Vol. 304, Issue 3, 1243-1250, March 2003

Effects of Large Clostridial Cytotoxins on Activation of RBL 2H3-hm1 Mast Cells Indicate Common and Different Roles of Rac in FcRI and M1-Receptor Signaling

Nabil Djouder, Eduardo Aneiros, Adolfo Cavalié and Klaus Aktories

Institut für Experimentelle und Klinische Pharmakologie und Toxikologie der Albert-Ludwigs, Universität Freiburg, Freiburg, Germany (N.D., K.A.); and Experimentelle und Klinische Pharmakologie und Toxikologie, Universität des Saarlandes, Homburg (Saar), Germany (E.A., A.C.)

Using Rho GTPases-inhibiting clostridial cytotoxins, we showed recently in RBL cells that the GTPase Rac is involved in FcRI (high-affinity receptor for IgE) signaling and receptor-mediated calcium mobilization, including influx via calcium release-activated calcium channels. Here, we studied the role of Rho GTPases in muscarinic M1 receptor signaling in RBL 2H3-hm1 cells. Clostridium difficile toxin B, which inactivates Rho, Rac, and Cdc42, and Clostridium sordellii lethal toxin, which inhibits Rac but not Rho, blocked M1-mediated exocytosis, indicating that Rac but not Rho is involved in the regulation of receptor-mediated exocytosis. Although antigen-induced FcRI stimulation caused tyrosine phosphorylation of the Rac guanine nucleotide exchange factor Vav, M1 stimulation by carbachol activated Rac independently of Vav. The Rac-inactivating toxins blocked M1 receptor-induced membrane translocation of the pleckstrin homology domain of protein kinase B, which is a phosphoinositide 3-kinase effector. The M1-induced calcium release from internal stores was not affected by toxin B; however, the subsequent calcium influx from the extracellular space was inhibited. The data suggest that besides capacitative calcium entry, the M1 signaling pathway activates further calcium entry channels with mechanisms that are not affected by the inhibition of Rac.