Comparative study of the cytological aspects of the mode of action of destruxins and other peptidic fungal metabolites on target epithelial cells

The cytopathological effects of cyclic fungal peptides, destruzins (dtxs) A and E, Cyclosporin A, and Linear peptaibols efrapeptins (=tolypin), on epithelial cells were studied by in vitro experiments with preparations of insect organs. The treatment of Malpighian tubules and midgut revealed the induction by dtx E of the formation of vesicles on microvilli of Malpighian cells and on the apical surface of midgut cells. This morphological alteration of the brush border was a Ca2+-dependent process, very strongly inhibited in Ca2+-free medium or when a combined treatment with cadmium chloride + dtx E was applied. However, cadmium chloride did not inhibit all the cytotoxic effects of dtx E. The ability to trigger the formation of vesicular structures is a property of biologically active dtxs, the inducing capacity showing, however, a decreasing intensity from dtx E to dtx A and dtx B. On the contrary, the other peptides, Cyclosporin A and efrapeptins, did not induce the formation of vesicles. The most obvious intracellular alterations consist of a pycnosis of the nucleus, and in changes of mitochondria, the contents of which show a decrease in density and are comparable to those observed in vivo. If mitochondria are target organelles for the three different types of toxins tested, the structural changes induced at this level were specific for dtxs, Cyclosporin A, and efrapeptins, respectively. The cytological data obtained strongly suggested that dtxs have a unique mode of action among fungal peptides and do not act mainly as ionophores, inducing the formation of pores in cellular membranes, or as mitochondrial ATPase inhibitors. (C) 1996 Academic Press, Inc.