Phospho-C5AR1 (S338) Antibody

What is C5AR1 and what is the significance of its phosphorylation at serine 338?

C5AR1 (Complement 5a Receptor 1, also known as CD88) is a G-protein coupled receptor that functions as the receptor for complement component 5a (C5a), an anaphylatoxin with potent proinflammatory and chemotactic properties. C5AR1 is widely expressed, particularly in myeloid cells such as neutrophils and macrophages, and couples predominantly to Gαi .

Phosphorylation at S338 has critical functional significance:

• It is essential for C5a-induced stable recruitment of β-arrestin

• It mediates internalization of C5AR1 following activation

• It contributes to the attenuation of C5a-induced calcium mobilization, Erk2 activity, and ROS production

The post-translational modification of S338 (along with S334) is a key regulatory mechanism that controls C5AR1 signaling dynamics and is performed by G protein-coupled receptor kinases (GRKs) and PKC following receptor activation by C5a .

How does phosphorylation at S338 differ from other C5AR1 phosphorylation sites?

While multiple serine and threonine residues in the C-terminal tail of C5AR1 can be phosphorylated, S338 has been identified as having particular importance:

• S338 phosphorylation, along with S334, is specifically critical for stable β-arrestin recruitment

• This site appears to be more selective for receptor internalization functions compared to other phosphorylation sites

• The Phospho-C5AR1 (S338) antibody specifically detects endogenous levels of CD88/C5AR1 protein only when phosphorylated at S338, distinguishing it from the non-phosphorylated form

Recent studies comparing S334 and S338 phosphorylation suggest potentially different dynamics in receptor trafficking, with research showing that both sites are important but may have distinct roles in fine-tuning receptor signaling and internalization .

What are the recommended protocols for using Phospho-C5AR1 (S338) Antibody in different applications?

Based on manufacturer specifications and research protocols, the following applications have validated methodologies:

Immunohistochemistry (IHC):

• Recommended dilution range: 1:100-1:300

• Optimal fixation: 4% paraformaldehyde for tissue sections

• Antigen retrieval: Citrate buffer (pH 6.0) heat-induced epitope retrieval

• Detection system: Use HRP or fluorophore-conjugated secondary antibodies

• Include appropriate negative controls (isotype control, secondary-only)

Immunofluorescence (IF):

• Recommended dilution: 1:50-1:200

• Cell fixation: 4% paraformaldehyde (10 minutes at room temperature)

• Permeabilization: 0.1% Triton X-100 (5 minutes)

• Blocking: 5% BSA or normal serum (1 hour)

• Counterstain: DAPI for nuclear visualization

ELISA:

• Recommended dilution: 1:40000

• Coating concentration: 1-2 μg/ml of capture antibody

• Secondary detection: HRP-conjugated secondary antibody

• Substrate: TMB with stop solution

• Standard curve generation recommended for quantification

How can I validate the specificity of the Phospho-C5AR1 (S338) Antibody in my experimental system?

Validating antibody specificity is critical for reliable results. Recommended approaches include:

• Phosphatase Treatment Control:

  • Split your sample and treat one portion with lambda phosphatase

  • The phospho-specific signal should be abolished in the treated sample

• Stimulation Experiments:

  • Compare unstimulated cells with C5a-stimulated cells (15-100 nM)

  • The S338 phosphorylation signal should increase upon stimulation

  • Time course experiments show peak phosphorylation occurs 2-5 minutes post-stimulation

• Genetic Validation:

  • If possible, use C5AR1 knockout cells or CRISPR-edited cells with S338A mutation

  • No signal should be detected in these negative controls

• Peptide Competition:

  • Pre-incubate the antibody with the immunizing phosphopeptide

  • This should block specific binding and eliminate the signal

• Cross-validation:

  • Compare results with another antibody recognizing a different epitope of C5AR1

  • Use total C5AR1 antibody alongside the phospho-specific antibody

What sample preparation methods best preserve C5AR1 phosphorylation status?

Phosphorylation is labile and can be rapidly lost during sample preparation. Follow these guidelines:

• Cell/Tissue Lysis:

  • Use ice-cold lysis buffer containing phosphatase inhibitors (sodium fluoride, sodium orthovanadate, β-glycerophosphate)

  • Include protease inhibitor cocktail

  • Maintain samples at 4°C throughout processing

• Stimulation Conditions:

  • For maximum S338 phosphorylation, stimulate cells with 15-100 nM C5a for 2-5 minutes

  • Rapidly stop stimulation by adding ice-cold PBS containing phosphatase inhibitors

• Fixation for Microscopy:

  • Fix cells immediately after stimulation (within 1-2 minutes)

  • Use freshly prepared 4% paraformaldehyde

  • Avoid over-fixation which can mask epitopes

• Storage Considerations:

  • For long-term storage, snap-freeze samples in liquid nitrogen

  • Store at -80°C with phosphatase inhibitors

  • Avoid repeated freeze-thaw cycles

How can the Phospho-C5AR1 (S338) Antibody be used to study receptor trafficking mechanisms?

This antibody is particularly valuable for investigating C5AR1 internalization and recycling dynamics:

• Receptor Internalization Studies:

  • Track phosphorylated C5AR1 following C5a stimulation using flow cytometry or microscopy

  • Phosphorylation at S338 precedes and is required for internalization

  • Time-course experiments can reveal the kinetics of internalization (typically 2-10 minutes)

  • Studies show the receptor is trafficked from plasma membrane into early endosomes (Eea1-positive) upon C5a stimulation, with peak accumulation at 2 minutes

• Endosomal Trafficking Analysis:

  • Use cellular fractionation to isolate different compartments:

    • 10,000g post-granule supernatant (PGS)

    • Cytosol fraction (Gapdh-positive)

    • Membrane fraction (Kras-positive)

    • Early endosome fraction (Eea1-positive)

  • Following C5a stimulation, C5AR1 decreases in plasma membrane fraction and increases in early endosome fraction

  • After 10 minutes, C5AR1 decreases in both fractions, suggesting trafficking to mature endosomes

• Receptor Recycling Quantification:

  • After stimulation with 100 nM C5a for 10 minutes, track receptor recycling back to cell surface over 60 minutes

  • Research shows approximately 40-70% recycling occurs within 30 minutes

  • The phospho-antibody allows distinguishing between newly synthesized vs. recycled receptors

What is the relationship between C5AR1 phosphorylation and inflammatory disease processes?

Research using phospho-specific antibodies has revealed important connections between C5AR1 phosphorylation and inflammation:

• Macrophage-Dependent Inflammatory Diseases:

  • C5AR1 signaling plays a critical role in sterile inflammation such as atherosclerosis

  • Phosphorylated C5AR1 has been identified on mitochondria in macrophages, regulating IL-1β production during crystal-induced inflammation

  • Research shows that blockade of C5AR1 can normalize inflammatory gene signatures in unstable atherosclerotic plaques

• Cancer Research Applications:

  • High levels of C5AR1 in tumor-associated macrophages (TAMs) predict poor prognosis in colorectal cancer

  • C5AR1 inhibition reprograms TAMs toward M1 polarization via AKT2-mediated PFKM stabilization

  • Studying C5AR1 phosphorylation states provides insight into macrophage polarization mechanisms

• Bacterial Infection Models:

  • Staphylococcus aureus toxins like PVL interact with C5AR1

  • Phosphorylation status of C5AR1 modulates macrophage cell death and inflammation following PVL exposure

  • The FBXO11 protein regulates C5AR1 expression and thereby modulates PVL toxicity

What factors might affect detection of phosphorylated C5AR1 (S338)?

Several experimental variables can influence detection efficacy:

• Stimulation Parameters:

  • C5a concentration is critical - 15 nM may show partial internalization while 100 nM induces maximal internalization

  • Time-dependent effects: peak phosphorylation occurs at 2-5 minutes post-stimulation

  • Cell type variation: neutrophils vs. macrophages may show different phosphorylation kinetics

• Sample Processing Factors:

  • Phosphatase activity during sample preparation can reduce signal

  • Inadequate fixation may cause loss of phosphorylated epitopes

  • Over-fixation can mask the phospho-epitope, reducing antibody accessibility

• Technical Considerations:

  • Antibody concentration needs optimization (see recommended dilutions in section 2.1)

  • Incubation conditions (temperature, time) affect binding efficiency

  • Detection system sensitivity may need adjustment for low-expression systems

How can I distinguish between specific signal and background when using this antibody?

Proper controls and optimization strategies include:

• Essential Controls:

  • Isotype control antibody at the same concentration

  • Unstimulated cells (baseline phosphorylation)

  • Phosphatase-treated samples (negative control)

  • Competitive blocking with immunizing phosphopeptide

• Signal Optimization:

  • Titrate antibody concentration to determine optimal signal-to-noise ratio

  • For IF/IHC: use antigen retrieval methods compatible with phospho-epitopes

  • For flow cytometry: optimize fixation and permeabilization conditions

  • For Western blot: test different blocking agents (BSA vs. milk - note that milk contains phosphatases)

• Validation Methods:

  • Compare phospho-specific antibody signal with total C5AR1 antibody

  • Confirm that signal increases after C5a stimulation

  • Verify reduction of signal with specific C5AR1 inhibitors (W-54011, PMX-53, avacopan)

How should I design experiments to study the kinetics of C5AR1 phosphorylation?

Effective experimental design includes:

• Stimulation Time Course:

  • Include multiple timepoints: 0, 1, 2, 5, 10, 15, 30 minutes after C5a addition

  • Use physiologically relevant C5a concentrations (15-100 nM)

  • Include rapid quenching of phosphorylation by ice-cold PBS with phosphatase inhibitors

• Dose-Response Analysis:

  • Test C5a concentration range (1-100 nM)

  • Different phosphorylation sites may show different dose sensitivities

  • Compare S338 with S334 phosphorylation dynamics

• Cellular Localization Analysis:

  • Track phosphorylated receptor movement between membrane fractions

  • Use cellular fractionation to isolate early endosomes (Eea1-positive)

  • Compare timing of phosphorylation with timing of internalization and trafficking

• Integrative Measurements:

  • Correlate phosphorylation with downstream signaling events

  • Measure calcium flux, ERK activation, and β-arrestin recruitment in parallel

  • Use phosphorylation-deficient mutants (S338A) as controls for specificity

How is Phospho-C5AR1 (S338) research contributing to therapeutic developments?

Recent research highlights several therapeutic avenues:

• Inflammatory Disease Interventions:

  • C5AR1 inhibition reduced colorectal cancer growth by reprogramming tumor-associated macrophages

  • High C5AR1 levels in TAMs predicted poor prognosis in patients

  • Understanding phosphorylation mechanisms provides targets for precise intervention

• Novel Therapeutic Antibodies:

  • Development of function-blocking antibodies against C5AR1

  • Monoclonal antibody 18-41-6 shows potential as a therapeutic candidate for inflammatory diseases

  • Cell-permeable C5AR1 antagonists normalize inflammatory gene signatures in atherosclerotic plaques

• Targeted Drug Development:

  • Phosphorylation-specific inhibitors may offer advantages over general C5AR1 antagonists

  • Drugs targeting the phosphorylation machinery (specific GRKs) rather than the receptor itself

  • Understanding how phosphorylation regulates β-arrestin recruitment opens pathways to biased ligand development

What are emerging methods for studying C5AR1 phosphorylation beyond traditional antibody-based approaches?

Cutting-edge techniques include:

• Phosphoproteomics:

  • Mass spectrometry-based detection of multiple phosphorylation sites simultaneously

  • Quantitative analysis of phosphorylation stoichiometry

  • Discovery of novel phosphorylation sites with functional significance

• CRISPR-Based Approaches:

  • Generation of phospho-null mutants (S338A) to study site-specific functions

  • Knock-in of fluorescently tagged C5AR1 for live-cell imaging

  • Creation of cell lines with endogenously tagged receptors at physiological expression levels

• Advanced Imaging Techniques:

  • Super-resolution microscopy to track individual phosphorylated receptors

  • FRET-based biosensors for real-time phosphorylation monitoring

  • Correlative light and electron microscopy to study phosphorylated receptor ultrastructural localization

• Systems Biology Approaches:

  • Mathematical modeling of phosphorylation/dephosphorylation kinetics

  • Network analysis of phosphorylation-dependent signaling pathways

  • Integration of phosphorylation data with transcriptomic and proteomic datasets