Restoration of mitochondrial function reverses developmental neuronal death in vitro

In a previous study characterizing morphological and functional features of cell death in trophically deprived chick ciliary ganglion neurons (Pena and Pilar [2000] J. Comp. Neurol. 424:377-396), we hypothesized that early cell death events might be targets for reversal, allowing for rescue of dying neurons. To test this hypothesis, ciliary ganglion (CG) neurons were cultured with or without trophic support (choroid, iris, and pigment epithelium soluble extract [CIPE]), or without trophic support for 11 or 18 hours and then exposed to trophic support. Prior to and at the onset of cell death commitment (11 hours) CIPE-deprived neurons exhibited increased membrane permeability, blebbing, cytoplasmic vacuolization, swollen mitochondria, low adenosine triphosphate levels, and release of cytochrome c from mitochondria. CIPE readdition at 11 hours reversed these changes. Between 11 and 18 hours, irreversible DNA fragmentation increased in CIPE-deprived neurons. Cyclosporin A and bongkrekic acid (inhibitors of mitochondrial transition permeability pores) prevented membrane permeability increases and delayed the progression to death in trophically deprived neurons by 12 hours; however, by 48 hours all neurons had died. BOC-Asp-CH2F (BAF), a pan-caspase inhibitor, did not prevent early events of cell death including increased membrane permeability and Cyto c release, but it inhibited DNA fragmentation and prolonged neuronal survival to 48 hours. We conclude that mitochondria changes occur early, prior to commitment and that the suppression of these changes can prevent all the downstream events of death, whereas caspase inhibitors have no effect on the early mitochondria/plasma membrane changes. Mitochondria thus play a critical role in the transition from reversible to irreversible commitment to developmental neuronal death. Furthermore, neuronal death is brought about by activation of one of two distinct pathways, one localized in mitochondria and the other dependent on activation of caspases.