Neither nocodazole neverless nor vinblastine did not increase the total amounts of lysosomes indicated by LAMP2, a lyso somal membrane associated protein. Treatment with bafilomycin A1 caused inhibition of lysosomal activity, but did not change the amount of lysosomal vesicles or LAMP2 levels dramatically. When lysosomal activity was inhibited, a large number of autolysosomes resulted from fusion of GFP LC3 labelled autophagosomes with lysosomes were preserved in the control and nocodazole treated cells causing overlap of more than 50% of GFP LC3 punctate foci with LAMP2 signal. In contrast, vinblastine reduced overlap to less than 20% when the amount of lysosomes were not increase. This suggested that vinblastine induced depolymerization of acetylated microtubules impairs the fusion of autophagosomes with lysosomes to form autolysosomes.

Discussion To form mature autophagosomes, microtubule associated LC3I is translocated to sites where it is conju gated with phosphatidylethanolamine to become LC3II that is inserted into isolation membranes. The iso lation membrane may be pre assembled in some uni dentified subcellular location and transported to sites where substrates and potential cargo exist. Alternatively, small fragments of isolation membrane or some pre autophagosomal structure may be transported to sites where substrates exist to assemble autophagosomes. Pre assembled isolation membranes may also remain on site waiting for substrates to appear, or both isolation membrane and substrates may be moved to sites such as microtubule organizing centers to form mature autophagosomes.

Independent of the precise mechanism cytoskeletal elements are required for the trafficking of pre autophagosomal structures, substrates and cargo and mature autophagosomes. Although both directly bind to the same b tubulin subunit, paclitaxel prevents while nocodazole promotes depolymerization of normal microtubules. Treatment with either of them results in a similar impact on autop hagy. There is no obvious influence on interphase cells cultured under normal conditions, but a similar inhibi tory effect on the conversion of LC3I to LC3II in mito tic cells. This suggests that basal levels of autophagy GSK-3 are highly efficient and independent of the status of regular microtubules so that interruption of the dynamics of regular microtubules causes no dramatic impact on overall autophagic influx under steady state conditions. However, consistent with its short duration, but extreme vulnerability to damaged organelles and particularly mitochondria, autophagic flux appears to intensify during mitosis.