caspase-2 is one of the closest mammalian homologues of the caenorhabditis elegans caspase ced-3 and shares significant homology with the drosophila nedd2-like caspase ( dronc ) , both of which are essential for developmentally programmed cell death [ 1 - 4 ] .
although several studies have implicated caspase-2 as a crucial mediator of apoptosis in mammalian cells , its apoptotic function has remained enigmatic , partly due to the fact that casp2 mice are viable and fertile with only minor apoptotic defects in some cell types [ 2,5 - 7 ] .
furthermore , lymphocytes and fibroblasts from mice lacking both initiator caspases , casp9 and casp2 , are no more resistant to apoptosis than cells from casp9 mice .
together , these findings indicate that the role of caspase-2 in developmental cell death is redundant and can be compensated by other caspases .
however , this does not rule out context - dependent and cell - specific caspase-2 functions .
for example , one study found an accumulation of oocytes in casp2 mice ( although this was not reported in a second casp2 strain ) and casp2 mice display premature ageing - related traits .
the activation of caspase-2 has been shown to occur both upstream ( by the piddosome ) and downstream ( by caspase-3 or -7 ) of mitochondrial outer membrane permeabilisation ( momp ) .
although this is also controversial since raidd ( receptor - interacting protein - associated ich-1/ced-3 homologous protein with a death domain ) and pidd ( p53-inducible protein with a death domain ) , the protein components of the piddosome , are dispensable for caspase-2 activation .
interestingly , caspase-2 activation can be mediated by caspase-8-induced cleavage following recruitment to the death receptor - inducing signaling complex ( disc ) . however , the importance of the disc as an activation platform is also unclear since caspase-2 dimerisation and self - processing are sufficient for its activation .
in addition , cells from casp2 mice are normally sensitive to death receptor - induced apoptosis , indicating that caspase-8-mediated cleavage of caspase-2 is not critical for its activation . while there is limited information on physiologically relevant substrates , caspase-2 can cleave and activate the protein bid , which provides a significant link between caspase-2 and momp .
furthermore , a unique feature of caspase-2 is its ability to localise to the nucleus in an importin - mediated fashion [ 18 - 20 ] .
this nuclear localisation of caspase-2 is likely associated with the recently found functions for caspase-2 in cell cycle regulation and cellular dna damage response .
in the absence of an overt phenotype in knockout mice , one may speculate that caspase-2 functions under specific contexts , such as under conditions of stress or in the fine - tuning of stress signaling , resulting in relatively minor aberrations in the whole animal physiology .
several recent studies showing caspase-2 functions in cell cycle regulation , dna damage response , and tumor suppression seem to be consistent with these predictions .
a role for caspase-2 in cell cycle regulation became apparent from observations that casp2-deficient murine embryonic fibroblasts ( mefs ) proliferate faster than their wild - type counterparts and that transformation of casp2 mefs with e1a / ras exacerbated this proliferative effect .
another recent study found that caspase-2 is involved in maintaining a g2/m cell cycle checkpoint in response to ionising radiation ( ir)-induced dna damage , with cells lacking casp2 unable to completely arrest in g2/m .
in addition , caspase-2 activation has been shown to be inhibited by cyclin - dependent kinase 1 ( cdk1)/cyclin - b1-mediated phosphorylation at ser340 during mitosis to allow for the repair of replication - induced dna damage . during mitotic arrest , prolonged activation of spindle assembly checkpoint results in apoptosis by 
mitotic slippage and prematurely exit mitosis with chromosomal abnormalities , resulting in genomically unstable aneuploid cells . in support of this ,
casp2 mefs show resistance to cell death induced by microtubule - disrupting drugs and also display increased genomic instability in culture compared with wild - type cells .
these findings indicate that deregulation of g2/m checkpoint in casp2 cells may contribute to the accumulation of cells with damaged dna .
the study by shi and colleagues   found that caspase-2 is involved in dna damage repair through its interaction with a nuclear complex comprising pidd and dna - dependent protein kinase catalytic subunit ( dna - pkc ) .
following ir - induced dna damage , this dna - pkc piddosome complex phosphorylates caspase-2 at ser122 , leading to its activation .
activated caspase-2 is then required for the repair of double - strand dna breaks by non - homologous end - joining ( nhej ) with cells lacking casp2 unable to efficiently repair dna breaks .
although it is unclear how caspase-2 mediates nhej , these important observations establish an additional non - apoptotic nuclear role for caspase-2 in dna damage signaling ( figure 1 ) . following double - strand dna breaks ( dsbs ) ,
the ataxia telangiectasia mutated ( atm ) and atm - related ( atr ) kinases are activated and in turn phosphorylate and activate several target proteins , including checkpoint kinase 1 ( chk1 ) and chk2 .
atr also activates chk1 , which can then act in a feedback loop to negatively regulate atr and inhibit further activation of nuclear caspase-2 .
dna - dependent protein kinase ( dna - pk ) is also activated by dsbs , presumably by atm / atr , and forms a complex with p53-inducible protein with a death domain ( pidd ) and caspase-2 ( dna - pk piddosome ) .
this complex serves to phosphorylate and activate caspase-2 , which is then required for the initiation of non - homologous end - joining ( nhej ) and dna repair .
cytosolic caspase-2 is also activated by other stress signals such as reactive oxygen species ( ros ) , metabolic stress , cytotoxic drugs , heat shock , or endoplasmic reticulum ( er ) stress . following heat shock , raidd ( receptor - interacting protein - associated ich-1/ced-3 homologous protein with a death domain )
can activate caspase-2 , which is inhibited by hsp90. ca / calmodulin - dependent kinase ii ( camkii ) acts to inhibit caspase-2 activation and cell death in oocytes .
activated cytosolic caspase-2 is able to cleave bid to its truncated form ( tbid ) , which ( via bax / bak ) can induce mitochondrial outer membrane permeability ( momp ) , activation of caspase-9 and -3 , and cell death .
sidi and colleagues   have described an unexpected nuclear function of caspase-2 in an apparently novel pathway of apoptosis in p53-deficient cells . using zebrafish as a model system , it was found that inhibition or loss of checkpoint kinase 1 ( chk1 ) restores -radiation - induced apoptosis in p53 mutant fish embryos . a similar ataxia telangiectasia mutated ( atm)/atm - related ( atr)/caspase-2-dependent pathway seems to be present in chk1-inhibited p53-deficient human tumor cells and in mefs following -irradiation .
these findings implicate caspase-2 in an apoptosis pathway downstream of atm / atr in response to dna damage , which is independent of p53 ( figure 1 ) . since atm / atr can induce phosphorylation and activation of dna - pkc , it would be of interest to investigate whether the regulation of caspase-2 phosphorylation in the nucleus finely tunes its function in either apoptosis or dna damage repair following ir exposure . the role of caspase-2 in specific cell death pathways other than dna damage is also emerging . in oocytes ,
the apoptotic activity of caspase-2 has been shown to be inhibited by ca / calmodulin - dependent kinase ii ( camkii)-mediated phosphorylation at ser135 .
the binding of 14 - 3 - 3 to phosphorylated caspase-2 prevents ser135 dephosphorylation , thereby promoting oocyte survival
. however , under nutrient - depleted conditions , the dephosphorylation at this site leads to caspase-2 activation and oocyte cell death .
these findings suggest that caspase-2 is an important player in metabolic regulation of oocyte cell death and that ser135 dephosphorylation is a sensor for caspase-2 activation .
caspase-2 has also been implicated in cell death induced by heat shock . a recent study assessed the real - time recruitment of caspase-2 to activation platforms during stress - induced apoptosis , including heat shock , cytoskeletal disruption or dna damage , and found that caspase-2 activation occurred in the cytosol , not the nucleus .
furthermore , heat shock - induced activation of caspase-2 occurred upstream of momp and was raidd - dependent and negatively regulated by hsp90 .
the caspase-2 functions in oxidative stress - induced apoptosis and ageing are also coming to light .
caspase-2 , along with bid and bak , were reported to be mediators of superoxide - induced cell death in muscle and in primary neurons .
in addition , caspase-2 has been shown to be involved in an age - related increase in muscle cell apoptosis in mice .
consistent with these findings , zhang and colleagues   found that casp2 mice show significantly higher levels of oxidised proteins in liver than wild - type mice .
this indicates that lack of casp2 can antagonise apoptosis induced by reactive oxygen species , leading to accumulation of cells with oxidative damage and consequently enhanced ageing phenotypes .
the reduced nhej activity in casp2 cells may also contribute to the premature ageing phenotype observed in casp2 mice .
our own studies using the e-myc transgenic mouse model of b - cell lymphoma found a potential role for caspase-2 in lymphoma suppression . specifically , the loss of even a single copy of casp2 resulted in increased tumor susceptibility and markedly accelerated tumor formation in e-myc transgenic mice .
these studies suggest that caspase-2 can suppress myc - induced lymphomagenesis . while the precise mechanism of caspase-2-induced tumor suppression remains unclear , it is tempting to speculate that its roles in cell cycle checkpoint , dna damage repair , and removal of oxidative damaged cells are important for this function .
while the recent observations have shed light on possible physiological functions of caspase-2 , it remains entirely speculative how caspase-2 might carry out some of the apparently unrelated functions in apoptotic and non - apoptotic contexts .
it is becoming clear that caspase-2 may act as a sensor to protect against cellular stress and that regulation of caspase-2 by phosphorylation or nuclear translocation or both may determine its role in cell death , the cell cycle , or nhej .
it will be important to establish whether these functions also contribute to the tumor suppressor mechanism of caspase-2 .
the major deficiency in caspase-2 research is that the targets of caspase-2 , which may mediate its various functions , remain largely unknown . while it remains a technical challenge to find proteins that are specifically cleaved by caspase-2 in specific contexts , identification of these substrates will be the key to unraveling the functional versatility of caspase-2
 caspase-2 is the most evolutionarily conserved of caspase family members , yet its physiological function has remained unclear and is a matter of considerable debate . 
 newly published data now suggest that caspase-2 is required for cell cycle regulation , repair of damaged dna , and in suppressing myc - induced lymphomagenesis . 
 additionally , loss of casp2 in mice leads to features of premature ageing . 
 these findings suggest that caspase-2 has non - apoptotic functions in addition to its context - dependent roles in cell death . 
Introduction and context
Major recent advances
Future directions
Competing interests