Cytochrome c Deficiency Causes Embryonic Lethality and Attenuates Stress-Induced Apoptosis (original) (raw)

or caspase-9 (Hakem et al., 1998; Kuida et al., and R. Sanders Williams* § # 1998) results in prenatal or perinatal death of homozy-* Department of Internal Medicine gous null animals, which exhibit prominent overgrowth † Department of Biochemistry of neural structures based on a failure of developmen- ‡ Department of Pathology tally mediated apoptosis. Cells derived from these § Department of Molecular Biology knockout mice demonstrate defects in response to a Howard Hughes Medical Institute variety of apoptotic stimuli. However, T lymphocytes University of Texas Southwestern Medical Center from mice lacking Apaf-1 or caspase-9 undergo apo-Dallas, Texas 75390 ptosis normally in response to TNF␣ or antibodies that function as agonists for Fas receptor signaling (Hakem et al., 1998; Yoshida et al., 1998). These observations Summary have supported the notion of discrete apoptotic signaling pathways (reviewed in Green and Reed, 1998; Vaux Cytochrome c released from mitochondria has been and Korsmeyer, 1999): a "cellular stress" or "mitochonproposed to be an essential component of an apodrial" pathway dependent on cytochrome c, Apaf-1, and ptotic pathway responsive to DNA damage and other caspase-9; and a "death ligand" or "death receptor" forms of cell stress. Murine embryos devoid of cytopathway mediated by other signaling proteins such as chrome c die in utero by midgestation, but cell lines FADD and caspase-8. Both pathways converge on established from early cytochrome c null embryos are caspase-3 and other proteases and nucleases that drive viable under conditions that compensate for defective the terminal events of programmed cell death. The two oxidative phosphorylation. As compared to cell lines pathways are thought to function in parallel, although established from wild-type embryos, cells lacking cythe death receptor pathway is amplified by the mitotochrome c show reduced caspase-3 activation and chondrial pathway in certain cell types through the proare resistant to the proapoptotic effects of UV irradiatein Bid (Li et al., 1998; Luo et al., 1998). tion, serum withdrawal, or staurosporine. In contrast, Unlike Apaf-1, caspase-9, and caspase-3, the biocells lacking cytochrome c demonstrate increased chemical function of cytochrome c proposed in this sensitivity to cell death signals triggered by TNF␣. model has not been subjected to the tests provided by These results define the role of cytochrome c in differthe analysis of cells or animals bearing null mutations. ent apoptotic signaling cascades. Since cytochrome c is required for mitochondrial respiration, genetic approaches that have been applied suc-Introduction cessfully in model organisms to characterize the function of other apoptotic signaling intermediates have not Cytochrome c (Cyt c), the only water-soluble component been useful in assessing the proposed function of cytoof the electron transfer chain, was unexpectedly found chrome c in apoptosis. to promote the activation of apoptotic caspases in cell-Here, we employ genetically modified mice to investifree extracts or when introduced ectopically into the gate the importance of cytochrome c in apoptotic signalcytoplasm of intact cells (Liu et al., 1996; Li et al., 1997; ing pathways. Cell lines devoid of cytochrome c were Zhivotovsky et al., 1998). In response to a variety of established from early embryos under conditions that death-promoting stimuli, cytochrome c is released from compensate for their defect in mitochondrial respiration, its normal position within the intermembrane space of as defined previously for culture of cells lacking mitomitochondria, in association with changes in mitochonchondrial DNA (King and Attardi, 1989). In comparison drial permeability, membrane potential, and ultrastrucwith wild-type cells, cells lacking cytochrome c are resisture (Heiskanen et al., 1999). Once in the cytosol, cytotant to death induced by UV irradiation or proapoptotic chrome c binds Apaf-1 with high affinity, an event that drugs and are partially resistant to the apoptotic effects triggers oligomerization of Apaf-1/cytochrome c in comof serum withdrawal. In contrast, the apoptotic response plexes that activate procaspase-9 (Zou et al., 1999).