Recombinant Mouse Protein orai-3 (Orai3)

What is Orai3 and what is its role in calcium signaling pathways?

Orai3 is one of three Orai family calcium channel proteins (Orai1, Orai2, and Orai3) that function as components of the calcium release-activated calcium (CRAC) channels. These channels mediate store-operated calcium entry (SOCE), a critical signaling mechanism in immune cells. Orai3 participates in regulating cytosolic calcium levels after endoplasmic reticulum (ER) calcium store depletion, contributing to calcium oscillations and downstream signaling events .

In mouse B cells, Orai3 works alongside Orai1 to mediate calcium influx following B cell receptor (BCR) stimulation, though its contribution appears to be less significant than that of Orai1. Research shows that Orai3 deletion alone reduces SOCE by approximately 28% in A20 B lymphoblasts, while Orai1 deletion causes a 62% reduction .

How does Orai3 expression compare to other Orai isoforms in immune cells?

Interestingly, Orai3 has been identified as the most highly expressed ORAI paralog in many immune cell subsets, including B cells and macrophages . This expression pattern suggests potential specialized functions for Orai3 in these cell types.

The expression of Orai isoforms is dynamically regulated in response to B cell activation. Studies show that stimulation with anti-IgM dramatically increases Orai1 expression, while Orai2 expression is significantly reduced under conditions that increase mitochondrial respiration (anti-IgM, anti-IgM + anti-CD40, LPS). Orai3 expression significantly increases only with anti-IgM + anti-CD40 or LPS stimulation, but not with anti-IgM or anti-CD40 alone .

What are the distinctive pharmacological properties of Orai3 compared to other Orai isoforms?

Orai channel isoforms demonstrate distinct pharmacological profiles and sensitivities to various CRAC channel modifiers. One extensively utilized modifier is 2-aminoethyl diphenyl borate (2-APB), which at high concentrations (25-50 μM) strongly inhibits Orai1, partially inhibits Orai2, but potentiates Orai3 channel activity .

This distinct pharmacological profile makes 2-APB a valuable tool for distinguishing between Orai isoform contributions to calcium signaling in experimental settings. Researchers can use this differential response to 2-APB to identify the relative contributions of Orai3 versus other isoforms in calcium entry pathways within specific cell types.

How does Orai3 contribute to calcium oscillations in B cells?

Research demonstrates that Orai3 plays a role in maintaining calcium oscillations following B cell receptor stimulation. In A20 B lymphoblasts, the combined deletion of Orai1 and Orai3 substantially reduced the frequency of calcium oscillations induced by anti-IgG stimulation, while deletion of either Orai1 or Orai3 individually only slightly reduced oscillation frequency .

This finding aligns with studies in other cell types showing that under conditions of physiological agonist stimulation causing modest ER store depletion while eliciting calcium oscillations, Orai2 and Orai3 were sufficient to maintain cytosolic calcium oscillations, while having relatively minor contributions to SOCE induced by maximal store depletion .

What insights have been gained from Orai3 knockout models?

Studies using Orai3 knockout mice have revealed several important findings:

• Single Orai3 deficiency had no significant effects on the numbers and phenotype of lymphoid and myeloid cells in primary or secondary lymphoid organs .

• Deletion of Orai3 alone did not affect SOCE in T and B cells following antigen-receptor stimulation but moderately enhanced SOCE in macrophages, potentially indicating an inhibitory role of Orai3 in CRAC channel regulation in these cells .

• Despite robust expression in lymphoid and myeloid cells, deletion of Orai3 alone did not impair their function in vitro or affect immune responses in vivo .

• In B cells specifically, Orai3 knockout maintained normal proliferation in response to anti-IgM stimulation (90.3% of control) but showed higher viability (34.8%) compared to controls .

How does combined Orai1/Orai3 knockout affect B cell function?

The combined deletion of both Orai1 and Orai3 has more profound effects than single knockouts:

• Nearly complete abrogation of SOCE (reduction by ~91%) in A20 B lymphoblasts .

• Substantial reduction in both viable (10%) and proliferating (60.8%) cells in response to anti-IgM stimulation .

• Significant downregulation of pathways governing survival and cell cycle progression, as revealed by gene set enrichment analysis (GSEA) .

• Strong inhibition of SOCE and impaired mitochondrial metabolism in B cells .

Interestingly, these defects in survival and proliferation of Orai1/Orai3-deficient B cells were partially rescued when cells were co-stimulated with anti-IgM + anti-CD40, suggesting alternative pathways can compensate for CRAC channel deficiency under certain stimulation conditions .

How does Orai3 influence metabolic reprogramming in activated B cells?

Orai3, particularly in conjunction with Orai1, plays a critical role in B cell metabolic reprogramming. Research shows that combined deletion of Orai1 and Orai3 strongly inhibits SOCE and hampers mitochondrial metabolism in B cells .

Transcriptome and metabolomic analyses have uncovered key signaling pathways regulated by SOCE and the phosphatase calcineurin that are essential for the efficient transition of B cells from a quiescent to a metabolically active state . This metabolic reprogramming is crucial for B cell proliferation and effector functions following antigen receptor stimulation.

The data suggests that calcium signaling mediated by Orai1 and Orai3 is integral to initiating the metabolic shifts required for B cell activation, including changes in both mitochondrial respiration and glycolysis.

What is the relationship between Orai3 and NFAT activation in B cells?

The combined loss of Orai1 and Orai3 strongly impairs SOCE and Nuclear Factor for Activated T cells (NFAT) activation in B cells . NFAT is a calcium-dependent transcription factor that regulates various aspects of immune cell function.

This connection between Orai3/Orai1-mediated calcium entry and NFAT activation represents an important mechanistic link between calcium signaling and transcriptional regulation in B cells. The impaired NFAT activation likely contributes to the defects in proliferation, survival, and metabolic reprogramming observed in Orai1/Orai3-deficient B cells.

What compensatory mechanisms exist in the absence of Orai3?

Research indicates several compensatory mechanisms in the absence of Orai3:

• Studies have shown that despite the high expression of Orai3 in B cells and macrophages, its deletion alone does not impair their function, suggesting that other Orai homologs compensate for the loss of Orai3 .

• Stimulation with either anti-CD40 or LPS results in similar percentages of viability and proliferation across control and Orai-knockout B cells, suggesting that activation of the toll-like receptor 4 (TLR4) signaling pathway can compensate for the loss of Orai1/Orai3 .

• Humoral immunity to influenza A virus infection remained unaltered in mice with B cell-specific deletion of both Orai1 and Orai3, suggesting that alternative calcium-independent signaling pathways activated in vivo through co-stimulatory receptors can overcome the loss of CRAC channel activity in B cells .

How do different stimuli affect Orai3-dependent calcium signaling?

The impact of Orai3 on calcium signaling varies depending on the stimulus:

• Anti-IgM stimulation alone: Orai3-deficient B cells show comparable proliferation to controls (90.3%) but higher viability (34.8%) .

• Anti-IgM + anti-CD40 co-stimulation: This partially rescues the defects in survival and proliferation observed in Orai1/Orai3-deficient B cells .

• Anti-CD40 or LPS stimulation alone: Similar percentages of viability and proliferation are observed across all experimental groups, including Orai1/Orai3-deficient cells .

• Expression pattern responses: Orai3 expression significantly increases only with anti-IgM + anti-CD40 or LPS stimulation, but not with anti-IgM or anti-CD40 alone .

This differential response to various stimuli suggests that the requirement for Orai3-mediated calcium signaling depends on the specific activation pathway engaged.

What are the optimal techniques for measuring Orai3-mediated calcium entry?

Based on the research methodologies described in the studies:

• Store depletion with thapsigargin: Using thapsigargin, an inhibitor of the sarcoplasmic/endoplasmic reticulum ATPase (SERCA), to passively deplete ER calcium stores and measure subsequent calcium entry provides a reliable method to assess SOCE in various cell types .

• Calcium oscillation measurements: Monitoring cytosolic calcium oscillations induced by anti-IgG stimulation provides insights into the physiological calcium signaling patterns that may be more relevant than maximal SOCE measurements for understanding Orai3 function .

• Pharmacological profiling with 2-APB: Utilizing 2-APB to distinguish between Orai isoforms based on their differential sensitivity (inhibition of Orai1 and Orai2 but potentiation of Orai3) can help delineate the contribution of each isoform to calcium signaling .

What considerations should be made when designing Orai3 knockout experiments?

When designing Orai3 knockout experiments, researchers should consider:

• Potential compensatory mechanisms: Single Orai3 knockout may have minimal effects due to compensation by other Orai isoforms. Combined knockouts (e.g., Orai1/Orai3 double knockout) may be necessary to observe significant functional effects .

• Cell type specificity: Generate cell-specific knockout models (e.g., B cell-specific Orai3 knockout using systems like Mb1-Cre) to avoid potential confounding effects from systemic knockout .

• Stimulation conditions: Test multiple stimulation conditions (e.g., anti-IgM alone, anti-IgM + anti-CD40, anti-CD40 alone, LPS) as the requirement for Orai3 may vary depending on the activation pathway engaged .

• In vitro vs. in vivo effects: Consider that in vitro observations may not always translate to in vivo immune responses due to additional compensatory mechanisms present in the complex in vivo environment .