Mechanisms of AIF-mediated apoptotic DNA degradation in Caenorhabditis elegans

Discovery, regulation, and action of the major apoptotic nucleases DFF40/CAD and endonuclease G

Toward the end of the 20th and beginning of the 21st centuries, clever in vitro biochemical complementation experiments and genetic screens from the laboratories of Xiaodong Wang, Shigekazu Nagata, and Ding Xue led to the discovery of two major apoptotic nucleases, termed DNA fragmentation factor (DFF) or caspase-activated DNase (CAD) and endonuclease G (Endo G). Both endonucleases attack chromatin to yield 3'-hydroxyl groups and 5'-phosphate residues, first at the level of 50-300 kb cleavage products and next at the level of internucleosomal DNA fragmentation, but these nucleases possess completely different cellular locations in normal cells and are regulated in vastly different ways. In non-apoptotic cells, DFF exists in the nucleus as a heterodimer, composed of a 45 kD chaperone and inhibitor subunit (DFF45) [also called inhibitor of CAD (ICAD-L)] and a 40 kD latent nuclease subunit (DFF40/CAD). Apoptotic activation of caspase-3 or -7 results in the cleavage of DFF45/ICAD and release of active DFF40/CAD nuclease. DFF40's nuclease activity is further activated by specific chromosomal proteins, such as histone H1, HMGB1/2, and topoisomerase II. DFF is regulated by multiple pre- and post-activation fail-safe steps, which include the requirements for DFF45/ICAD, Hsp70, and Hsp40 proteins to mediate appropriate folding during translation to generate a potentially activatable nuclease, and the synthesis in stoichiometric excess of the inhibitors (DFF45/35; ICAD-S/L). By contrast, Endo G resides in the mitochondrial intermembrane space in normal cells, and is released into the nucleus upon apoptotic disruption of mitochondrial membrane permeability in association with co-activators such as apoptosis-inducing factor (AIF). Understanding further regulatory check-points involved in safeguarding non-apoptotic cells against accidental activation of these nucleases remain as future challenges, as well as designing ways to selectively activate these nucleases in tumor cells.

Tumour necrosis factor-induced death of adult human oligodendrocytes is mediated by apoptosis inducing factor

Tumour necrosis factor (TNF)-induced death of oligodendrocytes, the cell type targeted in multiple sclerosis, is mediated by TNF receptor p55 (TNFR-p55). The ligation of TNFR-p55 induces several signal transduction pathways; however, the precise mechanism involved in human oligodendrocyte (hOL) death is unknown. We defined that TNF-induced death of hOLs is non-caspase dependent, as evidenced by lack of generation of caspases 8, 1 and 3 active subunits; lack of cleavage of caspases 1 and 3 fluorogenic substrates; and lack of hOL death inhibition by the general caspase inhibitor, ZVAD.FMK. Electrophoresis of TNF-exposed hOL DNA revealed large-scale DNA fragmentation characteristic of apoptosis-inducing factor (AIF)-mediated cell death, and co-localization experiments showed that AIF translocation to the nucleus occurred upon exposure to TNF. AIF depletion by an antisense strategy prevented TNF-induced hOL death. These results indicate that TNF-induced death of hOLs is dependent on AIF, information of significance for the design strategies to protect hOLs during immune-mediated demyelination.

The mitochondrial death squad: hardened killers or innocent bystanders?

Since the discovery that formation of the apoptosome in mammalian cells is triggered by cytochrome c released from the mitochondria, many other mitochondrial proteins have been suspected to be part of a conspiracy to cause cell death. AIF, EndoG, ANT, cyclophilin D, Bit1, p53AIP, GRIM-19, DAP3, Nur77/TR3/NGFB-1, HtrA2/Omi and Smac/Diablo have all been convicted as killers, but new genetic technology is raising questions about their guilt. Gene knockout experiments suggest that many were wrongly convicted on circumstantial evidence, and just happened to be in the wrong place at the wrong time.

The role of apoptosis in the development and function of T lymphocytes

Apoptosis plays an essential role in T cell biology. Thymocytes expressing nonfunctional or autoreactive TCRs are eliminated by apoptosis during development. Apoptosis also leads to the deletion of expanded effector T cells during immune responses. The dysregulation of apoptosis in the immune system results in autoimmunity, tumorogenesis and immunodeficiency. Two major pathways lead to apoptosis: the intrinsic cell death pathway controlled by Bcl-2 family members and the extrinsic cell death pathway controlled by death receptor signaling. These two pathways work together to regulate T lymphocyte development and function.

A possible role of mitochondria in the apoptotic-like programmed nuclear death of Tetrahymena thermophila

The ciliated protozoan Tetrahymena has a unique apoptosis-like process, which is called programmed nuclear death (PND). During conjugation, the new germinal micro- and somatic macro-nuclei differentiate from a zygotic fertilized nucleus, whereas the old parental macronucleus degenerates, ensuring that only the new macronucleus is responsible for expression of the progeny genotype. As is the case with apoptosis, this process encompasses chromatin cleavage into high-molecular mass DNA, oligonucleosomal DNA laddering, and complete degradation of the nuclear DNA, with the ultimate outcome of nuclear resorption. Caspase-8- and caspase-9-like activities are involved in the final resorption process of PND. In this report, we show evidence for mitochondrial association with PND. Mitochondria and the degenerating macronucleus were colocalized in autophagosome using two dyes for the detection of mitochondria. In addition, an endonuclease with similarities to mammalian endonuclease G was detected in the isolated mitochondria. When the macronuclei were incubated with isolated mitochondria in a cell-free system, DNA fragments of 150-400 bp were generated, but no DNA ladder appeared. Taking account of the present observations and the timing of autophagosome formation, we conclude that mitochondria might be involved in Tetrahymena PND, probably with the process of oligonucleosomal laddering.

Early nuclear translocation of endonuclease G and subsequent DNA fragmentation after transient focal cerebral ischemia in mice

We investigated whether the endonuclease G (endoG) translocated from mitochondria to nucleus after transient focal cerebral ischemia (tFCI), thereby contributed to subsequent DNA fragmentation. Adult male mice were subjected to 60min of focal cerebral ischemia by intraluminal suture blockade of the middle cerebral artery. Western blot analysis for endoG was performed at various time points of tFCI. Nuclear endoG was detected as early as 4h after tFCI in the ischemic brain, and correspondingly mitochondrial endoG showed a significant reduction at 4h after reperfusion (p<0.01). Immunohistochemistry of endoG confirmed that the nuclear translocation of endoG was detected as early as 4h after tFCI in the middle cerebral artery (MCA) territory of the ischemic brain. Double immunofluorescent staining with endoG and AIF showed that endoG was predominantly colocalized with AIF at 24h after tFCI. Double staining with endoG immunohistochemistry and TdT-mediated dUTP-biotin nick end labeling showed a spatial relationship between endoG expression and DNA fragmentation at 24h after tFCI. These data suggest that the early nuclear translocation of endoG occurs and could induce DNA fragmentation in the ischemic brain after tFCI.

Pathways of apoptosis and importance in development

The elimination of cells by programmed cell death is a fundamental event in development where multicellular organisms regulate cell numbers or eliminate cells that are functionally redundant or potentially detrimental to the organism. The evolutionary conservation of the biochemical and genetic regulation of programmed cell death across species has allowed the genetic pathways of programmed cell death determined in lower species, such as the nematode Caenorhabditis elegans and the fruitfly Drosophila melanogaster to act as models to delineate the genetics and regulation of cell death in mammalian cells. These studies have identified cell autonomous and non-autonomous mechanisms that regulate of cell death and reveal that developmental cell death can either be a pre-determined cell fate or the consequence of insufficient cell interactions that normally promote cell survival.

p53-dependent caspase-2 activation in mitochondrial release of apoptosis-inducing factor and its role in renal tubular epithelial cell injury

We demonstrate the role of p53-mediated caspase-2 activation in the mitochondrial release of apoptosis-inducing factor (AIF) in cisplatin-treated renal tubular epithelial cells. Gene silencing of AIF with its small interfering RNA (siRNA) suppressed cisplatin-induced AIF expression and provided a marked protection against cell death. Subcellular fractionation and immunofluorescence studies revealed cisplatin-induced translocation of AIF from the mitochondria to the nuclei. Pancaspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone or p53 inhibitor pifithrin-alpha markedly prevented mitochondrial release of AIF, suggesting that caspases and p53 are involved in this release. Caspase-2 and -3 that were predominantly activated in response to cisplatin provided a unique model to study the role of these caspases in AIF release. Cisplatin-treated caspase-3 (+/+) and caspase-3 (-/-) cells exhibited similar AIF translocation to the nuclei, suggesting that caspase-3 does not affect AIF translocation, and thus, caspase-2 may be involved in the translocation. Caspase-2 inhibitor benzyloxycarbonyl-Val-Asp-Val-Ala-Asp-fluoromethylketone or down-regulation of caspase-2 by its siRNA significantly prevented translocation of AIF. Caspase-2 activation was a critical response from p53, which was markedly induced and phosphorylated in cisplatin-treated cells. Overexpression of p53 not only resulted in caspase-2 activation but also mitochondrial release of AIF. The p53 inhibitor pifithrin-alpha or p53 siRNA prevented both cisplatin-induced caspase-2 activation and mitochondrial release of AIF. Caspase-2 activation was dependent on the p53-responsive gene, PIDD, a death domain-containing protein that was induced by cisplatin in a p53-dependent manner. These results suggest that caspase-2 activation mediated by p53 is an important pathway involved in the mitochondrial release of AIF in response to cisplatin injury.

L'apoptose : bases fondamentales et applications médicales

Apoptosis has become a most popular concept of cell death. What makes apoptosis particularly exciting for medicine is that its dysfunctions play a central role in the pathogenesis of several human diseases. This review summarizes the considerable knowledge about the cell death pathways. The purpose of this article is to provide a background of relevance to clinicians on apoptosis, and the rationale for future therapeutic interventions directed toward the apoptotic machinery.

Male sterility in mice lacking retinoic acid receptor alpha involves specific abnormalities in spermiogenesis

The severe degeneration of the germinal epithelium and subsequent male sterility observed in mice null for the retinoic acid receptor alpha (RARalpha) gene suggested its critical role in spermatogenesis, although the etiology and progression of these abnormalities remain to be determined. Previous studies have revealed that elongated spermatids in RARalpha(-/-) testes were improperly aligned at the tubular lumen and did not undergo spermiation at stage VIII(*). We now report a distinctive failure of step 8-9 spermatids to orient properly with regard to the basal aspect of Sertoli cells, resulting in stage VIII(*)-IX(*) tubules with randomly oriented spermatids. By in situ terminal deoxynucleotidyltransferase-mediated deoxy-UTP nick end labeling (TUNEL), we noted that elongating spermatids frequently underwent apoptosis. Immunohistochemical analysis revealed that while activated caspase-3, the primary effector caspase in the apoptotic cell death machinery, was detected in the nuclei of primary spermatocytes in the first wave of spermatogenesis and occasionally in spermatogonia of both normal and mutant testes, it was not involved in the death of elongating spermatids in RARalpha(-/-) testes. Thus, sterility in RARalpha(-/-) males was associated with specific defects in spermiogenesis, which may correlate with a failure in both spermatid release and spermatid orientation to the basal aspect of Sertoli cells at stage VIII(*) in young adult RARalpha(-/-) testis. Further, the resulting apoptosis in elongating spermatids appears to involve pathways other than that mediated by activated caspase-3.

Mediation of cell death by poly(ADP-ribose) polymerase-1

Poly(ADP-ribosyl)ation plays an important role in modulating the cellular response to stress. The extent of poly(ADP-ribosyl)ation, chiefly via the activation of the poly(ADP-ribose) polymerase-1 (PARP-1), correlates with the severity of genotoxic stress and this determines the cellular response. Under mild and moderate stress, it plays important roles in DNA processing and it participates in the proinflammatory/cellular defense via transcriptional regulation. However, severe stress following acute neuronal injury causes the overactivation of PARP-1, which results in unregulated poly(ADP-ribose) (PAR) synthesis and widespread neuronal cell death. Previously, this PARP-1-dependent cell death mechanism was manifest solely through necrosis, but apoptotic mechanisms are also evident. Poly(ADP-ribosyl)ation directly induces the nuclear translocation of apoptosis-inducing factor, which results in caspase-independent cell death significant in many neurodegenerative conditions. Further, the hydrolysis of PAR by poly(ADP-ribose) glycohydrolase (PARG) has a protective role, since the accumulation of PAR leads to cell death by apoptosis. Thus, PAR signaling, regulated by PARP-1 and PARG, mediates cell death. Accordingly, modulation of PAR synthesis or degradation through the targeting of PARP-1 or PARG holds particular promise in the treatment of conditions such as cancer, stroke, and Parkinson's disease.

AIF suppresses chemical stress-induced apoptosis and maintains the transformed state of tumor cells

Apoptosis-inducing factor (AIF) exhibits reactive oxygen species (ROS)-generating NADH oxidase activity of unknown significance, which is dispensable for apoptosis. We knocked out the aif gene in two human colon carcinoma cell lines that displayed lower mitochondrial complex I oxidoreductase activity and produced less ROS, but showed increased sensitivity to peroxide- or drug-induced apoptosis. AIF knockout cells failed to form tumors in athymic mice or grow in soft agar. Only AIF with intact NADH oxidase activity restored complex I activity and anchorage-independent growth of aif knockout cells, and induced aif-transfected mouse NIH3T3 cells to form foci. AIF knockdown in different carcinoma cell types resulted in lower superoxide levels, enhanced apoptosis sensitivity and loss of tumorigenicity. Antioxidants sensitized AIF-expressing cells to apoptosis, but had no effect on tumorigenicity. In summary, AIF-mediated resistance to chemical stress involves ROS and probably also mitochondrial complex I. AIF maintains the transformed state of colon cancer cells through its NADH oxidase activity, by mechanisms that involve complex I function. On both counts, AIF represents a novel type of cancer drug target.

Cell death independent of caspases: a review

Patterns of cell death have been divided into apoptosis, which is actively executed by specific proteases, the caspases, and accidental necrosis. However, there is now accumulating evidence indicating that cell death can occur in a programmed fashion but in complete absence and independent of caspase activation. Alternative models of programmed cell death (PCD) have therefore been proposed, including autophagy, paraptosis, mitotic catastrophe, and the descriptive model of apoptosis-like and necrosis-like PCD. Caspase-independent cell death pathways are important safeguard mechanisms to protect the organism against unwanted and potential harmful cells when caspase-mediated routes fail but can also be triggered in response to cytotoxic agents or other death stimuli. As in apoptosis, the mitochondrion can play a key role but also other organelles such as lysosomes and the endoplasmic reticulum have an important function in the release and activation of death factors such as cathepsins, calpains, and other proteases. Here we review the various models of PCD and their death pathways at molecular and organelle level and discuss the relevance of the growing knowledge of caspase-independent cell death pathways for cancer.

A Caenorhabditis elegans model of insulin resistance: altered macronutrient storage and dauer formation in an OGT-1 knockout

O-linked N-acetylglucosamine (O-GlcNAc) is an evolutionarily conserved modification of nuclear pore proteins, signaling kinases, and transcription factors. The O-GlcNAc transferase (OGT) catalyzing O-GlcNAc addition is essential in mammals and mediates the last step in a nutrient-sensing "hexosamine-signaling pathway." This pathway may be deregulated in diabetes and neurodegenerative disease. To examine the function of O-GlcNAc in a genetically amenable organism, we describe a putative null allele of OGT in Caenorhabditis elegans that is viable and fertile. We demonstrate that, whereas nuclear pore proteins of the homozygous deletion strain are devoid of O-GlcNAc, nuclear transport of transcription factors appears normal. However, the OGT mutant exhibits striking metabolic changes manifested in a approximately 3-fold elevation in trehalose levels and glycogen stores with a concomitant approximately 3-fold decrease in triglycerides levels. In nematodes, a highly conserved insulin-like signaling cascade regulates macronutrient storage, longevity, and dauer formation. The OGT knockout suppresses dauer larvae formation induced by a temperature-sensitive allele of the insulin-like receptor gene daf-2. Our findings demonstrate that OGT modulates macronutrient storage and dauer formation in C. elegans, providing a unique genetic model for examining the role of O-GlcNAc in cellular signaling and insulin resistance.

DNA degradation in development and programmed cell death

Most mammalian cells have nuclei that contain DNA, which replicates during cell proliferation. DNA is destroyed by various developmental processes in mammals. It is degraded during programmed cell death that accompanies mammalian development. The nuclei of erythrocytes and eye lens fiber cells are also removed during their differentiation into mature cells. If DNA is not properly degraded in these processes, it can cause various diseases, including tissue atrophy, anemia, cataract, and autoimmune diseases, which indicates that DNA can be a pathogenic molecule. Here, I present how DNA is degraded during programmed cell death, erythroid cell differentiation, and lens cell differentiation. I discuss what might be or will be learned from understanding the molecular mechanisms of DNA degradation that occurs during mammalian development.

Downregulation of apoptosis-inducing factor in harlequin mutant mice sensitizes the myocardium to oxidative stress-related cell death and pressure overload-induced decompensation

Apoptosis-inducing factor (AIF), or programmed cell death 8 (Pdcd8), is a highly conserved, ubiquitous flavoprotein localized in the mitochondrial intermembrane space. In vivo, AIF provides protection against neuronal apoptosis induced by oxidative stress. Conversely, in vitro, AIF has been demonstrated to have a proapoptotic role when, on induction of the mitochondrial death pathway, AIF translocates to the nucleus where it facilitates chromatin condensation and large scale DNA fragmentation. To determine the role of AIF in myocardial apoptotic processes, we examined cardiomyocytes from an AIF-deficient mouse mutant, Harlequin (Hq). Hq mutant cardiomyocytes demonstrated increased sensitivity to H2O2-induced cell death. Further, Hq hearts subjected to ischemia/reperfusion revealed more cardiac damage and, unlike wild-type mice, the amount of damage increased with the age of the animal. Aortic banding caused enhanced hypertrophy, increased cardiomyocyte apoptotic and necrotic cell death, and accelerated progression toward maladaptive left ventricular remodeling in Hq mutant mice compared with wild-type counterparts. These findings correlated with a reduced capacity of subsarcolemmal mitochondria from Hq mutant hearts to scavenge free radicals. Together, these data demonstrate a critical role for AIF as a cardiac antioxidant in the protection against oxidative stress-induced cell death and development of heart failure induced by pressure overload.

Galectin-1 induces nuclear translocation of endonuclease G in caspase- and cytochrome c-independent T cell death

Galectin-1, a mammalian lectin expressed in many tissues, induces death of diverse cell types, including lymphocytes and tumor cells. The galectin-1 T cell death pathway is novel and distinct from other death pathways, including those initiated by Fas and corticosteroids. We have found that galectin-1 binding to human T cell lines triggered rapid translocation of endonuclease G from mitochondria to nuclei. However, endonuclease G nuclear translocation occurred without cytochrome c release from mitochondria, without nuclear translocation of apoptosis-inducing factor, and prior to loss of mitochondrial membrane potential. Galectin-1 treatment did not result in caspase activation, nor was death blocked by caspase inhibitors. However, galectin-1 cell death was inhibited by intracellular expression of galectin-3, and galectin-3 expression inhibited the eventual loss of mitochondrial membrane potential. Galectin-1-induced cell death proceeds via a caspase-independent pathway that involves a unique pattern of mitochondrial events, and different galectin family members can coordinately regulate susceptibility to cell death.

The mitochondrial death pathway and cardiac myocyte apoptosis

Apoptosis has been causally linked to the pathogenesis of myocardial infarction and heart failure in rodent models. This death process is mediated by two central pathways, an extrinsic pathway involving cell surface receptors and an intrinsic pathway using mitochondria and the endoplasmic reticulum. Each of these pathways has been implicated in myocardial pathology. In this review, we summarize recent advances in the understanding of the intrinsic pathway and how it relates to cardiac myocyte death and heart disease.

The Caenorhabditis elegans pvl-5 gene protects hypodermal cells from ced-3-dependent, ced-4-independent cell death

Programmed cell death (PCD) is regulated by multiple evolutionarily conserved mechanisms to ensure the survival of the cell. Here we describe pvl-5, a gene that likely regulates PCD in Caenorhabditis elegans. In wild-type hermaphrodites at the L2 stage there are 11 Pn.p hypodermal cells in the ventral midline arrayed along the anterior-posterior axis and 6 of these cells become the vulval precursor cells. In pvl-5(ga87) animals there are fewer Pn.p cells (average of 7.0) present at this time. Lineage analysis reveals that the missing Pn.p cells die around the time of the L1 molt in a manner that often resembles the programmed cell deaths that occur normally in C. elegans development. This Pn.p cell death is suppressed by mutations in the caspase gene ced-3 and in the bcl-2 homolog ced-9, suggesting that the Pn.p cells are dying by PCD in pvl-5 mutants. Surprisingly, the Pn.p cell death is not suppressed by loss of ced-4 function. ced-4 (Apaf-1) is required for all previously known apoptotic cell deaths in C. elegans. This suggests that loss of pvl-5 function leads to the activation of a ced-3-dependent, ced-4-independent form of PCD and that pvl-5 may normally function to protect cells from inappropriate activation of the apoptotic pathway.

Deadly conversations: nuclear-mitochondrial cross-talk

Neuronal damage following stroke or neurodegenerative diseases is thought to stem in part from overexcitation of N -methyl-D-aspartate (NMDA) receptors by glutamate. NMDA receptors triggered neurotoxicity is mediated in large part by activation of neuronal nitric oxide synthase (nNOS) and production of nitric oxide (NO). Simultaneous production of superoxide anion in mitochondria provides a permissive environment for the formation of peroxynitrite (ONOO-). Peroxynitrite damages DNA leading to strand breaks and activation of poly(ADP-ribose) polymerase-1 (PARP-1). This signal cascade plays a key role in NMDA excitotoxicity, and experimental models of stroke and Parkinson's disease. The mechanisms of PARP-1-mediated neuronal death are just being revealed. While decrements in ATP and NAD are readily observed following PARP activation, it is not yet clear whether loss of ATP and NAD contribute to the neuronal death cascade or are simply a biochemical marker for PARP-1 activation. Apoptosis-inducing factor (AIF) is normally localized to mitochondria but following PARP-1 activation, AIF translocates to the nucleus triggering chromatin condensation, DNA fragmentation and nuclear shrinkage. Additionally, phosphatidylserine is exposed and at a later time point cytochrome c is released and caspase-3 is activated. In the setting of excitotoxic neuronal death, AIF toxicity is caspase independent. These observations are consistent with reports of biochemical features of apoptosis in neuronal injury models but modest to no protection by caspase inhibitors. It is likely that AIF is the effector of the morphologic and biochemical events and is the commitment point to neuronal cell death, events that occur prior to caspase activation, thus accounting for the limited effects of caspase inhibitors. There exists significant cross talk between the nucleus and mitochondria, ultimately resulting in neuronal cell death. In exploiting this pathway for the development of new therapeutics, it will be important to block AIF translocation from the mitochondria to the nucleus without impairing important physiological functions of AIF in the mitochondria.

Regulation of mitochondrial membrane permeabilization by BCL-2 family proteins and caspases

Mitochondria play an important role in the integration and transmission of cell death signals, activating caspases and other cell death execution events by releasing apoptogenic proteins from the intermembrane space. The BCL-2 family of proteins localize (or can be targeted) to mitochondria and regulate the permeability of the mitochondrial outer membrane to these apoptotic factors. Recent evidence suggests that multiple mechanisms may regulate the release of mitochondrial factors, some of which depend on the action of caspases.

Life's smile, death's grin: vital functions of apoptosis-executing proteins

Apoptosis is executed by caspases as well as caspase-independent death effectors. Caspases are expressed as inactive zymogens in virtually all animal cells and are activated in cells destined to undergo apoptosis. However, there are many examples where caspase activation is actually required for cellular processes not related to cell death, namely terminal differentiation, activation, proliferation, and cytoprotection. Several caspase-independent death effectors including apoptosis-inducing factor, endonuclease G and a serine protease (Omi/HtrA2) are released from the mitochondrial intermembrane space upon permeabilization of the outer membrane. Such proteins also have important roles in cellular redox metabolism and/or mitochondrial biogenesis. As a general rule, it thus appears that cell-death-relevant proteins, especially those involved in the core of the executing machinery, have a dual function in life and death. This has important implications for pathophysiology. The fact that the building blocks of the apoptotic machinery have normal functions not related to cell death may mean that essential parts of the apoptotic executioner cannot be lost and thus reduces the possibility of oncogenic mutations that block the apoptotic program. Moreover, therapeutic suppression of unwarranted cell death must be designed to target only the lethal (and not the vital) role of death effectors.

Calpain Mediates Excitotoxic DNA Fragmentation via Mitochondrial Pathways in Adult Brains

Calpain has been implicated in excitotoxic neurode-generation, but its mechanism of action particularly in adult brains remains unclear. We generated mutant mice lacking or overexpressing calpastatin, the only solely calpain-specific inhibitor ever identified or synthesized. Modulation of calpastatin expression caused no defect in the mice under normal conditions, indicating that calpastatin functions as a negative regulator of calpain only under pathological conditions. Kainate-evoked excitotoxicity in hippocampus resulted in proteolytic activation of a proapoptotic Bcl-2 subfamily member (Bid), nuclear translocation of mitochondria-derived DNA fragmentation factors (apoptosis-inducing factor and endonuclease G), DNA fragmentation, and nuclear condensation in pyramidal neurons. These apoptotic responses were significantly augmented by calpastatin deficiency. Consistently calpastatin overexpression suppressed them. No evidence of caspase-3 activation was detected. Our results demonstrated that calpain mediates excitotoxic signals through mobilization of proapoptotic factors in a caspase-independent manner. These mutant mice will serve as useful tools for investigating calpain involvement in various diseases.

Molecular cloning and characterization of a human AIF-like gene with ability to induce apoptosis

In this study, we cloned and characterized a human gene homologous to the apoptosis-inducing factor (AIF), which is named AIF-like (AIFL). Human AIFL has 598 amino acids, with a characteristic Rieske domain and a pyridine nucleotide-disulfide oxidoreductase domain (Pyr_redox). AIFL shares 35% homology with AIF, mainly in the Pyr_redox domain. Reverse transcriptase-PCR analysis showed the expression of AIFL mRNA in all tissues tested, i.e. brain, colon, heart, kidney, liver, lung, muscle, ovary, pancreas, placenta, small intestine, and testis. We developed antibodies against human AIFL using fusion proteins as antigens. The antibodies specifically recognized the antigen and heterologously expressed AIFL proteins. The expression of AIFL proteins in human tissues was also ubiquitous, demonstrated by immunohistochemistry in tissue array slides. Subcellular fractionation and immunofluorescence staining studies revealed that AIFL is predominantly localized to the mitochondria. Similar to AIF, overexpression of AIFL induced apoptosis, as shown by increased cytoplasmic nucleosomes and subdiploid cell populations in AIFL-transfected cells. The segment 1-190 containing the Rieske domain induced apoptosis, whereas the segment containing the Pyr_redox domain did not contribute to the pro-apoptotic function. The mitochondrial membrane potential of cells transfected with AIFL was significantly more depolarized than that of the control. AIFL transfection-induced cytochrome c release and cleavage of caspase 3. Furthermore, the pan-caspase inhibitor Z-VAD-fmk inhibited AIFL induced apoptosis. In summary, AIFL induces apoptosis in a caspase-dependent manner when heterologously expressed.

DRP-1-mediated mitochondrial fragmentation during EGL-1-induced cell death in C. elegans

Genetic analyses in Caenorhabditis elegans have been instrumental in the elucidation of the central cell-death machinery, which is conserved from C. elegans to mammals. One possible difference that has emerged is the role of mitochondria. By releasing cytochrome c, mitochondria are involved in the activation of caspases in mammals. However, there has previously been no evidence that mitochondria are involved in caspase activation in C. elegans. Here we show that mitochondria fragment in cells that normally undergo programmed cell death during C. elegans development. Mitochondrial fragmentation is induced by the BH3-only protein EGL-1 and can be blocked by mutations in the bcl-2-like gene ced-9, indicating that members of the Bcl-2 family might function in the regulation of mitochondrial fragmentation in apoptotic cells. Mitochondrial fragmentation is independent of CED-4/Apaf-1 and CED-3/caspase, indicating that it occurs before or simultaneously with their activation. Furthermore, DRP-1/dynamin-related protein, a key component of the mitochondrial fission machinery, is required and sufficient to induce mitochondrial fragmentation and programmed cell death during C. elegans development. These results assign an important role to mitochondria in the cell-death pathway in C. elegans.

Calpain I induces cleavage and release of apoptosis-inducing factor from isolated mitochondria

The translocation of apoptosis-inducing factor (AIF) from mitochondria to the nucleus has been implicated in the mechanism of glutamate excitotoxicity in cortical neurons and has been observed in vivo following acute rodent brain injuries. However, the mechanism and time course of AIF redistribution to the nucleus is highly controversial. Because elevated intracellular calcium is one of the most ubiquitous features of neuronal cell death, this study tested the hypothesis that cleavage of AIF by the calcium-activated protease calpain mediates its release from mitochondria. Both precursor and mature forms of recombinant AIF were cleaved near the amino terminus by calpain I in vitro. Mitochondrial outer membrane permeabilization by truncated Bid induced cytochrome c release from isolated liver or brain mitochondria but only induced AIF release in the presence of active calpain. Enzymatic inhibition of calpain by calpeptin precluded AIF release, demonstrating that proteolytic activity was required for release. Calpeptin and the mitochondrial permeability transition pore antagonist cyclosporin A also inhibited calcium-induced AIF release from mouse liver mitochondria, implicating the involvement of an endogenous mitochondrial calpain in release of AIF during permeability transition. Cleavage of AIF directly decreased its association with pure lipid vesicles of mitochondrial inner membrane composition. Taken together, these results define a novel mechanism of AIF release involving calpain processing and identify a potential molecular checkpoint for cytoprotective interventions.

Role of caspases in death and survival of the plaque macrophage

This review considers the role of macrophage cell death in formation of the necrotic core and in plaque progression, and lists many of the possible mediators of macrophage cell death. Among these, perhaps the most cited toxic agent is oxidized low-density lipoprotein (oxLDL). Whereas oxLDL can kill macrophage, and whereas the form of death is morphologically apoptotic, caspase inhibitors appear to be ineffective in preventing death. This finding is consistent with recent literature showing how the canonical caspase pathways are used for physiological cellular functions other than cell death. Plaque macrophages appear to be among the cells with this nonapoptotic signaling function for activated caspases. In many of the other cell types, caspase activation appears to play a critical role in cell differentiation. We discuss possible functions of plaque macrophage using the nondeath caspase pathway. Recent literature shows that physiological and developmental functions of many cell types require active caspases without progressing to cell death. We discuss the role of macrophage cell death in plaque progression, possible mediators of macrophage cell death, and the possible functions of plaque macrophage using the nondeath caspase pathway.

Apoptosis-inducing factor is a key factor in neuronal cell death propagated by BAX-dependent and BAX-independent mechanisms

Mitochondria release proteins that propagate both caspase-dependent and caspase-independent cell death pathways. AIF (apoptosis-inducing factor) is an important caspase-independent death regulator in multiple neuronal injury pathways. Presently, there is considerable controversy as to whether AIF is neuroprotective or proapoptotic in neuronal injury, such as oxidative stress or excitotoxicity. To evaluate the role of AIF in BAX-dependent (DNA damage induced) and BAX-independent (excitotoxic) neuronal death, we used Harlequin (Hq) mice, which are hypomorphic for AIF. Neurons carrying double mutations for Hq/Apaf1-/- (apoptosis proteases-activating factor) are impaired in both caspase-dependent and AIF-mediated mitochondrial cell death pathways. These mutant cells exhibit extended neuroprotection against DNA damage, as well as glutamate-induced excitotoxicity. Specifically, AIF is involved in NMDA- and kainic acid- but not AMPA-induced excitotoxicity. In vivo excitotoxic studies using kainic acid-induced seizure showed that Hq mice had significantly less hippocampal damage than wild-type littermates. Our results demonstrate an important role for AIF in both BAX-dependent and BAX-independent mechanisms of neuronal injury.

Clearance of apoptotic cells in Caenorhabditis elegans

Programmed cell death, or apoptosis, is a genetically controlled process of cell suicide that is a common fate during an animal's life. In metazoans, apoptotic cells are rapidly removed from the body through the process of phagocytosis. Genetic analyses probing the mechanisms controlling the engulfment of apoptotic cells were pioneered in the nematode Caenorhabditis elegans. So far, at least seven genes have been identified that are required for the recognition and engulfment of apoptotic cells and have been shown to function in two partially redundant signaling pathways. Molecular characterization of their gene products has lead to the finding that similar genes act to control the same processes in other organisms, including mammals. In this paper, we review these exciting findings in C. elegans and discuss their implications in understanding the clearance of apoptotic cells in mammals.

"The stress of dying": the role of heat shock proteins in the regulation of apoptosis

Heat shock proteins (Hsps) are a family of highly homologous chaperone proteins that are induced in response to environmental, physical and chemical stresses and that limit the consequences of damage and facilitate cellular recovery. The underlying ability of Hsps to maintain cell survival correlates with an inhibition of caspase activation and apoptosis that can, but does not always, depend upon their chaperoning activities. Several mechanisms proposed to account for these observations impact on both the "intrinsic", mitochondria-dependent and the "extrinsic", death-receptor-mediated pathways to apoptosis. Hsps can inhibit the activity of pro-apoptotic Bcl-2 proteins to prevent permeabilization of the outer mitochondrial membrane and release of apoptogenic factors. The disruption of apoptosome formation represents another mechanism by which Hsps can prevent caspase activation and induction of apoptosis. Several signaling cascades involved in the regulation of key elements within the apoptotic cascade are also subject to modulation by Hsps, including those involving JNK, NF-kappaB and AKT. The coordinated activities of the Hsps thus modulate multiple events within apoptotic pathways to help sustain cell survival following damaging stimuli.

Apoptotic and autophagic cell death induced by histone deacetylase inhibitors

Histone deacetylase (HDAC) inhibitors can induce programmed cell death in cancer cells, although the underlying mechanism is obscure. In this study, we show that two distinct HDAC inhibitors, butyrate and suberoylanilide hydroxamic acid (SAHA), induced caspase-3 activation and cell death in multiple human cancer cell lines. The activation of caspase-3 was via the mitochondria/cytochrome c-mediated apoptotic pathway because it was abrogated in mouse embryonic fibroblasts with knockout of Apaf-1, the essential mediator of the pathway. Overexpression of Bcl-XL in HeLa cells also blocked caspase activation by the HDAC inhibitors. Nevertheless, Apaf-1 knockout, overexpression of Bcl-XL, and pharmacological inhibition of caspase activity did not prevent SAHA and butyrate-induced cell death. The cells undergoing such caspase-independent death had unambiguous morphological features of autophagic cell death. Therefore, HDAC inhibitors can induce both mitochondria-mediated apoptosis and caspase-independent autophagic cell death. Induction of autophagic cell death by HDAC inhibitors has clear clinical implications in treating cancers with apoptotic defects.

AIF deficiency compromises oxidative phosphorylation

Apoptosis-inducing factor (AIF) is a mitochondrial flavoprotein that, after apoptosis induction, translocates to the nucleus where it participates in apoptotic chromatinolysis. Here, we show that human or mouse cells lacking AIF as a result of homologous recombination or small interfering RNA exhibit high lactate production and enhanced dependency on glycolytic ATP generation, due to severe reduction of respiratory chain complex I activity. Although AIF itself is not a part of complex I, AIF-deficient cells exhibit a reduced content of complex I and of its components, pointing to a role of AIF in the biogenesis and/or maintenance of this polyprotein complex. Harlequin mice with reduced AIF expression due to a retroviral insertion into the AIF gene also manifest a reduced oxidative phosphorylation (OXPHOS) in the retina and in the brain, correlating with reduced expression of complex I subunits, retinal degeneration, and neuronal defects. Altogether, these data point to a role of AIF in OXPHOS and emphasize the dual role of AIF in life and death.

Cross-linking of P-selectin glycoprotein ligand-1 induces death of activated T cells

Increasing evidence has shown that death signaling in T cells is regulated in a complicated way. Molecules other than death receptors can also trigger T-cell death. Here, we demonstrate for the first time that P-selectin glycoprotein ligand-1 (PSGL-1) or CD162 molecules cross-linked by an anti-PSGL-1 monoclonal antibody, TAB4, can trigger a death signal in activated T cells. In contrast to classic cell death, PSGL-1-mediated T-cell death is caspase independent. It involves translocation of apoptosis-inducing factor from mitochondria to nucleus and mitochondrial cytochrome c release. Ultrastructurally, both peripheral condensation of chromatin and apoptotic body were observed in PSGL-1-mediated T-cell death. Collectively, this study demonstrates a novel role for PSGL-1 in controlling activated T-cell death and, thus, advances our understanding of immune regulation.

Molecular mechanisms of light-induced photoreceptor apoptosis and neuroprotection for retinal degeneration

Human retinal dystrophies and degenerations and light-induced retinal degenerations in animal models are sharing an important feature: visual cell death by apoptosis. Studying apoptosis may thus provide an important handle to understand mechanisms of cell death and to develop potential rescue strategies for blinding retinal diseases. Apoptosis is the regulated elimination of individual cells and constitutes an almost universal principle in developmental histogenesis and organogenesis and in the maintenance of tissue homeostasis in mature organs. Here we present an overview on molecular and cellular mechanisms of apoptosis and summarize recent developments. The classical concept of apoptosis being initiated and executed by endopeptidases that cleave proteins at aspartate residues (Caspases) can no longer be held in its strict sense. There is an increasing number of caspase-independent pathways, involving apoptosis inducing factor, endonuclease G, poly-(ADP-ribose) polymerase-1, proteasomes, lysosomes and others. Similarly, a considerable number and diversity of pro-apoptotic stimuli is being explored. We focus on apoptosis pathways in our model: light-damage induced by short exposures to bright white light and highlight those essential conditions known so far in the apoptotic death cascade. In our model, the visual pigment rhodopsin is the essential mediator of the initial death signal. The rate of rhodopsin regeneration defines damage threshold in different strains of mice. This rate depends on the level of the pigment epithelial protein RPE65, which in turn depends on the amino acid (leucine or methionine) encoded at position 450. Activation of the pro-apoptotic transcription factor AP-1 constitutes an essential death signal. Inhibition of rhodopsin regeneration as well as suppression of AP-1 confers complete protection in our system. Furthermore, we describe observations in other light-damage systems as well as characteristics of animal models for RP with particular emphasis on rescue strategies. There is a vast array of different neuroprotective cytokines that are applied in light-damage and RP animal models and show diverging efficacy. Some cytokines protect against light damage as well as against RP in animal models. At present, the mechanisms of neuroprotective/anti-apoptotic action represent a "black box" which needs to be explored. Even though acute light damage and RP animal models show different characteristics in many respects, we hope to gain insights into apoptotic mechanisms for both conditions by studying light damage and comparing results with those obtained in animal models. In our view, future directions may include the investigation of different apoptotic pathways in light damage (and inherited animal models). Emphasis should also be placed on mechanisms of removal of dead cells in apoptosis, which appears to be more important than initially recognized. In this context, a stimulating concept concerns age-related macular degeneration, where an insufficiency of macrophages removing debris that results from cell death and photoreceptor turnover might be an important pathogenetic event. In acute light damage, the appearance of macrophages as well as phagocytosis by the retinal pigment epithelium are a consistent and conspicuous feature, which lends itself to the study of removal of cellular debris in apoptosis. We are aware of the many excellent reviews and the earlier work paving the way to our current knowledge and understanding of retinal degeneration, photoreceptor apoptosis and neuroprotection. However, we limited this review mainly to work published in the last 7-8 years and we apologize to all the researchers which have contributed to the field but are not cited here.

Mitochondrial fission proteins regulate programmed cell death in yeast

The possibility that single-cell organisms undergo programmed cell death has been questioned in part because they lack several key components of the mammalian cell death machinery. However, yeast encode a homolog of human Drp1, a mitochondrial fission protein that was shown previously to promote mammalian cell death and the excessive mitochondrial fragmentation characteristic of apoptotic mammalian cells. In support of a primordial origin of programmed cell death involving mitochondria, we found that the Saccharomyces cerevisiae homolog of human Drp1, Dnm1, promotes mitochondrial fragmentation/degradation and cell death following treatment with several death stimuli. Two Dnm1-interacting factors also regulate yeast cell death. The WD40 repeat protein Mdv1/Net2 promotes cell death, consistent with its role in mitochondrial fission. In contrast to its fission function in healthy cells, Fis1 unexpectedly inhibits Dnm1-mediated mitochondrial fission and cysteine protease-dependent cell death in yeast. Furthermore, the ability of yeast Fis1 to inhibit mitochondrial fission and cell death can be functionally replaced by human Bcl-2 and Bcl-xL. Together, these findings indicate that yeast and mammalian cells have a conserved programmed death pathway regulated by a common molecular component, Drp1/Dnm1, that is inhibited by a Bcl-2-like function.

Not Ions Alone: Barriers to Ion Permeation in Nanopores and Channels

A hydrophobic pore of subnanometer dimensions can appear impermeable to an ion even though its radius is still much wider than that of the ion. Pores of molecular dimensions can be found, for instance, in carbon nanotubes, zeolites, or ion channel proteins. We quantify this barrier to ion permeation by calculating the potential of mean force from umbrella-sampled molecular dynamics simulations and compare them to continuum-electrostatic Poisson-Boltzmann calculations. The latter fail to describe the ion barrier because they do not account for the properties of water in the pore. The barrier originates from the energetic cost to desolvate the ion. Even in wide pores, which could accommodate an ion and its hydration shell, a barrier of several kT remains because the liquid water phase is not stable in the hydrophobic pore. Thus, the properties of the solvent play a crucial role in determining permeation properties of ions in confinement at the molecular scale.

Cytochrome C-mediated apoptosis

Apoptosis, or programmed cell death, is involved in development, elimination of damaged cells, and maintenance of cell homeostasis. Deregulation of apoptosis may cause diseases, such as cancers, immune diseases, and neurodegenerative disorders. Apoptosis is executed by a subfamily of cysteine proteases known as caspases. In mammalian cells, a major caspase activation pathway is the cytochrome c-initiated pathway. In this pathway, a variety of apoptotic stimuli cause cytochrome c release from mitochondria, which in turn induces a series of biochemical reactions that result in caspase activation and subsequent cell death. In this review, we focus on the recent progress in understanding the biochemical mechanisms and regulation of the pathway, the roles of the pathway in physiology and disease, and their potential therapeutic values.

Emerging Role for ERK as a Key Regulator of Neuronal Apoptosis

Extracellular signal-regulated kinases (ERKs) are traditionally viewed as a survival factor in the mitogen-activated protein kinase (MAPK) family. On the other hand, some recent reports have suggested that ERK can also be responsible for neuronal cell death in various neurodegeneration models. In-depth studies on the action of ERK in apoptosis, however, have not been done. A recent study has revealed that ERK is a key apoptotic factor in potassium deprivation-induced neuronal cell death by showing that ERK inhibitors protect neurons from low potassium conditions, whereas constitutively activated ERK activates cell death. Most important, this study shows how ERK can promote neuronal cell death by causing plasma membrane and DNA damage that is independent of caspase-3 activity. Further studies on the mechanism of ERK in neuronal cell death will shed light on the possibility of using ERK as a therapeutic target in treating neurodegeneration.

Caspases: an ancient cellular sword of Damocles

Caspases are a family of cysteine proteases homologous to the Caenorhabditis elegans programmed cell death gene product CED-3. Caspases and their distant relatives, meta- and paracaspases, have been found in phylogenetically distant nonmetazoan groups, including plants, fungi and prokaryotes. This review summarizes the current information on the mechanisms and functions of non-mammalian caspases and their relatives in apoptotic and nonapoptotic processes, and explores the possible evolutionary origin of the caspase family.

Viral modulators of cell death provide new links to old pathways

By observing how viruses facilitate their parasitic relationships with host cells, we gain insights into key regulatory pathways of the cell. Not only are mitochondria key players in the regulation of programmed cell death, but many viral regulators of cell death also alter mitochondrial functions either directly or indirectly. Although cytomegalovirus vMIA and Epstein-Barr virus BHRF1 seem to have opposite effects on mitochondrial morphology, they both inhibit cell death. Drosophila Reaper, a regulator of developmental cell death, acts on IAP (inhibitor of apoptosis) proteins to activate caspases, but can regulate mitochondrial permeability in vitro. Despite its pivotal role in Drosophila, homologues of Reaper in other species were not previously known. Recently, amino acid sequence similarity was recognized between Drosophila Reaper and a protein known to be important for the replication and virulence of mosquito-borne bunyaviruses that cause human encephalitis. Thus, viral mechanisms for regulating apoptosis are diverse and not fully elucidated but promise to provide new insights.

Apoptosis-inducing factor determines the chemoresistance of non-small-cell lung carcinomas

Non-small-cell lung carcinomas (NSCLCs) are resistant to the induction of apoptosis by conventional anticancer treatment. However, NSCLC cell lines are sensitive to the action of the broad protein kinase inhibitor, staurosporine (STS). In the NSCLC cell line U1810, STS induced the mitochondrial release of apoptosis-inducing factor (AIF) and cytochrome c (Cyt c) followed by activation of caspases, nuclear condensation, DNA fragmentation and finally cell death. Although preincubation of U1810 cells with the broad-spectrum caspase inhibitor z-VAD.fmk delayed the occurrence of nuclear apoptosis induced by STS, it did not impede mitochondrial alterations (such as the release of Cyt c and AIF) and cell death to occur. Moreover, the microinjection of neither Cyt c nor recombinant active caspase-3 into the cytoplasm promoted nuclear apoptosis-related changes in U1810 cells. Evaluation of the role of the caspase-independent factor AIF in STS-mediated death revealed that, upon immunodepletion of AIF, cytosols from STS-treated U1810 lost their capacity to induce nuclear condensation when incubated with isolated nuclei. In addition, microinjection of an anti-AIF antibody prevented AIF from translocating to the nuclei of STS-treated U1810 cells and reduced STS-induced cell death. Finally, although the transfection-enforced overexpression of AIF was not sufficient to induce cell death, it did enhance STS-mediated cell killing. Altogether, these results indicate that activation of caspases is not sufficient to kill U1810 cells and rather suggests an important role for the AIF-mediated mitochondrial-mediated death pathway.

Mitochondrial function in apoptotic neuronal cell death

Apoptosis can be defined as the regulated death of a cell and is conducted by conserved pathways. Apoptosis of neurons after injury or disease differs from programed cell death, in the sense that neurons in an adult brain are not "meant" to die and results in a loss of function. Thus apoptosis is an honorable process by a neuron, a cell with limited potential to replace itself, choosing instead to commit suicide to save neighboring cells from release of cellular components that cause injury directly or trigger secondary injury resulting from inflammatory reactions. The excess of apoptosis of neuronal cells underlies the progressive loss of neuronal populations in neurodegenerative disorders and thus is harmful. Mitochondria are the primary source for energy in neurons but are also poised, through the "mitochondrial apoptosis pathway," to signal the demise of cells. This duplicity of mitochondria is discussed, with particular attention given to the specialized case of pathological neuronal cell death.

Apoptotic signaling pathways induced by nitric oxide in human lymphoblastoid cells expressing wild-type or mutant p53

Loss of p53 function by inactivating mutations results in abrogation of NO*induced apoptosis in human lymphoblastoid cells. Here we report characterization of apoptotic signaling pathways activated by NO* in these cells by cDNA microarray expression and immunoblotting. A p53-mediated transcriptional response to NO* was observed in p53-wild-type TK6, but not in closely related p53-mutant WTK1, cells. Several previously characterized p53 target genes were up-regulated transcriptionally in TK6 cells, including phosphatase PPM1D (WIP1), oxidoreductase homolog PIG3, death receptor TNFRSF6 (Fas/CD95), and BH3-only proteins BBC3 (PUMA) and PMAIP1 (NOXA). NO* also modulated levels of several gene products in the mitochondria-dependent and death-receptor-mediated apoptotic pathways. Inhibitors of apoptosis proteins X-chromosome-linked inhibitor of apoptosis, cellular inhibitor of apoptosis protein-1, and survivin were significantly down-regulated in TK6 cells, but not in WTK1 cells. Smac release from mitochondria was induced in both cell types, but release of apoptosis-inducing factor and endonuclease G was detected only in TK6 cells. Fas/CD95 was increased, and levels of the antiapoptotic proteins Bcl-2 and Bcl-x/L were reduced in TK6 cells. Activation of procaspases 3, 8, 9, and 10, as well as Bid and poly(ADP-ribose) polymerase cleavage, were observed only in TK6 cells. NO* treatment did not alter levels of death receptors 4 and 5, Fas-associated death domain or proapoptotic Bax and Bak proteins in either cell line. Collectively, these data show that NO* exposure activated a complex network of responses leading to p53-dependent apoptosis via both mitochondrial and Fas receptor pathways, which were abrogated in the presence of mutant p53.

Nuclear and mitochondrial conversations in cell death: PARP-1 and AIF signaling

Different cell-death mechanisms control many physiological and pathological processes in humans. Mitochondria play important roles in cell death through the release of pro-apoptotic factors such as cytochrome c and apoptosis-inducing factor (AIF), which activate caspase-dependent and caspase-independent cell death, respectively. Poly(ADP-ribose) polymerase 1 (PARP-1) is emerging as an important activator of caspase-independent cell death. PARP-1 generates the majority of long, branched poly(ADP-ribose) (PAR) polymers following DNA damage. Overactivation of PARP-1 initiates a nuclear signal that propagates to mitochondria and triggers the release of AIF. AIF then shuttles from mitochondria to the nucleus and induces peripheral chromatin condensation, large-scale fragmentation of DNA and, ultimately, cytotoxicity. Identification of the pro-death and pro-survival signals in the PARP-1-mediated cell-death program might provide novel therapeutic targets in human diseases.

Structural and functional characterization of mitochondrial EndoG, a sugar non-specific nuclease which plays an important role during apoptosis

Combining sequence analysis, structure prediction, and site-directed mutagenesis, we have investigated the mechanism of catalysis and substrate binding by the apoptotic mitochondrial nuclease EndoG, which belongs to the large family of DNA/RNA non-specific betabetaalpha-Me-finger nucleases. Catalysis of phosphodiester bond cleavage involves several highly conserved amino acid residues, namely His143, Asn174, and Glu182 required for water activation and metal ion binding, as well as Arg141 required for proper substrate binding and positioning, respectively. These results indicate that EndoG basically follows a similar mechanism as the Serratia nuclease, the best studied representative of the family of DNA/RNA non-specific nucleases, but that differences are observed for transition state stabilisation. In addition, we have identified two putative DNA/RNA binding residues of bovine EndoG, Arg135 and Arg186, strictly conserved only among mammalian members of the nuclease family, suggesting a similar mode of binding to single and double-stranded nucleic acid substrates by these enzymes. Finally, we demonstrate by ectopic expression of active and inactive variants of bovine EndoG in HeLa and CV1-cells that extramitochondrial active EndoG by itself induces cell death, whereas expression of an enzymatically inactive variant does not.

Apoptosis in human cancer cells

PURPOSE OF REVIEW

Apoptosis, or programmed cell death, is a vital physiologic process to eliminate damaged or unwanted cells. Defects in apoptosis promote tumor formation and make cancer cells resistant to therapy. This review provides an overview of recent advances in the understanding of apoptosis in human cancer cells.

RECENT FINDINGS

Recent studies revealed that the apoptotic machinery in humans consists of a molecular network of a large number of proteins. These proteins regulate a cascade of events in signaling, commitment and execution stages of apoptosis through multiple parallel pathways. Delineation of the basic mechanisms of apoptosis has shed light on how apoptosis is deregulated in human cancer cells. Therapeutic strategies based on apoptosis have also been designed to selectively target tumor cells.

SUMMARY

Understanding the basic mechanisms of apoptosis and determining how cancer cells evade apoptosis will afford discoveries of new molecular targets and better cancer therapies.

Toxic proteins released from mitochondria in cell death

A plethora of apoptotic stimuli converge on the mitochondria and affect their membrane integrity. As a consequence, multiple death-promoting factors residing in the mitochondrial intermembrane space are liberated in the cytosol. Pro- and antiapoptotic Bcl-2 family proteins control the release of these mitochondrial proteins by inducing or preventing permeabilization of the outer mitochondrial membrane. Once released into the cytosol, these mitochondrial proteins activate both caspase-dependent and -independent cell death pathways. Cytochrome c was the first protein shown to be released from the mitochondria into the cytosol, where it induces apoptosome formation. Other released mitochondrial proteins include apoptosis-inducing factor (AIF) and endonuclease G, both of which contribute to apoptotic nuclear DNA damage in a caspase-independent way. Other examples are Smac/DIABLO (second mitochondria-derived activator of caspase/direct IAP-binding protein with low PI) and the serine protease HtrA2/OMI (high-temperature requirement protein A2), which both promote caspase activation and instigate caspase-independent cytotoxicity. The precise mode of action and importance of cytochrome c in apoptosis in mammalian cells has become clear through biochemical, structural and genetic studies. More recently identified factors, for example HtrA2/OMI and Smac/DIABLO, are still being studied intensively in order to delineate their functions in apoptosis. A better understanding of these functions may help to develop new strategies to treat cancer.

Role of AIF in caspase-dependent and caspase-independent cell death

The major challenge in treating cancer is that many tumor cells carry mutations in key apoptotic genes such as p53, Bcl family proteins or those affecting caspase signaling. Such defects render treatment with traditional chemotherapeutic agents ineffective. Many studies have demonstrated the importance of caspase-independent cell death pathways in injury, degenerative diseases and tumor tissue. It is now recognized that in addition to their critical role in the production of cellular energy, mitochondria are also the source of key proapoptotic molecules involved in caspase activation. More recently, it has been discovered that in response to apoptotic stimuli, mitochondria can also release caspase-independent cell death effectors such as AIF and Endonuclease G. In this review, we examine the role of Bcl family proteins and poly(ADP-ribose) polymerase-1 signaling in the regulation of these apoptotic pathways and address the ongoing controversies in this field. Continued study of the mechanisms of apoptosis including caspase-independent death processes are likely to reveal novel therapeutic targets for the treatment of diverse human pathologies including cancer, neurodegenerative diseases and acute injuries such as stroke or myocardial infarction.