Ruriko Suzuki, Rinka Ogawa, Tadahide Furuno
Crosslinking of the high affinity IgE receptor (FcεRI) by multivalent antigen increases intracellular Ca2+ concentration ([Ca2+]i) through two sequential steps: (i) Ca2+ release from the endoplasmic reticulum (ER) and (ii) store-operated Ca2+ entry (SOCE) mediated by stromal interaction molecule 1 (STIM1)-Orai1 interaction. This process is essential for basophil and mast cell activation. We previously showed that FcεRI disaggregation by monomeric hapten rapidly and persistently inhibited [Ca2+]i increase although upstream signal disruption was transient in rat basophilic leukemia cells. However, the underlying inhibitory mechanisms remain unclear. In this study, we found that monomeric hapten dissociates STIM1-Orai1 interaction by promoting Orai1 re-phosphorylation. Although Ca2+ release from ER, the first step in [Ca2+]i increase, was blocked by monomeric hapten addition, phosphorylation of phospholipase Cγ1, a critical step for Ca2+ release from the ER, was inhibited only when the monomeric hapten was added immediately after multivalent antigen stimulation, but not when added several minutes later. However, STIM1-Orai1 interaction was dissociated by the monomeric hapten, regardless of the timing of its addition. Multivalent antigen induced Orai1 dephosphorylation, which is necessary for SOCE. In contrast, monomeric hapten addition promoted its re-phosphorylation. These results suggest that cessation of [Ca2+]i increase following FcεRI disaggregation primarily arises from immediate suppression of SOCE via STIM1-Orai1 dissociation.