Baculovirus apoptotic suppressor P49 is a substrate inhibitor of initiator caspases resistant to P35 in vivo

Caspases play a critical role in the execution of metazoan apoptosis and are thus attractive therapeutic targets for apoptosis-associated diseases. Here we report that baculovirus P49, a homolog of pancaspase inhibitor P35, prevents apoptosis in invertebrates by inhibiting an initiator caspase that is P35 insensitive. Consequently P49 blocked proteolytic activation of effector caspases at a unique step upstream from that affected by P35 but downstream from inhibitor of apoptosis Op-IAP. Like P35, P49 was cleaved by and stably associated with its caspase target. Ectopically expressed P49 blocked apoptosis in cultured cells from a phylogenetically distinct organism, Drosophila melanogaster. Furthermore, P49 inhibited human caspase-9, demonstrating its capacity to affect a vertebrate initiator caspase. Thus, P49 is a substrate inhibitor with a novel in vivo specificity for a P35-insensitive initiator caspase that functions at an evolutionarily conserved step in the caspase cascade. These data indicate that activated initiator caspases provide another effective target for apoptotic intervention by substrate inhibitors.     

Proapoptotic BAX and BAK control multiple initiator caspases

BAX and BAK operate at both the mitochondria and endoplasmic reticulum (ER) to regulate the intrinsic apoptotic pathway. An unresolved issue is whether any caspases can be activated in response to intrinsic apoptotic signals in the absence of BAX and BAK. Following organelle-specific intrinsic stress signals, including DNA damage and ER stress, we detected no activation of CARD-containing caspases (initiator CASP)-1, -2, -9, -11 and -12 in Bax(-/-)Bak(-/-) doubly deficient (DKO) cells. BCL-2 overexpression in these DKO cells provided no further protection to their already strong protection from DNA damage and ER stress. Moreover, there was no activation of effector CASP-3 and -7 in DKO cells, consistent with the lack of initiator caspase activity and disfavouring a BAX, BAK-independent intrinsic apoptotic pathway to activate initiator caspases. Thus, BAX and BAK confer an essential gateway for the activation of caspases in the intrinsic apoptotic pathway.     

Differential involvement of initiator caspases in apoptotic volume decrease and potassium efflux during Fas- and UV-induced cell death.

Caspase activation and apoptotic volume decrease are fundamental features of programmed cell death; however, the relationship between these components is not well understood. Here we provide biochemical and genetic evidence for the differential involvement of initiator caspases in the apoptotic volume decrease during both intrinsic and extrinsic activation of apoptosis. Apoptosis induction in Jurkat T lymphocytes by Fas receptor engagement (intrinsic) or ultraviolet (UV)-C radiation (extrinsic) triggered the loss of cell volume, which was restricted to cells with diminished intracellular K(+) ions. These characteristics kinetically coincided with the proteolytic processing and activation of both initiator and effector caspases. Although the polycaspase inhibitor benzyloxycarbonyl-Val-Ala-Asp fluoromethyl ketone completely inhibited the Fas-mediated apoptotic volume decrease and K(+) efflux, it was much less effective in preventing these processes during UV-induced cell death under conditions whereby caspase activities and DNA degradation were blocked. To define the roles of specific initiator caspases, we utilized Jurkat cells genetically deficient in caspase-8 or stably transfected with a dominant-negative mutant of caspase-9. The results show that the activation of caspase-8, but not caspase-9, is necessary for Fas-induced apoptosis. Conversely, caspase-9, but not caspase-8, is important for UV-mediated shrunken morphology and apoptosis progression. Together, these findings indicate that cell shrinkage and K(+) efflux during apoptosis are tightly coupled, but are differentially regulated by either caspase-8 or caspase-9 depending on specific pathways of cell death.     

Activation of initiator caspases through a stable dimeric intermediate

Structural and biochemical studies have revealed that procaspases form dimers prior to proteolytic activation. How the two procaspases interact in the dimer is unclear. To study the mechanisms of dimer-dependent caspase activation we used a heterodimeric system so that two caspase molecules can be specifically brought together. Surprisingly, only one caspase partner in the dimer needs to be enzymatically active for caspase processing and activation to occur. Caspase activation is inefficient in the dimer in the absence of intramolecular processing, suggesting that caspase activation is initiated via intramolecular processing. Homodimerization of caspase-8 or caspase-9 leads to the formation of a stable dimeric complex. However, heterodimerization between caspase-8 and caspases-3, -9, or -10 failed to induce stable dimer formation or caspase activation. Our data suggest that the formation of a stable dimeric intermediate initiates caspase activation.     

Expression of extended polyglutamine sequentially activates initiator and effector caspases

To date, eight neurodegenerative disorders, including Huntington's disease and dentatorubral-pallidoluysian atrophy, have been identified to be caused by expansion of a CAG repeat coding for a polyglutamine (polyQ) stretch. It is, however, unclear how polyQ expansion mediates neuronal cell death observed in these disorders. Here, we have established a tetracycline-regulated expression system producing 19 and 56 repeats of glutamine fused with green fluorescent protein. Induced expression of the 56 polyQ, but not of the 19 polyQ stretch caused marked nuclear aggregation and apoptotic morphological changes of the nucleus. In vitro enzyme assays and Western blotting showed that polyQ56 expression sequentially activated initiator and effector caspases, such as caspase-8 or -9, and caspase-3, respectively. Furthermore, using cell-permeable fluorogenic substrate, the activation of caspase-3-like proteases was demonstrated in intact cells with aggregated polyQ. This is the first direct evidence that the expression of extended polyQ activates caspases and together with the previous findings that some of the products of genes responsible for CAG repeat diseases are substrates of caspase-3 indicates an important role of caspases in the pathogenesis of these diseases.     

Apoptosome: a platform for the activation of initiator caspases

Apoptosome refers to the adaptor protein complex that mediates the activation of an initiator caspase at the onset of apoptosis. In mammalian cells, caspase-9, caspase-8, and caspase-2 rely on the apoptotic protease-activating factor 1 (Apaf-1)-apoptosome, death-inducing signaling complex (DISC), and PIDDosome, respectively, for activation. In Drosophila, activation of the caspase-9 homolog Dronc requires assembly of an apoptosome comprised of Dark/Hac-1/Dapaf-1. In Caenorhabditis elegans, activation of the caspase CED-3 is facilitated by the CED-4-apoptosome. Recent biochemical and structural investigation revealed significant insights into the assembly and function of the various apoptosomes. Nonetheless, conclusive mechanisms by which the initiator caspases are activated by the apoptosomes remain elusive. Several models have been proposed to explain the activation process. The induced proximity model summarizes the general process of initiator caspase activation. The proximity-driven dimerization model describes how initiator caspases respond to induced proximity and offers an explanation for their activation. Regardless of how initiator caspases are activated, enhanced activity must be correlated with altered active site conformation. The induced conformation model posits that the activated conformation for the active site of a given initiator caspase is attained through direct interaction with the apoptosome or through homo-oligomerization facilitated by the apoptosome.     

Mechanism-based inactivation of caspases by the apoptotic suppressor p35.

Caspases play a crucial role in the ability of animal cells to kill themselves by apoptosis. Caspase activity is regulated in vivo by members of three distinct protease inhibitor families, one of which--p35--has so far only been found in baculoviruses. P35 has previously been shown to rapidly form essentially irreversible complexes with its target caspases in a process that is accompanied by peptide bond cleavage. To determine the protease-inhibitory pathway utilized by this very selective protease inhibitor, we have analyzed the thermodynamic and kinetic stability of the protein. We show that the conformation of p35 is stabilized following cleavage within its reactive site loop. An inactive catalytic mutant of caspase 3 is bound by p35, but much less avidly than the wild-type enzyme, indicating that the protease catalytic nucleophile is required for stable complex formation. The inhibited protease is trapped as a covalent adduct, most likely with its catalytic Cys esterified to the carbonyl carbon of the scissile peptide bond. Together these data reveal that p35 is a mechanism-based inactivator that has adopted an inhibitory device reminiscent of the widely distributed serpin family, despite a complete lack of sequence or structural homology.     

Reduced cellular Ca availability enhances TDP-43 cleavage by apoptotic caspases

Accumulation of transactive response DNA binding protein (TDP-43) fragments in motor neurons is a post mortem hallmark of different neurodegenerative diseases. TDP-43 fragments are the products of the apoptotic caspases-3 and -7. Either excessive or insufficient cellular Ca²⁺ availability is associated with activation of apoptotic caspases. However, as far as we know, it is not described whether activation of caspases, due to restricted intracellular Ca²⁺, affects TDP-43 cleavage. Here we show that in various cell lineages with restricted Ca²⁺ availability, TDP-43 is initially cleaved by caspases-3 and -7 and then, also by caspases-6 and -8 once activated by caspase-3. Furthermore, we disclose the existence of a TDP-43 caspase-mediated fragment of 15 kDa, in addition to the well-known fragments of 35 and 25 kDa. Interestingly, with respect to the other two fragments this novel fragment is the major product of caspase activity on murine TDP-43 whereas in human cell lines the opposite occurs. This outcome should be considered when murine models are used to investigate TDP-43 proteinopathies.     

A pathway of signals regulating effector and initiator caspases in the developing Drosophila eye

Regulated cell death and survival play important roles in neural development. Extracellular signals are presumed to regulate seven apparent caspases to determine the final structure of the nervous system. In the eye, the EGF receptor, Notch, and intact primary pigment and cone cells have been implicated in survival or death signals. An antibody raised against a peptide from human caspase 3 was used to investigate how extracellular signals controlled spatial patterning of cell death. The antibody crossreacted specifically with dying Drosophila cells and labelled the activated effector caspase Drice. It was found that the initiator caspase Dronc and the proapoptotic gene head involution defective were important for activation in vivo. Dronc may play roles in dying cells in addition to activating downstream effector caspases. Epistasis experiments ordered EGF receptor, Notch, and primary pigment and cone cells into a single pathway that affected caspase activity in pupal retina through hid and Inhibitor of Apoptosis Proteins. None of these extracellular signals appeared to act by initiating caspase activation independently of hid. Taken together, these findings indicate that in eye development spatial regulation of cell death and survival is integrated through a single intracellular pathway.     

Role of factors downstream of caspases in nuclear disassembly during apoptotic execution

We used cytoplasmic extracts from chicken DU249 cells at various stages along the apoptotic pathway to analyse the events of apoptotic execution. So-called S/M extracts from morphologically normal 'committed-stage' cells induce apoptotic morphology and DNA cleavage in substrate nuclei. These apoptotic changes appear to require the function of multiple caspases (cysteine aspartases, a specialized class of proteases) acting in parallel. Extracts from 'execution-stage' apoptotic cells induce apoptotic events in added nuclei in a caspase-independent manner. Biochemical fractionation of these extracts reveals that a column fraction enriched in endogenous active caspases is unable to induce DNA fragmentation or chromatin condensation in substrate nuclei, whereas a caspase-depleted fraction induces both changes. 'Execution-stage' extracts contain an ICAD/DFF45-inhibitable nuclease resembling CAD, plus another activity that is required for the apoptotic chromatin condensation. 'Committed-stage' S/M extracts lack these downstream activities. These observations reveal that caspases act in an executive fashion, serving to activate downstream factors that disassemble the nucleus rather than disassembling it themselves. They also suggest that activation of the downstream factors (rather than the caspases) is the critical event that occurs at the transition from the latent to the execution phase of apoptosis.     

Overlapping cleavage motif selectivity of caspases: implications for analysis of apoptotic pathways

Caspases orchestrate the controlled demise of a cell after an apoptotic signal through specific protease activity and cleavage of many substrates altering protein function and ensuring apoptosis proceeds efficiently. Comparing a variety of substrates of each apoptotic caspase (2, 3, 6, 7, 8, 9 and 10) showed that the cleavage sites had a general motif, sometimes specific for one caspase, but other times specific for several caspases. Using commercially available short peptide-based substrates and inhibitors the promiscuity for different cleavage motifs was indicated, with caspase-3 able to cleave most substrates more efficiently than those caspases to which the substrates are reportedly specific. In a cell-free system, immunodepletion of caspases before or after cytochrome c-dependent activation of the apoptosome indicated that the majority of activity on synthetic substrates was dependent on caspase-3, with minor roles played by caspases-6 and -7. Putative inhibitors of individual caspases were able to abolish all cytochrome c-induced caspase activity in a cell-free system and inhibit apoptosis in whole cells through the extrinsic and intrinsic pathways, raising issues regarding the use of such inhibitors to define relevant caspases and pathways. Finally, caspase activity in cells lacking caspase-9 displayed substrate cleavage activity of a putative caspase-9-specific substrate underlining the lack of selectivity of peptide-based substrates and inhibitors of caspases.     

Proteolytic specificity of caspases is required to signal the appearance of apoptotic morphology

The caspase family of cysteine proteases is essential for implementation of physiological cell death. Since a wide variety of cellular proteins is cleaved by caspases during apoptosis, it has been predicted that digestion of proteins crucial to maintaining the life of a cell is central to apoptosis. To assess the role of the proteolytic destruction during apoptosis, we introduced the non-specific protease proteinase K into intact cells. This introduction led to extensive digestion of cellular proteins, including physiological caspase-substrates. Caspase-3-like activity was induced rapidly, followed by morphological signs of apoptosis such as membrane blebbing and nuclear condensation. The caspase inhibitor Z-VAD-fmk inhibited the appearance of these morphological changes without reducing the extent of intracellular proteolysis by proteinase K. Loss of integrity of the cell membrane, however, was not blocked by Z-VAD-fmk. This system thus generated conditions of extensive destruction of caspase substrates by proteinase K in the absence of apoptotic morphology. Taken together, these experiments suggest that caspases implement cell death not by protein destruction but by proteolytic activation of specific downstream effector molecules.     

Mammalian mitochondrial initiation factor 2 supports yeast mitochondrial translation without formylated initiator tRNA

Initiation of protein synthesis in mitochondria and chloroplasts is widely believed to require a formylated initiator methionyl-tRNA (fMet-tRNAfMet) in a process involving initiation factor 2 (IF2). However, yeast strains disrupted at the FMT1 locus, encoding mitochondrial methionyl-tRNA formyltransferase, lack detectable fMet-tRNAfMet but exhibit normal mitochondrial function as evidenced by normal growth on non-fermentable carbon sources. Here we show that mitochondrial translation products in Saccharomyces cerevisiae were synthesized in the absence of formylated initiator tRNA. ifm1 mutants, lacking the mitochondrial initiation factor 2 (mIF2), are unable to respire, indicative of defective mitochondrial protein synthesis, but their respiratory defect could be complemented by plasmid-borne copies of either the yeast IFM1 gene or a cDNA encoding bovine mIF2. Moreover, the bovine mIF2 sustained normal respiration in ifm1 fmt1 double mutants. Bovine mIF2 supported the same pattern of mitochondrial translation products as yeast mIF2, and the pattern did not change in cells lacking formylated Met-tRNAfMet. Mutant yeast lacking any mIF2 retained the ability to synthesize low levels of a subset of mitochondrially encoded proteins. The ifm1 null mutant was used to analyze the domain structure of yeast mIF2. Contrary to a previous report, the C terminus of yeast mIF2 is required for its function in vivo, whereas the N-terminal domain could be deleted. Our results indicate that formylation of initiator methionyl-tRNA is not required for mitochondrial protein synthesis. The ability of bovine mIF2 to support mitochondrial translation in the yeast fmt1 mutant suggests that this phenomenon may extend to mammalian mitochondria as well.     

Death receptor-induced apoptotic and necrotic cell death: differential role of caspases and mitochondria

In L929sAhFas cells, tumor necrosis factor (TNF) leads to necrotic cell death, whereas agonistic anti-Fas antibodies elicit apoptotic cell death. Apoptosis, but not necrosis, is correlated with a rapid externalization of phosphatidylserine and the appearance of a hypoploid population. During necrosis no cytosolic and organelle-associated active caspase-3 and -7 fragments are detectable. The necrotic process does not involve proteolytic generation of truncated Bid; moreover, no mitochondrial release of cytochrome c is observed. Bcl-2 overexpression slows down the onset of necrotic cell death. In the case of apoptosis, active caspases are released to the culture supernatant, coinciding with the release of lactate dehydrogenase. Following necrosis, mainly unprocessed forms of caspases are released. Both TNF-induced necrosis and necrosis induced by anti-Fas in the presence of the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone are prevented by the serine protease inhibitor N-tosyl-L-phenylalanine chloromethylketone and the oxygen radical scavenger butylated hydroxyanisole, while Fas-induced apoptosis is not affected.     

Crystal structure of caspase-2, apical initiator of the intrinsic apoptotic pathway

The cell death protease caspase-2 has recently been recognized as the most apical caspase in the apoptotic cascade ignited during cell stress signaling. Cytotoxic stress, such as that caused by cancer therapies, leads to activation of caspase-2, which acts as a direct effector of the mitochondrion-dependent apoptotic pathway resulting in programmed cell death. Here we report the x-ray structure of caspase-2 in complex with the inhibitor acetyl-Leu-Asp-Glu-Ser-Asp-aldehyde at 1.65-A resolution. Compared with other caspases, significant structural differences prevail in the active site region and the dimer interface. The structure reveals the hydrophobic properties of the S5 specificity pocket, which is unique to caspase-2, and provides the details of the inhibitor-protein interactions in subsites S1-S4. These features form the basis of caspase-2 specificity and allow the design of caspase-2-directed ligands for medical and analytical use. Another unique feature of caspase-2 is a disulfide bridge at the dimer interface, which covalently links the two monomers. Consistent with this finding, caspase-2 exists as a (p19/p12)2 dimer in solution, even in the absence of substrates or inhibitors. The intersubunit disulfide bridge stabilizes the dimeric form of caspase-2, whereas all other long prodomain caspases exist as monomers in solution, and dimer formation is driven by ligand binding. Therefore, the central disulfide bridge appears to represent a novel way of dimer stabilization in caspases.     

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.     

Inhibition of caspases inhibits the release of apoptotic bodies: Bcl-2 inhibits the initiation of formation of apoptotic bodies in chemotherapeutic agent-induced apoptosis

During apoptosis, the cell actively dismantles itself and reduces cell size by the formation and pinching off of portions of cytoplasm and nucleus as "apoptotic bodies." We have combined our previously established quantitative assay relating the amount of release of [3H]-membrane lipid to the degree of apoptosis with electron microscopy (EM) at a series of timepoints to study apoptosis of lymphoid cells exposed to vincristine or etoposide. We find that the [3H]-membrane lipid release assay correlates well with EM studies showing the formation and release of apoptotic bodies and cell death, and both processes are regulated in parallel by inducers or inhibitors of apoptosis. Overexpression of Bcl-2 or inhibition of caspases by DEVD inhibited equally well the activation of caspases as indicated by PARP cleavage. They also inhibited [3H]-membrane lipid release and release of apoptotic bodies. EM showed that cells overexpressing Bcl-2 displayed near-normal morphology and viability in response to vincristine or etoposide. In contrast, DEVD did not prevent cell death. Although DEVD inhibited the chromatin condensation, PARP cleavage, release of apoptotic bodies, and release of labeled lipid, DEVD-treated cells showed accumulation of heterogeneous vesicles trapped in the condensed cytoplasm. These results suggest that inhibition of caspases arrested the maturation and release of apoptotic bodies. Our results also imply that Bcl-2 regulates processes in addition to caspase activation.     

Cytokine response modifier a inhibition of initiator caspases results in covalent complex formation and dissociation of the caspase tetramer

Active caspases are generally composed of two catalytic domains, each containing a large (p20) and a small (p10) subunit so that a fully active caspase has the organization (p20-p10)(2). The cowpox serpin crmA suppresses host apoptosis and inflammation by inhibiting endogenous caspases. We report on the mechanism crmA uses to inhibit caspases 1, 6, and 8. Native PAGE showed formation of tight crmA-caspase complexes. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry provided evidence for a covalent crmA-p20 thioester linkage. SDS-PAGE of isolated complexes showed near complete loss of the p10 subunit from initiator caspases 1 and 8 but not from the executioner caspase-6. This was confirmed for caspase-1 by sequencing and Western blotting. Size exclusion chromatography indicated a size of approximately 60 kDa for complexes with caspases 1 and 8, consistent with a crmA.p20 species, suggesting that the p20-p10 interface and possibly the p10-p10 interface had been disrupted. In contrast, crmA.caspase-6 complex behaved as an equilibrium mixture of crmA(2).(p20-p10)(2) and crmA.(p20-p10). Complex deacylation rates were all slow, suggesting effective kinetic trapping of the covalent thioacyl intermediate. These results suggest a novel serpin inhibition mechanism through which crmA down-regulates apoptosis and inflammation. This involves (i) rapid formation of covalent complex with initiator caspases 8 or 1, (ii) very slow deacylation, and (iii) loss of the caspase p10 subunit for initiator but not for executioner caspases, so that any free p20 formed by deacylation of initiator caspases cannot reassociate to active heterotetramer, thus resulting in irreversible inhibition of apoptosis and inflammation.