Viral proteins E1B19K and p35 protect sympathetic neurons from cell death induced by NGF deprivation

Emerging Approaches to Functionalizing Cell Membrane-Coated Nanoparticles

There has been significant interest in developing cell membrane-coated nanoparticles due to their unique abilities of biomimicry and biointerfacing. As the technology progresses, it becomes clear that the application of these nanoparticles can be drastically broadened if additional functions beyond those derived from the natural cell membranes can be integrated. Herein, we summarize the most recent advances in the functionalization of cell membrane-coated nanoparticles. In particular, we focus on emerging methods, including (1) lipid insertion, (2) membrane hybridization, (3) metabolic engineering, and (4) genetic modification. These approaches contribute diverse functions in a nondisruptive fashion while preserving the natural function of the cell membranes. They also improve on the multifunctional and multitasking ability of cell membrane-coated nanoparticles, making them more adaptive to the complexity of biological systems. We hope that these approaches will serve as inspiration for more strategies and innovations to advance cell membrane coating technology.

Transfection methods for high-throughput cellular assays of voltage-gated calcium and sodium channels involved in pain

Chemical transfection is broadly used to transiently transfect mammalian cells, although often associated with cellular stress and membrane instability, which imposes challenges for most cellular assays, including high-throughput (HT) assays. In the current study, we compared the effectiveness of calcium phosphate, FuGENE and Lipofectamine 3000 to transiently express two key voltage-gated ion channels critical in pain pathways, Ca_V2.2 and Na_V1.7. The expression and function of these channels were validated using two HT platforms, the Fluorescence Imaging Plate Reader FLIPR^Tetra and the automated patch clamp QPatch 16X. We found that all transfection methods tested demonstrated similar effectiveness when applied to FLIPR^Tetra assays. Lipofectamine 3000-mediated transfection produced the largest peak currents for automated patch clamp QPatch assays. However, the FuGENE-mediated transfection was the most effective for QPatch assays as indicated by the superior number of cells displaying GΩ seal formation in whole-cell patch clamp configuration, medium to large peak currents, and higher rates of accomplished assays for both Ca_V2.2 and Na_V1.7 channels. Our findings can facilitate the development of HT automated patch clamp assays for the discovery and characterization of novel analgesics and modulators of pain pathways, as well as assisting studies examining the pharmacology of mutated channels.

NGF mediates protection of mesenchymal stem cells-conditioned medium against 2,5-hexanedione-induced apoptosis of VSC4.1 cells via Akt/Bad pathway

It has been shown that the conditioned medium of bone mesenchymal stem cells (BMSC-CM) can inhibit apoptosis of neural cells exposed to 2,5-hexanedione (HD), but its protective mechanism remains unclear. To investigate the underlying mechanism, VSC4.1 cells were given HD and 5, 10 and 15% BMSC-CM (v/v) in the current experiment. Our data showed that BMSC-CM concentration-dependently attenuated HD-induced cell apoptosis. Moreover, BMSC-CM remarkably decreased the mitochondrial cytochrome c (Cyt C) release and the caspase-3 activity in HD-given VSC4.1 cells. Given a relatively high expression of NGF in BMSCs and BMSC-CM, we hypothesized that NGF might be an important mediator of the protection of BMSC-CM against apoptosis induced by HD. To verify our hypothesis, the VSC4.1 cells were administrated with NGF and anti-NGF antibody in addition to HD. As expected, NGF could perfectly mimic BMSC-CM's protective role and these beneficial effects were abolished by anti-NGF antibody intervention. To further explore its mechanism, inhibitors of TrkA and Akt were given to the VSC4.1 cells and NGF/Akt/Bad pathway turned out to be involved in anti-apoptotic role of BMSC-CM. Based on these findings, it was revealed that BMSC-CM beneficial role was mediated by NGF and relied on the Akt/Bad pathway.

Shock wave-induced permeabilization of mammalian cells

Controlled permeabilization of mammalian cell membranes is fundamental to develop gene and cell therapies based on macromolecular cargo delivery, a process that emerged against an increasing number of health afflictions, including genetic disorders, cancer and infections. Viral vectors have been successfully used for macromolecular delivery; however, they may have unpredictable side effects and have been limited to life-threatening cases. Thus, several chemical and physical methods have been explored to introduce drugs, vaccines, and nucleic acids into cells. One of the most appealing physical methods to deliver genes into cells is shock wave-induced poration. High-speed microjets of fluid, emitted due to the collapse of microbubbles after shock wave passage, represent the most significant mechanism that contributes to cell membrane poration by this technique. Herein, progress in shock wave-induced permeabilization of mammalian cells is presented. After covering the main concepts related to molecular strategies whose applications depend on safer drug delivery methods, the physics behind shock wave phenomena is described. Insights into the use of shock waves for cell membrane permeation are discussed, along with an overview of the two major biomedical applications thereof-i.e., genetic modification and anti-cancer shock wave-assisted chemotherapy. The aim of this review is to summarize 30 years of data showing underwater shock waves as a safe, noninvasive method for macromolecular delivery into mammalian cells, encouraging the development of further research, which is still required before the introduction of this promising tool into clinical practice.

Programmed cell death during neuronal development: the sympathetic neuron model

Developing sympathetic neurons of the superior cervical ganglion are one of the best studied models of neuronal apoptosis. These cells require nerve growth factor (NGF) for survival at the time that they innervate their final target tissues during late embryonic and early postnatal development. In the absence of NGF, developing sympathetic neurons die by apoptosis in a transcription-dependent manner. Molecular studies of sympathetic neuron apoptosis began in the 1980s. We now know that NGF withdrawal activates the mitochondrial (intrinsic) pathway of apoptosis in sympathetic neurons cultured in vitro, and the roles of caspases, Bcl-2 (B-cell CLL/lymphoma 2) family proteins and XIAP (X-linked inhibitor of apoptosis protein) have been extensively studied. Importantly, a considerable amount has also been learned about the intracellular signalling pathways and transcription factors that regulate programmed cell death in sympathetic neurons. In this article, we review the key papers published in the past few years, covering all aspects of apoptosis regulation in sympathetic neurons and focusing, in particular, on how signalling pathways and transcription factors regulate the cell death programme. We make some comparisons with other models of neuronal apoptosis and describe possible future directions for the field.

Viral product trafficking to mitochondria, mechanisms and roles in pathogenesis

A wide variety of viruses cause significant morbidity and mortality in humans. However, targeted antiviral therapies have been developed for only a subset of these viruses, with the majority of currently licensed antiviral drugs targeting viral entry, replication or exit steps during the viral life cycle. Due to increasing emergence of antiviral drug resistant viruses, the isolation of multiple viral subtypes, and toxicities of existing therapies, there remains an urgent need for the timely development of novel antiviral agents, including those targeting host factors essential for viral replication. This review summarizes viral products that target mitochondria and their effects on common mitochondria regulated pathways. These viral products and the mitochondrial pathways affected by them provide potential novel targets for the rational design of antiviral drugs. Viral products alter oxidative balance, mitochondrial permeability transition pore, mitochondrial membrane potential, electron transport and energy production. Moreover, viruses may cause the Warburg Effect, in which metabolism is reprogrammed to aerobic glycolysis as the main source of energy. Finally, viral products affect proapoptotic and antiapoptotic signaling, as well as mitochondrial innate immune signaling. Because of their importance for the generation of metabolic intermediates and energy as well as cell survival, mitochondrial pathways are targeted by multiple independent viral products. Structural modifications of existing drugs targeted to mitochondrial pathways may lead to the development of novel antiviral drugs with improved efficacy and reduced toxicity.

Mammalian cell transfection: the present and the future

Transfection is a powerful analytical tool enabling study of the function of genes and gene products in cells. The transfection methods are broadly classified into three groups; biological, chemical, and physical. These methods have advanced to make it possible to deliver nucleic acids to specific subcellular regions of cells by use of a precisely controlled laser-microcope system. The combination of point-directed transfection and mRNA transfection is a new way of studying the function of genes and gene products. However, each method has its own advantages and disadvantages so the optimum method depends on experimental design and objective.

Array-based comparative genomic hybridization identifies a high frequency of copy number variations in patients with syndromic overgrowth

Overgrowth syndromes are a heterogeneous group of conditions including endocrine hormone disorders, several genetic syndromes and other disorders with unknown etiopathogenesis. Among genetic causes, chromosomal deletions and duplications such as dup(4)(p16.3), dup(15)(q26qter), del(9)(q22.32q22.33), del(22)(q13) and del(5)(q35) have been identified in patients with overgrowth. Most of them, however, remain undetectable using banding karyotype analysis. In this study, we report on the analysis using a 1-Mb resolution array-based comparative genomic hybridization (CGH) of 93 patients with either a recognizable overgrowth condition (ie, Sotos syndrome or Weaver syndrome) or an unclassified overgrowth syndrome. Five clinically relevant imbalances (three duplications and two deletions) were identified and the pathogenicity of two additional anomalies (one duplication and one deletion) is discussed. Altered segments ranged in size from 0.32 to 18.2 Mb, and no recurrent abnormality was identified. These results show that array-CGH provides a high diagnostic yield in patients with overgrowth syndromes and point to novel chromosomal regions associated with these conditions. Although chromosomal deletions are usually associated with growth retardation, we found that the majority of the imbalances detected in our patients are duplications. Besides their importance for diagnosis and genetic counseling, our results may allow to delineate new contiguous gene syndromes associated with overgrowth, pointing to new genes, the deregulation of which may be responsible for growth defect.

Identification of the Apoptosis Inhibitor Gene p49 of Spodoptera litura Multicapsid Nucleopolyhedrovirus

Baculoviruses possess two types of genes that suppressed apoptosis, p35 and inhibitor of apoptosis (iap). Computer-assisted analysis indicated that Spodoptera litura multicapsid nucleopolyhedrovirus (SpltMNPV) ORF55 (designated as the p49 gene) display 79 and 31% amino acid identity with Spodoptera littoralis (Spli)MNPV P49 and Autographa californica (Ac)MNPV P35, respectively, Splt MNPV putative P49 contains a peptide cleavage site TVTDG recognized by death caspases. In marker rescue assay, Splt-p49 was able to suppress apoptosis induced by infection of a mutant AcMNPV deficient in p35 and rescue the mutant virus replication from apoptosis in Sf-9 cells.

Deacetylation of p53 after nerve growth factor treatment in PC12 cells as a post-translational modification mechanism of neurotrophin-induced tumor suppressor activation

The tumor suppressor protein p53 is a transcription factor that regulates the response to cellular insults such as DNA damage and growth factor withdrawal. Transcriptional activity of p53 requires post-translational modification by phosphorylation and acetylation. This study used site-specific antibodies to demonstrate that nerve growth factor (NGF) treatment of PC12 cells results in p53 deacetylation at lysine (Lys) 382. Histone deacetylase (HDAC) activity, measured by a direct fluorescent assay, was increased after NGF treatment and peaked before p53 deacetylation. Inhibition of HDAC by trichostatin blocked the deacetylation of p53 and its transcriptional activity toward a reporter gene construct. Comparison of PC12 with PC12 cells containing a temperature-sensitive, dominant-negative construct showed that p53 deacetylation required functional p53. Inhibitors of MAP kinase that block p53 transactivation and inhibitors of TrkA receptor also abolished HDAC activation, indicating that deacetylation of p53 is an NGF-dependent post-translational mechanism of p53 activation. Finally, NGF or serum withdrawal did not lead to p53 deacetylation. A model is proposed in which the acetylation status of Lys 382 of p53 discriminates between cell cycle arrest and apoptosis.

Men are but worms: neuronal cell death in C elegans and vertebrates

Awarding the 2002 Nobel Prize in Physiology or Medicine to Sydney Brenner, H Robert Horvitz, and John E Sulston for 'their discoveries concerning the genetic regulation of organ development and programmed cell death (PCD)' highlights the significant contribution that the study of experimental organisms, such as the nematode Caenorhabditis elegans, has made to our understanding of human physiology and pathophysiology. Their studies of lineage determination in worms established the 'central dogma' of apoptosis: The BH3-only protein EGL-1 is induced in cells destined to die, interacts with the BCL-2-like inhibitor CED-9, displacing the adaptor CED-4, which then promotes activation of the caspase CED-3. The vast majority of cells undergoing PCD during development in C. elegans, as in vertebrates, are neurons. Accordingly, the genetic regulation of apoptosis is strikingly similar in nematode and vertebrate neurons. This review summarizes these similarities - and the important differences - in the molecular mechanisms responsible for neuronal PCD in C. elegans and vertebrates, and examines the implications that our understanding of physiological neuronal apoptosis may have for the diagnosis and treatment of acute and chronic human neurodegenerative disorders.

Viral Bcl-2 homologs and their role in virus replication and associated diseases

Cellular Bcl-2 family proteins regulate a critical step in the mammalian programmed cell death pathway by modulating mitochondrial permeability and function. Bcl-2 family proteins are also encoded by several large DNA viruses, including all known gamma herpesviruses, adenoviruses, and several other unrelated viruses. Viral Bcl-2 proteins can prevent cell death but often escape cellular regulatory mechanisms that govern their cellular counterparts. By evading the "altruistic" suicide of infected cells, viruses can ensure replication and propagation in the infected host, but sometimes in surprising ways. Many human cancers and other disorders are associated with viruses that encode Bcl-2 homologs. Here we consider the available mechanistic data for viral compared to cellular Bcl-2 protein function along with relevance to the virus life cycle and human disease states.

Cytosine arabinoside rapidly activates Bax-dependent apoptosis and a delayed Bax-independent death pathway in sympathetic neurons

Cytosine arabinoside (ara-C) is a nucleoside analog used in the treatment of hematologic malignancies. One of the major side effects of ara-C chemotherapy is neurotoxicity. In this study, we have further characterized the cell death induced by ara-C in sympathetic neurons. Similar to neurons undergoing trophic factor deprivation-induced apoptosis, ara-C-exposed neurons became hypometabolic before death and upregulated c-myb, c-fos, and Bim. Bax deletion delayed, but did not prevent, ara-C toxicity. Neurons died by apoptosis, indicated by the release of mitochondrial cytochrome-c and caspase-3 activation. p53-deficient neurons demonstrated decreased sensitivity to ara-C, but neither p53 nor multiple p53-regulated genes were induced. Mature neurons showed increased ara-C resistance. These results demonstrate that molecular mechanisms underlying ara-C-induced death are similar to those responsible for trophic factor deprivation-induced apoptosis. However, substantial differences in neuronal death after these two distinct stress stimuli exist since ara-C toxicity, unlike the developmental death, can proceed in the absence of Bax.

Caspase inhibitors as anti-inflammatory and antiapoptotic agents

The striking efficacy of Z-VAD-fmk in the various animal models presented above may reflect its ability to inhibit multiple enzymes including caspases. In accord with this, more selective, reversible inhibitors usually show low efficacy in multifactorial models such as ischaemia, but may offer some protection against NMDA-induced excitotoxicity and hepatitis. Importantly, caspase inhibitors may exhibit significant activity in vivo even when they are applied post insult. As far as the CNS is concerned, the first systemically active inhibitors have emerged. Functional recovery could be achieved in some ischaemia models, but long-term protection by caspase inhibitors is still being questioned. Recent developments in drug design enabled the first caspase inhibitors to enter the clinic. Although initially directed towards peripheral indications such as rheumatoid arthritis, caspase inhibitors will no doubt eventually be used to target CNS disorders. For this purpose the peptidic character of current inhibitors will have to be further reduced. Small molecule, nonpeptidic caspase inhibitors, which have appeared recently, indicate that this goal can be accomplished. Unfortunately, many fundamental questions still remain to be addressed. In particular, the necessary spectrum of inhibitory activity required to achieve the desired effect needs to be determined. There is also a safety aspect associated with prolonged administration. Therefore, the next therapeutic areas for broader-range caspase inhibitors are likely to involve acute treatment. Recent results with synergistic effects between MK-801 and caspase inhibitors in ischaemia suggest that caspase inhibitors may need to be used in conjunction with other drugs. It can be expected that, in the near future, research on caspases and their inhibitors will remain a rapidly developing area of biology and medicinal chemistry. More time, however, may be needed for the first caspase inhibitors to appear on the market.

A method utilizing differential culture and comparative RT-PCR for determining RNA expression in superior cervical ganglion neurones

In order to assess the neuronal expression of caspase mRNA in primary cultures of rat superior cervical ganglion (SCG) neurones a method of differential cell purification and comparative RT-PCR was devised. SCG primary cultures generally contain variable percentages of non-neuronal contaminants, which influence RT-PCR results. We optimised a neuronal purification method, allowing the preparation of both highly purified neuronal cultures and mixed cultures, enriched in non-neuronal contaminants. These two sets of cells were cultured in parallel and subsequently analysed by RT-PCR. The use of cell type specific oligonucleotides allowed evaluation of the relative distribution of neuronal (neurofilament) and non-neuronal transcripts in the two cultures. In parallel, specific oligonucleotides were used to detect the mRNA levels of caspase family members. The partition of neurofilament transcript between pure and mixed cultures was found to be statistically different from the partition of the non-neuronal markers. Therefore statistical difference from the partition of non-neuronal markers was taken as evidence for expression in neurones. We show that caspase-2, -3, -6, -7 and -9 transcripts are expressed in SCG neurones whereas caspase-1 is probably absent. Furthermore, none of these transcripts are upregulated during neuronal death induced by nerve growth factor withdrawal. This method could be applied to the analysis of other transcripts in SCG and other primary neuronal cultures containing significant percentages of contaminant cell types.

Mutational analyses of the p35-caspase interaction. A bowstring kinetic model of caspase inhibition by p35

Apoptosis is a highly regulated multistep process for programmed cellular destruction. It is centered on the activation of a group of intracellular cysteine proteases known as caspases. The baculoviral p35 protein effectively blocks apoptosis through its broad spectrum caspase inhibition. It harbors a caspase recognition sequence within a highly protruding reactive site loop (RSL), which gets cleaved by a target caspase before the formation of a tight complex. The crystal structure of the post-cleavage complex between p35 and caspase-8 shows that p35 forms a thioester bond with the active site cysteine of the caspase. The covalent bond is prevented from hydrolysis by the N terminus of p35, which repositions into the active site of the caspase to eliminate solvent accessibility of the catalytic residues. Here, we report mutational analyses of the pre-cleavage and post-cleavage p35/caspase interactions using surface plasmon resonance biosensor measurements, pull-down assays and kinetic inhibition experiments. The experiments identify important structural elements for caspase inhibition by p35, including the strict requirement for a Cys at the N terminus of p35 and the rigidity of the RSL. A bowstring kinetic model for p35 function is derived in which the tension generated in the bowstring system during the pre-cleavage interaction is crucial for the fast post-cleavage conformational changes required for inhibition.

In vivo role of caspases in excitotoxic neuronal death: generation and analysis of transgenic mice expressing baculoviral caspase inhibitor, p35, in postnatal neurons

Caspases, a family of cysteine proteases, are thought to be critical mediators of apoptosis. To examine the role of neuronal caspases in excitotoxic neurodegeneration in vivo, we have generated transgenic mice expressing the baculovirus protein p35, a potent viral caspase inhibitor, using the neuron-specific calmodulin dependent kinase-II alpha (CaMKII-alpha) promoter. The expression of p35 was confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting and immunohistochemistry. We analyzed caspase activation and cell death by employing an experimental paradigm, in which the excitotoxin kainate (KA) was injected into CA1 of hippocampus and the distribution of the caspase-generated actin fragment was detected immunohistochemically. While kainate treatment led to selective neuronal death in the CA1, CA3 and CA4 of non-transgenic control mice, we observed restricted caspase activation only in the CA3 sector. The transgenic expression of p35 consistently inhibited the kainate-induced caspase activation, but failed to influence the death of neurons to any extent. In addition, we observed concomitant early calpain activation in the specific areas where neurons underwent degeneration in both the transgenic and non-transgenic mice. These results indicate that p35-inhibitable caspases play rather minor roles in the kainate-induced excitotoxicity and that the relative contribution of calpain is likely to be greater than that of caspases.

The p35 relative, p49, inhibits mammalian and Drosophila caspases including DRONC and protects against apoptosis

This study characterized the ability of a new member of the p35 family, p49, to inhibit a number of mammalian and insect caspases. p49 blocked apoptosis triggered by treatment with Fas ligand (FasL), Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) or ultraviolet (UV) radiation but provided negligible protection against apoptosis induced by the chemotherapeutic drug cisplatin. The caspase cleavage site in p49 was determined, and mutation of the P1 residue of this site abolished the ability of p49 to inhibit caspases, implying that p49 inhibits caspases through an analogous suicide-substrate mechanism to p35. Unlike p35, p49 inhibited the upstream insect caspase DRONC.

DLPC decreases TGF-beta1-induced collagen mRNA by inhibiting p38 MAPK in hepatic stellate cells

Dilinoleoylphosphatidylcholine (DLPC), the active component of polyenylphosphatidylcholine extracted from soybeans, decreases collagen accumulation induced by TGF-beta1 in cultured hepatic stellate cells (HSCs). Because DLPC exerts antioxidant effects and TGF-beta1 generates oxidative stress, we evaluated whether the antifibrogenic effect of DLPC is linked to its antioxidant action. In passage 1 culture of rat HSCs, TGF-beta1 induced a concentration-dependent increase in procollagen-alpha(1)(I) mRNA levels and enhanced intracellular H(2)O(2) and superoxide anion formation and lipid peroxidation but decreased GSH levels. These changes were prevented by DLPC. Upregulation of collagen mRNA by TGF-beta1 was likewise inhibited by catalase and p38 MAPK inhibitor SB-203580, suggesting involvement of H(2)O(2) and p38 MAPK signaling in this process. TGF-beta1 or addition of H(2)O(2) to HSCs activated p38 MAPK with a rise in procollagen mRNA level; these changes were blocked by catalase and SB-203580 and likewise by DLPC. alpha-Smooth muscle actin abundance in HSCs was not altered by TGF-beta1 treatment (with or without DLPC), indicating that downregulation of procollagen mRNA by DLPC was not due to alteration in HSC activation. These results demonstrate that DLPC prevents TGF-beta1-induced increase in collagen mRNA by inhibiting generation of oxidative stress and associated H(2)O(2)-dependent p38 MAPK activation, which explains its antifibrogenic effect. DLPC, an innocuous phospholipid, may be considered for prevention and treatment of liver fibrosis.

Techniques for gene transfer into neurons

To illuminate the function of the thousands of genes that make up the complexity of the nervous system, it is critical to be able to introduce and express DNA in neurons. Over the past two decades, many gene transfer methods have been developed, including viral vectors, liposomes and electroporation. Although the perfect gene transfer technique for every application has not yet been developed, recent technical advances have facilitated the ease of neuronal gene transfer and have increased the accessibility of these techniques to all laboratories. In order to select a transfection method for any particular experiment, the specific advantages and disadvantages of each technique must be considered.

Biolistic transfection and morphological analysis of cultured sympathetic neurons

We have developed a protocol for introducing foreign genes into postmitotic peripheral neurons and their subsequent analysis by computer-assisted morphometry. The application of the biolistic approach results in transfection of approximately 10% of dissociated sympathetic neurons. Gold particles coated with plasmid DNA are propelled into sympathetic neuronal nuclei using a hand-held 'gene gun'. Transfection efficiency is optimized by choosing a microcarrier loading quantity of 0.7 mg of gold per experiment and a DNA loading ratio of 2 microg DNA per mg of gold. The gene gun is modified to minimize pressure-induced cell damage. Neurons are transfected with plasmids encoding enhanced green fluorescent protein (EGFP) or red fluorescent protein (DsRed), which serve as suitable markers for the cell body, axonal processes and growth cones including lamellipodia and filopodia. A detailed routine for quantitative morphometric image analysis using METAMORPH software is described. The protocol is demonstrated by comparing the effects of nerve growth factor (NGF) and dibutyryl cyclic adenosine monophosphate (dbcAMP) on neurite outgrowth of transfected sympathetic neurons plated on glass floor dishes.

c-Jun N-terminal kinase-3 (JNK3)/stress-activated protein kinase-beta (SAPKbeta) binds and phosphorylates the neuronal microtubule regulator SCG10

The neuronal growth-associated protein SCG10 is enriched in the growth cones of neurons where it destabilizes microtubules and thus contributes to the dynamic assembly and disassembly of microtubules. Since its microtubule-destabilizing activity is regulated by phosphorylation, SCG10 may link extracellular signals to rearrangements of the neuronal cytoskeleton. To identify signal transduction pathways that may lead to SCG10 phosphorylation, we tested a series of serine-threonine-directed protein kinases that phosphorylate SCG10 in vitro. We demonstrate that purified SCG10 can be phosphorylated by two subclasses of mitogen-activated protein (MAP) kinases, c-Jun N-terminal/stress-activated protein kinase (JNK/SAPK) and p38 MAP kinase. Moreover, SCG10 was found to bind tightly and specifically to JNK3/SAPKbeta. JNK3/SAPKbeta phosphorylation occurs at Ser-62 and Ser-73, residues that result in reduced microtubule-destabilizing activity for SCG10. Endogenous SCG10 also undergoes increased phosphorylation in sympathetic neurons at times of JNK3/SAPKbeta activation following deprivation from nerve growth factor. Together these observations indicate that activation of JNK/SAPKs provides a pathway for phosphorylation of SCG10 and control of growth cone microtubule formation following neuronal exposure to cellular stresses.

Crystal structure of baculovirus P35 reveals a novel conformational change in the reactive site loop after caspase cleavage

Baculovirus P35 is a universal suppressor of apoptosis that stoichiometrically inhibits cellular caspases in a novel cleavage-dependent mechanism. Upon caspase cleavage at Asp-87, the 10- and 25-kDa cleavage products of P35 remain tightly associated with the inhibited caspase. Mutations in the alpha-helix of the reactive site loop preceding the cleavage site abrogate caspase inhibition and antiapoptotic activity. Substitution of Pro for Val-71, which is located in the middle of this alpha-helix, produces a protein that is cleaved at the requisite Asp-87 but does not remain bound to the caspase. This loss-of-function mutation provided the opportunity to structurally analyze the conformational changes of the P35 reactive site loop after caspase cleavage. We report here the 2.7 A resolution crystal structure of V71P-mutated P35 after cleavage by human caspase-3. The structure reveals a large movement in the carboxyl-terminal side of the reactive site loop that swings down and forms a new beta-strand that augments an existing beta-sheet. Additionally, the hydrophobic amino terminus releases and extends away from the protein core. Similar movements occur when P35 forms an inhibitory complex with human caspase-8. These findings suggest that the alpha-helix mutation may alter the sequential steps or kinetics of the conformational changes required for inhibition, thereby causing P35 loss of function.

Stable cell lines expressing baculovirus P35: resistance to apoptosis and nutrient stress, and increased glycoprotein secretion

The baculovirus P35 protein is a caspase inhibitor that prevents the induction of apoptosis during infection of Sf21 cells by Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV). P35 inhibits the induction of apoptosis in a broad range of cells and circumstances. In this study, we examined the effects of constitutive cellular P35 expression on the response of cells to stressful culture conditions and on protein production in AcMNPV infected cells. Sf9 cell lines expressing AcMNPV P35 or an epitope-tagged P35 protein were generated using a double selection technique, involving selection in the antibiotic G418, followed by a second round of selection by exposure to actinomycin D, a potent inducer of apoptosis in Sf9 cells. Clonal cell lines were generated and examined for (1) resistance to actinomycin D induced apoptosis, (2) resistance to nutrient deprivation, and (3) baculovirus expression of intracellular and secreted proteins. When compared with Sf9 cells, two P35-expressing cell lines (Sf9P35AcV5-1 and Sf9P35AcV5-3) showed increased resistance to actinomycin D-induced apoptosis and a profound resistance to nutrient deprivation. When these cell lines were infected with a recombinant baculovirus expressing a secreted glycoprotein (secreted alkaline phosphatase), expression of the glycoprotein from these cells exceeded that from the parental Sf9 cells and was comparable to expression levels obtained from Tn5B1-4 cells, the best available cell line for high-level expression. Increased levels of protein secretion in Sf9P35AcV5-1 and Sf9P35AcV5-3 cells appear to result from a prolonged infection cycle and accumulation of the secreted glycoprotein.

Direct inhibition of c-Jun N-terminal kinase in sympathetic neurones prevents c-jun promoter activation and NGF withdrawal-induced death

c-Jun N-terminal kinases (JNKs) regulate gene expression by phosphorylating transcription factors, such as c-Jun. Studies with JNK: knockout mice suggest that JNK activity may be required for excitotoxin-induced apoptosis in the adult hippocampus and for apoptosis in the developing embryonic neural tube. Here we investigate the role of JNKs in classical neurotrophin-regulated developmental neuronal death by using nerve growth factor (NGF)-dependent sympathetic neurones. In this system, NGF withdrawal leads to an increase in JNK activity, an increase in c-Jun protein levels and c-Jun N-terminal phosphorylation before the cell death commitment point, and c-Jun activity is required for cell death. To inhibit JNK activity in sympathetic neurones we have used two different JNK inhibitors that act by distinct mechanisms: the compound SB 203580 and the JNK binding domain (JBD) of JNK interacting protein 1 (JIP-1). We demonstrate that JNK activity is required for c-Jun phosphorylation, c-jun promoter activation and NGF withdrawal-induced apoptosis. We also show that ATF-2, a c-Jun dimerization partner that can regulate c-jun gene expression, is activated following NGF deprivation. Finally, by co-expressing the JBD and a regulatable c-Jun dominant negative mutant we demonstrate that JNK and AP-1 function in the same pro-apoptotic signalling pathway after NGF withdrawal.

Activation of cell-cycle-associated proteins in neuronal death: a mandatory or dispensable path?

Cell-cycle-related proteins, such as cyclins or cyclin-dependent kinases, are re-expressed in neurons committed to death in response to a variety of insults, including excitotoxins, hypoxia and ischemia, loss of trophic support, or beta-amyloid peptide. In some of these conditions events that are typical of the mid-G1 phase, such as cyclin-dependent kinase 4/6 activation, are required for the induction of neuronal death. In other cases, the cycle must proceed further and recruit steps that are typical of the G1/S transition for death to occur. Finally, there are conditions in which cell-cycle proteins might be re-expressed, but do not contribute to neuronal death. We hypothesize that cell-cycle signaling becomes a mandatory component of neuronal demise when other mechanisms are not enough for neurons to reach the threshold for death. Under this scheme, the death threshold is set by the extent of DNA damage. Whenever the extent of DNA damage is below this threshold, a cell-cycle signaling becomes crucial for the induction of neuronal death through p53-dependent or -independent pathways.

Mediation of nerve growth factor-driven cell cycle arrest in PC12 cells by p53. Simultaneous differentiation and proliferation subsequent to p53 functional inactivation

Upon stimulation with nerve growth factor (NGF), PC12 cells extend neurites and cease to proliferate by influencing cell cycle proteins. Previous studies have shown that neuritogenesis and a block at the G(1)/S checkpoint correlate with the nuclear translocation of and an increase in the p53 tumor suppressor protein. This study was designed to determine if p53 plays a direct role in mediating NGF-driven G(1) arrest. A retroviral vector that overexpresses a temperature-sensitive p53 mutant protein (p53ts) was used to extinguish the function of endogenous p53 in PC12 cells in a dominant-negative manner at the nonpermissive temperature. NGF treatment led to transactivation of a p53 response element in a luciferase reporter construct in PC12 cells, whereas this response to NGF was absent in PC12(p53ts) cells at the nonpermissive temperature. With p53 functionally inactivated, NGF failed to activate growth arrest, as measured by bromodeoxyuridine incorporation, and also failed to induce p21/WAF1 expression, as measured by Western blotting. Since neurite outgrowth proceeded unharmed, 50% of the cells simultaneously demonstrated neurite morphology and were in S phase. Both PC12 cells expressing SV40 T antigen and PC12 cells treated with p53 antisense oligonucleotides continued through the cell cycle, confirming the dependence of the NGF growth arrest signal on a p53 pathway. Activation of Ras in a dexamethasone-inducible PC12 cell line (GSRas1) also caused p53 nuclear translocation and growth arrest. Therefore, wild-type p53 is indispensable in mediating the NGF antiproliferative signal through the Ras/MAPK pathway that regulates the cell cycle of PC12 cells.

c-Jun and the transcriptional control of neuronal apoptosis

There has been considerable interest in the molecular mechanisms of apoptosis in mammalian neurons because this form of neuronal cell death is important for the normal development of the nervous system and because inappropriate neuronal apoptosis may contribute to the pathology of human neurodegenerative diseases. The aim of recent research has been to identify the key components of the cell death machinery in neurons and understand how the cell death programme is regulated by intracellular signalling pathways activated by the binding of neurotrophins or death factors to specific cell surface receptors. The aim of this commentary was to review research that has investigated the role of the Jun N-terminal kinase (JNK)/c-Jun signalling pathway in neuronal apoptosis, focusing in particular on work carried out with developing sympathetic neurons. Experiments with sympathetic neurons cultured in vitro, as well as with cerebellar granule neurons and differentiated PC12 cells, have demonstrated that JNK/c-Jun signalling can promote apoptosis following survival factor withdrawal. In addition, experiments with Jnk(-/-) knockout mice have provided evidence that Jnk3 may be required for apoptosis in the hippocampus in vivo following injection of kainic acid, an excitotoxin, and that Jnk1 and Jnk2 are required for apoptosis in the developing embryonic neural tube. However, in the embryonic forebrain, Jnk1 and Jnk2 have the opposite function and are necessary for the survival of developing cortical neurons. These results suggest that JNKs and c-Jun are important regulators of the cell death programme in the mammalian nervous system, but that their biological effects depend on the neuronal type and stage of development.

Caspases determine the vulnerability of oligodendrocytes in the ischemic brain

Although oligodendrocytes (OLGs) are thought to be vulnerable to hypoxia and ischemia, little is known about the detailed mechanism by which these insults induce OLG death. From the clinical viewpoint, it is imperative to protect OLGs as well as neurons against ischemic injury (stroke), because they are the only myelin-forming cells of the central nervous system. Using the Cre/loxP system, we have established a transgenic mouse line that selectively expresses p35, a broad-spectrum caspase inhibitor, in OLGs. After hypoxia, cultured OLGs derived from wild-type mice exhibited significant upregulation of caspase-11 and substantial activation of caspase-3, which led to cell loss. Expression of p35 or elimination of caspase-11 suppressed the caspase-3 activation and conferred significant protection against hypoxic injury. Expression of p35 in OLGs in vivo resulted in significant protection from ischemia-induced cell injury, thus indicating that caspases are involved in the ischemia-induced cell death of OLGs. Furthermore, the induction of caspase-11 was evident in the ischemic brains of wild-type mice, and OLGs exhibited resistance to brain ischemia in mice deficient in caspase-11, suggesting that caspase-11 is critically implicated in the mechanism(s) underlying ischemia-induced OLG death. Caspases may therefore offer a good therapeutic target for reducing ischemia-induced damage to OLGs.

Conditional expression of anti-apoptotic protein p35 by Cre-mediated DNA recombination in cardiomyocytes from loxP-p35-transgenic mice

p35, a viral inhibitor of caspase, prevents cell death induced by various stimuli. We established an experimental system to study the involvement of caspases in cell death, using primary cultured cells from p35 transgenic mice in which the p35 open reading frame (ORF) had been disrupted by the insertion of a DNA segment flanked by loxP sites, the Cre recognition sites. In this system, p35 expression can be initiated by Cre recombinase. Cardiomyocytes, which are highly sensitive to hypoxic stress, were infected with an adenovirus carrying the cre gene (AxCANCre). Expression of p35 by infection with AxCANCre resulted in inhibition of caspase-3 activation and resistance to hypoxia-induced cell death. Hypoxia-induced cytochrome c release was also attenuated in p35-expressing cardiomyocytes. Our transgenic mice can be used as an experimental model for studying the involvement of caspases in various degenerative diseases as well as programmed cell death both in vitro and in vivo.

Bcl-xS and Bax induce different apoptotic pathways in PC12 cells

Apoptosis is regulated by the action of the Bcl-2 family of proteins, which includes anti- and pro-apoptotic members such as Bcl-xS and Bax. These proteins may differ from each other in structure, mechanism of action and interactions with anti-apoptotic signaling. The mechanism whereby Bax induces cell death has been studied in some cellular systems, but the mechanism of Bcl-xS-induced apoptosis is largely unknown. In this study we investigated and compared the apoptotic effects of Bcl-xS and Bax in the pheochromocytoma cell line, PC12 (a useful model system for studying neuronal apoptosis), and the extent to which they are protected by the survival factor, nerve growth factor (NGF). PC12 cells express endogenous Bcl-xS, Bax and Bcl-xL proteins. Subcellular fractionation revealed that Bax is presented mainly in the cytosolic and the heavy membrane fractions, Bcl-xS is present only in the cytosol, and the anti-apoptotic protein Bcl-xL is located mainly in the heavy membrane fraction. In contrast to the cytosolic localization of endogenous Bcl-xS, the exogenously overexpressed Bcl-xS is localized to the mitochondria. Overexpression of Bcl-xS or Bax induces cell death in the transfected cells. The cell death induced by overexpression of Bcl-xS was inhibited by coexpression of Bcl-xS with Bcl-2 or Bcl-xL, or by treatment with the broad-spectrum caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoro-methylketone (Z-VAD-FMK) or with NGF. The Bcl-2 mutants deltaC22, which lacks the transmembrane domain, and G145A (mI-3) were able to inhibit the death-inducing effect of Bcl-xS. These results therefore suggest that the apoptotic pathway induced by overexpression of Bcl-xS in PC12 cells can be controlled by Bcl-2 and Bcl-xL, is mediated by caspases, and can be inhibited by the NGF signaling pathway. The Bax-induced cell death was inhibited by co-expression of Bax with Bcl-2 or Bcl-xL, but was not inhibited by Z-VAD-FMK, NGF, or the Bcl-2 ml-3 or deltaC22 mutants. These results therefore suggest that Bax induces a caspase-independent cell death pathway which is blocked by Bcl-2 but not by the NGF signaling pathway. They further suggest that Bcl-xS and Bax induce different cell death pathways in PC12 cells.

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

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

The BH3 domain of Bcl-x(S) is required for inhibition of the antiapoptotic function of Bcl-x(L)

bcl-x is a member of the bcl-2 family of genes. The major protein product, Bcl-x(L), is a 233-amino-acid protein which has antiapoptotic properties. In contrast, one of the alternatively spliced transcripts of the bcl-x gene codes for the protein Bcl-x(S), which lacks 63 amino acids present in Bcl-x(L) and has proapoptotic activity. Unlike other proapoptotic Bcl-2 family members, such as Bax and Bak, Bcl-x(S) does not seem to induce cell death in the absence of an additional death signal. However, Bcl-x(S) does interfere with the ability of Bcl-x(L) to antagonize Bax-induced death in transiently transfected 293 cells. Mutational analysis of Bcl-x(S) was conducted to identify the domains necessary to mediate its proapoptotic phenotype. Deletion mutants of Bcl-x(S) which still contained an intact BH3 domain retained the ability to inhibit survival through antagonism of Bcl-x(L). Bcl-x(S) was able to form heterodimers with Bcl-x(L) in mammalian cells, and its ability to inhibit survival correlated with the ability to heterodimerize with Bcl-x(L). Deletion mutants of Bax and Bcl-2, which lacked BH1 and BH2 domains but contained a BH3 domain, were able to antagonize the survival effect conferred by Bcl-x(L). The results suggest that BH3 domains from both pro- and antiapoptotic Bcl-2 family members, while lacking an intrinsic ability to promote programmed cell death, can be potent inhibitors of Bcl-x(L) survival function.

Caspase inhibition by baculovirus P35 requires interaction between the reactive site loop and the beta-sheet core

Baculovirus P35 is a universal substrate-inhibitor of the death caspases. Stoichiometric inhibition by P35 is correlated with cleavage of its reactive site loop (RSL) and formation of a stable P35.caspase complex through a novel but undefined mechanism. The P35 crystal structure predicts that the RSL associates with the beta-sheet core of P35 positioning the caspase cleavage site at the loop's apex. Here we demonstrate that proper interaction between the RSL and the beta-sheet core is critical for caspase inhibition, but not cleavage. Disruption of RSL interaction with the beta-sheet by substituting hydrophobic residues of the RSL's transverse helix alpha1 with destabilizing charged residues caused loss of caspase inhibition, without affecting P35 cleavage. Restabilization of the helix/sheet interaction by charge compensation from within the beta-sheet partially restored anti-caspase potency. Mutational effects on P35 helix/sheet interactions were confirmed by measuring intermolecular helix/sheet association with the yeast two-hybrid system. Moreover, the identification of P35 oligomers in baculovirus-infected cells suggested that similar P35 interactions occur in vivo. These findings indicate that P35's anti-caspase potency depends on a distinct conformation of the RSL which is required for events that promote stable, post-cleavage interactions and inhibition of the target caspase.

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

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

Apoptosis-resistant E1B-19K-expressing NS/0 myeloma cells exhibit increased viability and chimeric antibody productivity under perfusion culture conditions

We have shown previously that recombinant NS/0 myelomas expressing sufficient amounts of E1B-19K were resistant to apoptosis occurring in the late phase of batch culture and under stressful conditions such as cultivation in glutamine-free medium or following heat shock. However, no significant increase in monoclonal antibodies (MAb) was observed during the prolonged stationary phase of these batch cultures. Here, we show that E1B-19K can enhance cell survival and improve MAb productivity in high cell density perfusion culture. Typically, lymphoid cells grown under steady state in perfusion exhibit decreasing viabilities with concomitant accumulation of apoptotic cells. By modulating the ability of these cells to resist to induction of apoptosis in low nutrient environment, a 3-fold decrease in specific death rate from 0.22 day-1 for NS/0 control to 0.07 day-1 for E1B-19K cells was achieved, resulting in a significant improvement in cell viability throughout perfusion. E1B-19K cells at the perfusion plateau phase also exhibited a 3-fold reduction in specific growth rate concomitant with a lower percentage of S and higher percentage of G1 phase cells. This was associated with a 40% decrease in specific oxygen consumption rate, likely related to a reduction in the specific consumption rates of limiting nutrient(s). Expression of E1B-19K consequently had a significant impact on the steady-state viable cell density, allowing maintenance of 11.5 x 10(6) E1B-19K cells/mL versus 5.9 x 10(6) control NS/0 cells/mL for the same amount of fresh medium brought into the system (half a volume per day). Whereas MAb concentrations found in perfusion culture of control NS/0 myelomas were almost 3-fold higher than those found in batch culture; in the case of E1B-19K-expressing myelomas, the MAb concentration in perfusion was more than 7-fold higher than in batch. This was attributable to the 2-fold increase in viable cell plateau and to a 40% increase in the perfusion to batch ratio of specific MAb productivity (2.2-fold for E1B-19K myelomas versus 1.6-fold for NS/0 control).

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

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

Crystal structure of baculovirus P35: role of a novel reactive site loop in apoptotic caspase inhibition

The aspartate-specific caspases are critical protease effectors of programmed cell death and consequently represent important targets for apoptotic intervention. Baculovirus P35 is a potent substrate inhibitor of metazoan caspases, a property that accounts for its unique effectiveness in preventing apoptosis in phylogenetically diverse organisms. Here we report the 2.2 A resolution crystal structure of P35, the first structure of a protein inhibitor of the death caspases. The P35 monomer possesses a solvent-exposed loop that projects from the protein's main beta-sheet core and positions the requisite aspartate cleavage site at the loop's apex. Distortion or destabilization of this reactive site loop by site-directed mutagenesis converted P35 to an efficient substrate which, unlike wild-type P35, failed to interact stably with the target caspase or block protease activity. Thus, cleavage alone is insufficient for caspase inhibition. These data are consistent with a new model wherein the P35 reactive site loop participates in a unique multi-step mechanism in which the spatial orientation of the loop with respect to the P35 core determines post-cleavage association and stoichiometric inhibition of target caspases.

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

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

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.