The lysosomotropic agent monodansylcadaverine also acts as a solvent polarity probe

The autofluorescent substance monodansylcadaverine has recently been report ed as a specific in vivo marker for autophagic vacuoles. However, the mecha nism for this specific labeling remained unclear. Our results reveal that t he common model of ion trapping in acidic compartments cannot completely ac count for the observed autophagic vacuole staining. Because autophagic vacu oles are characterized by myelin-like membrane inclusions, we tested whethe r this lipid-rich environment is responsible for the staining properties of monodansylcadaverine. In in vitro experiments using either liposomes or so lvents of different polarity, monodansylcadaverine showed an increased rela tive fluorescence intensity in a hydrophobic environment as well as a Stoke s shift dependent on the solvent polarity. To test the effect of autophagic vacuoles or autophagic vacuole lipids on monodansylcadaverine fluorescence , we isolated autophagic vacuoles and purified autophagic vacuole lipids de pleted of proteins. Entire autophagic vacuoles and autophagic vacuole lipid s had the same effect on monodansylcadaverine fluorescence properties, sugg esting lipids as the responsible component. Our results suggest that the in vivo fluorescence properties of monodansylcadaverine do not depend exclusi vely on accumulation in acidic compartments by ion trapping but also on an effective interaction of this molecule with autophagic vacuole membrane lip ids.