Murine ORAI2 splice variants form functional Ca2+ release-activated Ca2+ (CRAC) channels

The stimulation of membrane receptors coupled to the phopholipase C pathway leads to activation of the Ca2+ release-activated Ca2+ ( CRAC) channels. Recent evidence indicates that ORAI1 is an essential pore subunit of CRAC channels. STIM1 is additionally required for CRAC channel activation. The present study focuses on the genomic organization, tissue expression pattern, and functional properties of the murine ORAI2. Additionally, we report the cloning of the murine ORAI1, ORAI3, and STIM1. Two chromosomal loci were identified for the murine orai2 gene, one containing an intronless gene and a second locus that gives rise to the splice variants ORAI2 long (ORAI2L) and ORAI2 short (ORAI2S). Northern blots revealed a prominent expression of the ORAI2 variants in the brain, lung, spleen, and intestine, while ORAI1, ORAI3, and STIM1 appeared to be near ubiquitously expressed in mice tissues. Specific antibodies detected ORAI2 in RBL 2H3 but not in HEK 293 cells, whereas both cell lines appeared to express ORAI1 and STIM1 proteins. Co-expression experiments with STIM1 and either ORAI1 or ORAI2 variants showed that ORAI2L and ORAI2S enhanced substantially CRAC current densities in HEK 293 but were ineffective in RBL 2H3 cells, whereas ORAI1 strongly amplified CRAC currents in both cell lines. Thus, the capability of ORAI2 variants to form CRAC channels depends strongly on the cell background. Additionally, CRAC channels formed by ORAI2S were strongly sensitive to inactivation by internal Ca2+. When co-expressed with STIM1 and ORAI1, ORAI2S apparently plays a negative dominant role in the formation of CRAC channels.