HRH4 Antibody

What is HRH4 and where is it primarily expressed in human tissues?

The Histamine H4 Receptor (HRH4) is a low-molecular-weight G protein-coupled receptor (GPCR) that belongs to the 7-transmembrane domain superfamily. It is primarily expressed in hematopoietic cells including mast cells, eosinophils, dendritic cells, and T lymphocytes . Unlike the more ubiquitously expressed H1 and H2 receptors, HRH4 shows a more specialized distribution pattern, making it an important target for selective therapeutic interventions in immune-related conditions. Recent research has also detected HRH4 expression in brain endothelial cells, suggesting a broader tissue distribution than previously recognized . The receptor is encoded by the HRH4 gene located on chromosome 18q11, and several transcript variants have been identified across different tissues .

What experimental techniques are most reliable for detecting HRH4 expression?

Multiple complementary techniques should be employed for reliable HRH4 detection:

• Western Blot Analysis: Effective for detecting HRH4 protein in cell lysates, as demonstrated in studies of human chronic myelogenous leukemia (K562) and human promyelocytic leukemia (HL-60) cells .

• Flow Cytometry: Useful for cell surface detection of HRH4 in intact cells. For optimal results, use 5μg of anti-HRH4 antibody with appropriate secondary antibodies (e.g., goat-anti-rabbit-FITC) .

• RT-PCR and Quantitative PCR: Essential for detecting multiple HRH4 transcript variants. Primers targeting conserved regions include: HRH4 (sense): 5′-GTGGTTAGCATAGGTTATA C-3′, HRH4 (antisense): 5′-ATGCCACTGCACTCCTGC-3′ .

• Immunofluorescence Staining: Particularly useful for tissue sections, using anti-HRH4-antibody (1:50 dilution) and appropriate fluorophore-conjugated secondary antibodies .

• FISH (Fluorescence in situ Hybridization): For analyzing chromosomal abnormalities and copy number variations (CNVs) of the HRH4 gene in clinical samples .

How does HRH4 signaling differ from other histamine receptors?

HRH4 exhibits distinct signaling mechanisms compared to other histamine receptors:

HRH4 couples specifically to Gi/G0 proteins, and receptor activation leads to inhibition of adenylate cyclase, mobilization of calcium from intracellular stores, and activation of the mitogen-activated protein kinase (MAPK) cascade . This signaling pathway is particularly important in immune cell chemotaxis and inflammatory responses.

What are the key considerations when selecting anti-HRH4 antibodies for experiments?

When selecting anti-HRH4 antibodies, researchers should consider:

• Epitope Specificity: Target antibodies that recognize extracellular domains for live cell applications. For example, antibodies targeting the first extracellular loop (amino acids 75-87 of human HRH4) are effective for cell surface detection .

• Cross-Reactivity Testing: Validate specificity using appropriate blocking peptides. The search results indicate that preincubation with Human Histamine H4 Receptor/HRH4 extracellular Blocking Peptide can effectively eliminate signals in Western blot analysis .

• Application Suitability: Different applications require different antibody preparations. For instance, while blocking peptides can be used for Western blot validation, they may not be suitable for flow cytometry applications .

• Isoform Recognition: Consider whether the antibody can detect multiple splice variants of HRH4, which have been documented in various tissues .

• Species Reactivity: Ensure compatibility with your experimental model, as sequence differences exist between human, rat, and mouse HRH4.

How can researchers effectively investigate HRH4 expression changes in cancer models?

Investigating HRH4 expression changes in cancer models requires a multi-faceted approach:

• Expression Level Analysis: Quantify both protein and mRNA levels using Western blot and qPCR. In gastric carcinomas, attenuated expression levels of HRH4 protein correlate with advanced cancer stages compared to matched adjacent normal tissues (ANTs) .

• Copy Number Variation Analysis: Employ real-time PCR with the comparative Ct method to determine HRH4 gene copy numbers. Use cut-off values of 0.25, 0.75, 1.25, and 1.75 to define copy numbers as 0, 1, 2, and 3, respectively .

• Confirmation by FISH: Validate CNV findings using fluorescence in situ hybridization with chromosome 18q and 18q11-specific probes .

• Functional Analysis: Assess the biological significance of HRH4 expression changes using cell proliferation assays (WST-1), long-term cologenic assays, and cell cycle analysis by flow cytometry .

• Correlation with Clinical Parameters: Analyze the relationship between HRH4 expression levels and clinical features such as tumor stage, metastatic potential, and patient outcomes .

What methodological approaches can detect different HRH4 splice variants?

Detection of HRH4 splice variants requires specialized techniques:

• RT-PCR with Strategic Primer Design: Design primers that flank potential splice junctions. Research has identified two rat HRH4 gene transcripts in RBE4 cells: a full-length transcript (coding sequence 1173 bp) and a variant with a 164 bp deletion due to differential splicing with the second exon excluded .

• Sequencing Validation: Confirm splice variant identity through direct sequencing of PCR products .

• Isoform-Specific qPCR: Develop primers that specifically amplify individual isoforms for quantitative analysis of expression patterns across tissues.

• Splice Junction-Specific Antibodies: When available, use antibodies that specifically recognize epitopes created by alternative splicing events.

• Functional Characterization: Assess potential functional differences between splice variants using overexpression systems and signaling pathway analysis. For example, examine if variants differently affect the Erk1/2 MAPK pathway activation .

How do HRH4 expression patterns differ between normal and pathological states?

HRH4 expression shows significant alterations in various pathological states:

• Gastric Carcinoma: Significantly attenuated expression of HRH4 in advanced gastric carcinomas compared to matched adjacent normal tissues. Stage 3-4 tumors show more pronounced reduction than stage 0-2 tumors .

• Copy Number Variations in Cancer:

  • Stage 0-2 GC: 2/34 (5.9%) cases show HRH4 deletion

  • Stage 3 GC: 10/54 (18.5%) cases show HRH4 deletion

  • Stage 4 GC: 11/43 (25.6%) cases show HRH4 deletion

• CNV-Expression Correlation: Samples with deleted copies of HRH4 show significantly lower mRNA expression than those with unaltered copies (p=0.000109), suggesting that copy number loss contributes to down-regulation of HRH4 expression in gastric carcinomas .

• Brain Endothelial Cells: HRH4 expression in these cells, previously not well characterized, has functional implications for neuroinflammatory processes and blood-brain barrier function .

• Autoimmune Conditions: H4 receptor knockout mice develop more severe experimental autoimmune encephalomyelitis (EAE), suggesting HRH4's role in T cell recruitment and regulation of anti-inflammatory responses .

What are the critical controls needed when validating HRH4 antibody specificity?

Thorough validation of HRH4 antibody specificity requires multiple controls:

• Blocking Peptide Controls: Preincubate the anti-HRH4 antibody with a specific blocking peptide (e.g., Human Histamine H4 Receptor/HRH4 extracellular Blocking Peptide) to confirm signal specificity in Western blot analyses .

• Knockout/Knockdown Validation: Where possible, use HRH4 knockout tissues or cells with siRNA-mediated HRH4 knockdown as negative controls.

• Multiple Antibody Approach: Employ antibodies targeting different epitopes of HRH4 to confirm expression patterns.

• Cross-Species Validation: Test antibody reactivity across species if conducting comparative studies, noting potential differences in epitope sequences.

• Positive Control Tissues: Include tissues with established high HRH4 expression (e.g., spleen) as positive controls .

• Secondary Antibody-Only Controls: For immunofluorescence and flow cytometry, include samples treated only with secondary antibody to identify non-specific binding .

How can researchers distinguish between H3 and H4 receptor functions in experimental systems?

Distinguishing between histamine H3 and H4 receptor functions requires strategic approaches:

• Selective Pharmacological Tools:

  • H4-selective antagonist: JNJ 7777120 effectively blocks H4 receptor-mediated effects

  • H3-selective antagonist: Ciproxifan specifically blocks H3 receptor-mediated effects

  • Dual H3/H4 agonist: Immepip activates both receptors

• Receptor Expression Analysis: Characterize the specific expression patterns of H3R and H4R in your experimental system. For example, in RBE4 cells, both the full-length H3 receptor and a 144 bp deletion variant are expressed alongside H4 receptors .

• Signaling Pathway Discrimination: Monitor specific downstream pathways. Both receptors can activate the Erk1/2 MAPK pathway, but selective antagonists can differentiate the source of activation .

• Functional Readouts with Selective Inhibition: Measure functional responses (e.g., calcium mobilization, chemotaxis) with and without selective receptor blockade.

• Genetic Approaches: Use siRNA knockdown or CRISPR/Cas9 editing to selectively modulate each receptor type.

What are common challenges in detecting low-abundance HRH4 in tissue samples?

Researchers frequently encounter several challenges when detecting low-abundance HRH4:

• Signal Amplification Strategies: For tissues with low HRH4 expression, consider:

  • Using tyramide signal amplification for immunohistochemistry

  • Employing nested PCR approaches for transcript detection

  • Utilizing more sensitive detection methods such as droplet digital PCR

• Sample Preparation Optimization: Cell membrane proteins like HRH4 may require specialized extraction protocols. Ensure complete solubilization using appropriate detergents while maintaining the native conformation of the receptor.

• Background Reduction: Optimize blocking conditions (5% BSA or 5-10% normal serum from the same species as the secondary antibody) and include additional washing steps to improve signal-to-noise ratio in immunodetection methods .

• Antibody Concentration Titration: Perform systematic antibody dilution series to determine optimal concentration for specific detection. The research indicates successful flow cytometry using 5μg of anti-HRH4 antibody .

• Cross-Reactivity Management: Validate antibody specificity against other histamine receptor subtypes, particularly H3R which shares some sequence homology with H4R.

How should researchers interpret contradictory HRH4 expression data across different detection methods?

When facing contradictory HRH4 expression data:

• Methodological Considerations:

  • Protein vs. mRNA discrepancies: Post-transcriptional regulation may lead to differences between transcript and protein levels

  • Antibody epitope accessibility: Different antibodies may recognize epitopes with variable accessibility depending on tissue preparation

  • Detection thresholds: Methods vary in sensitivity, potentially leading to false negatives in less sensitive approaches

• Biological Variables:

  • Alternative splicing: Different detection methods may preferentially detect certain splice variants

  • Receptor internalization: Surface expression detected by flow cytometry may differ from total expression detected by Western blot

  • Pathological state influence: Disease progression may affect HRH4 expression patterns

• Resolution Strategies:

  • Employ multiple detection methods on the same samples

  • Use isoform-specific detection approaches

  • Include positive controls with known HRH4 expression patterns

  • Consider cell-specific expression analyses using single-cell approaches

What novel approaches can improve detection of HRH4 in complex tissue microenvironments?

Emerging technologies offer improved HRH4 detection in complex tissues:

• Multiplex Immunofluorescence: Simultaneously detect HRH4 alongside cell-type specific markers to characterize expression in heterogeneous tissues.

• Proximity Ligation Assay (PLA): Detect protein-protein interactions involving HRH4, providing insights into receptor complex formation and signaling.

• Single-Cell RNA Sequencing: Characterize HRH4 expression at single-cell resolution to identify specific cell populations expressing the receptor within complex tissues.

• CRISPR-Based Tagging: Generate endogenously tagged HRH4 to facilitate visualization without overexpression artifacts.

• Super-Resolution Microscopy: Employ techniques like STORM or PALM to visualize HRH4 distribution at nanoscale resolution in cellular membranes.

• Mass Cytometry (CyTOF): Use metal-conjugated antibodies against HRH4 and other markers for high-dimensional analysis of expression patterns across multiple cell types.

How does HRH4 function in neuroinflammation and CNS disorders?

Recent research has revealed important roles for HRH4 in neuroinflammation:

• Brain Endothelial Expression: Contrary to previous beliefs that HRH4 expression was largely restricted to hematopoietic cells, studies have identified functional HRH4 in brain endothelial cells, suggesting direct involvement in blood-brain barrier function .

• Anti-inflammatory Modulation: HRH4 appears to act as a modulator of anti-inflammatory responses within the CNS. HRH4-knockout mice develop more severe experimental autoimmune encephalomyelitis (EAE) upon myelin oligodendrocyte glycoprotein administration .

• T Cell Regulation: HRH4 is implicated in T cell recruitment and regulation of T cell chemotaxis and suppressor activity, thereby influencing anti-inflammatory responses in neuroinflammatory conditions .

• MAPK Pathway Activation: Both histamine and immepip (H3 and H4 receptor agonists) activate the Erk1/2 MAPK pathway in brain blood vessels in vivo by activating H4 receptors, which can be blocked by the H4 receptor-specific antagonist JNJ 7777120 .

• Splice Variant Functions: Multiple HRH4 transcript variants in brain endothelial cells suggest differential functions that may contribute to region-specific responses in neuroinflammation .

What is the significance of HRH4 copy number variations in cancer progression?

HRH4 copy number variations (CNVs) have emerging significance in cancer:

• Progressive HRH4 Deletion in Advanced Cancer: The frequency of HRH4 gene deletion increases with cancer stage progression in gastric carcinomas:

• Genotype-Phenotype Correlation: Samples with deleted copies of HRH4 show significantly lower mRNA expression levels compared to those with unaltered copies (p=0.000109), establishing a direct link between CNVs and receptor expression .

• Validation by Multiple Methods: Both real-time PCR-based CNV analysis and fluorescence in situ hybridization (FISH) confirm the deletion of the 18q11 region containing the HRH4 gene in gastric carcinoma tissues .

• Potential as Biomarker: The progressive increase in HRH4 deletion frequency with advancing cancer stages suggests its potential utility as a prognostic biomarker .

• Mechanistic Implications: While CNV deletion contributes to reduced HRH4 expression, the observation that some samples with unaltered copy numbers also show reduced expression indicates additional regulatory mechanisms are involved .

How do epigenetic modifications affect HRH4 expression and function?

While the provided search results don't directly address epigenetic regulation of HRH4, researchers should consider:

• Promoter Methylation Analysis: Investigate whether DNA methylation patterns in the HRH4 promoter region correlate with expression changes observed in cancer and other conditions.

• Histone Modification Profiling: Examine histone modifications (acetylation, methylation) at the HRH4 locus using ChIP-seq to identify epigenetic mechanisms controlling tissue-specific expression.

• miRNA Regulation: Analyze potential miRNA binding sites in HRH4 mRNA and test whether specific miRNAs modulate HRH4 expression post-transcriptionally in different physiological and pathological states.

• Epigenetic Drug Effects: Assess whether HDAC inhibitors, DNA methyltransferase inhibitors, or other epigenetic modulators affect HRH4 expression patterns.

• Integration with CNV Data: Consider how epigenetic modifications might interact with the copy number variations observed in cancer to collectively influence HRH4 expression levels .

What emerging therapeutic applications might target HRH4 in immune and inflammatory disorders?

Potential therapeutic applications targeting HRH4 include:

• Autoimmune Disease Modulation: Based on findings that H4 receptor-knockout mice develop more severe EAE, HRH4 agonists might help regulate T cell responses in multiple sclerosis and other autoimmune conditions .

• Blood-Brain Barrier Modulation: Given HRH4 expression in brain endothelial cells, selective modulators could potentially influence BBB permeability in neuroinflammatory conditions .

• Cancer Immunotherapy: Understanding the role of HRH4 in immune cell function could inform new approaches to enhance anti-tumor immune responses, particularly in contexts where HRH4 expression is altered .

• Allergy and Asthma Treatment: HRH4's role in mast cell and eosinophil function suggests potential applications in allergic inflammation beyond current antihistamines targeting H1 receptors .

• Anti-inflammatory Drug Development: HRH4-selective compounds could provide more targeted approaches to inflammation with potentially fewer side effects than current anti-inflammatory therapies.

What are promising protocols for functional characterization of HRH4 in primary human cells?

For functional characterization of HRH4 in primary human cells:

• Calcium Mobilization Assays: Monitor intracellular calcium levels using fluorescent indicators (Fluo-4, Fura-2) following stimulation with histamine or selective H4 agonists, with and without selective antagonists .

• MAPK Pathway Analysis: Assess Erk1/2 phosphorylation by Western blot or flow cytometry to quantify pathway activation in response to receptor stimulation .

• Migration and Chemotaxis Assays: Use Transwell or microfluidic systems to assess H4R-mediated cell migration in response to histamine gradients.

• Gene Expression Profiling: Perform RNA-seq after H4R stimulation or blockade to identify downstream gene expression changes.

• Receptor Internalization Studies: Use fluorescently labeled antibodies or tagged ligands to track receptor trafficking following activation.

• Electrophysiological Measurements: In appropriate cell types, assess ion channel modulation in response to H4R activation.

• Multiplex Cytokine Analysis: Measure secreted inflammatory mediators following receptor modulation to characterize functional outcomes in immune cells.