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Life's smile_ death's grin_ vital functions of apoptosis-executing



Life’s smile, death’s grin: vital functions of
apoptosis-executing proteins
Carmen Garrido1 and Guido Kroemer2

Apoptosis is executed by caspases as well as caspase-                 PT       permeability transition
independent death effectors. Caspases are expressed as                siRNA    small interfering RNA
                                                                      Smac     second mitochondria-derived activator of caspase
inactive zymogens in virtually all animal cells and are activated
in cells destined to undergo apoptosis. However, there are
many examples where caspase activation is actually required
for cellular processes not related to cell death, namely terminal
differentiation, activation, proliferation, and cytoprotection.
                                                                      Apoptosis is a regulated, energy-dependent form of cell
Several caspase-independent death effectors including
                                                                      death with a characteristic morphological appearance that
apoptosis-inducing factor, endonuclease G and a serine
                                                                      involves cellular shrinkage and chromatin condensation.
protease (Omi/HtrA2) are released from the mitochondrial
                                                                      Two main pathways can lead to apoptosis: the intrinsic
intermembrane space upon permeabilization of the outer
                                                                      pathway and the extrinsic pathway (Figure 1). The
membrane. Such proteins also have important roles in cellular
                                                                      extrinsic pathway involves a death receptor protein (pro-
redox metabolism and/or mitochondrial biogenesis. As a                totype: Fas/CD95) and an adaptor protein (e.g. FADD,
general rule, it thus appears that cell-death-relevant proteins,      Fas-associated death domain protein), which in turn
especially those involved in the core of the executing                interacts with the cysteine aspartate protease pro-cas-
machinery, have a dual function in life and death. This has           pase-8 (and often pro-caspase-10). Activation of cas-
important implications for pathophysiology. The fact that the         pase-8 culminates in the explosive activation of other
building blocks of the apoptotic machinery have normal                caspases (including the so-called executioner caspases-3,
functions not related to cell death may mean that essential           -6 and -7) or alternatively triggers the activation of caspase-
parts of the apoptotic executioner cannot be lost and thus            3 in an indirect fashion, through a pathway that involves
reduces the possibility of oncogenic mutations that block the         the mitochondrial release of cytochrome c. The intrinsic
apoptotic program. Moreover, therapeutic suppression of               pathway directly releases soluble proteins contained in the
unwarranted cell death must be designed to target only the            mitochondrial intermembrane space [1]. These mole-
lethal (and not the vital) role of death effectors.                   cules include cytochrome c, apoptosis inducing factor
                                                                      (AIF), endonuclease G (EndoG), Omi/HtrA2 and Smac/
  INSERM U-517, Faculty of Medicine and Pharmacy, 7 Boulevard         DIABLO. Cytochrome c, once in the cytosol, interacts
Jeanne d’Arc, 21033 Dijon, France                                     with Apaf-1 (apoptotic protease activation factor 1) and
e-mail:                                       pro-caspase-9 leading to the formation of the caspase-9
  CNRS-UMR 8125, Institut Gustave Roussy, 39 rue Camille-             activation complex. Activated caspase-9 triggers the
Desmoulins, F-94805 Villejuif, France
                                                                      maturation of pro-caspase-3. Smac/DIABLO and Omi/
                                                                      HtrA2 activate apoptosis by neutralizing the inhibitory
                                                                      activity of IAPs (inhibitory apoptosis proteins) that associ-
  Current Opinion in Cell Biology 2004, 16:639–646                    ate with and inhibit caspases. Caspases are therefore
  This review comes from a themed issue on                            implicated in different aspects of cell death. They initiate
  Cell division, growth and death                                     (in the case of caspase-8 and -9) the propagation of
  Edited by Steve Reed and Joel Rothman                               apoptotic signals and execute (in the case of caspase-3,
                                                                      -6 and -7) the apoptotic program through cleavage of an
  Available online 13th October 2004
                                                                      array of vital proteins. In addition, apoptosis can be
  0955-0674/$ – see front matter                                      induced by caspase-independent death effectors such
  # 2004 Elsevier Ltd. All rights reserved.                           as AIF, EndoG and Omi/HtrA2.
  DOI 10.1016/
                                     for development and adult tissue
                                                                 Apoptosis is essential
                                                                      homeostasis. An abnormal increase in apoptosis leading to
bNAC      beta subunit of the nascent polypeptide associate complex
                                                                      the unwarranted demise of cells is involved in many
AIF       apoptosis-inducing factor                                   pathological processes such as myocardial infarction,
DIABLO direct IAP-binding protein with low pI                         stroke, neurodegenerative disease and AIDS [2]. As a
EndoG     endonuclease G                                              result, targeting of essential elements of the apoptotic
FADD      Fas-associated death domain protein                         machinery has been considered a promising therapeutic
HtrA2     high temperature requirement A2
IAP       inhibitory apoptosis protein
                                                                      option, especially for the treatment of massive, life-threa-
MLS       mitochondrial localization sequence                         tening states of acute apoptotic death, for instance in
PARP      poly (ADP-ribose) polymerase                                septic shock or after ischemia of the heart or the brain.                                                                  Current Opinion in Cell Biology 2004, 16:639–646
640 Cell division, growth and death

Figure 1

                             Fas L
                                                                                  Function in normal metabolism
                              Fas                  Stress
                                                                                  • T cell proliferation
                             A                                                    • Cell cycle control
                             D                                                    • Placental trophoblast differentiation
                                                                                  • Axonal growth (Bcl-2)
                             Caspase-8                                            • T cell proliferation, cell cycle control ?
                                                 Bcl2 family                       (Bcl-2, Mcl-1, Bax)
                                                  proteins                        • ER Ca2+ release (Bcl-2, Bax, Bak)

                                                                                  • Embryonic cavitation
                                                                       AIF        • Oxidoreductase function
                                                                                  • Antioxidant function

                                                               EndoG              • Early embryogenesis
                                                                                  • Mitochondrial replication

                                                      Omi/HtrA2                   • Mitochondrial homeostasis
                                                                                  • Degeneration of striatal neurons

                                                                                  • Mitochondrial respiration
                                               Cytochrome c
                                                                                  • Sperm cell differentiation

                                                                                  • Cell differentiation: erythroblasts,
                                               Caspase-9                           keratinocytes, lens cells, skeletal muscle,
                                                                                   sperm cells, monocytes, osteoblasts
                                                                                  • Platelet formation
                                                                                  • B cell proliferation
                                                                                  • Brain development


                                                                                                   Current Opinion in Cell Biology

Schematic representation of apoptotic pathways. Indicated on the right are the non-lethal functions in which individual cell death proteins have
been implicated.

It would only be possible to target these components at an   inhibitors arrest the maturation of erythroid progenitors at
acceptable level of toxicity, however, if they had no        an early stage of differentiation before nuclear shrinkage
important function for normal cellular metabolism or vital   and chromatin condensation occur. Effector caspases such
signal transduction processes. This review will focus on     as caspase-3 are transiently activated through the mito-
the function of these proteins in processes other than cell  chondrial pathway during erythroblast differentiation.
death (Figure 1).                                            They cleave proteins involved in nuclear integrity (lami-
                                                             nin B of the envelope) and chromatin condensation
Implication of caspases in differentiation                   (acinus) but spare certain potential caspase substrates
processes                                                    (such as the major erythroid transcription factor GATA-
The involvement of caspases in differentiation (Table 1)     1) and do not kill the cells [4]. Intriguingly, caspase-
was initially thought to be associated with the enucleation  mediated degradation of GATA-1 has been postulated
process, which is regarded as a caspase-dependent incom-     as a mechanism for controlling the total red cell mass [5].
plete apoptotic process. Erythroblasts, keratinocytes and    This GATA-1 degradation would occur in late erythroid
                               erythroblasts and would be followed
lens epithelial cells lose their nucleus, as well as other   progenitors and early
organelles, during terminal differentiation, yet continue    by differentiation arrest and apoptosis [5]. Another tran-
to be metabolically active. For instance, erythropoiesis     scription factor target of caspases required for blood cell
involves the sequential formation in the bone marrow of      development is SCL/Tal-1 [6].
a series of erythrocyte precursors (proerythroblasts and
basophilic, polychromatophilic and orthochromatic ery-       There are also examples of caspase activation promoting
throblasts), which extrude their nuclei and enter the        differentiation in cells that do not undergo enucleation and
circulation as mature, anucleate red blood cells. During     instead manifest remodeling of the cytoplasm. Sperm cell
this maturation process, caspase activity is required for an terminal differentiation involves complex changes in the
as-yet-unspecified non-apoptotic function [3,4]. Caspase     cytoplasmic architecture, some of which are reminiscent of

Current Opinion in Cell Biology 2004, 16:639–646                                                                           
                                                              Vital functions of apoptosis-executing proteins Garrido and Kroemer 641

 Table 1

 Apoptosis-regulatory and vital functions of caspases.

 Caspase         Knock out phenotype (reviewed in [46])         Role in apoptosis                    Other roles (see text)
 Caspase-1       Animals develop normally                       Role in death-receptor-mediated      IL-1 production
 Caspase-2       Animals have excess oocytes                    Initiator or executor caspase        DNA repair
 Caspase-3       Perinatal Excess brain tissue lethality        Executor caspase                     Differentiation of erythroblasts,
                                                                                                     keratinocytes, monocytes, and
                                                                                                     epithelial, sperm, skeletal, muscle,
                                                                                                     osteoblast and trophoblast cells;
                                                                                                     B cells proliferation.
 Capase-6        Animals develop normally                       Executor caspase                     Not determined
 Caspase-7       Embryonic lethality                            Executor caspase                     Not determined
 Caspase-8       Embryonic lethality, impaired heart-muscle     Initiator caspase of the death       T cell proliferation; placental
                 development and decreased pool of              receptor pathway                     trophoblast differentiation;
                 hematopoietic precursors; in humans,                                                cell cycle control?
                 familial mutation leads to
 Caspase-9       Perinatal lethality Excess brain tissue        Initiator caspase                    In most cells differentiation
                                                                                                     processes in which caspase-3
                                                                                                     has been implicated
 Caspase-10      In humans, patients with familial              Initiator caspase of the death       Not determined
                 mutation have autoimmune                       receptor pathway
                 lymphoproliferative syndrome type II
 Caspase-11      Animals develop normally                       Initiator caspase of the death       IL-1 production
                                                                receptor pathway
 Caspase-12      Animals develop normally                       Initiator caspase in ER stress       Attenuates inflammation;
                                                                                                     innate immune response
 Caspase-14      Not determined                                 Not determined                       Terminal differentiation of keratinocytes

apoptosis. Arama et al. [7] demonstrated that caspase      are certainly apoptotic-like in many respects. Caspase
activity is indispensable for sperm differentiation in Dro-  activation has also been found to contribute to the differ-
sophila and that one of the two cytochrome c genes, cyt-c-d, entiation of human blood monocytes into macrophages in
is also required for spermatogenesis. Caspase activation is  the absence of any morphological signs of apoptosis. This
also involved in platelet formation. Platelets are formed    caspase activation involves the mitochondrial release of
from mature megakaryocytes and arise from the budding        cytochrome c and leads to the cleavage of the protein
of thin, long cytoplasmic extensions called proplatelets.    acinus, while poly (ADP-ribose) polymerase (PARP),
Localized caspase-3 and -9 processing, as well as mito-      another caspase-3 substrate, remains uncleaved. Pharma-
chondrial membrane permeabilization and cytochrome c         cological inhibitors of caspases or overexpression of Bcl-2
release within the central part of the cell, have been       inhibits the differentiation process [12]. Caspase activa-
implicated in platelet formation. Inhibitors of caspases     tion has also been implicated in osteoblastic differentia-
blocked platelet formation in vitro, as did Bcl-2 overex-    tion [13] and caspase-14 activation has been associated
pression, whereas calpeptin inhibitors had no effect [8,9].  with terminal differentiation of human keratinocytes [14].
Skeletal muscle differentiation reportedly relies on cas-    Finally, caspase-8 has been implicated in the differentia-
pase-3 activity and the caspase-mediated activation of       tion of human placental villous trophoblast, a process that
                            of cytotrophoblast to form syn-
Mammalian Sterile Twenty-like kinase. Retransfection         includes syncytial
of a truncated kinase (which mimics the result of caspase-   cytiotrophoblast. It has been reported that expression and
3-mediated digestion) into caspase-3-null myoblasts          activity of caspase 8 is a prerequisite for the differentia-
restores the differentiation phenotype [10]. Low levels      tion and syncytial fusion of cytotrophoblast cells [15].
of Raf signaling lead to accelerated differentiation,
whereas high levels of Raf signaling induce a transformed    Caspases in the immune system
morphology and abrogate differentiation [11].                Beyond the well-known function of the inflammatory
                                                             caspases-1 and -11 in interleukin-1 production, loss-of-
All the differentiation processes mentioned above that       function mutations in caspase-8 are linked to defects in
implicate executer caspases, although not cell death per se, the activation of T, B and NK cells that culminate in an                                                                       Current Opinion in Cell Biology 2004, 16:639–646
642 Cell division, growth and death

immunodeficiency syndrome [16]. These data, which              as N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone
have been obtained in the human system, have been               (Z-VAD.fmk), they are actually sensitized to cell death
corroborated in the mouse model. Genetic elimination of         induction in vitro. That caspase inhibition can be dele-
caspase-8 (and knockout of its death-domain-containing          terious has been confirmed in vivo by Cauwels and
cytosolic adaptor FADD) in mice results in impaired             co-workers [23], who show that pretreatment with
heart muscle development and defects in the immune              Z-VAD.fmk can sensitize mice to the lethal effect of
system at the level of hematopoietic precursors and T-cell      recombinant human or mouse TNFa. Furthermore, cas-
progenitors [17]. This demonstrates that FADD is              pase inhibitors may arrest apoptosis but promote other
required not only for death-receptor-mediated induction         forms of cell death, such as the recently demonstrated
of apoptosis but also for the development and prolifera-        autophagic cell death induced by caspase-8 [24]. Well
tion of T cells. Transgenic expression of a dominant-           beyond the role of caspases in lethal signaling or disman-
negative FADD mutant induces a more profound pro-               tling of dead corpses, caspases may have cytoprotective
liferation defect in CD8+ T cells than in CD4+ cells. A         effects in some (patho)physiological contexts. Thus, cas-
targeted caspase-8 deletion in T cells caused defects in        pase-2 has been speculated to participate in the activation
the activation-induced expansion of peripheral T cells, as      of the DNA repair machinery when it is activated by the
well as in the ability of T cells to clear lymphocyte           PIDDosome in the nucleus, as part of a multiprotein
choriomeningitis virus in vivo [18]. Caspase activation        complex containing PIDD (p53 induced gene with a death
linked to the activation of T or B lymphocytes reportedly       domain) and CRAD/TRADD (an adaptor protein) [25].
results in the intracellular digestion of a selected panel of
substrates including PARP, laminin B and Wee1 kinase            How can caspases be implicated in different cell pro-
(but not the DNA fragmentation factor DFF45 or repli-           cesses without killing the cell? The answer is, most
cation factor C [RFC]140, both of which are frequently          probably, by a selective processing of substrates in
cleaved in apoptosis). Thus, caspase-8 and FADD can, in         non-apoptotic cells. This substrate restriction may be
a highly regulated fashion, induce the selective cleavage       governed by specific subcellular compartmentalization
of a certain set of proteins that seem to be crucial for        of caspases in vesicle-like structures [8], differential
adaptive immunity [18].                                        accessibility of cleavable substrates, the existence of
                                                                scaffold proteins, or the activation of anti-apoptotic, cas-
Whereas caspase-8 activation is a physiological response        pase-inhibitory factors. For instance, dBruce, an E2-ubi-
required for T cell proliferation, caspase-3 regulates cell     quitin conjugating enzyme with a baculovirus IAP repeat
cycle progression in B cells [19]. The caspase-3 inhibitor    (BIR) domain, protects the nuclei of differentiating fly
c-FLIP(L) can act as a molecular switch between cell            spermatids from destruction, presumably by targeting
death and growth signals transmitted by the death recep-        caspases to their proteasomal degradation [7]. Finally,
tor CD95. Experiments with transgenic mice overexpres-          the uncoupling of nuclear and cytosolic events (i.e. failure
sing c-FLIP(L) in the T-cell compartment demonstrate            of molecules to migrate into the nucleus) could also
that c-FLIP(L) decreases the signaling threshold of the         separate the non-apoptotic versus the apoptotic role of
T-cell receptor [20].                                           activated caspases.

Caspase-12, initially reported to play a role in apoptosis Cytochrome c: essential in life and death
induced by endoplasmic reticulum stress [21], may          Cytochrome c may be considered as the quintessential
attenuate the inflammatory and innate immune response       example of a protein whose role in normal life — as an
to endotoxins, and this attenuation is a risk factor for   obligate electron shuttling protein between complexes
developing sepsis [22]. A single nucleotide polymorph-   III and IV of the respiratory chain — was well character-
ism in caspase-12 in humans results in the synthesis of    ized before its role in apoptotic demise was discovered.
either a truncated protein (Csp12-S) or a full-length      Only holo-cytochrome c (which contains the prosthetic
caspase proenzyme (Csp12-L). The read-through single       heme group) has a proapoptotic function. Substitution of
nucleotide polymorphism encoding Csp12-L is confined        the iron atom within heme (which abolishes electron
to populations of African descent, and the frequency of    transfer by cytochrome c) does not affect caspase activa-
the Csp12-L allele is increased in c, meaning that the redox function
                                       African American    tion by cytochrome
individuals with severe sepsis [22].                     and the pro-apoptotic effect of cytochrome c are governed
                                                           by distinct biochemical principles. Accordingly, cyto-
Caspases as cytoprotective agents?                         chrome c from Saccharomyces cerevisiae (which can substi-
It has been widely assumed that apoptotic death of         tute for mammalian cytochrome c in respiration) does not
mammalian cells would be closely associated with the       induce caspase activation in mammalian cells. Recently,
activation of caspases. In consequence, it would be        cytochrome c has been implicated in Drosophila sperm cell
expected that pharmacological inhibition of caspases       differentiation. The cyt-c-d gene (which is expressed
would abrogate apoptotic cell death. However, when         at a lower level than the other Drosophila cytochrome c
mammalian cells are treated with caspase inhibitors such   gene, cyt-c-p) does not seem to play an essential role in

Current Opinion in Cell Biology 2004, 16:639–646                                             
                                                      Vital functions of apoptosis-executing proteins Garrido and Kroemer 643

mitochondrial respiration, but just in caspase-dependent       quin mice. The oxidoreductase activity of AIF in vivo
spermatid differentiation. Mutant cyt-c-d males are viable     could affect reactive oxygen species detoxification in an
but sterile [7]. Obviously, disruption of the unique         indirect fashion, via an effect on glutathione content.
somatic cytochrome c gene causes embryonic lethality           Cande et al. [35] have recently demonstrated that HeLa
in mice [26]. The knock-in of a mutation that abolishes        and embryonic stem cells that have been manipulated by
cytochrome-c-mediated apoptosome activation, yet               homologous recombination or siRNA to lose AIF expres-
leaves cytochrome c-dependent respiration intact, has          sion form cytoplasmic stress granules much more easily
not yet been performed.                                        than the AIF positive cells do when stressed. Glutathione
                                                               is a major regulator of stress granule formation, and the
Apoptosis-inducing factor — a Janus protein                    oxidoreductase function of AIF is required for the main-
AIF is a phylogenetically conserved protein essential for      tenance of glutathione levels in stress conditions. This
embryonic development [27]. It is synthesized from a           latter function is separable from the apopotogenic func-
nuclear gene as an immature precursor with a 100-             tion of AIF. For instance, deletion of a C-terminal domain
amino-acid-long N-terminal mitochondrial localization          of AIF abolishes apoptosis induction by AIF, yet has no
sequence (MLS). Upon import into the mitochondrial             effect on the modulation of stress granules by AIF [35].
intermembrane space, the MLS is removed from the
protein, which refolds and incorporates flavine adenine        Other caspase-independent mitochondrial
nucleotide (FAD) [28]. The resulting 57 kDa AIF fla-          death effectors: EndoG and Omi/HtrA2
voprotein is redox-active and behaves as an NADH              EndoG is a mitochondrial nuclease encoded by a nuclear
oxidase. Upon apoptosis induction, AIF translocates from      gene. Once liberated into the cytosol, EndoG translocates
mitochondria to the nucleus, interacts with DNA and           to the nucleus where it causes oligonucleosomal DNA
induces chromatin condensation [29]. One of the positive      fragmentation even in the presence of caspase inhibitors
regulators of AIF-mediated apoptosis is cyclophilin A,        [36]. In mammalian cells, EndoG cooperates with exo-
which interacts with AIF to form an active DNase [30]. In     nuclease and DNase I to facilitate DNA processing [37],
addition, AIF interacts with HSP70, a cytoprotective          while it is thought to cooperate with AIF in C. elegans
protein that functions as an endogenous inhibitor of          [38,39]. EndoG homozygous mutant embryos die
AIF [31]. Mutational and biochemical analysis of AIF          between embryonic days 2.5 and 3.5. This essential
suggests that its apoptotic and redox functions are separ-    function of EndoG in early development is unrelated
able. The AIF oxidoreductase domain is homologous to          to mitochondrial DNA replication since mitochondrial
the ferrodoxin reductase component of bacterial biphenyl      DNA copy numbers in ovulated oocytes are similar in
dioxygenases, has NAD(P)H oxidase activity and gener-         EndoG heterozygous mutant and wild type mice [40].
ates superoxide anion radicals in vitro [32,33]. By contrast, Thus, the exact role of EndoG in mitochondrial physiol-
in vivo, in a mouse mutant termed Harlequin, low AIF          ogy remains an ongoing conundrum.
expression (due to a retroviral insertion into the first
intron of the AIF gene) leads to an impairment of free        Omi/HtrA2 belongs to a family of serine proteases that
radical homeostasis and to oxidative stress in neurons.       is well conserved from bacteria to humans. Omi/HtrA
These abnormalities in free radical pathways may con-         endoprotease is localized within the periplasmic space
tribute to the degeneration of AIF-deficient neurons, as       of bacteria and its presence is necessary for bacterial
evidenced by the increased and selective sensitivity of       thermotolerance [41]. Moreover, it has recently been
cultured Harlequin cerebellar granule neurons to hydro-       shown that bacterial Omi/HtrA has a dual role, acting as
gen-peroxide-induced apoptosis, a phenotype that is res-      a chaperone at normal temperature and as an active
cued by the expression of AIF [34]. This demonstrates       protease at high temperatures [42]. In human cells,
an interesting link between the loss of AIF’s oxidoreduc-     Omi/HtrA2 is confined to the mitochondrial intermem-
tase activity and neurodegeneration. The response of          brane space. Upon apoptosis induction, Omi/HtrA2 is
neurons depleted in AIF to oxidative stress correlated        released into the cytosol and promotes cell death via
with the reactivation of the cell cycle. Indeed, the cell     two different mechanisms: one relies on IAP inhibition
cycle is re-activated in the degenerating postmitotic         and involves a significant increase in caspase activity,
                             depends on its serine protease activity
neurons, as shown by the incorporation of bromodeox-          whereas the second
yuridine and the expression of DNA replication proteins       and is caspase-independent [28,43]. In normal, non-apop-
in these neurons. Abortive cell cycle re-entry might be a     totic conditions Omi/HtrA2 serine protease is involved in
fundamental mechanism by which postmitotic neurons            maintenance of mitochondria homeostasis. This has been
undergo apoptosis. Whether cell cycle re-entry truly          demonstrated by studying the mouse mnd2 mutation, a
constitutes the mechanism by which AIF deficiency              spontaneous loss-of-function mutation in the gene for
triggers neuronal degeneration and apoptosis in the Har-      Omi/HtrA that causes a severe neuromuscular degenera-
lequin mouse remains to be determined. It is also possible    tion that is due to a loss-of-function of the serine protease
that a mitochondrial dysfunction induced by the AIF           of Omi/HtrA2. Indeed, purified mitochondria from mnd2
defect leads to a selective neuronal phenotype in Harle-      mice are more susceptible to permeabilization in vitro in                                                           Current Opinion in Cell Biology 2004, 16:639–646
644 Cell division, growth and death

response to Ca2+ than control mitochondria from healthy                       This review discusses the apoptotic role of the so-called permeability
                                                                              transition (PT), which arises from the opening of a regulated proteaceous
mice. Thus, paradoxically, the inhibition of Omi/HtrA2                        pore, the mitochondrial megachannel (also called the PT pore). The PT
may actually enhance susceptibility to apoptosis, and this                    pore is triggered by Ca2+ influx into mitochondria and is modulated by a
                                                                              variety of factors that include metabolic factors and mediators of intra-
may explain the increased degeneration of striatal neurons                    cellular signaling. Persistent opening of the PT pore precipitates a
in mnd2 mice [44].                                                          bioenergetic crisis.
                                                                              2.   Kroemer G, Reed JC: Mitochondrial control of cell death.
Another mitochondrial protein with a dual function in                              Nat Med 2000, 6:513-519.
death and life is the beta subunit of the nascent poly-                       3.   Carlile GW, Smith DH, Wiedmann M: Caspase-3 has a
                                                                                  nonapoptotic function in erythroid maturation.
peptide associate complex (bNAC). As demonstrated in                               Blood 2004, 103:4310-4316.
Caenorhabditis elegans [45], bNAC has a role in apop-                         This paper confirms the need for caspase-3 in erythroid maturation, using
tosis — it suppresses CED-3 independent apoptosis —                           a siRNA approach.
and has an essential role in development particularly in                      4.   Zermati Y, Garrido C, Amsellem S, Fishelson S, Bouscary D,
                                                                                   Valensi F, Varet B, Solary E, Hermine O: Caspase activation is
the nervous system. In humans, bNAC is a caspase                                   required for terminal erythroid differentiation. J Exp Med 2001,
substrate that is rapidly eliminated in dying cells,                               193:247-254.
suggesting that bNAC’s apoptosis-suppressing activity                         5.   De Maria R, Zeuner A, Eramo A, Domenichelli C, Bonci D,
may be inactivated by caspases.                                                    Grignani F, Srinivasula SM, Alnemri ES, Testa U, Peschle C:
                                                                                   Negative regulation of erythropoiesis by caspase-mediated
                                                                                   cleavage of GATA-1. Nature 1999, 401:489-493.
Concluding remarks
                                                                              6.   Zeuner A, Eramo A, Testa U, Felli N, Pelosi E, Mariani G, Srinivasula
In evolutionary terms, it appears unlikely that any protein                        SM, Alnemri ES, Condorelli G, Peschle C et al.: Control of
has been generated with the exclusive function of killing                          erythroid cell production via caspase-mediated cleavage of
                                                                                   transcription factor SCL/Tal-1. Cell Death Differ 2003, 10:905-
cells. Indeed, it appears that most pro-apoptotic proteins,                        913.
perhaps with the exception of the pro-apoptotic proteins
                                                                              7.    Arama E, Agapite J, Steller H: Caspase activity and a specific
from the Bcl-2 family, have additional functions in normal                        cytochrome C are required for sperm differentiation in
metabolism (as opposed to apoptotic catabolism) or signal                           Drosophila. Dev Cell 2003, 4:687-697.
                                                                              This paper describes how in Drosophila the vital and cell-death-related
transduction that are not related to apoptosis. One exam-                     functions of cytochrome c are performed by two distinct cytochrome c
ple is AIF, which has a pro-apoptotic function, yet causes                    genes. The product of the cyt-c-p gene is necessary for caspase activa-
                                                                              tion during apoptosis, while that of the cyt-c-d gene is involved in caspase
the death of vulnerable cells when it is underexpressed.                      activation during spermatid differentiation. The paper provides a good
Thus, the mere removal of such pro-apoptotic proteins                         example of the paradoxical effect of caspases (death versus life), since
                                                                              the authors demonstrate that caspases are involved in the origin of life
from the organism is unlikely to be of therapeutic benefit                     itself.
when cytoprotection is the goal, and more subtle strate-
                                                                              8.   De Botton S, Sabri S, Daugas E, Zermati Y, Guidotti JE, Hermine O,
gies may have to be employed. In the case of AIF, the                              Kroemer G, Vainchenker W, Debili N: Platelet formation is the
lethal and vital roles have been attributed to distinct                            consequence of caspase activation within megakaryocytes.
                                                                                   Blood 2002, 100:1310-1317.
domains of the protein. Future attempts to intervene
pharmacologically with apoptosis induction by AIF will                        9.   Plenchette S, Moutet M, Benguella M, N’Gondara JP,
                                                                                   Guigner F, Coffe C, Corcos L, Bettaieb A, Solary E: Early
have to take into account these separate functions to                              increase in DcR2 expression and late activation of
avoid unwarranted (e.g. neurotoxic) side effects. It is                            caspases in the platelet storage lesion. Leukemia 2001,
theoretically conceivable that specific inhibitors acting
on the pro-death domain of AIF (rather than on the vital                      10. Fernando P, Kelly JF, Balazsi K, Slack RS, Megeney LA: Caspase
                                                                                  3 activity is required for skeletal muscle differentiation.
redox-active domain) would have some cytoprotective                               Proc Natl Acad Sci USA 2002, 99:11025-11030.
effects. The future will tell whether such a strategy will                    11. DeChant AK, Dee K, Weyman CM: Raf-induced effects on the
be applicable to AIF, or to other proteins with similar                           differentiation and apoptosis of skeletal myoblasts are
Janus-like behavior.                                                              determined by the level of Raf signaling: abrogation of
                                                                                  apoptosis by Raf is downstream of caspase 3 activation.
                                                                                  Oncogene 2002, 21:5268-5279.
Acknowledgements                                                              12. Sordet O, Rebe C, Plenchette S, Zermati Y, Hermine O,
The authors’ own work has been sponsored by a special grant from                   Vainchenker W, Garrido C, Solary E, Dubrez-Daloz L: Specific
League against Cancer, as well as by the French Ministry of Science and the         involvement of caspases in the differentiation of monocytes
EU Trans-Death consortium (to G.K.), ‘Fondation contre la Leucemie’,                into macrophages. Blood 2002, 100:4446-4453.
                                              ´          `
‘Ligue National contre le Cancer’ and ‘Comite de la Nievre de la Ligue        This is the first paper to demonstrate that caspase activation is involved in
contre le Cancer’ (to C.G). We thank S Gurbuxani, E Schmitt and               a differentiation process that bears no similarity to apoptosis.
A Parcellier for their help.
                                                                              13. Mogi M, Togari A: Activation of caspases is required for
                                                                                  osteoblastic differentiation. J Biol Chem 2003,
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Current Opinion in Cell Biology 2004, 16:639–646                                                                      
                                                                     Vital functions of apoptosis-executing proteins Garrido and Kroemer 645

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646 Cell division, growth and death

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Current Opinion in Cell Biology 2004, 16:639–646                                                          

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