A novel role for microtubules in apoptotic chromatin dynamics and cellular fragmentation

Abstract

Dramatic changes in cellular dynamics characterise the apoptotic execution phase, culminating in fragmentation into membrane-bound apoptotic bodies. Previous evidence suggests that actin-myosin plays dominant roles in apoptotic cellular remodelling, while all other cytoskeletal elements dismantle. We have used fixed and live-cell imaging to confirm that interphase microtubules rapidly depolymerise at the start of the execution phase. At around this time, pericentriolar components (pericentrin, ninein and γ-tubulin) are lost from the centrosomal region. Subsequently, however, extensive non-centrosomal bundles of densely packed, dynamic microtubules rapidly assemble throughout the cytoplasm in all cell-lines tested. These microtubules play important roles in the peripheral relocation of chromatin in the dying cell, because nocodazole treatment restricts the dispersal of condensed apoptotic chromatin into surface blebs, and causes the withdrawal of chromatin fragments back towards the cell centre. Importantly, nocodazole and taxol are both potent inhibitors of apoptotic fragmentation in A431 cells, implicating dynamic microtubules in apoptotic body formation. Live-cell imaging studies indicate that fragmentation is accompanied by the extension of rigid microtubule-rich spikes that project through the cortex of the dying cell. These structures enhance interactions between apoptotic cells and phagocytes in vitro, by providing additional sites for attachment to neighbouring cells