Bcl-xL overexpression blocks bax-mediated mitochondrial contact site formation and apoptosis in rod photoreceptors of lead-exposed mice

Bcl-XL inhibits membrane permeabilization by competing with Bax

Although Bcl-XL and Bax are structurally similar, activated Bax forms large oligomers that permeabilize the outer mitochondrial membrane, thereby committing cells to apoptosis, whereas Bcl-XL inhibits this process. Two different models of Bcl-XL function have been proposed. In one, Bcl-XL binds to an activator, thereby preventing Bax activation. In the other, Bcl-XL binds directly to activated Bax. It has been difficult to sort out which interaction is important in cells, as all three proteins are present simultaneously. We examined the mechanism of Bax activation by tBid and its inhibition by Bcl-XL using full-length recombinant proteins and measuring permeabilization of liposomes and mitochondria in vitro. Our results demonstrate that Bcl-XL and Bax are functionally similar. Neither protein bound to membranes alone. However, the addition of tBid recruited molar excesses of either protein to membranes, indicating that tBid activates both pro- and antiapoptotic members of the Bcl-2 family. Bcl-XL competes with Bax for the activation of soluble, monomeric Bax through interaction with membranes, tBid, or t-Bid-activated Bax, thereby inhibiting Bax binding to membranes, oligomerization, and membrane permeabilization. Experiments in which individual interactions were abolished by mutagenesis indicate that both Bcl-XL–tBid and Bcl-XL–Bax binding contribute to the antiapoptotic function of Bcl-XL. By out-competing Bax for the interactions leading to membrane permeabilization, Bcl-XL ties up both tBid and Bax in nonproductive interactions and inhibits Bax binding to membranes. We propose that because Bcl-XL does not oligomerize it functions like a dominant-negative Bax in the membrane permeabilization process.

During development and under stress, cells can become committed to die via programmed cell death (apoptosis). In most cases, the permeabilization of the outer mitochondrial membrane is a key component of this commitment. The membrane permeablization step is both positively and negatively regulated by members of the Bcl-2 family of proteins. One member of this protein family with only a BH3 region, such as tBid, activates another family member, Bax, causing it to form large complexes that generate membrane-spanning pores, hence making the membrane permeable. Antiapoptotic members of the Bcl-2 family, such as Bcl-XL, are structurally similar to Bax but inhibit the membrane permeabilization process by an unknown mechanism. Two mutually exclusive models have been proposed to explain how the Bcl-2 family is operating: one states that Bcl-XL binds to tBid, thereby preventing Bax activation, while the second suggests that Bcl-XL binds directly to activated Bax. It has been difficult to sort out which interaction is important in cells, where multiple members of all three protein families are present simultaneously. Here, we describe an in vitro system containing the three recombinant proteins and the use of mutagenesis to selectively remove individual interactions. We show that Bcl-XL inhibits Bax by competing with it for binding to membranes, tBid, and activated Bax. Because Bcl-XL does not form pores, it inhibits apoptosis by acting as if it is a dominant-negative version of Bax.

Bcl-XL and Bax are structurally similar members of the Bcl-2 family of cell-death-related proteins, and they compete for binding to membranes, as well as to Bcl-2 family member tBid and activated Bax. Unlike Bax, Bcl-XL is unable to oligomerize and form pores in membranes, so it inhibits membrane permeabilization--a key step during commitment to apoptosis--by functioning like a dominant-negative Bax.

Low-level human equivalent gestational lead exposure produces supernormal scotopic electroretinograms, increased retinal neurogenesis, and decreased retinal dopamine utilization in rats

BACKGROUND

Postnatal lead exposure in children and animals produces alterations in the visual system primarily characterized by decreases in the rod-mediated (scotopic) electroretinogram (ERG) amplitude (subnormality). In contrast, low-level gestational Pb exposure (GLE) increases the amplitude of scotopic ERGs in children (supernormality).

OBJECTIVES

The goal of this study was to establish a rat model of human equivalent GLE and to determine dose-response effects on scotopic ERGs and on retinal morphology, biochemistry, and dopamine metabolism in adult offspring.

METHODS

We exposed female Long-Evans hooded rats to water containing 0, 27 (low), 55 (moderate), or 109 (high) ppm of Pb beginning 2 weeks before mating, throughout gestation, and until postnatal day (PND) 10. We measured maternal and litter indices, blood Pb concentrations (BPb), retinal Pb concentrations, zinc concentrations, and body weights. On PND90, we performed the retinal experiments.

RESULTS

Peak BPb concentrations were < 1, 12, 24, and 46 microg/dL in control, low-, moderate- and high-level GLE groups, respectively, at PNDs 0-10. ERG supernormality and an increased rod photoreceptor and rod bipolar cell neurogenesis occurred with low- and moderate-level GLE. In contrast, high-level GLE produced ERG subnormality, rod cell loss, and decreased retinal Zn levels. GLE produced dose-dependent decreases in dopamine and its utilization.

CONCLUSIONS

Low- and moderate-level GLE produced persistent scotopic ERG supernormality due to an increased neurogenesis of cells in the rod signaling pathway and/or decreased dopamine utilization, whereas high-level GLE produced rod-selective toxicity characterized by ERG subnormality. The ERG is a differential and noninvasive biomarker of GLE. The inverted U-shaped dose-response curves reveal the sensitivity and vulnerability of the developing retina to GLE.

Low-level human equivalent gestational lead exposure produces sex-specific motor and coordination abnormalities and late-onset obesity in year-old mice

BACKGROUND

Low-level developmental lead exposure is linked to cognitive and neurological disorders in children. However, the long-term effects of gestational lead exposure (GLE) have received little attention.

OBJECTIVES

Our goals were to establish a murine model of human equivalent GLE and to determine dose-response effects on body weight, motor functions, and dopamine neurochemistry in year-old offspring.

METHODS

We exposed female C57BL/6 mice to water containing 0, 27 (low), 55 (moderate), or 109 ppm (high) of lead from 2 weeks prior to mating, throughout gestation, and until postnatal day 10 (PN10). Maternal and litter measures, blood lead concentrations ([BPb]), and body weights were obtained throughout the experiment. Locomotor behavior in the absence and presence of amphetamine, running wheel activity, rotarod test, and dopamine utilization were examined in year-old mice.

RESULTS

Peak [BPb] were < 1, < or = 10, 24-27, and 33-42 microg/dL in control, low-, moderate- and high-dose GLE groups at PN0-10, respectively. Year-old male but not female GLE mice exhibited late-onset obesity. Similarly, we observed male-specific decreased spontaneous motor activity, increased amphetamine-induced motor activity, and decreased rotarod performance in year-old GLE mice. Levels of dopamine and its major metabolite were altered in year-old male mice, although only forebrain utilization increased. GLE-induced alterations were consistently larger in low-dose GLE mice.

CONCLUSIONS

Our novel results show that GLE produced permanent male-specific deficits. The nonmonotonic dose-dependent responses showed that low-level GLE produced the most adverse effects. These data reinforce the idea that lifetime measures of dose-response toxicant exposure should be a component of the neurotoxic risk assessment process.

Metals and apoptosis: recent developments

Apoptosis, also known as programmed cell death is a highly regulated and crucial process found in all multicellular organisms. It is not only implicated in regulatory mechanisms of cells, but has been attributed to a number of diseases, i.e. inflammation, malignancy, autoimmunity and neurodegeneration. A variety of toxins can induce apoptosis. Carcinogenic transition metals, viz. cadmium, chromium and nickel promote apoptosis along with DNA base modifications, strand breaks and rearrangements. Generation of reactive oxygen species, accumulation of Ca(2+), upregulation of caspase-3, down regulation of bcl-2, and deficiency of p-53 lead to arsenic-induced apoptosis. In the case of cadmium, metallothionein expression determines the choice between apoptosis and necrosis. Reactive oxygen species (ROS) and p53 contribute in apoptosis caused by chromium. Immuno suppressive mechanisms contribute in lead-induced apoptosis whereas in the case of mercury, p38 mediated caspase activation regulate apoptosis. Nickel kills the cells by apoptotic pathways. Copper induces apoptosis by p53 dependent and independent pathways. Beryllium stimulates the formation of ROS that play a role in Be-induced macrophage apoptosis. Selenium induces apoptosis by producing superoxide that activates p53. Thus, disorders of apoptosis may play a critical role in some of the most debilitating metal-induced afflictions including hepatotoxicity, renal toxicity, neurotoxicity, autoimmunity and carcinogenesis. An understanding of metal-induced apoptosis will be helpful in the development of preventive molecular strategies.

Spatiotemporal regulation of ATP and Ca2+ dynamics in vertebrate rod and cone ribbon synapses

PURPOSE

In conventional neurons, Ca2+ enters presynaptic terminals during an action potential and its increased local concentration triggers transient exocytosis. In contrast, vertebrate photoreceptors are nonspiking neurons that maintain sustained depolarization and neurotransmitter release from ribbon synapses in darkness and produce light-dependent graded hyperpolarizing responses. Rods transmit single photon responses with high fidelity, whereas cones are less sensitive and exhibit faster response kinetics. These differences are likely due to variations in presynaptic Ca2+ dynamics. Metabolic coupling and cross-talk between mitochondria, endoplasmic reticulum (ER), plasma membrane Ca2+ ATPase (PMCA), and Na+-Ca2+ exchanger (NCX) coordinately control presynaptic ATP production and Ca2+ dynamics. The goal of our structural and functional studies was to determine the spatiotemporal regulation of ATP and Ca2+ dynamics in rod spherules and cone pedicles.

METHODS

Central retina tissue from C57BL/6 mice was used. Laser scanning confocal microscopy (LSCM) experiments were conducted on fixed-frozen vertical sections. Primary antibodies were selected for their tissue/cellular specificity and ability to recognize single, multiple or all splice variants of selected isoforms. Electron microscopy (EM) and 3-D electron tomography (ET) studies used our standard procedures on thin- and thick-sectioned retinas, respectively. Calibrated fluo-3-Ca2+ imaging experiments of dark- and light-adapted rod and cone terminals in retinal slices were conducted.

RESULTS

Confocal microscopy showed that mitochondria, ER, PMCA, and NCX1 exhibited distinct retinal lamination patterns and differential distribution in photoreceptor synapses. Antibodies for three distinct mitochondrial compartments differentially labeled retinal areas with high metabolic demand: rod and cone inner segments, previously undescribed cone juxtanuclear mitochondria and the two plexiform layers. Rod spherule membranes uniformly and intensely stained for PMCA, whereas the larger cone pedicles preferentially stained for NCX1 at their active zones and PMCA near their mitochondria. EM and ET revealed that mitochondria in rod spherules and cone pedicles differed markedly in their number, location, size, volume, and total cristae surface area, and cristae junction diameter. Rod spherules had one large ovoid mitochondrion located near its active zone, whereas cone pedicles averaged five medium-sized mitochondria clustered far from their active zones. Most spherules had one ribbon synapse, whereas pedicles contained numerous ribbon synapses. Fluo-3 imaging studies revealed that during darkness rod spherules maintained a lower [Ca2+] than cone pedicles, whereas during light adaptation pedicles rapidly lowered their [Ca2+] below that observed in spherules.

CONCLUSIONS

These findings indicate that ATP demand and mitochondrial ATP production are greater in cone pedicles than rod spherules. Rod spherules employ high affinity/low turnover PMCA and their mitochondrion to maintain a relatively low [Ca2+] in darkness, which increases their sensitivity and signal-to-noise ratio. In contrast, cone pedicles utilize low affinity/high turnover NCX to rapidly lower their high [Ca2+] during light adaptation, which increases their response kinetics. Spatiotemporal fluo-3-Ca2+ imaging results support our immunocytochemical results. The clustering of cone pedicle mitochondria likely provides increased protection from Ca2+ overload and permeability transition. In summary, these novel studies reveal that several integrated cellular and subcellular components interact to regulate ATP and Ca2+ dynamics in rod and cone synaptic terminals. These results should provide a greater understanding of in vivo photoreceptor synaptic terminal exocytosis/endocytosis, Ca2+ overload and therapies for retinal degenerations.

Reversal of lead-induced neuronal apoptosis by chelation treatment in rats: role of reactive oxygen species and intracellular Ca(2+)

Lead, a ubiquitous and potent neurotoxicant causes several neurophysiological and behavioral alterations. Toxic properties of lead have been attributed to its capability to mimic calcium and alter calcium homeostasis. In this study, we have addressed the following issues: 1) whether chelation therapy could circumvent the altered Ca(2+) homeostasis and prevent neuronal death in chronic lead-intoxicated rats, 2) whether chelation therapy could revert altered biochemical and behavioral changes, 3) whether combinational therapy using two different chelating agents was more advantageous over monotherapy in lead-treated rats, and 4) what could be the mechanism of neuronal apoptosis. Results indicated that lead caused a significant increase in reactive oxygen species, neuronal nitric-oxide synthetase, and intracellular free calcium levels along with altered behavioral abnormalities in locomotor activity, exploratory behavior, learning, and memory that were supported by changes in neurotransmitter levels. A fall in membrane potential, release of cytochrome c, and altered bcl(2)/bax ratio indicated mitochondrial-dependent apoptosis. Most of these alterations reverted toward normal level following combination therapy over monotherapy with calcium disodium EDTA (CaNa(2)EDTA) or monoisoamyl meso-2,3-dimercaptosuccinic acid (MiADMSA). It could be concluded from our present results that combined therapy with CaNa(2)EDTA and MiADMSA might be a better treatment protocol than monotherapy with these chelators in lead-induced neurological disorders. We for the first time report the role of Ca(2+) in regulating neurological dystrophy caused by chronic lead exposure in rats and its recovery with a two-course treatment regime of mono or combination therapy.

Pancortin-2 interacts with WAVE1 and Bcl-xL in a mitochondria-associated protein complex that mediates ischemic neuronal death

The actin-modulating protein Wiskott-Aldrich syndrome protein verprolin homologous-1 (WAVE1) and a novel CNS-specific protein, pancortin, are highly enriched in adult cerebral cortex, but their functions are unknown. Here we show that WAVE1 and pancortin-2 interact in a novel cell death cascade in adult, but not embryonic, cerebral cortical neurons. Focal ischemic stroke induces the formation of a protein complex that includes pancortin-2, WAVE1, and the anti-apoptotic protein Bcl-xL. The three-protein complex is associated with mitochondria resulting in increased association of Bax with mitochondria, cytochrome c release, and neuronal apoptosis. In pancortin null mice generated using a Cre-loxP system, ischemia-induced WAVE1-Bcl-xL interaction is diminished, and cortical neurons in these mice are protected against ischemic injury. Thus, pancortin-2 is a mediator of ischemia-induced apoptosis of neurons in the adult cerebral cortex and functions in a novel mitochondrial/actin-associated protein complex that sequesters Bcl-xL.

Comparison of lymphoblast mitochondria from normal subjects and patients with Barth syndrome using electron microscopic tomography

Barth syndrome (BTHS) is a mitochondrial disorder that is caused by mutations in the tafazzin gene, which affects phospholipid composition. To determine whether this defect leads to alterations in the internal three-dimensional organization of mitochondrial membranes, we applied electron microscopic tomography to lymphoblast mitochondria from BTHS patients and controls. Tomograms were formed from 50 and 150 nm sections of chemically fixed lymphoblasts and the data were used to manually segment volumes of relevant structural details. Normal lymphoblast mitochondria contained well-aligned, lamellar cristae with slot-like junctions to the inner boundary membrane. In BTHS, mitochondrial size was more variable and the total mitochondrial volume per cell increased mainly due to clusters of fragmented mitochondria inside nuclear invaginations. However, mitochondria showed reduced cristae density, less cristae alignment, and inhomogeneous cristae distribution. Three-dimensional reconstruction of BTHS mitochondria revealed zones of adhesion of the opposing inner membranes, causing obliteration of the intracrista space. We found small isolated patches of adhesion as well as extended adhesion zones, resulting in sheets of collapsed cristae packaged in multiple concentric layers. We also found large tubular structures (diameter 30-150 nm) that appeared to be derivatives of the adhesion zones. The data suggest that mitochondrial abnormalities of BTHS involve adhesions of inner mitochondrial membranes with subsequent collapse of the intracristae space.

Bcl-2 protein family: implications in vascular apoptosis and atherosclerosis

Apoptosis has been recognized as a central component in the pathogenesis of atherosclerosis, in addition to the other human pathologies such as cancer and diabetes. The pathophysiology of atherosclerosis is complex, involving both apoptosis and proliferation at different phases of its progression. Oxidative modification of lipids and inflammation differentially regulate the apoptotic and proliferative responses of vascular cells during progression of the atherosclerotic lesion. Bcl-2 proteins act as the major regulators of extrinsic and intrinsic apoptosis signalling pathways and more recently it has become evident that they mediate the apoptotic response of vascular cells in response to oxidation and inflammation either in a provocative or an inhibitory mode of action. Here we address Bcl-2 proteins as major therapeutic targets for the treatment of atherosclerosis and underscore the need for the novel preventive and therapeutic interventions against atherosclerosis, which should be designed in the light of molecular mechanisms regulating apoptosis of vascular cells in atherosclerotic lesions.

Electron tomography of membrane-bound cellular organelles

Electron microscope tomography produces three-dimensional reconstructions and has been used to image organelles both isolated and in situ, providing new insight into their structure and function. It is analogous to the various tomographies used in medical imaging. Compared with light microscopy, electron tomography offers an improvement in resolution of 30- to 80-fold and currently ranges from 3 to 8 nm, thus filling the gap between high-resolution structure determinations of isolated macromolecules and larger-scale studies on cells and tissues by light microscopy. Here, we provide an introduction to electron tomography and applications of the method in characterizing organelle architecture that also show its power for suggesting functional significance. Further improvements in labeling modalities, imaging tools, specimen preparation, and reconstruction algorithms promise to increase the quality and breadth of reconstructions by electron tomography and eventually to allow the mapping of the cellular proteomes onto detailed three-dimensional models of cellular structure.

Lead-induced apoptosis in PC 12 cells: involvement of p53, Bcl-2 family and caspase-3

It has been reported that lead could induce apoptosis in a variety of cell types. Although mitochondrion is regarded as the most pertinent pathway in mediating apoptosis, the exact mechanisms of lead-induced apoptosis are still largely unknown. Furthermore, there is little information about expressions and regulations of Bax, Bcl-2, and p53 in lead-induced apoptosis, which are critical regulators of mitochondrial stability. The present study was undertaken to determine whether lead could induce DNA damage and apoptosis in PC 12 cells, and the involvement of Bax, Bcl-2, p53, and caspase-3 in this process. The results showed that lead could induce DNA damage and apoptosis in PC 12 cells, accompanying with upregulation of Bax and downregulation of Bcl-2. Additionally, the expression of p53 increased, and caspase-3 was activated. Therefore, it suggests that lead can induce activation of p53 by DNA damage, which may lead to imbalance of Bax/Bcl-2 and mitochondrial dysfunction. Subsequently, after activation of caspase-3, lead-induced apoptosis occurres.

Regulation of Bcl-2 proteins and of the permeability of the outer mitochondrial membrane

In many apoptotic responses, pro-apoptotic members of the Bcl-2 family trigger the permeabilization of the outer mitochondrial membrane, thereby allowing the release of mitochondrial apoptogenic factors that contribute to caspase activation in the cytosol. The mechanisms that lead to the activation of pro-apoptotic Bcl-2 family members and to the permeabilization of the outer mitochondrial membrane are not yet completely understood. Here, we attempt to summarize our current view of the mechanisms that lead to these events, regarding both additional proteins that were recently suggested to be involved, and the roles of lipids.

Newcomers in the process of mitochondrial permeabilization

Under stress conditions, apoptogenic factors normally sequestered in the mitochondrial intermembrane space are released into the cytosol, caspases are activated and cells die by apoptosis. Although the precise mechanism that leads to the permeabilization of mitochondria is still unclear, the activation of multidomain pro-apoptotic proteins of the Bcl-2 family, such as Bax and Bak, is evidently crucial. Regulation of Bax and Bak by other members of the family has been known for a long time, but recent evidence suggests that additional unrelated proteins participate in the process, both as inhibitors and activators. The important rearrangements mitochondrial lipids undergo during apoptosis play a role in the permeabilization process and this role is probably more central than first envisioned.

Bcl-xL gene transfer inhibits Bax translocation and prolongs cardiac cold preservation time in rats

BACKGROUND

Apoptosis is an important cause of early graft loss after heart transplantation. Bcl-xL was reported to protect the heart against normothermic ischemia and reperfusion injury. In this study, we determined whether overexpression of Bcl-xL could inhibit tissue injury resulting from prolonged cold preservation followed by warm reperfusion of heart transplants.

METHODS AND RESULTS

Lewis rat hearts were transduced with an adenovirus vector harboring Bcl-xL cDNA (AxCAhBclxL) 4 days before collection of tissue. After preservation in University of Wisconsin solution at 4 degrees C for 24 hours, the heart was either perfused with a Langendorff device ex vivo or used for heterotopic heart transplantation in vivo. Bcl-xL gene transfer significantly reduced the infarct size (23.0+/-2.6% versus 47.7+/-7.0% in saline control and 48.6+/-6.1% in vector control, P<0.01) after 2-hour reperfusion at 37 degrees C with the Langendorff device and significantly decreased creatine kinase release (0.82+/-0.27 IU, versus 1.57+/-0.33 and 1.50+/-0.37 IU in saline and vector controls, respectively; P<0.05). In heart transplantation, overexpression of Bcl-xL inhibited Bax translocation from the cytosol to the mitochondria, resulting in decreased cytochrome c release from the mitochondria; it also significantly decreased cardiac cell apoptosis and improved graft survival rate after long cold preservation, followed by warm reperfusion.

CONCLUSIONS

Bcl-xL gene transfer inhibited the translocation of Bax and prolonged the cold preservation time of cardiac transplants. This may be a potential therapeutic method in clinical practice.

Activation of calcium-independent phospholipase A2 (iPLA2) in brain mitochondria and release of apoptogenic factors by BAX and truncated BID

Cleaved or truncated BID (tBID) is known to oligomerize both BAK and BAX. Previously, BAK and BAX lacing the C-terminal fragment (BAXDeltaC) were shown to induce modest cytochrome c (Cyt c) release from rat brain mitochondria when activated by tBID. We now show that tBID plus monomeric full-length BAX induce extensive release of Cyt c, Smac/DIABLO, and Omi/HtrA2 (but not endonuclease G and the apoptosis inducing factor) comparable to the release induced by alamethicin. This occurs independently of the permeability transition without overt changes in mitochondrial morphology. The mechanism of the release may involve formation of reactive oxygen species (ROS) and activation of calcium-independent phospholipase A(2) (iPLA(2)). Indeed, increased ROS production and activated iPLA(2) were observed prior to massive Cyt c release. Furthermore, the extent of inhibition of Cyt c release correlated with the degree of suppression of iPLA(2) by the inhibitors propranolol, dibucaine, 4-bromophenacyl bromide, and bromenol lactone. Consistent with a requirement for iPLA(2) in Cyt c release from brain mitochondria, synthetic liposomes composed of lipids mimicking the outer mitochondrial membrane (OMM) but lacing iPLA(2) failed to release 10 kDa fluorescent dextran (FD-10) in response to tBID plus BAX. We propose that tBID plus BAX activate ROS generation, which subsequently augments iPLA(2) activity leading to changes in the OMM that allow translocation of certain mitochondrial proteins from the intermembrane space.

Amelioration of lead toxicity on rat liver with Vitamin C and silymarin supplements

The aim of the present study was to investigate the impact of the combined administration of Vitamin C and silymarin on lead toxicity. Male albino rats were subdivided into three groups: the first was a control group, the second received lead acetate in diet as 500 mg/kg diet daily, the third received the same lead acetate dose and supplemented with Vitamin C (1 mg/100g body weight) and silymarin (1 mg/100g body weight) by gastric tube three times per week. Blood samples were taken after 2, 4 and 6 weeks of treatment. Significant lead-induced elevations in serum ALT, AST, GGT and ALP activities were observed after different periods of treatment. However, serum LDLc was decreased. The intensities of RNA and apoptotic fragments of DNA were measured as optical density by Gel-pro program. Lead acetate decreased the intensity of DNA at 6 weeks and induced apoptotic DNA fragments reversibly with time. After 2 weeks of lead administration dilation and congestion of terminal hepatic veins and portal vein branches were observed. Lead also induced hepatocyte proliferation without any localized distribution among zones 1-3. Portal inflammatory infiltrate with disruption of the limiting plates (interface hepatitis), steatosis, apoptosis and mild fibrosis were detected especially by sixth week of lead administration. Combined treatment of lead-exposed animals with Vitamin C and silymarin showed marked improvement of the biochemical, molecular and histopathological findings. These experimental results strongly indicate the protective effect of Vitamin C and silymarin against toxic effects of lead on liver tissue.

Cell membranes and apoptosis: role of cardiolipin, phosphatidylcholine, and anticancer lipid analogues

The apoptotic program utilizes cellular membranes to transduce and generate operative signals. Lipids are major components of cellular membranes and have the potential to control the effectiveness of the signal by directing it to the proper location, being a source of new signals or as mediators in the response. These possible lipid functions are illustrated in the present review, focussing on the role that two different phospholipids, cardiolipin and phosphatidyl choline, play in apoptosis. Mitochondria have a central role in apoptosis, and many important aspects of the process mediated by this organelle converge through its distinctive lipid cardiolipin. Specifically, changes in cardiolipin metabolism have been detected in early steps of the death program and it is postulated (i) to mediate recruitment of pro apoptotic proteins like Bid to the mitochondria surface and (ii) to actively participate in the release of proteins relevant for the execution phase of apoptosis, like cytochrome c. Unlike the organelle specific distribution of cardiolipin, phosphatidylcholine is widely distributed among all organelles of the cell. The importance of phosphatidylcholine in apoptosis has been approached mainly through the study of the mode of action of (i) phosphatidylcholine anticancer analogues such as edelfosine and (ii) molecules that alter phosphatidylcholine metabolism, such as farnesol. The contribution of phosphatidylcholine metabolism to the apoptotic program is discussed, analyzing the experimental evidence available and pointing out some controversies in the proposed mechanisms of action.

Deficiency of Bax and Bak protects photoreceptors from light damage in vivo

Photoreceptors of bax(-/-)bak(-/-) but neither bax(-/-) mice nor bak(-/-) mice are protected from developmental apoptosis, suggesting that bax(-/-)bak(-/-) photoreceptors may also be protected from pathologic apoptosis. To test this possibility, we exposed bax(-/-)bak(-/-) and bax(-/-) mice to bright light, which normally induces photoreceptor death. Photoreceptors in bax(-/-)bak(-/-) mice were protected from death compared to bax(-/-) mice as indicated by a reduction in the number of TUNEL-positive photoreceptor nuclei 24 h following light damage and almost complete preservation of photoreceptors 7 days following light damage. These results provide the first in vivo evidence that combined deficiency of Bax and Bak can rescue cells from a pathologic stimulus more effectively than Bax deficiency and suggest that combined deficiency of Bax and Bak may also protect cells from other insults.

Bax does not directly participate in the Ca(2+)-induced permeability transition of isolated mitochondria

The mitochondrial permeability transition pore and Bax have both been proposed to be involved in the release of pro-apoptotic factors from mitochondria in the "intrinsic" pathway of apoptosis. The permeability transition pore is widely thought to be a supramolecular complex including or interacting with Bax. Given the relevance of the permeability transition in vivo, we have verified whether Bax influences the formation and/or the properties of the Ca(2+)/P(i)-induced permeability transition by using mitochondriaisolated from isogenic human colon cancer bax(+/-) and bax(-/-) HCT116 cell lines. We used mitochondria isolated from both types of cells and from Bax(+) cells exposed to apoptotic stimuli, as well as Bax-less mitochondria into which exogenous Bax had been incorporated. All exhibited the same behavior and pharmacological profile in swelling and Ca(2+)-retention experiments. Mitochondria from a bax(-)/bak(-) cell line also underwent an analogous Ca(2+)/P(i)-inducible swelling. This similarity indicates that Bax hasno major role in regulating the Ca(2+)-induced mitochondrial permeability transition.

Translation initiation factor 4E blocks endoplasmic reticulum-mediated apoptosis

Eukaryotic translation initiation factor 4E (eIF4E) is the mRNA cap-binding protein required for translation of cellular mRNAs utilizing the 5' cap structure. The rate-limiting factor for mRNA recruitment to ribosomes, eIF4E is a major target for regulation of translation by growth factors, hormones, and other extracellular stimuli. When overexpressed, eIF4E exerts profound effects on cell growth and survival, leading to suppression of oncogene-dependent apoptosis, causing malignant transformation and conferring tumors with multiple drug resistance. We found previously that overexpressed eIF4E interdicts the apoptotic pathway induced by growth factor withdrawal and cytotoxic drugs by selectively activating the expression of Bcl-X(L), thus preventing mitochondrial release of cytochrome c. In this study, we examined the impact of ectopic eIF4E expression on apoptosis mediated by the endoplasmic reticulum (ER). Here we show that eIF4E rescued cells from the ER stressors brefeldin A, tunicamycin, thapsigargin, and the Ca(2+) ionophore A23187. In addition, we found that cells rescued from Ca(2+) ionophore-triggered apoptosis did not release calcium from their ER nor did they translocate caspase-12 from the ER to the cytoplasm. These data lend strong support to the concept that eIF4E functions as a pleiotropic regulator of cell viability and that integration of critical organelle-mediated checkpoints for apoptosis can be controlled by the cap-dependent translation apparatus.

The intra-mitochondrial cytochrome c distribution varies correlated to the formation of a complex between VDAC and the adenine nucleotide translocase: this affects Bax-dependent cytochrome c release

The mechanism of Bax-dependent cytochrome c release is still controversial and may also depend on the actual localisation of cytochrome C: (i) we studied the distribution of cytochrome c in sub-fractions of rat kidney mitochondria and found that 10-20% of the total cytochrome c was associated at the peripheral inner membrane and to some extent organised in the contact sites. (ii) Cytochrome c concentrations in the contact site fractions varied related to surface bound hexokinase activity. It decreased upon reduction of contact sites by glycerol or specific dissociation of the VDAC-ANT complexes by bongkrekate, whereas it increased upon induction of contacts by dextran or association of VDAC-ANT complexes by atractyloside. (iii) The outer membrane pore (VDAC) acquires high capacity for hexokinase binding by interacting with the ANT. Thus, surface-attached hexokinase protein indicated the frequency of VDAC-ANT complexes and the correlation between hexokinase activity and cytochrome c suggested association of the latter to the complexes. (iv) Substances affecting exclusively the structure of either hexokinase (glucose-6P) or cytochrome c (borate) led to a decrease only of the effected protein without changing the concentration of other contact site constituents. (v) Hexokinase was furthermore used as a tool to isolate the contact site forming complex of outer membrane VDAC and inner membrane ANT from Triton-dissolved membranes. Cytochrome c remained attached to the hexokinase VDAC-ANT complexes that were reconstituted in phospholipid vesicles. (vi) The vesicles were loaded with malate and BaxDeltaC released the endogenous cytochrome c from the reconstituted complexes without forming unspecific pores for malate. BaxDeltaC targeted a cytochrome c fraction associated at the VDAC-ANT complex. The cytochrome c organisation was dependent on the actual structure of VDAC and ANT. Thus, the BaxDeltaC effect was suppressed either by hexokinase utilising glucose and ATP or by bongkrekic acid both influencing the pore and ANT structure.

Bcl-2 overexpression corrects mitochondrial defects and ameliorates inherited desmin null cardiomyopathy

One of the hallmarks of cardiomyopathy and heart failure is pronounced and progressive cardiomyocyte death. Understanding the mechanisms involved in cardiomyocyte cell death is a topic of great interest for treatment of cardiac disease. Mice null for desmin, the muscle-specific member of the intermediate filament gene family, develop cardiomyopathy characterized by extensive cardiomyocyte death, fibrosis, calcification, and eventual heart failure. The earliest ultrastructural defects are observed in mitochondria. In the present study, we have demonstrated that these mitochondrial abnormalities are the primary cause of the observed cardiomyopathy and that these defects can be ameliorated by overexpression of bcl-2 in desmin null heart. Overexpression of bcl-2 in the desmin null heart results in correction of mitochondrial defects, reduced occurrence of fibrotic lesions in the myocardium, prevention of cardiac hypertrophy, restoration of cardiomyocyte ultrastructure, and significant improvement of cardiac function. Furthermore, we have found that loss of desmin also diminishes the capacity of mitochondria to resist exposure to calcium, a defect that can be partially restored by bcl-2 overexpression. These results point to a unique function for desmin in protection of mitochondria from calcium exposure that can be partially rescued by overexpression of bcl-2. We show that bcl-2 cardiac overexpression has provided significant improvement of an inherited form of cardiomyopathy, revealing the potential for bcl-2, and perhaps other genes in the family, as therapeutic agents for heart disease of many types, including inherited forms.

Pharmacological strategies to block rod photoreceptor apoptosis caused by calcium overload: a mechanistic target-site approach to neuroprotection

PURPOSE

Photoreceptor apoptosis and resultant visual deficits occur in humans and animals with inherited, and disease-, injury- and chemical-induced retinal degeneration. Our aims were three-fold: 1) to determine the kinetics of rod apoptosis and Ca2+ overload in Pde6b9rd1) mice and developmentally lead-exposed rats, 2) to establish a pathophysiologically-relevant model of Ca2+ overload/rod-selective apoptosis in isolated rat retina and 3) to examine different mechanistic based neuroprotective strategies that would abrogate or mollify rod Ca2+ overload/apoptosis.

METHODS

Retinal morphometry and elemental calcium content ([Ca]) determined the kinetics of rod apoptosis and Ca2+ overload. A multiparametric analysis of apoptosis including rod [Ca], a live/dead assay, rod oxygen consumption, cytochrome c immunoblots and caspase assays was combined with pharmacological studies of an isolated rat retinal model of rod-selective Ca2+ overload/apoptosis.

RESULTS

Ca2+ overload preceded rod apoptosis in mice and rats, although the extent and kinetics in each differed significantly. The isolated rat model of rod Ca2+ overload/apoptosis showed that blockade of Ca2+ entry through rod cGMP-activated channels with L-cis diltiazem was partially neuroprotective, whereas blockade of Ca2+ entry into rods through L-type Ca2+ channels with D-cis diltiazem or verapamil provided no protection. Inhibition of the mitochondrial Na+/Ca2+ exchanger with D-cis diltiazem provided no protection. CsA and NIM811, mitochondrial permeability transition pore (mPTP) inhibitors, blocked all Ca(2+)-induced apoptosis, whereas the caspase-3 inhibitor DEVD-fmk only blocked the downstream cytochrome c-induced apoptosis.

CONCLUSIONS

The successful pharmacological neuroprotective strategies for rod Ca2+ overload/apoptosis targeted the rod cGMP-activated channels or mPTP, but not the rod L-type Ca2+ channels.