DIFFERENTIAL REQUIREMENT FOR CASPASE-9 IN APOPTOTIC PATHWAYS IN-VIVO

Mutation of Caspase 9 (Casp9) results in embryonic lethality and defec tive brain development associated with decreased apoptosis. Casp9 (-/- ) embryonic stem cells and embryonic fibroblasts are resistant to seve ral apoptotic stimuli, including UV and gamma irradiation. Casp9(-/-) thymocytes are also resistant to dexamethasone- and gamma irradiation- induced apoptosis, but are surprisingly sensitive to apoptosis induced by UV irradiation or anti-CD95. Resistance to apoptosis is accompanie d by retention of the mitochondrial membrane potential in mutant cells . In addition, cytochrome c is translocated to the cytosol of Casp9(-/ -) ES cells upon UV stimulation, suggesting that Casp9 acts downstream of cytochrome c. Caspase processing is inhibited in Casp9(-/-) ES cel ls but not in thymocytes or splenocytes. Comparison of the requirement for Casp9 and Casp3 in different apoptotic settings indicates the exi stence of at least four different apoptotic pathways in mammalian cell s.