Association of caspase-2 with the promyelocytic leukemia protein nuclear bodies

Apoptotic cell death is executed by a family of cysteine proteases known as caspases. Synthesized as inactive precursors, caspases become activated sequentially in cascades. Activation of apical or initiator caspases in these cascades occurs in macromolecular complexes located in various compartments. One such complex is the plasma membrane-bound death-inducing signaling complex (DISC), formed upon engagement of death receptors, which recruits and activates caspase-8 and -10. Another complex is the cytosolic apoptosome, assembled in response to the release of mitochondrial cytochrome c, which recruits caspase-9. The other major human initiator caspase is caspase-2, which is activated in response to various lethal stimuli and has recently been shown to be required for DNA damage-induced apoptosis. The regulation of caspase-2 is not well understood. Here we present evidence that caspase-2 is localized to the promyelocytic leukemia protein nuclear bodies (PML-NBs), nuclear macro-molecular complexes that are involved in many scenarios of apoptosis including DNA damage. The localization of caspase-2 requires both the prodomain and protease domain but appears to be independent of its adaptor protein, CRADD/RAIDD. These data suggest the existence of a nuclear apoptosis pathway that involves both caspase-2 and the PML-NBs.

Genetic mapping and exome sequencing identify variants associated with five novel diseases

The Clinic for Special Children (CSC) has integrated biochemical and molecular methods into a rural pediatric practice serving Old Order Amish and Mennonite (Plain) children. Among the Plain people, we have used single nucleotide polymorphism (SNP) microarrays to genetically map recessive disorders to large autozygous haplotype blocks (mean = 4.4 Mb) that contain many genes (mean = 79). For some, uninformative mapping or large gene lists preclude disease-gene identification by Sanger sequencing. Seven such conditions were selected for exome sequencing at the Broad Institute; all had been previously mapped at the CSC using low density SNP microarrays coupled with autozygosity and linkage analyses. Using between 1 and 5 patient samples per disorder, we identified sequence variants in the known disease-causing genes SLC6A3 and FLVCR1, and present evidence to strongly support the pathogenicity of variants identified in TUBGCP6, BRAT1, SNIP1, CRADD, and HARS. Our results reveal the power of coupling new genotyping technologies to population-specific genetic knowledge and robust clinical data.

Are promyelocytic leukaemia protein nuclear bodies a scaffold for caspase-2 programmed cell death?

Promyelocytic leukaemia protein nuclear bodies (PML-NBs) are nuclear structures whose function is still poorly understood. They are implicated in various biological functions, such as viral infection, cellular transformation, innate immunity and growth control, and they might be dynamic hubs sensing stress and DNA damage. Data from PML(-/-) mice suggest that PML-NBs are involved in apoptosis via caspase-independent mechanisms, probably involving p53-dependent and independent pathways. However, the recently demonstrated co-localization of caspase-2 within the PML-NB nuclear structures presents a new paradigm for nuclear cell death. Here, we show that these nuclear structures have a protein known as SP100 that could contain a caspase recruitment domain (CARD). If verified experimentally, this discovery will suggest a mechanism by which caspase-2 could be recruited into the complex and ultimately lead to apoptosis.

Heat Shock Induces Apoptosis Independently of Any Known Initiator Caspase-activating Complex

Adaptive responses to mild heat shock are among the most widely conserved and studied in nature. More intense heat shock, however, induces apoptosis through mechanisms that remain largely unknown. Herein, we present evidence that heat shock activates an apical protease that stimulates mitochondrial outer membrane permeabilization and processing of the effector caspase-3 in a benzyloxycarbonyl-VAD-fluoromethyl ketone (polycaspase inhibitor)- and Bcl-2-inhibitable manner. Surprisingly, however, neither FADD.caspase-8 nor RAIDD.caspase-2 PIDDosome (p53-induced protein with a death domain) complexes were detected in dying cells, and neither of these initiator caspases nor the endoplasmic reticulum stress-activated caspases-4/12 were required for mitochondrial outer membrane permeabilization. Similarly, although cytochrome c was released from mitochondria following heat shock, functional Apaf-1.caspase-9 apoptosome complexes were not formed, and caspase-9 was not essential for the activation of caspase-3 or the induction of apoptosis. Thus, heat shock does not require any of the known initiator caspases or their activating complexes to promote apoptotic cell death but instead relies upon the activation of an apparently novel apical protease with caspase-like activity.

Identification of novel genes with prognostic value in childhood leukemia using cDNA microarray and quantitative RT-PCR

The aim of this study was to identify genes distinctively expressed or suppressed in childhood leukemia with different prognoses, using cDNA microarray and quantitative reverse transcription-polymerase chain reaction (RT-PCR). The expression levels of the selected genes by cDNA microarray were quantified in primary leukemic blasts from 44 patients (acute lymphoblastic leukemia, 28; acute myelogenous leukemia (AML), 13; transient myeloproliferative disorder, 3). The expression levels of CDKN2C, CRADD, and IGFBP-2 genes were significantly associated with the event-free survival of the patients in AML. The present results suggest that a combination of cDNA microarray and quantitative RT-PCR may be useful to identify novel genes with prognostic value in childhood AML.

PIDD mediates NF-kappaB activation in response to DNA damage

Activation of NF-kappaB following genotoxic stress allows time for DNA-damage repair and ensures cell survival accounting for acquired chemoresistance, an impediment to effective cancer therapy. Despite this clinical relevance, little is known about pathways that enable genotoxic-stress-induced NF-kappaB induction. Previously, we reported a role for the p53-inducible death-domain-containing protein, PIDD, in caspase-2 activation and apoptosis in response to DNA damage. We now demonstrate that PIDD plays a critical role in DNA-damage-induced NF-kappaB activation. Upon genotoxic stress, a complex between PIDD, the kinase RIP1, and a component of the NF-kappaB-activating kinase complex, NEMO, is formed. PIDD expression enhances genotoxic-stress-induced NF-kappaB activation through augmented sumoylation and ubiquitination of NEMO. Depletion of PIDD and RIP1, but not caspase-2, abrogates DNA-damage-induced NEMO modification and NF-kappaB activation. We propose that PIDD acts as a molecular switch, controlling the balance between life and death upon DNA damage.

Crystal structure of RAIDD death domain implicates potential mechanism of PIDDosome assembly

Caspase-2 is implicated in stress-induced apoptosis that acts as an upstream initiator of mitochondrial permeabilization. Recent studies have shown that caspase-2 activation requires a molecular complex known as the PIDDosome comprising the p53-inducible protein PIDD, the adapter protein RAIDD and caspase-2. RAIDD has an N-terminal caspase recruitment domain (CARD) that interacts with the CARD of caspase-2 and a C-terminal death domain (DD) that interacts with the DD in PIDD. As a first step towards elucidating the molecular mechanisms of caspase-2 activation, we report the crystal structure of RAIDD DD at 2.0 A resolution. The high-resolution structure reveals important features of RAIDD DD that may be important for DD folding and dynamics and for assembly of the PIDDosome.

In situ trapping of activated initiator caspases reveals a role for caspase-2 in heat shock-induced apoptosis

Activation of 'initiator' (or 'apical') caspases-2, -8 or -9 (refs 1-3) is crucial for induction of apoptosis. These caspases function to activate executioner caspapses that, in turn, orchestrate apoptotic cell death. Here, we show that a cell-permeable, biotinylated pan-caspase inhibitor (bVAD-fmk) both inhibited and 'trapped' the apical caspase activated when apoptosis was triggered. As expected, only caspase-8 was trapped in response to ligation of death receptors, whereas only caspase-9 was trapped in response to a variety of other apoptosis-inducing agents. Caspase-2 was exclusively activated in heat shock-induced apoptosis. This activation of caspase-2 was also observed in cells protected from heat-shock-induced apoptosis by Bcl-2 or Bcl-xL. Reduced sensitivity to heat-shock-induced death was observed in caspase-2(-/-) cells. Furthermore, cells lacking the adapter molecule RAIDD failed to activate caspase-2 after heat shock treatment and showed resistance to apoptosis in this setting. This approach unambiguously identifies the apical caspase activated in response to apoptotic stimuli, and establishes caspase-2 as a proximal mediator of heat shock-induced apoptosis.

Apoptosis caused by p53-induced protein with death domain (PIDD) depends on the death adapter protein RAIDD

The p53 tumor suppressor promotes cell cycle arrest or apoptosis in response to diverse stress stimuli. p53-mediated cell death depends in large part on transcriptional up-regulation of target genes. One of these targets, P53-induced protein with a death domain (PIDD), was shown to function as a mediator of p53-dependent apoptosis. Here we show that PIDD is a cytoplasmic protein, and that PIDD-induced apoptosis and growth suppression in embryonic fibroblasts depend on the adaptor protein receptor-interacting protein (RIP)-associated ICH-1/CED-3 homologous protein with a death domain (RAIDD). We provide evidence that PIDD-induced cell death is associated with the early activation of caspase-2 and later activation of caspase-3 and -7. Our results also show that caspase-2(-/-), in contrast to RAIDD(-/-), mouse embryonic fibroblasts, are only partially resistant to PIDD. Our findings suggest that caspase-2 contributes to PIDD-mediated cell death, but that it is not the sole effector of this pathway.

In situ analysis of Raidd-beta-galactosidase fusion gene expression in transgenic mouse midgestation embryos

Apoptosis and differentiation are tightly intertwined processes occurring at organ formation and remodelling during embryonic development. RAIDD (receptor-interacting protein [RIP]-associated ICH-1/CED-3-homologous protein with a death domain), a dual-domain adaptor protein has been shown to mediate the recruitment of CASPASE-2 to tumour necrosis factor receptor-1 (TNF-R1) signalling complex through RIP kinase. However, Raidd overexpression studies suggest that apart from the established role in apoptosis, Raidd may have an additional function in cell differentiation. In this study, we could not generate Raidd null adult mice suggesting that lack of function of Raidd might be embryonic lethal. Thus, to elucidate the role of Raidd during mouse embryogenesis when the processes of organogenesis are most dynamic, we studied the Raidd expression pattern in midgestation mouse embryos. We generated Raidd+/- transgenic mice with a reporter transgene encoding the bacterial Beta-galactosidase (beta-gal) under the control of Raidd promoter. During the midgestation period (E8.5-E12.5), Raidd is expressed in developing organs derived from the ectoderm such as lens, structures of the inner ear and the fourth brain ventricle in regions where differentiation takes place implicating Raidd role in this process. In addition, Raidd expression was found in developing mesenchyme organs like heart and kidney and in the endothelial lining of the midgut at the time when profound morphological changes take place in these organs. In developing heart and kidney Raidd expression patterns overlapped with known zones of cell death suggesting Raidd may be involved in apoptosis-mediated remodelling. The observed lethality of mice targeted at both Raidd alleles and Raidd expression patterns during midgestation period strongly suggest that Raidd plays an important role in mammalian development.

RAIDD is required for apoptosis of PC12 cells and sympathetic neurons induced by trophic factor withdrawal

Caspase 2 has been implicated in trophic deprivation-induced neuronal death. We have shown that overexpression of the caspase 2-binding protein RAIDD induces neuronal apoptosis, acting synergistically with trophic deprivation. Currently, we examine the role of endogenous RAIDD in apoptosis of PC12 cells and sympathetic neurons. Expression of a truncated caspase recruitment domain-only form of caspase 2, which presumably disrupts the RAIDD interaction with endogenous caspase 2, attenuated trophic deprivation-induced apoptosis. Furthermore, downregulation of RAIDD by small interfering RNA led to inhibition of trophic deprivation-induced death, whereas death induced by DNA damage, which is not caspase 2-mediated, was not inhibited. Therefore, RAIDD, likely through interaction with caspase 2, is involved in trophic deprivation-induced neuronal apoptosis. This is the first demonstration of the involvement of RAIDD in apoptosis, and provides further support for the idea that apoptotic pathways in the same system may differ depending on the initiating stimulus.

The PIDDosome, a protein complex implicated in activation of caspase-2 in response to genotoxic stress

Apoptosis is triggered by activation of initiator caspases upon complex-mediated clustering of the inactive zymogen, as occurs in the caspase-9-activating apoptosome complex. Likewise, caspase-2, which is involved in stress-induced apoptosis, is recruited into a large protein complex, the molecular composition of which remains elusive. We show that activation of caspase-2 occurs in a complex that contains the death domain-containing protein PIDD, whose expression is induced by p53, and the adaptor protein RAIDD. Increased PIDD expression resulted in spontaneous activation of caspase-2 and sensitization to apoptosis by genotoxic stimuli. Because PIDD functions in p53-mediated apoptosis, the complex assembled by PIDD and caspase-2 is likely to regulate apoptosis induced by genotoxins.

RAIDD aggregation facilitates apoptotic death of PC12 cells and sympathetic neurons

In human cell lines, the caspase 2 adaptor RAIDD interacts selectively with caspase 2 through its caspase recruitment domain (CARD) and leads to caspase 2-dependent death. Whether RAIDD induces such effects in neuronal cells is unknown. We have previously shown that caspase 2 is essential for apoptosis of trophic factor-deprived PC12 cells and rat sympathetic neurons. We report here that rat RAIDD, cloned from PC12 cells, interacts with rat caspase 2 CARD. RAIDD overexpression induced caspase 2 CARD- and caspase 9-dependent apoptosis of PC12 cells and sympathetic neurons. Apoptosis correlated with the formation of discrete perinuclear aggregates. Both death and aggregates required the expression of full-length RAIDD. Such aggregates may enable more effective activation of caspase 2 through close proximity. Following trophic deprivation, RAIDD overexpression increased death and aggregate formation. Therefore, RAIDD aggregation is important for its death-promoting effects and may play a role in trophic factor withdrawal-induced neuronal apoptosis.

Structural characterization of the mouse high growth deletion and discovery of a novel fusion transcript between suppressor of cytokine signaling-2 (Socs-2) and viral encoded semaphorin receptor (Plexin C1)

The high growth (HG) mouse mutation is a 460 Kb deletion of chromosome 10 which causes a 30-50% increase in growth in the homozygous animal. We have shotgun sequenced six bacterial artificial chromosomes which span the length of the deletion to an average depth of 13.2x to generate a 649,868 bp sequence. Sequence analysis revealed the presence of three genes, suppressor of cytokine signaling-2 (Socs-2), caspase and RIP adaptor with death domain (Raidd/Cradd), and viral encoded semaphorin receptor (Plexin C1, viral encoded semaphorin receptor). The two deletion breakpoints lie in within the second introns of both Socs-2 and Plexin C1, resulting in the formation of a novel expressed fusion transcript between Socs-2 and Plexin C1 in HG mice. Expression of the fusion transcript, the presence of four splice variants of Raidd/Cradd and the exon structure of Socs-2 were illustrated using polymerase chain reaction. Genomic comparisons of the mouse and human sequence were used to verify the sequence assembly.

Delineation of RAID1, the RACK1 interaction domain located within the unique N-terminal region of the cAMP-specific phosphodiesterase, PDE4D5

BACKGROUND

The cyclic AMP specific phosphodiesterase, PDE4D5 interacts with the beta-propeller protein RACK1 to form a signaling scaffold complex in cells. Two-hybrid analysis of truncation and mutant constructs of the unique N-terminal region of the cAMP-specific phosphodiesterase, PDE4D5 were used to define a domain conferring interaction with the signaling scaffold protein, RACK1.

RESULTS

Truncation and mutagenesis approaches showed that the RACK1-interacting domain on PDE4D5 comprised a cluster of residues provided by Asn-22/Pro-23/Trp-24/Asn-26 together with a series of hydrophobic amino acids, namely Leu-29, Val-30, Leu-33, Leu-37 and Leu-38 in a 'Leu-Xaa-Xaa-Xaa-Leu' repeat. This was done by 2-hybrid analyses and then confirmed in biochemical pull down analyses using GST-RACK1 and mutant PDE4D5 forms expressed in COS cells. Mutation of Arg-34, to alanine, in PDE4D5 attenuated its interaction with RACK1 both in 2-hybrid screens and in pull down analyses. A 38-mer peptide, whose sequence reflected residues 12 through 49 of PDE4D5, bound to RACK1 with similar affinity to native PDE4D5 itself (Ka circa 6 nM).

CONCLUSIONS

The RACK1 Interaction Domain on PDE4D5, that we here call RAID1, is proposed to form an amphipathic helical structure that we suggest may interact with the C-terminal beta-propeller blades of RACK1 in a manner akin to the interaction of the helical G-gamma signal transducing protein with the beta-propeller protein, G-beta.

Caspase-2 induces apoptosis by releasing proapoptotic proteins from mitochondria

Caspase-2 is one of the earliest identified caspases, but the mechanism of caspase-2-induced apoptosis remains unknown. We show here that caspase-2 engages the mitochondria-dependent apoptotic pathway by inducing the release of cytochrome c (Cyt c) and other mitochondrial apoptogenic factors into the cell cytoplasm. In support of these observations we found that Bcl-2 and Bcl-xL can block caspase-2- and CRADD (caspase and RIP adaptor with death domain)-induced cell death. Unlike caspase-8, which can process all known caspase zymogens directly, caspase-2 is completely inactive toward other caspase zymogens. However, like caspase-8, physiological levels of purified caspase-2 can cleave cytosolic Bid protein, which in turn can trigger the release of Cyt c from isolated mitochondria. Interestingly, caspase-2 can also induce directly the release of Cyt c, AIF (apoptosis-inducing factor), and Smac (second mitochondria-derived activator of caspases protein) from isolated mitochondria independent of Bid or other cytosolic factors. The caspase-2-released Cyt c is sufficient to activate the Apaf-caspase-9 apoptosome in vitro. In combination, our data suggest that caspase-2 is a direct effector of the mitochondrial apoptotic pathway.

A 500-kb YAC and BAC contig encompassing the high-growth deletion in mouse chromosome 10 and identification of the murine Raidd/Cradd gene in the candidate region

The mouse high growth (hg) gene was identified in a selection experiment for rapid growth. It produces a 30-50% increase in weight gain of homozygous individuals without resulting in obesity. Recently, hg was mapped to a deletion around marker D10Mit69. Here we report a map of yeast artificial chromosome and bacterial artificial chromosome clones spanning the entire region deleted in high-growth mice. The size of the deletion estimated from the clone lengths is approximately 500 kb. Using exon trapping, the murine Raidd/Cradd gene was identified in the hg region, and its cDNA sequence and expression pattern were determined. The human RAIDD/CRADD was mapped using radiation hybrid mapping to human chromosome 12, 2.9 cR distal to marker AFMB311WC5. The identified Raidd/Cradd gene, which is deleted in high-growth mice, presents a potential candidate for hg.

Death-inducing functions of ligands of the tumor necrosis factor family: a Sanhedrin verdict

Members of the tumor necrosis factor ligand family can kill cells in a rather straightforward manner. They induce their receptors to recruit and activate caspases, enzymes that are critically involved in the death process, and this activation is further amplified by intracellular mitochondria-associated mechanisms. The potentially hazardous expression of the ligands occurs widely in the body; it is antigen-restricted only in the lymphocytes. Yet, in addition to control modes affecting ligand expression, there are numerous inhibitory mechanisms that act within target cells, to make doubly sure of avoiding an undue 'death verdict', while allowing the cells to exhibit other, noncytocidal effects of the ligands.

CRADD, a novel human apoptotic adaptor molecule for caspase-2, and FasL/tumor necrosis factor receptor-interacting protein RIP

FADD/MORT1 is a death domain (DD)-containing adaptor/signaling molecule that interacts with the intracellular DD of FAS/APO-I (CD95) and tumor necrosis factor receptor 1 and the prodomain of caspase-8 (Mch5/MACH/FLICE). FADD engagement of caspase-8 presumably activates this caspase and leads to apoptosis. Another DD-containing adaptor/signaling molecule, CRADD, was identified and was shown to induce apoptosis. CRADD has a dual-domain structure similar to that of FADD. It has an NH2-terminal caspase homology domain that interacts with caspase-2 and a COOH-terminal DD that interacts with RIP. CRADD is constitutively expressed in many tissues and thus could play a role in regulating apoptosis in mammalian cells.