Role of caspases and mitochondria in the steroid-induced programmed cell death of a motoneuron during metamorphosis

Accessory planta retractor (APR) motoneurons of the hawk moth, Manduca sext a, undergo a segment-specific pattern of programmed cell death (PCD) 24 to 48 h after pupal ecdysis (PE). Cell culture experiments show that the PCD o f APRs in abdominal segment 6 [APR(6)s] is a cell-autonomous response to th e steroid hormone 20-hydroxyecdysone (20E) and involves mitochondrial demis e and cell shrinkage. Twenty-four hours before PE, at stage W3-noon, APR(6) s require further 20E exposure and protein synthesis (as tested with cycloh eximide) to undergo PCD, and death can be blocked by a broad-spectrum caspa se inhibitor. By PE, death is 20E- and protein synthesis-independent and th e caspase inhibitor blocks cell shrinkage but not loss of mitochondrial fun ction. Thus, the commitment to mitochondrial demise precedes the commitment to execution events. The phenotype of necrotic cell death induced by a mit ochondrial electron transfer inhibitor differs unambiguously from 20E-induc ed PCD. By inducing PCD pharmacologically, the readiness of APR(6)s to exec ute PCD was found to increase during the final larval instar. These data su ggest that the 20E-induced PCD of APR(6)s includes a premitochondrial phase which includes 20E-induced synthetic events and apical caspase activity, a mitochondrial phase which culminates in loss of mitochondrial function, an d a postmitochondrial phase during which effector caspases are activated an d APR(6) is destroyed. (C) 2000 Academic Press.