Three distinct stages of apoptotic nuclear condensation revealed by time-lapse imaging, biochemical and electron microscopy analysis of cell-free apoptosis

During apoptotic execution, chromatin undergoes a phase change from a heterogeneous, genetically active network to an inert highly condensed form that is fragmented and packaged into apoptotic bodies. We have previously used a cell-free system to examine the roles of caspases or other proteases in apoptotic chromatin condensation and nuclear disassembly. But so far, the role of DNase activity or ATP hydrolysis in this system has not yet been elucidated. Here, in order to better define the stages of nuclear disassembly in apoptosis, we have characterized the apoptotic condensation using a cell-free system and time-lapse imaging. We demonstrated that the population of nuclei undergoing apoptosis in vitro appears to follow a reproducible program of nuclear condensation, suggesting the existence of an ordered biochemical pathway. This enabled us to define three stages of apoptotic chromatin condensation: stage 1 ring condensation; stage 2 necklace condensation; and stage 3 nuclear collapse/disassembly. Electron microscopy revealed that neither chromatin nor detectable subnuclear structures were present inside the stage 1 ring-condensed structures. DNase activity was not essential for stage 1 ring condensation, which could occur in apoptotic extracts depleted of all detectable DNase activity. However, DNase(s) were required for stage 2 necklace condensation. Finally, we demonstrated that hydrolyzable ATP is required for stage 3 nuclear collapse/disassembly. This requirement for ATP hydrolysis further distinguished stage 2 from stage 3. Together, these experiments provide the first steps towards a systematic biochemical characterization of chromatin condensation during apoptosis.