Multiple death signals influence mitochondria during apoptosis, yet the critical initiating event for mitochondrial dysfunction in vivo has been unclear. tBID, the caspase-activated form of a "BH3-domain-only" BCL-2 family member, triggers the homooligomerization of "multidomain" conserved proapoptotic family members BAK or BAX, resulting in the release of cytochrome c from mitochondria. We find that cells lacking both Bax and Bak, but not cells lacking only one of these components, are completely resistant to tBID-induced cytochrome c release and apoptosis. Moreover, doubly deficient cells are resistant to multiple apoptotic stimuli that act through disruption of mitochondrial function: staurosporine, ultraviolet radiation, growth factor deprivation, etoposide, and the endoplasmic reticulum stress stimuli thapsigargin and tunicamycin. Thus, activation of a "multidomain" proapoptotic member, BAX or BAK, appears to be an essential gateway to mitochondrial dysfunction required for cell death in response to diverse stimuli.Members of the "BH3-domain-only" subset of BCL-2 family proteins connect proximal death signals to the core apoptotic pathway (1-5). After activation of CD95 (Fas) or TNFR1 death receptors, BID is cleaved and activated to p15 tBID (6-8), which, in a model system using purified mitochondria, serves as a death ligand that induces the oligomerization of BAK (9) and BAX (10). tBID does not cause release of cytochrome c from purified Bakdeficient mitochondria, suggesting that interaction of tBID and BAK is a critical event at least in vitro (9).Here we assess whether BAK is required for tBID-induced apoptosis of intact, whole cells by using a retroviral vector to express tBID in murine embryonic fibroblasts (MEFs). Numerous cells with shrunken, apoptotic morphology were detected in Bak-deficient as well † To whom correspondence should be addressed.

We report the isolation of bcl-x, a bcl-2-related gene that can function as a bcl-2-independent regulator of programmed cell death (apoptosis). Alternative splicing results in two distinct bcl-x mRNAs. The protein product of the larger mRNA, bcl-xL, is similar in size and predicted structure to Bcl-2. When stably transfected into an IL-3-dependent cell line, bcl-xL inhibits cell death upon growth factor withdrawal at least as well as bcl-2. Surprisingly, the second mRNA species, bcl-xS, encodes a protein that inhibits the ability of bcl-2 to enhance the survival of growth factor-deprived cells. In vivo, bcl-xS mRNA is expressed at high levels in cells that undergo a high rate of turnover, such as developing lymphocytes. In contrast, bcl-xL is found in tissues containing long-lived postmitotic cells, such as adult brain. Together these data suggest that bcl-x plays an important role in both positive and negative regulation of programmed cell death.

Two seemingly unrelated hallmarks of memory CD8(+) T cells are cytokine-driven proliferative renewal after pathogen clearance and a latent effector program in anticipation of rechallenge. Memory CD8(+) T cells and natural killer cells share cytotoxic potential and dependence on the growth factor interleukin 15. We now show that mice with compound mutations of the genes encoding the transcription factors T-bet and eomesodermin were nearly devoid of several lineages dependent on interleukin 15, including memory CD8(+) T cells and mature natural killer cells, and that their cells had defective cytotoxic effector programming. Moreover, T-bet and eomesodermin were responsible for inducing enhanced expression of CD122, the receptor specifying interleukin 15 responsiveness. Therefore, these key transcription factors link the long-term renewal of memory CD8(+) T cells to their characteristic effector potency.