Recombinant Rat CD9 antigen (Cd9)

What is Rat CD9 antigen and what are its key structural features?

CD9 is a member of the tetraspanin superfamily, a 24-27 kDa transmembrane protein predominantly localized to the plasma membrane. Rat CD9 shares 86.9% amino acid sequence identity with human CD9 and 77.4% with mouse CD9 . The protein typically appears as a 22-27 kDa band on Western blots due to variable glycosylation patterns . Structurally, CD9 contains four transmembrane domains with two extracellular loops and short intracellular amino and carboxyl termini. The large extracellular loop (EC2) is particularly important for protein-protein interactions and contains conserved cysteine residues that form disulfide bonds critical for proper protein folding and function .

How is recombinant Rat CD9 typically produced for research applications?

Recombinant Rat CD9 is commonly produced using bacterial expression systems, particularly E. coli, for specific domains (such as the E. coli-derived mouse CD9 recombinant protein from position T110-I193) . For full-length protein that requires proper folding and post-translational modifications, mammalian expression systems are preferred. The production process typically involves:

• Cloning the full-length or specific domain of CD9 into an appropriate expression vector (e.g., YOE-LV001 as mentioned in search result 3)

• Transfection into host cells using methods such as Lipofectamine 2000

• Selection of stable transfectants using appropriate antibiotics

• Verification of expression using RT-PCR and Western blotting with anti-CD9 antibodies

• Purification via affinity chromatography, typically using tags engineered into the recombinant protein

Expression validation should include both mRNA quantification by qRT-PCR and protein detection by Western blotting using specific anti-CD9 antibodies at approximately 1:1000 dilution .

How can I verify the functional activity of recombinant Rat CD9?

The functional verification of recombinant Rat CD9 requires multiple approaches:

• Binding assays: Confirm the ability of recombinant CD9 to interact with known binding partners, such as integrins or other tetraspanins.

• Cell-based functional assays: In transfected cell lines (such as RBL-2H3), assess CD9 function through:

  • Secretory response measurements following antibody cross-linking

  • Migration assays (e.g., Transwell assays as described in the research literature)

  • Co-immunoprecipitation with FcεRI to confirm complex formation

• Quantitative verification: Determine the antibody-binding capacity of CD9-expressing cells (e.g., CD9-RBL cells showed 1.6 × 10^5 binding sites per cell)

• Flow cytometry validation: Using verified anti-CD9 antibodies (e.g., at 1 μg per 10^6 cells) with appropriate secondary antibodies to confirm surface expression and compare with isotype controls .

What are the optimal conditions for detecting Rat CD9 in Western blot applications?

For optimal Western blot detection of Rat CD9:

• Sample preparation:

  • Lyse cells in RIPA buffer (such as Beyotime RIPA lysis buffer)

  • Determine protein concentration using BCA Protein Assay Kit

  • Load approximately 26 μg of protein per lane

• Electrophoresis and transfer conditions:

  • Separate proteins using SDS-PAGE

  • Transfer to PVDF membranes (Millipore or equivalent)

  • Block with 5% non-fat dry milk in TBST for 30 minutes

• Antibody incubation:

  • Primary antibody: Anti-CD9 antibody at 1:1000 dilution, incubated overnight at 4°C

  • Secondary antibody: HRP-conjugated secondary antibody at 1:5000 dilution for 2 hours at room temperature

  • Visualization using ECL detection system

• Expected results:

  • CD9 typically appears as a band between 22-27 kDa

  • Multiple bands may appear due to glycosylation variants

  • Validation should include positive controls (CD9-expressing cells) and negative controls

What experimental approaches are recommended for studying CD9 interactions with other membrane proteins?

To study CD9 interactions with other membrane proteins, researchers should consider:

• Co-immunoprecipitation under mild detergent conditions:

  • Use CHAPS detergent for membrane solubilization to preserve protein-protein interactions

  • Immunoprecipitate using specific antibodies against CD9 or its potential binding partners

  • Analyze co-precipitated proteins by Western blotting

• Proximity ligation assays:

  • Visualize protein interactions in situ with single-molecule resolution

  • Use pairs of primary antibodies against CD9 and potential interaction partners

  • Detect with species-specific secondary antibodies linked to complementary oligonucleotides

• FRET (Förster Resonance Energy Transfer) analysis:

  • Tag CD9 and potential binding partners with compatible fluorophores

  • Measure energy transfer between molecules in close proximity (typically <10 nm)

• Cross-linking studies:

  • Use chemical cross-linkers of defined spacer lengths to capture transient interactions

  • Employ F(ab')2 fragments of anti-CD9 antibodies to distinguish between direct binding and Fc-mediated effects

Evidence from RBL cell studies shows that CD9 can form stable complexes with FcεRI prior to antibody cross-linking, and these complexes can be isolated by immunoprecipitation under mild detergent conditions .

How can I establish stable cell lines expressing recombinant Rat CD9 for functional studies?

To establish stable cell lines expressing recombinant Rat CD9:

• Vector selection and cloning:

  • Choose an appropriate expression vector with a strong promoter (e.g., YOE-LV001)

  • Clone the full-length CD9 sequence into the vector

  • Verify the construct by sequencing

• Transfection and selection:

  • Transfect target cells (e.g., LN229 or RBL cells) using Lipofectamine 2000 or equivalent reagent

  • Include appropriate control transfections (empty vector)

  • Select stable transfectants using an appropriate antibiotic (e.g., G418)

  • Grow cells continuously in selection media containing G418

• Validation of expression:

  • Perform multiple rounds of FACS selection, isolating the top 10% of fluorescent cells after staining with fluorescently labeled anti-CD9 antibodies

  • Verify expression by:

    • Flow cytometry (e.g., using DyLight®488 conjugated anti-rabbit IgG at 5-10 μg/10^6 cells)

    • RT-qPCR using β-actin as a reference gene

    • Western blotting with specific anti-CD9 antibodies

    • Immunofluorescence microscopy to confirm plasma membrane localization

• Functional testing:

  • Compare cellular responses in CD9-expressing and control cells

  • Assess antibody-binding capacity (e.g., 1.6 × 10^5 binding sites per cell as reported for CD9-RBL cells)

How does CD9 participate in receptor-mediated signaling pathways, and what experimental approaches best capture these dynamics?

CD9 participates in receptor-mediated signaling through several mechanisms that can be studied using specific experimental approaches:

• Complex formation with signaling receptors:

  • CD9 forms pre-existing complexes with FcεRI that can be activated by antibody cross-linking

  • These complexes can be detected by co-immunoprecipitation under mild detergent conditions (CHAPS)

  • Quantitative analysis shows increased association of CD9 with FcεRI following activation with polyvalent antigens (DNP-HSA)

• Biphasic activation response:

  • Anti-CD9 antibodies (IgG1 isotype) can activate CD9-transfected RBL cells through a biphasic dose-response curve

  • Maximal stimulation occurs at 10-100 nM antibody concentrations

  • Activity decreases at higher concentrations (100-1000 nM), indicating a requirement for multivalent binding

• Inhibition studies to determine specificity:

  • Co-incubation with monomer murine IgE inhibits the secretory response to anti-CD9 antibodies

  • IgG1 isotype controls do not inhibit this response

  • This suggests involvement of the endogenous high-affinity IgE receptor (FcεRI)

• Domain-specific interaction studies:

  • F(ab')2 fragments of anti-CD9 antibodies bind to CD9 but fail to stimulate degranulation

  • Cross-linking these fragments with intact anti-F(ab')2 antibodies restores the response

  • This indicates that both CD9 binding and Fc receptor involvement are necessary

What are the critical considerations when designing experiments to study CD9's role in cancer progression using recombinant proteins?

When designing experiments to study CD9's role in cancer progression:

• Expression analysis across cancer types:

  • CD9 shows differential expression across cancer types, with significant differences observed in 11 cancer types in TCGA data

  • In gliomas, CD9 expression correlates with patient survival rates, with high expression associated with lower survival

  • Experimental design should include comparison of CD9 expression across:

    • Tumor vs. normal tissue

    • Different tumor grades

    • Different molecular subtypes (e.g., IDH mutation status in gliomas)

• Functional assays relevant to cancer biology:

  • Cell migration assays (e.g., Transwell assays) to assess CD9's impact on invasiveness

  • Proliferation assays following CD9 overexpression or knockdown

  • Tumor sphere formation to assess effects on cancer stem cell properties

  • In vivo xenograft models with CD9-modified cells

• Mechanistic studies:

  • Gene set enrichment analysis (GSEA) to identify pathways affected by CD9

  • Immune-related analysis to assess CD9's role in tumor immune microenvironment

  • Focus on neutrophil involvement, which shows the strongest correlation with CD9 expression (cor = 0.30, P < 0.05)

• Overexpression and knockdown validation:

  • Use both approaches to establish causality

  • For overexpression, clone full-length CD9 into appropriate vectors (e.g., YOE-LV001)

  • For knockdown, use shRNA with non-targeting controls

  • Verify expression changes at both mRNA level (qRT-PCR) and protein level (Western blot)

• Translational relevance:

  • Correlate experimental findings with clinical parameters from cancer databases (TCGA, CGGA)

  • Assess CD9 as a potential biomarker using ROC curve analysis (AUC > 0.7 indicates high discriminative ability)

  • Evaluate implications for immunotherapy response through analysis of immune checkpoint expression and TIDE scores

How can discrepancies in CD9 functional studies between different cellular contexts be reconciled?

Reconciling discrepancies in CD9 functional studies requires systematic analysis of context-dependent factors:

• Species-specific differences:

  • Rat CD9 shares 86.9% amino acid sequence identity with human and 77.4% with mouse CD9

  • These differences may impact functional interactions and signaling outcomes

  • Cross-species studies should account for sequence variations in key interaction domains

• Expression level considerations:

  • Cellular responses may vary with CD9 expression levels

  • Biphasic activation curves observed with anti-CD9 antibodies suggest that optimal CD9 density is critical for function

  • Standardize expression levels across experimental systems using quantitative measures:

    • Antibody binding capacity (e.g., 1.6 × 10^5 binding sites per cell in CD9-RBL cells)

    • Flow cytometric quantification with calibrated standards

    • Western blot quantification normalized to housekeeping proteins

• Contextual protein interactions:

  • CD9 function depends on its interaction partners, which vary across cell types

  • In RBL cells, CD9 interacts with FcεRI, facilitating degranulation responses

  • In cancer cells, CD9 correlates with immune cell infiltration, particularly neutrophils

  • Comprehensive interactome analysis should be performed for each cellular context

• Methodological variations:

  • Detergent conditions critically affect the maintenance of CD9-containing complexes

    • CHAPS detergent preserves CD9-FcεRI complexes

    • Harsher detergents may disrupt important interactions

  • Antibody selection affects outcomes:

    • IgG1 isotype anti-CD9 antibodies can activate cells through FcεRI

    • IgG2a isotypes (e.g., ALMA 3) fail to stimulate despite binding to CD9

  • Standardize methodologies across studies and explicitly report critical parameters

How can recombinant CD9 be utilized to study its role in immune cell regulation and potential immunotherapeutic applications?

Recombinant CD9 offers several approaches to investigate immune regulation and immunotherapeutic applications:

• Immune checkpoint interaction studies:

  • Data shows CD9 correlates with 39 immune checkpoints, with the strongest correlation with CD44 (cor = 0.51)

  • Recombinant CD9 can be used to:

    • Map binding domains between CD9 and immune checkpoints

    • Develop blocking peptides that disrupt these interactions

    • Screen for small molecule modulators of these interactions

• Immune cell infiltration models:

  • CD9 expression correlates with neutrophil infiltration (cor = 0.30, P < 0.05)

  • High CD9 expression groups show higher rejection responses and TIDE scores

  • Experimental approaches should include:

    • Co-culture systems with immune cells and CD9-expressing targets

    • Analysis of immune cell activation markers and cytokine profiles

    • In vivo immune infiltration studies in CD9-manipulated tumor models

• Predictive biomarker development:

  • Recombinant CD9 can be used to develop standardized assays for:

    • Detection of circulating CD9+ exosomes as liquid biopsy markers

    • Screening patient samples for CD9 expression levels

    • Correlation with immunotherapy response parameters

• Immunomodulatory therapeutic approaches:

  • Recombinant CD9 domains or CD9-derived peptides may have therapeutic potential by:

    • Blocking key protein-protein interactions

    • Modulating immune cell trafficking or function

    • Altering tumor cell interactions with the immune microenvironment

Evidence suggests that CD9 plays a role in immune escape, with high CD9 expression potentially predicting lower success rates with immunotherapy .

What are the current challenges in translating CD9 research from basic science to clinical applications?

Translating CD9 research to clinical applications faces several challenges:

• Contextual complexity of CD9 function:

  • CD9 exhibits both tumor-promoting and tumor-suppressive roles depending on cancer type

  • In gliomas, high CD9 expression correlates with lower survival rates

  • In other cancers, CD9 may have opposite effects

  • Solution approach: Develop cancer-specific models and comprehensive biomarker panels

• Technical challenges in protein production and stability:

  • Full-length CD9 with proper folding and post-translational modifications is difficult to produce

  • Membrane proteins like CD9 present challenges for structural studies

  • Solution approach: Focus on specific domains (e.g., EC2) or develop stable cell-based assay systems

• Immunogenicity concerns:

  • Recombinant proteins may trigger immune responses in therapeutic applications

  • Species differences between rat and human CD9 (77.4% sequence identity) may limit direct translation

  • Solution approach: Humanize key domains or develop human-compatible mimetic peptides

• Standardization of biomarker assessment:

  • Variable detection methods affect CD9 quantification

  • Different antibodies recognize different epitopes with varying sensitivity

  • Solution approach: Develop standardized protocols and reference materials for CD9 detection

• Complex interaction network:

  • CD9 functions within a network of tetraspanins and other membrane proteins

  • Manipulating CD9 alone may have unpredictable effects on this network

  • Solution approach: Systems biology approaches to model interaction networks

What methodological approaches are most effective for studying CD9's role in extracellular vesicle biology?

Studying CD9's role in extracellular vesicle (EV) biology requires specialized methodological approaches:

• Isolation and characterization of CD9-positive EVs:

  • Differential ultracentrifugation combined with CD9 immunoaffinity capture

  • Size exclusion chromatography to separate vesicle populations

  • Nanoparticle tracking analysis for size distribution and concentration

  • Confirm CD9 presence by Western blotting and flow cytometry of captured vesicles

• Functional analysis of CD9-positive vs. CD9-negative EVs:

  • Comparative RNA-seq and proteomics of separated vesicle populations

  • Cellular uptake studies using labeled EVs

  • Functional assays measuring target cell responses:

    • Migration/invasion assays

    • Signaling pathway activation

    • Phenotypic changes

• Manipulating CD9 in EVs:

  • Generate EVs from cells with CD9 overexpression or knockdown

  • Use CRISPR-Cas9 to introduce mutations in specific CD9 domains

  • Compare EV production, content, and target cell effects

• Visualization techniques:

  • Super-resolution microscopy to study CD9 distribution on EVs

  • Cryo-electron microscopy for structural analysis

  • Correlative light and electron microscopy to track CD9+ EVs

• In vivo tracking of CD9+ EVs:

  • Label EVs with lipophilic dyes or CD9-fluorescent protein fusions

  • Track biodistribution using intravital microscopy

  • Assess functional effects on recipient tissues

The methodological approaches should be tailored to answer specific questions about CD9's role in EV biogenesis, cargo loading, target cell recognition, or functional effects on recipient cells.