Intestinal-specific activatable Myb initiates colon tumorigenesis in mice

Transcription factor Myb is overexpressed in most colorectal cancers (CRC). Patients with CRC expressing the highest Myb are more likely to relapse. We previously showed that mono-allelic loss of Myb in an Adenomatous polyposis coli (APC)-driven CRC mouse model (Apc^Min/+) significantly improves survival. Here we directly investigated the association of Myb with poor prognosis and how Myb co-operates with tumor suppressor genes (TSGs) (Apc) and cell cycle regulator, p27. Here we generated the first intestinal-specific, inducible transgenic model; a MybER transgene encoding a tamoxifen-inducible fusion protein between Myb and the estrogen receptor-α ligand-binding domain driven by the intestinal-specific promoter, Gpa33. This was to mimic human CRC with constitutive Myb activity in a highly tractable mouse model. We confirmed that the transgene was faithfully expressed and inducible in intestinal stem cells (ISCs) before embarking on carcinogenesis studies. Activation of the MybER did not change colon homeostasis unless one p27 allele was lost. We then established that MybER activation during CRC initiation using a pro-carcinogen treatment, azoxymethane (AOM), augmented most measured aspects of ISC gene expression and function and accelerated tumorigenesis in mice. CRC-associated symptoms of patients including intestinal bleeding and anaemia were faithfully mimicked in AOM-treated MybER transgenic mice and implicated hypoxia and vessel leakage identifying an additional pathogenic role for Myb. Collectively, the results suggest that Myb expands the ISC pool within which CRC is initiated while co-operating with TSG loss. Myb further exacerbates CRC pathology partly explaining why high MYB is a predictor of worse patient outcome.

Loss of c-REL but not NF-κB2 prevents autoimmune disease driven by FasL mutation

FASL/FAS signaling imposes a critical barrier against autoimmune disease and lymphadenopathy. Mutant mice unable to produce membrane-bound FASL (FasL(Δm/Δm)), a prerequisite for FAS-induced apoptosis, develop lymphadenopathy and systemic autoimmune disease with immune complex-mediated glomerulonephritis. Prior to disease onset, FasL(Δm/Δm) mice contain abnormally high numbers of leukocytes displaying activated and elevated NF-κB-regulated cytokine levels, indicating that NF-κB-dependent inflammation may be a key pathological driver in this multifaceted autoimmune disease. We tested this hypothesis by genetically impairing canonical or non-canonical NF-κB signaling in FasL(Δm/Δm) mice by deleting the c-Rel or NF-κB2 genes, respectively. Although the loss of NF-κB2 reduced the levels of inflammatory cytokines and autoantibodies, the impact on animal survival was minor due to substantially accelerated and exacerbated lymphoproliferative disease. In contrast, a marked increase in lifespan resulting from the loss of c-REL coincided with a striking reduction in classical parameters of autoimmune pathology, including the levels of cytokines and antinuclear autoantibodies. Notably, the decrease in regulatory T-cell numbers associated with loss of c-REL did not exacerbate autoimmunity in FasL(Δm/Δm)c-rel(-/-) mice. These findings indicate that selective inhibition of c-REL may be an attractive strategy for the treatment of autoimmune pathologies driven by defects in FASL/FAS signaling that would be expected to circumvent many of the complications caused by pan-NF-κB inhibition.

Actions of leukaemia inhibitory factor on megakaryocyte and platelet formation

Leukaemia inhibitory factor (LIF) is able to potentiate megakaryocyte colony formation in cultures of mouse bone marrow cells in the presence of multi-CSF (interleukin 3). Membrane receptors for LIF are present on mouse megakaryocytes and receptor numbers increase with increasing maturation of the cells. When injected into normal mice at doses of 0.2-2 micrograms two to three times daily, LIF induced a rise in platelet numbers, which reached up to twice normal values during the second week of injections. This rise was preceded by a rise first in megakaryocyte progenitor numbers, then in mature megakaryocytes in the bone marrow and spleen. Injections of LIF also marginally accelerated platelet regeneration in mice pre-injected with 5-fluorouracil or subjected to whole-body irradiation and transplantation of marrow cells. In view of similar responses to LIF in parallel studies in primates, clinical trial of LIF in patients with thrombocytopenia is warranted.

Fluorescence in situ hybridization (FISH) subgroup analysis of TRIBUTE, a phase III trial of erlotinib plus carboplatin and paclitaxel in NSCLC

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Background: TRIBUTE was a phase III, placebo-controlled study of patients with previously untreated advanced NSCLC. Patients received erlotinib (E) (150 mg/d) or placebo, plus a course of 6 cycles of carboplatin and paclitaxel (CP), followed by maintenance E monotherapy (for those in the CP+E arm who were responding to treatment). 1,059 patients were randomized and treated (526 E; 533 placebo). There were no significant differences in OS, RR, or TTP between the two arms. In subgroup analyses of the pivotal 2nd line trial (BR21) of E in patients with relapsed NSCLC in which E significantly increased median survival (HR=0.71, p <0.0001), patients who scored positively for increased EGFR copy number by fluorescence in situ hybridization (FISH) exhibited prolonged OS (HR=0.44, p=0.008). Here we report on a similar subgroup analysis for TRIBUTE. Methods: FISH analysis was performed on all available tissue samples. FISH+ samples had a high level of polysomy (=four copies of the gene in =40% of cells), or gene amplification (presence of tight gene clusters, gene/chromosome per cell ratio =2, or =15 copies of the genes per cell in =10% of analyzed cells). Results: FISH analysis was successfully performed on 245 patients (121 E, 124 placebo). Outcome in the placebo patients from this subgroup was better than the overall population, suggesting that this subgroup may not be representative. Of the 100 patients (41%) that were FISH+, 33 had amplification and 67 had high polysomy. In FISH+ patients, OS was similar between those treated with CP+E and those treated with CP alone. However, FISH+ patients in the CP+E arm experienced a decrease in RR compared to those in the CP alone arm, and had a marginally significant longer TTP (HR=0.59, 0.35–0.99). The benefit in TTP appeared after approximately 6 months, during the maintenance portion of the trial. Conclusions: In this retrospective analysis, FISH+ did not predict survival benefit in TRIBUTE. A longer TTP, but a lower RR, was observed in the FISH+ patients. The lower RR in the CP+E arm in this group, taken together with the improved TTP during maintenance therapy, suggests that a non-concurrent combination approach (CP followed by E) warrants further investigation.

No significant financial relationships to disclose.

Lack of augmentation of tumor spectrum or severity in dual heterozygous Men1 and Rb1 knockout mice

To identify possible genetic interactions between the mechanisms of tumor suppression of menin and pRb, we intercrossed mice with targeted deletions of Men1 and Rb1, and compared tumor development in cohorts of animals carrying single or dual mutations of these tumor-suppressor genes. In mice lacking one copy of Men1, pancreatic islet and anterior pituitary adenomas are common. In animals lacking one copy of Rb1, intermediate pituitary and thyroid tumors occur at high frequency, with less frequent development of pancreatic islet hyperplasia and parathyroid lesions. In mice heterozygous for both Men1 and Rb1, pancreatic hyperplasia and tumors of the intermediate pituitary and thyroid occurred at high frequency. Serum measurements of calcium and glucose did not vary significantly between genotypic groups. Loss of heterozygosity at the Rb1 locus was common in pituitary and thyroid tumors, whereas loss of menin was observed in pancreatic and parathyroid lesions. The tumor spectrum in the double heterozygotes was a combination of pathologies seen in each of the individual heterozygotes, without decrease in age of onset, indicating independent, non-additive effects of the two mutations. Together with the lack of increased tumor spectrum, this suggests that menin and pRb function in a common pathway of tumor suppression.

Histopathological features of breast cancer in carriers of ATM gene variants

AIMS

Germline variants in the ataxia telangiectasia mutated (ATM) gene have been implicated in increased breast cancer risk. The aim of this study was to determine whether the histopathology of breast cancers occurring in ATM variant carriers is distinctive or resembles the described BRCA1 mutation-associated phenotype.

METHODS

The histopathological features of breast cancers occurring in ATM variant carriers from multiple-case breast cancer families were compared with matched controls. The test group included 21 cases of in situ and/or invasive cancer from carriers of either the IVS10-6T-->G, 2424V-->G or 1420L-->F ATM variants in the absence of BRCA1 or BRCA2 mutations. An additional four invasive cancers from carriers of a pathogenic BRCA1 mutation in the context of a familial ATM variant were also examined.

RESULTS

The histopathology of breast cancers in ATM variant-only carriers was not significantly different from controls and known features of BRCA1 mutation-associated cancer were rarely seen. In contrast, these features were prominent in the small group of cases with a pathogenic BRCA1 mutation.

CONCLUSIONS

Breast cancer occurring in carriers of ATM variants is not associated with distinctive histopathological features and does not resemble the tumour phenotype commonly observed in BRCA1 mutation carriers.

Genetic, functional, and histopathological evaluation of two C-terminal BRCA1 missense variants

BACKGROUND

The vast majority of BRCA1 missense sequence variants remain uncharacterized for their possible effect on protein expression and function, and therefore are unclassified in terms of their pathogenicity. BRCA1 plays diverse cellular roles and it is unlikely that any single functional assay will accurately reflect the total cellular implications of missense mutations in this gene.

OBJECTIVE

To elucidate the effect of two BRCA1 variants, 5236G>C (G1706A) and 5242C>A (A1708E) on BRCA1 function, and to survey the relative usefulness of several assays to direct the characterisation of other unclassified variants in BRCA genes.

METHODS AND RESULTS

Data from a range of bioinformatic, genetic, and histopathological analyses, and in vitro functional assays indicated that the 1708E variant was associated with the disruption of different cellular functions of BRCA1. In transient transfection experiments in T47D and 293T cells, the 1708E product was mislocalised to the cytoplasm and induced centrosome amplification in 293T cells. The 1708E variant also failed to transactivate transcription of reporter constructs in mammalian transcriptional transactivation assays. In contrast, the 1706A variant displayed a phenotype comparable to wildtype BRCA1 in these assays. Consistent with functional data, tumours from 1708E carriers showed typical BRCA1 pathology, while tumour material from 1706A carriers displayed few histopathological features associated with BRCA1 related tumours.

CONCLUSIONS

A comprehensive range of genetic, bioinformatic, and functional analyses have been combined for the characterisation of BRCA1 unclassified sequence variants. Consistent with the functional analyses, the combined odds of causality calculated for the 1706A variant after multifactorial likelihood analysis (1:142) indicates a definitive classification of this variant as "benign". In contrast, functional assays of the 1708E variant indicate that it is pathogenic, possibly through subcellular mislocalisation. However, the combined odds of 262:1 in favour of causality of this variant does not meet the minimal ratio of 1000:1 for classification as pathogenic, and A1708E remains formally designated as unclassified. Our findings highlight the importance of comprehensive genetic information, together with detailed functional analysis for the definitive categorisation of unclassified sequence variants. This combination of analyses may have direct application to the characterisation of other unclassified variants in BRCA1 and BRCA2.

Defective gp130-mediated signal transducer and activator of transcription (STAT) signaling results in degenerative joint disease, gastrointestinal ulceration, and failure of uterine implantation

The receptor subunit gp130 transduces multiple cell type-specific activities of the leukemia inhibitory factor (LIF)/interleukin (IL)-6 family of cytokines through the signal transducer and activator of transcription (STAT) and src homology 2 domain-bearing protein tyrosine phosphatase (SHP)-2/ras/Erk pathways. To define STAT-dependent physiological responses, we generated mice with a COOH-terminal gp130(DeltaSTAT) "knock-in" mutation which deleted all STAT-binding sites. gp130(DeltaSTAT) mice phenocopyed mice deficient for IL-6 (impaired humoral and mucosal immune and hepatic acute phase responses) and LIF (failure of blastocyst implantation). However, unlike mice with null mutations in any of the components in the gp130 signaling pathway, gp130(DeltaSTAT) mice also displayed gastrointestinal ulceration and a severe joint disease with features of chronic synovitis, cartilaginous metaplasia, and degradation of the articular cartilage. Mitogenic hyperresponsiveness of synovial cells to the LIF/IL-6 family of cyto-kines was caused by sustained gp130-mediated SHP-2/ras/Erk activation due to impaired STAT-mediated induction of suppressor of cytokine signaling (SOCS) proteins which normally limits gp130 signaling. Therefore, the joint pathology in gp130(DeltaSTAT) mice is likely to arise from the disturbance of the otherwise balanced activation of the SHP-2/ras/Erk and STAT signaling cascades emanating from gp130.

Atm knock-in mice harboring an in-frame deletion corresponding to the human ATM 7636del9 common mutation exhibit a variant phenotype

ATM, the gene mutated in the human immunodeficiency disorder ataxia-telangiectasia (A-T), plays a central role in recognizing ionizing radiation damage in DNA and in controlling several cell cycle checkpoints. We describe here a murine model in which a nine-nucleotide in-frame deletion has been introduced into the Atm gene by homologous recombination followed by removal of the selectable marker cassette by Cre-loxP site-specific, recombination-mediated excision. This mouse, Atm-DeltaSRI, was designed as a model of one of the most common deletion mutations (7636del9) found in A-T patients. The murine Atm deletion results in the loss of three amino acid residues (SRI; 2556-2558) but produces near full-length detectable Atm protein that lacks protein kinase activity. Radiosensitivity was observed in Atm-DeltaSRI mice, whereas the immunological profile of these mice showed greater heterogeneity of T-cell subsets than observed in Atm(-/-) mice. The life span of Atm-DeltaSRI mice was significantly longer than that of Atm(-/-) mice when maintained under nonspecific pathogen-free conditions. This can be accounted for by a lower incidence of thymic lymphomas in Atm-DeltaSRI mice up to 40 weeks, after which time the animals died of other causes. The thymic lymphomas in Atm-DeltaSRI mice were characterized by extensive apoptosis, which appears to be attributable to an increased number of cells expressing Fas ligand. A variety of other tumors including B-cell lymphomas, sarcomas, and carcinomas not seen in Atm(-/-) mice were observed in older Atm-DeltaSRI animals. Thus, expression of mutant protein in Atm-DeltaSRI knock-in mice gives rise to a discernibly different phenotype to Atm(-/-) mice, which may account for the heterogeneity seen in A-T patients with different mutations.

Cell death mediated by alloreactive cytotoxic T cells via the granule exocytosis or the Fas pathway is independent of p34cdc2 kinase: Fas dependent killing of cells arrested in the cell cycle

Inappropriate activation of p34cdc2 kinase has been shown to occur during apoptosis induced by cytotoxic T-cell derived perforin and fragmentin. We analysed the effect of two inhibitors of p34cdc2 kinase on alloreactive Tc-cell-mediated lysis and DNA fragmentation of P815 and L1210 target cells. Olomoucine, a specific inhibitor of cyclin dependent kinases, did not affect DNA fragmentation in the target cells. Lysis of olomoucine-treated target cells as assessed by 51Cr release over a typical 8-h period was also unaffected. We also examined the effects of thapsigargin on target cell death. This toxin causes increased intracellular calcium rises that then result in irreversible inhibition of cyclin dependent kinases, including p34cdc2 kinase. The same extent of specific cell lysis was induced by cytotoxic T cells from perforin(-/-), granzyme B(-/-), granzyme A(-/-), perforin(-/-) X granzymeB(-/-) X granzymeA(-/-) KO mice or normal mice in untreated target cells or target cells treated with either olomoucine or thapsigargin. Similarly DNA fragmentation measured by release of tritiated DNA was also unaffected. Thus inhibition of p34cdc2 kinase affects neither the Fas nor the perforin/granzyme pathways of alloreactive cytotoxic T-cell killing as measured by DNA fragmentation or chromium release. P815 cells treated with olomoucine were arrested in the cell cycle after 12-16 h exposure to the toxin. After cell cycle arrest, target cells now showed enhanced 51Cr release induced by effector cytotoxic T cells (CTL) derived from perforin(-/-) mice compared to untreated cells. This lysis was accompanied by an increase in cell surface Fas expression. Olomoucine induced cell cycle arrest and expression of Fas was reversible and when cells re-entered the cell cycle, surface expression of Fas was lost.

Redox sensitive epidithiodioxopiperazines in biological mechanisms of toxicity

Epidithiodioxopiperazines (ETPs) are a class of secondary metabolites characterized by a bridged disulfide linkage across the alpha,alpha'-positions of the dioxopiperazine ring. This class of compounds displays a range of biological activities, attributed to the sulfur moiety in the oxidized disulfide form and/or the reduced dithiol form. The underlying mechanisms of toxicity of the ETP metabolites are still a matter of debate and this review presents an overview of the evidence for the possible pathways of toxicity.

Evidence for gliotoxin-glutathione conjugate adducts

The equilibrium constant for the gliotoxin/glutathione pair was found to be 1200+/-100 M(-1) at pH 7.0 at 25 degrees C. Under conditions where the reaction was quenched rapidly with the addition of acid, gliotoxin-glutathione conjugate adducts were detected.

Cytotoxic T cells specifically induce Fas on target cells, thereby facilitating exocytosis-independent induction of apoptosis

Cytotoxic T (Tc) cells deficient in perforin lyse Fas-negative targets after lengthy incubation periods. This process is independent of granzymes, and killing occurs via the Fas pathway for the following reasons. Interaction of perforin-deficient Tc cells with Fas-negative targets leads to an up-regulation of Fas that is dependent on Ag recognition, de novo synthesis, and transport of proteins to the target cell surface. Treatment of effectors with brefeldin A, but not with the exocytosis inhibitor concanamycin, inhibited this process. Lysis of targets is inhibited by anti-Fas Abs, soluble mouse Fas-Fc, and the caspase-cascade inhibitor, crm-A. Targets from Fas-mutant lpr mice are refractory to lysis, and Tc cells from mice deficient in Fas- and perforin-mediated lysis do not lyse Fas-negative targets. The possible relevance of this exocytosis-independent cytolytic process in the regulation of T cell activity and control of pathogens is discussed.

Inactivation of rabbit muscle creatine kinase by reversible formation of an internal disulfide bond induced by the fungal toxin gliotoxin

The biological activity of gliotoxin is dependent on the presence of a strained disulfide bond that can react with accessible cysteine residues on proteins. Rabbit muscle creatine kinase contains 4 cysteines per 42-kDa subunit and is active in solution as a dimer. Only Cys-282 has been identified as essential for activity. Modification of this residue results in loss of activity of the enzyme. Treatment of creatine kinase with gliotoxin resulted in a time-dependent loss of activity abrogated in the presence of reducing agents. Activity was restored when the inactivated enzyme was treated with reducing agents. Inactivation of creatine kinase by gliotoxin was accompanied by the formation of a 37-kDa form of the enzyme. This oxidized form of creatine kinase was rapidly reconverted to the 42-kDa species by the addition of reducing agents concomitant with restoration of activity. A 1:1 mixture of the oxidized and reduced monomer forms of creatine kinase as shown on polyacrylamide gel electrophoresis was equivalent to the activity of the fully reduced form of the enzyme consistent with only one reduced monomer of the dimer necessary for complete activity. Conversion of the second monomeric species of the dimer to the oxidized form by gliotoxin correlated with loss of activity. Our data are consistent with gliotoxin inducing the formation of an internal disulfide bond in creatine kinase by initially binding and possibly activating a cysteine residue on the protein, followed by reaction with a second neighboring thiol. The recently published crystal structure of creatine kinase suggests the disulfide is formed between Cys-282 and Cys-73.

Skeletal muscle ryanodine receptor channels are activated by the fungal metabolite, gliotoxin

Interactions between the reactive disulfide fungal metabolite, gliotoxin (GTX), and rabbit skeletal ryanodine receptor (RyR) calcium release channels have been examined. RyRs in terminal cisternae vesicles formed a covalent complex with 100 microm (35)S-GTX, which was reversed by 1 mm dithiothreitol (DTT) or 1 mm glutathione. GTX (80-240 microm), added to either cytoplasmic (cis) or luminal (trans) solutions, increased the rate of Ca(2+) release from SR vesicles and the frequency of opening of single RyR channels in lipid bilayers. Channel activation was reversed upon addition of 2 mm DTT to the cis solution, showing that the activation was due to an oxidation reaction (2 mm DTT added to the cis solution in the absence of GTX did not affect RyR activity). Furthermore, RyRs were not activated by trans GTX if the cis chamber contained DTT, suggesting that GTX oxidized a site in or near the membrane. In contrast to cis DTT, 2 mm DTT in the trans solution increased RyR activity when added either alone or with 200 microm trans GTX. The results suggest that (i) GTX increases RyR channel activity by oxidizing cysteine residues that are close to the membrane and located on RyR, or associated proteins, and (ii) a disulfide bridge or nitrosothiol, accessible only from the luminal solution, normally suppresses RyR channel activity. Some of the actions of GTX in altering Ca(2+) homeostatsis might depend on its modification of RyR calcium channels.

Granzymes are the essential downstream effector molecules for the control of primary virus infections by cytolytic leukocytes

Analysis of perforin-deficient mice has identified the cytolytic pathway and perforin as the preeminent effector molecule in T cell-mediated control of virus infections. In this paper, we show that mice lacking both granzyme A (gzmA) and granzyme B (gzmB), which are, beside perforin, key constituents of cytolytic vesicles, are as incapable as are perforin-deficient mice of controlling primary infections by the natural mouse pathogen ectromelia, a poxvirus. Death of gzmAxgzmB double knockout mice occurred in a dose-dependent manner, despite the expression of functionally active perforin and the absence of an intrinsic defect to generate splenic cytolytic T cells. These results establish that both gzmA and gzmB are indispensable effector molecules acting in concert with perforin in granule exocytosis-mediated host defense against natural viral pathogens.

Cell death induced by the Fas/Fas ligand pathway and its role in pathology

Engagement of the cell death surface receptor Fas by Fas ligand (FasL) results in apoptotic cell death, mediated by caspase activation. Cell death mediated via Fas/FasL interaction is important for homeostasis of cells in the immune system and for maintaining immune-privileged sites in the body. Killing via the Fas/FasL pathway also constitutes an important pathway of killing for cytotoxic T cells. Fas ligand is induced in activated T cells, resulting in activation-induced cell death by the Fas/FasL pathway. Recently it has been shown that the Fas receptor can also be up-regulated following a lesion to the cell, particularly that induced by DNA-damaging agents. This can then result in killing of the cell by a Fas/FasL-dependent pathway. Up-regulation of Fas receptor following DNA damage appears to be p53 dependent.

Increased cell surface exposure of phosphatidylserine on propidium iodide negative thymocytes undergoing death by necrosis

Phosphatidylserine (PS) exposure on propidium iodide negative cells using FITC labelled annexin-V has been used to quantify apoptosis in vitro and in vivo. Detection of PS within cells undergoing necrosis is also possible if labelled annexin-V specific for PS enters the cell following early membrane damage. Necrotic or late apoptotic cells can be excluded from flow cytometric analysis using propidium iodide which enters and stains cells with compromised membrane integrity. Here we show that thymocytes undergoing death exclusively by necrosis show early exposure of PS prior to loss of membrane integrity. This early exposure of PS occurs in cells treated with agents which both raise intracellular calcium levels and are also capable of interacting with protein thiol groups. We also demonstrate that PS exposure in thymocytes induced to undergo apoptosis by three different agents does not correlate with calcium rises but correlates with and precedes DNA fragmentation.

Reprogramming of intestinal differentiation and intercalary regeneration in Cdx2 mutant mice

The homeobox gene Cdx2, a homologue of the Drosophila gene caudal, has been implicated in the control of cell differentiation in the intestinal epithelium. Recently, we showed that mice in which one allele of the Cdx2 gene had been inactivated by homologous recombination developed multiple intestinal polyp-like lesions that did not express Cdx2 and that contained areas of squamous metaplasia in the form of keratinizing stratified squamous epithelium, similar to that occurring in the mouse esophagus and forestomach. We have now examined colonic lesions from 98 Cdx2+/- mice and report that the lesions are composed of heterotopic stomach and small intestinal mucosa. We conclude that Cdx2 directs endodermal differentiation toward a caudal phenotype and that haploinsufficient levels of expression in the developing distal intestine lead to homeotic transformation to a more rostral endodermal phenotype, such as forestomach epithelium that does not express Cdx2 during normal development. Intercalary growth (epimorphic regeneration), which previously has never been described in mammals, then occurs, resulting in the ordered "filling in" of tissue types at the discontinuity between the gastric and colonic epithelia. This intercalary growth in a restricted space results in the formation of the polypoid lesions observed.

Increased cell surface exposure of fucose residues is a late event in apoptosis

Apoptosis is a well defined physiological process characterised by many specific features including DNA fragmentation and protease activation. Cell membrane-associated changes such as the altered exposure of phosphatidylserine and glycosylation patterns have also been described. We investigate here the change in exposure of surface alpha-L-fucose residues during thymocyte and P815 cell apoptosis. We show that apoptosis in these cells induced by dexamethasone, gliotoxin or thapsigargin was associated with an increase in the exposure of terminal fucose residues. Furthermore, this increase in fucose exposure occurred late in the apoptotic process. The observation of increased fucose exposure in two different cell types by three different apoptosis-inducing agents suggests it may be part of the normal apoptotic process.

Apoptosis induced by gliotoxin is preceded by phosphorylation of histone H3 and enhanced sensitivity of chromatin to nuclease digestion

The fungal toxin gliotoxin induces apoptotic cell death in a variety of cells. Apoptosis induced in thymocytes by gliotoxin is rapid, and DNA fragmentation is observable within 4 h treatment. Apoptosis induced by gliotoxin is calcium-independent and unaffected by protein synthesis inhibitors. We have previously shown that gliotoxin results in phosphorylation of a 16.3-kDa protein within 10 min treatment of thymocytes. Here we show that this protein is histone H3 and phosphorylation occurs on Ser-10. Cyclic AMP levels and activity of protein kinase A (PKA) are raised in cells treated with gliotoxin. Apoptosis is inhibited by genistein which also inhibits PKA and histone H3 phosphorylation. Apoptosis is also inhibited by a number of specific inhibitors of PKA suggesting apoptosis induced by gliotoxin is modulated by this kinase. The agents forskolin and cholera toxin do not induce rapid phosphorylation of H3 although some increase in phosphorylation of H3 does occur after 8 h with these agents. Forskolin and cholera toxin also induce apoptosis but over a longer time course than gliotoxin. In all cases levels of apoptosis correlate with degree of H3 phosphorylation. Cells treated with gliotoxin show an early sensitivity to micrococcal nuclease and DNase I digestion indicating a functional relationship between DNA fragmentation and H3 phosphorylation.

Gliotoxin and related epipolythiodioxopiperazines

1. Gliotoxin belongs to the epipolythiodioxopiperazine class of secondary metabolites. These compounds show a diverse range of biological activity including antimicrobial, antifungal and antiviral properties. They also display potent in vitro and in vivo immunomodulating activity. 2. Their properties resulted in a number of early studies designed to exploit their possible chemotherapeutic value, although the general toxicity of most members of this class has precluded clinical use. 3. Most recently, their selective immunosuppressive properties have led to the possibility of ex vivo treatment of tissue to selectively remove immune cells responsible for tissue rejection. The mode of action of gliotoxin appears to be via covalent interaction to proteins through mixed disulphide formation and gliotoxin has been shown to inhibit a number of thiol requiring enzymes. 4. Gliotoxin is also a potent inducer of apoptotic cell death in a number of cells. Gliotoxin and other members of this class of toxins may be produced in vivo during the course of fungal infections and contribute to the aetiology of the disease.

Thapsigargin induces mitochondrial dysfunction and apoptosis in the mastocytoma P815 cell line and in mouse thymocytes

Thapsigargin is a plant-derived inhibitor of the endoplasmic reticulum Ca(2+)-ATPase.Treatment with thapsigargin leads to a rapid, large and prolonged increase in the intracellular calcium ion concentration ([Ca(2+)](i)). Previously thapsigargin has been shown to inhibit proliferation and induce apoptosis. Here we report the results of thapsigargin treatment in thymocytes harvested from 10-day-old mice and in the P815 mastocytoma cell line. In thapsigargin-treated cells we observed enlarged mitochondria with disrupted cristae structure. These mitochondria closely resembled those observed after the induction of phase transition. To determine if the mitochondria were functioning normally the cells were stained with rhodamine 123 (R123) and analysed with flow cytometry. After thapsigargin treatment the R123 staining decreased, indicative of a loss of mitochondrial membrane potential. Furthermore intracellular ATP concentrations were also found to be reduced in cells treated with thapsigargin. Taken together these results indicate an increase in the [Ca(2+)](i) caused by thapsigargin treatment results in dysfunctional mitochondria and reduced ATP. We propose that this decrease in the concentration of ATP provokes the onset of thapsigargin-induced apoptosis. To investigate the effect of thapsigargin treatment on the cell cycle, rapidly cycling P815 cells were sorted into populations enriched for either G(0)/G(1) or S/G(2)/M phases, and these populations were then treated with thapsigargin. Thapsigargin treatment induced a cell cycle block before S phase. We propose that the block in the cell cycle induced by thapsigargin was a result of the decreased intracellular ATP concentration interfering with the energy requiring processes of DNA replication. The block could also be related to the high intracellular calcium ion concentration that would interfere with the subtle calcium transients involved in the cell's preparations for replication and mitosis. Apoptosis occurred to an equal extent in both populations of cells.

Cyclosporin A rescues thymocytes from apoptosis induced by very low concentrations of thapsigargin: effects on mitochondrial function

Raising intracellular calcium levels can induce apoptosis or programmed cell death in many cells. While early rises in intracellular calcium are not universally associated with apoptotic cell death, calcium clearly plays a key role in many of the biochemical events which occur during apoptosis. In this paper we have determined intracellular calcium rises induced by 2, 10, and 100 nM thapsigargin in mouse thymocytes. These concentrations cause increases in cytosolic calcium of 100-250, 400-600, and > 1000 nM, respectively. These rises are sustained for at least 85 min and the ratio between the maximum rise caused by 10 nM compared to 2 nM thapsigargin is 2.1 +/- 0.4 (n = 6). Both 2 and 10 nM thapsigargin cause apoptosis at 24 h as shown by DNA fragmentation and morphology when examined by electron microscopy. Cyclosporin A (CsA) inhibits apoptosis caused by 2 nM thapsigargin but not that caused by 10 nM thapsigargin. Electron microscopy of thymocytes treated with 2 nM thapsigargin at 24 h shows intact mitochondria although with altered morphology. There is no loss of ATP or decrease in the ATP/ADP ratio in these cells over 12 h. Mitochondria in cells treated with 10 nM thapsigargin, however, are swollen by 6 h and many are lost by 24 h. These cells show greatly diminished ATP content by 12 h and a decrease in ATP/ADP ratio. Examination of the effects of PMA, an activator of the plasma membrane calcium ATPase pump, on cells treated with 10 nM thapsigargin suggests that two pools of calcium may be responsible for the differential effects of the two calcium levels in the cells. Probing of the mitochondrial membrane potential (MMP) by rhodamine 123 staining of live cells shows that the collapse of the MMP caused by 10 nM thapsigargin is unaffected by CsA. The MMP is also reduced in cells treated with 2 nM thapsigargin but this is restored by CsA. Cells are also rescued from apoptosis caused by 2 nM thapsigargin by incubation with FK506. This immunosuppressive agent has no effect on the membrane permeability transition induced in isolated mitochondria. These results suggest that very low rises in intracellular calcium in thymocytes cause activation-induced cell death inhibited by CsA and FK506 and are without effect on ATP levels and therefore do not involve irreversible mitochondrial damage. Exceeding these calcium levels by only two-fold results in apoptosis accompanied by reduced ATP levels and mitochondrial damage, although apoptotic cell death in this instance is unaffected by the classic inhibitor of mitochondrial permeability transition, CsA.

Exacerbation of invasive aspergillosis by the immunosuppressive fungal metabolite, gliotoxin

Invasive aspergillosis is a significant cause of death in immunocompromised individuals. The majority of strains of the main causative agent, Aspergillus fumigatus, produce gliotoxin, a secondary metabolite with demonstrated in vitro immunosuppressive activity. Pretreatment of normally resistant mice with a single injection of a sublethal dose of gliotoxin was sufficient to make them susceptible to infection and subsequent death, after challenge with A. fumigatus spores. Animals infected with the non-gliotoxin producing strain survived significantly longer than those infected with a gliotoxin producer. We propose that the release of gliotoxin by A. fumigatus hyphae during infection can exacerbate the pathogenesis of aspergillosis.

Evidence that gliotoxin enhances lymphocyte activation and induces apoptosis by effects on cyclic AMP levels

Gliotoxin is a secondary metabolite produced by several pathogenic fungi. It has potential clinical applications as an immunosuppressive agent in preventing allograft rejection. At low doses (< 30 nM) gliotoxin displays co-mitogenic activity, but at higher doses induces apoptosis in cells. Here we demonstrate that gliotoxin, although not mitogenic in its own right, enhances activation in preactivated splenocytes by a calcium-independent mechanism. The enhancement in activation correlates with a decrease in intracellular cyclic AMP levels. This property is inhibited by dibutyryl-cAMP. Increasing the concentration of gliotoxin to levels that caused apoptosis produced a dose-related increase in cAMP levels. Thus, the effects of gliotoxin on cell activation and the induction of apoptosis may both be mediated by changed levels of cAMP.

A decrease in intracellular glutathione concentration precedes the onset of apoptosis in murine thymocytes

Free radical damage has been implicated in the induction of apoptosis in some cells. We investigated whether the status of a cell's oxidant defence system is involved in the signalling pathways triggering apoptosis. We used three unrelated agents, dexamethasone, thapsigargin and gliotoxin to induce apoptosis in thymocytes from 10-day-old BALB/c mice. With all stimuli there was a correlation between the percentage of cells undergoing apoptosis (as measured with propidium iodide DNA staining) and the percentage of cells with lowered [GSH]i. Treatment with either 1 mM reduced glutathione or 10 nM thapsigargin inhibited dexamethasone-induced apoptosis in thymocytes at 6 h, as well as the rise in the percentage of cells with lowered [GSH]i that normally accompanied the onset of apoptosis. Furthermore, following treatment of thymocytes with oxidized glutathione, a normal product of the action of the cell's oxidant defence system, high levels of apoptosis were observed. This suggested that the onset of apoptosis was not simply the result of a loss of GSH from the cytosol. From our evidence we suggest that a decrease in [GSH]i, or an increase in [GSSG]i or perhaps a change in the ratio of [GSH]i to [GSSG]i constitutes a trigger for apoptosis.

DNA synthesis precedes gliotoxin-induced apoptosis

The toxin gliotoxin induces apoptosis or programmed cell death in a variety of immune cells including thymocytes. Apoptosis induced by gliotoxin in thymocytes is unaffected by protein synthesis inhibitors nor is it associated with early changes in intracellular calcium levels (Beaver and Waring, 1994). This work shows that the cell lines P815 and WEHI7 and murine thymocytes when treated with gliotoxin show an early incorporation of tritiated thymidine over the concentration range which causes apoptosis. Proliferating cell nuclear antigen (PCNA), a marker for S phase, is elevated in cells following gliotoxin treatment and S phase DNA content is increased. Thymidine incorporation is inhibited by hydroxyurea, an inhibitor of replicative DNA synthesis not repair. Free radical scavangers have no effect on apoptosis induced by gliotoxin in thymocytes. Hydrogen peroxide-treated cells showed no enhanced thymidine incorporation and no apoptosis. Thus oxidative stress does not appear to be a factor in gliotoxin-induced apoptosis. Thymocytes treated with gliotoxin show increased phosphorylation of a 16.3 kDa protein, and apoptosis is inhibited by the tyrosine kinase inhibitor genistein, which also inhibited the increased thymidine incorporation in P815 cells. We conclude that one mechanism by which gliotoxin can cause apoptosis may be the induction of inappropriate entry of cells into the cell cycle followed by death.

Gliotoxin inactivates alcohol dehydrogenase by either covalent modification or free radical damage mediated by redox cycling

The fungal metabolite gliotoxin shows selective toxicity to cells of the immune system and has been implicated in the aetiology of invasive aspergillosis. The related toxin sporidesmin is the causative agent of facial eczema in sheep. The toxicity of these compounds has been related to their ability to redox cycle intracellularly and thus produce damaging free radicals. These toxins are also potentially capable of forming mixed disulphides with thiol groups on proteins by virtue of their bridged disulphide structure. We show here that gliotoxin can inactivate horse liver alcohol dehydrogenase by either oxidative damage or covalent modification of thiol groups on the enzyme. Either Cys-281 or Cys-282 is selectively modified. Neither of these residues are at the active site. Covalent modification occurs in the absence of reducing agents such as dithiothreitol. In the presence of dithiothreitol no protection is observed and the rate of inactivation is enhanced although as expected no covalent modification occurs. Gliotoxin can therefore inhibit alcohol dehydrogenase by either pathway and this will depend on the availability of reducing agents such as glutathione and/or how readily the reactive oxygen species generated are removed.

Extracellular calcium is not required for gliotoxin or dexamethasone-induced DNA fragmentation: a reappraisal of the use of EGTA

The immunomodulating agent gliotoxin and related toxins cause apoptotic cell death in a variety of cell types including macrophages and thymocytes [Waring et al. (1988) J. biol. Chem., 263, 18,493-18,499; Waring et al. (1990) Int. J. Immunopharmac., 12, 445-457]. The mechanism of induction of apoptosis by gliotoxin is not yet known, although it does not require protein synthesis [Waring (1990) J. biol. Chem., 14,476-14,480], unlike dexamethasone-induced apoptosis in thymocytes. Because of the reported requirement for extracellular calcium in apoptosis induced by dexamethasone, we studied the effects of extracellular calcium on gliotoxin-induced apoptosis in macrophages. Initial experiments using calcium-depleted media showed no inhibition of apoptotic DNA fragmentation by gliotoxin. By measuring residual 45Ca2+ remaining in cells pulsed with labelled calcium over the time period required for DNA fragmentation, we could demonstrate some uptake of extracellular calcium into treated cells as assessed by residual, slowly exchanging calcium. When cells were treated with the calcium chelator EGTA at 0.5-2 mM, calcium uptake was abolished but DNA fragmentation was unaffected. EGTA at higher concentrations, up to 8 mM, did inhibit DNA fragmentation without any additional inhibition of calcium uptake. Similar results were found for dexamethasone-treated thymocytes. Thymocytes treated with 8 mM EGTA, however, were not rescued from apoptosis but died by necrosis. These results indicate that extracellular calcium is not essential for apoptosis induced by these agents and that the use of high concentrations of EGTA to establish a requirement for extracellular calcium in apoptosis should be treated with caution.

Lack of correlation between early intracellular calcium ion rises and the onset of apoptosis in thymocytes

Apoptosis, a well-recognized process of cell death, is usually defined by chromatin condensation, plasma membrane blebbing, reduction in cell volume, and in many cell types the cleavage of DNA into nucleosomal multiples, and finally the formation of apoptotic bodies. We have characterized the time of onset and the range of concentrations at which the toxins gliotoxin and thapsigargin induce apoptosis in thymocytes. We also looked for early changes in cytosolic calcium ion concentration ([Ca2+]i). Three methods were used to detect apoptosis: cellular morphology, DNA fragmentation and a flow cytometric method using ethidium bromide. Calcium fluxes were measured using both flow cytometry and bulk cell fluorimetry. Gliotoxin concentrations of 50 nmol/L to 10 mumol/L induced significant numbers of cells to become apoptotic in a dose dependent manner. At these concentrations there was no observable increase in [Ca2+]i as determined by flow cytometry or in bulk cells. However, when thymocytes were treated with gliotoxin at concentrations greater than 500 mumol/L, rises in [Ca2+]i were apparent, but these cells died by necrosis. Thapsigargin induced low levels of apoptosis in thymocytes; the maximum effect observable after a 10 nmol/L treatment. Thapsigargin is known to inhibit the Ca(2+)-ATPase in the endoplasmic reticulum thereby causing a sustained increase in [Ca2+]i in thymocytes. The rise in [Ca2+]i observed was quantitatively similar when thymocytes were treated with thapsigargin concentrations ranging between 10 and 100 nmol/L. These results led us to investigate the effect of dexamethasone on [Ca2+]i. In these experiments thymocytes showed no rises in [Ca2+]i above the control over 85 min following treatment with 10 mumol/L dexamethasone.

Cellular uptake and release of the immunomodulating fungal toxin gliotoxin

Uptake of the immunomodulating agent gliotoxin into a panel of cells using biosynthetically radiolabelled 35S toxin showed rapid association of the toxin with all cell types studied with 70-85% of the total counts in the media becoming cell associated. A difference in kinetics was observed for cell lines when compared to the primary cells thymocytes, activated T-cells and macrophages. In the latter uptake was maximal after 10-15 min and radiolabel was lost from the cells as early as 100 min. In the cell lines studied, uptake was complete in less than 1 min with no loss of label after 100 min. The exception to this was a Wilms tumour line. Analysis of the fate of gliotoxin taken up into sensitive (activated T-cells) and resistant (human fibroblast) cells by HPLC showed: (a) up to 30% of the original gliotoxin taken up by sensitive cells was released as free gliotoxin over a 22 hr period. The remainder was metabolized to inorganic sulphate; (b) in T-cells gliotoxin is reduced to the dithiol form in significant amounts and this reduction may be modulated by glutathione; and (c) no reduced gliotoxin could be detected in the resistant fibroblast cell line 27Sk even though up to 50% of the original gliotoxin was still present in the free form in these cells at 22 hr. Gliotoxin became covalently associated with macromolecules in both cell types studied. Very little free gliotoxin is released into extracellular medium by the fibroblast cell line. Gliotoxin at 500 nM was found to induce apoptosis or programmed cell death in the Wilms tumour cell line but not in any other cell line studied, and this may account for the different kinetics of release of the toxin from the Wilms tumour cell line.

In vivo immunosuppressive activity of gliotoxin, a metabolite produced by human pathogenic fungi

Aspergillosis is a disease caused by the opportunistic pathogen Aspergillus fumigatus and other related fungi. It occurs mainly in immunosuppressed people and causes very high mortality rates. A fumigatus and other pathogenic fungi have been shown to produce a metabolite, gliotoxin, which has immunosuppressive properties in vitro, but little is known about its in vivo activity. Here we report that gliotoxin has increased toxicity in mice after irradiation. A single injection of gliotoxin delayed the recovery of immune cells after immunosuppression by sublethal irradiation by 2 weeks. Study of the morphology of cells of the thymus, spleen, and mesenteric lymph nodes by light microscopy and electron microscopy and agarose gel electrophoresis of DNA from these organs showed that the injection of gliotoxin induced apoptosis in cells of the immune system in vivo. Thus, gliotoxin does have immunosuppressive activity in vivo and could potentially play a significant role in the pathogenesis of aspergillosis and other fungal diseases.

Macrophage adherence prevents apoptosis induced by ricin

Ricin, a lectin with potent protein synthesis inhibitory properties, has been known to cause morphological changes in epithelial cells typical of apoptosis (P. Waring et al., Med. Res. Rev. 11, 1-17 (1991)). In earlier preliminary experiments from this laboratory with murine macrophages and T-blasts (P. Waring, J. Biol. Chem. 265, 14,476-14,480 (1990)), it was shown that ricin induces regular DNA fragmentation, a biochemical event also associated with apoptosis. Here we confirm morphologically and by examination of DNA fragmentation that macrophages undergo apoptosis when treated with ricin in a dose-dependent manner. Ricin also inhibits adherence of macrophages to plastic surfaces but does not affect adherence of preadhered macrophages after 7 h of treatment. We also report that adherence significantly diminishes DNA fragmentation induced in macrophages by ricin but has no effect on ricin-induced inhibition of protein synthesis. From these results we may conclude that the property of ricin to induce apoptosis may not be related to its ability to inhibit protein synthesis in macrophages. Moreover, the anti-phagocytic activity of ricin may be a direct consequence of its ability to fragment DNA and induce apoptosis and not of its ability to inhibit protein synthesis. We also observed no immediate increase in Ca2+ concentration when macrophages were treated with ricin indicating that ricin-induced apoptosis may not involve the activation of a Ca2+ dependent endonuclease(s).

Production of leukemia inhibitory factor by human articular chondrocytes and cartilage in response to interleukin-1 and tumor necrosis factor alpha

OBJECTIVE

To investigate the production of leukemia inhibitory factor (LIF) by human articular chondrocytes and cartilage.

METHODS

Chondrocytes and cartilage were cultured with and without added cytokines, and the conditioned media assayed for LIF by a specific radioreceptor competition assay.

RESULTS

Interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF alpha) stimulated LIF production by chondrocyte monolayers and cartilage. LIF was generally not detected in unstimulated cultures. Northern blot analysis showed increased amounts of LIF messenger RNA in IL-1- and TNF alpha-treated chondrocyte cultures.

CONCLUSION

Chondrocytes, stimulated by IL-1 and/or TNF alpha, are potential contributors to the elevated levels of LIF observed in the synovial fluids of patients with rheumatoid arthritis and other inflammatory arthritides.

Synthesis and Activity of New Epipolythiopiperazine-2,5-dione Compounds. I

The new lipophilic epipolythiopiperazine-2,5-diones 1,4-dibutylepidithiopiperazine-2,5-dione. 1,4-dibenzylepidithiopiperazine-2,5-dione, 1-benzyl-4-methylepidithiopiperazine-2,5-dione and 3,3′-dithiobis(1,4-dimethylpiperazine-2,5-dione) were synthesized. Unlike the parent compound, the fungal toxin and immunomodulating agent gliotoxin, these compounds did not affect macrophage adherence and cell proliferation in vitro. Like the reduced form of gliotoxin and other simple analogues, these new compounds induce oxidative damage to naked DNA. We conclude that an increase in lipophilicity plays no part in the biological activity of these compounds and in fact abrogates almost all activity. We also synthesized an intermolecular disulfide analogue which also lacked activity.

The effects of leukaemia inhibitory factor on platelet function

Leukaemia inhibitory factor (LIF) is able to promote megakaryocytopoiesis in vitro and elevate platelet counts in vivo, and is a potential new therapeutic agent for the treatment of thrombocytopenia. To determine whether platelets released under conditions of LIF-stimulated megakaryocytopoiesis have intact function, we compared aggregation responses of platelets from mice with constitutively elevated LIF levels (FD/LIF mice) and mice injected with recombinant murine LIF (rmLIF mice) with their respective control mice. We report that ex vivo platelet aggregability and thromboxane B2 release were intact in the LIF-treated mice, and were significantly enhanced in some situations. LIF-treated mice also had significantly increased platelet counts (FD/LIF mice: 1302 +/- 173 x 10(9)/l compared to 1012 +/- 99 x 10(9)/l for FD mice; rmLIF mice: 1460 +/- 193 x 10(9)/l compared to 985 +/- 67 x 10(9)/l for FCS/NS mice), increased platelet volumes and elevated plasma fibrinogen and calcium levels. The platelet hyperreactivity seen in the LIF-treated mice is likely to reflect the larger platelet volumes and/or the effect of plasma components such as fibrinogen, elevated levels of which were due to the concomitant action of LIF as a stimulant of acute phase protein synthesis.

Morbidity of low-birthweight infants with intrauterine cocaine exposure

The effects of intrauterine cocaine exposure on 158 low-birthweight infants (500 g to 2500 g) were compared with 536 low-birthweight infants not known to be exposed to cocaine who were admitted to our neonatal intensive care unit. Univariate analyses indicated that drug-exposed infants had lower birthweights when compared with control infants. Frequency of necrotizing enterocolitis differed between the drug-exposed infants and controls (11% versus 6%). Frequency of severe respiratory distress syndrome, severe intraventricular hemorrhage, and bronchopulmonary dysplasia did not differ between groups of similar birthweight. We conclude that the most evident risks of prenatal cocaine exposure are low birthweight and increased incidence of necrotizing enterocolitis.

Leukemia inhibitory factor levels are elevated in septic shock and various inflammatory body fluids

Leukemia inhibitory factor (LIF) has many biological actions which parallel those of IL-1, IL-6 and tumor necrosis factor-alpha, but its role in the pathogenesis of human disease is unknown. A specific radioreceptor competition assay capable of detecting LIF at concentrations above 1 ng/ml (45 pM) was developed. To identify disease states in which LIF might be involved, a cross-sectional survey of serum and body fluids from approximately 1,500 subjects with a variety of diseases was performed using the LIF radioreceptor competition assay. Serum LIF concentrations were transiently elevated (2-200 ng/ml) in six subjects with meningococcal or Gram-negative septic shock, and in a subject with idiopathic fulminant hepatic failure. Moderately elevated LIF concentrations (> 10 ng/ml) were detected in cerebrospinal fluid from subjects with bacterial meningitis, in effusions associated with pneumonia and peritonitis, and in amniotic fluid from a woman with chorioamnionitis. Low LIF concentrations (1-10 ng/ml) were present in synovial fluid from subjects with inflammatory arthritis, amniotic fluid from women in labor, and some reactive, chronic inflammatory and malignant effusions and cyst fluids, but rarely in transudates. These initial findings suggest that LIF might be involved in the pathogenesis of inflammation and septic shock.

Fragmentation of DNA in the retina of chicken embryos coincides with retinal ganglion cell death

Neuronal cell death was studied in the developing retina of the chicken embryo. One of the most characteristic indices of the form of cell death termed apoptosis is regular, apparently internucleosomal fragmentation of DNA. When retinae of eight to seventeen day old chicken embryos were dissected out and the DNA from this tissue size fractionated on agarose gels, fragmentation typical of apoptosis was observed on day ten. The maximal amount of fragmentation was reached around day eleven and twelve and declined from day 15 to 17. These findings correlate in time with previous histological data on retinal cell death and demonstrate for the first time the occurrence of DNA fragmentation typical of apoptosis in the developing nervous system.