Omnitemporal choreographies of IP3R and all five STIM/Orai underlie the complexity of mammalian Ca2+ signaling

Abstract

Invertebrates express one endoplasmic reticulum (ER)-resident Ca2+-sensing stromal-interaction molecule (Stim) and one Orai plasma membrane channel protein. Stim conveys store depletion to Orai, mediating the evolutionarily conserved Ca2+ release-activated Ca2+ (CRAC) current. The crucial role of their vertebrate homologues, STIM1 and Orai1 in mediating CRAC activity in mammals is well-established. However, mammals possess two STIM and three Orai isoforms and the choreography of their interactions under physiological receptor activation is unknown. We show that the five mammalian STIM1/2 and Orai1/2/3 isoforms have non-redundant functions. Yet, all five isoforms are always required together to ensure the graded diversity of mammalian Ca2+ signaling events in response to the full spectrum of agonist strengths. Receptor-activated Ca2+ signaling across the range of stimulus intensities requires functional interactions between not only STIM1/2 and Orai1/2/3, but also IP3R, ensuring that receptor-mediated Ca2+ release is precisely tailored to Ca2+ entry and activation of nuclear factor of activated T-cells (NFAT). This is orchestrated by two interdependent and counterbalancing paradigms: the N-termini Ca2+-binding ER-luminal domains of unactivated STIM1/2 inhibit IP3R-evoked Ca2+ release. Gradual increase in agonist intensity leads to gradual STIM1/2 activation and relief of IP3R inhibition. Concomitantly, the cytosolic C-termini of activated STIM1/2 differentially interact with Orai1/2/3 proteins as agonist intensity increases. Thus, coordinated and omnitemporal functions of all five STIM/Orai proteins and IP3Rs at the ER-lumen and cytosol translate the strength of agonist stimulation to precise levels of Ca2+ release, Ca2+ entry and NFAT induction, ensuring the diversity and fidelity of complex mammalian Ca2+ signaling. HighlightsO_LIAll five STIM/Orai and IP3R are always required together in mammalian Ca2+ signalling C_LIO_LIUnactivated STIM1/2 inhibit IP3R and activated STIM1/2 cooperatively activate Orai1/2/3 C_LIO_LISTIM1 contribution increases and that of STIM2 decreases as agonist intensifies C_LIO_LIGraded IP3R disinhibition and Orai activation tailor receptor activity to NFAT induction C_LI O_FIG O_LINKSMALLFIG WIDTH=192 HEIGHT=200 SRC="FIGDIR/small/325480v1_ufig1.gif" ALT="Figure 1"> View larger version (65K): org.highwire.dtl.DTLVardef@f6172dorg.highwire.dtl.DTLVardef@6bbb09org.highwire.dtl.DTLVardef@c45c80org.highwire.dtl.DTLVardef@194d7e4_HPS_FORMAT_FIGEXP M_FIG C_FIG