Prevention of vertebrate neuronal death by the crmA gene

Molecular mechanism of ultraviolet-induced keratinocyte apoptosis

This article reviews advances in the study of the molecular mechanisms for ultraviolet (UV)-induced keratinocyte apoptosis, with particular reference to the cytokines tumor necrosis factor-alpha (TNF-alpha) and Fas ligand (FasL). TNF-alpha and FasL induce their respective receptors and then activate caspase enzymes that are critically involved in the apoptotic process. This activation is further amplified by intracellular mitochondria-associated mechanisms. Using gene-targeted knockout mice lacking either the TNF-Rp55 or the TNF-Rp75, we have shown that TNF-alpha plays an important role in UV-induced keratinocyte apoptosis via TNF-Rp55. TNF-Rp55 shares homology with Fas and contains an intracellular death domain. UV seems to directly stimulate cross-linking of Fas, resulting in the engagement of the death machinery. Fas-associated death domain protein (FADD) acts as an adapter protein in both the TNF-Rp55 and Fas death-inducing cascades and is responsible for downstream signal transduction by recruiting caspases. Moreover, signaling of p53 contributes to the induction of apoptosis by regulating Bcl-2 family expression and increasing surface Fas expression. In addition to induction mechanisms of apoptosis, there are numerous inhibitory molecules that play a role in restricting the apoptotic pathway. Thus, the ultimate determination of whether or not a cell undergoes apoptosis after UV radiation is based on the balance between agonist and antagonist pathways.

Increased expression of mRNA encoding interleukin-1beta and caspase-1, and the secreted isoform of interleukin-1 receptor antagonist in the rat brain following systemic kainic acid administration

Kainic acid, an analogue of glutamate, injected systemically to rats evokes seizures that are accompanied by nerve cell damage primarily in the limbic system. In the present study, we have analyzed the temporal profile of the expression of the cytokines interleukin-1beta (IL-1beta) and IL-1 receptor antagonist (IL-1ra), and the related IL-1beta-converting enzyme (ICE/caspase-1), in different regions of the rat brain in response to peripheral kainic acid administration (10 mg/kg, i.p.). In situ hybridization histochemistry experiments revealed that IL-1beta mRNA-expressing cells, morphologically identified as microglial cells, were mainly localized to regions showing pronounced neuronal degeneration; hippocampus, thalamus, amygdala, and certain cortical regions. The strongest expression of IL-1beta mRNA was observed after 12 hr in these regions. A weak induction of the IL-1beta mRNA expression was observed already at 2 hr. Similar results were obtained by RT-PCR analysis, showing a significantly increased expression of IL-1beta mRNA in the hippocampus and amygdala after 12 hr. In addition, RT-PCR analysis revealed that IL-1ra mRNA, and specifically mRNA encoding the secreted isoform of IL-1ra (sIL-1ra), was strongly induced in the hippocampus and amygdala at 12 and 24 hr post-injection. RT-PCR analysis of mRNA encoding caspase-1 showed a significantly increased expression in the amygdala after 12 hr. In conclusion, in response to systemic kainic acid injection IL-1beta mRNA is rapidly induced and followed by induction of IL-1ra mRNA and caspase-1 mRNA, supporting a role of the IL-1 system in the inflammatory response during excitotoxic damage.

Intracellular and extracellular leukemia inhibitory factor proteins have different cellular activities that are mediated by distinct protein motifs

Although many growth factors and cytokines have been shown to be localized within the cell and nucleus, the mechanism by which these molecules elicit a biological response is not well understood. The cytokine leukemia inhibitory factor (LIF) provides a tractable experimental system to investigate this problem, because translation of alternatively spliced transcripts results in the production of differentially localized LIF proteins, one secreted from the cell and acting via cell surface receptors and the other localized within the cell. We have used overexpression analysis to demonstrate that extracellular and intracellular LIF proteins can have distinct cellular activities. Intracellular LIF protein is localized to both nucleus and cytoplasm and when overexpressed induces apoptosis that is inhibited by CrmA but not Bcl-2 expression. Mutational analysis revealed that the intracellular activity was independent of receptor interaction and activation and reliant on a conserved leucine-rich motif that was not required for activation of cell surface receptors by extracellular protein. This provides the first report of alternate intracellular and extracellular cytokine activities that result from differential cellular localization of the protein and are mediated by spatially distinct motifs.

Involvement of caspase 3 in apoptotic death of cortical neurons evoked by DNA damage

Previous reports have shown that DNA-damage-evoked death of embryonic cortical neurons is delayed by general caspase inhibitors and is accompanied by an increase in DEVD-AFC cleavage activity. We show here that this cleavage activity is lacking in camptothecin-treated caspase 3-deficient neurons. Moreover, we report that death of camptothecin-treated caspase 3-deficient neurons cultured from E16 embryos is delayed and that no significant increase in survival is observed with cotreatment with the general caspase inhibitor BAF. These results indicate that caspase-dependent death of camptothecin-treated cortical neurons requires caspase 3 activity. The delay in death is accompanied by impairment of DNA fragmentation. However, Bax-dependent cytochrome c release still occurs in camptothecin-treated caspase 3-deficient cortical neurons. Accordingly, we hypothesize that the delayed death which occurs in the absence of caspase 3 activity may be due to mitochondrial dysfunction. Finally, we show that the delay in death observed with E16 caspase 3-deficient neurons does not occur in neurons cultured from E19 embryos. This suggests that the requirement for caspase 3 in death of neurons evoked by DNA damage may differ depending upon the developmental state of the cell.

Estrogen-regulated developmental neuronal apoptosis is determined by estrogen receptor subtype and the Fas/Fas ligand system

Adult sexual dimorphism in neuronal cell number is controlled by estrogen exposure during a tightly defined period of rat brain development. The mechanisms of estrogen's effect are unknown; one possibility is regulation of programmed cell death (apoptosis). In this study we have shown that estradiol can function as a neuroprotective agent or an inducer of apoptosis, depending on the estrogen receptor-subtype present in the cell. Thus, ERalpha has a neuroprotective effect, while ERbeta mediates the induction of apoptosis in neuronal cells. Moreover, we show that estrogen-induced apoptosis through ER-beta requires the expression of Fas- and Fas ligand (FasL) proteins, since the absence of FasL in neurons prevents this effect. Furthermore, we demonstrate that microglia-secreted products induce the expression of FasL necessary to mediate estradiol-ERbeta apoptotic effect. These findings may explain the dichotomous effect of fetal estradiol on the adult neuronal number.

IL-1beta and ICE mRNA are not altered upon beta-amyloid(25-35) induced neurotoxicity in human neuroblastoma cells

The specific beta-amyloid(25-35) fragment induced cellular degradation of the human neuroblastoma cell line SH-SY5Y, but did not elicit an effect on the levels of interleukin-1beta and interleukin-1beta converting enzyme, as determined by semiquantitative reverse transcription-polymerase chain reaction and immunocytochemical analysis. The assays revealed constitutive expression of these proteins both at mRNA and protein level. It is conceivable that in the absence of glial elements, such as in the present neuroblastoma cell line, beta-amyloid triggers the toxicity through a direct action and/or through the production of other harmful molecules.

Intracerebral injection of caspase-3 inhibitor prevents neuronal apoptosis after kainic acid-evoked status epilepticus

In the aftermath of prolonged continuous seizure activity (status epilepticus, SE), neuronal cell death occurs in the brain regions through which the seizure propagates. Recent studies have implicated apoptotic processes in this seizure-related injury. Because activation of caspase-3-like cysteine proteases plays a crucial role in mammalian neuronal apoptosis, we explored the possibility that activation of caspase-3 is involved in the neuronal apoptotic cell death that occurs in rat brain following SE induced by systemic kainic acid. Caspase-3 activity was determined immunocytochemically using CM1 antibodies specific for catalytically active subunit (p17) of the enzyme. We found an induction of caspase-3 activity in rhinal cortex and amygdala at 24 h after SE. To determine whether activation of caspase-3-like proteases is a necessary component of the injury process, we delivered a caspase-3 inhibitor, z-DEVD-fmk, into the lateral ventricle prior to, and following SE. z-DEVD-fmk treatment substantially attenuated apoptotic cell death after SE, both in hippocampus and rhinal cortex, as evaluated by analysis of internucleosomal DNA fragmentation and neuronal nuclear morphology. Our findings implicate caspase-3 cysteine protease in the neurodegenerative response to SE and suggest that this degeneration can be attenuated by inhibition of caspase-3-like enzyme activity.

Adenovirus-mediated transfer of Bcl-X(L) protects neuronal cells from Bax-induced apoptosis

Bax-mediated apoptosis in neurons is involved in many pathologic conditions affecting the central nervous system, including degenerative diseases, stroke, and trauma. Two molecules belonging to the Bcl-2 family, Bcl-2 and Bcl-X(L), protect cells from Bax-induced apoptosis and show distinct expression patterns in adult neurons, with downregulated Bcl-2 and highly upregulated Bcl-X(L) expression. To investigate the biological functions of these two molecules in Bax-mediated apoptosis in neurons, we transduced various levels of Bcl-X(L) or Bcl-2 via adenoviral vectors into nerve growth factor (NGF)-treated PC12 cells. Overexpression of Bax induced drastic apoptosis in NGF-treated PC12 cells. Bcl-X(L) expressed at a wide range of levels conferred a high level of protection against Bax-mediated apoptosis. In contrast, Bcl-2 at various levels conferred far less protection against apoptosis. Moreover, Bcl-X(L) protected PC12 cells from apoptosis induced by NGF withdrawal. These data indicate that Bcl-X(L)-mediated protection is the major pathway that suppresses apoptosis in NGF-treated PC12 cells and that Bcl-X(L) would be a more relevant target of manipulation in future treatment strategies, including gene therapies.

Fas-mediated apoptosis in T cells involves the dephosphorylation of the retinoblastoma protein by type 1 protein phosphatases

Apoptosis is triggered by a number of different stimuli including the activation of Fas antigen, a member of the TNF family, by the Fas ligand. The signal transduction events implicated in apoptosis are complex and remain only partially understood. In this study, we used calyculin A, a potent inhibitor of serine/threonine (ser/thr) phosphatases types 1 and 2A, to investigate the role of ser/thr phosphatases in Fas-induced apoptosis. We showed that calyculin A inhibited Fas-induced DNA fragmentation and cytolysis in Jurkat cells and that this inhibition was not due to the modulation of Fas. Okadaic acid also inhibited Fas-induced apoptosis of Jurkat cells, but at much higher concentrations (microM level), thus implicating that type 1 phosphatases rather than type 2A are inhibited at nM concentrations. Cross-linking Fas led to the dephosphorylation of the retinoblastoma gene product (Rb) within 5 min, and to PARP cleavage within 2 h. Both events were inhibited by calyculin A indicating that apoptotic death triggered by Fas cross-linking involves the activation of type 1 ser/thr phosphatases.

Mutational analysis of the Caenorhabditis elegans cell-death gene ced-3

Mutations in the gene ced-3, which encodes a protease similar to interleukin-1beta converting enzyme and related proteins termed caspases, prevent programmed cell death in the nematode Caenorhabditis elegans. We used site-directed mutagenesis to demonstrate that both the presumptive active-site cysteine of the CED-3 protease and the aspartate residues at sites of processing of the CED-3 proprotein are required for programmed cell death in vivo. We characterized the phenotypes caused by and the molecular lesions of 52 ced-3 alleles. These alleles can be ordered in a graded phenotypic series. Of the 30 amino acid sites altered by ced-3 missense mutations, 29 are conserved with at least one other caspase, suggesting that these residues define sites important for the functions of all caspases. Animals homozygous for the ced-3(n2452) allele, which is deleted for the region of the ced-3 gene that encodes the protease domain, seemed to be incompletely blocked in programmed cell death, suggesting that some programmed cell death can occur independently of CED-3 protease activity.

Molecular mechanisms of cell death in the ear

Damage to sensory cells and neurones of the inner ear, induced by ototoxic drugs, physical stimulation, or even normal aging, occurs by mechanisms that are not yet fully elucidated. However, for the individual the consequent loss of hearing can be devastating. This the concept that, in some instances, this brain cell death may be preventable has provided an exciting and novel route to thwart the degenerative process. Signals that trigger cell death can be as diverse as the removal of essential growth factors, damage by exogenous toxins (including ototoxic drugs), free radicals, and excitotoxins. An important facet to the newly discovered death cascade is that it can be halted, and such interventions may rescue the dying cell. The question now remains whether any of the cell death observed on ototoxic or noise-induced hearing loss occurs by an "active" programmed mechanism (apoptosis), as physiological cell death does in the developing ear, and if so, whether it can be prevented.

SPI-1-dependent host range of rabbitpox virus and complex formation with cathepsin G is associated with serpin motifs

Serpins are a superfamily of serine proteinase inhibitors which function to regulate a number of key biological processes including fibrinolysis, inflammation, and cell migration. Poxviruses are the only viruses known to encode functional serpins. While some poxvirus serpins regulate inflammation (myxoma virus SERP1 and cowpox virus [CPV] crmA/SPI-2) or apoptosis (myxoma virus SERP2 and CPV crmA/SPI-2), the function of other poxvirus serpins remains unknown. The rabbitpox virus (RPV) SPI-1 protein is 47% identical to crmA and shares all of the serpin structural motifs. However, no serpin-like activity has been demonstrated for SPI-1 to date. Earlier we showed that RPV with the SPI-1 gene deleted, unlike wild-type virus, fails to grow on A549 or PK15 cells (A. Ali, P. C. Turner, M. A. Brooks, and R. W. Moyer, Virology 202:306-314, 1994). Here we demonstrate that in the absence of a functional SPI-1 protein, infected nonpermissive cells which exhibit the morphological features of apoptosis fail to activate terminal caspases or cleave the death substrates PARP or lamin A. We show that SPI-1 forms a stable complex in vitro with cathepsin G, a member of the chymotrypsin family of serine proteinases, consistent with serpin activity. SPI-1 reactive-site loop (RSL) mutations of the critical P1 and P14 residues abolish this activity. Viruses containing the SPI-1 RSL P1 or P14 mutations also fail to grow on A549 or PK15 cells. These results suggest that the full virus host range depends on the serpin activity of SPI-1 and that in restrictive cells SPI-1 inhibits a proteinase with chymotrypsin-like activity and may function to inhibit a caspase-independent pathway of apoptosis.

Proteolytic activation of protein kinase C delta and epsilon by caspase-3 in U937 cells during chemotherapeutic agent-induced apoptosis

Protein kinase C (PKC) family members play pivotal roles in cellular signal transduction and nPKCdelta and theta are known to be subjected to restrictive proteolysis during apoptosis. Here we show that nPKCepsilon was specifically cleaved and generates 43-kDa and 36-kDa C-terminal fragments during chemotherapeutic drug-induced apoptosis. The proteolytic cleavage of nPKCdelta and epsilon was completely inhibited by pretreatment with Ac-DEVD-cho, a specific inhibitor of caspase-3 family enzymes. Furthermore, nPKCepsilon in non-treated U937 cell lysates was cleaved by purified recombinant caspase-3 to generate the 43-kDa fragment, identical in size to the fragment observed in vivo. This cleavage was prevented by the addition of Ac-DEVD-cho. These results suggest that caspase-3 specifically cleaves nPKCepsilon. These findings suggest the possibility that nPKC subfamily members are generally involved in the execution of apoptosis but they are regulated diversely depending on the different apoptotic stimuli.

A new savior for neurons

How NGF promotes neuronal survival is unclear. New findings suggest that the mechanism involves induction of an anti-apoptotic protein called ITA.

Immunohistochemical localization of interleukin-1beta, interleukin-1 receptor antagonist and interleukin-1beta converting enzyme/caspase-1 in the rat brain after peripheral administration of kainic acid

The temporal and anatomical distribution of members of the interleukin-1 system in the rat brain following intraperitoneal kainic acid administration was studied in relation to neurodegeneration as detected with in situ end labelling. Kainic acid administration (10 mg/kg, i.p.) resulted in the induced expression of interleukin-1beta, interleukin- receptor antagonist and caspase-1p10 immunoreactivity in areas known to display neuronal and tissue damage upon excitotoxic lesions. The induction of these proteins was transient. Interleukin-1 immunoreactivity appeared at 5 h, and the interleukin-1 receptor antagonist-immunoreactive cells were first detected at 12 h, whereas the induction of caspase- 1p10 expression was first detected 24 h after kainic acid injection. Double labelling with the microglial marker Ox42 confirmed that both interleukin-1beta and interleukin-1 receptor antagonist were mainly localized in microglial cells. The regional distribution of in situ end-labelled neurons was similar to the distribution of cells expressing interleukin-1beta and interleukin-1 receptor antagonist, whereas the distribution of caspase-1 was more limited. The in situ end-labelled neurons, were, similarly to the interleukin-1beta-positive cells, first detected at 5 h, which is earlier than the induction of caspase-1. Our results show that the induction of IL-1beta and IL-1 receptor antagonist proteins after kainic acid are closely associated with the temporal as well as the anatomical distribution of in situ end-labelled neurons, whereas the induction of caspase-1 protein exhibited a delayed temporal profile and limited distribution. Since cytokine production occurs in activated microglial cells, the inflammatory component seems to be a strong mediator of this type of excitotoxic damage. The late onset of the caspase-1 expression would seem to indicate that this enzyme has no fundamental role in directly causing neuronal cell death induced by systemic kainic acid.

Recent advances on neuronal caspases in development and neurodegeneration

In view of a large and growing literature, this overview emphasizes recent advances in neuronal caspases and their role in cell death. To provide historical perspective, morphology and methods are surveyed with emphasis on early studies on interleukin converting enzyme (ICE) as a prototype for identifying zymogen subunits. The unexpected homology of ICE (caspase-1) to Caenorhabditis elegans death gene CED-3 provided early clues linking caspases to programmed cell death, and led later to discovery of bcl-2 proteins (CED-9 homologs) and 'apoptosis associated factors' (Apafs). Availability of substrates, inhibitors, and cDNAs led to identification of up to 16 caspases as a new superfamily of unique cysteine proteinases targeting Asp groups. Those acting as putative death effectors dismantle neurons by catabolism of proteins essential for survival. Caspases degrade amyloid precursor protein (APP), presenilins (PS1, PS2), tau, and huntingtin, raising questions on their role in neurodegeneration. Brain contains 'inhibitors of apoptosis proteins' (IAPs) survivin and NAIP associated also with some neuronal disorders. Apoptotic stress in neurons initiates a chain of events leading to activation of distal caspases by pathways that remain to be fully mapped. Neuronal caspases play multiple roles for initiation and execution of cell death, for morphogenesis, and in non-mitotic neurons for homeostasis. Recent studies focus on cytochrome c as pivotal in mediating conversion of procaspase-9 as a major initiator for apoptosis. Identifying signaling pathways and related events paves the way to design useful therapeutic remedies to prevent neuronal loss in disease or aging.

Caspase activation is downstream of commitment to apoptosis of Ntera-2 neuronal cells

Death by apoptosis is widespread among different cell types, including neurones. Apoptosis consists of a phase during which cells commit to die, followed by an execution phase, characterized by conserved morphological changes. To prevent neuronal loss during disease, it is important to identify the events which define irreversible commitment to death. The present study has investigated the events accompanying the commitment and execution phases of the neuronal cell line Ntera-2. In response to serum starvation, Ntera-2 cells enter the execution phase and detach into the culture supernatant with an apoptotic morphology. This phase is associated with activation of caspases. The remaining adherent cells have a normal morphology and can adhere to extracellular matrix substrates. However, after 96 h of serum deprivation, 95% of these adherent cells fail to form colonies in a single cell cloning assay. When refed with serum, 70% of these cells become apoptotic within 24 h, suggesting that they had previously committed to die. A further 20% of the cells escape from commitment to apoptosis by beginning to differentiate. Inhibition of caspases fails to delay commitment. In response to serum deprivation, therefore, neuronal cells either differentiate or commit to cell death, and events upstream of caspase activation regulate this irreversible commitment. These results have significant therapeutic implications since they suggest that caspase inhibitors may not promote long-term survival of every neuronal cell type in every situation.

Collapse of the inner mitochondrial transmembrane potential is not required for apoptosis of HL60 cells

Apoptotic cell death involves a series of morphological and biochemical changes orchestrated by activated proteases belonging to the caspase family. Recent studies have suggested that the activation of this process of execution is dependent upon events associated with the loss of mitochondrial inner transmembrane potential (Deltapsi(m)), as a consequence of the formation of the permeability transition (PT) pore. This has led to the proposal that mitochondrial depolarization represents a central irreversible checkpoint in the apoptotic program. Here, we present evidence that HL-60 cells undergo apoptosis in response to the cytotoxic insults of actinomycin-D, etoposide, and staurosporine without showing significant changes in Deltapsi(m). Instead, the loss of Deltapsi(m) could be detected only later in the cell death pathway. In addition, the uncoupling agent CCCP produced an early mitochondrial depolarization in HL-60s but these cells showed few signs of apoptosis up to 8 h after the insult. Furthermore, examination of these cells in response to staurosporine revealed the release of mitochondrial cytochrome c into the cytosol over time, corresponding to caspase activation irrespective of mitochondrial depolarization. In summary, our data suggest that the collapse of Deltapsi(m) as a consequence of PT is not a universal early marker for apoptosis and, moreover, it is not part of the central apoptotic machinery.

Bcl-xL is a negative regulator of caspase-3 activation in immature neurons during development

Caspases and Bcl-xL, the mammalian homologues of the Caenorhabditis elegans (C. elegans) ced-3 and ced-9 genes, respectively, regulate apoptosis of various cells. Caspase-3 is processed into an active form (p20 or p17 and p12) during apoptosis. We investigated the relation between caspase-3 and Bcl-xL during development by examining activation of caspase-3 and apoptotic cells in Bcl-x-deficient (bcl-x(-/-)) mice at embryonic (E) day 11.5. We used a double-staining technique with a cleavage site-directed antibody against caspase-3 (anti-p20/17) and terminal-deoxytransferase-mediated deoxyuridine triphosphate nick-end labeling (TUNEL). Bcl-xL-deficiency increased both numbers of p20/17-positive and -negative apoptotic cells in dorsal root ganglia (DRG); the numbers of p20/17-positive apoptotic cells in the caudal parts of the ventral hindbrain and ventral spinal cord; and the numbers of p20/17-negative apoptotic cells in the dorsal midbrain, dorsal hindbrain, and dorsal spinal cord. Thus, Bcl-xL blocks the caspase-3-dependent apoptotic pathway in the restricted regions of the nervous system during development. Furthermore, these observations suggest that Bcl-xL protects against activation of the caspase-3-independent apoptotic pathway. Other caspases or apoptotic mechanisms may also be activated in the nervous systems of bcl-x(-/-) mice.

Etiology of Parkinson's disease

Controversy over the etiology and pathogenesis of Parkinson's disease (PD) has continued for many years and while the details have changed, the uncertainty persists. Although heritability was most emphatically refuted a decade ago by many investigators, recent progress firmly indicates that genetic factors at least play a role, although probably to a variable degree from one individual to another. Evidence for a variety of other etiological factors is amassed from epidemiological studies, animal models, molecular and cellular biology. Genetic factors, infectious and immunological abnormalities, the effects of ageing, toxins (endogenous as well as exogenous) and other environmental factors may all contribute to the development of PD. Loss of nigral dopaminergic neurons may be mediated by varying combinations of oxidative free radical toxicity, impaired mitochondrial function, "weak excitotoxicity" and abnormal handling of cytoskeletal proteins, all of which may shift the balance regulating apoptotic cell death.

Signaling of neuronal cell death by the p75NTR neurotrophin receptor

The neurotrophin receptor (p75NTR) is best known for mediating tropic support by participating in the formation of high-affinity nerve growth factor (NGF) receptor complexes with trkA, however, p75NTR more recently has been shown to act as a bona fide death-signaling receptor, which can signal independently of trkA. This article discusses the evidence for an active role of p75NTR in neuronal cell death and the mechanisms controlling this process, including roles for Bcl-2 family members, the c-jun stress kinase JNK, the transcription factor nuclear factor kappa B (NFkappaB), and caspases.

Immunolocalization of caspase proteolysis in situ: evidence for widespread caspase-mediated apoptosis of neurons and glia in the postnatal rat brain

Activation of a member of the caspase family of cysteine proteases is thought to be required for the execution of apoptosis in neurons and other cell types. We describe here an antibody (Ab127) reactive with a neoantigenic site on caspase substrate proteins degraded during apoptosis, and its characterization as a biochemical and histochemical probe for apoptosis-associated proteolysis in growth factor-deprived neural cells in vitro and the developing postnatal rat brain. Neuronally differentiated PC12 cells became strongly Ab127 immunoreactive only during apoptosis following nerve growth factor withdrawal. Apoptosis-associated caspase proteolysis was detectable on western blots as markedly increased immunoreactivity of a approximately 46,000 mol. wt polypeptide, a product also generated by caspase-3 treatment of cell-free extracts. In the postnatal rat brain, intense immunoreactivity indicative of caspase activation was exhibited by small proportions of neurons and glia in distinct regional and temporal patterns. The degenerating nature of these cells was confirmed by their argyrophilia, cytoplasmic immunoreactivity for c-jun and fragmented processes. Combined immunofluorescence and Hoechst 33342 staining demonstrated that cells immunopositive for caspase activation have apoptotic nuclear morphologies. Caspase proteolysis was observed throughout the neuraxis in a minority of progenitor cells in germinal zones, postmitotic neurons in the parenchyma, and glia in the corpus callosum and other white matter tracts, but was observed rarely in the adult brain. These data characterize a new approach for evaluating apoptosis in physiological and pathological neurodegeneration, and demonstrate that caspase-associated apoptosis is a widespread mechanism for the programmed death of neurons and glia in the postnatal rat brain.

Nerve growth factor signaling through p75 induces apoptosis in Schwann cells via a Bcl-2-independent pathway

Apoptosis is involved in the regulation of Schwann cell numbers during normal development and after axonal damage, but the molecular regulation of Schwann cell death remains unknown. We have used stably transfected rat Schwann cell lines to study the potential roles of nerve growth factor (NGF), the antiapoptotic protein Bcl-2 and the cytokine response modifier A (CrmA) in modulating Schwann cell death in vitro. Bcl-2 inhibited Schwann cell apoptosis induced by survival factor withdrawal, whereas CrmA did not. In contrast, Bcl-2-transfected Schwann cells were susceptible to apoptosis in response to exogenous NGF, whereas CrmA-expressing cell lines were resistant. Demonstration of high levels of the low-affinity neurotrophin receptor p75 but not the high-affinity TrkA receptor on the Bcl-2-transfected cell lines suggested that the NGF-induced killing was mediated by p75. This was confirmed by resistance of Schwann cells isolated from p75 knockout mice to the NGF-induced cell death. Nerve growth factor also promoted the death of wild-type mouse and rat Schwann cells in the absence of survival factor withdrawal. Endogenous Bcl-2 mRNA was expressed by wild-type Schwann cells in all conditions that promoted survival but was downregulated to undetectable levels after survival factor withdrawal. In conclusion, our results demonstrate the existence of two separate pathways that expedite apoptosis in Schwann cells: a Bcl-2-blockable pathway initiated on loss of trophic support, and a Bcl-2-independent, CrmA-blockable pathway mediated via the p75 receptor.

Involvement of interleukin-1beta in the mechanism of human immunodeficiency virus type 1 (HIV-1) recombinant protein gp120-induced apoptosis in the neocortex of rat

The effect of subchronic intracerebroventricular injection of the human immunodeficiency virus type 1 (HIV-1) recombinant protein gp120 (100 ng, given daily for up to seven consecutive days) on interleukin-1beta expression was studied by immunohistochemistry in the brain of adult rats. In comparison to control, bovine serum albumin (300 ng, given intracerebroventricularly for up to seven days) -treated animals (n=6), interleukin-1beta immunoreactivity increased in the brain cortex and hippocampus of rats (n=6) receiving a single injection of the viral protein 24 h before analysis with more substantial increases being observed in these regions of the brain (n=6) after seven days treatment. Double-labelling immunofluorescence experiments support a neuronal and, possibly, a microglial cell origin for gp120-enhanced interleukin-1beta expression. Transmission electron microscopy analysis of brain tissue sections revealed that combination treatments (given intracerebroventricularly daily for seven days) with gp120 (100 ng) and interleukin-1 receptor antagonist (80 ng) or with the interleukin converting enzyme inhibitor II (100 pmol), but not with leupeptin (100 pmol), prevented apoptotic death of rat (n=6/group) brain cortical cells typically elicited by the viral protein. These data demonstrate that gp120 enhances interleukin-1beta expression in the brain and this may be involved in the mechanism underlying apoptosis induced by gp120 in the brain cortex of rat. Further support to this hypothesis comes from the evidence that intracerebroventricular injection of murine recombinant interleukin-1beta (200 U, given daily for seven consecutive days) produces DNA fragmentation in the brain cortex of rat (n=6). Interestingly, the latter treatment enhanced nerve growth factor level in the hippocampus but not in the cerebral cortex and this coincides with a similar effect recently reported in identical brain areas of rats treated likewise with gp120. In conclusion, the present data demonstrate that treatment with gp120 enhances interleukin-1beta expression and this participates in the mechanism of apoptotic cell death in the brain cortex of rat. By contrast, in the hippocampus, gp120-enhanced interleukin-1beta expression elevates nerve growth factor that may prevent or delay apoptosis in this plastic region of the rat brain.

The central effectors of cell death in the immune system

The immune system relies on cell death to maintain lymphoid homeostasis and avoid disease. Recent evidence has indicated that the caspase family of cysteine proteases is a central effector in apoptotic cell death and is absolutely responsible for many of the morphological features of apoptosis. Cell death, however, can occur through caspase-independent and caspase-dependent pathways. In the case of cells that are irreversibly neglected or damaged, death occurs even in the absence of caspase activity. In contrast, healthy cells require caspase activation to undergo cell death induced by surface receptors. This review summarizes the current understanding of these two pathways of cell death in the immune system.

Sequential cleavage of poly(ADP-ribose)polymerase and appearance of a small Bax-immunoreactive protein are blocked by Bcl-X(L) and caspase inhibitors during staurosporine-induced dopaminergic neuronal apoptosis

To assess the role of Bcl-X(L) and its splice derivative, Bcl-X(S), in staurosporine-induced cell death, we used a dopaminergic cell line, MN9D, transfected with bcl-xL (MN9D/Bcl-X(L)), bcl-xS (MN9D/Bcl-X(S)), or control vector (MN9D/Neo). Only 8.6% of MN9D/Neo cells survived after 24 h of 1 microM staurosporine treatment. Caspase activity was implicated because a caspase inhibitor, N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (Z-VAD-fmk), attenuated staurosporine-induced cell death. Bcl-X(L) rescued MN9D cells from death (89.4% viable cells), whereas Bcl-X(S) had little or no effect. Bcl-X(L) prevented morphologically apoptotic changes as well as cleavage of poly(ADP-ribose)polymerase (PARP) induced by staurosporine. It is interesting that a small Bax-immunoreactive protein appeared 4-8 h after PARP cleavage in MN9D/Neo cells. The appearance of the small Bax-immunoreactive protein, however, may be cell type-specific as it was not observed in PC12 cells after staurosporine treatment. The sequential cleavage of PARP and the appearance of the small Bax-immunoreactive protein in MN9D cells were blocked either by Z-VAD-fmk or by Bcl-X(L). Thus, our present study suggests that Bcl-X(L) but not Bcl-X(S) prevents staurosporine-induced apoptosis by inhibiting the caspase activation that may be directly or indirectly responsible for the appearance of the small Bax-immunoreactive protein in some types of neurons.

Recruitment of apoptotic cysteine proteases (caspases) in influenza virus-induced cell death

Influenza virus infection induces apoptosis in cultured cells with an augmented expression of Fas (APO-1/CD95). Caspases, a family of cysteine proteases structurally related to interleukin-1-beta-converting enzyme (ICE), play crucial roles in apoptosis induced by various stimuli, including Fas. However, activation of the caspase-cascade seems to be different in various pathways of apoptotic stimuli. We therefore examined the involvement of caspases in influenza virus-induced apoptosis using caspase inhibitors. We found that z-VAD-fmk and z-IETD-fmk effectively inhibited virus-induced apoptosis, whereas Ac-DEVD-CHO and Ac-YVAD-CHO showed partial and little effect on virus-induced cell death, respectively. Consistently, caspase-3-like activity, but not caspase-1-like activity, was increased in the virus-infected cells. The transfection of plasmids encoding viral inhibitors of caspase (v-FLIP or crmA) into HeLa cells inhibited apoptosis by virus infection. The peptide inhibitors of caspases used in this study did not inhibit viral replication. We conclude that influenza virus infection activates some caspases, and that this activation may be downstream of viral replication.

Contribution of p53-dependent caspase activation to neuronal cell death declines with neuronal maturation

Caspases play a pivotal role in neuronal cell death during development and after trophic factor withdrawal. However, the mechanisms regulating caspase activity and the role played by caspase activation in response to neuronal injury is poorly understood. The tumor suppressor gene p53 has been implicated in the loss of neuronal viability caused by excitotoxic and DNA damaging agents. In the present study we determined if p53-mediated neuronal cell death required caspase activation. DNA damage increased caspase activity in both cultured embryonic telencephalic and postnatal cortical neurons in a p53-dependent manner. Caspase inhibitors protected embryonic telencephalic neurons, but not postnatal cortical neurons, from DNA damage-induced cell death as measured by direct cell counting and annexin V staining. In marked contrast to the caspase inhibitors, an inhibitor of the DNA repair enzyme, poly(ADP-ribose) polymerase, conferred significant protection from genotoxic and excitotoxic cell death on postnatal cortical neurons but had no effect on embryonic neurons. Glutamate-mediated excitotoxicity in postnatal neurons was not associated with measurable changes in caspase activity, consistent with the failure of caspase inhibitors to prevent cell death under these conditions. Moreover, adenovirus-mediated overexpression of p53 killed embryonic and postnatal neurons without activating caspases. Thus, p53-mediated neuronal cell death may occur via both caspase-dependent and caspase-independent pathways. These results demonstrate that p53 is required for caspase activation in response to some forms of neuronal injury. However, the relative importance of caspase activation in neurons depends on the developmental status of the cell and the specific nature of the death stimulus.

Generation of reactive oxygen species, release of L-glutamate and activation of caspases are required for oxygen-induced apoptosis of embryonic hippocampal neurons in culture

Oxygen-induced cell death in embryonic neurons is a useful in vitro model of neuronal apoptosis to study the molecular mechanisms underlying the cell death induced by oxidative stress. In the present study, we examined the involvement of reactive oxygen species and glutamate in the high (50%) oxygen-induced death of cultured hippocampal neurons. During the course of cell death, increases in O2- and hydrogen peroxide (H2O2) levels were observed. On the other hand, superoxide dismutase (SOD), catalase and deferoxamine (DFX), which have inhibitory effects on the generation of O2-, H2O2 and hydroxyl radicals, respectively, protected the neurons. These results suggested that both O2- and H2O2 play important roles in this apoptosis. Antagonists of NMDA and AMPA/kinate (AMPA/KA) receptors and an inhibitor of glutamate release partially prevented the apoptosis, suggesting that exposure to high oxygen enhances glutamate release, which results in activation of NMDA receptor and AMPA/KA receptor. In addition, specific nitric oxide (NO) scavenger and NO synthetase inhibitors blocked the apoptosis, indicating that NO and/or peroxynitrite are involved in this mechanism of cell death. Caspase inhibitors also blocked the neuronal apoptosis. These results suggested that multiple effectors including generation of reactive oxygen species, release of L-glutamate and activation of caspases are activated during the death induced by high oxygen.

Immunomodulation by viruses: the myxoma virus story

Myxoma virus is a poxvirus pathogen of rabbits that has evolved to replicate successfully in the presence of an active immune response by an infected host. To accomplish this, the virus has developed a variety of strategies to avoid detection by or obstruct specific aspects of the antiviral response whose consolidated action is antagonistic to virus survival. We describe two distinct viral strategies carried out by viral proteins with which myxoma virus subverts the host immune response. The first strategy is the production of virus-encoded proteins known as viroceptors or virokines that mimic host receptors or cytokines. These seek to actively block extracellular immune signals required for effective virus clearance and produce a local environment in the infected tissue that is "virus friendly". The second strategy, carried out by intracellular viral proteins, seeks to retard the innate antiviral responses such as apoptosis, and hinder attempts by the infected cell to communicate with the cellular arm of the immune system. By studying these viral strategies of immune evasion, the myxoma system can provide insights into virus-host interactions and also provide new insights into the complex immune system.

The Intracellular Serpin Proteinase Inhibitor 6 Is Expressed in Monocytes and Granulocytes and Is a Potent Inhibitor of the Azurophilic Granule Protease, Cathepsin G

The monocyte and granulocyte azurophilic granule proteinases elastase, proteinase 3, and cathepsin G are implicated in acute and chronic diseases thought to result from an imbalance between the secreted proteinase(s) and circulating serpins such as 1-proteinase inhibitor and 1-antichymotrypsin. We show here that the intracellular serpin, proteinase inhibitor 6 (PI-6), is present in monocytes, granulocytes, and myelomonocytic cell lines. In extracts from these cells, PI-6 bound an endogenous membrane-associated serine proteinase to form an sodium dodecyl sulfate (SDS)-stable complex. Using antibodies to urokinase, elastase, proteinase 3, or cathepsin G, we demonstrated that the complex contains cathepsin G. Native cathepsin G and recombinant PI-6 formed an SDS-stable complex in vitro similar in size to that observed in the extracts. Further kinetic analysis demonstrated that cathepsin G and PI-6 rapidly form a tight 1:1 complex (ka = 6.8 ± 0.2 × 106mol/L−1s−1 at 17°C;Ki = 9.2 ± 0.04 × 10−10 mol/L). We propose that PI-6 complements 1-proteinase inhibitor and 1-antichymotrypsin (which control extracellular proteolysis) by neutralizing cathepsin G that leaks into the cytoplasm of monocytes or granulocytes during biosynthesis or phagocytosis. Control of intracellular cathepsin G may be particularly important, because it has recently been shown to activate the proapoptotic proteinase, caspase-7.

Caspase-3 inhibits growth in Saccharomyces cerevisiae without causing cell death

Caspase-3, a member of the caspase family of cell death proteases, cleaves cytoplasmic and nuclear substrates and promotes apoptotic cell death in mammalian cells. Although yeast homologs of apoptotic genes have not been identified, some components of apoptotic pathways retain function in yeast. Here we show that the expression of caspase-3 delays cell growth in Saccharomyces cerevesiae without causing cell death. Mutation of the caspase-3 QACRG active site abolished effects on yeast growth. Co-expression of caspase inhibitors alleviated growth inhibition in yeast as did the tripeptide caspase inhibitor ZVAD-fmk. These results suggest that substrates for caspase-3 are present in S. cerevesiae and may participate in the normal cell growth and division processes.

Caspase-8 is required for cell death induced by expanded polyglutamine repeats

We show here that caspase-8 is required for the death of primary rat neurons induced by an expanded polyglutamine repeat (Q79). Expression of Q79 recruited and activated caspase-8. Inhibition of caspase-8 blocked polyglutamine-induced cell death. Coexpression of Q79 with the caspase inhibitor CrmA, a dominant-negative mutant of FADD (FADD DN), Bcl-2, or Bcl-xL, but not an N-terminally tagged Bcl-xL, prevented the recruitment of caspase-8 and inhibited polyglutamine-induced cell death. Furthermore, Western blot analysis revealed the presence of activated caspase-8 in the insoluble fraction of affected brain regions from Huntington's disease (HD) patients but not in those from neurologically unremarkable controls, suggesting the relocation and activation of caspase-8 during the pathogenesis of HD. These results suggest an essential role of caspase-8 in HD-related neural degenerative diseases.

Serpins and regulation of cell death

Proteolysis is a key feature of programmed cell death. Extracellular proteinases can activate cell surface receptors which trigger apoptosis, and the effector machinery requires the activation and activity of numerous intracellular proteinases (primarily caspases). Effective control of proteolysis is essential for homeostasis and can occur at two levels: regulation of proteinase activation, and regulation of the activated proteinase. Serpins control activated proteinases and several have been implicated in the regulation of cell death. Serpins that inhibit intracellular processes include the viral proteins CrmA and SPI-1, as well as the granzyme B inhibitor, PI-9. Another endogenous serpin, PN-I, prevents the delivery of an apoptotic signal by inhibiting an extracellular proteinase from cleaving a cell surface receptor. There is evidence to suggest that PAI-2 may target an extracellular as well as an intracellular proteinase. Much of our knowledge of proteolysis within apoptotic cells has come from studies using the poxvirus serpin CrmA/SPI-2. CrmA prevents cytokine processing by inhibiting caspase-1, and protects against Fas-, TNF- and TRAIL-mediated apoptosis by inhibiting an unidentified proteinase specific to these pathways. Work with CrmA has also clearly demonstrated that there are separable effector mechanisms within cells, and that those triggered by growth factor withdrawal, matrix dissociation or cytotoxic ligands are different in several respects to those triggered by radiation, chemicals or steroid hormones. It is likely that analysis of other poxvirus serpins with different inhibitory profiles (especially SPI-1) will yield further insights into these processes. Prospecting for intracellular serpin genes in other virus species may also be fruitful. Finally, all of the serpins known to regulate intracellular proteolysis are members of the ovalbumin subgroup. It remains to be seen whether the more recently described "orphan" ovalbumin serpins (Riewald and Schleef 1995; Sprecher et al. 1995; Sun et al. 1997) also have roles in the regulation of cell death.

Regulation of pro-apoptotic leucocyte granule serine proteinases by intracellular serpins

Caspase activation and apoptosis can be initiated by the introduction of serine proteinases into the cytoplasm of a cell. Cytotoxic lymphocytes have evolved at least one serine proteinase with specific pro-apoptotic activity (granzyme B), as well as the mechanisms to deliver it into a target cell, and recent evidence suggests that other leucocyte granule proteinases may also have the capacity to kill if released into the interior of cells. For example, the monocyte/granulocyte proteinase cathepsin G can activate caspases in vitro, and will induce apoptosis if its entry into cells is mediated by a bacterial pore-forming protein. The potent pro-apoptotic activity of granzyme B and cathepsin G suggests that cells producing these (or other) proteinases would be at risk from self-induced death if the systems involved in packaging, degranulation or targeting fail and allow proteinases to enter the host cell cytoplasm. The purpose of the present review is to describe recent work on a group of intracellular serine proteinase inhibitors (serpins) which may function in leucocytes to prevent autolysis induced by the granule serine proteinases.

Constitutive expression of interleukin-1beta (IL-1beta) in rat oligodendrocytes

The RT-PCR analysis of RNA from progenitor and differentiated primary rat oligodendrocytes, and from the oligodendrocyte CG-4 cell line, shows the presence of the IL-1beta mRNA, the type I IL-1beta receptor and the IL-1 receptor accessory protein in these cells. In situ hybridization of a rat IL-1beta probe to primary progenitor and differentiated rat oligodendrocytes results in a positive signal. The double hybridization of the IL-1beta probe, together with an oligodendrocyte-specific differentiation marker, to sections of postnatal rat brain at different stages of differentiation is also positive. The double immuno-labelling technique utilized indicates coincidence of the signals on the brain slices. The results show that IL-1beta mRNA is constitutively expressed in rat brain oligodendrocytes from 1 day after birth onward. In agreement with this observation, CG-4 cells, primary progenitor and differentiated rat oligodendrocytes are positively stained by antibodies against IL-1beta. Postnatal brain slices from 1 and 4 day old and adult rats, labelled with a double immunofluorescence technique, are also stained by antibodies against IL-1beta. This signal coincides with that of antibodies against oligodendrocyte-specific surface markers. We conclude that IL-1beta is constitutively expressed in rat brain progenitor and differentiated oligodendrocytes.

Caspase-1 is not involved in CD95/Fas-induced apoptosis in Jurkat T cells

It is now well established that the caspases, a family of cysteine proteases, play a key role in apoptosis. Although overexpressing each of the caspases in cells triggered apoptosis, the precise role and contribution of individual caspases are still unclear. Caspase-1, the first caspase discovered, was initially implicated in mammalian apoptosis because of its similarity to the gene product ced-3. Using whole cells as well as an in vitro system to study apoptosis, the role of caspase-1 in Fas-mediated apoptosis in Jurkat T cells was examined in greater detail. Using various peptide-based caspase inhibitors, our results showed that N-acetyl-Tyr-Val-Ala-Asp chloromethyl ketone and benzyloxycarbonyl-Val-Ala-Asp (OMe) fluoromethyl ketone efficiently blocked Fas-mediated apoptosis in Jurkat T cells, whereas N-acetyl-Tyr-Val-Ala-Asp aldehyde, which is more specific for caspase-1, had little effect. Cell lysates derived from anti-Fas-stimulated cells, which readily induced apoptotic nuclei morphology and DNA fragmentation in isolated thymocyte nuclei, had no caspase-1 activity using proIL-1beta as a substrate. Time-course studies showed no caspase-1 activity during the activation of apoptosis in Jurkat cells by agonistic Fas antibodies. Furthermore, no pro-caspase-1 protein nor activated form of the protein was detected in normal or apoptotic Jurkat cells. In contrast, both caspase-2 and caspase-3 were readily detected as proenzymes in control cells and their activated forms were detected in apoptotic cells. Incubation of recombinant active caspase-1 with control cell lysates did not activate the apoptotic cascade as shown by the lack of detectable apoptotic nuclei promoting activity using isolated nuclei as substrate. However, under similar conditions proIL-1beta was readily processed into the mature cytokine, indicating that the recombinant caspase-1 remained active in the presence of control cell lysates. Taken together our results demonstrate that caspase-1 is not required for the induction of apoptosis in Jurkat T cells mediated by the Fas antigen.

Differential expression and translocation of protein tyrosine phosphatase 1B-related proteins in ME-180 tumor cells expressing apoptotic sensitivity and resistance to tumor necrosis factor: potential interaction with epidermal growth factor receptor

Tumor necrosis factor (TNF)-induced apoptosis can be inhibited by overexpression of specific tyrosine kinases or activation of tyrosine kinase cascades, suggesting potential antagonism between apoptotic and tyrosine kinase signaling processes. In this report, the effects of TNF on EGF receptor tyrosine phosphorylation in ME-180 cell variants selected for apoptotic sensitivity (Sen) or resistance (Res) to TNF, previously shown to differentially express EGFr, were examined. Prior to the onset of apoptosis, TNF caused a significant reduction in the level of EGFr tyrosine phosphorylation in Sen cells but mediated only limited suppression of EGFr tyrosine phosphorylation in apoptotically resistant Res cells. In vitro incubation of cellular membranes with TNF derived from Sen cells stimulated a resident protein tyrosine phosphatase (PTP) activity which was able to dephosphorylate EGFr or tyrosine phosphopeptides mimicking an EGFr autophosphorylation site. In membrane preparations, PTPIB complexed with tyrosine phosphorylated EGFr and this association was disrupted by TNF through an apparent stimulation of PTP activity and turnover of phosphotyrosine. Intrinsic enzymatic activity of PTP1B was 2-3-fold higher in Sen versus Res cell lysates and a family of PTP1B-related proteins with altered C-termini was found to be highly expressed in Sen cells but absent or expressed at reduced levels in Res cells. Cytoplasmic extracts of Sen cells contained PTP1B-like proteins and TNF incubation resulted in the time dependent accumulation of PTP1B-like proteins in Sen cells but did not effect these proteins in Res cells. Together, these results suggest that specific changes in expression and subcellular distribution of phosphotyrosine modulatory proteins may play a role in conveying intrinsic apoptotic sensitivity to TNF in some tumor cell types.

Ordering the cytochrome c-initiated caspase cascade: hierarchical activation of caspases-2, -3, -6, -7, -8, and -10 in a caspase-9-dependent manner

Exit of cytochrome c from mitochondria into the cytosol has been implicated as an important step in apoptosis. In the cytosol, cytochrome c binds to the CED-4 homologue, Apaf-1, thereby triggering Apaf-1-mediated activation of caspase-9. Caspase-9 is thought to propagate the death signal by triggering other caspase activation events, the details of which remain obscure. Here, we report that six additional caspases (caspases-2, -3, -6, -7, -8, and -10) are processed in cell-free extracts in response to cytochrome c, and that three others (caspases-1, -4, and -5) failed to be activated under the same conditions. In vitro association assays confirmed that caspase-9 selectively bound to Apaf-1, whereas caspases-1, -2, -3, -6, -7, -8, and -10 did not. Depletion of caspase-9 from cell extracts abrogated cytochrome c-inducible activation of caspases-2, -3, -6, -7, -8, and -10, suggesting that caspase-9 is required for all of these downstream caspase activation events. Immunodepletion of caspases-3, -6, and -7 from cell extracts enabled us to order the sequence of caspase activation events downstream of caspase-9 and reveal the presence of a branched caspase cascade. Caspase-3 is required for the activation of four other caspases (-2, -6, -8, and -10) in this pathway and also participates in a feedback amplification loop involving caspase-9.

Multiple pathways of apoptosis in PC12 cells. CrmA inhibits apoptosis induced by beta-amyloid

Stable transfectants of PC12 cells expressing bcl-2 or crmA were generated and tested for their susceptibility to various apoptotic insults. Bcl-2 expression conferred resistance to apoptosis induced by staurosporine and by oxidative insults including hydrogen peroxide and peroxynitrite, but was less effective in inhibition of activation-induced programmed cell death induced by concanavalin A. Concanavalin A-induced apoptosis was abated, however, in cells expressing very high levels of bcl-2. In contrast, cells expressing crmA were protected from concanavalin A-induced apoptosis, but were as susceptible as control cells to apoptosis induced by staurosporine and oxidative insults. Therefore, at least two apoptotic pathways in PC12 cells can be discerned by their differential sensitivity to blockade by bcl-2 and crmA. The ability of beta-amyloid (Abeta) to induce apoptosis in these cells was assessed. CrmA transfectants were protected from apoptosis induced by Abeta1-42, but only cells expressing very high levels of bcl-2 were similarly protected. These results suggest that the apoptotic pathway activated by Abeta1-42 in PC12 cells can be differentiated from the apoptotic pathway activated by oxidative insults. Gene transfer experiments also demonstrated that expression of crmA in primary cultures of hippocampal neurons is protective against cell death induced by Abeta1-42. Together these results support the hypothesis that Abeta-induced apoptosis occurs through activation-induced programmed cell death.

The progression and topographic distribution of interleukin-1beta expression after permanent middle cerebral artery occlusion in the rat

The cytokine interleukin-1 (IL-1) has been implicated in the exacerbation of ischemic damage in the brains of rodents. This study has ascertained the cellular localization and chronologic and topographic distribution of pro/mature interleukin-1beta (IL-1beta) protein 0.5, 1, 2, 6, 24, and 48 hours after ischemia by subjecting rats to permanent unilateral occlusion of the middle cerebral artery. Interleukin-1beta was localized immunocytochemically in vibratome sections of perfusion-fixed brains. The cells that expressed IL-1beta had the morphologic features of microglia and macrophages. Interleukin-1beta was first detected 1 hour after occlusion in ipsilateral meningeal macrophage-like cells. By 6 hours, pro/mature IL-1beta-immunoreactive (IL-1(beta)ir) putative microglia were present in the ischemic cerebral cortex, corpus callosum, caudoputamen, and surrounding tissue. By 24 and 48 hours after ischemia, the number and spread of IL-1(beta)ir cells increased greatly, including those resembling activated microglia and macrophages, as the core of the infarct became infiltrated. Interleukin-1(beta)ir cells also were present in apparently undamaged tissue, adjacent to the lesion ipsilaterally, and contralaterally in the cerebral cortex, dorsal corpus callosum, dorsal caudoputamen, and hippocampus. These results support the functional role of IL-1 in ischemic brain damage and reveal a distinct temporal and spatial expression of IL-1beta protein in cells believed to be microglia and macrophages.

Adenovirus-mediated gene transfer of inhibitors of apoptosis protein delays apoptosis in cerebellar granule neurons

The inhibitor of apoptosis (IAP) family of antiapoptotic genes, originally discovered in baculovirus, exists in animals ranging from insects to humans. Here, we investigated the ability of IAPs to suppress cell death in both a neuronal model of apoptosis and excitotoxicity. Cerebellar granule neurons undergo apoptosis when switched from 25 to 5 mM potassium, and excitotoxic cell death in response to glutamate. We examined the endogenous expression of four members of the IAP family, X chromosome-linked IAP (XIAP), rat IAP1 (RIAP1), RIAP2, and neuronal apoptosis inhibitory protein (NAIP), by semiquantitative reverse PCR and immunoblot analysis in cultured cerebellar granule neurons. Cerebellar granule neurons express significant levels of RIAP2 mRNA and protein, but expression of RIAP1, NAIP, and XIAP was not detected. RIAP2 mRNA content and protein levels did not change when cells were switched from 25 to 5 mM potassium. To determine whether ectopic expression of IAP influenced neuronal survival after potassium withdrawal or glutamate exposure, we used recombinant adenoviral vectors to target XIAP, human IAP1 (HIAP1), HIAP2, and NAIP into cerebellar granule neurons. We demonstrate that forced expression of IAPs efficiently blocked potassium withdrawal-induced N-acetyl-Asp-Glu-Val-Asp-specific caspase activity and reduced DNA fragmentation. However, neurons were only protected from apoptosis up to 24 h after potassium withdrawal, but not at later time points, suggesting that IAPs delay but do not block apoptosis in cerebellar granule neurons. In contrast, treatment with 100 microM or 1 mM glutamate did not induce caspase activity and adenoviral-mediated expression of IAPs had no influence on subsequent excitotoxic cell death.

Programmed cell death and the caspases

Members of a family of cysteine proteases known as caspases orchestrate the intracellular biochemical events that enable animal cells to kill themselves by apoptosis. To counteract the apoptotic response to infection, some viruses have adapted and evolved proteins that specifically block caspases. More recently, it has been demonstrated that endogenous proteins belonging to the IAP family can regulate apoptosis by directly inactivating some of the caspases involved in initiating and executing programmed cell death.

The pharmacology of apoptosis

Apoptosis is an area of intense scientific interest, which encompasses the study of and triggers mechanisms involved in mediating the cell biology of programmed cell death. A number of low molecular weight compounds have been used to inhibit or enhance this fundamental cellular process and so apoptosis has now become amenable to pharmacological manipulation. In this review Ross Kinloch, Mark Treherne, Mike Furness and Iradj Hajimohamadreza will focus on the current literature describing the pharmacology of apoptosis, with particular reference to the therapeutic potential that could arise from the development of pro- and anti-apoptotic drugs. The pivotal role of apoptosis in such diverse pathological processes as tumour growth, the immune response and neurodegeneration suggests that an understanding of how apoptosis can be regulated by drugs will become increasingly important to the pharmaceutical industry.

The 2-5A system in viral infection and apoptosis

The 2-5A system is an established endogenous antiviral pathway. Interferon treatment of cells leads to an increase in basal, but latent, levels of 2-5A-dependent RNase (RNase L) and the family of 2'-5' oligoadenylate synthetases (OAS). Double-stranded RNA, thought to be derived from viral replication intermediates, activates OAS. Activated OAS converts ATP into unusual short 2'-5' linked oligoadenylates called 2-5A [ppp5'(A2'p5')2A]. The 2-5A binds to and activates RNase L which cleaves single stranded RNA with moderate specificity for sites 3' of UpUp and UpAp sequences, and thus leads to degradation of cellular rRNA. During apoptosis, generalized cellular RNA degradation, distinct from the differential expression of mRNA species that may regulate specific gene expression during apoptosis, has been observed. The mechanism of RNA breakdown during apoptosis has been commonly considered a non-specific event that reflects the generalized shut down of translation and homeostatic regulation during cell death. Due to the similar RNA degradation that occurs during both apoptosis and viral infection we investigated the potential role of RNase L in apoptosis. To investigate whether RNase L activity could lead to apoptosis, NIH3T3 cells were transfected with a lac-inducible vector containing the human RNase L gene. Treatment of these cells with isopropylthiogalactoside (IPTG) caused loss of cell viability that was confirmed as an apoptotic cell death by morphological and biochemical criteria. Similarly, specific allosteric activation of endogenous RNase L by introduction of 2-5A directly into L929 cells also induced apoptosis. In L929 cells poly(I).poly(C) treatment in combination with interferon caused an increase in apoptosis whereas neither interferon or double stranded RNA alone altered cell viability. Therefore, increased expression or activation of RNase L causes apoptosis. Inhibition of RNase L, specifically with a dominant negative mutant, suppressed poly(I)Ypoly(C)-induced apoptosis in interferon-primed fibroblasts. Poliovirus, a picornovirus with a single-stranded RNA genome, causes apoptosis of HeLa cells. Expression of the dominant negative inhibitor of RNase L in HeLa prevented virus-induced apoptosis and maintained cell viability. Thus, reduction or inhibition of RNase L activity prevents apoptosis. Both apoptosis and the 2-5A system can provide defense against viral infection in multicellular organisms by preventing production and therefore spread of progeny virus. RNase L appears to function in both mechanisms, therefore, initiation of apoptosis may be one mechanism for the antiviral activity of the 2-5A system.

Inhibition of human caspases by peptide-based and macromolecular inhibitors

Studies with peptide-based and macromolecular inhibitors of the caspase family of cysteine proteases have helped to define a central role for these enzymes in inflammation and mammalian apoptosis. A clear interpretation of these studies has been compromised by an incomplete understanding of the selectivity of these molecules. Here we describe the selectivity of several peptide-based inhibitors and the coxpox serpin CrmA against 10 human caspases. The peptide aldehydes that were examined (Ac-WEHD-CHO, Ac-DEVD-CHO, Ac-YVAD-CHO, t-butoxycarbonyl-IETD-CHO, and t-butoxycarbonyl-AEVD-CHO) included several that contain the optimal tetrapeptide recognition motif for various caspases. These aldehydes display a wide range of selectivities and potencies against these enzymes, with dissociation constants ranging from 75 pM to >10 microM. The halomethyl ketone benzyloxycarbonyl-VAD fluoromethyl ketone is a broad specificity irreversible caspase inhibitor, with second-order inactivation rates that range from 2.9 x 10(2) M-1 s-1 for caspase-2 to 2.8 x 10(5) M-1 s-1 for caspase-1. The results obtained with peptide-based inhibitors are in accord with those predicted from the substrate specificity studies described earlier. The cowpox serpin CrmA is a potent (Ki < 20 nM) and selective inhibitor of Group I caspases (caspase-1, -4, and -5) and most Group III caspases (caspase-8, -9, and -10), suggesting that this virus facilitates infection through inhibition of both apoptosis and the host inflammatory response.

Oxygen toxicity induces apoptosis in neuronal cells

1. A high oxygen atmosphere induced apoptosis in cultured neuronal cells including PC12 cells and rat embryonic cortical, hippocampal, and basal forebrain neurons associated with DNA fragmentation and nuclear condensation. 2. The sensitivity of CNS neurons to a high-oxygen atmosphere was the following order; cortex > basal forebrain > hippocampus. 3. Cycloheximide and actinomycin-D inhibited the apoptosis, indicating that it depends on new macromolecular synthesis. In contrast, cultured postnatal CNS neurons were resistant to oxidative stress. 4. Neurotrophic factors such as nerve growth factor (NGF), fibroblast growth factor (FGF), and epidermal growth factor (EGF) blocked the apoptosis induced by a high-oxygen atmosphere.