Prevention of vertebrate neuronal death by the crmA gene

Increased cerebellar Purkinje cell numbers in mice overexpressing a human bcl-2 transgene

The Purkinje cell is a primary organizer in the development of the cerebellum. Purkinje cells may provide positional information cues that regulate afferent innervation, and Purkinje cell target size controls the adult number of afferent olivary neurons and granule cells. While Purkinje cells are necessary for the survival of olivary neurons and granule cells during periods of programmed cell death, little is known about the survival requirements of Purkinje cells in vivo. To determine if Purkinje cells are subject to programmed cell death during development we have analyzed Purkinje cell numbers in two lines of transgenic mice that overexpress a human gene for bcl-2 (Hu-bcl-2). Bcl-2 is a protooncogene that inhibits apoptosis in many cell types. Overexpression of bcl-2 in vitro and in vivo rescues neurons from trophic factor deprivation or naturally occurring cell death. In the mice analyzed in this study, transgene expression is driven by the neuron-specific enolase promoter that is first expressed embryonically in most regions of the brain in one line and postnatally in the second line. We have counted Purkinje cells in three adult control mice, five early overexpressing transgenics, and three late expressing transgenics. The number of Purkinje cells in the Hu-bcl-2 transgenic mice is significantly increased above control numbers, with an increase of 43% in the embryonically overexpressing line and an increase of 27% in the postnatally overexpressing line. Because bcl-2 overexpression has been shown to rescue other neurons from programmed cell death, the increase in Purkinje cell numbers in overexpressing bcl-2 transgenics suggests that Purkinje cells undergo a period of cell death during normal development.

Dissecting processing and apoptotic activity of a cysteine protease by mutant analysis

We have compared the behavior of wild-type mouse NEDD-2, a neural precursor cell-expressed, developmentally down-regulated cysteine protease gene, to various mutant forms of the gene in both apoptotic activity in neuronal cells and proteolytic cleavage in the Semliki Forest virus and rabbit reticulocyte protein expression systems. Our results confirm that NEDD-2 processing and apoptotic activity are linked phenomena. They identify aspartate residues as likely targets for autocatalytic cleavage. They establish that cleavage events only occur at specific sites. Finally, they pinpoint differential effects of individual mutations on the overall proteolytic cleavage patterns, raising interesting questions related to the mechanisms of subunit assembly.

Activation of an interleukin 1 converting enzyme-dependent apoptosis pathway by granzyme B

Cytotoxic T lymphocytes (CTL) can induce apoptosis through a granzyme B-based killing mechanism. Here we show that in cells undergoing apoptosis by granzyme B, both p45 pro-interleukin 1 beta converting enzyme (ICE) and pro-CPP32 are processed. Using ICE deficient (ICE -/-) mice, embryonic fibroblasts exhibit high levels of resistance to apoptosis by granzyme B or granzyme 3, while B lymphoblasts are granzyme B-resistant, thus identifying an ICE-dependent apoptotic pathway that is activated by CTL granzymes. In contrast, an alternative ICE-independent pathway must also be activated as ICE -/- thymocytes remain susceptible to apoptosis by both granzymes. In ICE -/- B cells or HeLa cells transfected with mutant inactive ICE or Ich-1S that exhibit resistance to granzyme B, CPP32 is processed to p17 and poly(ADP-ribose) polymerase is cleaved indicating that this protease although activated was not associated with an apoptotic nuclear phenotype. Using the peptide inhibitor Ac-DEVD-CHO, apoptosis as well as p45 ICE hydrolysis are suppressed in HeLa cells, suggesting that a CPP32-like protease is upstream of ICE. In contrast, p34cdc2 kinase, which is required for granzyme B-induced apoptosis, remains inactive in ICE -/- B cells indicating it is downstream of ICE. We conclude that granzyme B activates an ICE-dependent cell death pathway in some cell types and requires a CPP32-like Ac-DEVD-CHO inhibitable protease acting upstream to initiate apoptosis.

ICE-like proteases execute the neuronal death program

The past year has witnessed significant advances in our understanding of the mechanisms that kill neurons during programmed cell death. The executioners are members of a family of proteases founded by ced-3, the product of a gene that is required for programmed cell death in the nematode Caenorhabditis elegans, and by mammalian interleukin-1 beta-converting enzyme. These proteases represent interesting novel targets for the therapy of acute and chronic pathologies of the nervous system associated with neuronal death.

Kainate induces apoptosis in neurons

Growing evidence suggests that non-N-methyl-D-aspartate receptor activation may contribute to neuronal death in both acute and chronic neurological diseases. The intracellular processes that mediate this form of neuronal death are poorly understood. We have previously characterized a model of kainate neurotoxicity using cerebellar granule cell neurons in vitro and we sought to determine the mechanism of kainate-induced neurons degeneration. We found DNA, and chromatin condensation using a fluorescent DNA intercalating dye, in cerebellar granule cells following exposure to kainate (100 microM). Aurintricarboxylic acid protected cerebellar granule cells from kainate-induced death. While the morphological and biochemical features of neuronal death induced by kainate resembled low-K(+)-induced apoptosis in cerebellar granule cells; the time interval from the institution of the death-promoting condition to neuronal death was briefer with kainate and did not require new protein or RNA synthesis. These results demonstrate that kainate receptor activation can induce transcription-independent apoptosis in neurons. This in vitro model should be useful in identifying the intracellular pathways that link kainate receptor activation with apoptosis.

Ordering the cell death pathway. Differential effects of BCL2, an interleukin-1-converting enzyme family protease inhibitor, and other survival agents on JNK activation in serum/nerve growth factor-deprived PC12 cells

Previous studies indicate that activation of c-Jun kinase (JNK) is necessary for apoptosis of trophic factor-deprived PC12 cells and that death in this system is suppressed by multiple agents, including BCL2, inhibitors of the interleukin-1-converting enzyme (ICE) family of proteases, blockers of transcription, and a variety of small molecules with differing modes of action. Here, we determine the order in which these agents block apoptosis relative to JNK activation. Overexpression of BCL2 promotes PC12 cell survival and blocks JNK activation caused by trophic factor withdrawal. Similarly, the survival-promoting agents aurintricarboxylic acid, N-acetylcysteine, the nitric oxide generator diethylenetriamine nitric oxide, 8-bromo-cGMP, and 8-(4-chlorophenylthio)-cAMP act upstream to inhibit JNK activation. In contrast, zVAD-fluoromethylketone (a permeant ICE family inhibitor), actinomycin D, and the G1/S cell cycle inhibitor deferoxamine, all promote survival after trophic factor withdrawal, but do not affect JNK activation. These findings are consistent with the presence of an ordered cell death pathway triggered by trophic factor deprivation in which 1) BCL2 and a number of survival-promoting agents act upstream of JNK, 2) ICE family protease actions, regulated genes required for cell death, and certain cell cycle blockers lie either downstream of JNK or on independent pathways required for apoptotic death.

Morphological and biochemical changes in neurons: apoptosis versus mitosis

Apoptosis and mitosis are often thought to share certain morphological similarities and therefore to be regulated by similar sets of enzymes. In this study, the Golgi apparatus and nuclear lamina were examined in PC12 cells and rat superior cervical ganglion neurons undergoing apoptosis in response to withdrawal of nerve growth factor or addition of staurosporine. We found that the Golgi apparatus disperses during apoptosis, without obvious degradation, in a manner similar to that occurring in mitosis. In contrast, the nuclear lamina did not become completely solubilized during apoptosis, as occurs in mitosis, but remained as a distinct structure around the nucleus, although some degradation of nuclear lamins was seen. To assess the integrity of the nuclear envelope, fluorescent probes were introduced into the cytoplasm of live and dying cells. High molecular weight tracers were still excluded from the nuclei of apoptotic cells, demonstrating the continued existence of a functional nuclear barrier. These data suggest, therefore, that cell death is unlikely to occur simply as a result of inappropriate activation of cell cycle enzymes.

ICE inhibitor YVADcmk is a potent therapeutic agent against in vivo liver apoptosis

In liver, apoptosis is a physiological process involved in the clearance of injured cells and in homeostatic control [1]. However, in patients with viral fulminant hepatitis or with nonacute liver diseases [2], dramatic liver failure or secondary cirrhosis results from the death of hepatocytes, which express the cell-surface receptor Fas, by apoptosis. To date, treatment of fulminant hepatitis relies mainly on orthotopic liver transplantation, which is limited by immunological complications and graft availability. Unravelling the molecular mechanisms that underlie acute liver failure could allow the design of an appropriate therapy. Ligand-bound Fas and tumour necrosis factor alpha (TNF-alpha) induce hepatic apoptosis in mice [3-6]. In various cell types, Fas- or TNF-alpha-induced apoptosis is blocked by viral proteins (such as p35 and CrmA) as well as by a decoy peptide (YVADcmk) [7-11], suggesting that these mechanisms of apoptosis involve ICE (interleukin-1 beta converting enzyme)-like proteases. Here, we report that, in vivo, pre-treatment of mice with YVADcmk protects them from the lethal effect of anti-Fas antibody and from liver failure induced by injection of TNF-alpha. Remarkably, YVADcmk administration is also highly effective in rescuing mice that have been pretreated with anti-Fas antibody from rapid death, despite extensive hepatic apoptosis. This dramatic curative effect could be of clinical benefit for the treatment of viral and inflammatory liver diseases.

Characterization of a myxoma virus-encoded serpin-like protein with activity against interleukin-1 beta-converting enzyme

A genomic library of myxoma virus (MV) DNA, a leporipoxvirus that causes myxomatosis, was constructed and screened by in vitro transcription-translation. A clone was selected on the basis of its strong reactivity with MV antiserum. Analysis of the corresponding DNA sequence and the deduced amino acid sequence revealed an open reading frame coding for a 34-kDa protein with strong homologies to members of the serpin superfamily. The gene encoding this new protein, called serp2, was localized on the MV genome. Interestingly, this gene is deleted in an attenuated strain. We constructed a baculovirus vector to produce recombinant Serp2 protein and raised specific antisera that allowed the characterization of Serp2 expression during the MV cycle. The biological relevance of this new serpin from MV was monitored, and it was shown that Serp2 could inhibit human interleukin-1 beta-converting enzyme activity.

Protection against apoptosis by the vaccinia virus SPI-2 (B13R) gene product

Vaccinia virus contains a gene, termed SPI-2 or B13R, that is closely related in its sequence to a potent inhibitor of apoptosis from cowpox virus (crmA). Infection by vaccinia virus protects HeLa cells against apoptosis that is induced by an immunoglobulin M antibody against the fas receptor or by tumor necrosis factor alpha. This effect is profoundly reduced when the SPI-2 gene is deleted. The SPI-2 gene, when transiently expressed in these cells, can also protect against apoptosis mediated by these agents. Given the similarity to crmA, it seems likely that SPI-2 functions in an analogous fashion, inhibiting the activity of ICE protease family members and blocking the onset of apoptosis.

Identification and characterization of Ich-3, a member of the interleukin-1beta converting enzyme (ICE)/Ced-3 family and an upstream regulator of ICE

We report here the isolation and characterization of a new member of the ice/ced-3 family of cell death genes, named ich-3. The predicted amino acid sequence of Ich-3 protein shares 54% identity with murine interleukin-1beta converting enzyme (ICE). Overexpression of ich-3 in Rat-1 and HeLa cells induces apoptosis, which can be inhibited by CrmA and Bcl-2. The mRNA and proteins of ich-3 are dramatically induced in vivo upon stimulation with lipopolysaccharide, an inducer of septic shock. The ich-3 gene product can be cleaved by cytotoxic T cells granule serine protease granzyme B, suggesting that Ich-3 may mediate apoptosis induced by granzyme B. Ich-3 does not process proIL-1beta directly but does promote proIL-1beta processing by ICE. These results suggest that Ich-3 may play a very important role in apoptosis and inflammatory responses and may be an upstream regulator of ICE.

Differential inhibition of the Fas- and granule-mediated cytolysis pathways by the orthopoxvirus cytokine response modifier A/SPI-2 and SPI-1 protein

Cytotoxic T lymphocytes are important effectors of antiviral immunity, and they induce target cell death either by secretion of cytoplasmic granules containing perforin and granzymes or by signaling through the Fas cell surface antigen. Although it is not known whether the granule-mediated and Fas-mediated cytolytic mechanisms share common components, proteinase activity has been implicated as an important feature of both pathways. The orthopoxviruses cowpox virus and rabbitpox virus each encode three members of the serpin family of proteinase inhibitors, designated SPI-1, SPI-2, and SPI-3. Of these, SPI-2 (also referred to as cytokine response modifier A in cowpox virus) has been shown to inhibit the proteolytic activity of both members of the interleukin 1 beta converting enzyme family and granzyme B. We report here that cells infected with cowpox or rabbitpox viruses exhibit resistance to cytolysis by either cytolytic mechanism. Whereas mutation of the cytokine response modifier A/SPI-2 gene was necessary to relieve inhibition of Fasmediated cytolysis, in some cell types mutation of SPI-1, in addition to cytokine response modifier A/SPI-2, was necessary to completely abrogate inhibition. In contrast, viral inhibition of granule-mediated killing was unaffected by mutation of cytokine response modifier A/SPI-2 alone, and it was relieved only when both the cytokine response modifier A/SPI-2 and SPI-1 genes were inactivated. These results suggest that an interleukin 1 beta converting enzyme-like enzymatic activity is involved in both killing mechanisms and indicate that two viral proteins, SPI-1 and cytokine response modifier A/SPI-2, are necessary to inhibit both cytolysis pathways.

Functional role of interleukin 1 beta (IL-1 beta) in IL-1 beta-converting enzyme-mediated apoptosis

Prointerleukin-1 beta (pro-IL-1 beta) is the only known physiologic substrate of the interleukin-1 beta (IL-1 beta)-converting enzyme (ICE), the founding member of the ICE/ced-3 cell death gene family. Since secreted mature IL-1 beta has been detected after apoptosis, we investigated whether this cytokine, when produced endogenously, plays a role in cell death. We found that hypoxia-induced apoptosis can be inhibited by either the IL-1 receptor antagonist (IL-1Ra) or by neutralizing antibodies to IL-1 or to its type 1 receptor. IL-1Ra also inhibits apoptosis induced by trophic factor deprivation in primary neurons, as well as by tumor necrosis factor alpha in fibroblasts. In addition, during the G1/S phase arrest, mature IL-1 beta induces apoptosis through a pathway independent of CrmA-sensitive gene activity. We also demonstrate that Ice, when expressed in COS cells, requires the coexpression of pro-IL-1 beta for the induction of apoptosis, which is inhibited by IL-1Ra. Interestingly, we found that mature IL-1 beta has antiapoptotic activity when added exogenously before the onset of hypoxia, which we found is caused in part by its ability to downregulate the IL-1 receptor. Our findings demonstrate that pro-IL-1 beta is a substrate of ICE relevant to cell death, and depending on the temporal cellular commitment to apoptosis, mature IL-1 beta may function as a positive or negative mediator of cell death.

Fas-induced activation of the cell death-related protease CPP32 Is inhibited by Bcl-2 and by ICE family protease inhibitors

The human proto-oncogene bcl-2 and its Caenorhabditis elegans homologue ced-9 inhibit programmed cell death. In contrast, members of the human interleukin-1beta converting enzyme (ICE) family of cysteine proteases and their C. elegans homologue CED-3 promote the death program. Genetic experiments in C. elegans have shown that ced-9 is formally a negative regulator of ced-3 function, but neither those studies nor others have determined whether CED-9 or Bcl-2 proteins act biochemically upstream or downstream of CED-3/ICE proteases. CPP32, like all known members of the CED-3/ICE family, is synthesized as a proenzyme that is subsequently processed into an active protease with specificity for cleavage at Asp-X peptide bonds. In this report, we demonstrate that the CPP32 proenzyme is proteolytically processed and activated in Jurkat cells induced to die by Fas ligation. CPP32 activation is blocked by cell-permeable inhibitors of aspartate-directed, cysteine proteases, suggesting that pro-CPP32 is cleaved by active CPP32 or by other ICE family members. Heterologous expression of Bcl-2 in Jurkat cells prevents Fas-induced cell death as well as proteolytic processing and activation of CPP32. Thus, Bcl-2 acts at or upstream of the CPP32 activation step to inhibit apoptosis induced by Fas stimulation.

Bcl-2 down-regulates the activity of transcription factor NF-kappaB induced upon apoptosis

Among the many target genes of the transcription factor NF-kappaB are p53 and c-myc, both of which are involved in apoptosis. This prompted us to investigate the role of NF-kappaB in this process. We report that NF-kappaB is potently activated upon serum starvation, a condition leading to apoptosis in 293 cells. Similar to Bcl-2, a transdominant-negative mutant of the NF-kappaB p65 subunit partially inhibited apoptosis, indicating a direct involvement of the transcription factor in induction of cell death. As expected, the p65 mutant suppresses kappaB-dependent gene expression. Surprisingly, transiently or stably overexpressed Bcl-2 had the same effect. The transcription inhibitory activity of the two proteins correlated with their cell death protective potential. Like Bcl-2, the related protein Bcl-xL but not Bcl-xS was able to suppress kB-dependent transcription. Bcl-2 inhibited NF-kappaB activity by an unusual mechanism. It did not prevent the release of IkappaB in the cytoplasm but down-modulated the transactivating potential of nuclear p65. These data show that NF-kappaB can participate in apoptosis. We suggest that at least part of the anti-apoptotic potential of Bcl-2 may be explained from a hitherto undiscovered activity of Bcl-2 in controlling nuclear gene expression.

Constitutive expression of the machinery for programmed cell death

In the presence of cycloheximide (CHX) to inhibit protein synthesis, a high concentration of staurosporine (STS) induces almost all cells in explant cultures of 8/8 types of newborn mouse organs and 3/3 types of adult mouse organs to die with the characteristic features of apoptosis. Eggs and blastomeres also die in this way when treated with STS and CHX, although they are less sensitive to this treatment than trophectoderm or inner cell mass cells whose sensitivity resembles that of other developing cells. Human red blood cells are exceptional in being completely resistant to treatment with STS and CHX. As (STS plus CHX)-induced cell deaths have been shown to display the characteristic features of programmed cell death (PCD), we conclude that all mammalian nucleated cells are capable of undergoing PCD and constitutively express all the proteins required to do so. It seems that the machinery for PCD is in place and ready to run, even though its activation often depends on new RNA and protein synthesis.

Role of Ced-3/ICE-family proteases in staurosporine-induced programmed cell death

In the accompanying paper by Weil et al. (1996) we show that staurosporine (STS), in the presence of cycloheximide (CHX) to inhibit protein synthesis, induces apoptotic cell death in a large variety of nucleated mammalian cell types, suggesting that all nucleated mammalian cells constitutively express all of the proteins required to undergo programmed cell death (PCD). The reliability of that conclusion depends on the evidence that STS-induced, and (STS + CHS)-induced, cell deaths are bona fide examples of PCD. There is rapidly accumulating evidence that some members of the Ced-3/Interleukin-1 beta converting enzyme (ICE) family of cysteine proteases are part of the basic machinery of PCD. Here we show that Z-Val-Ala-Asp-fluoromethylketone (zVAD-fmk), a cell-permeable, irreversible, tripeptide inhibitor of some of these proteases, suppresses STS-induced and (STS + CHX)-induced cell death in a wide variety of mammalian cell types, including anucleate cytoplasts, providing strong evidence that these are all bona fide examples of PCD. We show that the Ced-3/ICE family member CPP32 becomes activated in STS-induced PCD, and that Bcl-2 inhibits this activation. Most important, we show that, in some cells at least, one or more CPP32-family members, but not ICE itself, is required for STS-induced PCD. Finally, we show that zVAD-fmk suppresses PCD in the interdigital webs in developing mouse paws and blocks the removal of web tissue during digit development, suggesting that this inhibition will be a useful tool for investigating the roles of PCD in various developmental processes.

The contrasting roles of ICE family proteases and interleukin-1beta in apoptosis induced by trophic factor withdrawal and by copper/zinc superoxide dismutase down-regulation

We compare here the mechanisms of apoptotic death of PC12 cells induced by down-regulation of Cu2+,Zn2+ superoxide dismutase (SOD1) and withdrawal of trophic support (serum/nerve growth factor). Our previous results indicated that the initiating causes of death are different in each paradigm. However, bcl-2 rescues cells in either paradigm, suggesting common downstream elements to the cell death pathway. To determine whether the ICE [interleukin 1beta converting enzyme] family of proteases, which is required for apoptosis on trophic factor withdrawal, is also required for apoptosis induced by oxidative stress, we have developed a novel peptide inhibitor that mimics the common catalytic site of these enzymes and thereby blocks their access to substrates. This differs from the more usual pseudosubstrate approach to enzyme inhibition. Blockade of ICE family proteases by either this inhibitor or by a permeant competitive ICE family antagonist rescues PC12 cells from apoptotic death following apoptosis induced by down-regulation of SOD1, as well as from trophic factor/nerve growth factor deprivation. SOD1 down-regulation results in an increase in interleukin 1beta (IL- 1beta) production by the cells, and cell death under these conditions can be prevented by either blocking antibodies against IL-1beta or the IL-1 receptor antagonist (IL-1Ralpha). In contrast, trophic factor withdrawal does not increase IL-1beta secretion, and the blocking antibody failed to protect PC12 cells from trophic factor withdrawal, whereas the receptor antagonist was only partially protective at very high concentrations. There were substantial differences in the concentrations of pseudosubstrate inhibitors which rescued cells from SOD1 down-regulation and trophic factor deprivation. These results suggest the involvement of different members of the ICE family, different substrates, or both in the two different initiating causes of cell death.

Bc1-2 protects mice against fatal alphavirus encephalitis

Virus-induced apoptosis has been well characterized in vitro, but the role of apoptosis in viral pathogenesis is not well understood. The suicide of a cell in response to viral infection is postulated to be an important host defense for the organism, leading to a reduction in its total viral burden. However, virus-induced death of nonregenerating cells in the central nervous system may be detrimental to the host. Therefore, to investigate the role of apoptosis in the pathogenesis of fatal encephalitis, we constructed a recombinant alphavirus chimera that expresses the antiapoptotic gene, bcl-2, in virally infected neural cells. Infection of neonatal mice with the alphavirus chimera expressing human bcl-2 [Sindbis virus (SIN)/bcl-2] resulted in a significantly lower mortality rate (7.5%) as compared with infection with control chimeric viruses containing a chloramphenicol acetyltransferase (CAT) reporter gene (SIN/CAT) (78.1%) or bcl-2 containing a premature stop codon (SIN/bcl-2stop) (72.1%) (P < 0.001). Viral titers were reduced 5-fold 1 day after infection and 10-fold 6 days after infection in the brains of SIN/bcl-2-infected mice as compared to SIN/CAT or SIN/bcl-2stop-infected mice. In situ end labeling to detect apoptotic nuclei demonstrated a reduction in the number of foci of apoptotic cells in the brains of mice infected with SIN/bcl-2 as compared with SIN/bcl-2stop. The reduction in apoptosis was associated with a reduction in the number of foci of cells expressing alphavirus RNA. Thus, the antiapoptotic gene, bcl-2, suppresses viral replication and protects against a lethal viral disease, suggesting an interaction between cellular genetic control of viral replication and cell death.

Processing and activation of CMH-1 by granzyme B

Granzyme B plays an essential role in cytotoxic T lymphocyte (CTL)-mediated cell killing. Recent studies suggest that granzyme B may exert its effect by cleaving and activating CPP32, a member of the interleukin-1 beta-converting enzyme/Ced-3 family of cysteine proteases. We have examined the processing and activation of CMH-1, a close homologue of CPP32, by granzyme B in vitro. We have found that granzyme B specifically cleaves CMH-1 at Asp198-Ser199 between the p20 and p12 and activates the cysteine protease. Cleavage between p20 and the prosequence of CMH-1 at Asp23-Ala24 is autocatalytic and is not required for CMH-1 activity in vitro. The cleavage and activation of CMH-1 by granzyme B in vitro sugge st that, in addition to CPP32, CMH-1 may also play a role in CTL-mediated cell killing.

Neurogenesis and cell death in olfactory epithelium

The olfactory epithelium (OE) of the mammal is uniquely suited as a model system for studying how neurogenesis and cell death interact to regulate neuron number during development and regeneration. To identify factors regulating neurogenesis and neuronal death in the OE, and to determine the mechanisms by which these factors act, investigators studied OE using two major experimental paradigms: tissue culture of OE; and ablation of the olfactory bulb or severing the olfactory nerve in adult animals, procedures that induce cell death and a subsequent surge of neurogenesis in the OE in vivo. These studies characterized the cellular stages in the olfactory receptor neuron (ORN) lineage, leading to the realization that at least three distinct stages of proliferating neuronal precursor cells are employed in generating ORNs. The identification of a number of factors that act to regulate proliferation and survival of ORNs and their precursors suggests that these multiple developmental stages may serve as control points at which cell number is regulated by extrinsic factors. In vivo surgical studies, which have shown that all cell types in the neuronal lineage of the OE undergo apoptotic cell death, support this idea. These studies, and the possible coregulation of neuronal birth and apoptosis in the OE, are discussed.

Mechanisms of neuronal cell death. An overview

Neuronal cell death is both a vital component of the embryo-genesis of the nervous system and forms the basis of all neurodegenerative diseases. This overview explores the fundamental mechanisms underlying neuronal cell death at a cellular and molecular level. The significance of the mode of neuronal death is compared with respect to physiological (developmental) and pathological neuronal loss.

The cell-death machine

Apoptosis, or programmed cell death, is the physiological process whereby individual cells are deliberately eliminated to achieve homeostasis and proper metazoan development. Numerous genes have recently been identified that are involved in apoptosis: some are believed to encode death effectors, whereas others encode positive or negative regulators of the cell-death machine. Precisely how these various proteins interact in the molecular mechanism of apoptosis remains to be discovered.

Sequential activation of ICE-like and CPP32-like proteases during Fas-mediated apoptosis

Binding of Fas ligand or an agonistic anti-Fas antibody induces apoptosis in Fas-bearing cells. The interleukin-1Beta-converting enzyme (ICE) is a cysteine protease that is involved in apoptosis induced by various stimuli, including Fas-mediated apoptosis. Several ICE homologues have been identified, and these are subdivided into three groups (ICE-, CPP32-, and Ich-1-like proteases). We show here that specific inhibitors of ICE- or CPP32-like proteases can inhibit Fas-mediated apoptosis. Transient ICE-like activity was found in the cytosolic fraction of Fas-activated cells, whereas ICE-dependent, CPP32-like activity gradually accumulated in the cytosol. Cell lysates from mouse lymphoma supplemented with either recombinant ICE or CPP32 induced apoptosis of nuclei. The CPP32 inhibitor inhibited ICE- or CPP32-induced apoptosis in the cell-free system, whereas the ICE-inhibitor only inhibited ICE-induced apoptosis. Cell extracts from thymocytes from ICE-null mice induced apoptosis in the cell-free system when it was supplemented with CPP32. These results indicate that Fas sequentially activates ICE- and CPP32-like proteases, and that downstream CPP32, together with a component(s) in the cytoplasm, causes apoptosis of nuclei.

Molecular mechanisms of lymphocyte-mediated cytotoxicity and their role in immunological protection and pathogenesis in vivo

Studies with perforin-deficient mice have demonstrated that two independent mechanisms account for T cell-mediated cytotoxicity: A main pathway is mediated by the secretion of the pore-forming protein perforin by the cytotoxic T cell, whereas an alternative nonsecretory pathway relies on the interaction of the Fas ligand that is upregulated during T cell activation with the apoptosis-inducing Fas molecule on the target cell. NK cells use the former pathway exclusively. The protective role of the perforin-dependent pathway has been shown for infection with the noncytopathic lymphocytic choriomeningitis virus, for infection with Listeria monocytogenes, and for the elimination of tumor cells by T cells and NK cells. In contrast, perforin-dependent cytotoxicity is not involved in protection against the cytopathic vaccinia virus and vesicular stomatitis virus. LCMV-induced immunopathology and autoimmune diabetes have been found to require perforin-expression. A contribution of perforin-dependent cytotoxicity to the rejection of MHC class I-disparate heart grafts has also been observed. Its absence is efficiently compensated in rejection of fully allogeneic organ or skin grafts. So far, evidence for a role of Fas-dependent cytotoxicity as a T cell effector mechanism in vivo is lacking. Current data suggest that the main function of Fas may be in regulation of the immune response and apparently less at the level of an effector mechanism in host defense. Further analysis is necessary, however, to settle this point finally.

One step ahead of the game: viral immunomodulatory molecules

For decades cell biologists have relied on viruses to facilitate the study of complex cellular function. More recently, the tragedy of the AIDS epidemic has focused considerable human and financial resources on both virology and immunology, resulting in the generation of new information relating these disciplines. As the miracle of the mammalian immune system unfolds in the laboratory, the elegance of the mechanisms used by co-evolving viruses to circumvent detection and destruction by the host becomes inescapably obvious. Although many observation of virus-induced phenomena that likely contribute to the virus's escape of immune surveillance are still empirical, many other such phenomena have now been defined at the molecular level and confirmed in in vivo models. Immune modulators encoded within viral genomes include proteins that regulate antigen presentation, function as cytokines or cytokine antagonists, inhibit apoptosis, and interrupt the complement cascade. The identification of such gene products and the elucidation of their function have substantially strengthened our understanding of specific virus-host interactions and, unexpectedly, have contributed to the recognition of potent synergy between viruses, which can result in an unpredictable exacerbation of disease in co-infected individuals. Because many viral immune modulators clearly have host counterparts, viruses provide a valuable method for studying normal immune mechanisms. It is conceivable that an improved understanding of virus-encoded immunomodulators will enhance our ability to design reagents for use in therapeutic intervention in disease and in vaccine development.

Negative regulatory activity of a prostaglandin F2 alpha receptor associated protein (FPRP)

The cDNA has been cloned for a protein which copurifies with and colocalizes with [3H]PGF2 alpha binding activity. This cloning was based on prior purification of the [3H]PGF2 alpha binding complex from pregnant corpus luteum, antibody production against the protein of interest, and antibody screening of a rat ovary cDNA expression library. Here I report on the activity of this prostaglandin F2 alpha receptor (FP) associated protein (FPRP). Expression of the FPRP cDNA in COS cells results in production of a full length (approximately 130 kD) immunoreactive molecule with an endoplasmic reticulum and Golgi network distribution similar to that seen in granulosa lutein cells. COS cell expressed FPRP inhibits binding of [3H]PGF2 alpha to FP of COS cell origin or FP expressed from cotransfected rat or mouse FP cDNA in a dose-dependent manner. This inhibition of [3H]PGF2 alpha binding by FPRP occurs only when the FPRP cDNA is expressed in the same cell as the FP resides, reaches a maximum of approximately 80%, and is unaffected by second messenger perturbing agents such as phorbol ester, 8-Br-cAMP, calcium ionophore A23187, and okadaic acid. Scatchard analysis indicates that FPRP induces a decrease in receptor number rather than affinity constant, suggesting a non-competitive means of inhibition. Molecular dissection of the FPRP protein indicates that two portions of the molecule play a role in the inhibition of FP. Whether FPRP is an FP-associated regulatory molecule, an FP subunit, or a receptor for a PGF2 alpha-antagonistic ligand is presently unknown. Physiological relevance and significance of FPRP are discussed. During the course of these experiments it was necessary to clone the rat FP cDNA.

Temporal changes in chromatin, intracellular calcium, and poly(ADP-ribose) polymerase during Sindbis virus-induced apoptosis of neuroblastoma cells

Sindbis virus (SV) induces apoptosis in many vertebrate cells, but the mechanism is unknown. To gain insight into this mechanism, the nature and time course of intracellular changes related to programmed cell death were studied in SV-infected mouse neuroblastoma cells. New virus production began at 5 h after infection and reach a peak at 12 h. Hoechst 33342 staining of DNA analyzed by flow cytometry demonstrated changes in chromatin beginning 6 h after infection. These chromatin changes were cell cycle dependent, affecting cells in G0/G1 but not S phase. Apoptosis was not dependent on increases in intracellular Ca2+ and occurred more rapidly in the absence of extracellular Ca2+. Nuclear changes were accompanied by activation of the DNA repair enzyme poly(ADP-ribose) polymerase (PARP), resulting in increased consumption of NAD which was apparent by 10 h after infection. SV-induced apoptosis also involved the proteolytic cleavage of PARP. This cleavage was detectable at 16 h after infection approximately the same time that DNA fragmentation was apparent by agarose gel electrophoresis. We conclude that SV-induced apoptosis of neuroblastoma cells is dependent on viral replication, is not dependent on a rise in intracellular Ca2+, and is accompanied by activation of PARP and of a protease that cleaves PARP.

Regionally specific induction of ICE mRNA and enzyme activity in the rat brain and adrenal gland by LPS

Pro interleukin-1 beta converting enzyme (ICE) activity in the pituitary was found to be significantly increased 4 h after intraperitoneal injection of E. coli lipopolysaccharides, when distribution and inducibility of the enzyme was studied in the adult rat brain and the adrenal gland, using an artificial fluorescence peptide substrate. The same lipopolysaccharide treatment induced ICE mRNA levels in the pituitary, adrenal gland and hypothalamus as studied by reverse transcript-polymerase chain reaction.

Proteolytic activation of the cell death protease Yama/CPP32 by granzyme B

The serine protease granzyme B, which is secreted by cytotoxic cells, is one of the major effectors of apoptosis in susceptible targets. To examine the apoptotic mechanism of granzyme B, we have analyzed its effect on purified proteins that are thought to be components of death pathways inherent to cells. We demonstrate that granzyme B processes interleukin 1beta-converting enzyme (ICE) and the ICE-related protease Yama (also known as CPP32 or apopain) by limited proteolysis. Processing of ICE does not lead to activation. However, processing by granzyme B leads directly to the activation of Yama, which is now able to bind inhibitors and cleave the substrate poly(ADP-ribose) polymerase whose proteolysis is a marker of apoptosis initiated by several other stimuli. Thus ICE-related proteases can be activated by serine proteases that possess the correct specificity. Activation of pro-Yama by granzyme B is within the physiologic range. Thus the cytotoxic effect of granzyme B can be explained by its activation of an endogenous protease component of a programmed cell death pathway.

Cleavage of actin by interleukin 1 beta-converting enzyme to reverse DNase I inhibition

Three of the predominant features of apoptosis are internucleosomal DNA fragmentation, plasma membrane bleb formation, and retraction of cell processes. We demonstrate that actin is a substrate for the proapoptotic cysteine protease interleukin 1beta-converting enzyme. Actin cleaved by interleukin 1beta-converting enzyme can neither inhibit DNase I nor polymerize to its filamentous form as effectively as intact actin. These findings suggest a mechanism for the coordination of the proteolytic, endonucleolytic, and morphogenetic aspects of apoptosis.

DNA damage and repair in central nervous system injury: National Institute of Neurological Disorders and Stroke Workshop Summary

BACKGROUND AND PURPOSE

DNA damage and repair are areas of research with important implications for stroke and cerebral trauma. DNA damage is present in central nervous system (CNS) injury, and defects in repair mechanisms are associated with neurodegenerative disease.

METHODS

A workshop, DNA Damage and Repair in CNS Injury, was organized by the National Institute of Neurological Disorders and Stroke in Bethesda, Md, on September 11, 1995. The objective of this workshop was to promote inquiry and to foster application of research in DNA damage and repair after stroke and trauma.

RESULTS

The participants discussed the connection between the fields of DNA damage and repair and stroke and trauma and identified gaps in knowledge to be filled to expand research of DNA damage and repair in CNS injury. Specific recommendations were made targeting research opportunities in the areas of DNA repair and damage in stroke and trauma.

CONCLUSIONS

Research in the science of DNA injury and repair will likely provide new and important information on mechanisms of cell damage and provide opportunities for the development of novel and effective therapies to reduce CNS injury in stroke and trauma.

ICE/CED3-like proteases as therapeutic targets for the control of inappropriate apoptosis

Excessive or failed apoptosis is a prominent morphological feature of several human diseases. Many of the key biochemical players that contribute to the highly ordered process of apoptotic cell death have recently been identified. These include members of the emerging family of cysteine proteases related to mammalian interleukin-1 beta converting enzyme (ICE) and to CED-3, the product of a gene that is necessary for programmed cell death in the nematode C. elegans. Among a growing number of potential molecular targets for the control of human diseases where inappropriate apoptosis is prominent, ICE/CED-3-like proteases may be an attractive and tangible point for therapeutic intervention.

Developing Caenorhabditis elegans neurons may contain both cell-death protective and killer activities

We developed a method for examining the effects of overexpressing cell-death-related genes in specific Caenorhabditis elegans neurons that normally live. Using this method, we demonstrated that the cell-death genes ced-3, ced-4, and ced-9 all can act cell autonomously to control programmed cell death. Our observations indicate further that not only the protective activity of ced-9 but also the killer activities of ced-3 and ced-4 are likely to be present in cells that normally live. We propose that both in C. elegans and in other organisms a competition between antagonistic protective and killer activities determines whether specific cells will live or die. Our results suggest a genetic pathway for programmed cell death in C. elegans in which ced-4 acts upstream of or in parallel to ced-3 and ced-9 negatively regulates the activity of ced-4.

Suppression of interleukin-1 beta-converting enzyme-mediated cell death by insulin-like growth factor

COS cells are resistant to cell death induced either by interleukin-1beta-converting enzyme (*ICE) and ICE homolog (ICH-1L) overexpression or by serum deprivation. COS cells deprived of serum undergo apoptosis after transfection with an ICE expression construct, but not an ICH-1L construct. ICE-mediated apoptosis of COS cells in serum-free medium is suppressed by insulin-like growth factor (IGF)-1 and insulin. Viability of Rat-1 cell line (Rat-1/ICE) expressing low levels of ICE-LacZ fusion protein is lower than those of cell lines expressing either both Bcl-2 and ICE or mutant ICEGly-->Ser during serum deprivation. Enzymatic activation and processing of ICE are observed in cells induced to die by serum deprivation, which are suppressed by IGF-1. IGF-1 or insulin suppresses ICE-mediated cell death without affecting the expression levels of Bcl-2, Bcl-x, or Bax. Taken together, these results indicate that ICE is activated by growth factor deprivation, and IGF-1 is able to suppress ICE-mediated cell death through a mechanism independent of the expression of Bcl-2, Bcl-x, or Bax.

Apoptosis: molecular regulation of cell death

The field of apoptosis is unusual in several respects. Firstly, its general importance has been widely recognised only in the past few years and its surprising significance is still being evaluated in a number of areas of biology. Secondly, although apoptosis is now accepted as a critical element in the repertoire of potential cellular responses, the picture of the intra-cellular processes involved is probably still incomplete, not just in its details, but also in the basic outline of the process as a whole. It is therefore a very interesting and active area at present and is likely to progress rapidly in the next two or three years. This review emphasises recent work on the molecular mechanisms of apoptosis and, in particular, on the intracellular interactions which control this process. This latter area is of crucial importance since dysfunction of the normal control machinery is likely to have serious pathological consequences, probably including oncogenesis, autoimmunity and degenerative disease. The genetic analysis of programmed cell death during the development of the nematode Caenorhabditis elegans has proved very useful in identifying important events in the cell death programme. Recently defined genetic connections between C. elegans cell death and mammalian apoptosis have emphasised the value of this system as a model for cell death in mammalian cells, which, inevitably, is more complex. The signals inducing apoptosis are very varied and the same signals can induce differentiation and proliferation in other situations. However, some pathways appear to be of particular significance in the control of cell death; recent analysis of the apoptosis induced through the cell-surface Fas receptor has been especially important for immunology. Two gene families are dealt with in particular detail because of their likely importance in apoptosis control. These are, first, the genes encoding the interleukin-1 beta-converting enzyme family of cysteine proteases and, second, those related to the proto-oncogene bcl-2. Both of these families are homologous to cell death genes in C. elegans. In mammalian cells the number of members of both families which have been identified is growing rapidly and considerable effort is being directed towards establishing the roles played by each member and the ways in which they interact to regulate apoptosis. Other genes with established roles in the regulation of proliferation and differentiation are also important in controlling apoptosis. Several of these are known proto-oncogenes, e.g. c-myc, or tumour suppressors, e.g. p53, an observation which is consistent with the importance of defective apoptosis in the development of cancer. Viral manipulation of the apoptosis of host cells frequently involves interactions with these cellular proteins. Finally, the biochemistry of the closely controlled cellular self-destruction which ensues when the apoptosis programme has been engaged is also very important. The biochemical changes involved in inducing phagocytosis of the apoptotic cell, for example, allow the process to be neatly integrated within the tissues, under physiological conditions. Molecular defects in this area too may have important pathological consequences.

Biochemistry of cell death

Apoptosis is a common feature of the nervous system, occurring physiologically during development and pathologically in several diseases. In view of the latter, pathways that regulate apoptosis in neurones are of particular interest, and recent advances in this field implicate several signalling pathways in the induction of apoptosis, after withdrawal of nerve growth factor. Recent information drawn from both mammalian and invertebrate models enables us to assign components of the apoptotic pathway as either regulators or effectors.

Apoptosis--molecular mechanisms and biomedical implications

Apoptosis is a distinct form of cell death of importance in tissue development and homeostasis and in several diseases. This review summarizes current knowledge about the regulation and molecular mechanisms of apoptosis and discusses the potential role of disregulated apoptosis in several major diseases. Finally, we speculate that modulation of apoptosis may be a target in future drug therapy.

Programmed cell death in invertebrates

Genetic studies of programmed cell death in Caenorhabditis elegans and Drosophila melanogaster have led to the identification of several invertebrate cell death genes. In C. elegans, ced-3 and ced-4 function to kill cells, whereas ced-9 protects cells from death. In Drosophila, the genes reaper and hid act in parallel to promote cell death. Characterization of these genes has revealed that the process of programmed cell death is evolutionarily conserved and has shed light on the molecular nature of the apoptotic machinery.

ICE-related proteases in apoptosis

Apoptotic execution involves numerous enzymatic pathways, all of which appear to be triggered by the activation of one or more ICE-related proteases (IRPs). Considerable effort is currently being expended in the identification and functional characterization of the rapidly expanding superfamily of IRPs. Important questions that remain unsolved include the identity of the vertebrate IRP that triggers the apoptotic cascade and the identities of the crucial substrates whose cleavage results in the dramatic morphological changes during apoptosis.

Identification and characterization of CPP32/Mch2 homolog 1, a novel cysteine protease similar to CPP32

We have identified and characterized a novel cysteine protease named CMH-1 that is a new member of the interleukin 1 beta converting enzyme (ICE) family of proteases with substrate specificity for Asp-X. CMH-1 has the highest similarity to CPP32 (52% amino acid identity) and MCH2 (31% identical). CMH-1 shares conserved amino acid residues that form the core structure of ICE as well as those residues involved in catalysis and in the P1 aspartate binding. Overexpression of CMH-1 in COS cells resulted in the processing of CMH-1 and the induction of apoptosis of transfected cells. Coexpression of CMH-1 with poly(ADP-ribose) polymerase (PARP) also resulted in a specific cleavage of PARP. Purified recombinant CMH-1 cleaved PARP but not interleukin 1 beta precursor in vitro.

Microfilament reorganization during apoptosis: the role of Gas2, a possible substrate for ICE-like proteases

Gas2, a component of the microfilament system, belongs to the class of gas genes whose expression is induced at growth arrest. After serum or growth factor addition to quiescent NIH 3T3 cells, Gas2 is hyperphosphorylated and relocalized at the membrane ruffles. By overexpressing gas2wt and a series of deletion mutants of the C-terminal region, we have analysed its role in the organization of the actin cytoskeleton in different cell lines. Overexpression of Gas2 deleted at its C-terminal region (delta 276-314 and delta 236-314), but not its wild-type form, induces dramatic changes in the actin cytoskeleton and cell morphology. These effects are not due to interference of the deleted forms with the endogenous Gas2wt function but could be ascribed to a gain of function. We demonstrate that during apoptosis the C-terminal domain of Gas2 is removed by proteolytic cleavage, resulting in a protein that is similar in size to the described delta 276-314. Moreover, by using in vitro mutagenesis, we also demonstrate that the proteolytic processing of Gas2 during apoptosis is dependent on an aspartic acid residue at position 279. The evidence accumulated here could thus represent a first example of a mechanism linking apoptosis with the co-ordinated microfilament-dependent cell shape changes, as possibly mediated by an interleukin-1 beta-converting enzyme (ICE)-like dependent proteolytic cleavage of the Gas2 protein.

Apoptosis and gastrointestinal pharmacology

Apoptosis is a fundamental biological process that regulates cell number and removes cells that are neoplastic or infected by viruses. This review summarises present knowledge of the mechanisms and genetic regulation of apoptosis in the gastrointestinal tract and highlights areas of pharmacological relevance. In the intestine, apoptosis occurs in the crypt and possibly at the villus tip. Abnormal apoptosis plays a role in a number of gastrointestinal disease including colorectal cancer. The effects of cytotoxic drugs, chemical carcinogens, nonsteroidal anti-inflammatory drugs, short-chain fatty acids, bile salts and anthraquinolones on apoptosis in the gastrointestinal tract are reviewed.