A protein from Naegleria amoebae causes apoptosis in chick embryo and CHO cells after they become confluent

Exposure for less than an hour to a protein isolated from Naegleria amoebae initiates a process that has no apparent effect on the appearance or growth of chick embryo or CHO cell cultures for 4 to 9 days; after the development of confluency, at some unknown signal, all of the cells undergo an apoptotic death within a 12- to 24-hour period. Abnormalities detected among the last mitotic cells include chromosomal breakage and early reversal in metaphase to telo/interphase daughter nuclei with irregular shapes. Additional events in the dying cultures include the development of a cytoplasmic amoebic-related immunogen, gross DNA fragmentation, cell blebbing, shrinkage, and apoptotic body formation. Culture death included all cells, those present in confluent cultures when the protein was added, and in other cultures, those formed during a more than 30-fold increase in cells as the cultures became confluent. The increase in the number of cells followed by the uniformity and synchrony of their death pattern indicates that the signal to kill has increased and spread throughout the culture; upon an unknown condition related to confluency, events are initiated that lead to the unusual apoptotic death of the culture.