Rac-mediated Stimulation of Phospholipase Cγ2 Amplifies B Cell Receptor-induced Calcium Signaling

Abstract The Rho GTPase Rac is crucially involved in controlling multiple B cell functions, including those regulated by the B-cell-receptor (BCR) through increased cytosolic Ca2+. The underlying molecular mechanisms and their relevance to the functions of intact B cells have thus far remained unknown. We have previously shown that the activity of phospholipase C-γ2 (PLCγ2), a key constituent of the BCR signalosome, is stimulated by activated Rac through direct protein-protein-interaction. Here, we use a Rac-resistant mutant of PLCγ2 to functionally reconstitute cultured PLCγ2-deficient DT40 B cells and to examine the effects of the Rac.PLCγ2 interaction on BCR-mediated changes of intracellular Ca2+ and regulation of Ca2+- and NFAT-regulated gene transcription at the level of single, intact B cells. The results show that the functional Rac.PLCγ2 interaction causes marked increases in (i) the sensitivity of B cells to BCR ligation, (ii) the BCR-mediated Ca2+ release from intracellular stores, (iii) the Ca2+ entry from the extracellular compartment, and (iv) the nuclear translocation of the Ca2+-regulated transcription factor NFAT. Hence, Rac-mediated stimulation of PLCγ2 activity serves to amplify B cell receptor-induced Ca2+ signaling.