CHROMATIN CONDENSATION IS NOT ASSOCIATED WITH APOPTOSIS

Apoptosis plays an important role in the survival of an organism, and substantial work has been done to understand the signaling pathways th at regulate this process. Characteristic changes in chromatin organiza tion accompany apoptosis and are routinely used as markers for cell de ath. We have examined the organization of chromatin in apoptotic PC12 and HeLa cells by indirect immunofluorescence and electron spectroscop ic imaging. Our results indicate that de novo chromatin condensation n ormally seen during mitosis does not occur when cells undergo apoptosi s. Instead, the condensed chromatin typically observed results from ag gregation of the heterochromatin. We present evidence that, early in a poptosis, there is a rapid degradation of the nuclease-hypersensitive euchromatin that contains hyperacetylated histones. This occurs coinci dent with the loss of nuclear integrity due to degradation of lamins a nd reorganization of intranuclear protein matrix. These events lead to collapse of the nucleus and aggregation of heterochromatin to produce the appearance of condensed apoptotic chromatin. This heterochromatin aggregate is then digested by nucleases to produce the oligonucleosom al DNA ladder that is a hallmark of late apoptosis. Unlike mitosis, we have not seen any evidence for the requirement of phosphorylated hist ones H1 and H3 to maintain the chromatin in the condensed state.