P53-induced protein with a death domain (PIDD): master of puppets?

The Molecular Mechanisms in Senescent Cells Induced by Natural Aging and Ionizing Radiation

This review discusses the relationship between cellular senescence and radiation exposure. Given the wide range of ionizing radiation sources encountered by people in professional and medical spheres, as well as the influence of natural background radiation, the question of the effect of radiation on biological processes, particularly on aging processes, remains highly relevant. The parallel relationship between natural and radiation-induced cellular senescence reveals the common aspects underlying these processes. Based on recent scientific data, the key points of the effects of ionizing radiation on cellular processes associated with aging, such as genome instability, mitochondrial dysfunction, altered expression of miRNAs, epigenetic profile, and manifestation of the senescence-associated secretory phenotype (SASP), are discussed. Unraveling the molecular mechanisms of cellular senescence can make a valuable contribution to the understanding of the molecular genetic basis of age-associated diseases in the context of environmental exposure.

Cold Stress Induces Apoptosis in Silver Pomfret via DUSP-JNK Pathway

We cultured silver pomfret for 20 days, decreasing water temperature from 18 to 8 ℃, and sampled muscle every 5 days. Muscle fiber degeneration and apoptosis began to increase at 13 ℃ detected by HE and TUNEL staining. Further analysis of transcriptome revealed that several apoptosis-related pathways were highly enriched by differentially expressed genes (DEGs). We analyzed 10 DEGs from these pathways by RT-qPCR during the temperature-decreasing process. JNK1, PIDD, CytC, Casp 3, and GADD45 were up-regulated after 15 and 20 days, while DUSP3, JNK2, and PARP genes were down-regulated after 15 and 20 days. DUSP5 was up-regulated from 10 to 20 days, and C-JUN was up-regulated after 20 days. We analyzed apoptosis in PaM cells under different temperatures (26 ℃, 23 ℃, 20 ℃, 17 ℃, and 14 ℃). The cell viability significantly declined from 14 to 20 ℃; the TUNEL and IHC results showed that the apoptosis signal increased with the temperature dropping, especially in 17 ℃ and 14 ℃; DUSP5, JNK1, CytC, C-JUN, Casp 3, and GADD45 were up-regulated at 17 ℃ and 14 ℃, and PIDD was up-regulated at 20 ℃, 17 ℃, and 14 ℃. DUSP3 was up-regulated at 20 ℃ but down-regulated at 17 ℃ and 14 ℃, and PARP was down-regulated at 17 ℃ and 14 ℃. JNK2 was up-regulated at 20 ℃ but down-regulated at 17 ℃ and 14 ℃. Our results suggest that DUSP could help inhibit apoptosis in the initial stage of cold stress, but low temperature could down-regulate it and up-regulate JNK-C-JUN, inducing apoptosis in a later stage. These data provide a basis for the study of the response mechanism of fish to cold.

Total recall: the role of PIDDosome components in neurodegeneration

The PIDDosome is a multiprotein complex that includes p53-induced protein with a death domain 1 (PIDD1), receptor-interacting protein-associated ICH-1/CED-3 homologous protein with a death domain (RAIDD), and caspase-2, the activation of which is driven by PIDDosome assembly. In addition to the key role of the PIDDosome in the regulation of cell differentiation, tissue homeostasis, and organogenesis and regeneration, caspase-2, RAIDD and PIDD1 engagement in neuronal development was shown. Here, we focus on the involvement of PIDDosome components in neurodegenerative disorders, including retinal neuropathies, different types of brain damage, and Alzheimer's disease (AD), Huntington's disease (HD), and Lewy body disease. We also discuss pathogenic variants of PIDD1, RAIDD, and caspase-2 that are associated with intellectual, behavioral, and psychological abnormalities, together with prospective PIDDosome inhibition strategies and their potential clinical application.

Research Progress on the Inflammatory Effects of Long Non-coding RNA in Traumatic Brain Injury

Globally, traumatic brain injury (TBI) is an acute clinical event and an important cause of death and long-term disability. However, the underlying mechanism of the pathophysiological has not been fully elucidated and the lack of effective treatment a huge burden to individuals, families, and society. Several studies have shown that long non-coding RNAs (lncRNAs) might play a crucial role in TBI; they are abundant in the central nervous system (CNS) and participate in a variety of pathophysiological processes, including oxidative stress, inflammation, apoptosis, blood-brain barrier protection, angiogenesis, and neurogenesis. Some lncRNAs modulate multiple therapeutic targets after TBI, including inflammation, thus, these lncRNAs have tremendous therapeutic potential for TBI, as they are promising biomarkers for TBI diagnosis, treatment, and prognosis prediction. This review discusses the differential expression of different lncRNAs in brain tissue during TBI, which is likely related to the physiological and pathological processes involved in TBI. These findings may provide new targets for further scientific research on the molecular mechanisms of TBI and potential therapeutic interventions.

Neat1 decreases neuronal apoptosis after oxygen and glucose deprivation

Studies have shown that downregulation of nuclear-enriched autosomal transcript 1 (Neat1) may adversely affect the recovery of nerve function and the increased loss of hippocampal neurons in mice. Whether Neat1 has protective or inhibitory effects on neuronal cell apoptosis after secondary brain injury remains unclear. Therefore, the effects of Neat1 on neuronal apoptosis were observed. C57BL/6 primary neurons were obtained from the cortices of newborn mice and cultured in vitro, and an oxygen and glucose deprivation cell model was established to simulate the secondary brain injury that occurs after traumatic brain injury in vitro. The level of Neat1 expression in neuronal cells was regulated by constructing a recombinant adenovirus to infect neurons, and the effects of Neat1 expression on neuronal apoptosis after oxygen and glucose deprivation were observed. The experiment was divided into four groups: the control group, without any treatment, received normal culture; the oxygen and glucose deprivation group were subjected to the oxygen and glucose deprivation model protocol; the Neat1 overexpression and Neat1 downregulation groups were treated with Neat1 expression intervention techniques and were subjected to the in oxygen and glucose deprivation protocol. The protein expression levels of neurons p53-induced death domain protein 1 (PIDD1, a pro-apoptotic protein), caspase-2 (an apoptotic priming protein), cytochrome C (a pro-apoptotic protein), and cleaved caspase-3 (an apoptotic executive protein) were measured in each group using the western blot assay. To observe changes in the intracellular distribution of cytochrome C, the expression levels of cytochrome C in the cytoplasm and mitochondria of neurons from each group were detected by western blot assay. Differences in the cell viability and apoptosis rate between groups were detected by cell-counting kit 8 assay and terminal deoxynucleotidyl transferase dUTP nick-end labeling assay, respectively. The results showed that the apoptosis rate, PIDD1, caspase-2, and cleaved caspase-3 expression levels significantly decreased, and cell viability significantly improved in the Neat1 overexpression group compared with the oxygen and glucose deprivation group; however, Neat1 downregulation reversed these changes. Compared with the Neat1 downregulation group, the cytosolic cytochrome C level in the Neat1 overexpression group significantly decreased, and the mitochondrial cytochrome C level significantly increased. These data indicate that Neat1 upregulation can reduce the release of cytochrome C from the mitochondria to the cytoplasm by inhibiting the PIDD1-caspase-2 pathway, reducing the activation of caspase-3, and preventing neuronal apoptosis after oxygen and glucose deprivation, which might reduce secondary brain injury after traumatic brain injury. All experiments were approved by the Animal Ethics Committee of the First Affiliated Hospital of Chongqing Medical University, China, on December 19, 2020 (approval No. 2020-895).

Radiotherapy-Induced Digestive Injury: Diagnosis, Treatment and Mechanisms

Radiotherapy is one of the main therapeutic methods for treating cancer. The digestive system consists of the gastrointestinal tract and the accessory organs of digestion (the tongue, salivary glands, pancreas, liver and gallbladder). The digestive system is easily impaired during radiotherapy, especially in thoracic and abdominal radiotherapy. In this review, we introduce the physical classification, basic pathogenesis, clinical characteristics, predictive/diagnostic factors, and possible treatment targets of radiotherapy-induced digestive injury. Radiotherapy-induced digestive injury complies with the dose-volume effect and has a radiation-based organ correlation. Computed tomography (CT), MRI (magnetic resonance imaging), ultrasound (US) and endoscopy can help diagnose and evaluate the radiation-induced lesion level. The latest treatment approaches include improvement in radiotherapy (such as shielding, hydrogel spacers and dose distribution), stem cell transplantation and drug administration. Gut microbiota modulation may become a novel approach to relieving radiogenic gastrointestinal syndrome. Finally, we summarized the possible mechanisms involved in treatment, but they remain varied. Radionuclide-labeled targeting molecules (RLTMs) are promising for more precise radiotherapy. These advances contribute to our understanding of the assessment and treatment of radiation-induced digestive injury.

The expression and role of PIDD in retina after optic nerve crush

To examine the expression of P53-induced protein with a death domain (PIDD) at retina in animal model of optic nerve crush (ONC) and to investigate the role of PIDD in retinal glial activation and NF-κB activation induced by optic nerve damage, ONC animal model was established in Sprague-Dawley rats. PIDD has three isoforms (Isof); Western blot was performed to examine the expression of PIDD (Isof-1, Isof-2, and Isof-3, respectively) in retina at different time points after ONC. Retinal glial activation is closely associated with retinal neuronal death and is monitored by the expression of GFAP+ glial cells and IBA1+ microglia, then activated microglia leads to inflammatory cytokine production. NF-kB activation in glial cells also can promote neuronal death. In our study, the role of PIDD in retinal glial activation and NF-kB activation was investigated with PIDD inhibition selectively. PIDD expression (Isof-1 and Isof-3) was dramatically increased, and peaked at 3 days after ONC, while Isof-2 did not show any difference. In the ONC animal model, the number of GFAP+ glial cells and IBA1+ microglia in retinal layers was increased significantly, inflammatory cytokine production was upregulated, and NF-κB in glial cell was also activated. Moreover, those responses induced by optic nerve damage were attenuated with PIDD inhibition, which indicated that PIDD could regulate retinal glial activation, neuro-inflammation, and NF-κB activation. These results provided the direct demonstration that the PIDD (Isof-1and Isof-3) was overexpressed in retina after ONC, and PIDD may be involved in retinal neurodegenerative diseases by regulating retinal glial activation and NF-κB activation.

Involvement of Upregulated P53-Induced Death Domain Protein in Retinal Ganglion Cells Apoptosis After Optic Nerve Crush

Purpose:

Retinal ganglion cells (RGCs) apoptosis is a common characteristic of optic neuropathies. p53-induced protein with a death domain (PIDD) is a well-known regulator of genotoxic stress-induced apoptosis, which is constitutively cleaved into three main fragments: PIDD-N, PIDD-C and PIDD-CC. Thus, we aim to determine the physiological relevance of PIDD in RGCs apoptosis in an optic nerve crush (ONC) model.

Methods:

All animals were evenly randomized into four groups: sham-control group, con-siRNA group, ONC group, and PIDD-siRNA group (ONC +PIDD-siRNA). Expressions of PIDD, caspase-2, Brn3a and tBid in ONC model were analyzed by Western blot and immunofluorescence. Mean densities of RGCs/mm2 were calculated with Fluoro-Gold (FG). Moreover, we tested the effect of PIDD-siRNA on ONC-induced RGCs apoptosis using TUNEL staining.

Results:

The level of full-length PIDD was weakly present and showed no significant differences at any time points. PIDD-CC and PIDD-C were significantly up-regulated in the retina at 3 days after ONC. Meanwhile, the expression of PIDD was significantly increased in Brn3a (a marker of RGCs) positive cells, indicating that the localization of PIDD appeared to be confined to RGCs. Furthermore, inhibition of PIDD prevented RGCs apoptosis by inhibiting caspase-2 and tBid activation.

Conclusions:

Taken together, PIDD may play a crucial role in RGCs apoptosis after ONC, and this process may be relevant to caspase-2 and tBid.

PIDD interaction with KEAP1 as a new mutation-independent mechanism to promote NRF2 stabilization and chemoresistance in NSCLC

Chemotherapy resistance is a major problem in non-small cell lung cancer (NSCLC) treatment. A major mechanism of chemoresistance involves stabilization of the NRF2 transcription factor. NRF2 levels are normally tightly regulated through interaction with KEAP1, an adaptor that targets NRF2 to the CUL3 E3 ubiquitin ligase for proteolysis. In NSCLC, aberrant NRF2 stabilization is best understood through mutations in NRF2, KEAP1, or CUL3 that disrupt their interaction. Biochemical studies, however, have revealed that NRF2 can also be stabilized through expression of KEAP1-interacting proteins that competitively sequester KEAP1 away from NRF2. Here, we have identified PIDD, as a novel KEAP1-interactor in NSCLC that regulates NRF2. We show that this interaction allows PIDD to reduce NRF2 ubiquitination and increase its stability. We also demonstrate that PIDD promotes chemoresistance in NSCLC cells both in vitro and in vivo, and that this effect is dependent on NRF2. Finally, we report that NRF2 protein expression in a NSCLC cohort exceeds the typical incidence of combined NRF2, KEAP1, and CUL3 mutations, and that NRF2 expression in this cohort is correlated with PIDD levels. Our data identify PIDD as a new NRF2 regulator, and suggest that variations in PIDD levels contribute to differential chemosensitivities among NSCLC patients.

E1A oncogene induced sensitization to NK cell induced apoptosis requires PIDD and Caspase-2

Expression of the adenovirus E1A oncogene sensitizes tumor cells to innate immune rejection by NK cells. This increased NK sensitivity is only partly explained by an E1A-induced increase in target cell surface expression of NKG2D ligands. The post-recognition mechanisms by which E1A sensitizes cells to the apoptotic cell death response to NK injury remains to be defined. E1A sensitizes cells to apoptotic stimuli through two distinct mechanisms-repression of NF-κB-dependent antiapoptotic responses and enhancement of caspase-2 activation and related mitochondrial injury. The current studies examined the roles of each of these post-NKG2D-recognition pathways in the increased sensitivity of E1A-positive target cells to NK killing. Sensitization to NK-induced apoptosis was independent of E1A-mediated repression of cellular NF-κB responses but was dependent on the expression of both caspase-2 and the upstream, caspase-2 activating molecule, PIDD. Target cells lacking caspase-2 or PIDD expression retained E1A-induced increased expression of the NKG2D ligand, RAE-1. NK cell-induced mitochondrial injury of E1A-expressing cells did not require expression of the mitochondrial molecules, Bak or Bax. These results define a PIDD/caspase-2-dependent pathway, through which E1A sensitizes cells to NK-mediated cytolysis independently of and complementarily to E1A-enhanced NKG2D/RAE-1 ligand expression.

The resurrection of the PIDDosome - emerging roles in the DNA-damage response and centrosome surveillance

The PIDDosome is often used as the alias for a multi-protein complex that includes the p53-induced death domain protein 1 (PIDD1), the bipartite linker protein CRADD (also known as RAIDD) and the pro-form of an endopeptidase belonging to the caspase family, i.e. caspase-2. Yet, PIDD1 variants can also interact with a number of other proteins that include RIPK1 (also known as RIP1) and IKBKG (also known as NEMO), PCNA and RFC5, as well as nucleolar components such as NPM1 or NCL. This promiscuity in protein binding is facilitated mainly by autoprocessing of the full-length protein into various fragments that contain different structural domains. As a result, multiple responses can be mediated by protein complexes that contain a PIDD1 domain. This suggests that PIDD1 acts as an integrator for multiple types of stress that need instant attention. Examples are various types of DNA lesion but also the presence of extra centrosomes that can foster aneuploidy and, ultimately, promote DNA damage. Here, we review the role of PIDD1 in response to DNA damage and also highlight novel functions of PIDD1, such as in centrosome surveillance and scheduled polyploidisation as part of a cellular differentiation program during organogenesis.

The long non-coding RNA Neat1 is an important mediator of the therapeutic effect of bexarotene on traumatic brain injury in mice

OBJECTIVE

Bexarotene treatments exert neuroprotective effects on mice following traumatic brain injury (TBI). The present study aims to investigate the potential roles of the long noncoding RNA Neat1 in the neuroprotective effects of bexarotene.

MATERIALS AND METHODS

Adult male C57BL/6J mice (n=80) and newborn mice (within 24h after birth) (n=20) were used to generate a "controlled cortical impact" (CCI) model and harvest primary cortex neurons, respectively. The HT22 cell line and the BV2 cell line were cultured under "normal" or "oxygen/glucose-deprived" (OGD) conditions. The relationship between RXR-α and the Neat1 promoter was clarified using ChIP-qPCR and dual-luciferase reporter gene assays. The mRNA alterations induced by Neat1 knockdown were measured using next-generation RNA sequencing. Proteins were captured by Neat1, pulled down and subjected to mass spectrometry. The neurological severity score, rotarod test and water maze test were employed to measure the animals' motor and cognitive functions.

RESULTS

Bexarotene prominently up-regulated the Neat1 level in an RXR-α-dependent manner. Neat1 knockdown induced significant changes in mRNA expression, and the altered mRNAs were involved in many biological processes, including synapse formation and axon guidance. In primary neurons, Neat1 knockdown inhibited and Neat1 over-expression prompted axon elongation. Multiple proteins, including Pidd1, were captured by Neat1. Neat1 inhibited cell apoptosis and restricted inflammation by capturing Pidd1. The in vitro anti-apoptotic and anti-inflammatory effects of Neat1 were further confirmed in C57BL/6 mice, which resulted in better motor and cognitive function after TBI.

CONCLUSION

Bexarotene up-regulates the lncRNA Neat1, which inhibits apoptosis and inflammation, thereby resulting in better functional recovery in mice after TBI.

The PIDDosome activates p53 in response to supernumerary centrosomes

In this study, Fava et al. show that an increase in the number of mature centrosomes (the main microtubule-organizing centers in animal cells), generated by disrupting cytokinesis or forcing centrosome overduplication, triggers the activation of the PIDDosome multiprotein complex, leading to Caspase-2-mediated MDM2 cleavage, p53 stabilization, and p21-dependent cell cycle arrest.

Centrosomes, the main microtubule-organizing centers in animal cells, are replicated exactly once during the cell division cycle to form the poles of the mitotic spindle. Supernumerary centrosomes can lead to aberrant cell division and have been causally linked to chromosomal instability and cancer. Here, we report that an increase in the number of mature centrosomes, generated by disrupting cytokinesis or forcing centrosome overduplication, triggers the activation of the PIDDosome multiprotein complex, leading to Caspase-2-mediated MDM2 cleavage, p53 stabilization, and p21-dependent cell cycle arrest. This pathway also restrains the extent of developmentally scheduled polyploidization by regulating p53 levels in hepatocytes during liver organogenesis. Taken together, the PIDDosome acts as a first barrier, engaging p53 to halt the proliferation of cells carrying more than one mature centrosome to maintain genome integrity.

NPM1 directs PIDDosome-dependent caspase-2 activation in the nucleolus

The PIDDosome (PIDD-RAIDD-caspase-2 complex) is considered to be the primary signaling platform for caspase-2 activation in response to genotoxic stress. Yet studies of PIDD-deficient mice show that caspase-2 activation can proceed in the absence of PIDD. Here we show that DNA damage induces the assembly of at least two distinct activation platforms for caspase-2: a cytoplasmic platform that is RAIDD dependent but PIDD independent, and a nucleolar platform that requires both PIDD and RAIDD. Furthermore, the nucleolar phosphoprotein nucleophosmin (NPM1) acts as a scaffold for PIDD and is essential for PIDDosome assembly in the nucleolus after DNA damage. Inhibition of NPM1 impairs caspase-2 processing, apoptosis, and caspase-2-dependent inhibition of cell growth, demonstrating that the NPM1-dependent nucleolar PIDDosome is a key initiator of the caspase-2 activation cascade. Thus we have identified the nucleolus as a novel site for caspase-2 activation and function.

Caspase 2 in mitotic catastrophe: The terminator of aneuploid and tetraploid cells

Mitotic catastrophe is an oncosuppressive mechanism that targets cells experiencing defective mitoses via the activation of specific cell cycle checkpoints, regulated cell death pathways and/or cell senescence. This prevents the accumulation of karyotypic aberrations, which otherwise may drive oncogenesis and tumor progression. Here, we summarize experimental evidence confirming the role of caspase 2 (CASP2) as the main executor of mitotic catastrophe, and we discuss the signals that activate CASP2 in the presence of mitotic aberrations. In addition, we summarize the main p53-dependent and -independent effector pathways through which CASP2 limits chromosomal instability and non-diploidy, hence mediating robust oncosuppressive functions.

p53 Modulates the Fate of Cardiac Progenitor Cells Ex Vivo and in the Diabetic Heart In Vivo

p53 is an important modulator of stem cell fate, but its role in cardiac progenitor cells (CPCs) is unknown. Here, we tested the effects of a single extra-copy of p53 on the function of CPCs in the presence of oxidative stress mediated by doxorubicin in vitro and type-1 diabetes in vivo. CPCs were obtained from super-p53 transgenic mice (p53-tg), in which the additional allele is regulated in a manner similar to the endogenous protein. Old CPCs with increased p53 dosage showed a superior ability to sustain oxidative stress, repair DNA damage and restore cell division. With doxorubicin, a larger fraction of CPCs carrying an extra-copy of the p53 allele recruited γH2A.X reestablishing DNA integrity. Enhanced p53 expression resulted in a superior tolerance to oxidative stress in vivo by providing CPCs with defense mechanisms necessary to survive in the milieu of the diabetic heart; they engrafted in regions of tissue injury and in three days acquired the cardiomyocyte phenotype. The biological advantage provided by the increased dosage of p53 in CPCs suggests that this genetic strategy may be translated to humans to increase cellular engraftment and growth, critical determinants of successful cell therapy for the failing heart.

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p53 improves the ability of CPCs to sustain oxidative stress.

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p53 promotes the restoration of DNA integrity and cell division.

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p53 enhances the engraftment of CPCs in the diabetic heart.

Ongoing clinical trials with autologous cardiac stem cells (CSCs) are faced with a critical limitation which is related to the modest amount of retained cells within the damaged myocardium. We have developed a strategy that overcomes in part this problem enhancing the number of CSCs able to engraft within the pathologic organ. Additionally, these genetically modified CSCs can be generated in large number, raising the possibility that multiple temporally distinct deliveries of cells can be introduced to restore the structural and functional integrity of the decompensated heart.

Gambogic acid inhibits the growth of ovarian cancer tumors by regulating p65 activity

Ovarian cancer patients often have poor prognosis, therefore, it is important to search for more effective therapeutic strategies to treat them. Gambogic acid (GA) exhibits an anti-tumor effect through various mechanisms, and has multiple targets in tumor cells. The present study aimed to elucidate the efficacy of GA in the treatment of ovarian cancer both in vivo and in vitro by analyzing its impact on cell survival and tumor growth through cell cycle and apoptosis analysis. GA inhibited the growth of ovarian cancer cells in a dose and time dependent manner, and arrested the cell cycle in ovarian cancer cells. Furthermore, GA increased caspase-3 and caspase-9 activity and inhibited RELA/NF-κB p65 (p65) DNA binding activity. Finally, GA suppressed tumor growth in vivo. Therefore, the current study suggests that GA inhibits the growth of ovarian cancer by regulating p65 activity, and may be developed as a novel therapeutic strategy to treat ovarian cancer.

Inhibition of NF-κB by deoxycholic acid induces miR-21/PDCD4-dependent hepatocellular apoptosis

MicroRNAs (miRNAs/miRs) are key regulators of liver metabolism, while toxic bile acids participate in the development of several liver diseases. We previously demonstrated that deoxycholic acid (DCA), a cytotoxic bile acid implicated in the pathogenesis of non-alcoholic fatty liver disease, inhibits miR-21 expression in hepatocytes. Here, we investigated the mechanisms by which DCA modulates miR-21 and whether miR-21 contributes for DCA-induced cytotoxicity. DCA inhibited miR-21 expression in primary rat hepatocytes in a dose-dependent manner, and increased miR-21 pro-apoptotic target programmed cell death 4 (PDCD4) and apoptosis. Both miR-21 overexpression and PDCD4 silencing hampered DCA-induced cell death. Further, DCA decreased NF-κB activity, shown to represent an upstream mechanism leading to modulation of the miR-21/PDCD4 pathway. In fact, NF-κB overexpression or constitutive activation halted miR-21-dependent apoptosis by DCA while opposite results were observed upon NF-κB inhibition. In turn, DCA-induced oxidative stress resulted in caspase-2 activation and NF-κB/miR-21 inhibition, in a PIDD-dependent manner. Finally, modulation of the NF-κB/miR-21/PDCD4 pro-apoptotic pathway by DCA was also shown to occur in the rat liver in vivo. These signalling circuits may constitute appealing targets for bile acid-associated liver pathologies.

An Inhibitor of PIDDosome Formation

The PIDDosome-PIDD-RAIDD-caspase-2 complex-is a proapoptotic caspase-activation platform of elusive significance. DNA damage can initiate complex assembly via ATM phosphorylation of the PIDD death domain (DD), which enables RAIDD recruitment to PIDD. In contrast, the mechanisms limiting PIDDosome formation have remained unclear. We identify the mitotic checkpoint factor BubR1 as a direct PIDDosome inhibitor, acting in a noncanonical role independent of Mad2. Following its phosphorylation by ATM at DNA breaks, "primed" PIDD relocates to kinetochores via a direct interaction with BubR1. BubR1 binds the PIDD DD, competes with RAIDD recruitment, and negates PIDDosome-mediated apoptosis after ionizing radiation. The PIDDosome thus sequentially integrates DNA damage and mitotic checkpoint signals to decide cell fate in response to genotoxic stress. We further show that by sequestering PIDD at the kinetochore, BubR1 acts to delay PIDDosome formation until the next cycle, defining a new mechanism by which cells evade apoptosis during mitosis.

The tumor-modulatory effects of Caspase-2 and Pidd1 do not require the scaffold protein Raidd

The receptor-interacting protein-associated ICH-1/CED-3 homologous protein with a death domain (RAIDD/CRADD) functions as a dual adaptor and is a constituent of different multi-protein complexes implicated in the regulation of inflammation and cell death. Within the PIDDosome complex, RAIDD connects the cell death-related protease, Caspase-2, with the p53-induced protein with a death domain 1 (PIDD1). As such, RAIDD has been implicated in DNA-damage-induced apoptosis as well as in tumorigenesis. As loss of Caspase-2 leads to an acceleration of tumor onset in the Eμ-Myc mouse lymphoma model, whereas loss of Pidd1 actually delays onset of this disease, we set out to interrogate the role of Raidd in cancer in more detail. Our data obtained analyzing Eμ-Myc/Raidd^−/− mice indicate that Raidd is unable to protect from c-Myc-driven lymphomagenesis. Similarly, we failed to observe a modulatory effect of Raidd deficiency on DNA-damage-driven cancer. The role of Caspase-2 as a tumor suppressor and that of Pidd1 as a tumor promoter can therefore be uncoupled from their ability to interact with the Raidd scaffold, pointing toward the existence of alternative signaling modules engaging these two proteins in this context.

The apoptotic initiator caspase-8: its functional ubiquity and genetic diversity during animal evolution

The caspases, a family of cysteine proteases, play multiple roles in apoptosis, inflammation, and cellular differentiation. Caspase-8 (Casp8), which was first identified in humans, functions as an initiator caspase in the apoptotic signaling mediated by cell-surface death receptors. To understand the evolution of function in the Casp8 protein family, casp8 orthologs were identified from a comprehensive range of vertebrates and invertebrates, including sponges and cnidarians, and characterized at both the gene and protein levels. Some introns have been conserved from cnidarians to mammals, but both losses and gains have also occurred; a new intron arose during teleost evolution, whereas in the ascidian Ciona intestinalis, the casp8 gene is intronless and is organized in an operon with a neighboring gene. Casp8 activities are near ubiquitous throughout the animal kingdom. Exogenous expression of a representative range of nonmammalian Casp8 proteins in cultured mammalian cells induced cell death, implying that these proteins possess proapoptotic activity. The cnidarian Casp8 proteins differ considerably from their bilaterian counterparts in terms of amino acid residues in the catalytic pocket, but display the same substrate specificity as human CASP8, highlighting the complexity of spatial structural interactions involved in enzymatic activity. Finally, it was confirmed that the interaction with an adaptor molecule, Fas-associated death domain protein, is also evolutionarily ancient. Thus, despite structural diversity and cooption to a variety of new functions, the ancient origins and near ubiquitous distribution of this activity across the animal kingdom emphasize the importance and utility of Casp8 as a central component of the metazoan molecular toolkit.

Death of p53-defective cells triggered by forced mitotic entry in the presence of DNA damage is not uniquely dependent on Caspase-2 or the PIDDosome

Much effort has been put in the discovery of ways to selectively kill p53-deficient tumor cells and targeting cell cycle checkpoint pathways has revealed promising candidates. Studies in zebrafish and human cell lines suggested that the DNA damage response kinase, checkpoint kinase 1 (Chk1), not only regulates onset of mitosis but also cell death in response to DNA damage in the absence of p53. This effect reportedly relies on ataxia telangiectasia mutated (ATM)-dependent and PIDDosome-mediated activation of Caspase-2. However, we show that genetic ablation of PIDDosome components in mice does not affect cell death in response to γ-irradiation. Furthermore, Chk1 inhibition largely failed to sensitize normal and malignant cells from p53^−/− mice toward DNA damaging agents, and p53 status did not affect the death-inducing activity of DNA damage after Chk1 inhibition in human cancer cells. These observations argue against cross-species conservation of a Chk1-controlled cell survival pathway demanding further investigation of the molecular machinery responsible for cell death elicited by forced mitotic entry in the presence of DNA damage in different cell types and model organisms.

Loss of PIDD limits NF-κB activation and cytokine production but not cell survival or transformation after DNA damage

Activation of NF-κB (nuclear factor of kappa light chain gene enhancer in B cells) in response to DNA damage is considered to contribute to repair of genetic lesions, increased cell survival and cytokine release. The molecular mechanisms orchestrating this cytoplasmic event involve core components of the nuclear DNA damage response machinery, including ATM-kinase (ataxia telangiectasia mutated kinase) and PARP-1 (poly (ADP-ribose) polymerase 1). The physiological consequences of defective NF-κB activation in this context, however, remain poorly investigated. Here we report on the role of the 'p53-induced protein with a death domain', PIDD, which appears rate limiting in this process, as is PARP-1. Despite impaired NF-κB activation, DNA damage did not increase cell death or reduce clonal survival of various cell types lacking PIDD, such as mouse embryonic fibroblasts or stem and progenitor cells of the hematopoietic system. Furthermore, lymphomagenesis induced by γ-irradiation (IR) was unaffected by deficiency for PIDD or PARP-1, indicating that loss of DNA damage-triggered NF-κB signalling does not affect IR-driven tumorigenesis. However, loss of either gene compromised cytokine release after acute IR injury. Hence, we propose that NF-κB's most notable function after DNA damage in primary cells is related to the release of cytokines, thereby contributing to sterile inflammation.

Live or let die: manipulation of cellular suicide programs by murine cytomegalovirus

Cytomegaloviruses (CMVs) are large double-stranded DNA viruses that replicate slowly and cause life-long persisting infections in their hosts. To achieve this, the CMVs had to evolve numerous countermeasures against innate and adaptive immune responses. Induction of programmed cell death is one important host defense mechanism against intracellular pathogens such as viruses. For a multicellular organism, it is advantageous to let infected cells die in order to thwart viral replication and dissemination. For a virus, by contrast, it is better to inhibit cell death and keep infected cells alive until the viral replication cycle has been completed. As a matter of fact, the CMVs encode a number of proteins devoted to interfering with different forms of programmed cell death: apoptosis and necroptosis. In this review, we summarize the known functions of the four best characterized cell death inhibitors of murine cytomegalovirus (MCMV), which are encoded by open reading frames, M36, m38.5, m41.1, and M45. The viral proteins interact with key molecules within different cell death pathways, namely caspase-8, Bax, Bak, and RIP1/RIP3. In addition, we discuss which events during MCMV infection might trigger apoptosis or necrosis and how MCMV's countermeasures compare to those of other herpesviruses. Since both, MCMV and its natural host, are amenable to genetic manipulation, the mouse model for CMV infection provides a particularly suitable system to study mechanisms of cell death induction and inhibition.