A Stim1-dependent, noncapacitative Ca²⁺-entry pathway is activated by B-cell-receptor stimulation and depletion of Ca²⁺

The depletion of intracellular Ca²⁺ stores activates capacitative Ca²⁺ entry (CCE), which is a Ca²⁺-selective and La³⁺-sensitive entry pathway. Here, we report a novel mechanism of La³⁺-resistant Ca²⁺ entry that is synergistically regulated by B-cell-receptor (BCR) stimulation and Ca²⁺ store depletion. In DT40 cells, stimulation of BCRs with anti-IgM antibodies induced Ca²⁺ release and subsequent Ca²⁺ entry in the presence of 0.3 μM La³⁺, a condition in which CCE is completely blocked. This phenomenon was not observed in inositol 1,4,5-trisphosphate receptor-deficient DT40 (IP3R-KO) cells. However, in response to thapsigargin pretreatment, BCR stimulation induced La³⁺- resistant Ca²⁺ entry into both wild-type and IP3R-KO cells. These results indicate that BCR stimulation alone does not activate Ca²⁺ entry, whereas BCR stimulation and depleted Ca²⁺ stores (either due to IP3R-mediated Ca²⁺ release or Ca²⁺ uptake inhibition) work in concert to activate La³⁺-resistant Ca²⁺ entry. This Ca²⁺ entry was inhibited by genistein. In addition, BCR-mediated Ca²⁺ entry was completely abolished in Stim1-deficient DT40 cells and was restored by overexpression of YFP-Stim1, but was unaffected by double knockdown of Orai1 and Orai2. These results demonstrate a unique non-CCE pathway, in which Ca²⁺ entry depends on Stim1- and BCR-mediated activation of tyrosine kinases.