REDUCED APOPTOSIS AND CYTOCHROME-C-MEDIATED CASPASE ACTIVATION IN MICE LACKING CASPASE-9

Caspases are essential components of the mammalian cell death machiner y. Here we test the hypothesis that Caspase 9 (Casp9) is a critical up stream activator of caspases through gene targeting in mice. The major ity of Casp9 knockout mice die perinatally with a markedly enlarged an d malformed cerebrum caused by reduced apoptosis during brain developm ent. Casp9 deletion prevents activation of Casp3 in embryonic brains i n vivo, and Casp9-deficient thymocytes show resistance to a subset of apoptotic stimuli, including absence of Casp3-like cleavage and delaye d DNA fragmentation. Moreover, the cytochrome c-mediated cleavage of C asp3 is absent in the cytosolic extracts of Casp9-deficient cells but is restored after addition of in vitro-translated Casp9. Together, the se results indicate that Casp9 is a critical upstream activator of the caspase cascade in vivo.