Identification of the primary caspase 3 cleavage site in alpha II-spectrin during apoptosis

Chitinase-like proteins promoting tumorigenesis through disruption of cell polarity via enlarged endosomal vesicles

Introduction

Chitinase-like proteins (CLPs) are associated with tissue-remodeling and inflammation but also with several disorders, including fibrosis, atherosclerosis, allergies, and cancer. However, CLP's role in tumors is far from clear.

Methods

Here, we utilize Drosophila melanogaster and molecular genetics to investigate the function of CLPs (imaginal disc growth factors; Idgf's) in Ras^V12 dysplastic salivary glands.

Results and discussion

We find one of the Idgf's members, Idgf3, is transcriptionally induced in a JNK-dependent manner via a positive feedback loop mediated by reactive oxygen species (ROS). Moreover, Idgf3 accumulates in enlarged endosomal vesicles (EnVs) that promote tumor progression by disrupting cytoskeletal organization. The process is mediated via the downstream component, aSpectrin, which localizes to the EnVs. Our data provide new insight into CLP function in tumors and identifies specific targets for tumor control.

Hormonal Regulation of Programmed Cell Death in Sea Urchin Metamorphosis

Programmed cell death (PCD) has been identified as a key process in the metamorphic transition of indirectly developing organisms such as frogs and insects. Many marine invertebrate species with indirect development and biphasic life cycles face the challenge of completing the metamorphic transition of the larval body into a juvenile when they settle into the benthic habitat. Some key characteristics stand out during this transition in comparison to frogs and insects: (1) the transition is often remarkably fast and (2) the larval body is largely abandoned and few structures transition into the juvenile stage. In sea urchins, a group with a drastic and fast metamorphosis, development and destruction of the larval body is regulated by endocrine signals. Here we provide a brief review of the basic regulatory mechanisms of PCD in animals. We then narrow our discussion to metamorphosis with a specific emphasis on sea urchins with indirect life histories and discuss the function of thyroid hormones and histamine in larval development, metamorphosis and settlement of the sea urchin Strongylocentrotus purpuratus. We were able to annotate the large majority of PCD related genes in the sea urchin S. purpuratus and ongoing studies on sea urchin metamorphosis will shed light on the regulatory architecture underlying this dramatic life history transition. While we find overwhelming evidence for hormonal regulation of PCD in animals, especially in the context of metamorphosis, the mechanisms in many marine invertebrate groups with indirect life histories requires more work. Hence, we propose that studies of PCD in animals requires functional studies in whole organisms rather than isolated cells. We predict that future work, targeting a broader array of organisms will not only help to reveal important new functions of PCD but provide a fundamentally new perspective on its use in a diversity of taxonomic, developmental, and ecological contexts.

Identification of potential chemical compounds enhancing generation of enucleated cells from immortalized human erythroid cell lines

Immortalized erythroid cell lines are expected to be a promising source of ex vivo manufactured red blood cells (RBCs), however the induction of enucleation in these cell lines is inefficient at present. We utilized an imaging-based high-throughput system to identify chemical compounds that trigger enucleation of human erythroid cell lines. Among >3,300 compounds, we identified multiple histone deacetylase inhibitors (HDACi) inducing enucleated cells from the cell line, although an increase in membrane fragility of enucleated cells was observed. Gene expression profiling revealed that HDACi treatment increased the expression of cytoskeletal genes, while an erythroid-specific cell membrane protein, SPTA1, was significantly down-regulated. Restoration of SPTA1 expression using CRISPR-activation partially rescued the fragility of cells and thereby improved the enucleation efficiency. Our observations provide a potential solution for the generation of mature cells from erythroid cell lines, contributing to the future realization of the use of immortalized cell lines for transfusion therapies.

In an imaging-based screen of >3,300 compounds compounds, Soboleva et al identify HDAC inhibitors as mediators of erythroid cell enucleation. They further show that the erythroid-specific cell membrane protein, SPTA1, is downregulated in HDAC inhibited cells and that restoration of SPTA1 expression using CRISPR-activation partially rescues the fragility of cells, improving enucleation efficiency.

Quantitative studies of caspase-3 catalyzed αII-spectrin breakdown

Under various physiological and patho-physiological conditions, spectrin breakdown reactions generate several spectrin breakdown products (SBDPs)-in particular SBDPs of 150 kDa (SBDP150) and 120 kDa (SBDP120). Recently, numerous studies have shown that reactions leading to SBDPs are physiologically relevant, well regulated, and complex. Yet molecular studies on the mechanism of the SBDP formation are comparatively scarce. We have designed basic systems to allow us to follow the breakdown of αII-spectrin model proteins by caspase-3 in detail with gel electrophoresis, fluorescence and mass spectrometry methods. Amongst the predicted and reported sites, our results show that caspase-3 cleaves after residues D1185 and D1478, but not after residues D888, D1340 and D1475. We also found that the cleavage at these two sites is independent of each other. It may be possible to inhibit one site without affecting the other site. Cleavage after residue D1185 in intact αII-spectrin leads to SBDP150, and cleavage after D1478 site leads to SBDP120. Our results also show that the cleavage after the D1185 residue is unusually efficient, with a kcat/KM value of 40,000 M(-1) s(-1), and the cleavage after the D1478 site is more similar to most of the other reported caspase-3 substrates, with a kcat/KM value of 3000 M(-1) s(-1). We believe that this study lays out a methodology and foundation to study caspase-3 catalyzed spectrin breakdown to provide quantitative information. Molecular understanding may lead to better understanding of brain injuries and more precise and specific biomarker development.

NAD+ and nicotinamide: sex differences in cerebral ischemia

BACKGROUND

Previous literature suggests that cell death pathways activated after cerebral ischemia differ between the sexes. While caspase-dependent mechanisms predominate in the female brain, caspase-independent cell death induced by the activation of poly(ADP-ribose) polymerase (PARP) predominates in the male brain. PARP-1 gene deletion decreases infarction volume in the male brain, but paradoxically increases damage in PARP-1 knockout females.

PURPOSE

This study examined stroke-induced changes in NAD+, a key energy molecule involved in PARP-1 activation in both sexes.

METHODS

Mice were subjected to middle cerebral artery occlusion and NAD+ levels were assessed. Caspase-3 activity and nuclear translocation were assessed 6h after ischemia. In additional cohorts, Nicotinamide (500 mg/kg i.p.) a precursor of NAD+ or vehicle was administered and infarction volume was measured 24h after ischemia.

RESULTS

Males have higher baseline NAD+ levels than females. Significant stroke-induced NAD+ depletion occurred in males and ovariectomized females but not in intact females. PARP-1 deletion prevented the stroke-induced loss in NAD+ in males, but worsened NAD+ loss in PARP-1 deficient females. Preventing NAD+ loss with nicotinamide reduced infarct in wild-type males and PARP-1 knockout mice of both sexes, with no effect in WT females. Caspase-3 activity was significantly increased in PARP-1 knockout females compared to males and wild-type females, this was reversed with nicotinamide.

CONCLUSIONS

Sex differences exist in baseline and stroke-induced NAD+ levels. Nicotinamide protected males and PARP knockout mice, but had minimal effects in the wild-type female brain. This may be secondary to differences in energy metabolism between the sexes.

Allosteric peptides bind a caspase zymogen and mediate caspase tetramerization

The caspases are a family of cytosolic proteases with essential roles in inflammation and apoptosis. Drug discovery efforts have focused on developing molecules directed against the active sites of caspases, but this approach has proved challenging and has not yielded any approved therapeutics. Here we describe a new strategy for generating inhibitors of caspase-6, a potential therapeutic target in neurodegenerative disorders, by screening against its zymogen form. Using phage display to discover molecules that bind the zymogen, we report the identification of a peptide that specifically impairs the function of caspase-6 in vitro and in neuronal cells. Remarkably, the peptide binds at a tetramerization interface that is uniquely present in zymogen caspase-6, rather than binding into the active site, and acts via a new allosteric mechanism that promotes caspase tetramerization. Our data illustrate that screening against the zymogen holds promise as an approach for targeting caspases in drug discovery.

Proteome analysis reveals protein candidates involved in early stages of brain regeneration of teleost fish

Exploration of the molecular dynamics underlying regeneration in the central nervous system of regeneration-competent organisms has received little attention thus far. By combining a cerebellar lesion paradigm with differential proteome analysis at a post-lesion survival time of 30 min, we screened for protein candidates involved in the early stages of regeneration in the cerebellum of such an organism, the teleost fish Apteronotus leptorhynchus. Out of 769 protein spots, the intensity of 26 spots was significantly increased by a factor of at least 1.5 in the lesioned hemisphere, relative to the intact hemisphere. The intensity of 9 protein spots was significantly reduced by a factor of at least 1.5. The proteins associated with 15 of the spots were identified by peptide mass fingerprinting and/or tandem mass spectrometry, resulting in the identification of a total of 11 proteins. Proteins whose abundance was significantly increased include: erythrocyte membrane protein 4.1N, fibrinogen gamma polypeptide, fructose-biphosphate aldolase C, alpha-internexin neuronal intermediate filament protein, major histocompatibility complex class I heavy chain, 26S proteasome non-ATPase regulatory subunit 8, tubulin alpha-1C chain, and ubiquitin-specific protease 5. Proteins with significantly decreased levels of abundance include: brain glycogen phosphorylase, neuron-specific calcium-binding protein hippocalcin, and spectrin alpha 2. We hypothesize that these proteins are involved in energy metabolism, blood clotting, electron transfer in oxidative reactions, cytoskeleton degradation, apoptotic cell death, synaptic plasticity, axonal regeneration, and promotion of mitotic activity.

miR-23a regulation of X-linked inhibitor of apoptosis (XIAP) contributes to sex differences in the response to cerebral ischemia

It is increasingly recognized that the mechanisms underlying ischemic cell death are sexually dimorphic. Stroke-induced cell death in males is initiated by the mitochondrial release of apoptosis-inducing factor, resulting in caspase-independent cell death. In contrast, ischemic cell death in females is primarily triggered by mitochondrial cytochrome c release with subsequent caspase activation. Because X-linked inhibitor of apoptosis (XIAP) is the primary endogenous inhibitor of caspases, its regulation may play a unique role in the response to injury in females. XIAP mRNA levels were higher in females at baseline. Stroke induced a significant decrease in XIAP mRNA in females, whereas no changes were seen in the male brain. However, XIAP protein levels were decreased in both sexes after stroke. MicroRNAs (miRNAs) predominantly induce translational repression and are emerging as a major regulators of mRNA and subsequent protein expression after ischemia. The miRNA miR-23a was predicted to bind XIAP mRNA. miR-23a directly bound the 3' UTR of XIAP, and miR-23a inhibition led to an increase in XIAP mRNA in vitro, demonstrating that XIAP is a previously uncharacterized target for miR-23a. miR-23a levels differed in male and female ischemic brains, providing evidence for sex-specific miRNA expression in stroke. Embelin, a small-molecule inhibitor of XIAP, decreased the interaction between XIAP and caspase-3 and led to enhanced caspase activity. Embelin treatment significantly exacerbated stroke-induced injury in females but had no effect in males, demonstrating that XIAP is an important mediator of sex-specific responses after stroke.

Increased apoptosis of myoblasts in Drosophila model for the Walker-Warburg syndrome

Walker-Warburg syndrome, a progressive muscular dystrophy, is a severe disease with various kinds of symptoms such as muscle weakness and occasional seizures. The genes of protein O-mannosyltransferases 1 and 2 (POMT1 and POMT2), fukutin, and fukutin-related protein are responsible for this syndrome. In our previous study, we cloned Drosophila orthologs of human POMT1 and POMT2 and identified their activity. However, the mechanism of onset of this syndrome is not well understood. Furthermore, little is known about the behavioral properties of the Drosophila POMT1 and POMT2 mutants, which are called rotated abdomen (rt) and twisted (tw), respectively. First, we performed various kinds of behavioral tests and described in detail the muscle structures by using these mutants. The mutant flies exhibited abnormalities in heavy exercises such as climbing or flight but not in light movements such as locomotion. Defective motor function in mutants appeared immediately after eclosion and was exaggerated with aging. Along with motor function, muscle ultrastructure in the tw mutant was altered, as seen in human patients. We demonstrated that expression of RNA interference (RNAi) for the rt gene and the tw mutant was almost completely lethal and semi-lethal, respectively. Flies expressing RNAi had reduced lifespans. These findings clearly demonstrate that Drosophila POMT mutants are models for human muscular dystrophy. We then observed a high density of myoblasts with an enhanced degree of apoptosis in the tw mutant, which completely lost enzymatic activity. In this paper, we propose a novel mechanism for the development of muscular dystrophy: POMT mutation causes high myoblast density and position derangement, which result in apoptosis, muscle disorganization, and muscle cell defects.

Aggregation of spectrin and PKCtheta is an early hallmark of fludarabine/mitoxantrone/dexamethasone-induced apoptosis in Jurkat T and HL60 cells

It has been shown that changes in spectrin distribution in early apoptosis preceded changes in membrane asymmetry and phosphatidylserine (PS) exposure. PKCtheta was associated with spectrin during these changes, suggesting a possible role of spectrin/PKCtheta aggregation in regulation of early apoptotic events. Here we dissect this hypothesis using Jurkat T and HL60 cell lines as model systems. Immunofluorescent analysis of alphaIIbetaII spectrin arrangement in Jurkat T and HL60 cell lines revealed the redistribution of spectrin and PKCtheta into a polar aggregate in early apoptosis induced by fludarabine/mitoxantrone/dexamethasone (FND). The appearance of an alphaIIbetaII spectrin fraction that was insoluble in a non-ionic detergent (1% Triton X-100) was observed concomitantly with spectrin aggregation. The changes were observed within 2 h after cell exposure to FND, and preceded PS exposure. The changes seem to be restricted to spectrin and not to other cytoskeletal proteins such as actin or vimentin. In studies of the mechanism of these changes, we found that (i) neither changes in apoptosis regulatory genes (e.g., Bcl-2 family proteins) nor changes in cytoskeleton-associated proteins were detected in gene expression profiling of HL60 cells after the first hour of FND treatment, (ii) caspase-3, -7, -8, and -10 had minor involvement in the early apoptotic rearrangement of spectrin/PKCtheta, and (iii) spectrin aggregation was shown to be partially dependent on PKCtheta activity. Our results indicate that spectrin/PKCtheta aggregate formation is related to an early stage in drug-induced apoptosis and possibly may be regulated by PKCtheta activity. These findings indicate that spectrin/PKCtheta aggregation could be considered as a hallmark of early apoptosis and presents the potential to become a useful diagnostic tool for monitoring efficiency of chemotherapy as early as 24 h after treatment.

The SH3 domain of alphaII spectrin is a target for the Fanconi anemia protein, FANCG

The structural protein nonerythroid alpha spectrin (alphaIISp) plays a role in the repair of DNA interstrand cross-links and is deficient in cells from patients with Fanconi anemia (FA), in which there is a defect in ability to repair such cross-links. We have proposed a model in which alphaIISp, whose stability is dependent on FA proteins, acts as a scaffold to aid in recruitment of repair proteins to sites of damage. In order to get a clearer understanding of the proposed role of FA proteins in maintaining stability of alphaIISp, yeast two-hybrid analysis was carried out to determine whether FA proteins directly interact with alphaIISp and, if so, to map the sites of interaction. Four overlapping regions of alphaIISp were constructed. FANCG interacted with one of these regions and specifically with the SH3 domain in this region of alphaIISp. The site of interaction in FANCG was mapped to a motif that binds to SH3 domains and contains a consensus sequence with preference for the SH3 domain of alphaIISp. This site of interaction was confirmed using site-directed mutagenesis. Two FA proteins that did not contain motifs that bind to SH3 domains, FANCC and FANCF, did not interact with the SH3 domain of alphaIISp. These results demonstrate that one of the FA proteins, FANCG, contains a motif that interacts directly with the SH3 domain of alphaIISp. We propose that this binding of FANCG to alphaIISp may be important for the stability of alphaIISp in cells and the role alphaIISp plays in the DNA repair process.

Comparative proteomics analysis of caspase-9-protein complexes in untreated and cytochrome c/dATP stimulated lysates of NSCLC cells

Apoptosis is a process of cellular suicide executed by caspases. Impaired activation of caspase-9 may contribute to chemoresistance in cancer. Activation of caspase-9 occurs after binding to Apaf-1 and formation of the apoptosome in the presence of cytochrome c/(d)ATP. We used a proteomics approach to identify proteins in caspase-9-protein complexes in extracts derived from NSCLC cells with(out) cytochrome c/dATP. Using co-immunoprecipitation, one-dimensional gel electrophoresis and tandem mass spectrometry, 38 proteins were identified of which 24 differential interactors. The differential interactors can be functionally assigned to cytoskeletal (re)organization and cell motility, catalytic activity, and transcriptional processes and apoptosis. The interaction of caspase-9 with Apaf-1 was confirmed and acetylserotonin-O-methyltransferase-like protein was identified as a candidate substrate of caspase-9. Novel interactors were found including galectin-3, swiprosin-1 and the membrane-cytoskeleton linkers Ezrin/Radixin/Moesin. Co-immunoprecipitation and Western blot experiments confirmed the interaction of caspase-9 with several identified binding partners. A large number of cytoskeletal proteins associated with unprocessed caspase-9 may indicate a scaffold function of this structure and/or may act as caspase substrates during apoptosis. Together, our results indicate that proteomic analysis of the caspase-9-associated protein complexes is a powerful exploratory approach to identify novel caspase substrates and/or regulators of caspase-9-dependent apoptosis.

E2F2 expression induces proliferation of terminally differentiated cardiomyocytes in vivo

AIMS

In previous experiments we have demonstrated that expression of the transcription factors E2F2 and E2F4 is sufficient to induce proliferation of isolated primary cardiomyocytes from newborn rats and mice. We now wanted to analyse whether E2F2 or E2F4 are also able to promote cell cycle progression of adult cardiomyocytes in vivo, which unlike cardiomyocytes from newborn rodents lack the ability to undergo cell proliferation.

METHODS AND RESULTS

E2F2 or E2F4 was expressed in hearts of mice at different developmental stages using adenoviral vectors. Effects regarding proliferation, hypertrophy, and apoptosis were analysed on histological sections, and quantitative assessment of cell cycle regulatory genes was performed by real-time PCR (polymerase chain reaction) and western blot. We found that both E2F2 and E2F4 can stimulate hypertrophic cell growth of cardiomyocytes. However, only directed expression of E2F2 but not of E2F4 was sufficient to induce proliferation of cardiomyocytes. Expression of E2F2 in vivo did not increase the percentage of apoptotic cardiomyocytes but down-regulated the expression of the pro-apoptotic genes caspase-6 and apaf-1. Further analysis of the cell cycle regulatory machinery revealed that expression of E2F2 caused a strong induction of cyclin A and E while the expression of cyclin-dependent kinase inhibitors (CKIs) such as p21 was not affected.

CONCLUSION

We conclude that a limited induction of cardiomyocyte cell proliferation can be achieved by E2F2-mediated stimulation of cyclin A and E expression without a reduction of CKIs.

Prostaglandin-induced VASP phosphorylation controls alpha II-spectrin breakdown in apoptotic cells

In pathological conditions, the inflammatory mediator prostaglandin E2 (PGE2) has been shown to induce apoptosis through a cAMP-dependent pathway. However, underlying mechanisms have remained illusive. Irrespective whether apoptosis is induced by the intrinsic or extrinsic pathway, the cysteine protease caspase-3 becomes activated and cleaves many key proteins including spectrins. Cleavage of the plasma membrane-associated spectrins leads to cell shrinkage, membrane blebbing, the formation of apoptotic bodies, and irreversible cell death. Recently, we identified a novel interaction between alpha II-spectrin and vasodilator-stimulated phosphoprotein (VASP), which is abrogated by the cAMP-dependent protein kinase (PKA)-mediated phosphorylation of VASP. In the present study we investigated whether VASP binding to alpha II-spectrin affects spectrin breakdown in PGE2-induced apoptosis. PGE2 dose- and time-dependently triggered VASP phosphorylation. Following induction of apoptosis, caspase-3-mediated alpha II-spectrin breakdown and membrane blebbing were markedly delayed in wild-type as compared to VASP-deficient endothelial cells. This suggests that VASP binding to alpha II-spectrin attenuates alpha II-spectrin cleavage in apoptotic cells and that PGE2-induced VASP phosphorylation regulates this process. Our findings may therefore provide the molecular basis for PGE2-induced apoptosis in pathological events.

Neuroprotective Mechanism of Taurine due to Up-regulating Calpastatin and Down-regulating Calpain and Caspase-3 during Focal Cerebral Ischemia

AIMS

Taurine as an endogenous substance possesses a number of cytoprotective properties. In the study, we have evaluated the neuroprotective effect of taurine and investigated whether taurine exerted neuroprotection through affecting calpain/calpastatin or caspase-3 actions during focal cerebral ischemia, since calpain and caspase-3 play central roles in ischemic neuronal death.

METHODS

Male Sprague-Dawley rats were subjected to 2 h of middle cerebral artery occlusion (MCAo), and 22 h of reperfusion. Taurine was administrated intravenously 1 h after MCAo. The dose-responses of taurine to MCAo were determined. Next, the effects of taurine on the activities of calpain, calpastatin and caspase-3, the levels of calpastatin, microtubule-associated protein-2 (MAP-2) and alphaII-spectrin, and the apoptotic cell death in penumbra were evaluated.

RESULTS

Taurine reduced neurological deficits and decreased the infarct volume 24 h after MCAo in a dose-dependent manner. Treatment with 50 mg/kg of taurine significantly increased the calpastatin protein levels and activities, and markedly reduced the m-calpain and caspase-3 activities in penumbra 24 h after MCAo, however, it had no significant effect on mu-calpain activity. Moreover, taurine significantly increased the MAP-2 and alphaII-spectrin protein levels, and markedly reduced the ischemia-induced TUNEL staining positive score within penumbra 24 h after MCAo.

CONCLUSIONS

Our data demonstrate the dose-dependent neuroprotection of taurine against transient focal cerebral ischemia, and suggest that one of protective mechanisms of taurine against ischemia may be blocking the m-calpain and caspase-3-mediated apoptotic cell death pathways.

Dose- and time-dependent effects of TNFα and actinomycin D on cell death incidence and embryo growth in mouse blastocysts

This study was undertaken to obtain information about characteristics of different types of induced apoptosis in preimplantation embryos. Freshly isolated mouse blastocysts were culturedin vitrowith the addition of two apoptotic inductors – TNFα and actinomycin D – at various doses and times. The average number of nuclei and the percentage of dead cells were evaluated in treated embryos. Classification of dead cells was based on morphological assessment of their nuclei evaluated by fluorescence microscopy, the detection of specific DNA degradation (TUNEL assay), the detection of active caspase-3 and cell viability assessed by propidium iodide staining. The addition of both apoptotic inductors into culture media significantly increased cell death incidence in blastocysts. Their effects were dose and time dependent. Lower concentrations of inductors increased cell death incidence, usually without affecting embryo growth after 24 h culture. Higher concentrations of inductors caused wider cell damage and also retarded embryo development. In all experiments, the negative effect of actinomycin D on blastomere survival and blastocyst growth was greater than the effect of TNFα. Furthermore, the addition of actinomycin D into culture media increased cell death incidence even after 6 h culture. Differences resulted probably from diverse specificity of apoptotic inductors. The majority of dead cells in treated blastocysts were of apoptotic origin. Morphological and biochemical features of apoptotic cell death induced by both TNFα and actinomycin D were similar and had homologous profile. In blastomeres, similarly to somatic cells, the biochemical pathways of induced apoptosis included activation of caspase-3 and internucleosomal DNA fragmentation.

A mutant alphaII-spectrin designed to resist calpain and caspase cleavage questions the functional importance of this process in vivo

alpha- and beta-spectrins are components of molecular scaffolds located under the lipid bilayer and named membrane skeletons. Disruption of these scaffolds through mutations in spectrins demonstrated that they are involved in the membrane localization or the maintenance of proteins associated with them. The ubiquitous alphaII-spectrin chain bears in its central region a unique domain that is sensitive to several proteases such as calpains or caspases. The conservation of this region in vertebrates suggests that the proteolysis of alphaII-spectrin by these enzymes could be involved in important functions. To assess the role of alphaII-spectrin cleavage in vivo, we generated a murine model in which the exons encoding the region defining this cleavage sensitivity were disrupted by gene targeting. Surprisingly, homozygous mice expressing this mutant alphaII-spectrin appeared healthy, bred normally, and had no histological anomaly. Remarkably, the mutant alphaII-spectrin assembles correctly into the membrane skeleton, thus challenging the notion that this region is required for the stable biogenesis of the membrane skeleton in nonerythroid cells. Our finding also argues against a critical role of this particular alphaII-spectrin cleavage in either major cellular functions or in normal development.

Interferon-gamma sensitizes the human salivary gland cell line, HSG, to tumor necrosis factor-alpha induced activation of dual apoptotic pathways

Activated immune cells secrete proinflammatory cytokines such as tumor necrosis factor alpha (TNF-alpha), interferon-gamma (IFN-gamma) and Fas ligand (FasL) and these cytokines have been reported to induce apoptosis in numerous cell types. Apoptotic cell death has been associated with the progression of numerous autoimmune diseases. Proinflammatory cytokines are reportedly involved in apoptosis in the salivary glands of patients with Sjögren's syndrome (SS); an autoimmune disorder characterized by the destruction of salivary and lachrymal glands. In this study, we used the HSG cell line to determine if exposure to proinflammatory cytokines induces apoptosis in human salivary gland cells. In addition, we identified the mediators controlling the apoptotic process in response to TNF alpha and IFN gamma. TNF-alpha and IFN-gamma induced apoptosis in HSG cells and resulted in the activation of caspase 8 and the "death receptor" pathway. We further determined that caspase 9 and the "mitochondrial" pathway was also activated. Induction of the intrinsic and extrinsic pathways in HSG cells resulted in substrate cleavage by effector caspases, in particular the cleavage of alpha II spectrin, an autoantigen in Sjögren's syndrome. Our results suggest that HSG cells provide a model system to study processes regulating proinflammatory cytokine-induced apoptotic cell death.

Hypoxia-Induced Cytoskeleton Disruption in Alveolar Epithelial Cells

Alveolar hypoxia, a common feature of many respiratory disorders, has been previously reported to induce functional changes, particularly a decrease of transepithelial Na and fluid transport. In polarized epithelia, cytoskeleton plays a regulatory role in transcellular and paracellular transport of ions and fluid. We hypothesized that exposure to hypoxia could damage cytoskeleton organization, which in turn, may adversely affect ion and fluid transport. Primary rat alveolar epithelial cells (AEC) were exposed to either mild (3% O(2)) or severe (0.5% O(2)) hypoxia for 18 h or to normoxia (21% O(2)). First, mild and severe hypoxia induced a disorganization of actin, a major protein of the cytoskeleton, reflected by disruption of F-actin filaments. Second, alpha-spectrin, an apical cytoskeleton protein, which binds to actin cytoskeleton and Na transport proteins, was cleaved by hypoxia. Pretreatment of AEC by a caspase inhibitor (z-VAD-fmk; 90 microM) blunted hypoxia-induced spectrin cleavage as well as hypoxia-induced decrease in surface membrane alpha-ENaC and concomitantly induced a partial recovery of hypoxia-induced decrease of amiloride-sensitive Na transport at 3% O(2). Finally, tight junctions (TJs) proteins, which are linked to actin and are a determinant of paracellular permeability, were altered by mild and severe hypoxia: hypoxia induced a mislocalization of occludin from the TJ to cytoplasm and a decrease in zonula occludens-1 protein level. These modifications were associated with modest changes in paracellular permeability at 0.5% O(2,) as assessed by small 4-kD dextran flux and transepithelial resistance measurements. Together, these findings indicate that hypoxia disrupted cytoskeleton and TJ organization in AEC and may participate, at least in part, to hypoxia-induced decrease in Na transport.

Structure of the calmodulin alphaII-spectrin complex provides insight into the regulation of cell plasticity

AlphaII-spectrin is a major cortical cytoskeletal protein contributing to membrane organization and integrity. The Ca2+-activated binding of calmodulin to an unstructured insert in the 11th repeat unit of alphaII-spectrin enhances the susceptibility of spectrin to calpain cleavage but abolishes its sensitivity to several caspases and to at least one bacterially derived pathologic protease. Other regulatory inputs including phosphorylation by c-Src also modulate the proteolytic susceptibility of alphaII-spectrin. These pathways, acting through spectrin, appear to control membrane plasticity and integrity in several cell types. To provide a structural basis for understanding these crucial biological events, we have solved the crystal structure of a complex between bovine calmodulin and the calmodulin-binding domain of human alphaII-spectrin (Protein Data Bank ID code 2FOT). The structure revealed that the entire calmodulin-spectrin-binding interface is hydrophobic in nature. The spectrin domain is also unique in folding into an amphiphilic helix once positioned within the calmodulin-binding groove. The structure of this complex provides insight into the mechanisms by which calmodulin, calpain, caspase, and tyrosine phosphorylation act on spectrin to regulate essential cellular processes.

Caspase-8 promotes cell motility and calpain activity under nonapoptotic conditions

Significant caspase-8 activity has been found in normal and certain tumor cells, suggesting that caspase-8 possesses an alternative, nonapoptotic function that may contribute to tumor progression. In this article, we report that caspase-8 promotes cell motility. In particular, caspase-8 is required for the optimal activation of calpains, Rac, and lamellipodial assembly. This represents a novel nonapoptotic function of caspase-8 acting at the intersection of the caspase-8 and calpain proteolytic pathways to coordinate cell death versus cell motility signaling.

Pharmacologically induced calcium oscillations protect neurons from increases in cytosolic calcium after trauma

Increases in cytosolic calcium ([Ca(2+)](i)) following mechanical injury are often considered a major contributing factor to the cellular sequelae in traumatic brain injury (TBI). However, very little is known on how developmental changes may affect the calcium signaling in mechanically injured neurons. One key feature in the developing brain that may directly impact its sensitivity to stretch is the reduced inhibition which results in spontaneous [Ca(2+)](i) oscillations. In this study, we examined the mechanism of stretch-induced [Ca(2+)](i) transients in 18-days in vitro (DIV) neurons exhibiting bicuculline-induced [Ca(2+)](i) oscillations. We used an in vitro model of mechanical trauma to apply a defined uniaxial strain to cultured cortical neurons and used increases in [Ca(2+)](i) as a measure of the neuronal response to the stretch insult. We found that stretch-induced increases in [Ca(2+)](i) in 18-DIV neurons were inhibited by pretreatment with either the NMDA receptor antagonist, APV [D(-)-2-Amino-5-phosphonopentanoic acid], or by depolymerizing the actin cytoskeleton prior to stretch. Blocking synaptic NMDA receptors prior to stretch significantly attenuated most of the [Ca(2+)](i) transient. In comparison, cultures with pharmacologically induced [Ca(2+)](i) oscillations showed a substantially reduced [Ca(2+)](i) peak after stretch. We provide evidence showing that a contributing factor to this mechanical desensitization from induced [Ca(2+)](i) oscillations is the PKC-mediated uncoupling of NMDA receptors (NMDARs) from spectrin, an actin-associated protein, thereby rendering neurons insensitive to stretch. These results provide novel insights into how the [Ca(2+)](i) response to stretch is initiated, and how reduced inhibition - a feature of the developing brain - may affect the sensitivity of the immature brain to trauma.

Changes in spectrin organisation in leukaemic and lymphoid cells upon chemotherapy

The aim of the present study was to investigate changes in spectrin and protein kinase C theta; (PKC theta;) organisation in human lymphoid and leukaemic cells undergoing chemotherapeutically induced apoptosis. An analysis of spectrin arrangement in human peripheral lymphoid (non-Hodgkin lymphoma) and leukaemic (acute lymphoblastic leukaemia) cells before and after chemotherapy revealed radical differences in the distribution of this protein. By using immunofluorescent technique, in lymphocytes isolated before chemotherapy, we found spectrin evenly distributed in the cytoplasm and the plasma membrane, while after the therapy changes in spectrin organisation occurred. Moreover, in lymphocytes after chemotherapy, extraction with buffer containing non-ionic detergent (Triton X-100) revealed presence of an insoluble fraction of spectrin. In normal or malignant cells before chemotherapy spectrin was totally soluble, however it should be mentioned that in total cell extracts and supernatants (but not in pellets) apoptotic fragments of spectrin (in addition to intact alpha and beta chains) were also found. In malignant cells after chemotherapy changes in PKC theta; organisation, similar to this observed in the case of spectrin, were shown by the immunofluorescence technique. In contrast, no differences in the distribution of other isoforms of protein kinase C: betaI and betaII, before and after chemotherapy, were found. Apoptotic phosphatidyloserine (PS) externalisation, as well as cell shrinkage, membrane protrusions and blebbing were observed in lymphocytes after chemotherapy and treatment with cytostatics in vitro. The overall results may suggest that spectrin redistribution/aggregation is the phenomenon involved in programmed cell death (PCD) of normal and neoplastic lymphocytes and lymphoblasts, however molecular basis of this phenomenon should be further investigated.

AlphaII-spectrin is an in vitro target for caspase-2, and its cleavage is regulated by calmodulin binding

The spectrin-actin scaffold underlying the lipid bilayer is considered to participate in cell-shape stabilization and in the organization of specialized membrane subdomains. These structures are dynamic and likely to undergo frequent remodelling during changes in cell shape. Proteolysis of spectrin, which occurs during apoptosis, leads to destabilization of the scaffold. It is also one of the major processes involved in membrane remodelling. Spectrins, the main components of the membrane skeleton, are the targets for two important protease systems: m- and micro-calpains (Ca2+-activated proteases) and caspase-3 (activated during apoptosis). In this paper, we show that caspase-2 also targets spectrin in vitro, and we characterize Ca2+/calmodulin-dependent regulation of spectrin cleavage by caspases. Yeast two-hybrid screening reveals that the large isoform (1/L) of procaspase-2 specifically binds to alphaII-spectrin, while the short isoform does not. Like caspase-3, caspase-2 cleaves alphaII-spectrin in vitro at residue Asp-1185. This study emphasizes a role of executioner caspase for caspase-2. We also demonstrated that the executioner caspase-7 but not caspase-6 cleaves spectrin at residue Asp-1185 in vitro. This spectrin cleavage by caspases 2, 3 and 7 is inhibited by the Ca2+-dependent binding of calmodulin to spectrin. In contrast, calmodulin binding enhances spectrin cleavage by calpain at residue Tyr-1176. These results indicate that alphaII-spectrin cleavage is highly influenced by Ca2+ homoeostasis and calmodulin, which therefore represent potential regulators of the stability and the plasticity of the spectrin-based skeleton.