Recombinant Mouse High affinity immunoglobulin epsilon receptor subunit alpha (Fcer1a)

What is the structural composition of Fcer1a in mice?

Mouse Fc epsilon RI alpha (Fcer1a) cDNA encodes 250 amino acids including a 23 amino acid signal sequence, a 181 amino acid extracellular domain containing two Ig-like domains, a 19 amino acid transmembrane domain, and a 27 amino acid cytoplasmic sequence. The protein shares 52% and 71% amino acid sequence identity with human and rat Fc epsilon RI alpha, respectively . The receptor itself exists as a tetrameric complex (αβγ₂) on mast cells and basophils, consisting of one alpha subunit, one beta subunit, and two gamma subunits . Unlike humans, mice do not express the alternate trimeric form (αγ₂) that is found on human mast cells, basophils, eosinophils, and antigen-presenting cells .

How does Fcer1a function in IgE-mediated immune responses?

Fcer1a is responsible for high-affinity binding with the Fc portion of IgE, which is critical for IgE-dependent disease responses including allergic reactions and anti-parasitic immunity . When IgE binds to Fcer1a, it increases surface expression of the receptor. Cross-linking of IgE/Fc epsilon RI complexes by allergens initiates receptor internalization and signaling . This activation in mast cells and basophils leads to degranulation, resulting in the release of histamine, leukotrienes, prostaglandins, and other mediators of immediate-type and late-phase allergic reactions . Experimental models demonstrate that Fcer1a plays a crucial role in both cutaneous anaphylaxis and immune responses to parasitic infections like Schistosoma japonicum .

What role does the natural antisense transcript FCER1A-AS play in regulating Fcer1a expression?

Recent research has uncovered a novel regulatory pathway for Fcer1a expression involving a natural antisense transcript called FCER1A-AS. This transcript is co-expressed with the sense transcript (FCER1A-S) in both interleukin (IL)-3-induced Fcer1a-expressing cells and in the high Fcer1a-expressing cell line MC/9 . When FCER1A-AS is selectively knocked down using CRISPR/RfxCas13d (CasRx) in MC/9 cells, the expression of both FCER1A-S mRNA and proteins is markedly decreased .

This regulatory mechanism appears to occur through cis-regulation, as FCER1A-AS deficiency is associated with lack of FCER1A-S expression in vivo . The presence of FCER1A-AS is essential for sense transcript expression in mast cells and basophils, but not for the differentiation of these cells . This represents a significant advancement in understanding the post-transcriptional regulation of Fcer1a expression and provides potential new targets for therapeutic intervention in IgE-mediated disorders.

How do FCER1A knockout and FCER1A-AS deficient mice compare phenotypically?

Homozygous mice deficient in FCER1A-AS demonstrate a similar phenotype to FCER1A knockout mice in two key experimental models:

• Schistosoma japonicum infection model: FCER1A-KO mice showed an 88% mortality rate, significantly higher than the 61% mortality rate in wild-type mice. Notably, homozygous FCER1A-AS deficient mice (FCER1A dtr/dtr) exhibited a similar mortality rate of 84%, despite the protein-encoding sequences for Fcer1a not being directly altered during gene editing .

• IgE-Fcer1a-mediated cutaneous anaphylaxis: In passive cutaneous anaphylaxis (PCA) tests, which rely entirely on the presence of Fcer1a, both FCER1A-KO and FCER1A dtr/dtr mice displayed approximately 10-fold less Evans blue extravasation compared to wild-type mice .

The table below summarizes the phenotypic comparison:

These findings underscore the critical role of FCER1A-AS in maintaining normal Fcer1a expression and function in vivo .

What approaches are effective for studying FCER1A-AS and FCER1A-S co-expression?

For investigators studying the co-expression of FCER1A-AS and FCER1A-S, several methodological approaches have proven successful:

• Cell culture systems: The MC/9 cell line provides an excellent model for studying high Fcer1a expression . Additionally, IL-3-induced bone marrow-derived mast cells and basophils can be used to study the co-expression of these transcripts in primary cells.

• Selective knockdown techniques: The CRISPR/RfxCas13d (CasRx) approach has been successfully employed for selective knockdown of FCER1A-AS without directly affecting the FCER1A-S genomic sequence . This RNA-targeting CRISPR system allows for precise manipulation of antisense RNA levels.

• Quantitative analysis: Combining flow cytometry for protein detection (using fluorescence-labeled anti-Fcer1a monoclonal antibodies like MAR-1) with RT-qPCR for transcript level assessment provides comprehensive analysis of both sense and antisense transcript expression and their relationship to protein levels .

How can recombinant Fcer1a proteins be optimally handled and used in experimental settings?

Recombinant mouse Fc epsilon RI alpha proteins require specific handling conditions to maintain their biological activity:

• Reconstitution: Lyophilized recombinant Fcer1a should be reconstituted at a concentration of 100 μg/mL in water . This concentration provides a stable stock solution for experimental use.

• Storage conditions: Use a manual defrost freezer and avoid repeated freeze-thaw cycles to maintain protein integrity . For long-term storage, aliquot the reconstituted protein to minimize freeze-thaw cycles.

• Shipping and handling: The product is typically shipped at ambient temperature but should be stored immediately at the recommended temperature upon receipt .

• Carrier considerations: Carrier-free (CF) formulations are available for applications where the presence of bovine serum albumin (BSA) might interfere with experimental outcomes . Standard formulations containing BSA offer enhanced protein stability, increased shelf-life, and allow for more dilute storage concentrations.

• Functional validation: The binding ability of recombinant Fcer1a can be measured in functional ELISA assays. For example, recombinant mouse Fc epsilon RI alpha mFc chimera protein binds to human Fc epsilon RI alpha antibody with an ED₅₀ of 80.0-800 ng/mL .

What models are available for studying Fcer1a function in vivo?

Several genetically modified mouse models have been developed to study Fcer1a function:

• FCER1A knockout mice: These mice have the entire protein-encoding sequence of Fcer1a replaced with DTR-P2A-GFP (diphtheria toxin receptor-P2A-green fluorescent protein), resulting in complete absence of Fcer1a expression . They show significant impairment in IgE-mediated responses and increased susceptibility to Schistosoma japonicum infection.

• FCER1A-DTR mice: These mice were constructed by incorporating sequences encoding human DTR and self-cleaving peptide (P2A) before the 3′ UTR without disturbing protein-encoding sequences of Fcer1a . Unexpectedly, these mice display defective Fcer1a expression despite the intact coding sequence, highlighting the importance of non-coding regulatory elements.

• Passive cutaneous anaphylaxis model: This experimental approach allows for quantitative assessment of IgE-Fcer1a-mediated allergic responses through measurement of Evans blue extravasation following allergen challenge . The model involves skin sensitization via subcutaneous injection of anti-DNP IgE into mouse ears, followed by intravenous challenge with DNP antigen and Evans blue dye.

When selecting an appropriate model, researchers should consider the specific aspects of Fcer1a biology under investigation and the potential confounding factors associated with each model system.

How can Fcer1a expression be accurately measured in different cell populations?

Multiple complementary approaches can be employed to comprehensively assess Fcer1a expression:

• Flow cytometry: Fluorescence-labeled anti-Fcer1a monoclonal antibodies (such as MAR-1) can be used to detect Fcer1a-positive cells in peripheral blood, bone marrow cultures, and tissue samples . This technique allows for quantification of both the percentage of positive cells and the expression level per cell.

• Transcript analysis: RT-qPCR can be used to measure FCER1A-S and FCER1A-AS transcript levels. Proper primer design is critical to distinguish between sense and antisense transcripts .

• Cell identification markers: When analyzing mixed cell populations, additional markers should be included to identify specific Fcer1a-expressing cell types. For example, basophils can be identified using CD200R3 and CD49b markers in peripheral blood .

• Induction protocols: IL-3 treatment of bone marrow cells for 6 days provides a reliable method to induce Fcer1a expression in vitro . This approach can be used to study the regulation of expression and the impact of genetic or pharmacological interventions.

When implementing these methods, appropriate controls should be included, such as FCER1A knockout cells/tissues as negative controls and known high-expressing cells like MC/9 as positive controls.

What factors influence Fcer1a expression levels in experimental systems?

Several key factors have been identified that regulate Fcer1a expression:

• IL-3 signaling: The IL-3-GATA-2 pathway promotes Fcer1a expression during cell differentiation . IL-3 treatment of bone marrow cells induces Fcer1a expression and is commonly used in experimental protocols.

• Natural antisense transcript: FCER1A-AS plays a crucial role in maintaining FCER1A-S expression in mast cells and basophils through cis-regulation . This represents a novel regulatory mechanism distinct from the known IL-3-GATA-2 pathway.

• IgE binding: Binding of IgE alone increases surface expression of Fc epsilon RI, while crosslinking of IgE/Fc epsilon RI complexes by allergens initiates receptor internalization and signaling . This dynamic regulation affects the detectable levels of surface receptor.

• Beta subunit expression: The beta subunit of Fc epsilon RI increases the half-life of the receptor complex on the cell surface, affecting steady-state expression levels . An isoform of the beta subunit, beta T, can block processing of the alpha subunit and its cell surface expression .

• Pathological conditions: Certain disease states, such as Schistosoma japonicum infection, can elevate the number of Fcer1a-positive cells . These changes should be considered when designing experiments using disease models.

Understanding these regulatory factors is essential for properly interpreting experimental results and designing interventions targeting Fcer1a expression.

How can research on FCER1A-AS inform new approaches to allergic disease treatment?

The discovery of FCER1A-AS as a critical regulator of Fcer1a expression opens new avenues for therapeutic intervention in allergic diseases:

• RNA-based therapeutics: Targeting the FCER1A-AS/FCER1A-S regulatory axis could provide a novel approach to modulating IgE-mediated allergic responses without directly targeting the protein or its ligand . This might offer more specific control of Fcer1a expression in relevant cell types.

• Cell-specific targeting: Since FCER1A-AS affects Fcer1a expression but not the differentiation of mast cells and basophils , therapeutics targeting this pathway might modulate allergic responses without depleting important immune cell populations.

• Biomarker development: Expression levels of FCER1A-AS could potentially serve as biomarkers for predicting the severity of allergic responses or susceptibility to certain parasitic infections, based on its association with Fcer1a expression levels .

• Combination approaches: Understanding the interplay between the IL-3-GATA-2 pathway and FCER1A-AS regulation could inform combination therapies that more effectively downregulate Fcer1a expression in allergic conditions .

The development of these approaches requires further research into the molecular mechanisms by which FCER1A-AS regulates FCER1A-S expression and the factors that control FCER1A-AS expression itself.

What are the implications of mouse Fcer1a research for understanding human allergic conditions?

While there are important similarities between mouse and human Fcer1a biology, several key differences must be considered when translating findings:

• Receptor structure differences: Mice exclusively express the tetrameric (αβγ₂) form of Fc epsilon RI on mast cells and basophils, whereas humans also express a trimeric (αγ₂) form on additional cell types, including eosinophils and antigen-presenting cells . This structural difference affects cell type-specific responses to IgE.

• Expression pattern differences: The broader expression of Fc epsilon RI in humans (on DCs and Langerhans cells) means that therapies targeting this receptor may have different immunological effects across species . Human Fc epsilon RI on antigen-presenting cells mediates uptake and processing of allergens for presentation by class II MHC .

• Regulatory mechanisms: While FCER1A-AS has been identified as a critical regulator in mice , confirmation of similar regulatory mechanisms in humans is needed before therapeutic approaches can be developed.

• Disease model relevance: Mouse models of anaphylaxis and parasitic infection have provided valuable insights into Fcer1a function , but human allergic conditions have additional complexity that must be considered, including genetic factors and environmental variables.

These considerations highlight the importance of validating mouse findings in human systems before developing therapeutic applications based on Fcer1a research.