cells by the C3-specific uptake mechanism via acidified endosomal vesicles. Regarding its Rho-selective ADPribosyltransferase activity and the cellular effects, C2IN-C3lim behaves like C3lim. Since C3 toxins are the only known Rho-inhibitors and selectively target cells from the monocyte/macrophage-line, C3 toxins and C3-derived 760981-83-7 fusion toxins such as C2IN-C3lim are ideal tools for investigation and targeted pharmacological manipulation of Rho-dependent signal transduction in cells which are related to this cell line such as osteoclasts. In vivo, monocytes/macrophages and osteoclasts are derived from pluripotent hematopoietic stem cells and in vitro, osteoclasts differentiate from macrophage-like RAW 264.7 cells. Osteoclasts form a tight sealing zone on mineralized surfaces which is essential for resorption of matrix, during reorganization of bone tissue. It was reported earlier that the C3-catalyzed ADP-ribosylation of Rho in osteoclasts resulted in disruption of their sealing zone and their resorption activity, indicating that Rho plays a crucial role for osteoclast activity. Here, we demonstrate that C2IN-C3lim is efficiently taken up into the cytosol of RAW 264.7 cells and derived osteoclasts. Treatment with C2IN-C3lim inhibited the resorption activity of already differentiated osteoclasts but also the formation of osteoclasts from RAW 264.7 cells due to the C3-catalyzed ADP-ribosylation of Rho. In contrast, C2IN-C3lim had no effects on other cultured bone cell types such as murine pre-osteoblastic MC3T3 cells, GLYX-13 confirming its monocyte/macrophage-selective mode of action. When the effect of increasing concentrations of C3bot1, C3lim and C2IN-C3lim on the morphology of RAW 264.7 was compared after a 24h treatment, C2IN-C3lim was most efficient. Treatment of RAW 264.7 cells with C2INC3lim resulted in a decreased number of cells after 2 and 3 days of incubation. When the viable cells were determined by MTT assay, there was a decreased amount after C2IN-C3
