Tomomi Kuwana

Bid, Bax, and Lipids Cooperate to Form Supramolecular Openings in the Outer Mitochondrial Membrane

Bcl-2 family proteins regulate the release of proteins like cytochrome c from mitochondria during apoptosis. We used cell-free systems and ultimately a vesicular reconstitution from defined molecules to show that outer membrane permeabilization by Bcl-2 family proteins requires neither the mitochondrial matrix, the inner membrane, nor other proteins. Bid, or its BH3-domain peptide, activated monomeric Bax to produce membrane openings that allowed the passage of very large (2 megadalton) dextran molecules, explaining the translocation of large mitochondrial proteins during apoptosis. This process required cardiolipin and was inhibited by antiapoptotic Bcl-x(L). We conclude that mitochondrial protein release in apoptosis can be mediated by supramolecular openings in the outer mitochondrial membrane, promoted by BH3/Bax/lipid interaction and directly inhibited by Bcl-x(L).

Heat-shock protein 70 inhibits apoptosis by preventing recruitment of procaspase-9 to the Apaf-1 apoptosome

The cellular-stress response can mediate cellular protection through expression of heat-shock protein (Hsp) 70, which can interfere with the process of apoptotic cell death. Stress-induced apoptosis proceeds through a defined biochemical process that involves cytochrome c, Apaf-1 and caspase proteases. Here we show, using a cell-free system, that Hsp70 prevents cytochrome c/dATP-mediated caspase activation, but allows the formation of Apaf-1 oligomers. Hsp70 binds to Apaf-1 but not to procaspase-9, and prevents recruitment of caspases to the apoptosome complex. Hsp70 therefore suppresses apoptosis by directly associating with Apaf-1 and blocking the assembly of a functional apoptosome.

Apoptosis Induction by Caspase-8 Is Amplified through the Mitochondrial Release of Cytochrome c

Apoptosis often involves the release of cytochrome c from mitochondria, leading to caspase activation. However, in apoptosis mediated by CD95 (Fas/APO-1), caspase-8 (FLICE/MACH/Mch5) is immediately activated and, in principle, could process other caspases directly. To investigate whether caspase-8 could also act through mitochondria, we added active caspase-8 to aXenopus cell-free system requiring these organelles. Caspase-8 rapidly promoted the apoptotic program, culminating in fragmentation of chromatin and the nuclear membrane. In extracts devoid of mitochondria, caspase-8 produced DNA degradation, but left nuclear membranes intact. Thus, mitochondria were required for complete engagement of the apoptotic machinery. In the absence of mitochondria, high concentrations of caspase-8 were required to activate downstream caspases. However, when mitochondria were present, the effects of low concentrations of caspase-8 were vastly amplified through cytochromec-dependent caspase activation. Caspase-8 promoted cytochrome c release indirectly, by cleaving at least one cytosolic substrate. Bcl-2 blocked apoptosis only at the lowest caspase-8 concentrations, potentially explaining why CD95-induced apoptosis can often evade inhibition by Bcl-2.

Sphingolipid metabolism cooperates with BAK and BAX to promote the mitochondrial pathway of apoptosis.

Mitochondria are functionally and physically associated with heterotypic membranes, yet little is known about how these interactions impact mitochondrial outer-membrane permeabilization (MOMP) and apoptosis. We observed that dissociation of heterotypic membranes from mitochondria inhibited BAK/BAX-dependent cytochrome c (cyto c) release. Biochemical purification of neutral sphingomyelinases that correlated with MOMP sensitization suggested that sphingolipid metabolism coordinates BAK/BAX activation. Using purified lipids and enzymes, sensitivity to MOMP was achieved by in vitro reconstitution of the sphingolipid metabolic pathway. Sphingolipid metabolism inhibitors blocked MOMP from heavy membrane preparations but failed to influence MOMP in the presence of sphingolipid-reconstituted, purified mitochondria. Furthermore, the sphingolipid products, sphingosine-1-PO(4) and hexadecenal, cooperated specifically with BAK and BAX, respectively. Sphingolipid metabolism was also required for cellular responses to apoptosis. Our studies suggest that BAK/BAX activation and apoptosis are coordinated through BH3-only proteins and a specific lipid milieu that is maintained by heterotypic membrane-mitochondrial interactions.

Opa1-Mediated Cristae Opening Is Bax/Bak and BH3 Dependent, Required for Apoptosis, and Independent of Bak Oligomerization

Controversy surrounds the role and mechanism of mitochondrial cristae remodeling in apoptosis. Here we show that the proapoptotic BH3-only proteins Bid and Bim induced full cytochrome c release but only a subtle alteration of crista junctions, which involved the disassembly of Opa1 complexes. Both mitochondrial outer membrane permeabilization (MOMP) and crista junction opening (CJO) were caspase independent and required a functional BH3 domain and Bax/Bak. However, MOMP and CJO were experimentally separable. Pharmacological blockade of MOMP did not prevent Opa1 disassembly and CJO; moreover, expression of a disassembly-resistant mutant Opa1 (Q297V) blocked cytochrome c release and apoptosis but not Bax activation. Thus, apoptosis requires a subtle form of Opa1-dependent crista remodeling that is induced by BH3-only proteins and Bax/Bak but independent of MOMP.

Mechanism of apoptosis induction by inhibition of the anti-apoptotic BCL-2 proteins

Normal cellular lifespan is contingent upon preserving outer mitochondrial membrane (OMM) integrity, as permeabilization promotes apoptosis. BCL-2 family proteins control mitochondrial outer membrane permeabilization (MOMP) by regulating the activation of the pro-apoptotic BCL-2 effector molecules, BAX and BAK. Sustainable cellular stress induces proteins (e.g., BID, BIM, and cytosolic p53) capable of directly activating BAX and/or BAK, but these direct activators are sequestered by the anti-apoptotic BCL-2 proteins (e.g., BCL-2, BCL-xL, and MCL-1). In the event of accumulated or marked cellular stress, a coordinated effort between previously sequestered and nascent BH3-only proteins inhibits the anti-apoptotic BCL-2 repertoire to promote direct activator protein-mediated MOMP. We examined the effect of ABT-737, a BCL-2 antagonist, and PUMA, a BH3-only protein that inhibits the entire anti-apoptotic BCL-2 repertoire, with cells and mitochondria that sequestered direct activator proteins. ABT-737 and PUMA cooperated with sequestered direct activator proteins to promote MOMP and apoptosis, which in the absence of ABT-737 or PUMA did not influence OMM integrity or cellular survival. Our data show that the induction of apoptosis by inhibition of the anti-apoptotic BCL-2 repertoire requires “covert” levels of direct activators of BAX and BAK at the OMM.

Granzyme B-mediated Apoptosis Proceeds Predominantly through a Bcl-2-inhibitable Mitochondrial Pathway

Cytotoxic T lymphocytes kill virus-infected and tumor cell targets through the concerted action of proteins contained in cytolytic granules, primarily granzyme B and perforin. Granzyme B, a serine proteinase with substrate specificity similar to the caspase family of apoptotic cysteine proteinases, is capable of cleaving and activating a number of death proteins in target cells. Despite the ability to engage the death pathway at multiple entry points, the preferred mechanism for rapid induction of apoptosis by granzyme B has yet to be clearly established. Here we use time lapse confocal microscopy to demonstrate that mitochondrial cytochromec release is the primary mode of granzyme B-induced apoptosis and that Bcl-2 is a potent inhibitor of this pivotal event. Caspase activation is not required for cytochrome crelease, an activity that correlates with cleavage and activation of Bid, which we have found to be cleaved more readily by granzyme B than either caspase-3 or caspase-8. Bcl-2 blocks the rapid destruction of targets by granzyme B by blocking mitochondrial involvement in the process.

BH3 Domains other than Bim and Bid Can Directly Activate Bax/Bak

Bcl-2 family proteins regulate a critical step in apoptosis referred to as mitochondrial outer membrane permeabilization (MOMP). Members of a subgroup of the Bcl-2 family, known as the BH3-only proteins, activate pro-apoptotic effectors (Bax and Bak) to initiate MOMP. They do so by neutralizing pro-survival Bcl-2 proteins and/or directly activating Bax/Bak. Bim and Bid are reported to be direct activators; however, here we show that BH3 peptides other than Bim and Bid exhibited various degrees of direct activation of the effector Bax or Bak, including Bmf and Noxa BH3s. In the absence of potent direct activators, such as Bim and Bid, we unmasked novel direct activator BH3 ligands capable of inducing effector-mediated cytochrome c release and liposome permeabilization, even when both Bcl-xL- and Mcl-1-type anti-apoptotic proteins were inhibited. The ability of these weaker direct activator BH3 peptides to cause MOMP correlated with that of the corresponding full-length proteins to induce apoptosis in the absence of Bim and Bid. We propose that, in certain contexts, direct activation by BH3-only proteins other than Bim and Bid may significantly contribute to MOMP and apoptosis.

Apoptosis Regulation at the Mitochondrial Outer Membrane

Mitochondria play a critical role in apoptosis, or programmed cell death, by releasing apoptogenic factors from the intermembrane space. This process, known as mitochondrial outer membrane permeabilization (MOMP), is tightly regulated by the Bcl‐2 family proteins. Pro‐apoptotic Bcl‐2 family members, Bax and Bak, change their conformation when activated by BH3 domain‐only proteins in the family and permeabilize the MOM, whereas pro‐survival members inhibit permeabilization. The precise nature of the apoptotic pore in the MOM is unknown, but is probably lipidic. Furthermore, it has been realized that there is another layer of MOMP regulation by a protein factor termed the catalyst in the MOM in order for Bax/Bak to achieve efficient and complete membrane permeabilization. Mitochondrial dynamics do not affect MOMP directly, but seem closely coordinated with MOMP for swift protein efflux from mitochondria. This review will present current views on the molecular mechanisms and regulation of MOMP and conclude with recent developments in clinical applications based on the knowledge gleaned from the investigation. J. Cell. Biochem. 115: 632–640, 2014.

Reaper-induced apoptosis in a vertebrate system.

The reaper protein of Drosophila melanogaster has been shown to be a central regulator of apoptosis in that organism. However, it has not been shown to function in any vertebrate nor have the cellular components required for its action been defined. In this report we show that reaper can induce rapid apoptosis in vitro using an apoptotic reconstitution system derived from Xenopus eggs. Moreover, we show that a subcellular fraction enriched in mitochondria is required for this process and that reaper, acting in conjunction with cytosolic factors, can trigger mitochondrial cytochrome c release. Bcl‐2 antagonizes these effects, but high levels of reaper can overcome the Bcl‐2 block. These results demonstrate that reaper can function in a vertebrate context, suggesting that reaper‐responsive factors are conserved elements of the apoptotic program.