Download The Jekyll and Hyde Functions of Caspases

Developmental Cell
Review
The Jekyll and Hyde Functions of Caspases
Caroline H. Yi1 and Junying Yuan1,*
1Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA
*Correspondence: [email protected]
DOI 10.1016/j.devcel.2008.12.012
Apoptosis is an ancient form of regulated cell death that functions under pathological and nonpathological
contexts in all metazoans. More than a decade of intense research has led to extensive characterization of
the core molecular mechanisms for apoptotic cell death. This includes the identification of a family of cysteine
proteases, caspases, which are critical for the execution of apoptosis. Whereas completion of the proteolytic
caspase cascade leads to elimination of a cell by apoptosis, caspase activation, when finely tuned, directs
alternative cellular functions independent of cell death. Exciting recent developments have focused on uncovering nonapoptotic roles of caspases ranging from immune regulation to spermatogenesis, in highly
specialized cellular frameworks.
Introduction
Apoptosis is a highly regulated form of cell death that is known to
sculpt tissues during development and maintain tissue homeostasis by eliminating unnecessary or harmful cells. Genetic analysis of the nematode C. elegans provided the first insights into
the molecular mechanism of apoptosis (Yuan et al., 1993). The
canonical apoptotic pathways are well conserved in all metazoans from invertebrates to vertebrates (Figure 1) (Degterev
et al., 2003; Miura et al., 1993). Extensive research in the past
15 years established the critical role of caspases, a family of
cysteine proteases, in mediating the signal transduction and
execution of apoptosis. While CED-3 is the sole caspase
required for programmed cell death in C. elegans, multiple caspases are required for apoptosis in more complex organisms,
such as flies (4 caspases) and mammals (10 caspases in mouse;
11 caspases in human). Such expansion of the caspase family
during evolution might have arisen for dual purposes: the execution of apoptotic cell death and the role in carrying out multiple
cellular processes that do not necessarily involve cell death.
Members of the caspase family are subdivided into upstream
‘‘initiator’’ caspases (in fly, Dredd and Dronc; in mammals, caspase-1, -2, -4, -5, -8, -9, -10, -11, and -12), which respond to
proapoptotic signals, and downstream ‘‘executioner’’ caspases
(in fly, Drice and Dcp-1; in mammals, caspase-3, -6, -7, and -14).
Initiator caspases are molecularly defined by the presence of
a long N-terminal prodomain, which mediates the formation of
protein complexes that provide the molecular platform for caspase activation and inhibition. Caspases are synthesized as
zymogens and their activation requires allosteric conformational
changes, specific cleavage after a selective aspartate residue, or
both. The initial processing of inactive caspase separates the
large and small subunits, which is followed by removal of the
N-terminal domain to form the catalytically active protease (Degterev et al., 2003). Apoptosis ensues when the active caspases
cleave their respective substrates, leading to characteristic
morphological features of apoptotic cell death, including
membrane blebbing, pyknotic nuclei, cell rounding, and formation of apoptotic vesicles (Clarke, 1990).
In order to assemble a large multimeric platform for caspase
activation, adaptor proteins recruit specific initiator caspases
via homotypic interaction with the long prodomain of caspases,
which is either a caspase-recruitment domain (CARD; in
C. elegans, CED-3; in D. melanogaster, Dronc; in mammals, caspase-1, -2, -4, -5, -9, -11, and -12) or death-effector domain
(DED; in D. melanogaster, Dredd; in mammals, caspase-8 and
-10) (Degterev et al., 2003). In response to specific developmental cues, the adaptor protein CED-4, which is composed of
an N-terminal CARD and a nucleotide-binding domain (NBD,
also known as nucleotide-oligomerization domain [NOD]), oligomerizes to promote the activation of CED-3 (Yang et al., 1998).
Drosophila apoptotic protease-activating factor-1 (Apaf-1) (also
called Dark, Hac-1, or Dapaf-1), a CED-4-like molecule, is
responsible for the activation of the major fly initiator caspase,
Dronc, in response to cellular stresses such as DNA damage
(Kanuka et al., 1999; Rodriguez et al., 1999; Zhou et al., 1999).
Drosophila Fas-associated protein with death domain (dFADD)
is an additional adaptor protein that promotes Dredd activation
during the innate immune response to microbial infection (Hu
and Yang, 2000; Kanuka et al., 1999; Rodriguez et al., 1999;
Zhou et al., 1999). Whereas a single adaptor protein, CED-4, is
required for the activation of CED-3 in C. elegans, multiple
adaptor proteins are required for caspase activation in mammals
by promoting the formation of unique protein complexes under
different conditions.
In mammals, four specific caspase activating complexes have
been characterized. These complexes include the apoptosome,
which mediates the activation of caspase-9 via interaction with
the adaptor APAF-1 in the presence of cytochrome c; the
death-inducing signaling complex (DISC), which mediates the
activation of caspase-8 via interaction with the adaptor FADD;
the inflammasomes, which mediate the activation of caspase-1
and caspase-5 via interaction with the adaptor apoptosisassociated speck-like protein containing a CARD (ASC) or the
family of NBD- and NOD-like receptors (NLRs); and the PIDDosome, which mediates the activation of caspase-2 via interaction
with the adaptors receptor-interacting protein- (RIP-) associated
ICH-1/CED-3 homologous protein with a death domain (RAIDD;
also called CRADD) (Duan and Dixit, 1997) and p53-induced
protein with a death domain (PIDD) (Festjens et al., 2006). Importantly, the functions of these complexes are highly specialized
such that they are activated in response to distinct proapoptotic
and proinflammatory signals. The apoptosome is formed in
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Figure 1. The Canonical Apoptotic Pathway
Is Well Conserved in C. elegans, Drosophila,
and Mammals
Functional orthologs are color-coded. In
C. elegans inhibition of the antiapoptotic Bcl-2
family member CED-9 by the proapoptotic BH3only protein EGL-1 releases CED-4 to promote
activation of the caspase CED-3. In Drosophila,
the proapoptotic multidomain Bcl-2 family
members Debcl and Buffy promote Dark-mediated activation of Dronc followed by induction of
the caspase cascade. Similarly, mammalian proapoptotic BH3-only proteins inhibit the antiapoptotic Bcl-2 family members Bcl-2 and Bcl-xL
to promote oligomerization of the multidomain
Bcl-2 family members Bax and Bak. Adaptor
proteins, such as Apaf-1, promote caspase activation, leading to apoptotic cell death. Note that
in mammals, the caspase family has expanded
substantially (see text). Bcl, B cell lymphoma;
CED, cell death abnormal; BH3, Bcl-2 homology
3; EGL, egg-laying defective; Dark, Drosophila
Apaf-1 related killer.
response to cytochrome c release resulting from the loss of mitochondrial integrity, while the DISCs assemble after the stimulation of the death receptors by their respective ligands such as
Fas ligand (FasL) or tumor necrosis factor-a (TNFa). In addition,
the inflammasomes are involved in the activation of innate
immune response to intracellular pathogens, whereas the PIDDosome responds to DNA damage (Tinel and Tschopp, 2004)
as well as other cellular stresses. The complexity of environmental stresses at both cellular and organismal levels faced by
mammals may have been a strong selective force that led to
the specialization of these caspase complexes.
Although the critical function of caspases in apoptosis is firmly
established, an increasing number of studies have surprisingly
revealed various nonapoptotic functions of capsases (Table 1).
The first indication of a nonapoptotic role for caspases was the
initial characterization of caspase-1 in the processing of the
proinflammatory cytokine, proIL-1b, into mature IL-1b in
response to bacterial infection in mammals (Cerretti et al.,
1992; Thornberry and Molineaux, 1995). It has become increasingly clear that caspases and their respective adaptor proteins
mediate multiple cellular processes far beyond apoptotic cell
death per se. In this review, we would like to discuss the
emerging evidence for nonapoptotic functions of caspases in
mediating immunity, cell fate specification, cell survival, cell
cycle regulation, cell proliferation, and cell migration.
22 Developmental Cell 16, January 20, 2009 ª2009 Elsevier Inc.
Caspase Function in Immunity
Caspases in Invertebrate Immunity:
Regulation of Antimicrobial
Peptides
The function of caspases in mediating
the innate immune response is well
conserved from worm and fly to mammals.
Feeding C. elegans on a lawn of pathogenic bacteria, such as Salmonella
typhimurium, leads to substantial gonadal
cell death and eventually death of the
organism. Interestingly, ced-3 and ced-4
may play an important role in defending worms against pathogenic bacteria because ced-3 and ced-4 mutants are hypersensitive to the killing induced by S. typhimurium infection (Aballay and
Ausubel, 2001). The host defense role of CED-3 and CED-4 may
not be restricted to pathogenic bacteria because replication of
vaccinia virus is also enhanced in ced-3 and ced-4 mutants (Liu
et al., 2006). Thus, ced-3 and ced-4 may play an important role
in restricting both bacterial and viral pathogens in C. elegans.
However, it is still not clear whether CED-3 and CED-4 function
in the host response by directly promoting cell death in a cellautonomous manner or by indirectly regulating the immune
response in a cell nonautonomous manner.
In Drosophila, two signaling networks define the innate
immune system: the Toll pathway, which responds to Grampositive bacteria and pathogenic fungi, and the immune deficiency pathway (IMD), which primarily defends against infection by Gram-negative bacteria (Ferrandon et al., 2007).
Drosophila can discriminate between Gram-positive and
Gram-negative bacteria by their unique composition of peptidoglycan, which forms a polymeric layer of glycan molecules
in the outer coat of Gram-positive bacteria or the inner coat
of Gram-negative bacteria. Two members of the family of
peptidoglycan-recognition proteins (PGRPs), PGRP-LC and
-LE, specifically detect Gram-negative bacteria and activate
the IMD signaling pathway. Engagement of the transmembrane
Developmental Cell
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Table 1. The Function of Caspases in Nonapoptotic Processes
Organism
Caspase
Nonapoptotic Function
C. elegans
CED-3
immunity
Drosophila
Dronc
compensatory proliferation, dendritic
pruning, spermatogenesis, neuronal
differentiation, border cell migration
Dredd
immunity
Drice
compensatory proliferation,
spermatogenesis, neuronal
differentiation
Mammals
Dcp-1
compensatory proliferation
Caspase-1
innate immunity, protein secretion
Caspase-2
not determined
Caspase-3
adaptive immunity; lymphocyte
proliferation; erythroblast, macrophage,
megakaryocyte, lens epithelial cell,
skeletal muscle cell, osteoblast, and
placental trophoblast differentiation;
stem cell maintenance
Caspase-4
not determined
Caspase-5
innate immunity
Caspase-6
not determined
Caspase-7
not determined
Caspase-8
innate and adaptive immunity,
lymphocyte proliferation, macrophage
differentiation
Caspase-9
not determined
Caspase-10
not determined
Caspase-11
innate immunity, macrophage cell
migration
Caspase-12
not determined
Caspase-14
keratinocyte differentiation
receptor PGRP-LC (Gottar et al., 2002) and the intracellular
receptor PGRP-LE recruits IMD, which is similar to the
mammalian RIPs, most likely by an unknown intermediate
(Kaneko et al., 2006). IMD then initiates the formation of molecular complexes that facilitate the transcriptional activation of a
major immune regulator, the NF-kB factor-like molecule Relish
(Georgel et al., 2001).
Dredd, the Drosophila caspase-8 ortholog, functions in the
IMD pathway downstream of the innate immune receptors by
regulating the expression of antimicrobial peptides, an important
host defense mechanism (Leulier et al., 2000). dredd mutant flies
are highly sensitive to E. coli infection and fail to produce antimicrobial peptides, a phenotype similar to flies deficient in dFADD,
the fly ortholog of the caspase-8 adaptor protein FADD (Leulier
et al., 2002). These data suggest a model wherein IMD recruitment to PGRP-LC provides a scaffold for FADD and Dredd
complex formation, inducing the activation of Relish and the
expression of antimicrobial peptides (Stoven et al., 2003).
Thus, Dredd performs an important host defense response by
promoting the expression of antimicrobial peptides upon bacterial challenge. Interestingly, this function of Dredd may be highly
similar to that of its mammalian ortholog, caspase-8, as
described below.
Caspases in Mammalian Immunity
Mammalian Caspases and Innate Immunity: More than IL-1b.
Compared to their counterparts in Drosophila and C. elegans,
mammalian caspases play a more dominant and intricate role
in regulating the host inflammatory response. In mammals, two
major types of caspase complexes function in regulating host
immunity: DISC, which is the intracellular signaling complex
formed upon stimulation of the death receptors, and the inflammasomes, which are the protein complexes responsible for the
activation of caspase-1.
Once an appropriate proinflammatory cytokine, such as TNFa,
engages a death receptor in the TNF receptor (TNF-R) superfamily, DISC formation is initiated. This leads to aggregation
and internalization of the TNF-R complex, including TRADD,
FADD, and caspase-8 or -10. Although this protein complex is
primarily described in mediating caspase-dependent cell death,
nonapoptotic functions of caspase-8 have been elucidated
recently, including NF-kB activation in immunity, monocyte cell
differentiation, and T and B cell proliferation. In this section we
will focus on the role of caspase-8 in regulating the proinflammatory response following the activation of NF-kB, which is strikingly
reminiscent of the IMD pathway in Drosophila innate immunity.
One indication that caspase-8 regulates the immune response
is the observation that caspase-8-deficient mice are unable to
clear a viral infection when compared with control mice (Salmena
et al., 2003). Furthermore, Listeria monocytogenes was found to
colonize the liver of liver-specific caspase-8-deficient mice more
efficiently than that of wild-type mice, suggesting that caspase-8
limits the growth of this intracellular pathogen in vivo (Ben Moshe
et al., 2007). This may be due to ablated NF-kB activation as caspase-8 has been implicated in mediating the transcriptional
response to antigen stimulation of antigen receptors, Fc receptors, or Toll-like receptors in mammalian lymphocytes (Hu and
Yang, 2000; Lemmers et al., 2007). Caspase-8 was also found
to be associated with the IkB kinase (IKK) complex inducing the
transcriptional activation of NF-kB. Thus, mammalian caspase8 has a role in regulating innate immunity similar to that of Dredd
in the fly IMD pathway. In addition to promoting NF-kB activation,
caspase-8 might directly regulate proinflammatory cytokines.
Such a role could be considered because stimulation of FasL
was found to induce the processing and secretion of IL-1b in peritoneal exudate cells in a caspase-1-independent manner (Miwa
et al., 1998). However, the mechanism by which caspase-8 mediates cytokine secretion has not been explored.
The signaling events regulated by the family of proinflammatory
caspases define an additional branch of caspase-mediated
immune regulation, which includes possibly five CARD-containing
members (human caspase-1, -4, and -5; mouse caspase-1, -11,
and -12). A large body of work focuses on understanding the molecular regulation of caspase-1 during inflammation. The first insight
into the regulation of caspase-1 activation was the observation
that caspase-11-deficient mice show a remarkably similar phenotype to caspase-1-deficient mice (Wang et al., 1998). These mice
are highly resistant to lipopolysaccharide (LPS)-induced septic
shock and defective in IL-1 production, indicating that caspase11 is required for caspase-1 activation. Interestingly, endogenous
caspase-11 and caspase-1 form heterodimers, suggesting that
a caspase complex may be necessary for proper IL-1 processing.
Indeed, a subsequent study describes a proinflammatory caspase
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complex containing human caspase-1 and caspase-5, which is
closely related to caspase-11, and additional adaptor proteins
(Martinon et al., 2002). Consistent with the previous observations
in mice, the presence of both human caspases leads to efficient
production of IL-1b. The authors of the study coined the term inflammasome to describe this caspase-1-containing complex,
which will be discussed below.
Currently, four caspase-1-activating complexes have been
characterized in promoting a proinflammatory response in
response to infection. Despite their highly specialized capacity
to recognize specific classes of invasive microbes, these
complexes share a similar molecular composition of an NLR
family protein, ASC, and caspase-1. Once an invasive pathogen
is sensed in the cytosol, oligomerization of these protein
complexes is induced, leading to caspase-1-mediated processing of proIL-1b into mature IL-1b. These complexes include the
NALP1 (nucleotide-binding and oligomerization domain
[NACHT-], leucine-rich repeat domain [LRR-], and pyrin domain
[PYD]) inflammasome, the NALP3 (Cyropyrin) inflammasome,
the Ice protease-activating factor (IPAF) inflammasome, and
the Francisella-sensing inflammasome.
The characterization of the NALP1 inflammasome in a cell-free
system defined the first caspase-1-activating complex (Martinon
et al., 2002). In response to LPS treatment, caspase-1 and caspase-5 are recruited to NALP1 through a CARD-CARD interaction with the adaptor protein ASC (Figure 2). Importantly, the
formation of this multimeric complex provides the platform for
caspase activation and IL-1b maturation. The physiological
significance of this caspase-1 complex was indicated by the
identification of nalp1b as a Bacillus anthracis lethal toxin (LT)
susceptibility gene in mice (Boyden and Dietrich, 2006). Loss
of NALP1b or caspase-1 provided protection against LTinduced cell death in LT-sensitive macrophages, suggesting
that LT induces caspase-1-dependent cell death. However, the
mechanism by which NALP1b promotes caspase-1 activation
needs further clarification. Unlike human NALP1, mouse
NALP1b lacks the PYD domain, which is required for recruiting
ASC, suggesting that the mechanism of NALP1-caspase-1
complex formation is species dependent. Nevertheless, it is
clear that these bacterial products directly or indirectly induce
the formation of the NALP1 inflammasome. One study provides
strong evidence that muramyl dipeptide (MDP), a component of
peptidoglycan, induces assembly of the NALP1 inflammasome
in vitro, indicating that NALP1 directly senses MDP (Faustin
et al., 2007). It seems unlikely that NALP1 is also a direct microbial sensor for LPS and LT, which are structurally distinct. Two
possibilities can explain LPS- and LT-dependent caspase-1
activation by NALP1—these bacterial products may be recognized by unknown host factors that in turn activate the inflammasome, or alternatively, MDP may be present in the commercial
preparations of LPS or LT. Indeed, contamination of MDP in
LPS preparations has been previously reported (Faustin et al.,
2007; Girardin et al., 2003; Inohara et al., 2003). It will be important to reconcile these two possibilities.
Interestingly, the NALP3 inflammasome promotes caspase-1
activation in response to a plethora of microbial products and
host products secreted by dying cells. Such bacterial products
include LPS (Mariathasan et al., 2006; Sutterwala et al., 2006),
bacterial RNA (Kanneganti et al., 2006b), viral RNA (Kanneganti
24 Developmental Cell 16, January 20, 2009 ª2009 Elsevier Inc.
et al., 2006a), and bacterial lipopeptide (Mariathasan et al.,
2006). The NALP3 inflammasome may be specific for Grampositive bacteria infection as NALP3-deficient macrophages infected with Gram-negative bacteria normally secrete IL-1b.
Furthermore, NALP3-dependent IL-1b production is implicated
in the inflammatory response to the cellular deposition of uric
acid crystals in the autoinflammatory disease gout (Martinon
et al., 2006). A recent study proposes that the NALP3 inflammasome mediates the immunostimulatory properties of aluminum
adjuvants, which can form insoluble particles, in a similar manner
as described for uric acid crystals (Eisenbarth et al., 2008).
Because NALP3 responds to a multitude of foreign and cellular
signals, it is most likely that additional host factors, which function as direct microbial sensors, promote complex formation.
This seems to be a likely possibility as there is currently no
evidence for the aforementioned NALP3-stimulating agents in
direct assembly of the NALP3-caspase-1 complex (Kanneganti
et al., 2007).
Unlike the NALP3 inflammasome, the IPAF inflammasome
promotes caspase-1 activation in response to both Gramnegative and Gram-positive bacteria, including S. typhimurium
(Mariathasan et al., 2004), Legionella pneumophila (Amer et al.,
2006), Pseudomonas aeruginosa (Franchi et al., 2007; Miao
et al., 2008; Sutterwala et al., 2007), and L. monocytogenes
(Warren et al., 2008). Subsequent studies have identified flagellin,
the proteinaceous component of flagella, as the bacterial factor
that promotes NALP3-induced caspase-1 activation (Amer
et al., 2006; Franchi et al., 2007; Miao et al., 2008; Ren et al.,
2006). In particular, bacterial pathogens deficient in flagellin are
unable to induce IL-1b processing in macrophages. Concordantly, direct delivery of purified flagellin into the host cytosol
induces caspase-1 activation in an IPAF-dependent manner,
demonstrating the sufficiency of flagellin to elicit an innate
immune response. These findings indicate that the IPAF inflammasome may function as a microbial sensor or may respond to
a host factor that specifically recognizes bacterial flagellin.
The inflammasome that responds to the intracellular Gramnegative pathogen Francisella tularensis is the least understood
caspase-1-activating complex. Although ASC is required for
caspase-1 activation induced by Francisella infection, the
NLR family member that senses the pathogen is currently
unknown as NALP3 and IPAF seem to play negligible roles (Mariathasan et al., 2005, 2006). Cytosolic Francisella may be
necessary for host recognition because vacuolar escape
mutants cannot stimulate caspase-1 activation (Gavrilin et al.,
2006). This suggests that the microbial product that triggers
an immune response may not easily bypass the vacuolar
membrane or may not be actively secreted by the pathogen.
Moreover, caspase-1 activation is abrogated in macrophages
deficient in the type I interferon-receptor, which is known to
respond to cytosolic pathogens, suggesting that a component
of the type I interferon signaling network may promote inflammasome assembly in response to Francisella infection (Henry
and Monack, 2007).
One conundrum in the field is the observation that caspase-1
influences the secretion of IL-1a and IL-1b, but only proIL-1b is
a direct caspase-1 substrate (Howard et al., 1991; Li et al.,
1995). Although the cleavage sites for proIL-1a and proIL-1b
are unique (cleavage of proIL-1b is Asp dependent), production
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Figure 2. Inflammatory Caspases Mediate
an Innate Immune Response to Microbial
Invasion
Cellular recognition of foreign invaders leads to
formation of inflammatory caspase-containing
protein complexes, called the inflammasomes.
The best characterized is the NALP1 inflammasome, which consists of an NLR family member,
an adaptor protein called ASC, and caspase-1/5.
Inflammasome assembly leads to activation of
caspase-1 and maturation of IL-1b. A recent study
has implicated caspase-1 in regulating nonconventional protein secretion, which may be
intimately involved in the inflammatory response
(Keller et al., 2008). However, the exact mechanism for caspase-1 in mediating protein secretion
remains to be determined.
of both cytokines is curiously reduced in caspase-1-deficient
mice upon stimulation. Furthermore, the secretion of proIL-1a
and proIL-1b does not require the classical secretory pathway,
which pertains to the export of proteins tagged with a signal
peptide in an ER/Golgi-dependent manner. This suggests that
the maturation of IL-1 depends on a completely independent
mechanism. More than a decade later, one group has set out
to unravel this mystery. Using a proteomic approach, these
authors identified almost 1,000 extracellular proteins whose
secretion depends on caspase-1 activity rather than the classic
protein export system (Figure 2) (Keller et al., 2008). Such leaderless proteins include fibroblast growth factor-2 (FGF-2), caspase-1, proIL-1a, and proIL-1b. Although FGF-2 and proIL-1a
are not direct substrates of caspase-1, both proteins can interact
with the protease, suggesting that caspase-1 can directly and
indirectly promote protein secretion. It remains to be seen,
however, how caspase-1 regulates the secretion of such
substrates. The ability of caspase-1 to regulate the secretion of
this staggering number of proteins underscores that the important nonapoptotic functions of this caspase are yet to be fully
understood. It also raises the question of whether other caspases might also regulate protein secretion.
Mammalian Caspases in Adaptive Immunity: The Role of
Secretion? Specific to vertebrates, adaptive immunity is characterized by the exquisite specificity for foreign molecules and the
ability to remember certain microbes in
anticipation of future infections, such
that a highly potent defense can be
mounted. Negative regulation of adaptive
immunity by eliminating activated or
autoreactive immune cells is a wellestablished apoptotic role for caspases.
Surprisingly, a number of caspases have
also been found to play positive roles in
the adaptive immune response by
promoting proliferation and activation of
lymphocytes.
In addition to defects in lymphocyte
apoptosis, individuals with an inherited
genetic deficiency in caspase-8 exhibit
reduced activation of T cells, B cells,
and natural killer cells after stimulation (Chun et al., 2002). These
defects lead to immunodeficiency characterized by increased
susceptibility to viral infections in affected individuals. Consistent with a positive role of caspase-8 in mediating T cell activation, T-cell-specific caspase-8-deficient mice show a decrease
in the number of peripheral T cells and impaired T cell response
to stimulation (Salmena et al., 2003). The effect of caspase-8 in
mediating T cell proliferation may be coordinated with FADD,
as FADD / T cells also exhibit a similar proliferation defect
(Zhang et al., 1998). On the other hand, caspase-8 may not be
required for activation-induced B cell proliferation as mice with
a deletion of casapse-8 specifically in B cells manifest normal
lymphocyte development (Lemmers et al., 2007). However,
attenuated antibody production is observed after viral infection,
suggesting that caspase-8 is still important for a proper humoral
response. Surprisingly, B cell development is impaired in
FADD-deficient mice, in striking contrast to that of caspase-8deficient B cells, suggesting that FADD might have a caspase8-independent effect on B cell proliferation (Zhang et al., 1998).
Thus, caspase-8 may regulate activation-induced lymphocyte
proliferation in both a FADD-dependent and FADD-independent
manner.
Recently two studies have proposed a new hypothesis for the
requirement of caspase-8 in the activation and clonal expansion
of T cells (Bell et al., 2008; Ch’en et al., 2008). Surprisingly, the
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Figure 3. The Role of Caspase-8 in Maintaining the Survival of Activated T Cells
Caspase-8-deficient T cells undergo necroptosis in response to antigen stimulation, which can be inhibited by necrostatin-1 (Nec-1). Autocrine secretion of TNFa
is hypothesized to mediate necroptosis in activated caspase-8-deficient T cells.
authors found that although caspase-8- or FADD-deficient
T cells are able to undergo apparently normal proliferation in
response to antigen stimulation, these cells subsequently die
through a RIP1 kinase-dependent, but caspase-independent,
necrotic cell death pathway. This type of necrotic cell death,
which has been termed necroptosis (Degterev et al., 2005),
can be effectively inhibited by a RIP1 kinase inhibitor, necrostatin-1 (Nec-1) (Bell et al., 2008; Ch’en et al., 2008). Interestingly,
caspase-8 deficiency in T cells may lead to increased RIP1
kinase activity (Ch’en et al., 2008). Thus, caspase-8 may be
required to promote the survival of activated T cells by suppressing RIP1 kinase activity. It remains to be seen, however, if
defects in the activation and expansion of T cells, B cells, and
natural killer cells in individuals with an inherited genetic deficiency in caspase-8 (Chun et al., 2002) are due to activation of
necroptosis. Since inhibition of caspases by the pan caspase
inhibitor z-VAD.fmk leads to activation of necroptosis by autocrine secretion of TNFa (Hitomi et al., 2008), we speculate that
caspase-8 deficiency may lead to activation of necroptosis in
immune cells by increased secretion of TNFa (Figure 3).
In addition to caspase-8, caspase-3 may also influence the
adaptive immune response by regulating B cell proliferation.
Loss of caspase-3 in mice results in lymphadenopathy and
splenomegaly due to hyperproliferation of both B and T lymphocytes (Woo et al., 2003). The increased proliferation of peripheral
T cells in caspase-3-deficient mice has been found to be due to
defects in apoptosis (Woo et al., 1998). However, hyperproliferation of peripheral B cells in caspase-3-deficient mice may result
from altered cell cycle regulation rather than apoptosis resistance (Woo et al., 2003). Interestingly, caspase-3-deficient B
cells display an increase in the levels of PCNA, a processivity
factor for DNA polymerase d and 3, and elevated cyclin-dependent kinase activity, which is associated with increased cell cycle
progression. Surprisingly, p21 protein levels are also increased
in caspase-3-deficient B cells, which may be due to the lack of
cleavage since p21 is a caspase-3 substrate (Levkau et al.,
1998). Although it may seem counterintuitive that an increase
26 Developmental Cell 16, January 20, 2009 ª2009 Elsevier Inc.
in p21—a cyclin-dependent kinase inhibitor—is observed in
hyperproliferating cells, a deficiency of p21 inhibits the proliferation of caspase-3-deficient B cells in vivo, suggesting that p21
may have an alternative role in promoting cell cycle progression
(Woo et al., 2003). Taken together, the findings described above
elucidate the complex roles of caspases in influencing multiple
aspects of the mammalian immune response.
We still have much to learn as to how caspases function in
regulating cell proliferation. We speculate that caspase-3 and
caspase-8 could mediate an extracellular mechanism, such as
secretion of paracrine or autocrine factors, and contribute to
the intracellular signaling events described above, leading to
increased cell proliferation. Since caspase-1 has a role in regulating FGF-2 secretion (Keller et al., 2008), it is not too farfetched
that caspase-3 and caspase-8 might also influence the secretion
of certain growth factors that can promote cell proliferation in
a cell nonautonomous manner. In fact, there are suggestions
from studies on compensatory proliferation during Drosophila
development that caspases may regulate the expression of
non-cell-autonomous mitogenic pathways.
Caspases in Other Physiological Processes
In the remaining sections, we will discuss the growing body of
work that reveals the physiological functions of caspases during
fly and mammalian development, namely tissue regeneration
after injury, cell differentiation, and cell migration and motility.
Caspases Regulate Compensatory Proliferation:
Regulation of Mitogenic Pathways
Maintaining tissue homeostasis after injury is a critical survival
response for metazoans. We are most likely familiar with the
ability of our skin epithelium to regenerate after tissue damage
caused by ultraviolet radiation from the sun and other injuries
(Fuchs, 2007). Cell growth and proliferation has been the primary
focus in the study of tissue regeneration; however, it is the integration of cell death (or cell loss) and cell proliferation that shapes
the course of tissue maintenance. Fascinatingly, dying cells in
Drosophila stimulate proliferation of neighboring cells in a cell
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nonautonomous manner, termed compensatory proliferation. In
particular the larval imaginal disc, which develops into the adult
wing, has a remarkable capacity to recover from extensive tissue
damage: even destroying 40%–60% of cells by lethal irradiation
is not enough to prevent normal wing size development (Haynie
and Bryant, 1977; James and Bryant, 1981; Milan et al., 1997).
Surprisingly, this staggering cellular feat may be under the
control of caspases.
The utilization of an effector caspase inhibitor from baculovirus, p35, revealed that the activation of upstream caspase
Dronc, but not downstream caspases Dcp-1 or Drice, is a prerequisite for compensatory proliferation in the developing wing (Huh
et al., 2004a; Kondo et al., 2006; Ryoo et al., 2004; Wells et al.,
2006). Because phagocytes rapidly remove dying cells in vivo,
expression of p35 allows the execution of certain upstream
apoptotic events without cell death. Interestingly, the activation
of upstream apoptotic signaling leads to the upregulation of
Wingless (Wg; fly ortholog of Wnt) and Decapentaplegic (Dpp;
fly ortholog of Transforming Growth Factor-b [TGF-b]) (Huh
et al., 2004a; Perez-Garijo et al., 2004, 2005; Ryoo et al.,
2004), secreted mitogens that are known to play a role during
development and tissue regeneration (Stoick-Cooper et al.,
2007). Activation of the Wg pathway by dying cells in turn triggers
the proliferation of surrounding tissue in a non-cell-autonomous
manner. Thus, this might provide another example of caspases
mediating the secretion of extracellular factors. However, the
mechanism by which Dronc mediates the secretion of Wg and
Dpp remains to be understood.
In developing Drosophila larval imaginal eye disc, caspases
also play an important role in mediating apoptosis-induced
compensatory proliferation. Apoptosis-induced compensatory
proliferation in proliferating and differentiating tissues within
the developing fly eye requires independent pathways mediated
by upstream or downstream caspases. In the anterior proliferating eye disc, Dronc plays a dominant role in regulating
compensatory proliferation, whereas in the posterior differentiating eye disc, Dcp-1 and Drice activity are required to promote
proliferation of nearby undifferentiated cells (Fan and Bergmann,
2008). As described for the wing disc, Dronc-dependent induction of compensatory proliferation in the anterior eye disc corresponds to an increased expression of Wg and Dpp in the
apoptotic cells. In the differentiating posterior eye imaginal
disc, however, Hedgehog (Hh) is induced in the apoptotic photoreceptor neurons, stimulating a mitogenic pathway distinct from
that of Wg and Dpp. Thus, the mechanism by which compensatory proliferation is induced (Wg/Wnt or Hh pathway) may
depend on the differentiated state of the tissue.
Taken together, these findings indicate that caspase activation
drives apoptosis-induced compensatory proliferation; however,
many questions remain to be answered. What is the regulatory
mechanism that controls the induction of growth promoting
signals by dying cells? More importantly, how does caspase
activity regulate different mitogenic pathways to mediate
compensatory proliferation? Caspases may process proteins
that influence transcriptional activation of Dmp53 and in turn
promote compensatory proliferation (Wells et al., 2006). A more
parsimonious explanation is that caspases may directly or indirectly influence the secretion of mitogenic factors. Such mitogenic
factors may require cleavage by caspases for their bioactivity.
The ability of apoptotic cells to regulate the expression of
secreted mitogenic factors as a mechanism mediating compensatory proliferation is very interesting. If similar phenomenon can
be observed in mammalian cells, one may raise the question of
whether inducing apoptosis of cancer cells might lead to proliferation of remaining cancer cells. Given that caspase-1 regulates
the unconventional protein secretion of a large number of extracellular proteins involved in tissue repair and inflammation (Keller
et al., 2008), it may not be surprising that multiple caspases have
a similar capacity in regulating protein secretions under different
conditions.
Apoptosis-induced compensatory proliferation in developing
tissues has an uncanny resemblance to tissue regeneration of
adult tissues, a process that utilizes adult stem cells to replace
lost tissue. The developing fly eye tissue provides an interesting
system to study regeneration of differentiated tissue as apoptotic
neurons are implicated in inducing proliferation of the neighboring undifferentiated cells (Fan and Bergmann, 2008). It would
be exciting if an analogous mechanism also regulates regeneration of adult tissue. An emerging system to study adult tissue
maintenance is the freshwater planarian Schmidtea mediterranea, which demonstrates a remarkable regenerative capacity.
However, it is unclear whether cell death pathways control the
regenerative capacity of planarians because elucidation of these
pathways is currently underway (Pellettieri and Sanchez Alvarado, 2007). An enticing area of research could be the role of
crosstalk between cell death and cell proliferation in the potential
regulation of adult tissue regeneration in planarians. Identification
of such signaling effectors could help to rebuild damaged tissue
for biomedical applications.
Caspases in ‘‘Subcellular’’ Apoptosis
Whereas full caspase activation mediates total cellular destruction during apoptotic cell death, caspase activation in a temporally and spatially restricted manner mediates localized cellular
remodeling. Such caspase-mediated remodeling of organelles
and cellular structures may be considered a highly specialized
form of ‘‘subcellular’’ apoptosis in that caspases carry out strictly
defined functions independent of cell death. In this regard, it is
possible that the ability of caspases to regulate the protein
secretion may also be an example of subcellular compartmentalization of caspase activation.
Caspases in the Enucleation Process. Terminal differentiation
of certain cell types, such as lens epithelial cells, erythroblasts,
megakaryocytes, and keratinocytes, requires elimination of
nuclei. Interestingly, caspases have been found to play a role
in this ‘‘nuclear’’ apoptosis process. For example, the enucleation process of lens epithelial cells is associated with extensive
nuclear DNA fragmentation, a typical marker for caspase activation (Bassnett and Mataic, 1997; Ishizaki et al., 1998). Inhibition
of caspase activity by the pan caspase inhibitor, z-VAD.fmk,
prevents the enucleating process, suggesting that caspases
are involved in the specific elimination of nuclei as opposed to
elimination of the whole cell (Ishizaki et al., 1998). From these
studies, Raff and colleagues proposed that caspases might be
necessary in general for the removal of nuclei or other organelles
and structures in differentiating cells.
The terminal maturation of erythroblasts into anucleate red
blood cells also requires caspase activity. Although erythroblasts
are sensitive to apoptotic cell death upon stimulation of death
Developmental Cell 16, January 20, 2009 ª2009 Elsevier Inc. 27
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Review
receptors (De Maria et al., 1999), transient activation of effector
caspases, such as caspase-3, is detected during normal
erythroid maturation (Carlile et al., 2004; Zermati et al., 2001).
Moreover, the loss of caspase-3 blocks the enucleation process
in a human erythroid culture system (Carlile et al., 2004). Caspase
activation during enucleation is associated with the cleavage of
lamin B and acinus, which are necessary for the maintenance
of nuclear integrity and condensed chromatin, respectively (Zermati et al., 2001). It is still not clear how differentiating cells
restrain activated caspases to selectively remove the nucleus
without compromising cellular integrity. In this regard, one recent
study suggests that the molecular chaperone Hsp70 may protect
GATA-1, a master regulator of erythroid maturation, from caspase-3-mediated cleavage during the enucleation process
(Ribeil et al., 2007). It is unclear, however, whether the selective
masking of caspase substrates by Hsp70 provides a universal
control mechanism for substrate specificity. Localized caspase
activation may offer an alternative mechanism for mediating
cleavage of distinct substrates during enucleation. Because
a staggering number of caspase substrates have been systematically identified (Mahrus et al., 2008; Van Damme et al., 2005), the
question regarding the mode for selective caspase activity in
nonapoptotic processes in general is still ripe for discovery. It
remains to be seen whether such activation involves one of the
caspase adaptor complexes discussed above.
Enucleation is important during the differentiation of megakaryocytes into mature platelets. Unlike that of lens epithelial cells
and erythroblasts, this process is considered more akin to cytokinesis, where cytoplasmic fragmentation of megakaryocytes
leads to the formation of anucleate cells rather than removal of
the nucleus per se (Radley and Scurfield, 1980). Interestingly
this fragmentation process may be under the control of caspase-3 and caspase-9 (De Botton et al., 2002). Activated caspase-3 was detected in maturing megakaryocytes with punctate
cytoplasmic distribution and outer mitochondrial membrane permeabilization as assessed by the release of cytochrome c but no
detectable DNA fragmentation. Thus, caspase activation in the
enucleation process may involve the destruction of nucleus,
such as during the differentiation of lens epithelial cells, or
without nuclear destruction, such as during the differentiation
of megakaryocytes into platelets. In either case, the mechanism
by which nuclear apoptosis is orchestrated without destruction
of the cytoplasmic compartment remains to be determined.
Caspase activation is also involved in the terminal differentiation of keratinocytes into anucleate cells that form the outer layer
of the skin. The implicated caspase, caspase-14, was first identified for its unusual expression pattern in embryonic tissue and
adult skin (Van de Craen et al., 1998). Further characterization
revealed that caspase-14 expression is highly specific for keratinocytes during the enucleation process (Lippens et al., 2000).
A recent study sheds some light into the mechanism by which
caspase-14 controls skin development. Caspase-14-deficient
mice present a unique phenotype of shiny and lichenified skin
(Denecker et al., 2007). A key event in keratinocyte differentiation
is the cleavage and subsequent aggregation of filaggrin, an
abundant protein in the epidermis, leading to the collapse of the
anucleate cells and formation of the outer skin layer (Smith et al.,
2006). Although it was previously thought that caspase-14
lacked protease activity (Van de Craen et al., 1998), caspase28 Developmental Cell 16, January 20, 2009 ª2009 Elsevier Inc.
Figure 4. Local Caspase Activation Influences Dendritic Pruning
during Drosophila Metamorphosis
Extensive severing of neuronal dendrites occurs in the class IV dendritic arborization (C4 da) neurons within the fly peripheral nervous system during metamorphosis (Kuo et al., 2006). Local activation of caspases is restricted to
dendrites targeted for degradation at 4 hr after pupae formation (APF) as indicated by antibody staining with anti-activated caspase-3. Activated caspase3 is not observed at early metamorphosis or white pupae stage (WP). Scale
bar, 3 mm. Images courtesy of Yuh-Nung Jan.
14 was found to directly cleave filaggrin in vitro (Denecker
et al., 2007). These findings indicate that caspase-14 plays
a surprising role in maintaining the integrity of skin, though it is
unclear whether caspase-14 directly regulates the enucleation
process of keratinocytes.
Caspases in Dendritic Pruning. An emerging area of interest is
the possible role of caspases in influencing neural connectivity.
Selective removal of neuronal dendrites, or dendritic pruning,
is essential for normal brain function. This multistep process
involves cytoskeletal destabilization, leading to severing of
dendrites from the soma and subsequent engulfment by glia;
thus, pruning of dendritic branches might also be considered
‘‘dendritic’’ apoptosis. Interestingly, two recent studies indicate
that localized caspase activity is important for the selective
removal of sensory neuron dendrites during fly metamorphosis
(Figure 4) (Kuo et al., 2006; Williams et al., 2006). Dronc activity
is restricted to the dendrites that are eventually removed by
glia, suggesting that Dronc is involved in initiating dendritic
pruning during fly larval development. However, it is unclear
whether Dronc activation is required for severing of dendrites
prior to engulfment (Kuo et al., 2006) or required for targeting
dendrites to glia after dendrite severing (Williams et al., 2006).
Further characterization regarding the regulation of caspase
activation revealed that the ubiquitin-proteasome system
(UPS) promotes the degradation of Drosophila Inhibitor of
Apoptosis Protein 1 (DIAP1), a potent inhibitor of caspases,
and releases caspases from inhibition (Kuo et al., 2006). Thus,
localized degradation of DIAP1 might spatially control caspase
activation, though it remains to be determined how ubiquitindependent degradation of DIAP1 is initiated and restricted to
dendrites.
The ability of caspases in regulating dendritic pruning may
have direct consequence on brain function. For example, caspase-3 has been proposed to play a role in the habituation of
birdsong in zebra finch, which offers an in vivo setting to understand learning and memory (Huesmann and Clayton, 2006).
Developmental Cell
Review
Reminiscent to that of fly, caspase-3 activity is confined to
dendritic spines within the auditory forebrain of zebra finch and
may be necessary for proper memory consolidation of a song
stimulus. Thus, caspase activity may influence postsynaptic remodeling in response to certain environmental stimuli. Elucidating a possible mechanism for caspase-mediated dendritic
pruning during learning and memory could be an interesting
avenue of study.
Caspases and Spermatogenesis. Localized caspase activity
has also been found to play an important role during sperm
development. Dramatic removal of bulk cytoplasm occurs in
the terminal differentiation of spermatid into individual sperm,
which could be considered the ‘‘apoptotic cytoplasm.’’ Surprisingly, multiple fly caspases have been implicated in this highly
specialized cellular event, known as spermatid individualization.
Drice activity is observed at the onset of spermatid individualization, though this caspase may be more important for driving
cytoplasmic elimination (Arama et al., 2003). drice mutants,
however, show partial defects in sperm maturation (Muro et al.,
2006) similar to that of dFadd and dredd mutants (Huh et al.,
2004b). On the other hand, RNAi against dark or overexpression
of dominant-negative Dronc results in more severe phenotypes,
where the cytoplasm is mostly unaffected and individualization is
blocked (Arama et al., 2003; Huh et al., 2004a, 2004b). Thus,
cytoplasmic removal during spermatogenesis may be dependent on Dronc and Dark activity, though other caspases may
contribute to the nearly complete removal of cytoplasmic
material. Importantly, a recent study points to the UPS in the
regulation of caspase activation by mediating degradation of
the IAP-like protein dBruce, leading to the subsequent release
of caspases from inhibition (Arama et al., 2007). Coincidently,
this mechanism is similar to the one proposed for the regulation
of caspase activation during dendritic pruning (see above). It will
be interesting to determine how dBruce destabilization leads to
local caspase activity.
Caspase Function in Cell Differentiation
Cell differentiation also involves dramatic cellular reorganization
without subcellular remodeling, such as cell fusion events.
Emerging evidence suggests that this type of cell differentiation
is also regulated by caspases.
Caspases and Neuronal Differentiation. A well-known observation in the apoptosis field is that differentiating cells are more
refractory to apoptosis compared with less differentiated cells.
As differentiating neurons are largely apoptosis resistant, the
finding that caspase activity, when finely tuned, can influence
the differentiation of neuronal precursor cells into neurons is
quite intriguing. Curiously, one additional sensory organ
precursor (SOP) cell, equivalent to the mammalian sensory
neuron, is found in dark fly mutants (Kanuka et al., 2005). In addition, overexpression of dominant-negative Dronc or silencing of
Dronc by RNAi leads to a similar phenotype. The survival of an
extra neuron in animals deficient in caspase activity was found
to be due to a more complex cause than a simple failure in
apoptosis. Miura and colleagues discovered that cleavagedependent activation of a glycogen synthase kinase-3b-like
protein, Sgg46, by Dronc or Drice negatively controls SOP cell
development (Kanuka et al., 2005). Activation of Sgg46 leads
to downregulation of the Wg pathway, which is essential for
the generation of a sensory bristle from an SOP cell. Thus, Dronc
or Drice may have an instructive role in the regulation of neuronal
development by antagonizing the Wg pathway.
If caspases can have instructive roles in regulating neuronal
development, one might expect that there should be regulatory
mechanisms that restrict caspase activity in neuronal development independent of cell death. The identification of DmIKK3
(also called Ik2), the fly ortholog of the noncanonical IKK family
(IKK3 or NAK), in a dominant-modifier screen in Drosophila for
suppressors of caspase activation begins to address this question (Figure 5) (Kuranaga et al., 2006). DmIKK3 was found to
regulate Dronc activity by promoting phosphorylation and
subsequent degradation of DIAP1. Importantly, RNAi against
DmIKK3 leads to the development of extra SOP cells, which phenocopies dronc and dark mutants. Thus, DmIKK3 may control
the levels of DIAP1 to induce a certain threshold of caspase
activity that is still compatible with cell survival. This regulation
of caspase activation may be conserved in mammalian cells as
NAK controls XIAP, a mammalian ortholog of DIAP1, in a similar
fashion (Kuranaga et al., 2006). This mechanism could apply to
mammalian neuronal development as increased caspase-3
activity has been observed during the differentiation of neural
progenitor cells derived from mouse embryos (Fernando et al.,
2005).
Caspases and Other Cell Types. Caspases have been implicated in the differentiation process for a number of other cell
types; however, the regulatory mechanism is largely unknown
for these cases. For example, caspase activation is observed
during the differentiation of monocytes into macrophages but
not into dendritic cells (Netea et al., 2008; Sordet et al., 2002).
Concordantly, loss of caspase-8 leads to more severe defects
in the development of macrophages than that of dendritic cells
(Kang et al., 2004). Identification of possible caspase substrates
that are cleaved during macrophage differentiation has been
initiated (Cathelin et al., 2006). Caspase-8 has also been associated with specialized cell fusion events that are necessary for the
differentiation of human placental villous trophoblast, an epithelial layer important for transporting nutrients directly from mother
to fetus (Black et al., 2004). Other cell differentiation processes
may require caspase-3 activation. Caspase-3 activation is
observed during neuronal differentiation (Fernando et al., 2005)
and in glial cells within the developing cerebellum (Oomman
et al., 2006). Appropriate skeletal myogenesis characterized by
a series of myoblast fusion events may also depend on caspase-3 activity (Fernando et al., 2002). Cleavage-dependent
activation of Mammalian Sterile Twenty-like 1 (MST1) kinase
by caspase-3 may drive skeletal muscle differentiation since
expression of the truncated kinase rescues the differentiation
defect in caspase-3-deficient myoblasts. It is intriguing that caspase-3 has been linked to skeletal muscle differentiation as well
as osteogenic differentiation (Miura et al., 2004) because both
are necessary for a functional skeletal system.
Caspases and Stem Cells. The function of caspases in cell
differentiation may extend beyond the terminal differentiation
to include stem cell pluripotency and hematopoiesis. A defining
characteristic of stem cells is their dual capacity to self-renew
and to differentiate into specific somatic cells. Surprisingly, two
recent studies implicate caspase-3 in controlling the fate of
both embryonic stem cells (ESCs) and adult hematopoietic
stem cells (HSCs).
Developmental Cell 16, January 20, 2009 ª2009 Elsevier Inc. 29
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Figure 5. Nonapoptotic Function of Caspases in Drosophila Neural Development
Low-level caspase activity, which would otherwise fall below the threshold of detection in some assays, was measured using a fluorescence resonance energy
transfer (FRET)-based probe containing enhanced cyan fluorescence protein (ECFP) as the FRET donor and a variant of enhanced yellow fluorescence protein,
Venus, as the FRET acceptor. A caspase cleavage site was designed within the peptide linker that connects ECFP and Venus (Takemoto et al., 2003). In situ
confocal imaging of the FRET emission ratio (Venus/ECFP) reveals caspase activity in wing imaginal discs (A), including the specific subset of cells that have
the potential to give rise to scutellar sensory organ precursors ([B], upper panel). This level of caspase activation is dependent on DmIKK3-activated degradation
of DIAP1 and is insufficient to induce apoptosis. Rather, it is required for proper regulation of sensory organ precursor cell number (Kuranaga et al., 2006).
Suppression of caspase activation via loss of DmIKK3 results in the eventual formation of an extra sensory bristle, which costains with a neuronal marker, antisenseless ([B], lower panels). Images courtesy of Masayuki Miura.
The self-renewing state of ESCs is controlled by a set of
transcription factors, Oct4, Sox2, and Nanog, which constitutes
a robust autoregulatory signaling circuit (Boyer et al., 2005).
Increased caspase activity is observed in differentiating ESCs
and is associated with caspase-3-mediated cleavage of human
Nanog (Fujita et al., 2008). ESCs expressing a caspase-cleavageresistant Nanog mutant are refractory to differentiation induction
similar to that of cells deficient in caspase-3, suggesting that
Nanog may be the key caspase substrate for limiting the selfrenewing capacity of ESCs. Moreover, loss of caspase-3 leads
to increased proliferation of long-term repopulating HSCs with
a concordant defect in differentiation due to altered extracellular
signal-related kinase (ERK) signaling (Janzen et al., 2008). This
finding suggests that caspase-3 may help to maintain the quiescent state of HSCs. In light of these studies, the role of caspase-3
in stem cell maintenance may go hand in hand with the role of this
caspase in promoting the differentiation of stem cells. Determining whether caspases function at the molecular bifurcation
of the signal transduction pathways that direct cell lineage
specification and simultaneously block stem cell self-renewal
properties is a key unanswered question.
Caspases in Cell Migration and Motility
A tractable system to study cell migration in vivo is the migratory
behavior of border cells, which are the follicle cells within the
30 Developmental Cell 16, January 20, 2009 ª2009 Elsevier Inc.
egg chamber, during fly oogenesis. Surprisingly, a screen for
suppressors of the migration defect caused by overexpression
of dominant-negative Rac GTPase led to the identification of
DIAP1 (Geisbrecht and Montell, 2004). Further characterization
revealed that overexpression of dominant-negative Dronc and
Dark also rescued this migration defect. However, expression
of p35 was unable to promote border cell migration. These findings suggest that the upstream caspase Dronc may inhibit Racdependent cell motility. One hint as to how cell migration might
be affected by Dronc is the observation of a disorganized actin
cytoskeleton in the oocytes of flies expressing a hypomorphic
allele of DIAP1 (Geisbrecht and Montell, 2004). In addition, regulation of DIAP1 levels by DmIKK3 has also been implicated in the
organization of actin cytoskeleton at the cell edge (Oshima et al.,
2006). DmIKK3 mutants show increased branching of the actin
cytoskeleton, leading to enhanced membrane ruffling, whereas
hyperactivation of DmIKK3 results in decreased actin assembly.
Though it is unclear how Dronc might regulate actin dynamics,
one possibility is that caspases have a conserved role in actin
depolymerization.
An important aspect of a proper host response is appropriate
immune cell migration and motility. In this paradigm, our laboratory discovered that caspase-11 has a surprising role in regulating
actin dynamics in murine macrophages during inflammation. In
Developmental Cell
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particular, caspase-11 interacts with actin interacting protein 1
(Aip1) to promote cofilin-mediated actin depolymerization (Li
et al., 2007). Conversely, Flightless-I, a member of the gelsolin
superfamily of actin remodeling proteins, inhibits caspase-11and caspase-1-mediated IL-1b maturation by controlling the
localization and activation of these proinflammatory caspases
(Li et al., 2008). The link between caspases and actin dynamics
might provide an important regulatory venue for organization of
the actin cytoskeleton and localized caspase activity.
This work was supported in part by an NIH Director’s Pioneer Award
(DP1OD000580) and a grant from the National Institute of Aging (R37012859) to J.Y., and a Kirchstein National Research Service Award from
the National Institute of Neurological Disorders and Stroke (grant
1F31NS057872-01) to C.H.Y.
Concluding Remarks
It is remarkable how the identification of a simple cell death
machinery in C. elegans paved the path for elucidating the
complex apoptotic machinery in mammals. Though extensive
research has characterized caspases as key executioners of
apoptotic cell death, recent advances implicate caspases in
diverse nonapoptotic processes. While full activation of the caspase cascades associated with cell death is rare in long-lived cell
types, such as neurons, localized or limited activation of caspases may be more common and persistent in order to carry
out their nonapoptotic functions.
Much remains to be learned as to how caspases are regulated
in these highly specialized cellular processes. Such regulatory
mechanisms will be intricate in nature in order to provide a suitable balance between sufficient caspase activation to execute
a certain nonapoptotic function and maintenance of cellular
integrity. In Drosophila, it is clear that protein levels of DIAP1 influences caspase activation by regulating either localization of
caspase activity or the extent of caspase activation. To date,
a similar mechanism has yet to be implicated in regulating the
activation of mammalian caspases in nonapoptotic processes,
such as subcellular apoptosis. Interestingly, posttranslational
modifications, including phosphorylation and S-nitrosylation,
can directly regulate caspase activity and influence apoptotic
cell death (Allan and Clarke, 2007; Allan et al., 2003; Cardone
et al., 1998; Hess et al., 2005; Nutt et al., 2005). Furthermore these
modifications are influenced by specific proapoptotic stimuli—
for example, dephosphorylation of caspase-2 is induced upon
metabolic stress (Nutt et al., 2005). This mechanism could
provide the specificity required for appropriate caspase activation in highly specialized cellular contexts. A more speculative
possibility is that activated caspases might be unstable in living
cells. After executing their nonapoptotic function in a localized
manner, such short-lived active caspases could be eliminated
quickly by cellular degradative mechanisms. It will be of great
interest to test the temporal regulation of caspase activation as
well as the threshold of caspase activity under nonapoptotic
conditions. Refinement of the currently available fluorescent
caspase reporters will be useful in teasing apart such mechanisms (Bardet et al., 2008; Takemoto et al., 2003). With our
current technology and knowledge, the time is ripe for further
exploration of the nonapoptotic roles of caspases and for the
systematic identification of caspase substrates that might function as critical effectors for these cellular processes (Dix et al.,
2008; Keller et al., 2008).
Allan, L.A., and Clarke, P.R. (2007). Phosphorylation of caspase-9 by CDK1/
cyclin B1 protects mitotic cells against apoptosis. Mol. Cell 26, 301–310.
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ACKNOWLEDGMENTS
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We apologize to our colleagues whose work could not be cited due to space
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