Constitutive p53 heightens mitochondrial apoptotic priming and favors cell death induction by BH3 mimetic inhibitors of BCL-xL

Proapoptotic molecules directly targeting the BCL-2 family network are promising anticancer therapeutics, but an understanding of the cellular stress signals that render them effective is still elusive. We show here that the tumor suppressor p53, at least in part by transcription independent mechanisms, contributes to cell death induction and full activation of BAX by BH3 mimetic inhibitors of BCL-xL. In addition to mildly facilitating the ability of compounds to derepress BAX from BCL-xL, p53 also provides a death signal downstream of anti-apoptotic proteins inhibition. This death signal cooperates with BH3-induced activation of BAX and it is independent from PUMA, as enhanced p53 can substitute for PUMA to promote BAX activation in response to BH3 mimetics. The acute sensitivity of mitochondrial priming to p53 revealed here is likely to be critical for the clinical use of BH3 mimetics.

Mitochondria and cancer: is there a morphological connection?

Mitochondria are key players in several cellular functions including growth, division, energy metabolism, and apoptosis. The mitochondrial network undergoes constant remodelling and these morphological changes are of direct relevance for the role of this organelle in cell physiology. Mitochondrial dysfunction contributes to a number of human disorders and may aid cancer progression. Here, we summarize the recent contributions made in the field of mitochondrial dynamics and discuss their impact on our understanding of cell function and tumorigenesis.

Bcl-2 prevents activation of CPP32 cysteine protease and cleavage of poly (ADP-ribose) polymerase and U1-70 kD proteins in staurosporine-mediated apoptosis

Members of the the Bcl-2 and ICE/ced-3 gene families have been implicated as essential components in the control of the cell death pathway. Bcl-2 overexpression can prevent programmed cell death (PCD) in different cell types. ICE/ced-3-like proteases are synthesized as pro-enzymes and are activated by limited proteolysis. When overexpressed in diverse cell types, they trigger PCD. Bcl-2 can inhibit PCD mediated by these proteases, although as yet it is not clear at what specific step in the cell death pathway the protein acts. Here, we demonstrate that CPP32/Yama/Apopain, a member of the ICE/Ced-3 gene family, is processed during staurosporine-induced apoptosis in HeLa cells and that concomitant with CPP32 activation, two other proteins, poly (ADP-ribose) polymerase (PARP) and the U1-70 K small ribonucleoprotein, also undergo proteolysis. Overexpression of Bcl-2 prevents cleavage of CPP32, PARP and U1-70 K and protects HeLa cells from PCD. These results demonstrate that Bcl-2 controls PCD, by acting upstream of CPP32/Yama/Apopain.

Major role of BAX in apoptosis during retinal development and in establishment of a functional postnatal retina

Apoptosis plays a major role in the development of the central nervous system. Previous studies of apoptosis induction during retinal development are difficult to interpret, however, because they explored different mouse strains, different developmental periods, and used different assays. Here, we first established a comprehensive sequential pattern of cell death during the whole development of the C57BL/6J mouse retina, from E10.5 to postnatal day (P) 21 by using the terminal deoxynucleotidyl transferase (TdT) -mediated deoxyuridine triphosphate (dUTP)-biotinylated nick end labeling (TUNEL) assay. We confirmed the existence of three previously described apoptotic peaks and identified another, later peak at P15, in both the outer nuclear layer, in which the photoreceptors differentiate, and the ganglion cell layer. Comparison of wild-type C57BL/6 mice, gld mice, defective in the death ligand fasL, and bax-/- mice, defective in the pro-apoptotic BAX protein, revealed a minor role for FAS ligand but a crucial role for BAX in both apoptosis and normal retinal development. The lack of BAX resulted in thicker than normal inner neuroblastic and ganglion cell layers in adults, with larger numbers of cells and an impaired electroretinogram response related to a decreased number of responsive cells. Our findings indicate that cell death during normal retinal development is important for the modeling of a functional vision organ and showed that the pro-apoptotic BAX protein plays a crucial role in this process.

The p75 neurotrophin receptor: effects on neuron survival in vitro and interaction with death domain-containing adaptor proteins

The p75 neurotrophin receptor (p75NTR) is a death domain (DD) containing receptor of the TNF/FAS(APO-1) family. p75NTR has recently been shown to mediate apoptosis in certain types of neurons as well as in oligodendrocytes. The molecular mechanisms by which p75NTR stimulates apoptosis are still unknown. Here, we have tested whether overexpression of p75NTR could modulate survival of sympathetic neurons cultured in the presence or absence of NGF. Moreover, using the yeast two-hybrid system, we tested whether p75NTR intracellular domain was able to dimerize or interact with known DD-containing proteins including FADD, RIP, RAIDD and TRADD. We found that over-expression of p75NTR had no effect on the survival of sympathetic neurons cultured in the presence of NGF but instead delayed neuronal death following NGF deprivation. These results strongly support the finding that p75NTR is not involved in the apoptosis process induced by NGF deprivation in sympathetic neurons. We also foun d that the intracellular domain of p75NTR failed to associate either with itself or with other known DD-containing proteins. This suggests that the mechanisms by which p75NTR triggers apoptosis in certain cell types are different from those used by other receptors of the TNF/FAS family.

Mitochondria: regulating the inevitable

Apoptosis is a form of programmed cell death important in the development and tissue homeostasis of multicellular organisms. Abnormalities in cell death control can lead to a variety of diseases, including cancer and degenerative disorders. Hence, the process of apoptosis is tightly regulated through multiple independent signalling pathways that are initiated either from triggering events within the cell or at the cell surface. In recent years, mitochondria have emerged as the central components of such apoptotic signalling pathways and are now known to control apoptosis through the release of apoptogenic proteins. In this review we aim to give an overview of the role of the mitochondria during apoptosis and the molecular mechanisms involved.

Phosphorylation of bid by casein kinases I and II regulates its cleavage by caspase 8

Bid plays an essential role in Fas-mediated apoptosis of the so-called type II cells. In these cells, following cleavage by caspase 8, the C-terminal fragment of Bid translocates to mitochondria and triggers the release of apoptogenic factors, thereby inducing cell death. Here we report that Bid is phosphorylated by casein kinase I (CKI) and casein kinase II (CKII). Inhibition of CKI and CKII accelerated Fas-mediated apoptosis and Bid cleavage, whereas hyperactivity of the kinases delayed apoptosis. When phosphorylated, Bid was insensitive to caspase 8 cleavage in vitro. Moreover, a mutant of Bid that cannot be phosphorylated was found to be more toxic than wild-type Bid. Together, these data indicate that phosphorylation of Bid represents a new mechanism whereby cells control apoptosis.

Transgenic mice with neuronal overexpression of bcl-2 gene present navigation disabilities in a water task

In the CNS, Bcl-2 is an antiapoptotic gene involved in the regulation of neuronal death. Transgenic mice overexpressing the human gene Bcl-2 (Hu-bcl-2 mice) showed delayed acquisition in two tasks requiring them to find a hidden platform starting from either a random or a constant starting location. The same mice were not deficient in another task requiring them to find a visible platform suggesting that the delay observed was not due to motor, visual or motivational deficits in the water. The delay observed in Hu-bcl-2 mice was more important in the random starting test in which the allocentric demand for navigation was stronger. The results suggested that allocentric navigation is particularly sensitive to abnormal CNS maturation following the overexpression of the bcl-2 gene. The specific deficits (motor learning, fear-related behavior and allocentric navigation) observed in Hu-bcl-2 mice suggest that the regulation of developmental neuronal death is crucial for multisensorial learning and emotional behavior.

Bax is present as a high molecular weight oligomer/complex in the mitochondrial membrane of apoptotic cells

Bax is a Bcl-2 family protein with proapoptotic activity, which has been shown to trigger cytochrome c release from mitochondria both in vitro and in vivo. In control HeLa cells, Bax is present in the cytosol and weakly associated with mitochondria as a monomer with an apparent molecular mass of 20,000 Da. After treatment of the HeLa cells with the apoptosis inducer staurosporine or UV irradiation, Bax associated with mitochondria is present as two large molecular weight oligomers/complexes of 96,000 and 260,000 Da, which are integrated into the mitochondrial membrane. Bcl-2 prevents Bax oligomerization and insertion into the mitochondrial membrane. The outer mitochondrial membrane protein voltage-dependent anion channel and the inner mitochondrial membrane protein adenosine nucleotide translocator do not coelute with the large molecular weight Bax oligomers/complexes on gel filtration. Bax oligomerization appears to be required for its proapoptotic activity, and the Bax oligomer/complex might constitute the structural entirety of the cytochrome c-conducting channel in the outer mitochondrial membrane.

Involvement of mitochondria in apoptosis

Like in many other cell types, apoptosis can be induced by different stress in cells isolated from the cardiovascular system. The mitochondrial apoptotic pathway can be activated by serum deprivation, (9, 66) staurosporine treatment, (110) and oxidative stress. (14) The cytokine pathway is activated by TNF or Fas. (43, 52, 107) Immunohistochemical analysis of endomyocardial biopsies from patients with congestive heart failure, acute myocardial infarction, ischemic cardiomyopathies, and myocarditis, have led to the identification of apoptotic cardiomyocytes. (15 41, 74) Therefore, the pre-existing death program evidenced in isolated cardiomyocytes also may be activated in cardiomyopathies. Apoptosis also has been detected in vascular diseases, such as atherosclerosis, hypertension, and restenosis.49 It is likely that mitochondria, through permeabilization of their outer membrane, play a major role in many apoptotic responses leading to cardiomyocyte apoptosis. Elucidation of the mechanism whereby mitochondrial cell-death effectors are released in the cytosol should open the opportunity of developing compounds able to regulate the progression of apoptosis. The development of drugs acting on the mitochondrion may allow the prevention or the limitation of the seriousness of many cardiovascular diseases in which apoptosis has been detected.

Breaking the mitochondrial barrier

Pro- and anti-apoptotic members of the Bcl-2 family control the permeability of the outer mitochondrial membrane. They could do this either by forming autonomous pores in the membrane or by collaborating with components of the permeability transition pore. Here we discuss why we favour the first of these possibilities.

Mitochondria as the central control point of apoptosis

Mitochondria play a major role in apoptosis triggered by many stimuli. They integrate death signals through Bcl-2 family members and coordinate caspase activation through the release of cytochrome c as a result of the outer mitochondrial membrane becoming permeable. The mechanisms that lead to this permeability are not yet completely understood. Here, we attempt to summarize our current view of the mechanisms that lead to the efflux of many proteins from mitochondria during apoptosis and the role played by Bcl-2 family proteins in the control of this event.

The destabilization of lipid membranes induced by the C-terminal fragment of caspase 8-cleaved bid is inhibited by the N-terminal fragment

Bid is a proapoptotic, BH3-domain-only member of the Bcl-2 family. In Fas-induced apoptosis, Bid is activated through cleavage by caspase 8 into a 15.5-kDa C-terminal fragment (t(c)Bid) and a 6.5 kDa N-terminal fragment (t(n)Bid). Following the cleavage, t(c)Bid translocates to the mitochondria and promotes the release of cytochrome c into the cytosol by a mechanism that is not understood. Here we report that recombinant t(c)Bid can act as a membrane destabilizing agent. t(c)Bid induces destabilization and breaking of planar lipid bilayers without appearance of ionic channels; its destabilizing activity is comparable with that of Bax and at least 30-fold higher than that of full-length Bid. Consistently, t(c)Bid, but not full-length Bid, permeabilizes liposomes at physiological pH. The destabilizing effect of t(c)Bid on liposomes and planar bilayers is independent of the BH3 domain. In contrast, mutations in the BH3 domain impair t(c)Bid ability to induce cytochrome c release from mitochondria. The permeabilizing effect of t(c)Bid on planar bilayers, liposomes, and mitochondria can be inhibited by t(n)Bid. In conclusion, our results suggest a dual role for Bid: BH3-independent membrane destabilization and BH3-dependent interaction with other proteins. Moreover, the dissociation of Bid after cleavage by caspase 8 represents an additional step at which apoptosis may be regulated.

Cytochrome c release from mitochondria: all or nothing

During the process of apoptosis, cytochrome c is released from mitochondria into the cytosol where it helps to activate the caspases, a family of killer proteases. The release of cytochrome c is a rapid, complete and kinetically invariant event.

Bid induces the oligomerization and insertion of Bax into the outer mitochondrial membrane

In many types of apoptosis, the proapoptotic protein Bax undergoes a change in conformation at the level of the mitochondria. This event always precedes the release of mitochondrial cytochrome c, which, in the cytosol, activates caspases through binding to Apaf-1. The mechanisms by which Bax triggers cytochrome c release are unknown. Here we show that following binding to the BH3-domain-only proapoptotic protein Bid, Bax oligomerizes and then integrates in the outer mitochondrial membrane, where it triggers cytochrome c release. Bax mitochondrial membrane insertion triggered by Bid may represent a key step in pathways leading to apoptosis.

Bax oligomerization is required for channel-forming activity in liposomes and to trigger cytochrome c release from mitochondria

Bax is a Bcl-2-family protein with pro-apoptotic activity that can form channels in lipid membranes. The protein has been shown to trigger cytochrome c release from mitochondria both in vitro and in vivo. Recombinant human Bax isolated in the presence of detergent was found to be present as an oligomer with an apparent molecular mass of approx. 160000 Da on gel filtration. When Bax was isolated in the absence of detergent the purified protein was monomeric with an apparent molecular mass of 22000 Da. Bax oligomers formed channels in liposomes and triggered cytochrome c release from isolated mitochondria, whereas monomeric Bax was inactive in both respects. Incubation of the monomeric Bax with 2% octyl glucoside induced formation of oligomers that displayed channel-forming activity in liposomes and triggered cytochrome c release from mitochondria. Triton X-100, Nonidet P-40 and n-dedecyl maltoside also activated monomeric Bax, whereas CHAPS had no activating effect. In cytosolic extracts from mouse liver, Bax migrated at a molecular mass of 24000 Da on gel filtration, whereas after incubation of the cytosol with 2% octyl glucoside Bax migrated at approximately 140000 Da. These results show that oligomeric Bax possesses channel-forming activity whereas monomeric Bax has no such activity.

Differential effects of Bcl-2 overexpression on fibre outgrowth and survival of embryonic dopaminergic neurons in intracerebral transplants

The causes of death of transplanted neurons are not known in detail, but apoptotic mechanisms involving caspase activation are likely to play a role. We examined whether overexpression of the anti-apoptotic protein Bcl-2 may enhance the survival of dopaminergic [tyrosine hydroxylase (TH)-immunoreactive] grafted neurons. For this purpose, we prepared cells from embryonic day 13 ventral mesencephalon (VM) of mice overexpressing human Bcl-2, or from their wild-type littermates. The bcl-2 transgene was strongly expressed in these cells, and resulted in protection of neuronal cultures from death triggered by serum deprivation or exposure to staurosporine. To model pretransplantation stress more closely in vitro, we stored dissociated embryonic mesencephalic cells for 8 h in the same type of medium used for intracerebral transplantation. This resulted in massive cell death as quantified by lactate dehydrogenase (LDH) release, and increased DNA fragmentation. Although this cell loss was strongly reduced by a caspase inhibitor, Bcl-2 had no significant protective effect. Finally, mesencephalic cell suspensions were xenografted into the striatum of immunosuppressed hemiparkinsonian rats. Neither the survival of TH-immunopositive transplanted neurons nor the functional recovery of the rats was improved by Bcl-2, although the Bcl-2 protein was strongly expressed in transgenic grafts 5 weeks after implantation, and dopaminergic fibre outgrowth from the grafts was significantly improved. These data suggest that cell death in neuronal transplants involves apoptotic mechanisms that can bypass negative regulation by Bcl-2.

Overproduction of Cu/Zn-superoxide dismutase or Bcl-2 prevents the brain mitochondrial respiratory dysfunction induced by glutathione depletion

Recent work has focused attention on the role of oxidative stress in various acute and chronic neurodegenerative diseases. Low concentrations of the powerful antioxidant glutathione (GSH) and impaired brain energy metabolism, particularly in the substantia nigra, are key features of Parkinson's disease (PD). The main goal of this study was to better characterize the deleterious effects of brain GSH depletion on mitochondrial function. We depleted GSH in the brains of newborn wild-type (WT) and transgenic (Tg) mice overproducing either human Cu/Zn-superoxide dismutase (h-CuZnSOD) or human Bcl2 (h-Bcl-2), by subcutaneous injection of l-buthionine sulfoximine (BSO), a specific inhibitor of gamma-glutamylcysteine synthetase. GSH was 97% depleted in brain homogenates and 90% depleted in brain mitochondria for both WT and Tg mice. This depletion of brain GSH led to a decrease in the activity of the GSH-dependent antioxidant enzyme glutathione peroxidase, both in WT and in Tg animals. BSO treatment decreased the activities of respiratory complexes I, II, and IV in the brain homogenates of WT mice. BSO-treated h-CuZnSOD or h-Bcl-2 Tg mice had no respiratory chain deficiencies. Thus, brain GSH depletion leads to the impairment of mitochondrial respiratory chain activity. The protection of mitochondrial respiratory function by overproduction of Bcl-2 may result from a decrease in the generation of reactive oxygen species (ROS) or lipid peroxidation. The protection of mitochondria by overproduction of CuZnSOD is consistent with the involvement of superoxide or superoxide-derived ROS in the mitochondrial dysfunction caused by brain GSH depletion. This study demonstrates that the antioxidant balance is critical for maintenance of brain mitochondrial function, and its disruption may contribute to the pathogenesis of PD.

Transgenic murine cortical neurons expressing human Bcl-2 exhibit increased resistance to amyloid beta-peptide neurotoxicity

The generation of reactive oxygen species has been implicated in the neurotoxicity of amyloid beta-peptide, the main constituent of the senile plaques that accumulates in the brain of Alzheimer's disease victims. In this study, we have compared the toxicity of amyloid beta-peptide on cultured cortical neurons from control mice and transgenic mice expressing either human copper-zinc superoxide dismutase or human Bcl-2, two proteins that protect cells against oxidative damage. Copper-zinc superoxide dismutase overexpression failed to protect cortical neurons against the toxicity of amyloid beta-peptide(25-35) [the minimal cytotoxic fragment of amyloid beta-peptide(1-42)] as assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction and an enzyme-linked immunoabsorbent assay using an antibody directed against microtubule-associated protein-2 (a specific neuronal protein), ruling out a role for superoxide anion and peroxynitrite in amyloid beta-peptide-evoked neurotoxicity. On the contrary, cortical neurons expressing human copper-zinc superoxide dismutase exhibited increased apoptotic nuclei in both untreated and amyloid beta-peptide(25-35)-exposed neurons. Transgenic neurons expressing human Bcl-2 were partially protected against amyloid beta-peptide-induced neuronal death. This neuroprotection appears to be related to the complete inhibition of apoptosis induced by both amyloid beta-peptide(25-35) and amyloid beta-peptide(1-42). This study may be relevant for developing neuroprotective gene therapy to inhibit neuronal apoptosis in Alzheimer's disease.

Hu-Bcl-2 transgenic mice with supernumerary neurons exhibit timing impairment in a complex motor task

Programmed neuronal cell death is common during development, and is thought to be important in the elimination of errors in axonal projection, cell position and sculpting of neuronal circuits. However, the potential importance of programmed cell death for complex behaviour in the adult animal has never been addressed. We studied motor abilities in a strain of transgenic mice with neuronal overexpression of the human Bcl-2 protein, which have supernumerary neurons due to reduced developmental cell death. Our results show that these mice have a clear deficiency in fine timing of motor coordination without impairment of basic motor functions. This is the first indication that altered developmental cell death and the consequent neuronal surplus can impair complex behaviour in the adult animal.

The release of cytochrome c from mitochondria during apoptosis of NGF-deprived sympathetic neurons is a reversible event

During apoptosis induced by various stimuli, cytochrome c is released from mitochondria into the cytosol where it participates in caspase activation. This process has been proposed to be an irreversible consequence of mitochondrial permeability transition pore opening, which leads to mitochondrial swelling and rupture of the outer mitochondrial membrane. Here we present data demonstrating that NGF-deprived sympathetic neurons protected from apoptosis by caspase inhibitors possess mitochondria which, though depleted of cytochrome c and reduced in size, remained structurally intact as viewed by electron microscopy. After re-exposure of neurons to NGF, mitochondria recovered their normal size and their cytochrome c content, by a process requiring de novo protein synthesis. Altogether, these data suggest that depletion of cytochrome c from mitochondria is a controlled process compatible with function recovery. The ability of sympathetic neurons to recover fully from trophic factor deprivation provided irreversible caspase inhibitors have been present during the insult period, has therapeutical implications for a number of acute neuropathologies.

Bid-induced conformational change of Bax is responsible for mitochondrial cytochrome c release during apoptosis

Here we report that in staurosporine-induced apoptosis of HeLa cells, Bid, a BH3 domain containing protein, translocates from the cytosol to mitochondria. This event is associated with a change in conformation of Bax which leads to the unmasking of its NH2-terminal domain and is accompanied by the release of cytochrome c from mitochondria. A similar finding is reported for cerebellar granule cells undergoing apoptosis induced by serum and potassium deprivation. The Bax-conformational change is prevented by Bcl-2 and Bcl-xL but not by caspase inhibitors. Using isolated mitochondria and various BH3 mutants of Bid, we demonstrate that direct binding of Bid to Bax is a prerequisite for Bax structural change and cytochrome c release. Bcl-xL can inhibit the effect of Bid by interacting directly with Bax. Moreover, using mitochondria from Bax-deficient tumor cell lines, we show that Bid- induced release of cytochrome c is negligible when Bid is added alone, but dramatically increased when Bid and Bax are added together. Taken together, our results suggest that, during certain types of apoptosis, Bid translocates to mitochondria and binds to Bax, leading to a change in conformation of Bax and to cytochrome c release from mitochondria.

Oxidative stress is attenuated in mice overexpressing BCL-2

The protooncogene Bcl-2 inhibits apoptosis in neural cells, which may involve mitochondrial stabilization and decreased generation of reactive oxygen species. Using in vivo microdialysis we found that following administration of the mitochondrial toxin 3-nitropropionic acid (3-NP) there was a significant increase in the conversion of 4-hydroxybenzoic acid (4-HBA) to 3,4-dihydroxybenzoic acid (3,4-DHBA) in control mice, but not in Bcl-2 overexpressing mice. Striatal lesions were observed in littermate control mice, whereas, lesions were minimal or absent in Bcl-2 overexpressing mice. This shows that Bcl-2 overexpression in vivo attenuates the generation of reactive oxygen species.

Amelioration of hippocampal neuronal damage after global ischemia by neuronal overexpression of BCL-2 in transgenic mice

BACKGROUND AND PURPOSE

Reports suggesting the involvement of apoptosis in ischemic neuronal damage have been accumulating, and protection against apoptotic death by BCL-2 has been shown in many types of cells. Overexpression of BCL-2 has been shown to reduce infarct size after focal ischemia. The purpose of the present study was to assess whether BCL-2 exerted its effect on selective neuronal vulnerability after transient global ischemia.

METHODS

Transgenic mice overexpressing BCL-2 in neurons and their littermates were subjected to transient forebrain ischemia for 12 minutes, and the hippocampus was examined 7 days later with conventional histology, immunohistochemistry, and in situ terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling of fragmented DNA.

RESULTS

Although both types of mice showed a similar degree of ischemic insult, transgenic mice showed a lesser degree of neuronal death together with DNA fragmentation in the hippocampus than their littermates.

CONCLUSIONS

Overexpression of BCL-2 in neurons mitigates selective neuronal vulnerability in the hippocampus of transgenic mice after transient global ischemia.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyride neurotoxicity is attenuated in mice overexpressing Bcl-2

The proto-oncogene Bcl-2 rescues cells from a wide variety of insults. Recent evidence suggests that Bcl-2 protects against free radicals and that it increases mitochondrial calcium-buffering capacity. The neurotoxicity of 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyride (MPTP) is thought to involve both mitochondrial dysfunction and free radical generation. We therefore investigated MPTP neurotoxicity in both Bcl-2 overexpressing mice and littermate controls. MPTP-induced depletion of dopamine and loss of [3H]mazindol binding were significantly attenuated in Bcl-2 overexpressing mice. Protection was more profound with an acute dosing regimen than with daily MPTP administration over 5 d. 1-Methyl-4-phenylpyridinium (MPP+) levels after MPTP administration were similar in Bcl-2 overexpressing mice and littermates. Bcl-2 blocked MPP+-induced activation of caspases. MPTP-induced increases in free 3-nitrotyrosine levels were blocked in Bcl-2 overexpressing mice. These results indicate that Bcl-2 overexpression protects against MPTP neurotoxicity by mechanisms that may involve both antioxidant activity and inhibition of apoptotic pathways.

Bax-induced cytochrome C release from mitochondria is independent of the permeability transition pore but highly dependent on Mg2+ ions

Bcl-2 family members either promote or repress programmed cell death. Bax, a death-promoting member, is a pore-forming, mitochondria-associated protein whose mechanism of action is still unknown. During apoptosis, cytochrome C is released from the mitochondria into the cytosol where it binds to APAF-1, a mammalian homologue of Ced-4, and participates in the activation of caspases. The release of cytochrome C has been postulated to be a consequence of the opening of the mitochondrial permeability transition pore (PTP). We now report that Bax is sufficient to trigger the release of cytochrome C from isolated mitochondria. This pathway is distinct from the previously described calcium-inducible, cyclosporin A-sensitive PTP. Rather, the cytochrome C release induced by Bax is facilitated by Mg2+ and cannot be blocked by PTP inhibitors. These results strongly suggest the existence of two distinct mechanisms leading to cytochrome C release: one stimulated by calcium and inhibited by cyclosporin A, the other Bax dependent, Mg2+ sensitive but cyclosporin insensitive.

Extended therapeutic window for caspase inhibition and synergy with MK-801 in the treatment of cerebral histotoxic hypoxia

In rats, striatal histotoxic hypoxic lesions produced by the mitochondrial toxin malonate resemble those of focal cerebral ischemia. Intrastriatal injections of malonate induced cleavage of caspase-2 beginning at 6 h, and caspase-3-like activity as identified by DEVD biotin affinity-labeling within 12 h. DEVD affinity-labeling was prevented and lesion volume reduced in transgenic mice overexpressing BCL-2 in neuronal cells. Intrastriatal injection of the tripeptide, N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (zVAD-fmk), a caspase inhibitor, at 3 h, 6 h, or 9 h after malonate injections reduced the lesion volume produced by malonate. A combination of pretreatment with the NMDA antagonist, dizocilpine (MK-801), and delayed treatment with zVAD-fmk provided synergistic protection compared with either treatment alone and extended the therapeutic window for caspase inhibition to 12 h. Treatment with cycloheximide and zVAD-fmk, but not with MK-801, blocked the malonate-induced cleavage of caspase-2. NMDA injections alone resulted in a weak caspase-2 cleavage. These results suggest that malonate toxicity induces neuronal death by more than one pathway. They strongly implicate early excitotoxicity and delayed caspase activation in neuronal loss after focal ischemic lesions and offer a new strategy for the treatment of stroke.

Overexpression of a Hu-bcl-2 transgene in Lurcher mutant mice delays Purkinje cell death

Cerebellar Purkinje cells in the heterozygous Lurcher mutant undergo cell autonomous degeneration beginning in the second week of postnatal development and becoming almost total around 30-45 days. The Lurcher mutation was recently identified as gain-of-function defect in the delta 2 glutamate receptor causing a constitutive current leak, suggesting that +/Lc Purkinje cells die by an excitotoxic mechanism. In previous studies we have shown that overexpression of bcl-2, a key regulator of cell death, in the heterozygous Lurcher mutant does not prevent +/Lc Purkinje cell death. To investigate further the mechanisms of +/Lc Purkinje cell death, we have crossed +/Lc mutants with a second line of Hu-bcl-2 transgenics (NSE73a) that shows an earlier onset of transgene expression and higher expression levels. Analysis of eight +/Lc-NSE73a mutants (4 at 2 months and 4 at 5-6 months) showed that Hu-bcl-2 overexpression delayed, but ultimately could not prevent +/Lc Purkinje cell death.

Purification and biochemical properties of soluble recombinant human Bax

Bax is a member of the Bcl-2 protein family with proapoptotic properties. The proteins of this family contain three highly conserved regions termed BH1, BH2, and BH3 as well as a hydrophobic COOH-terminal domain, which is responsible for the membrane attachment of the proteins. We have expressed human Bax truncated of the 20 amino acid COOH-terminal hydrophobic domain to obtain large amounts of soluble protein suitable for biochemical and structural studies. The truncated protein was expressed as a glutathione S-transferase (GST) fusion protein in Escherichia coli. The GST-Bax fusion protein was bound to glutathione-Sepharose, and Bax was released by thrombin cleavage and further purified by sequential chromatography on heparin-Sepharose and DEAE-Sepharose. The purified protein was present in solution as a heptamer and multimers of the heptamer complex. Limited tryptic digestion cleaved the protein in the region preceding the BH3 domain and produced a specific stable protein fragment of 15 kDa. Phosphorylation has been proposed as a possible regulatory mechanism of the bcl-2 proteins. The Bax protein was an in vitro substrate for specific serine/threonine protein kinases.

Death-signalling cascade in mouse cerebellar granule neurons

Molecular mechanisms of neuronal cell death are still largely unknown. In the present study, the signal transduction pathway of cell death in cerebellar granule neurons was examined by employing various death-preventative agents. When death was induced by the depletion of serum and a depolarizing level of potassium, transient increase in active c-Jun, mitochondrial membrane potential (deltapsi) loss, activation of caspase-3 (-like) proteases, and nuclear condensation and fragmentation were observed. The protein synthesis inhibitor cycloheximide blocked all these phenomena, whereas RNA synthesis inhibitor actinomycin-D, survival factor such as insulin-like growth factor-1, brain-derived neurotrophic factor, high K+ (25 mM) and overproduced antiapoptotic protein Bcl-2, prevented deltapsi, loss, caspase activation, and nuclear change, but not an increase in active c-Jun. The caspase inhibitor z-Asp-CH2-DCB (carbobenzoxy-L-aspartyl-alpha-[(2,6-dichlorobenzoyl) oxy]methane) only inhibited activation of caspases and nuclear change. These results suggest that the death signal in cerebellar granule neurons is sequentially transduced in the order of c-Jun activation, de novo RNA synthesis, mitochondrial deltapsi loss, activation of caspase-3 (-like) proteases and nuclear change.

Bax and Bak proteins require caspase activity to trigger apoptosis in sympathetic neurons

We show that the pro-apoptotic proteins Bax and Bak trigger apoptosis when over-expressed in sympathetic neurons cultured in the presence of NGF. This effect can be blocked with z-VAD-fmk, a peptide inhibitor of caspases, but not with anti-apoptotic chemical compounds such as antioxidants or proteasome inhibitors. These results demonstrate that in sympathetic neurons Bax and Bak are sufficient to induce apoptosis in the absence of any other apparent cell death stimulus and that their effect is mediated by caspases but does not require reactive oxygen species nor activity of the proteasome.

Postnatal expression of Hu-bcl-2 gene in Lurcher mutant mice fails to rescue Purkinje cells but protects inferior olivary neurons from target-related cell death

The Lurcher mutant has been extensively studied as a model for cell-autonomous and target-related cell death, yet there are still many unknowns concerning the mechanisms of neuronal degeneration in this mutant. As a key regulator of apoptosis, a bcl-2 transgene has been overexpressed in the heterozygous Lurcher mutant to investigate the effects of BCL-2 on two types of in vivo neuronal cell loss in Lurcher: cell-autonomous Purkinje cell degeneration and target-related olivary neuron death. Six adult +/Lc mutants expressing a human bcl-2 transgene (Hu-bcl-2) were generated by crossing +/Lc mutants with NSE71 Hu-bcl-2 transgenic mice. Analysis of these brains showed that bcl-2 overexpression did not prevent +/Lc Purkinje cell degeneration, but it did rescue most olivary neurons from target-related cell death. Although the number of olivary neurons was equivalent to wild-type numbers, the inferior olive nucleus was significantly shorter in its rostrocaudal extent, suggesting that olivary neurons are atrophied. We propose that Lurcher gene action causes Purkinje cell degeneration independently of a BCL-2-mediated pathway. Furthermore, although bcl-2 overexpression rescues olivary neurons from target-related cell death, it does not prevent the atrophy associated with the loss of target-related trophic support.

Bcl-2 undergoes phosphorylation by c-Jun N-terminal kinase/stress-activated protein kinases in the presence of the constitutively active GTP-binding protein Rac1

We have studied the phosphorylation of the Bcl-2 family of proteins by different mitogen-activated protein (MAP) kinases. Purified Bcl-2 was found to be phosphorylated by the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) p54-SAPKbeta, and this is specific insofar as the extracellular signal-regulated kinase 1 (ERK1) and p38/RK/CSBP (p38) catalyzed only weak modification. Bcl-2 undergoes similar phosphorylation in COS-7 when coexpressed together with p54-SAPKbeta and the constitutive Rac1 mutant G12V. This is seen by both 32PO4 labeling and the appearance of five discrete Bcl-2 bands with reduced gel mobility. As anticipated, both intracellular p54-SAPKbeta activation and Bcl-2 phosphorylation are blocked by co-transfection with the MAP kinase specific phosphatase MKP3/PYST1. MAP kinase specificity is also seen in COS-7 cells as Bcl-2 undergoes only weak phosphorylation when co-expressed with enzymatically activated ERK1 or p38. Four critical residues undergoing phosphorylation in COS-7 cells were identified by expression of the quadruple Bcl-2 point mutant T56A,S70A,T74A, S87A. Sequencing phosphopeptides derived from tryptic digests of Bcl-2 indicates that purified GST-p54-SAPKbeta phosphorylates identical sites in vitro. This is the first report of Bcl-2 phosphorylation by the JNK/SAPK class of MAP kinases and could indicate a key modification allowing control of Bcl-2 function by cell surface receptors, Rho family GTPases, and/or cellular stresses.

[Caenorhabditis elegans and neuronal death in mammals]

The development of the nervous system implies not only the generation of neurons, but also their death. This neuronal death can occur through several mechanisms, one of them being apoptosis. This type of cell death seems to be also implicated in some neurodegenerative diseases. This study of the nematode Caenorhabditis elegans has led to the discovery of several genes controlling apoptosis in neurons. Two of them, the pro-apoptotic ced3 and the anti-apoptotic ced9, have mammalian homologs. The mammalian homologs to Ced9 form the Bcl-2 family and can be either pro-apoptotic or anti-apoptotic. Some of them, Bcl-x, and Bax have been shown to be involved in neuronal death during development in some pathological situations. The first mammalian homolog of Ced3 to be described was the Interleukin-1b Converting Enzymes (ICE). Since then, many other homologs of the proteases Ced3 and ICE have been discovered constituting the Caspases family. These Cysteinyl Aspartate Specific Proteases are pro-apoptotic in many different systems. Several studies using viral or peptidic inhibitors of the Caspases have demonstrated their role in neuronal death in vitro. In vivo, CPP32, a member of the Caspases family, has been shown to be clearly involved in the development of the nervous system. Finally, the analysis of apoptosis in Caenorhabditis elegans has led to the discovery of two families of genes involved in the cascade of events inducing neuronal death in mammals. Indeed, the Caspases seem to be controlled by the Bcl-2 family, as Ced3 is by Ced9.

Fear decrease in transgenic mice overexpressing bcl-2 in neurons

Neuronal destruction in the amygdala, hypothalamus and cerebellum provokes a diminution in anxiety and neophobia. In transgenic mice that express the human bcl-2 gene under the control of neuron specific enolase promotor (Hu-bcl-2), BCL-2 overexpression reduces the naturally occurring neuronal death, producing an increase of the number of neurons and brain size. Since BCL-2 over-expression has been observed in different parts of the brain and especially in the amygdaloid nuclei, the hypothalamus and the cerebellum, we studied the fear-related behavior of these transgenic mice. Hu-bcl-2 transgenic mice showed a decrease in anxiety and neophobia, indicating that, for this particular behavior, supernumerary neurons elicit the same modification as that observed after neuronal destruction.

Inhibition of Bax channel-forming activity by Bcl-2

Proteins of the Bcl-2 family are intracellular membrane-associated proteins that regulate programmed cell death (apoptosis) either positively or negatively by as yet unknown mechanisms. Bax, a pro-apoptotic member of the Bcl-2 family, was shown to form channels in lipid membranes. Bax triggered the release of liposome-encapsulated carboxyfluorescein at both neutral and acidic pH. At physiological pH, release could be blocked by Bcl-2. Bcl-2, in contrast, triggered carboxyfluorescein release at acidic pH only. In planar lipid bilayers, Bax formed pH- and voltage-dependent ion-conducting channels. Thus, the pro-apoptotic effects of Bax may be elicited through an intrinsic pore-forming activity that can be antagonized by Bcl-2.

Increased inferior olivary neuron and cerebellar granule cell numbers in transgenic mice overexpressing the human Bcl-2 gene

Neuron-target interactions during development are critical for determining the final numbers of neurons in the nervous system. To investigate the role of Purkinje cells and programmed cell death in the regulation of afferent neuron numbers, we have counted olivary neurons and granule cells in two lines of transgenic mice (NSE73a and NSE71) that overexpress a human gene for bcl-2 (Hu-bcl-2) in Purkinje cells and olivary neurons, but not in granule cells. Bcl-2 overexpression in vivo reduces naturally occurring neuronal cell death and cell death following axotomy, target removal, or ischemia. Olivary neuron numbers in NSE73a and NSE71 transgenic mice are significantly increased compared to controls by 28% and 27%, respectively, while granule cell numbers are only increased in NSE73a mice (29% above controls). We have previously shown that Purkinje cell number is increased by 43% in NSE73a transgenics and by 23% in NSE71 transgenics. The ratio of Purkinje cells to olivary neurons is not significantly different between the control and transgenic mice, while the ratio of granule cells to Purkinje cells is significantly decreased in the NSE71 transgenic mice compared to controls and NSE73a transgenics. The increased numbers of olivary neurons suggest that bcl-2 overexpression rescues these neurons from programmed cell death. The increase in granule cell number in only one transgenic line is discussed with respect to hypotheses that Purkinje cells regulate both granule cell progenitor proliferation and the survival of differentiated granule cells.

Mouse vaginal opening is an apoptosis-dependent process which can be prevented by the overexpression of Bcl2

In the mouse, opening of the vaginal cavity to the skin is a late event, occurring around the fifth week of life; it can be induced in sexually immature mice by beta-estradiol injections. We have generated two lines of transgenic mice expressing the human Bcl2 protein in a variety of tissues. The vaginal cavity of the transgenic females remained permanently closed, a condition completely resistant to beta-estradiol injections; this was accompanied by a considerable distension of the genital tract. Histologic studies of vaginal sections at the time of opening to the skin in normal mice showed, by the TUNEL method which detects nuclei with fragmented DNA characteristic of apoptosis, that this event coincides with extensive apoptosis in the lower part of the vaginal mucosa, a process prevented in the bcl2 transgenic mice, which express Bcl2 in suprabasal epithelial cells and in subepithelial cells of the vaginal mucosa. In contrast, two lines of mice bearing a Bcl2 transgene placed under the control of a K10 keratin promoter, whose expression is restricted to the suprabasal layers of the epidermis, had a normal phenotype. Eyelids' formation and opening of the external ear canals, which also occur after birth in the mouse, were not altered in any of these transgenic lines; histological study of eye and ear sections at the time of these events failed to detect apoptosis. In conclusion, the tissue remodeling required to complete maturation of the mouse female genital tract at the time of puberty is an hormonally triggered apoptosis-dependent process.

Susceptibility of cerebellar granule neurons derived from Bcl-2-deficient and transgenic mice to cell death

Overproduced Bcl-2 oncoprotein has been shown to suppress cell death induced by a variety of stimuli in many cell types, including neuronal cells. Because bcl-2 is expressed in the nervous system where massive cell death is observed during development, endogenous Bcl-2 is likely to be involved in regulating neuronal cell death. Here we examined the possible role of endogenous Bcl-2 in the regulation of neuronal cell survival in the central nervous system using primary cultured cerebellar granule neurons from bcl-2-deficient, wild-type and NSE-bcl-2-transgenic mice. Cerebellar granule neurons from bcl-2-deficient mice were more susceptible than those from normal littermates to death induced by reducing the K+ concentration of the medium from high (25 mM) to low (5 mM), and neurons from bcl-2-transgenic mice were least susceptible. Similar results were obtained when cell death was induced by serum withdrawal under high K+ conditions or by the presence of etoposide, A23187 or nimodipine. Consistently, bcl-2 deficiency reduced the number of cerebellar granule neurons per mouse. These results indicate that Bcl-2 impedes neuronal cell death induced by various stimuli in a dose-dependent manner, and that endogenous levels of Bcl-2 are able to regulate neuronal cell survival in the central nervous system.

Bcl-2 promotes regeneration of severed axons in mammalian CNS

Most neurons of the mammalian central nervous system (CNS) lose the ability to regenerate severed axons in vivo after a certain point in development. At least part of this loss in regenerative potential is intrinsic to neurons. Although embryonic retinal ganglion cells (RGCs) can grow axons into tectum of any age, most RGCs from older animals fail to extend axons into CNS tissue derived from donors of any age, including the embryonic tectum. Here we report that the proto-oncogene bcl-2 plays a key role in this developmental change by promoting the growth and regeneration of retinal axons. This effect does not seem to be an indirect consequence of its well-known anti-apoptotic activity. Another anti-apoptotic drug, ZVAD, supported neuronal survival but did not promote axon regeneration in culture. This finding could lead to new strategies for the treatment of injuries to the CNS.

Anti-apoptotic effect of Bcl-2 overexpression in RIN cells infected with Semliki Forest virus

RIN cells were infected with recombinant Semliki Forest virus (SFV) particles containing the LacZ gene. X-gal staining showed 100% infectivity of the cell cultures and high-level expression of bacterial beta-galactosidase in these cells. The cytopathogenic effects of the SFV infection were studied by measuring the viability of the RIN cells. Comparisons between control RIN cells and Bcl-2 overexpressing RIN cells were done 72 h post-infection. Significant differences in viability levels were observed. The control RIN cells showed in the MTT assay a mean value of 0.156+/-0.017 compared to 0.347+/-0.057 for the RIN/Bcl-2 cells. FACS analysis of cells labelled with propidium iodide indicated that only an average of 4.5+/-0.5% of the control cells were viable 72 h post-infection, while 44.5+/-3.5% of the RIN/Bcl-2 cells were still alive. Thus, the Bcl-2 overexpression clearly protected the SFV-infected cells from undergoing apoptosis.

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.

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.

Expression of the CTL-associated protein TIA-1 during murine embryogenesis

TIA-1 is a T cell-associated protein that binds poly(A) in vitro and induces apoptosis in permeabilized thymocytes. It may be involved in the induction of apoptosis in target cells during lymphocyte attack. To elucidate the role of TIA-1 in mammalian development, a cDNA-encoding mouse TIA-1 was cloned. The predicted mouse TIA-1 protein contains three RNA binding domains at the amino terminus and a putative lysosomal targeting sequence at the carboxyl terminus. The mouse sequence shows 96% overall identity with the human TIA-1 homologue. During murine embryogenesis, abundant mouse TIA-1 mRNA is detectable from 12.5 days of development onward in the brain and the retina, where it is selectively expressed within neuronal cells. Transcripts are also found in the lung, kidney, and thymus. TIA-1 in the adult mouse is expressed mainly in T cells and NK cells. The expression of TIA-1 during mouse embryogenesis is endogenous to tissues in which apoptotic cell death occurs. The conservation of this RNA-binding protein throughout evolution implicates its importance in embryogenesis, and in particular neuronal development.