Do all programmed cell deaths occur via apoptosis?

Nerve growth factor withdrawal induces the apoptotic death of developing septal cholinergic neurons in vitro: protection by cyclic AMP analogue and high potassium

Nerve growth factor regulates the developmental programmed cell death of certain neurons in the peripheral nervous system. The functions of nerve growth factor in the central nervous system are less well characterized. Nerve growth factor withdrawal results in the protein synthesis-dependent death of a large percentage of developing septal cholinergic neurons in sandwich tissue culture. In this study double labelling techniques were used to demonstrate that septal cholinergic neurons subjected to nerve growth factor withdrawal exhibit condensed chromatin and fragmented nuclei, and are labelled intensely for fragmented DNA. These degenerative changes are characteristic of apoptotic cell death. Half of the cholinergic neurons were committed to die and could no longer be rescued by nerve growth factor reapplication following approximately 16.5 h of nerve growth factor deprivation, whereas half of the cholinergic neurons could no longer be rescued by cycloheximide addition after only 9 h of nerve growth factor deprivation, suggesting that nerve growth factor and cycloheximide effect rescue by distinct mechanisms. Addition of a cyclic AMP analogue or depolarization with high K+, but not the general nuclease inhibitor aurintricarboxylic acid, prevented the death of cultured septal cholinergic neurons subjected to nerve growth factor withdrawal. Furthermore, these agents are capable of rescuing cholinergic neurons subjected to a period of nerve growth factor withdrawal after which addition of cycloheximide is no longer protective. Thus, nerve growth factor, cyclic AMP and high K+ can effect rescue after inhibition of translation ceases to be protective. These findings suggest that under defined conditions in vitro, withdrawal of nerve growth factor from septal cholinergic neurons during a critical period of development results in the apoptotic death of these CNS neurons, which can be prevented at the post-translational level by nerve growth factor, cyclic AMP and high K+.

Mechanisms of phosphatidylserine exposure, a phagocyte recognition signal, on apoptotic T lymphocytes

The appearance of phosphatidylserine (PS) on the cell surface during apoptosis in thymocytes and cytotoxic T lymphocyte cell lines provokes PS-dependent recognition by activated macrophages. Flow cytometric analysis of transbilayer lipid movements in T lymphocytes undergoing apoptosis reveals that downregulation of the adenosine triphosphate-dependent amino-phospholipid translocase and activation of a nonspecific lipid scramblase are responsible for PS reaching the surface from its intracellular location. Both mechanisms are expressed at the same time, and precede DNA degradation, zeiosis, and cell lysis in the apoptotic pathway.

Spatial, temporal and hormonal regulation of programmed muscle cell death during metamorphosis of the frog Xenopus laevis

No examination to date has been made of apoptosis during vertebrate muscle development. The authors recently reported programmed muscle cell death to be important in tail degeneration as well as in the larval-to-adult conversion of the dorsal body muscles of Xenopus laevis during metamorphosis [30]. In the present study, we examined programmed cell death (PCD) of the dorsal body and tail muscle morphologically and biochemically, with special attention to whether apoptotic processes, such as chromatin fragmentation and apoptotic body-formation actually occur, and whether triiodothyronine (T3) induces such processes. Light microscopic observation indicated muscle fibers break down into short fragments (sarcolytes or muscle apoptotic bodies) during the metamorphic climax, not only in tail but also in larval-type fibers of dorsal body muscles. Apoptotic bodies first appeared near the base of the tail in early climax (stage 59) when the T3 level is quite high, and thereafter expanded in an anterior direction in the dorsal body and posteriorly in the tail. The ratio of apoptotic area to total muscle area became maximum (10%-30% in dorsal body muscles and 50% in the tail) at the climax (stages 63-64). During these stages, genomic DNA fragmented into oligonucleosome-sized units (200 bp, 400 bp, 600 bp ...) in both body and tail muscles. To confirm whether this chromatin fragmentation is associated with apoptotic bodies, in situ DNA nick end labeling (TUNEL) was applied to sections of the dorsal body and tail muscles. Labeled muscle nuclei could be found only in muscle apoptotic bodies but not in intact muscle fibers, indicating DNA fragmentation was associated with cell fragmentation during metamorphosis. It thus follows that morphological (apoptotic body formation) and biochemical (fragmentation of chromatin) processes occurring during PCD of dorsal body and tail muscles are identical. To determine whether T3 regulates programmed muscle cell death, the effects of T3 on DNA ladder formation were examined in tails cultured in vitro. The oligonucleosomal DNA ladder was found to form only in tails incubated with T3, thus showing T3 to induce programmed muscle cell death without interaction with other endocrine organs during metamorphosis.

Identification, characterization, and purification of a tobacco endonuclease activity induced upon hypersensitive response cell death

Programmed cell death (pcd) is activated during the hypersensitive response (HR) of plants to avirulent pathogens. We have recently shown that, similar to pcd in animal cells, nuclei of cells undergoing HR cell death contain fragmented nuclear DNA (nDNA). Here, we report that cell death occurring during the HR is accompanied by an increase in the activity of several deoxyribonucleases. Induction of nuclease activities was coordinated with cell death and may account for the degradation of nDNA during the HR. HR-associated nuclease activities were not induced during senescence, following necrotic cell death resulting from abiotic stress, or in response to induction of plant defense mechanisms by salicylic acid. HR-associated nuclease activities were stimulated by Ca2+ and inhibited by EGTA, EDTA, and Zn2+. At least one of the HR-associated nuclease activities was detected in nuclei purified from leaves undergoing pcd. A nuclease with an electrophoretic mobility similar to that of the nuclease activity found in nuclei isolated from leaves undergoing HR cell death was purified. Our findings are in accordance with some of the biochemical events that occur during pcd in animal cells. However, further analysis of the pattern of nDNA fragmentation and the corresponding structural changes that occur in the nuclei of tobacco cells undergoing HR cell death revealed that these features may have differences from those that take place during apoptosis in animal cells.

Relationship between severity, necrosis, and apoptosis in five models of experimental acute pancreatitis

In an effort to elucidate factors that determine the severity of an attack of acute pancreatitis, we have quantitated the extent of necrosis and of apoptosis in five different models of experimental acute pancreatitis. Severe pancreatitis was induced by obstructing the opossum common bile-pancreatic duct, by administering to mice 12 hourly injections of a supramaximally stimulating dose of caerulein, and by feeding young female mice a choline-deficient, ethionine-supplemented diet. In each of these models of severe pancreatitis, marked necrosis but very little apoptosis was found. Mild pancreatitis was induced by obstructing the rat common bile-pancreatic duct and by infusing rats with a supramaximally stimulating dose of caerulein. In contrast to our findings in severe pancreatitis, mild pancreatitis was characterized by very little necrosis but a high degree of apoptosis. Our finding that the severity of acute pancreatitis is inversely related to the degree of apoptosis suggests that apoptosis may be a teleologically beneficial response to acinar cell injury in general and especially in acute pancreatitis.

Differences in DNA fragmentation following transient cerebral or decapitation ischemia in rats

The time course of appearance of cells with DNA damage was studied in rats following transient severe forebrain ischemia. This DNA damage could be detected by in situ end-labeling on brain sections. The breaks in DNA appeared selectively by day 1 in the striatum and later in the CA1 region of the hippocampus. It was possible by double labeling to show that there was no DNA damage in astrocytes. The DNA breaks consisted of laddered DNA fragments indicative of an ordered apoptotic type of internucleosomal cleavage, which persisted without smearing for up to 7 days of reperfusion. In contrast, the DNA breaks following ischemia induced by decapitation were random and, after gel electrophoresis, consisted of smeared fragments of multiple sizes. There was some early regional cellular death, restricted to the dentate of the hippocampus, prior to the pannecrotic degeneration. It is concluded that transient forebrain ischemia leads to a type of neuronal destruction that is not random necrosis but that shares some component of the apoptotic cell death pathway.

Dynamics of follicular growth and atresia of large follicles during the ovarian cycle of the guinea pig: fate of the degenerating follicles, a quantitative study

BACKGROUND

A quantitative integrated study of healthy ovarian follicles of different sizes and their mitotic activity and of clearly defined atretic stages of involuting large growing follicles at different stages of the guinea pig ovarian cycle is not available in the literature. We considered that such a study would reveal new aspects of ovarian tissue dynamics and provide new information in an organ with a continuous phenotypic transformation of its cellular components.

METHODS

Ovaries from guinea pigs were removed on days 1 (opening of the vagina), 3, 6, 9, 13, and 16 of the cycle, and the following were measured in serial sections: (1) total number of healthy follicles falling into categories based on the volume occupied by granulosa cells, (2) total number of atretic follicles falling into clearly defined morphological stages of the degenerative and involutionary process affecting medium to large follicles, and (3) proportion of metaphase-arrested granulosa cells, after colcemid injection, in healthy follicles of different size categories.

RESULTS

Dynamic patterns of follicular growth and degeneration were revealed that permitted the following main conclusions and observations: (1) small to middle-size follicles can reach the maximal category mass of granulosa mass within 6-7 days, and the number of granulosa cells can increase 6-7-fold during this interval, (2) the cohort that gives rise to 2-6 preovulatory follicles and to the large follicles that will undergo atresia during each cycle varied from 68 to 108 follicles, (3) cell death starts in the granulosa cell layers of large follicles even when neighbouring cells maintain a high mitotic activity and it spreads rapidly; dead granulosa cells are cleare by nucleolysis and cytolysis in the absence of blood leucocytes or neovascularization, (4) foci of atresia are observed also in a few preovulatory follicles, (5) antral cavities of follicles with dead granulosa cells in the process of being lysed shrink and are filled within 2-3 days with large fibroblast-like cells arising from phenotypic transformation of inner layers of theca interna, with no evidence of mitotic activity or angiogenesis; the outer layers of theca interna involute, and by progressive atrophy and a process of cell death, minute nodular structures arise with remnants of the ovum and zona pellucida, and (6) a transient wave of degeneration affects a proportion of small and middle-size follicles during the metestrous period. This process does not resemble the morphological phenomenology of follicular involution, which affects only large follicles.

CONCLUSIONS

This study contributes to a fuller understanding of the dynamics and time relationships of follicular growth and loss in the guinea pig ovary and provides new morphogenetic information on the atretic process. It would be valuable for the design of experiments on endocrine and paracrine interactions involved in follicular growth and atresia.

Ubiquitin in homeostasis, development and disease

Ubiquitin is the most phylogenetically conserved protein known. This 8,500 Da polypeptide can be covalently attached to cellular proteins as a posttranslational modification. In most cases, the addition of multiple ubiquitin adducts to a protein targets it for rapid degradation by a multisubunit protease known as the 26S proteasome. While the ubiquitin/26S proteasome pathway is responsible for the degradation of the bulk of cellular proteins during homeostasis, it may also be responsible for the rapid loss of protein during the programmed death of certain cells, such as skeletal muscle during insect metamorphosis. In addition, alterations in the expression and regulation of ubiquitin may play significant roles in pathological disorders. For example, dramatic increases in ubiquitin and ubiquitin-protein conjugates are observed in a wide variety of neurodegenerative disorders, including Alzheimer's disease. Patients suffering from the autoimmune disease systemic lupus erythematosus generate antibodies reacting with ubiquitin and ubiquitinated histones. At present, it is not known whether these changes in ubiquitin expression and regulation initiate pathological changes in these diseases or if they are altered as a consequence of these disorders.

Activation of a 15-kDa endonuclease in hypoxia/reoxygenation injury without morphologic features of apoptosis

Hypoxia/reoxygenation is an important cause of tissue injury in a variety of organs and is classically considered to be a necrotic form of cell death. We examined the role of endonuclease activation, considered a characteristic feature of apoptosis, in hypoxia/reoxygenation injury. We demonstrate that subjecting rat renal proximal tubules to hypoxia/reoxygenation results in DNA strand breaks and DNA fragmentation (both by an in situ technique and by agarose gel electrophoresis), which precedes cell death. Hypoxia/reoxygenation resulted in an increase in DNA-degrading activity with an apparent molecular mass of 15 kDa on a substrate gel. This DNA-degrading activity was entirely calcium dependent and was blocked by the endonuclease inhibitor aurintricarboxylic acid. The protein extract from tubules subjected to hypoxia/reoxygenation cleaved intact nuclear DNA obtained from normal proximal tubules into small fragments, which further supports the presence of endonuclease activity. Despite unequivocal evidence of endonuclease activation, the morphologic features of apoptosis, including chromatin condensation, were not observed by light and electron microscopy. Endonuclease inhibitors, aurintricarboxylic acid and Evans blue, provided complete protection against DNA damage induced by hypoxia/reoxygenation but only partial protection against cell death. Taken together, our data provide strong evidence for a role of endonuclease activation as an early event, which is entirely responsible for the DNA damage and partially responsible for the cell death that occurs during hypoxia/reoxygenation injury. Our data also indicate that in hypoxia/reoxygenation injury endonuclease activation and DNA fragmentation occur without the morphological features of apoptosis.

Apoptosis and necrosis: two distinct events induced, respectively, by mild and intense insults with N-methyl-D-aspartate or nitric oxide/superoxide in cortical cell cultures

N-Methyl-D-aspartate (NMDA) receptor-mediated neurotoxicity may depend, in part, on the generation of nitric oxide (NO.) and superoxide anion (O2.-), which react to form peroxynitrite (OONO-). This form of neurotoxicity is thought to contribute to a final common pathway of injury in a wide variety of acute and chronic neurologic disorders, including focal ischemia, trauma, epilepsy, Huntington disease, Alzheimer disease, amyotrophic lateral scelerosis, AIDS dementia, and other neurodegenerative diseases. Here, we report that exposure of cortical neurons to relatively short durations or low concentrations of NMDA, S-nitrosocysteine, or 3-morpholinosydnonimine, which generate low levels of peroxynitrite, induces a delayed form of neurotoxicity predominated by apoptotic features. Pretreatment with superoxide dismutase and catalase to scavenge O2.- partially prevents the apoptotic process triggered by S-nitrosocysteine or 3-morpholinosydnonimine. In contrast, intense exposure to high concentrations of NMDA or peroxynitrite induces necrotic cell damage characterized by acute swelling and lysis, which cannot be ameliorated by superoxide dismutase and catalase. Thus, depending on the intensity of the initial insult, NMDA or nitric oxide/superoxide can result in either apoptotic or necrotic neuronal cell damage.

Apoptosis of a fibrosarcoma induced by protein-free culture involves DNA cleavage to large fragments but not internucleosomal fragmentation

A murine fibrosarcoma clone, Gc-4 SD, grows depending on fetal calf serum. In MTT assay, protein-free cultivation resulted in a reduction of the viable cell number time-dependently. Electron-microscopic and flow-cytometric analyses revealed that the reduction in growth was accompanied by the appearance of apoptotic cells. However, no internucleosomal fragmentation was observed even after SI-nuclease treatment. On the other hand, pulse field gel electrophoresis revealed that cleavage of DNA into high-molecular-weight fragments estimated as 50 to 150 kilobase pairs (kbp), with a peak of 100 kbp, was found in the serum-deprived cells. Large fragments disappeared from the DNA extracts when the smaller cells with high blue fluorescence with Hoechst 33342 were removed by flow cytometry, suggesting direct correlation between the large DNA fragmentation and apoptosis. The addition of aurintricarboxylic acid neither abolished the large DNA fragmentation nor inhibited the reduction in the number of viable cells. Both cycloheximide and actinomycin D enhanced the reduction in the number of viable cells as well as the large DNA fragmentation. These results suggest that apoptosis of a fibrosarcoma induced by protein-free culture involves a specific endogenous endonuclease, which may be distinct from and independent of the ATA-sensitive endonuclease producing internucleosomal DNA fragmentation.

Apoptosis and loss of renal tissue in polycystic kidney diseases

BACKGROUND

Polycystic kidney disease is characterized by the enlargement of renal cysts, interstitial fibrosis, and gradual loss of normal renal tissue in association with progressive deterioration of renal function. The process causing the progressive loss of renal tissue is unknown, but it could be the result of a form of programmed cell death known as apoptosis.

METHODS

We assayed apoptotic DNA fragmentation in normal and polycystic kidneys biochemically by gel electrophoresis and histochemically by in situ end-labeling. A DNA-specific dye, Hoechst 33258, was used to detect morphologic apoptosis in renal samples from patients with normal kidneys, polycystic kidney disease, and other kidney diseases.

RESULTS

Apoptotic DNA fragmentation was detected in polycystic kidneys from 5 patients without renal failure and 11 patients with renal failure but not in kidneys from 12 patients with no renal disease. In situ end-labeling revealed apoptotic cells in glomeruli, in cyst walls, and in both cystic and noncystic tubules of the polycystic kidneys. No tubular apoptosis was detected in renal-biopsy specimens from five patients with IgA nephropathy, three patients with nephrosclerosis, two patients with focal glomerulosclerosis, one patient with diabetic nephropathy, six patients with acute tubular necrosis, or four patients with acute and four patients with chronic renal-transplant rejection. The capacity of polycystic kidney cells to undergo apoptosis was retained in vitro in the absence of uremia, ischemia, and other confounding pathologic conditions.

CONCLUSIONS

Apoptotic loss of renal tissue may be associated with the progressive deterioration of renal function that occurs in patients with polycystic kidney disease.

Lens epithelial cell apoptosis appears to be a common cellular basis for non-congenital cataract development in humans and animals

Cataract is a major ocular disease that causes blindness in many developing countries of the world. It is well established that various factors such as oxidative stress, UV, and other toxic agents can induce both in vivo and in vitro cataract formation. However, a common cellular basis for this induction has not been previously recognized. The present study of lens epithelial cell viability suggests such a general mechanism. When lens epithelial cells from a group of 20 cataract patients 12 to 94 years old were analyzed by terminal deoxynucleotidyl transferase (TdT) labeling and DNA fragmentation assays, it was found that all of these patients had apoptotic epithelial cells ranging from 4.4 to 41.8%. By contrast, in eight normal human lenses of comparable age, very few apoptotic epithelial cells were observed. We suggest that cataract patients may have deficient defense systems against factors such as oxidative stress and UV at the onset of the disease. Such stress can trigger lens epithelial cell apoptosis that then may initiate cataract development. To test this hypothesis, it is also demonstrated here that hydrogen peroxide at concentrations previously found in some cataract patients induces both lens epithelial cell apoptosis and cortical opacity. Moreover, the temporal and spatial distribution of induced apoptotic lens epithelial cells precedes development of lens opacification. These results suggest that lens epithelial cell apoptosis may be a common cellular basis for initiation of noncongenital cataract formation.

Platelet-derived growth factor is a survival factor for PSA-NCAM+ oligodendrocyte pre-progenitor cells

Mature oligodendroglia, which synthesize and express lipids and proteins characteristic of myelin, are generated from precursor cells which are formed in germinal matrix, then migrate widely through the neuraxis. We now demonstrate that these precursor cells can be recognized at a very early stage by their surface expression of polysialylated neural cell adhesion molecules (PSA-NCAM), and only later bind anti-ganglioside antibodies that had previously been used to recognize "O-2A" oligodendroglial precursor cells. PSA-NCAM expression by these cells is likely to be of functional significance, since a recent study demonstrated that O-2A cells become immobile when stripped of PSA-NCAM. Platelet-derived growth factor (PDGF) proved to be a survival factor for these PSA-NCAM+cells, and in a defined medium, PDGF was sufficient to ensure maturation of immunopurified PSA-NCAM+cells to oligodendroglia.

Structure of the embryonic primate spinal cord at the closure of the first reflex arc

Early development of the spinal cord was studied in macaque monkey embryos, using light- and electron microscopy, Golgi impregnation and [3H]thymidine radioautography. All neurons engaged in the first reflex arc are generated before E27 in the 165 day gestation period. The earliest-generated cells differentiate into motoneurons while the commissural and association neurons are generated later. Synaptogenesis starts at E27 in the basal plate, and 2 days later in the alar plate. The first synapses appear as symmetrical junctions situated on the neuronal perikarya and proximal dendrites. Closure of the first spinal reflex are is established within 2 days and follows an antidromic pattern as related to the physiological spread of nerve impulses: synapses on motoneuronal somata and primary dendrites in the basal plate appear first and are followed by synapses in the alar plate, between dorsal root axon collaterals and somata of borderline (commissural/association) neurons. Commissural axons grow towards the floor plate, cross the midline and proceed caudo-rostrally, while association fibers remain ipsilateral. The first wave of apoptosis (programmed cell death) thins out dense populations of nerve and glial cells by E30. Some of the early-generated borderine cells that form commissural and association interneurons, seem to play the role of transient target cells and die once the definitive axonal pathways are established. Since transient cells form numerous synapses, deprivation from the afferent impulses is not a likely cause of their elimination. The present results indicate that the initial developmental events, including formation of the first reflex arc in the primate spinal cord, occur considerably earlier in respect to birth than in other mammals, but that the schedule of cellular events and cellular mechanisms seem to be the same.

Apoptosis in human atherosclerosis and restenosis

BACKGROUND

Apoptosis has been recognized in normal, including rapidly proliferating, cell populations and is inferred to be potentially responsible for the maintenance of stable cell numbers in tissues with various degrees of proliferative activity. Previous studies performed in rats indicated that despite the persistence of a relatively high level of injury-induced proliferative activity, total arterial smooth muscle content at 12 weeks remained unchanged from that measured at 2 weeks, suggesting that accrual of vascular smooth muscle cells is mitigated by cell death. The extent to which apoptosis may be observed in human atherosclerosis and/or restenosis, however, has not been previously established.

METHODS AND RESULTS

We performed immunohistochemical studies on 56 specimens retrieved from patients undergoing directional atherectomy for primary atherosclerotic lesions or recurrent arterial narrowing after percutaneous revascularization (restenosis). Immunohistochemical staining disclosed evidence of apoptosis in 35 (63%) of the 56 specimens studied. When present, immunohistochemical evidence of apoptosis was typically limited to < 2% of cells in the specimen. The finding of apoptosis, however, was not distributed equally among four groups of specimens studied. Specimens retrieved from patients with restenosis were more frequently observed to contain foci of apoptosis than specimens retrieved from patients with primary atherosclerotic lesions. Among 14 peripheral arterial specimens from patients with restenosis, 13 (93%) contained foci of apoptosis; in contrast, apoptosis was observed in only 6 (43%) of 14 peripheral specimens from patients with primary lesions (P = .0046). Among coronary arterial specimens, apoptosis was observed in 12 (86%) of 14 specimens from patients with restenosis versus 6 (29%) of 14 specimens from patients with primary obstructions (P < .0075).

CONCLUSIONS

Apoptosis is a feature of human vascular pathology, including restenotic lesions and, to a lesser extent, primary atherosclerotic lesions. The findings of the present study suggest that apoptosis may modulate the cellularity of lesions that produce human vascular obstruction, particularly those with evidence of more extensive proliferative activity.

Association of nbl gene expression and glucocorticoid-induced apoptosis in mouse thymus in vivo

A gene of unknown biological function, nbl, was originally isolated by virtue of its abundance in a Namalwa Burkitt Lymphoma cDNA library. nbl expression was initially found to be higher in tissues which exhibited internucleosomal DNA cleavage characteristic of apoptosis, than in tissues which did not exhibit a 'DNA ladder'. nbl expression was therefore examined in mouse thymus in vivo, in which apoptosis is induced by the glucocorticoid, dexamethasone. nbl expression was markedly enhanced by dexamethasone treatment and then sharply decreased prior to the occurrence of maximal 'DNA ladder' formation. In contrast, expression of myc, which is believed to be involved in apoptosis in other cell systems, declined as thymic apoptosis increased. Thymic apoptosis was blocked by the transcriptional inhibitor actinomycin D, if administered when nbl expression was enhanced, but not before or after the peak of nbl expression. These results suggest that nbl expression is associated with thymic apoptosis.

Ultrastructural detection of DNA strand breaks in apoptotic neural cells by in situ end-labelling techniques

Recently developed techniques based on 'in situ end-labelling' (ISEL) of DNA strand breaks may help to identify apoptotic cells in tissue sections. We have applied ISEL techniques at the electron microscopic (EM) level, in order to verify if ultrastructural features of apoptosis are indeed associated with evidence of DNA fragmentation, and whether cells committed to, but which have not yet entered the stage of cell death are also labelled. Terminal transferase and DNA polymerase assays were applied to thin sections of Araldite and LR Gold-embedded medulloblastomas and embryonic mouse dorsal root ganglia. Digoxigenin-labelled nucleotides were used; incorporation was demonstrated by immunogold staining. Apoptotic cells in various stages of the death process were easily labelled in both tissues. In addition, DNA fragmentation was demonstrated in cells with initial chromatin condensation, but otherwise indistinguishable from adjacent unstained cells. Our results show that EM-ISEL techniques effectively demonstrate the occurrence of DNA strand breaks in apoptotic and possibly 'pre-apoptotic' cells in neural tissues. Since the labelling is easily obtained on tissue that is routinely processed for electron microscopy, this technique may allow retrospective studies on archival material.

Coordinated induction of the ubiquitin conjugation pathway accompanies the developmentally programmed death of insect skeletal muscle

The developmentally programmed cell death of abdominal intersegmental muscles in the tobacco hawk-moth Manduca sexta is coincident with a 10-fold induction of the polyubiquitin gene as a hormonally regulated event (Schwartz, L. M., Myer, A., Kosz, L., Engelstein, M., and Maier, C. (1990) Neuron 5, 411-419). Solid phase immunochemical assays measuring intersegmental muscle pools of free and conjugated ubiquitin reveal that the induction of polyubiquitin mRNA is accompanied by a proportional increase in total ubiquitin polypeptide. Ubiquitin conjugate pools increase 10-fold at eclosion, during which loss of muscle protein mass is maximum. A smaller but measurable increase in ubiquitin conjugates is observed earlier in pupal development coincident with a modest enhanced degradation of myofibrillar proteins. Accumulation of ubiquitin conjugates is accompanied by induction in the pathway for polypeptide ligation, including the activating enzyme (E1), several carrier protein (E2) isoforms, and ubiquitin:protein isopeptide ligase (E3). Both accumulation of ubiquitin polypeptide and the enzymes of the conjugation pathway are subject to regulation by declining titers of the insect molting hormone 20-hydroxyecdysone, which signals onset of programmed cell death in the intersegmental muscles. Thus, programmed cell death within the intersegmental muscles is accomplished in part by stimulation of the ubiquitin-mediated degradative pathway through a coordinated induction of ubiquitin and the enzymes responsible for its conjugation to yield proteolytic intermediates. This suggests enzymes required for ubiquitin conjugation may represent additional genes recruited for developmentally programmed death.

Apoptosis among CD45RA-/low CD3+ progeny accompanies differentiation of human multinegative thymocytes

Apoptotic cell death is widely believed to be the fate of negatively selected cells in the thymus. In this work we have used multiparameter flow cytometry to analyse reductions in DNA content that occur among differentiating human CD3-4-8- multinegative (MN) thymocytes as they acquire CD3/TCR during in vitro culture. DNA content was measured as the intensity of the DNA-binding dye DAPI. The position of the diploid peak was identified by comparison to chicken red blood cells and to unfractionated uncultured thymocytes which have a sharply defined diploid peak. Apoptosis, was defined as a reduction in DNA content. Apoptotic cell death occurred continuously throughout the 7 day culture period and at the latest stages of culture DNA fragmentation was apparent on gels. Although both CD3- and CD3+ progeny became apoptotic, it was more frequent among the CD3+ progeny of the MN thymocytes. Apoptotic progeny included 60-70% CD3+ cells, while progeny remaining diploid were 8-36% CD3+. Fifty five per cent of CD3+ TCR delta 1+ progeny had less than 75% of the diploid DNA content, while for CD3+ TCR delta 1- progeny only 28% were in this category. CD3+ TCR delta 1+ progeny also comprised 66% of the cycling cells at days 6-7 of culture, suggesting a pattern of rapid cell division followed by apoptotic cell death for this subset. A lack of positive selection in culture may trigger apoptosis among the TCR delta 1 progeny. In contrast, TCR alpha beta progeny arising in culture appear to be less susceptible to apoptosis, perhaps due to their lack of CD4 and CD8. The expression of CD45RA and CD45R0 isoforms were assessed on the progeny of MN thymocytes based on their DNA content. Although 30% of apoptotic progeny expressed CD45RA, it was present at relatively low density compared to that on diploid or cycling cells, 55% of which were CD45RAhi. The majority of apoptotic cells expressed neither CD45RA or CD45R0, but were CD45+, indicating the presence of an isoform not detected by our monoclonal antibodies (MoAbs). This is consistent with speculations that apoptotic cell death among thymocytes is preceded by a transition in CD45 isoform expression. These conditions may model early selective events resulting from high avidity TCR engagement that is independent of CD4 and/or CD8.

Correlation of micronucleus and apoptosis assays with reproductive cell death

The relationship between ionizing radiation-induced cell killing and DNA damage measured by the micronucleus and apoptosis assays was determined in three established cell lines (L929, HL-60, and Chang). Irradiation experiments revealed a dose-dependent increase of micronucleated cells until a certain dose was reached. Above this dose no further increase of the micronucleus frequency was observed, but in HL-60 and Chang cells additional DNA fragmentation was detected by morphological criteria, characteristic of apoptosis. This change was detected at different doses for the three cell lines examined, suggesting the existence of a cell-type-dependent upper limit for the employment of the micronucleus assay. However, the sum of both kinds of cellular DNA damage (e.g. micronucleation and morphological-like apoptosis) led to a significant cell-type-independent correlation with cell survival, even above the dose where micronuclei levels saturated. Therefore, a total cell damage assay, involving the inclusion of micronuclei and morphological-like apoptotic events, should be considered when evaluating the use of a predictor assay for ionizing radiation-induced cell killing, especially in conditions when apoptosis (-like) processes may occur.

Apoptosis occurs in the oligodendroglial lineage, and is prevented by basic fibroblast growth factor

During the perinatal period, oligodendroglial precursor cells proliferate rapidly, then cease dividing and differentiate into oligodendroglia. Many of these newly formed oligodendroglia are destined to die. We now demonstrate that oligodendroglia generated in passaged cultures of rat forebrain oligodendroglial precursor cells after removal of basic fibroblast growth factor (basic FGF) from the medium often undergo internucleosomal DNA nicking and nuclear fragmentation, features characteristic of apoptosis. These alterations are rare in cultures maintained continuously in basic FGF. As in many other cellular lineages susceptible to apoptosis, these degenerative changes can be prevented by treatment with the endonuclease antagonist, aurintricarboxylic acid, or by inhibiting de novo RNA or protein synthesis. Supplementation of the basic FGF-free medium with insulin, insulin-like growth factor-1, platelet-derived growth factor, or ciliary neuronotrophic growth factor also diminishes DNA nicking. Both oligodendroglial differentiation and DNA nicking are induced in basic FGF-treated cultures by inhibiting DNA synthesis with aphidicholin or excess thymidine, thus suggesting a close linkage between the anti-apoptotic, anti-differentiation, and mitogenic effects of basic FGF on the oligodendroglial lineage.

DNA staining changes associated with apoptosis and necrosis in blood lymphocytes of individuals with HIV infection

We used flow cytometry to quantitate cells that die by apoptosis or necrosis. The method uses low concentrations of two DNA binding dyes that allow one to establish selective regions for live, apoptotic, and necrotic cells in a rat thymocytes model. Quantitative analysis of blood lymphocyte death in individuals with HIV infection by this technique shows the presence of nonviable cells that exhibit a spectrum of changes in staining by DNA binding dyes. These changes range from typical features of cells undergoing programmed cell death or apoptosis to changes observed in cells that die by accidental death or necrosis. The proportion of cells exhibiting these lethal changes increases significantly in patients who progress to AIDS, but, although cells with staining features associated with apoptosis and necrosis were both found to be increased in in vitro-activated cells from AIDS patients, spontaneous in vivo activation preferentially leads to apoptotic changes without a significant increase of cells exhibiting the staining changes associated with necrosis.

Apoptosis-inducing and apoptosis-preventing functions of poliovirus

Data showing that an apoptotic reaction (the exit into the cytoplasm and nucleolytic internucleosomal degradation of chromosomal DNA, compaction and fragmentation of chromatin, cellular shrinkage, and cytoplasmic blebbing) developed in a subline of HeLa-S3 cells upon nonpermissive poliovirus infection with either a guanidine-sensitive poliovirus in the presence of guanidine, a guanidine-dependent mutant in the absence of guanidine, or certain temperature-sensitive mutants at a restrictive temperature are presented. Essentially, no apoptotic reaction occurred upon permissive infection of these cells. Both permissive and nonpermissive infections resulted in the inhibition of host protein synthesis. Actinomycin D or cycloheximide also elicited a rapid apoptotic reaction in uninfected cells. However, preinfection or coinfection with poliovirus prevented the apoptotic response to the addition of actinomycin D, and preinfection blocked cycloheximide-induced apoptosis as well. These data fit a model in which the cells used are prepared to develop apoptosis, with their viability due to the presence of certain short-lived mRNA and protein species. Poliovirus infection turns on two oppositely directed sets of reactions. On the one hand, the balance is driven toward apoptosis, probably via the shutoff of host macromolecular synthesis. On the other hand, viral protein exhibits antiapoptotic activity, thereby preventing premature cell death. To our knowledge, this is the first description of an antiapoptotic function for an RNA virus.

Apoptosis (programmed cell death) in arteries of the neonatal lamb

We have examined whether cell death contributes to postnatal remodeling of arteries in lambs. First, abdominal aortic smooth muscle cell proliferation rates fell from 2.87 +/- 0.08% per day at 3 days of age to 1.75 +/- 0.15% per day at 21 days. These proliferation rates would yield a 50% increase in DNA content in the absence of cell death. No increase in DNA content was observed (P < .05 for predicted versus measured accumulation); therefore, significant cell death was inferred. The same analysis did not indicate high cell-death rates in the carotid, renal, or iliac arteries; however, cell death was detected in situ by end-labeling partially degraded DNA with terminal deoxynucleotidyl transferase or by nuclear labeling with propidium iodide, a fluorescent dye that permeates only nonviable cells. Nuclei were labeled in all arteries, although labeling was most frequent in the abdominal aorta, a vessel that regresses substantially after birth. Cell death was apoptotic because DNA extracted from arteries and end-labeled with [32P]dCTP produced a series of low molecular weight bands (DNA ladder) on an agarose gel, a hallmark of apoptosis. The ladder was strong for neonatal abdominal aorta but weak for other arteries. Only weak laddering was observed for fetal abdominal aortas in late gestation, confirming that high apoptosis rates in this vessel were initiated after birth. Intense DNA ladders and frequent in situ labeling indicated high rates of apoptosis in the postnatal intra-abdominal umbilical artery, another vessel that regresses after birth. We conclude that apoptosis contributes to postpartum arterial remodeling. This contribution is greatest in arteries that regress after birth.

Identification of a novel serine/threonine kinase and a novel 15-kD protein as potential mediators of the gamma interferon-induced cell death

Programmed cell death is often triggered by the interaction of some cytokines with their cell surface receptors. Here, we report that gamma interferon (IFN-gamma) induced in HeLa cells a type of cell death that had cytological characteristics of programmed cell death. In this system we have identified two novel genes whose expression was indispensable for the execution of this type of cell death. The rescue was based on positive growth selection of cells after transfection with antisense cDNA expression libraries. The antisense RNA-mediated inactivation of the two novel genes protected the cells from the IFN-gamma-induced cell death but not from the cytostatic effects of the cytokine or from a necrotic type of cell death. One of those genes (DAP-1) is expressed as a single 2.4-kb mRNA that codes for a basic, proline-rich, 15-kD protein. The second is transcribed into a single 6.3-kb mRNA and codes for a unique 160-kD calmodulin-dependent serine/threonine kinase (DAP kinase) that carries eight ankyrin repeats. The expression levels of the two DAP proteins were selectively reduced by the corresponding antisense RNAs. Altogether, it is suggested that these two novel genes are candidates for positive mediators of programmed cell death that is induced by IFN-gamma.

Probing the molecular program of apoptosis by cancer chemopreventive agents

This paper provides a rational molecular basis for studies intended to clarify the interactions between cancer chemopreventive agents and apoptosis, one of the natural forms of cell death that overlaps molecular mechanisms with other forms such as programmed cell death and specialized forms of physiological cell death. Molecular details of the process show the existence of distinct molecular pathways leading to the activation of critical effector elements (apoptosis gene products) functioning under the control of a network of negative regulatory elements. Dysregulation of either apoptosis or anti-apoptosis genes has a significant role in multistage carcinogenesis. Inhibition of apoptosis is one of the underlying mechanisms of the action of tumor promoters. The network of apoptosis and anti-apoptosis gene products provides multiple targets for compounds with cancer chemopreventive potential. Many data in the literature show initiating, potentiating or inhibitory effects of such compounds on apoptosis. However, the molecular mechanism of these effects is largely unknown. We initiated a series of studies using mouse thymocytes which undergo apoptosis through distinct molecular mechanisms after T-cell receptor activation (TCR pathway), following the addition of glucocorticoids (DEX pathway) or DNA damaging agents (p53 pathway). All trans-and 9-cis-retinoic acid induced apoptosis, elicited through the DEX pathway, inhibited the TCR pathway, and did not affect p53- initiated apoptosis. N-acetylcysteine can inhibit all forms. Sodium salicylate enhanced spontaneous cell death, decreased p53-dependent apoptosis, and did not affect the DEX and TCR pathways. These preliminary results, which show differential effects of the studied compounds on distinct molecular pathways of apoptosis, warrant further investigations in the effort to utilize the molecular elements of apoptosis in proper cancer chemoprevention, and find biochemical targets for apoptosis-related surrogate endpoint biomarker assays of chemoprevention.

Programmed cell death in development

Although cell death has long been recognized to be a significant element in the process of embryonic morphogenesis, its relationships to differentiation and its mechanisms are only now becoming apparent. This new appreciation has come about not only through advances in the understanding of cell death in parallel immunological and pathological situations, but also through progress in developmental genetics which has revealed the roles played by death in the cell lineages of invertebrate embryos. In this review, we discuss programmed cell death as it is understood in developmental situations, and its relationship to apoptosis. We describe the morphological and biochemical features of apoptosis, and some methods for its detection in tissues. The occurrence of programmed cell death during invertebrate development is reviewed, as well as selected examples in vertebrate development. In particular, we discuss cell death in the early vertebrate embryo, in limb development, and in the nervous system.

Programmed cell death in Dictyostelium

Programmed cell death (PCD) of Dictyostelium discoideum cells was triggered precisely and studied quantitatively in an in vitro system involving differentiation without morphogenesis. In temporal succession after the triggering of differentiation, PCD included first an irreversible step leading to the inability to regrow at 8 hours. At 12 hours, massive vacuolisation was best evidenced by confocal microscopy, and prominent cytoplasmic condensation and focal chromatin condensation could be observed by electron microscopy. Membrane permeabilization occurred only very late (at 40–60 hours) as judged by propidium iodide staining. No early DNA fragmentation could be detected by standard or pulsed field gel electrophoresis. These traits exhibit some similarity to those of previously described non-apoptotic and apoptotic PCD, suggesting the hypothesis of a single core molecular mechanism of PCD emerging in evolution before the postulated multiple emergences of multicellularity. A single core mechanism would underly phenotypic variations of PCD resulting in various cells from differences in enzymatic equipment and mechanical constraints. A prediction is that some of the molecules involved in the core PCD mechanism of even phylogenetically very distant organisms, e.g. Dictyostelium and vertebrates, should be related.

Cell nucleus and DNA fragmentation are not required for apoptosis

Apoptosis is the predominant form of cell death and occurs under a variety of physiological and pathological conditions. Cells undergoing apoptotic cell death reveal a characteristic sequence of cytological alterations including membrane blebbing and nuclear and cytoplasmic condensation. Activation of an endonuclease which cleaves genomic DNA into internucleosomal DNA fragments is considered to be the hallmark of apoptosis. However, no clear evidence exists that DNA degradation plays a primary and causative role in apoptotic cell death. Here we show that cells enucleated with cytochalasin B still undergo apoptosis induced either by treatment with menadione, an oxidant quinone compound, or by triggering APO-1/Fas, a cell surface molecule involved in physiological cell death. Incubation of enucleated cells with the agonistic monoclonal anti-APO-1 antibody revealed the key morphological features of apoptosis. Moreover, in non-enucleated cells inhibitors of endonuclease blocked DNA fragmentation, but not cell death induced by anti-APO-1. These data suggest that DNA degradation and nuclear signaling are not required for induction of apoptotic cell death.

Differential expression of several E2-type ubiquitin carrier protein genes at different developmental stages in Arabidopsis thaliana and Nicotiana sylvestris

We characterized three genes encoding different E2-type ubiquitin carrier proteins involved in the ubiquitin-mediated proteolytic pathway: UbcAt3 shows homologies to the yeast CDC34 gene and Ub-cAt4a and UbcAt4b are two different genes homologous to the Ubc1/4/5 subfamily in yeast. Their accumulation was analysed and compared with that of the different families encoding polyubiquitins, as well as the monoubiquitin fusion protein, which is considered as a marker for cell division, during various developmental stages including G0/S transition and senescence of higher plant cells. Our results imply that these Ubc genes are under the control of complex mechanisms, and are differentially regulated, but not necessarily co-regulated with ubiquitin genes. Even the closely related UbcAt4a and UbcAt4b genes of the same multigene subfamily are controlled by distinct regulatory mechanisms.

Motoneurons deprived of trophic support in vitro require new gene expression to undergo programmed cell death

During normal development, large numbers of neurons die by programmed cell death. This phenomena has been extensively studied in the lateral motor column of chick embryos, where approximately 50% of the motoneurons that are initially produced, subsequently die due in part to competition for a limited supply of target-derived trophic support. Inhibitors of RNA and protein synthesis block this cell loss in vivo, indicating a requirement for new gene expression (Oppenheim et al., 1990). Prior to their commitment to death, motoneurons can be isolated as a relatively pure population from chick spinal cord for in vitro study. Cells plated with muscle extract, a potent source of target-derived trophic support, survive, and have large, phase-bright cell bodies and extensive neurite outgrowth. In contrast, motoneurons cultured in the absence of muscle extract die within 48 h. This death can be blocked by the RNA synthesis inhibitor actinomycin D, at the time when the cells become committed to die, suggesting that new gene expression is required for cell death. DNA fragmentation and nuclear condensation indicate that some of these cells die by apoptosis. Therefore, it appears that many aspects of motoneuron development observed in vivo can be reconstituted in vitro. These cultures can be used as a model system for studying neuronal death and may contribute to an understanding of the molecular mechanisms that mediate programmed cell death during neuronal development.

Programmed cell death during the earliest stages of spinal cord development in the chick embryo: a possible means of early phenotypic selection

The spatiotemporal distribution of cell death in the chick embryo neural tube and spinal cord (brachial region) was examined between stage (St.) 12 and 22, in plastic semithin sections. Between St. 12 and 16, the total number of pycnotic cells per segment was low, whereas after St. 16 the number of pycnotic cells was substantially increased. Between St. 17 and 19 three cell death foci or regions could be recognized. One region, the dorsal pycnotic zone, was located in the most dorsal part of the spinal cord, including the neural crest, with the highest number of pycnotic cells observed at St. 18. The second region, or ventral pycnotic zone, was located between motoneurons and the floor plate and had the highest number of dying cells at St. 17. The third region, the floor plate pycnotic zone, was located in the midportion of the floor plate and had the greatest amount of cell death at St. 19. Although low numbers of pycnotic cells were also observed in other regions between St. 17 and 19, no pycnotic cells were found in the ventrolateral region that gives rise to motoneurons. Ultrastructural observations as well as data from in situ nick end labeling indicate that the pycnotic cells observed in the neural tube die by apoptosis and that the debris from the dead cells is phagocytized primarily by adjacent healthy neuroepithelial cells. Although the spatiotemporal distribution of pycnotic cells suggests that cell death at these early stages could play a role in establishing the pioneer axonal pathway for spinal commissural neurons, preliminary observations following perturbations of cell death do not support this notion. Alternatively, early cell death may be involved in the regulation of cellular patterning along the dorsoventral axis of the neural tube by a kind of negative selection of specific progenitor cells.

Is apoptosis mediated by reactive oxygen species?

Apoptosis is a common mode of programmed cell death occurring during development as well as in many pathological conditions, in which the cell plays an active role in its own demise. Although the morphological and biochemical hallmarks of apoptosis are conserved across phyla and cell type, the mechanism(s) of apoptosis is unknown. However, data recently published demonstrate that expression of the anti-apoptotic gene bcl-2 decreases the net cellular generation of reactive oxygen species, and that reactive oxygen species serve as mediators of apoptosis in at least some cases.

Ubiquitin and neurofilament expression in anterior horn cells in amyotrophic lateral sclerosis: possible clues to the pathogenesis

Cytoskeletal abnormalities are a prominent pathological feature of anterior horn cells in amyotrophic lateral sclerosis (ALS), and are thought to be involved in the process of motor neuron death. Skein-like filamentous inclusions have been detected by immunocytochemical staining for ubiquitin, a stress protein involved in targeting abnormal proteins for proteolysis. So far, identification of the target protein has been elusive. We have studied the ultrastructural localization of ubiquitin and neurofilaments by post-embedding immunogold staining. In skein-like arrays, strong ubiquitin labelling was concentrated on abnormally formed 15-20 nm filaments; neurofilament labelling was localized on 10 nm filaments adjacent or in continuity with the abnormal filaments. In addition, Bunina bodies were a major site of ubiquitin accumulation. Our results suggest that ubiquitinated filaments in skein-like inclusions might originate from abnormally aggregated neurofilament proteins, which are no longer recognized by antibodies to neurofilament epitopes. Furthermore, the presence of ubiquitin in Bunina bodies suggests that, in addition to its protective role, ubiquitin might be directly implicated in the mechanism of programmed neuronal death in ALS.

Localization of immunoreactive ubiquitin in the nervous system of the Manduca sexta moth

Selective neuronal death is a normal component of metamorphosis in the moth, Manduca sexta. In particular, the three unfused abdominal ganglia of the ventral nerve cord serve as a useful experimental preparation in which to study the regulation of the molecular mechanisms that mediate programmed cell death. Ubiquitin, a highly conserved 76-amino acid protein found in all eukaryotic cells, has previously been shown to be present in increased amounts in some tissues undergoing programmed cell death (e.g., larval intersegmental muscles in Manduca sexta moths, dying cells in developing tunicates), but not in others (T-cells, Drosophila ommatidial cells, cultured sympathetic neurons deprived of nerve growth factor). It has been hypothesized that the need for ubiquitin-dependent proteolysis is increased in dying cells, and that the accumulation of ubiquitin might serve as an early marker for cells committed to die. Immunohistochemical localization of ubiquitin at the light microscopic level in the abdominal ganglia of Manduca sexta suggests that this protein plays a number of important roles in neuronal physiology and may be associated with the death of some neurons in this tissue. The most intense staining of neuronal cytoplasm, however, was found not in dying neurons, but instead in sets of persisting neurons that may serve a primarily neurosecretory or neuromodulatory function. The staining obtained in these cells with antibodies directed against ubiquitin was developmentally regulated.

Morphological and molecular characterization of the response of differentiated PC12 cells to calcium stress

The mechanisms that lead ultimately to neuronal death in pathological ageing of the brain remain mostly unknown as in the case of Parkinson's disease where there is a progressive and selective loss of dopaminergic neurons within the substantia nigra. Dopamine-expressing PC12 cells that were neuronally differentiated by nerve growth factor treatment were chosen as a culture model in which to study some of the changes that may occur during the course of the degenerative process. They were exposed to the calcium ionophore A23187 in order to produce a sustained rise in cytoplasmic calcium, a phenomenon related to various pathological conditions. The degenerative effects of the ionophore were dose- and time-dependent. They were characterized by early fragmentation of the neurites followed ultimately by a loss in cell viability. Biochemical changes, such as a decrease in [3H]dopamine uptake and modulations of the tyrosine hydroxylase gene, were detected before macroscopic evidence of cell suffering (e.g. neurite fragmentation) could be observed. Although an ongoing degenerative process was occurring in cell somata, PC12 cells were able to recover upon ionophore withdrawal. Characteristics of apoptosis such as chromatin condensation and DNA fragmentation were detectable in a small population of dying cells. DNA fragmentation could be prevented by the endonuclease inhibitor aurintricarboxylic acid. New protein synthesis was not required, as cycloheximide failed to prevent degeneration. Taken together, these results suggest that differentiated PC12 cells react to calcium stress through a sequence of regulatory processes which appears to be independent of the apoptotic pathway.

Apoptosis in adult retinal ganglion cells after axotomy

Lesions to the mature mammalian central nervous system cause irreversible degeneration, in which neurons have been previously thought to be passive victims. In this study, axon-lesioned adult rat neurons are shown instead to actively degrade themselves through the process of apoptosis: a programmed type of cell death in which the cellular apparatus is actively involved in the degradation process. To investigate whether retinal ganglion cells of an adult mammal follow an apoptotic type of death when their axons are severed, DNA breaks in nuclei were labeled in situ, using a method that specifically incorporates biotinylated deoxynucleotides by exogenous terminal deoxynucleotidyl transferase on the 3'-OH ends of DNA. The active nature of the death mechanism was demonstrated by the reduction in biotin-labeled nuclei after administering the protein synthesis inhibitor cycloheximide. Our results suggest that retinal ganglion cells of the adult rat die through apoptosis when axotomized. This raises new possibilities in the treatment of CNS injuries, by the potential interruptibility of a program for neuronal death.

Characterization of apoptosis in cultured rat sympathetic neurons after nerve growth factor withdrawal

Sympathetic neurons depend on nerve growth factor (NGF) for their survival both in vivo and in vitro. In culture, the neurons die after NGF withdrawal by an autonomous cell death program but whether these neurons die by apoptosis is under debate. Using vital DNA stains and in situ nick translation, we show here that extensive chromatin condensation and DNA fragmentation occur before plasma membrane breakdown during the death of NGF-deprived rat sympathetic neurons in culture. Furthermore, kinetic analysis of chromatin condensation events within the cell population is consistent with a model which postulates that after NGF deprivation nearly all of the neurons die in this manner. Although the dying neurons display membrane blebbing, cell fragmentation into apoptotic bodies does not occur. Apoptotic events proceed rapidly at around the time neurons become committed to die, regardless of neuronal culture age. However the duration of NGF deprivation required to commit neurons to die, and the rate at which apoptosis occurs, increase with culture age. Thus, within the first week of culture, apoptosis is the predominant form of cell death in sympathetic neurons.

Evidence supporting a role for programmed cell death in focal cerebral ischemia in rats

BACKGROUND AND PURPOSE

Cells die by one of two mechanisms, necrosis or programmed cell death. Necrosis has been implicated in stroke and occurs when the cytoplasmic membrane is compromised. Programmed cell death requires protein synthesis and often involves endonucleolytic cleavage of the cellular DNA. We assessed the potential contribution of programmed cell death to ischemia-induced neuronal death.

METHODS

Cycloheximide (protein synthesis inhibitor; 1 mg/kg per 24 hours) or vehicle (1 mL/kg per 24 hours) was continuously infused into the right cerebral ventricle of spontaneously hypertensive rats. Neocortical focal ischemia was produced by tandem occlusion of the right common carotid artery and the ipsilateral middle cerebral artery. After 24 hours the brain was stained with 2% 2,3,5-triphenyltetrazolium and the ischemic zone quantitated. Protein synthesis was determined by [3H]methionine incorporation into acid-precipitated protein. DNA integrity was determined in isolated DNA by gel electrophoresis and in whole cells by flow cytometry.

RESULTS

Continuous cycloheximide infusion caused approximately 70% reduction in cortical protein synthesis. Cycloheximide also reduced the size of the infarction produced by focal cerebral ischemia when compared with controls (ischemic brain volume, 147.5 +/- 25.9 and 188.7 +/- 16.8 mm3 for cycloheximide and saline, respectively; P < .01), suggesting that protein synthesis may contribute to cell death. Purified DNA from the ischemic zone showed evidence of endonucleolytic degradation when fractionated by gel electrophoresis. Flow cytometric analysis demonstrated increased propidium iodide fluorescence in intact cells isolated from ischemic cortex, indicating an increased accessibility of degraded DNA to the intercalating dye.

CONCLUSIONS

New protein synthesis appears to contribute to ischemic cell death in which endonucleolytic DNA degradation is apparent. These observations implicate programmed cell death in ischemic injury and may open unique therapeutic approaches for the preservation of neurons in stroke.

Ubiquitin pathway involvement in human lymphocyte gamma-irradiation-induced apoptosis

Apoptosis (the classical type of programmed cell death) can be triggered in many cell types by widely diverse stimuli. gamma rays, at low doses, can induce apoptosis in vitro in interphase human lymphocytes. In this type of apoptosis induction, activated gene expression is necessary for the fulfillment of the death program. In this report, we present evidence for a relationship between ubiquitin gene expression or ubiquitination and gamma-irradiation-mediated apoptosis in normal circulating human lymphocytes. Using in vitro nuclear transcription assays (run-on), Northern (RNA) blot analysis, immunolocalization studies, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis after immunoprecipitation, we demonstrate that (i) the ubiquitin mRNA level is increased as a consequence of the activation of ubiquitin gene transcription 15 to 90 min after initiation of apoptosis; (ii) specifically in apoptotic cells, and not in all irradiated cells, nuclear proteins are highly ubiquitinated; and (iii) ubiquitin sequence-specific antisense oligonucleotide inhibition results in a decreased level of ubiquitinated nuclear proteins and considerably diminishes the proportion of cells exhibiting the apoptotic death pattern. Each of these results might be explained by different modifications occurring in irradiated cells. Their convergence strongly suggests that the ubiquitin gene is one of the genes with induced activity in the apoptotic death program and that ubiquitination of nuclear proteins might be involved in chromatin disorganization and oligonucleosomal fragmentation, which are among the key events occurring in apoptosis.

Biochemical events in naturally occurring forms of cell death

Several molecular elements of programmed cell death and apoptosis have recently been revealed. The function of gene products which deliver the lethal 'hit' is still not known. Well-characterized and newly discovered cell surface structures (e.g. antigen receptors, FAS/APO-1), as well as transcriptional factors (steroid receptor, c-myc, P53, retinoblastoma protein and others), have been implicated in the initiation of the death pathway. Negative regulators of the process (ced-9 gene product in programmed death of cells in Caenorhabditis elegans and bcl-2 protein in apoptosis) have been described. Biochemical mechanisms responsible for the silent nature of natural deaths of cells include their rapid engulfment (mainly through integrin receptors), transglutaminase-catalyzed cross-linking of cellular proteins, and fragmentation of DNA. Several lines of evidence suggest that distinct molecular mechanisms may operate in various forms of natural cell death.