequent activation of downstream signaling pathways that lead to pro-inflammatory and chemotactic responses .

What is the difference between C5a and C5a-desArg in activating C5AR1?

C5a and C5a-desArg (C5a without the C-terminal arginine) both activate C5AR1 but with different potencies and potentially through slightly different mechanisms:

Researchers should consider these differences when designing experiments, as physiological levels of C5a-desArg (5-10 ng/ml; ~1 nM) can induce substantial cellular responses comparable to C5a, especially in assays measuring integrated cellular responses rather than single parameters .

What are the best methods for studying C5AR1 activation in vitro?

Several complementary methods can be employed to study C5AR1 activation:

• Calcium Mobilization Assays: Measure intracellular calcium release using fluorescent calcium indicators (Fluo-4, Fura-2) in C5AR1-expressing cells following stimulation with C5a or C5a-desArg.

• Chemotaxis Assays: Use transwell migration chambers with cells seeded in the upper compartment and C5a in the lower compartment. Pre-incubation with C5AR antagonists (e.g., PMX53, A8D 71-73) can confirm specificity .

• Cell Signaling Analysis: Evaluate downstream signaling using phospho-specific antibodies against ERK1/2, p38 MAPK, or Akt via Western blot or flow cytometry.

• Receptor Internalization: Monitor C5AR1 internalization following stimulation using fluorescently labeled antibodies and flow cytometry or confocal microscopy.

• Functional Readouts: Measure respiratory burst (superoxide production) using chemiluminescence or fluorescence-based assays, or evaluate degranulation by measuring released enzymes like myeloperoxidase.

• G Protein Coupling Assays: Use pertussis toxin (PTX) to inhibit Gαi signaling and assess contribution of different G protein pathways to cellular responses .

• Biased Signaling Analysis: Compare β-arrestin recruitment versus G protein activation to identify biased ligands or receptor mutants .

How can I express and purify recombinant C5AR1 for structural studies?

Expressing and purifying functional C5AR1 for structural studies requires specific approaches due to its nature as a membrane protein:

• Expression Systems:

  • Mammalian Cells: HEK293 or CHO cells provide proper folding and post-translational modifications

  • Insect Cells: Baculovirus-infected Sf9 or Hi5 cells offer high yield

  • Virus-Like Particles (VLPs): Clone C5AR1 with C-terminal tags (e.g., 10xHis) alongside VLP-forming viral capsid proteins

• Construct Design:

  • Include N-terminal signal peptides

  • Add purification tags (His, FLAG)

  • Consider fusion partners (T4 lysozyme, BRIL) for stability

  • Truncate or modify potentially disordered regions

• Solubilization and Purification:

  • Solubilize membranes with mild detergents (DDM, LMNG)

  • Purify using affinity chromatography (e.g., Ni-NTA for His-tags)

  • Further purify by size exclusion chromatography

  • Consider reconstitution into nanodiscs or lipid cubic phase for stability

• Quality Control:

  • Verify proper folding through ligand binding assays

  • Assess homogeneity via size exclusion chromatography

  • Confirm activity through functional assays (e.g., G protein coupling)

  • Use transmission electron microscopy (TEM) to verify VLP formation

For structural studies like cryo-EM, complexing C5AR1 with stabilizing binding partners such as Gi protein and scFv16 has proven successful, as demonstrated in recent structural determinations of the C5a-C5AR1-Gi-scFv16 complex .

What methods are available for analyzing C5AR1 expression in tissue samples?

Several complementary techniques can be used to analyze C5AR1 expression in tissue samples:

• Immunohistochemistry (IHC):

  • Use validated anti-C5AR1 antibodies (monoclonal preferred)

  • Include proper isotype controls

  • Consider antigen retrieval methods for formalin-fixed paraffin-embedded tissues

  • Quantify using digital image analysis

• Immunofluorescence (IF):

  • Allows co-localization studies with other markers

  • Available with conjugated antibodies (FITC, APC/CY7)

  • Superior for cellular localization (surface vs. intracellular)

• In Situ Hybridization (ISH):

  • Detects C5AR1 mRNA directly in tissues

  • RNAscope technology offers single-molecule sensitivity

  • Allows correlation with protein expression when combined with IHC

• Flow Cytometry:

  • For dissociated tissue samples or blood

  • Quantifies expression levels on specific cell populations

  • Multiple markers can be assessed simultaneously

  • PE-conjugated anti-C5AR1 antibodies are available

• RT-qPCR:

  • Quantifies C5AR1 mRNA expression from tissue extracts

  • Highly sensitive but loses spatial information

  • Requires careful primer design and reference gene selection

• Western Blotting:

  • Confirms protein size and specificity

  • Semi-quantitative assessment of expression levels

  • Less informative about cellular distribution

• Transcriptome Analysis:

  • RNA sequencing provides comprehensive gene expression profiling

  • Can reveal correlations with other genes and pathways

When analyzing disease tissues, comparing perilesional and non-lesional samples from the same patient along with appropriate controls is recommended for accurate assessment of disease-related changes in C5AR1 expression .

How does Pan troglodytes C5AR1 compare to human C5AR1?

Pan troglodytes (chimpanzee) C5AR1 shares high sequence homology with its human counterpart, making it valuable for comparative studies and as a preclinical model for human C5AR1-targeted therapeutics:

The high conservation of C5AR1 between humans and chimpanzees reflects the evolutionary importance of complement-mediated immune responses. Researchers can utilize recombinant Pan troglodytes C5AR1 expressed in various systems (E. coli, yeast, baculovirus, or mammalian cells) to conduct comparative studies or as a surrogate for human C5AR1 in certain experimental contexts .

What experimental models are available for studying C5AR1 function across species?

Several experimental models across different species are available for studying C5AR1 function:

• Mouse Models:

  • Conventional C5ar1 knockout mice

  • Human C5AR1 knock-in (HuC5aR1 KI) mice that express human C5AR1 exclusively on CD11b+ myeloid cells

  • Conditional C5ar1 knockout mice for tissue-specific deletion

  • Mouse models of various diseases (sepsis, inflammatory disorders, Alzheimer's)

• Rat Models:

  • Widely used for inflammatory and autoimmune disease models

  • ELISA kits available for rat C5a and C5AR1 detection

• Non-human Primates:

  • Pan troglodytes (chimpanzee) C5AR1 recombinant proteins

  • Macaque models for infectious and inflammatory diseases

  • ELISA kits available for monkey C5a and C5AR1 detection

• Fish Models:

  • Zebrafish and starry flounder models show C5ar expression

  • C5ar upregulation observed in vaccinated rainbow trout responding to Yersinia ruckeri infection

  • Studies in grass carp demonstrated pro-inflammatory activation after C5a binding

• Cell Culture Systems:

  • Transfected cell lines (RBL, COS-7, HEK293)

  • Primary cells from different species

  • Cell lines expressing species-specific C5AR1 variants

• In vitro Systems:

  • Reconstituted proteoliposomes containing purified C5AR1

  • Cell-free expression systems

For comparative studies, researchers should consider species-specific differences in C5a/C5AR1 interactions, signaling pathways, and antagonist sensitivity when interpreting results and extrapolating to human biology .

What is the role of C5AR1 in inflammatory diseases and potential therapeutic approaches?

C5AR1 plays pivotal roles in various inflammatory diseases, with distinct therapeutic implications:

Therapeutic targeting of C5AR1 must consider its dual pro- and anti-inflammatory roles, tissue-specific effects, and timing of intervention relative to disease progression. For example, in sepsis, C5AR1 inhibition may be beneficial in early or mild disease but counterproductive in severe cases .

How can C5AR1 antagonists be evaluated for efficacy in preclinical models?

Evaluation of C5AR1 antagonists in preclinical models involves a systematic approach:

• In Vitro Screening:

  • Binding Assays: Measure displacement of radiolabeled or fluorescent C5a from C5AR1

  • Functional Assays: Assess inhibition of C5a-induced calcium mobilization, ERK phosphorylation, or chemotaxis

  • Selectivity Profiling: Test against related receptors (C5AR2) and species orthologs

  • Biased Antagonism Analysis: Determine if compounds selectively block G protein vs. β-arrestin pathways

• Ex Vivo Tissue Analysis:

  • Human Blood Assays: Measure inhibition of C5a-induced neutrophil activation

  • Tissue Explant Cultures: Assess effects on inflammatory mediator production

• In Vivo Disease Models:

  • Acute Inflammation: Air pouch, peritonitis, or lung injury models

  • Disease-Specific Models:

    • Sepsis: Cecal ligation and puncture (CLP) model in wild-type or HuC5aR1 KI mice

    • ARDS: Intranasal C5a instillation in HuC5aR1 KI mice

    • Autoimmune Disease: Experimental autoimmune encephalomyelitis or glomerulonephritis

    • Neurodegeneration: Alzheimer's disease mouse models

• Efficacy Parameters:

  • Survival: In lethal disease models like sepsis

  • Inflammatory Markers: Cytokine levels, neutrophil infiltration, tissue damage

  • Disease-Specific Readouts: Proteinuria in kidney disease, cognitive performance in neurological models

  • Mechanistic Biomarkers: Changes in downstream signaling pathways

• PK/PD Correlation:

  • Pharmacokinetics: Drug exposure in plasma and tissues

  • Target Engagement: Receptor occupancy assays

  • Pharmacodynamic Endpoints: Inhibition of C5a-induced inflammation

The PMX series of antagonists (PMX53, PMX205) has been evaluated in multiple disease models, demonstrating the validity of this approach. For example, avdoralimab blocked the development of acute lung inflammation and injury in HuC5aR1 KI mice receiving intranasal C5a, as evidenced by reduced immune cell infiltration and decreased alveolar-capillary permeability .

How does biased signaling of C5AR1 impact its biological functions?

C5AR1 exhibits biased signaling that significantly impacts its biological functions:

• Molecular Basis of Biased Signaling:

  • The "ligand P6-C5aR1 IWI pattern" contributes significantly to biased signaling

  • Peptide ligands derived from the C-terminal tail of C5a can exhibit differential efficacy toward G protein vs. β-arrestin pathways

  • C5a peptide (C5a pep) shows G protein-biased efficacy compared to unbiased full-length C5a

  • BM213 is another G protein-biased agonist with therapeutic potential

  • Single or combined substitutions of P6-P7 ligand side chains may provide novel drug candidates with functional selectivity

• Conformational Changes:

  • Cryo-EM structures reveal unusual conformational changes in the intracellular end of transmembrane domain 7 and helix 8 upon agonist binding

  • These changes suggest differential signal transduction processes depending on the ligand

• G Protein Coupling Profiles:

  • C5a and C5a desArg may differentially engage Gαi vs. Gαs proteins

  • This differential G protein engagement impacts cAMP regulation

  • Ligand concentration can determine preferential activation of Gαs vs. Gαi proteins

  • The balance between these pathways may determine pro- vs. anti-inflammatory outcomes

• Physiological Consequences:

  • Biased signaling may explain the context-dependent effects of C5AR1 activation

  • Different tissue microenvironments may favor distinct signaling pathways

  • In sepsis, C5AR1 shows both pro- and anti-inflammatory effects depending on disease severity

  • C5AR1 deficiency increases production of pro-inflammatory IFN-γ while decreasing anti-inflammatory IL-10 in sepsis models

Understanding these biased signaling mechanisms provides opportunities to develop pathway-selective therapeutics that maintain beneficial functions while blocking detrimental effects of C5AR1 activation.

What are the latest structural insights into C5AR1 activation and antagonist binding?

Recent structural studies have provided unprecedented insights into C5AR1 activation mechanisms and antagonist binding:

• Cryo-EM Structures of Active C5AR1 Complexes:

  • Structures of wild-type C5AR1-Gi protein complex bound to:

    • Full-length C5a (2.9 Å resolution)

    • C5a peptide (C5a pep) (3.2 Å resolution)

    • G protein-biased agonist BM213 (2.9 Å resolution)

  • Structure of C5AR1 I116A mutant-Gi signaling activation complex induced by C089 (2.8 Å resolution)

  • These structures reveal the conformational changes associated with receptor activation

• Three-Site Binding Mode of C5a:

  • Site 1: C5a's positively charged H2-H4 cavity interacts with membrane-proximal N-terminal region of C5AR1

  • Site 2: C-terminus of C5a penetrates the core of the helical bundle upon receptor binding

  • Site 3: ECL2 region of C5AR1 occupies the C5a cavity that previously accommodated the C-terminal tail

  • This three-site model expands on the traditional two-site binding model from previous mutagenesis studies

• Critical Molecular Interactions:

  • R^P8 side chain projects over the aromatic side chain of Y258^6.51, forming hydrophobic cation-π interaction

  • A critical hydrogen bond between R^P8 and D282^7.35 is essential for receptor activation

  • Replacement of D282^7.35 with alanine significantly impairs efficacy of C5a- and C5a pep-mediated activation

  • The IWI pattern (I116^3.40, W213^5.49, I263^6.56) is crucial for receptor activation

• Activation Mechanism:

  • Activation is governed by interruption of the "zipper interface"

  • Recruitment of Gi protein occurs via noncanonical active conformation of intracellular TM7 and H8

  • Unusual conformational changes in the intracellular end of TM7 and H8 suggest differential signal transduction

These structural insights provide a foundation for rational design of novel therapeutics targeting specific aspects of C5AR1 function, potentially leading to more selective interventions with fewer side effects.

What methodological challenges exist in studying C5AR1 in complex disease environments?

Researchers face several methodological challenges when studying C5AR1 in complex disease environments:

• Temporal Dynamics of C5AR1 Expression and Function:

  • C5a is rapidly converted to C5a-desArg within 5 minutes in plasma

  • C5AR1 undergoes rapid internalization after activation

  • Challenge: Developing time-resolved experimental approaches to capture these dynamics

  • Solution: Time-course studies with multiple sampling points; real-time imaging techniques

• Heterogeneity of Cell Types Expressing C5AR1:

  • Expression on multiple immune and non-immune cell types

  • Cell type-specific effects may be masked in whole-tissue analyses

  • Challenge: Distinguishing cell-specific roles of C5AR1

  • Solution: Single-cell RNA-seq; conditional knockout models; cell type-specific reporter systems

• Integration with Other Complement Receptors:

  • C5AR1 functions in concert with C5AR2 and other complement receptors

  • Compensatory mechanisms may obscure phenotypes in knockout models

  • Challenge: Dissecting specific contributions of C5AR1 versus other receptors

  • Solution: Combined receptor knockouts; selective antagonists; receptor chimeras

• Tissue-Specific Microenvironmental Factors:

  • Local concentrations of C5a/C5a-desArg vary across tissues

  • Extracellular matrix components may modulate receptor function

  • Challenge: Recreating physiologically relevant microenvironments

  • Solution: Organoid cultures; tissue-specific in vivo imaging; microdialysis sampling

• Epigenetic Regulation of C5AR1 Expression:

  • DNA methylation can modify C5AR1 promoter activity

  • Challenge: Assessing epigenetic contributions to C5AR1 dysregulation

  • Solution: Methylation analysis of C5AR1 promoter; chromatin immunoprecipitation studies

• Translational Relevance of Animal Models:

  • Species differences in C5AR1 structure and function

  • Challenge: Extrapolating from animal models to human disease

  • Solution: Humanized mouse models (HuC5aR1 KI); comparative studies with human and animal C5AR1

• Integration of Multi-omics Data:

  • Complexity of signaling networks downstream of C5AR1

  • Challenge: Interpreting complex datasets to identify key regulatory nodes

  • Solution: Systems biology approaches; combined analysis of transcriptomics, proteomics, and metabolomics data

Addressing these challenges requires interdisciplinary approaches and the development of new methodologies that can capture the complexity of C5AR1 biology in disease contexts.

How can genetic manipulation of C5AR1 be used to study its function?

Genetic manipulation offers powerful approaches to study C5AR1 function:

• Gene Knockout Strategies:

  • Conventional Knockouts: Complete deletion of C5ar1 gene has revealed roles in sepsis and autoimmunity

  • Conditional Knockouts: Cell type-specific deletion using Cre-loxP system

  • Inducible Knockouts: Temporal control of gene deletion using tamoxifen-inducible Cre

  • Knockdown Approaches: siRNA or shRNA for transient C5ar1 reduction

• Humanized Animal Models:

  • HuC5aR1 KI mice: Express human C5AR1 exclusively on CD11b+ myeloid cells

  • Allow testing of species-specific antagonists in vivo

  • Enable more translatable preclinical studies

• Point Mutations and Structure-Function Analysis:

  • Site-Directed Mutagenesis: Systematic mutation of key residues

    • D282^7.35A: Disrupts essential hydrogen bond with R^P8 of C5a

    • I116^3.40A: Converts antagonist to agonist

  • Domain Swapping: Exchange domains between C5AR1 and related receptors

• Reporter Systems:

  • Promoter-Reporter Constructs: C5ar1 promoter driving fluorescent proteins or luciferase

  • Receptor-Fluorescent Protein Fusions: Visualize receptor trafficking

  • BRET/FRET Biosensors: Monitor conformational changes or protein interactions

• Genome Editing Applications:

  • CRISPR/Cas9: Precise editing of C5ar1 in cell lines or animals

  • Base Editing: Introduce specific coding variants without double-strand breaks

  • Knock-in Mutations: Create disease-relevant variants

• Epigenetic Modifications:

  • In vitro methylation of C5ar1 promoter significantly reduces transcriptional activity

  • CpG methyltransferase (M.SssI) can modify recombinant plasmids in vitro to study methylation effects

• Experimental Validation Methods:

  • Dual luciferase reporter assays to confirm promoter activity changes

  • Restriction enzyme (e.g., HpaII) digestion to verify methylation status

  • Functional assays to assess receptor signaling alterations

These genetic approaches, particularly when combined with pharmacological interventions, provide complementary strategies to dissect C5AR1 function in complex biological systems.

What new therapeutic approaches targeting C5AR1 are under development?

Several innovative therapeutic approaches targeting C5AR1 are under development:

• Small Molecule Antagonists:

  • PMX Series: PMX53 and PMX205 show efficacy in multiple disease models

  • Biased Antagonists: Compounds that selectively block G protein or β-arrestin pathways

  • Allosteric Modulators: Target non-orthosteric binding sites for improved selectivity

  • Dual Antagonists: A8D 71-73 blocks both C5AR1 and C5AR2

• Biologics:

  • Monoclonal Antibodies: Specifically target C5AR1 (e.g., avdoralimab)

  • Nanobodies: Smaller antibody fragments with improved tissue penetration

  • Engineered C5a Variants: Modified C5a peptides that act as competitive antagonists

• Novel Delivery Strategies:

  • Cell-Specific Targeting: Nanoparticle delivery to specific cell populations

  • Blood-Brain Barrier Penetration: Modified antagonists designed for CNS penetration

  • Local Delivery Systems: Reduce systemic effects while maintaining local efficacy

• Combination Therapies:

  • Complement Cascade Inhibitors: Combine C5AR1 antagonists with upstream complement inhibitors

  • Anti-inflammatory Agents: Synergize with conventional treatments

  • Immuno-oncology Combinations: Enhance efficacy of cancer immunotherapies

• Precision Medicine Approaches:

  • Biomarker-Guided Therapy: Target patients with elevated C5a/C5AR1 activity

  • Genetic Stratification: Identify patients most likely to respond based on genetic profiles

  • Disease Stage-Specific Intervention: Different approaches for early vs. late disease

• Emerging Preclinical Evidence:

  • C5AR1 antagonists show promise in models of COVID-19, reducing lung inflammation

  • Targeting C5AR1 decreases amyloid pathology and improves cognitive function in Alzheimer's disease models

  • C5AR1 inhibition improves outcomes in experimental autoimmune conditions

The diverse biological roles of C5AR1 offer multiple therapeutic opportunities, but also present challenges in terms of achieving desired specificity and avoiding disruption of beneficial immune functions. Future development will likely focus on context-specific interventions that selectively target pathological C5AR1 activation while preserving homeostatic functions.

What computational methods are available for predicting C5AR1-ligand interactions?

Advanced computational methods are increasingly valuable for predicting C5AR1-ligand interactions:

• Structure-Based Approaches:

  • Molecular Docking: Virtual screening of compound libraries against C5AR1 structures

  • Molecular Dynamics Simulations: Model dynamic interactions between C5AR1 and ligands

  • Free Energy Calculations: Estimate binding affinities (MM/PBSA, FEP)

  • Structure-Activity Relationship (SAR) Analysis: Correlate structural features with activity data

• Machine Learning Applications:

  • Quantitative Structure-Activity Relationship (QSAR) Models: Predict antagonist activities

  • Deep Learning: Neural networks trained on C5AR1 binding data

  • Proteochemometric Modeling: Integrate both protein and ligand descriptors

  • Artificial Intelligence Drug Design: Generate novel C5AR1 ligands with desired properties

• Biased Signaling Prediction:

  • Molecular Interaction Fingerprints: Identify patterns associated with G protein vs. β-arrestin bias

  • Conformational Ensemble Analysis: Predict ligand effects on receptor conformational states

  • Molecular Dynamics Simulations: Identify conformational changes linked to biased signaling

• Integrative Modeling:

  • Multi-Scale Modeling: Combine atomistic, coarse-grained, and systems-level approaches

  • Network Analysis: Map C5AR1 signaling networks and predict intervention points

  • Systems Pharmacology: Model effects of C5AR1 modulation on entire biological systems

• Data Resources and Tools:

  • Cryo-EM Structures: Recent structures of C5AR1 bound to various ligands provide templates

  • Homology Models: For species variants lacking experimental structures

  • Specialized Software: GPCR-specific tools (GPCRdb, GPCR-ModSim)

  • Web Servers: Online platforms for GPCR structure prediction and analysis

• Experimental Validation Strategies:

  • Site-Directed Mutagenesis: Test computationally predicted interaction sites

  • Biophysical Methods: Validate binding modes using NMR, HDX-MS, or crosslinking

  • Functional Assays: Confirm predicted functional consequences of specific interactions