Extracellular signals can affect caspase activity either through the activation of caspase zymogens, or through inhibition of caspase enzymatic activity. Cleavage of caspase zymogen permits assembly of active caspase heterodimer from proteolytically

During development, programmed cell death is regulated by cell-cell signals that either promote or inhibit caspase activation (Meier et al., 2000a). The biochemical mechanisms of extracellular regulation are incompletely known. Here, they have been studied using an antibody to detect caspase activation in vivo. Caspases are a class of intracellular cysteine proteases that cause cell death by cleaving a variety of intracellular proteins after aspartate residues (Chinnaiyan and Dixit, 1996). Accordingly, inhibitors of the caspase enzymes can prevent cell death (Bump et al., 1995; Xue and Horvitz, 1995; Ekert et al., 1999). Extracellular signals can affect caspase activity either through the activation of caspase zymogens, or through inhibition of caspase enzymatic activity. Cleavage of caspase zymogen permits assembly of active caspase heterodimer from proteolytically generated p10 and p20 subunits. Caspases are broadly divided into two classes. Initiator caspases do not require other caspases for cleavage of their zymogens, but are thought to interact with other regulators through their prodomains. Initiator caspases may be activated by cell surface receptors, cytochrome c release from mitochondria, the balance of positively and negatively acting Bcl family members, and ced-4/Apaf like proteins. Effector caspases are activated by other caspases, lack extensive prodomains and are thought to be responsible for the bulk of cellular proteolysis. It is thought that effector caspases may cleave their own zymogens to provide positive feedback once apoptosis has been initiated (Cryns and Yuan, 1998; Kumar, 1999; Budihardjo et al., 1999; Salvesen and Dixit, 1999; Song and Steller, 1999). In Drosophila, proapoptotic genes have been found that are required for most programmed cell death in the embryo. Three genes reaper (rpr), grim and head involution defective (hid) map close together on chromosome 3 (bands 75C1,2), and all are deleted by a small deficiency called H99. There is no programmed cell death in H99 homozygous embryos (White et al., 1994; Song and Steller, 1999). The rpr and grim genes are transcribed exclusively in cells fated to die, and their promoters must respond to extracellular signals controlling apoptosis (Nordstrom et al., 1996). The hid gene is also expressed in cells that survive, and could be regulated through MAP kinase by extracellular signals and receptor tyrosine kinases (Bergmann et al., 1998; Kurada and White, 1998). rpr, grim and hid encode intracellular proteins that promote apoptosis by inhibiting Inhibitor of Apoptosis Proteins (IAPs). 3269 Development 129, 3269-3278 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV5018