Inhibition of Calcium Release-activated Calcium Current by Rac/Cdc42-inactivating Clostridial Cytotoxins in RBL Cells

Using large clostridial cytotoxins as tools, the role of Rho GTPases in activation of RBL 2H3 hm1 cells was studied.Clostridium difficile toxin B, which glucosylates Rho, Rac, and Cdc42 and Clostridium sordellii lethal toxin, which glucosylates Rac and Cdc42 but not Rho, inhibited the release of hexosaminidase from RBL cells mediated by the high affinity antigen receptor (FcεRI). Additionally, toxin B and lethal toxin inhibited the intracellular Ca2+ mobilization induced by FcεRI-stimulation and thapsigargin, mainly by reducing the influx of extracellular Ca2+. In patch clamp recordings, toxin B and lethal toxin inhibited the calcium release-activated calcium current by about 45%. Calcium release-activated calcium current, the receptor-stimulated Ca2+ influx, and secretion were inhibited neither by the Rho-ADP-ribosylating C3-fusion toxin C2IN-C3 nor by the actin-ADP-ribosylating Clostridium botulinum C2 toxin. The data indicate that Rac and Cdc42 but not Rho are not only involved in late exocytosis events but are also involved in Ca2+ mobilization most likely by regulating the Ca2+ influx through calcium release-activated calcium channels activated via FcεRI receptor in RBL cells.