Jcb_201704114 1521..1524

1521 The Rockefeller University Press $30.00 J. Cell Biol. Vol. 216 No. 6 1521–1523 https://doi.org/10.1083/jcb.201704114 Caspase-2 is a critical component of the cellular machinery designed to remove damaged cells, preventing disease. Consistent with a function in the apoptosis pathway, caspase-2, like caspase-3, is a tumor suppressor (Puccini et al., 2013). Although caspase-2 is required to initiate an effective apoptotic response following stimuli, depletion of caspase-2 has multiple cellular consequences, such as genomic instability, altered cell cycle regulation, activation of the oxidative stress response, and premature ageing (Dorstyn et al., 2012; Shalini et al., 2012). Interestingly, all of these pathways are also implicated in the development and evolution of cancer. Indeed, genomic instability is a universal hallmark of cancer and the induction of genomic instability by depletion of caspase-2 suggested that the DNA damage response pathway signals through caspase-2. p53 is an important responder to genomic instability and caspase-2 is activated after genomic stress, in both a p53-dependent and -independent manner (Lassus et al., 2002; Sidi et al., 2008). The evolutionary conserved caspase-2 is important in signaling cell death, but how caspase-2 responds to different death stimuli is not clear. Caspase-2, like caspase-9, caspase-8, and caspase-1, is considered an initiator caspase, with an apical position in signaling of the stress stimuli. Some stimuli trigger caspase-2 activation through the PIDDosome complex—a high-molecular weight protein assembly that is well characterized to be an “activation platform” for caspase-2 and contains the PIDD1 and RAI DD proteins (Tinel et al., 2007). Although it was initially thought to be a DNA damage–responsive complex, loss of the PIDDosome function in mice does not impair the DNA damage response, DNA damage-induced apoptosis, or DNA damage-induced tumor formation. The exact role of the PIDDosome is still unclear. Additionally, it was shown that caspase-2 is activated in a PIDDosome-independent mechanism in response to other stimuli (Bouchier-Hayes et al., 2009), raising the possibility that caspase-2 is involved in different cellular pathways. In new research published in this issue, Ando et al. used a proximity fluorescence caspase-2 activation reporter to show that different caspase-2 activation platforms assemble in the cytoplasm and in the nucleolus and their requirement for PIDDosome proteins differs. Ando et al. (2017) determined that different caspase-2– activating stimuli had different cellular consequences for caspase-2. The authors demonstrated that DNA-damaging agents, such as topoisomerase I inhibitors, topoisomerase II inhibitors, camptothecin, irinotecan, and topotecan, resulted in a predominantly nucleolar activation of caspase-2, whereas treatment with the microtubule inhibitor vincristine resulted in cytoplasmic activation of caspase-2. The researchers were able to decipher which stimulus required nucleolar localization of caspase-2 for caspase-2 activation using mutant constructs with and without the nuclear localization sequence of caspase-2 and 3D imaging. They found that cytoplasmic activation of caspase-2 using heat shock did not require the nuclear localization sequence, whereas nucleolus activation of caspase-2 by DNA damage did. This data supported the hypothesis that caspase-2 had two modes of activation, one based on a response to genotoxic agents that is confined in the nucleolus and the other in response to other stress stimuli, occurring in the cytoplasm and nucleus. To understand this mechanism further, Ando et al. (2017) used immunofluorescence microscopy to study the behavior of caspase-2 after various stimuli in more detail. After nongenomic stress, with vincristine and etopiside, the formation of predominantly cytoplasmic caspase 2 puncta was observed, whereas treatment with DNA-damaging agents resulted in predominantly large nucleolar puncta. The nucleolar punta suggested that caspase-2 localized to a specific nucleolar substructure. There are three molecularly defined nucleolar substructures: the fibrillar center, the dense fibrillar center, and the granular component. Localization studies determined that caspase-2 accumulated predominantly in the fibrillar center, suggesting that caspase-2 activation platforms assemble predominantly in the fibrillar center, with some localization at the internal boundary of the granular component. Intriguingly, Ando et al. (2017) also observed that, after DNA damage, the abundant nucleolar protein NPM1 also changed location from the granular component to a ring structure surrounding caspase-2, indicating that NPM1 may be involved in the caspase-2 response. These data suggest that caspase-2 has two distinct cellular localization and activation profiles. Ando et al. (2017) sought to understand how the PIDDosome functions in this process. The researchers dissected the requirement for each PIDDosome component—PIDD1 and RAI DD—in nucleolar caspase-2 Caspase-2 triggers apoptosis, but how it is activated by different stimuli is unclear. In this issue, Ando et al. (2017. J. Cell Biol. https ://doi .org /10 .1083 /jcb .201608095) delineate two pathways of caspase-2 activation and show that, in response to DNA damage, caspase-2 forms a complex with the PIDDosome and NPM1 within the nucleolus. Nucleolar caspase-2: Protecting us from DNA damage