Alternative titles; symbols
HGNC Approved Gene Symbol: CASP9
Cytogenetic location: 1p36.21 Genomic coordinates (GRCh38): 1:15,491,401-15,524,912 (from NCBI)
Li et al. (1997) reported the purification from HeLa cells of a third apoptotic activation factor, APAF3, that participates in caspase-3 (600636) activation in vitro. APAF3 was identified as a member of the caspase family and designated caspase-9 (CASP9).
Crystal Structure
Shiozaki et al. (2003) reported the crystal structure of caspase-9 in an inhibitory complex with the BIR3 domain of XIAP at 2.4-angstrom resolution. The structure revealed that the BIR3 domain forms a heterodimer with a caspase-9 monomer. The surface of caspase-9 that interacts with BIR3 also mediates its homodimerization. Monomeric caspase-9 is catalytically inactive due to the absence of a supporting sequence element that could be provided by homodimerization. The authors concluded that XIAP sequesters caspase-9 in a monomeric state, which serves to prevent catalytic activity.
Li et al. (1997) determined that caspase-9 and APAF1 (602233) bind to each other via their respective NH2-terminal CED-3 homologous domains in the presence of cytochrome c (123970) and dATP, an event that leads to caspase-9 activation. Activated caspase-9 in turn cleaves and activates caspase-3. Depletion of caspase-9 from S-100 extracts diminished caspase-3 activation. Mutation of the active site of caspase-9 (cys287 to ala) attenuated the activation of caspase-3 and cellular apoptotic response in vivo, indicating that caspase-9 is the most upstream member of the apoptotic protease cascade that is triggered by cytochrome c and dATP.
Activation of procaspase-9 by APAF1 in the cytochrome c/dATP-dependent pathway requires proteolytic cleavage to generate the mature caspase molecule. Srinivasula et al. (1998) showed that deletion of the APAF1 WD40 repeats makes APAF1 constitutively active and capable of processing procaspase-9 independent of cytochrome c and dATP. APAF1-mediated processing of procaspase-9 occurs at asp315 by an intrinsic autocatalytic activity of procaspase-9 itself. Srinivasula et al. (1998) provided evidence that APAF1 can form oligomers and may facilitate procaspase-9 autoactivation by oligomerizing its precursor molecules. Once activated, caspase-9 can initiate a biochemical cascade involving the downstream executioners caspase-3, -6, and -7.
XIAP (300079) interacts with caspase-9 and inhibits its activity, whereas SMAC (605219) relieves this inhibition through interaction with XIAP. Srinivasula et al. (2001) demonstrated that XIAP associates with the active caspase-9-APAF1 holoenzyme complex through binding to the amino terminus of the linker peptide on the small subunit of caspase-9, which becomes exposed after proteolytic processing of procaspase-9 at asp315. Supporting this observation, point mutations that abrogate the proteolytic processing but not the catalytic activity of caspase-9, or deletion of the linker peptide, prevented caspase-9 association with XIAP and its concomitant inhibition. Srinivasula et al. (2001) noted that the N-terminal 4 residues of caspase-9 linker peptide share significant homology with the N-terminal tetrapeptide in mature SMAC and in the Drosophila proteins Hid/Grim/Reaper, defining a conserved class of IAP-binding motifs. Consistent with this finding, binding of the caspase-9 linker peptide and SMAC to the BIR3 domain of XIAP is mutually exclusive, suggesting that SMAC potentiates caspase-9 activity by disrupting the interaction of the linker peptide of caspase-9 with BIR3. Srinivasula et al. (2001) concluded that their studies reveal a mechanism in which binding to the BIR3 domain of XIAP by 2 conserved peptides, one from SMAC and the other from caspase-9, has opposing effects on caspase activity and apoptosis.
Marsden et al. (2002) established that the cell death pathway controlled by BCL2 (151430) does not require caspase-9 or its activator APAF1. In keeping with their evidence that neither is required for hematopoietic homeostasis, in which the BCL2 family has major roles, deletion of thymocytes with self-reactivity depends on BIM (603827) but not on APAF1. Because apoptosis was at most slightly delayed by the absence of APAF1 or caspase-9, Marsden et al. (2002) concluded that the apoptosome is not an essential trigger for apoptosis but is rather a machine for amplifying the caspase cascade. They found that BCL2 overexpression increased lymphocyte numbers in mice and inhibited many apoptotic stimuli, but the absence of APAF1 and caspase-9 did not. Caspase activity was still discernible in cells lacking APAF1 or caspase-9 and a potent caspase antagonist both inhibited apoptosis and retarded cytochrome c release. Marsden et al. (2002) concluded that BCL2 regulates a caspase activation program independently of the cytochrome c/APAF1/caspase-9 apoptosome, which seems to amplify rather than initiate the caspase cascade.
CASP9 activity increases dramatically upon association with the apoptosome complex. Yin et al. (2006) and Pop et al. (2006) demonstrated that the apoptosome activates CASP9 by causing its dimerization due to induced proximity. In addition, Yin et al. (2006) found that the apoptosome enhanced the affinity of CASP9 for proCASP3, either by directly interacting with proCASP3 or by inducing a conformational change in bound CASP9.
Chew et al. (2009) presented a highly validated set of targets that is necessary for apoptosis provoked by several stimuli in Drosophila. Among these, Tango7, whose human homolog is PCID1 (609641), was identified as a new effector. Cells depleted for this gene resisted apoptosis at a step before the induction of effector caspase activity, and the directed silencing of Tango7 in Drosophila prevented caspase-dependent programmed cell death. Unlike known apoptosis regulators in this model system, Tango7 activity did not influence stimulus-dependent loss of Drosophila DIAP1 (human homolog DIAPH1, 602121), but instead regulated levels of the apical caspase Dronc (human homolog CASP9). Similarly, Chew et al. (2009) found that human PCID1 impinged on caspase-9, revealing a novel regulatory axis affecting the apoptosome.
Hadano et al. (1999) determined that the CASP9 gene contains 9 exons and spans approximately 35 kb of genomic DNA.
By FISH, Hadano et al. (1999) mapped the CASP9 gene to chromosome 1p36.3-p36.1.
Kuida et al. (1998) created mice with a targeted disruption of the mouse homolog of the human CASP9 gene. The majority of Casp9 knockout mice died perinatally with a markedly enlarged and malformed cerebrum caused by reduced apoptosis during brain development. Casp9 deletion prevents activation of Casp3 in embryonic brains in vivo, and Casp9-deficient thymocytes show resistance to a subset of apoptotic stimuli, including absence of Casp3-like cleavage and delayed DNA fragmentation. Moreover, the cytochrome c-mediated cleavage of Casp3 is absent in the cytosolic extracts of Casp9-deficient cells but is restored after addition of in vitro-translated Casp9. These results indicate that Casp9 is a critical upstream activator of the caspase cascade in vivo.
Hakem et al. (1998) also created knockout mice with disrupted Casp9. Casp9 -/- embryonic stem cells and embryonic fibroblasts were resistant to several apoptotic stimuli, including UV and gamma-irradiation. Casp9 -/- thymocytes are also resistant to dexamethasone- and gamma-irradiation-induced apoptosis, but were sensitive to apoptosis induced by UV irradiation or anti-CD95. Resistance to apoptosis was accompanied by retention of the mitochondrial membrane potential in mutant cells. In addition, cytochrome c was translocated to the cytosol of Casp9 -/- embryonic stem (ES) cells upon UV stimulation, suggesting that Casp9 acts downstream of cytochrome c. Caspase processing was inhibited in Casp9 -/- ES cells but not in thymocytes or splenocytes. Comparison of the requirement for Casp9 and Casp3 in different apoptotic settings indicated the existence of at least 4 different apoptotic pathways in mammalian cells.
Chew, S. K., Chen, P., Link, N., Galindo, K. A., Pogue, K., Abrams, J. M. Genome-wide silencing in Drosophila captures conserved apoptotic effectors. Nature 460: 123-127, 2009. [PubMed: 19483676] [Full Text: https://doi.org/10.1038/nature08087]
Hadano, S., Nasir, J., Nichol, K., Rasper, D. M., Vaillancourt, J. P., Sherer, S. W., Beatty, B. G., Ikeda, J.-E., Nicholson, D. W., Hayden, M. R. Genomic organization of the human caspase-9 gene on chromosome 1p36.1-p36.3. Mammalian Genome 10: 757-760, 1999. [PubMed: 10384055] [Full Text: https://doi.org/10.1007/s003359901086]
Hakem, R., Hakem, A., Duncan, G. S., Henderson, J. T., Woo, M., Soengas, M. S., Elia, A., de la Pompa, J. L., Kagi, D., Khoo, W., Potter, J., Yoshida, R., Kaufman, S. A., Lowe, S. W., Penninger, J. M., Mak, T. W. Differential requirement for caspase 9 in apoptotic pathways in vivo. Cell 94: 339-352, 1998. [PubMed: 9708736] [Full Text: https://doi.org/10.1016/s0092-8674(00)81477-4]
Kuida, K., Haydar, T. F., Kuan, C.-Y., Gu, Y., Taya, C., Karasuyama, H., Su, M. S.-S., Rakic, P., Flavell, R. A. Reduced apoptosis and cytochrome c-mediated caspase activation in mice lacking caspase 9. Cell 94: 325-327, 1998. [PubMed: 9708735] [Full Text: https://doi.org/10.1016/s0092-8674(00)81476-2]
Li, P., Nijhawan, D., Budihardjo, I., Srinivasula, S. M., Ahmad, M., Alnemri, E. S., Wang, X. Cytochrome c and dATP-dependent formation of Apaf-1/caspase-9 complex initiates an apoptotic protease cascade. Cell 91: 479-489, 1997. [PubMed: 9390557] [Full Text: https://doi.org/10.1016/s0092-8674(00)80434-1]
Marsden, V. S., O'Connor, L., O'Reilly, L. A., Silke, J., Metcalf, D., Ekert, P. G., Huang, D. C. S., Cecconi, F., Kuida, K., Tomaselli, K. J., Roy, S., Nicholson, D. W., Vaux, D. L., Bouillet, P., Adams, J. M., Strasser, A. Apoptosis initiated by Bcl-2-regulated caspase activation independently of the cytochrome c/Apaf-1/caspase-9 apoptosome. Nature 419: 634-637, 2002. [PubMed: 12374983] [Full Text: https://doi.org/10.1038/nature01101]
Pop, C., Timmer, J., Sperandio, S., Salvesen, G. S. The apoptosome activates caspase-9 by dimerization. Molec. Cell 22: 269-275, 2006. [PubMed: 16630894] [Full Text: https://doi.org/10.1016/j.molcel.2006.03.009]
Shiozaki, E. N., Chai, J., Rigotti, D. J., Riedl, S. J., Li, P., Srinivasula, S. M., Alnemri, E. S., Fairman, R., Shi, Y. Mechanism of XIAP-mediated inhibition of caspase-9. Molec. Cell 11: 519-527, 2003. [PubMed: 12620238] [Full Text: https://doi.org/10.1016/s1097-2765(03)00054-6]
Srinivasula, S. M., Ahmad, M., Fernandes-Alnemri, T., Alnemri, E. S. Autoactivation of procaspase-9 by Apaf-1-mediated oligomerization. Molec. Cell 1: 949-957, 1998. [PubMed: 9651578] [Full Text: https://doi.org/10.1016/s1097-2765(00)80095-7]
Srinivasula, S. M., Hegde, R., Saleh, A., Datta, P., Shiozaki, E., Chai, J., Lee, R.-A., Robbins, P. D., Fernandes-Alnemri, T., Shi, Y., Alnemri, E. S. A conserved XIAP-interaction motif in caspase-9 and Smac/DIABLO regulates caspase activity and apoptosis. Nature 410: 112-116, 2001. Note: Erratum: Nature 411: 1081 only, 2001. [PubMed: 11242052] [Full Text: https://doi.org/10.1038/35065125]
Yin, Q., Park, H. H., Chung, J. Y., Lin, S.-C., Lo, Y.-C., da Graca, L. S., Jiang, X., Wu, H. Caspase-9 holoenzyme is a specific and optimal procaspase-3 processing machine. Molec. Cell 22: 259-268, 2006. [PubMed: 16630893] [Full Text: https://doi.org/10.1016/j.molcel.2006.03.030]