Review
The mitochondrial apoptosome: a killer unleashed by the cytochrome seas

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Abstract

The caspase family of cysteine proteases have emerged as central regulators of apoptosis. Diverse cellular stresses trigger caspase activation by promoting release of mitochondrial components, including cytochrome c, into the cytoplasm. In turn, cytochrome c promotes the assembly of a caspase-activating complex termed the apoptosome. In this article, the apoptosome and its role in life and death decisions of cells are discussed.

Section snippets

Caspase activation from within: the mitochondrial pathway

Until recently, our understanding of how mammals regulate caspase activation was largely restricted to cell-death pathways initiated through engagement of certain members of the tumor necrosis factor receptor family 5 (Fig. 1). More recently, an alternative and somewhat more circuitous route to caspase activation, involving mitochondria, has emerged 6. In this pathway, divergent cellular stresses including DNA damage, heat shock and oxidative stress, result in caspase activation through the

The worm turns

Before we discuss the mammalian apoptosome and its assembly in detail, it is useful to describe how caspase activation and apoptosis are regulated in the simple nematode worm Caenorhabditis elegans. Much of the recent progress that has been made in our understanding of mammalian apoptosis has originated from genetic screens performed by Horvitz and colleagues 14 for cell-death regulatory genes operative in the worm. These screens implicated three gene products, CED-3, CED-4 and CED-9, as having

Evolutionary conservation of the apoptosome

Mammalian homologues of CED-3 (the mammalian caspases), and of CED-9 and EGL-1 (the Bcl-2 family) were discovered in the early 1990s. However, mammalian CED-4 homologues proved much more elusive and have only emerged in the past few years. The identification of the first mammalian CED-4 homologue by Xiaodong Wang and colleagues 20 proved to be a major breakthrough in the field as the context of this discovery was particularly revealing. Pursuing their earlier observation that cytochrome c could

Breaching the hull: the role of the ‘BH3-only’ Bcl-2 family members

To activate Apaf-1 and realize its death-promoting activity, it is clear that cytochrome c must leave mitochondria and enter the cytosol. But how is this achieved?

Uncoupling of mitochondrial oxidative phosphorylation is commonly observed during apoptosis, resulting in a loss of mitochondrial transmembrane potential 13, 21. Because the latter event has been linked to the opening of mitochondrial permeability transition (PT) pores, or megachannels, this suggested that the release of cytochrome c

The end is nigh: assembly of the apoptosome

Exit from the mitochondrion signals a new role for cytochrome c as a life-taker, as opposed to its normal life-sustaining role in the electron transport chain. As already mentioned, the cytosolic target of cytochrome c is the mammalian CED-4 homologue Apaf-1 (Ref. 20). So how does the complex of cytochrome c and Apaf-1 activate the caspase cascade?

Fig. 3 illustrates a working model of apoptosome assembly. Following exposure of the cell to a stimulus that provokes the release of cytochrome c

Apaf-1 as an allosteric regulator of caspase-9

An intriguing possibility is that, in addition to facilitating caspase-9 activation, Apaf-1 is required for full catalytic activity of the mature caspase-9 protease. A key question is whether the apoptosome has bona fide proteolytic activity as a multimolecular complex, akin to the proteasome, or whether it serves only as an oligomerization platform for procaspase-9, which is released upon proteolytic maturation. Because several groups have observed the release of catalytically active caspase-9

Manning the lifeboats: IAPs and heat shock proteins

Having assembled the apoptosome, is a cell irreversibly committed to death? The answer to this question would appear to be context dependent. As discussed earlier, the C. elegans CED-9 protein is likely to prevent the assembly of the worm CED-4–CED-3 apoptosome by sequestering CED-4 to mitochondrial membranes 16, 17. Although it is tempting to envisage a similar scenario in mammalian cells, in which Bcl-2 or its relatives complex with Apaf-1, there is conflicting evidence as to whether Bcl-2

Smac: just when you thought it was safe to go back into the cytoplasm

As we have seen, life (and death) on the cytochrome seas is not always plain sailing. However, given the barrier to caspase activation posed by the IAPs, it is not surprising that several mechanisms exist to counter their effects and allow apoptosis to proceed. First, it has been observed that XIAP is a caspase substrate under death-stimulatory conditions and this might provide a caspase-dependent mechanism for XIAP disablement. Second, c-IAP-1 and XIAP might self-destruct via the E3-ubiquitin

Sinking the ship: amplification of the caspase cascade

If, in maritime terms, the activation of caspase-9 is akin to receiving a gash beneath the waterline, the cellular events that follow could be equated with the subsequent flooding that breaks the back of the ship. During apoptosis, activation of caspase-9 by the apoptosome sets in motion a cascade of caspase activation events that proceed to wreck the cellular infrastructure and trigger the plasma membrane changes that promote engulfment by phagocytes. This constitutes the final blow that seals

Concluding remarks

It is now clear that mitochondria act to integrate diverse proapoptotic stimuli by releasing death promoting factors such as cytochrome c, Smac/Diablo and the flavoprotein AIF (Ref. 50). The role of AIF in apoptosis, or in alternative (caspase-independent) death pathways, remains to be further defined. The mechanisms by which the release of mitochondrial proteins is regulated by Bcl-2 family members is now emerging, but further work is required to fully understand this process. The role of the

Acknowledgements

We thank The Wellcome Trust, Enterprise Ireland and The Health Research Board of Ireland for support of some of the work discussed in this article. We apologize to our colleagues for failing to cite many relevant primary papers because of space constraints.

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