ReviewThe Bcl-2 protein family: sensors and checkpoints for life-or-death decisions
Introduction
Programmed cell death (apoptosis) is an evolutionarily conserved process of eliminating unwanted, damaged, aged and misplaced cells during embryonic development and tissue homeostasis (Meier et al., 2000, Vaux and Korsmeyer, 1999). Abnormal resistance to apoptosis can lead to disorders such as autoimmunity or cancer due to the persistence of superfluous, self-specific or mutated cells (Green and Evan, 2002, Thompson, 1995, Zornig et al., 2001). In contrast, enhanced apoptosis contributes to acute diseases such as infection by toxin-producing microorganisms, ischemia–reperfusion damage or infarction, as well as to chronic pathologies such as neurodegenerative, neuromuscular diseases and AIDS (Mattson, 2000, Rathmell and Thompson, 2002). The Bcl-2 family of proteins are central regulators of apoptosis because they integrate diverse survival and death signals that are generated outside and inside the cell (Adams and Cory, 2001, Strasser et al., 2000). The family is subdivided into two classes: anti-apoptotic members such as Bcl-2 and Bcl-xL (the Bcl-2-like survival factors) which protect cells from apoptosis, and pro-apoptotic members such as Bax and Bak (the Bax-like death factors) and the large group of BH3-only death proteins (Antonsson, 2001, Huang and Strasser, 2000) which trigger or sensitize for apoptosis. Mutual interactions between pro- and anti-apoptotic members set the threshold that determines whether a cell should die or not. Thus, Bcl-2 family members act like checkpoints through which survival and death signals must pass before they determine the cell fate. In the last decade we have learned much about where Bcl-2 family proteins act within cells (e.g. on the outer mitochondrial and nuclear/endoplasmic reticulum (ER) membranes) and what type of reactions they control (e.g. caspase activation/inhibition). But we still do not fully understand how these proteins issue life permits or death sentences at the molecular level. Here I critically review recent studies on the molecular analysis of Bcl-2 family members, take the most convincing findings to build model systems about how these proteins may work, and then illustrate the importance of these actions for life-or-death decisions in the immune system.
Section snippets
The caspase death proteases: indirect targets of Bcl-2 family members
To understand the mode of action of Bcl-2 family proteins at the molecular level, we first need to look at the key players which execute the apoptotic process. These are cysteine proteases of the caspase family that cleave vital cellular substrates after aspartate residues (Earnshaw et al., 1999, Nicholson, 1999). These enzymes are minimally active in healthy cells and require further activation in response apoptotic stimuli (Shi, 2002). They are divided into two categories: the initiator
Classification of the Bcl-2 family
While C. elegans encodes for only two members of the Bcl-2 family CED-9 and EGL-1, higher eukaroytes possess up to 30 homologs (Adams and Cory, 2001, Gross et al., 1999, Puthalakath and Strasser, 2002). They can be grouped into three categories: (i) CED-9-like survival factors such as Bcl-2, Bcl-xL, Bcl-w, Mcl-1, A1/Bfl-1, NR-13, Boo/Diva/Bcl2-L-10 and Bcl-B (Bcl-2-like survival factors, Fig. 4A); (ii) EGL-like pro-apoptotic proteins such as Bik/Nbk, Blk, Hrk/DP5, BNIP3, BimL/Bod, Bad, Bid,
Bcl-2-like survival factors: membrane-bound scavengers of pro-apoptotic proteins
Like CED-9, the Bcl-2-like survival factors contain three to four so-called Bcl-2 homology domains (BH1–BH4) which are absolutely required for their survival functions (Borner et al., 1994a, Borner et al., 1994b; Chittenden et al., 1995a, Farrow and Brown, 1996, Hanada et al., 1995, Kelekar and Thompson, 1998, Yin et al., 1994). These domains do not have any enzymatic activity but mediate the interaction of Bcl-2-like survival factors with other protein partners (Fig. 4A). The solution
Bax-like death factors: triggers of apoptosis through conformational change and channel/pore formation
Strikingly, Bcl-2-like survival factors are converted into pro-apoptotic proteins after proteolytic removal of the N-terminal BH4 domain. This has been seen with endogenous and overexpressed proteins after alphavirus infection as well as in response to certain apoptotic stimuli such as staurosporine (Cheng et al., 1997, Grandgirard et al., 1998). Similarly, CED-9 enhances (rather than suppresses) programmed cell death in C. elegans carrying a mutation in CED-3 that reduces but does not abolish
The BH3-only death factors: the sensors and mediators of apoptosis
The BH3-only death factors are so-called because they share with each other, and with the other members of the Bcl-2 family of proteins, only the short BH3 domain (Huang and Strasser, 2000, Lutz, 2000, Puthalakath and Strasser, 2002). In worms, only one member of this subfamily, EGL-1, has so far been found (Conradt and Horvitz, 1998, Conradt and Horvitz, 1999). This protein plays a dominant and essential role in the induction of programmed cell death of somatic cells. Genetic and biochemical
The mode of actions of Bcl-2- and Bax-like factors: more than regulating mitochondria and caspases?
When cells from Bax/Bak double knock-outs are compared to those isolated from cytochrome c, Apaf-1 or caspase-9 deficient mice (which are also all embryonic lethal), the former are more resistant to a variety of apoptotic stimuli. This indicates that Bax-like factors may provoke the release of pro-apoptotic components that perform yet other functions than the formation of a cytochrome c-stimulated Apaf-1/caspase-9 apoptosome.
Principle of lymphocyte homeostasis and consequences of its failure
Lymphocytes undergo constant renewal from hematopoietic progenitor cells and are subjected to cyclic expansions and contractions as they participate in host defense (reviewed in Rathmell and Thompson, 2002). Physiological regulation of cell death is essential for the removal of potentially autoreactive lymphocytes during development and for the removal of excess, eventually damaged cells after the completion of an immune response. Failure to remove autoimmune cells that arise during development
Outlook
Until anti- or pro-apoptotic strategies can be exploited to treat diseases such as, for example, autoimmunity or cancer, a number of questions have yet to be answered. One critical question is whether Bcl-2-like survival factors are only regulated by BH3-only and Bax-like proteins or whether the binding and release of CED-4-like proteins also plays a role. Since BH3-only proteins are essential “sensors” and triggers of apoptosis, it will also be necessary to determine which BH3-only protein is
Acknowledgements
I apologize for my inability to cite all the contributing primary literature for this manuscript due to space constraints. I thank Ivonne Petermann for critically reading the manuscript.
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