A system for characterizing cellular and molecular events in programmed neuronal cell death

Abstract

A model system has been established in which PC12 cells are converted to neuronal-like cells that undergo transcription-dependent cell death following removal of NGF. Nineteen sublines of PC12 cells were tested to establish parameters for making cells dependent on NGF for survival. In most sublines, a relatively small percentage of cells become dependent on NGF for survival, and following removal of NGF, most of the cells begin proliferating in serum-containing medium. In several sublines, however, a significant percentage of cells die following removal of NGF. One of these sublines, PC6–3, can be grown under conditions in which 90% of the cells undergo transcription-dependent cell death following removal of NGF in either serum-free or serum- containing medium. Fourteen hours after removing NGF, 50% of the cells are committed to die, while initial morphological signs of cell death as determined by time-lapse videomicroscopy occur 2–6 hr later and include loss of neurites followed by a 1–3 hr period of active membrane “blebbing” and protrusions. Cell death can be blocked by the RNA synthesis inhibitor actinomycin D, the protein synthesis inhibitor cycloheximide, KCl, basic fibroblast growth factor, or dibutryl-cAMP, but not by epidermal growth factor, leupeptin, or the endonuclease inhibitor aurintricarboxylic acid (ATA). Removal of NGF activates an endonuclease that causes nucleosomal laddering of the DNA; however, endonuclease activity does not appear to be required for cell death. In agreement with previous studies (Batistatou and Greene, 1991; Rukenstein et al., 1991) demonstrating that naive PC12 cells undergo transcription-independent cell death when shifted into serum-free medium in the absence of growth factors, all cell lines tested except for one die when cultured in RPMI medium lacking growth factors. DNA fragmentation is a prominent feature of transcription-independent cell death, and death can be blocked with NGF, ATA, and dibutryl-cAMP but not with actinomycin D or KCl. The PC12 model system described here should be useful for identifying cell death genes and for characterizing cellular and molecular events in programmed neuronal cell death.