Recombinant Mouse Calcium release-activated calcium channel protein 1 (Orai1)

What is the functional significance of Orai1 in calcium signaling?

Orai1 functions as a calcium channel subunit that facilitates store-operated calcium entry (SOCE) in various cell types. It plays a critical role in coordinating intracellular calcium dynamics following depletion of endoplasmic reticulum calcium stores. Research has shown that Orai1 is the predominant SOCE channel supporting membrane Ca²⁺ entry within the plane of adhesive contact with inflammatory substrates . In immune cells, Orai1-mediated calcium influx is essential for processes such as immune synapse formation in T cells and contributes significantly to neutrophil arrest and polarization during inflammatory responses .

How do Orai1 knockout models affect calcium signaling in mice?

Studies utilizing heterozygous Orai1 knockout mice (Orai1⁻/⁺) have demonstrated approximately 47% reduced peak calcium upregulation compared to wild-type littermates, confirming Orai1's function as a primary store-operated channel . In B cells from B cell-specific Orai1 knockout (Orai1ᶠˡ/ᶠˡ Mb1-Cre/+) mice, SOCE is reduced by approximately 69% compared to controls . Interestingly, while Orai1 knockout significantly decreases SOCE, some studies have found that it may be dispensable for maintaining calcium oscillations in response to physiological agonist stimulation in B cells .

What are the phenotypic consequences of Orai1 deletion in mice?

Complete homozygous deletion of Orai1 (Orai1⁻/⁻) in mice on the C57BL/6 background results in perinatal lethality . Research with heterozygous Orai1 (G100S/+) mice, which model gain-of-function mutations, shows that homozygosity (G100S/G100S) causes early embryonic lethality . These findings highlight the essential role of properly regulated Orai1 function in development. In conditional knockout models, Orai1 deletion in specific tissues reveals its importance in various physiological processes, including skin homeostasis, where targeted disruption of the Orai1 gene affects epidermal function .

What methods are most effective for generating Orai1 mutant mouse models?

CRISPR/Cas9 gene editing has proven effective for generating Orai1 mutant mice. For example, Orai1 (G100S) mice were successfully generated using CRISPR/Cas9 by the Mouse Genome Editing Resource Facility at the University of Rochester Medical Center . The approach involved introducing a glycine to serine mutation at amino acid 100 (G100S), analogous to the G98S mutation found in humans with tubular aggregate myopathy (TAM).

For creating knockout models, homologous recombination in embryonic stem cells has been successfully employed. Gene targeting of the Orai1 gene has been performed in B6/3 embryonic stem cells derived from C57BL/6 mice, with chimeric mice generated by blastocyst injection of heterozygous Orai1ⁿᵉᵒ/⁺ ES cell clones . Cell-specific conditional knockouts, such as B cell-specific Orai1 knockout mice (Orai1ᶠˡ/ᶠˡ Mb1-Cre/+), have been generated using Cre-loxP technology, achieving approximately 97% reduction in Orai1 mRNA expression .

How can researchers distinguish between different Orai isoforms in experimental systems?

Distinguishing between Orai isoforms (Orai1, Orai2, Orai3) requires careful experimental design:

• mRNA quantification: Quantitative PCR with isoform-specific primers can effectively measure relative expression levels of each Orai isoform. For example, studies have used a prevalidated TaqMan gene expression assay to quantify Orai1 mRNA levels in HL-60 cells and to confirm siRNA-dependent knockdown .

• Protein detection: Western blotting can distinguish between Orai1 isoforms when properly optimized. For instance, native ORAI1 isoforms (ORAI1α and ORAI1β) from HEK293 cells can be resolved as two distinct bands, but only after protein samples are deglycosylated with PNGase F .

• Functional analysis: While antibody cross-reactivity can be an issue (with approximately 30% non-specific binding of some Orai1 antibodies to other Orai homologs due to shared epitopes ), functional studies combined with genetic approaches offer more definitive distinction. For example, comparing SOCE in wild-type versus Orai knockout cells helps determine the contribution of each isoform to calcium signaling.

What experimental systems are optimal for studying Orai1 function in vitro?

Several experimental systems have proven effective for studying Orai1 function:

• Cell lines with CRISPR/Cas9-mediated Orai1 knockout: ORAI1-knockout HEK293 cells provide a clean background for expressing and studying individual Orai1 isoforms .

• Primary cells from Orai1 knockout mice: Primary cells isolated from tissue-specific Orai1 knockout mice offer physiologically relevant systems. For example, mouse primary keratinocytes can be isolated from Orai1⁻/⁻ mice using a standard protocol involving collagenase/dispase digestion .

• Yeast expression systems: Recombinant ORAI1 protein has been successfully expressed in yeast (sec6-4 strain) for in vitro functional studies, allowing investigation of channel gating mechanisms .

• siRNA knockdown in appropriate cell lines: For example, Orai1 function has been assessed in HL-60 cells following siRNA knockdown, offering an alternative to genetic knockout models .

How does Orai1 interact with STIM1 to facilitate calcium entry?

Orai1 interacts directly with STIM1 through protein-protein binding to facilitate store-operated calcium entry. Studies using recombinant proteins have provided unambiguous proof of a direct interaction between the C-terminal cytoplasmic tail of ORAI1 (residues 259-301) and the cytoplasmic domain of STIM1 . This interaction is essential for the recruitment of ORAI1 to puncta by full-length STIM1 and subsequent channel activation.

What is known about the clustering behavior of Orai1 during activation?

Orai1 forms distinct spatial arrangements at the plasma membrane that change during activation. Studies using label quantification in STEM (Scanning Transmission Electron Microscopy) images have revealed:

• Resting state: Even at rest, Orai1 forms supra-molecular clusters with dimensions exceeding that of individual channels .

• Activated state: Upon SOCE activation, Orai1 partially relocates into distinct accumulation areas called puncta . These puncta have dimensions of approximately 2 ± 0.5 μm² as measured directly from STEM images .

• Label density changes: The average label density increases from 155/μm² ± 87/μm² in resting cells to 334/μm² ± 108/μm² inside regions of puncta with sub-maximal activation, and further increases to 640/μm² ± 193/μm² with maximal activation .

• Structure within puncta: Even within the dense ORAI1 accumulation areas, elongated cluster arrangements can be recognized, suggesting that puncta include chain-like supra-molecular ORAI1 clusters .

How do Orai isoforms (Orai1, Orai2, Orai3) functionally differ in calcium signaling?

The Orai family consists of three members (Orai1, Orai2, and Orai3) with distinct functional roles:

What are the key considerations when studying Orai1 gain-of-function mutations in mouse models?

Studying gain-of-function Orai1 mutations requires careful experimental design and awareness of several challenges:

• Embryonic lethality: Homozygous gain-of-function mutations such as G100S/G100S in Orai1 result in early embryonic lethality, complicating the generation of viable mouse models . From 35 embryos genotyped from heterozygous crosses, researchers identified 13 wild-type mice, 22 heterozygous (G100S/+) mice, but zero homozygous G100S/G100S Orai1 mice, deviating significantly from the expected Mendelian ratio .

• Anatomical focus: Since pathologies like tubular aggregate myopathy (TAM) affect specific tissues, models must be designed to study the affected cells. For example, Orai1 G100S/+ mice were developed to model the G98S mutation in ORAI1 found in humans with TAM, which results in a severe, childhood-onset form identified in at least three unrelated TAM families .

• Functional validation: Confirming that the mouse model properly reflects the human condition is essential. This includes verifying similar calcium flux abnormalities, cellular pathology, and physiological consequences.

• Technical considerations: The mutation position is critical - for example, the G98/G100 residue (human/mouse) is positioned within the first transmembrane region of Orai1 and serves as the gating hinge in a rigid section of the Orai1 pore, making it critical for channel opening upon activation .

How can researchers accurately measure Orai1-dependent calcium currents in different cell types?

Accurately measuring Orai1-dependent calcium currents requires specialized techniques:

• Real-time calcium imaging: This approach has been successfully used to assess calcium transients as cells (such as PMNs) are stimulated to roll, arrest, and migrate on substrates like E-selectin and ICAM-1 in shear flow . This allows for dynamic assessment of calcium signaling during cellular processes.

• Pharmacological tools: Using specific inhibitors can help distinguish Orai1-mediated SOCE from other calcium entry pathways. For example, studies have employed 2-APB to block SOCE and U73122 to inhibit phospholipase C (PLC) mediated calcium store release .

• Genetic approaches: Comparing calcium responses in cells with reduced Orai1 function (via heterozygous knockout or siRNA knockdown) to control cells provides a direct measure of Orai1 contribution. In experiments with PMNs from Orai1⁻/⁺ mice, a 47% reduced peak calcium upregulation was observed compared with wild-type littermates .

• Store depletion protocol: A standardized approach involves using thapsigargin to deplete intracellular calcium stores, followed by readdition of extracellular calcium to specifically measure store-operated calcium entry .

What technical challenges exist in distinguishing native Orai1 isoforms in mouse tissues?

Researchers face several technical challenges when attempting to distinguish native Orai1 isoforms:

• Antibody cross-reactivity: Studies have shown that antibodies against Orai1 may detect approximately 30% non-specific signal due to binding to other Orai homologs (Orai2, Orai3) that partially share the peptide epitope recognized by the anti-Orai1 antibody .

• Post-translational modifications: Native ORAI1 isoforms can only be resolved as distinct bands in western blots if protein samples are first deglycosylated by PNGase F, highlighting the importance of considering post-translational modifications .

• Expression level variations: Expression levels of Orai1 can vary between different cell types. For instance, in control Mb1-Cre/+ mice, Orai1 expression was comparable between B220+ B cells and CD8+ T cells, with CD4+ T cells showing slightly lower signal .

• Alternative translation start sites: A single Orai1 mRNA can produce two protein isoforms - full-length Orai1α starting at methionine-1 and a shorter Orai1β protein starting at methionine-64 . This complicates protein detection and functional analysis.

What are the most effective approaches for isolating primary cells from Orai1 knockout mice?

Isolation of primary cells from Orai1 knockout mice requires tissue-specific protocols:

For mouse primary keratinocytes from Orai1⁻/⁻ mice:

• Dissect skin and wash in PBS completed with 1% Penicillin/Streptavidin

• Fragment skin and incubate in a vol/vol solution of Collagenase I (8 mg/mL) and Dispase (5 mg/mL) for 3 hours at 37°C

• Add Trypsin for 15 minutes

• Filter the mixture using a 40-μm filter

• Centrifuge at 1,000 × g for 5 minutes

• Resuspend in Defined Epidermal Keratinocyte Medium (CnT-07, PCT) and incubate at 37°C and 5% CO₂ for one week

• Replace medium with CnT-57, PCT and change every 3 days

For B cell isolation from B cell-specific Orai1 knockout mice, standard protocols involving magnetic or flow cytometry-based separation of B220+ cells from spleen or bone marrow can be employed, as referenced in studies with Orai1ᶠˡ/ᶠˡ Mb1-Cre/+ mice .

What genotyping strategies are recommended for Orai1 mutant mice?

Effective genotyping of Orai1 mutant mice can be performed using PCR-based methods:

• DNA extraction: Extract PCR-ready genomic DNA from tail biopsies using standardized methods such as the QuickExtractDNA Extraction Solution .

• Primer design: For Orai1 knockout mice, a three-primer strategy has been successfully employed:

  • WT Forward: 5′-GGGTGTGGCGTATGCAAATAACCT-3′

  • WT Reverse: 5′-ACTCGAGCCGGTCTCCC-3′

  • KO Reverse: 5′-TCGTACCACCTTCTTGGGACTTGA-3′

• PCR amplification: Amplify the extracted DNA using the designed primers and appropriate DNA polymerase (such as Taq Choice from Applied Biosystems) .

• Result interpretation: Wild-type and knockout alleles will yield distinct band patterns when separated by gel electrophoresis, allowing clear identification of genotypes.

For point mutations such as the G100S mutation in Orai1, sequence-specific primers or restriction fragment length polymorphism (RFLP) analysis may be employed, though specific details were not provided in the search results .

How can researchers quantify changes in Orai1 expression in response to different stimuli?

Several methods can accurately quantify changes in Orai1 expression:

• Quantitative PCR (qPCR): This technique effectively measures Orai1 mRNA expression levels. For example, studies have used qPCR to demonstrate that B cells from Orai1ᶠˡ/ᶠˡ Mb1-Cre/+ mice show a near abrogation of Orai1 mRNA (to approximately 3% of control) compared to Mb1-Cre/+ control mice . The method involves:

  • RNA extraction from cells (e.g., using RNeasy mini kit)

  • cDNA synthesis

  • Real-time PCR with Orai1-specific primers and appropriate control genes (such as ribosomal protein L)

  • Analysis using the comparative CT method

• Flow cytometry: Surface expression of Orai1 can be quantified using flow cytometry with specific antibodies. Studies have shown that compared to Orai1ᶠˡ/ᶠˡ controls, the average surface expression of Orai1 was significantly reduced on B220+ B cells from Orai1ᶠˡ/ᶠˡ Mb1-Cre/+ mice by approximately 70% .

• Western blotting: For protein-level quantification, western blotting can determine relative Orai1 protein levels across different conditions. As noted earlier, deglycosylation with PNGase F may be necessary to resolve different Orai1 isoforms .

• RNA-seq and differential expression analysis: For genome-wide expression analysis, RNA-seq followed by differential expression analysis using tools like edgeR can identify changes in Orai1 expression patterns along with related genes. This approach has been used to analyze B cells from Orai1ᶠˡ/ᶠˡ Mb1-Cre/+ mice after stimulation with anti-IgM .

What are promising approaches for studying Orai1 clustering and dynamics in living cells?

Advanced imaging techniques offer promising approaches for studying Orai1 dynamics:

• Super-resolution microscopy: Techniques such as STORM (Stochastic Optical Reconstruction Microscopy) or PALM (Photo-Activated Localization Microscopy) could provide nanoscale resolution of Orai1 clustering beyond what conventional microscopy allows.

• Scanning Transmission Electron Microscopy (STEM): This technique has already been used successfully to visualize and quantify ORAI1 clustering patterns. In studies of ORAI1 distribution, researchers have identified distinct accumulation areas (puncta) with dimensions of approximately 2 ± 0.5 μm² and characterized changes in label density during activation states .

• Förster Resonance Energy Transfer (FRET): This approach has been used as a "molecular ruler" to identify protein-protein interactions within 75 Å, confirming direct binding between TRPC4 and TRPC1, and detecting Orai1 within the TRPC4 complex . Similar approaches could further elucidate Orai1's interactions with other proteins.

• Live-cell calcium imaging: Real-time calcium imaging combined with fluorescently tagged Orai1 could correlate channel clustering with functional calcium entry, as has been demonstrated in studies examining calcium transients during neutrophil rolling, arrest, and migration .

How might understanding Orai1 function lead to novel therapeutic approaches for calcium-related disorders?

Understanding Orai1 function has significant therapeutic implications:

• Tubular Aggregate Myopathy (TAM): Research on Orai1 gain-of-function mutations, such as the G100S mutation in mice (analogous to the human G98S mutation), provides insights into the pathophysiology of TAM . This could lead to targeted therapies aiming to modulate aberrant calcium entry.

• Immune disorders: Given Orai1's critical role in immune cell function, including B cell survival and proliferation , and neutrophil arrest and polarization , targeted modulation of Orai1 could potentially treat autoimmune conditions or inflammatory disorders.

• Skin disorders: Orai1's importance in skin homeostasis suggests that therapies targeting Orai1 might be effective for certain dermatological conditions.

• Combined targeting approaches: Research showing functional redundancy between Orai1 and Orai3 in B cells indicates that effective therapeutic strategies might need to target multiple Orai isoforms simultaneously for conditions involving B cell dysregulation.

What are the most important unresolved questions regarding Orai1 structure and function?

Several critical questions remain unanswered about Orai1:

• Native channel composition: The oligomeric state of native CRAC channels in cells like B cells and whether Orai1 and Orai3 form homomeric or heteromeric assemblies or both in primary cells remains an open question .

• Regulatory mechanisms: While the direct interaction between STIM1 and Orai1 is established , the complete regulatory network controlling Orai1 activation, including potential roles for additional proteins, remains to be fully elucidated.

• Isoform-specific functions: The distinct physiological roles of Orai1α (starting at methionine-1) versus the shorter Orai1β (starting at methionine-64) are not fully understood .

• Tissue-specific roles: While some tissue-specific functions have been identified, the comprehensive roles of Orai1 across different tissues and developmental stages require further investigation, particularly given the embryonic lethality of homozygous Orai1 knockouts .

• Therapeutic targeting: The optimal approaches for selectively modulating Orai1 function in specific tissues or disease states without affecting essential physiological processes remain to be determined.