Figure 5.

Annonacin causes fragmentation of microtubules. A, Control cultures and cultures exposed for 48 h to 50 nm annonacin, immunolabeled with antibodies against tau (AD2, red) and the dendritic marker MAP2 (green) or the axonal marker β-III-tubulin (β-III-T, green). Under control conditions, AD2⁺ tau is localized predominantly in axons, and its distribution is homogenous. In contrast, in annonacin-treated cultures, AD2⁺ tau has accumulated in the neuronal soma (white arrowheads), and the distribution of tau remaining in β-III-T⁺ axons is discontinuous (red arrows), leaving long stretches of the axons devoid of tau (green arrows). B, Ultrastructure of microtubules in neurons in control cultures and cultures exposed for 48 h to 50 nm annonacin. Overviews (left) and enlargements of the boxed areas (right) show that microtubules (highlighted by dotted lines) in neurites of a representative control neuron form parallel arrays and are continuous over long distances, whereas in annonacin-treated cultures, they are fragmented and disordered. Scale bar, 50 nm.