Apoptosis is induced in post-mitotic rat sympathetic neurons by arabinosides and topoisomerase II inhibitors in the presence of NGF

Cytosine arabinoside induces phosphorylation of histone H2AX in hippocampal neurons via a noncanonical pathway

Cytosine arabinoside (Ara-C), an anticancer drug, is known to inhibit DNA replication in mitotic cells. Ara-C is also considered to induce DNA damage, leading to neuronal cell death. To identify the mechanism by which Ara-C kills neurons, we assessed the levels of phosphorylated histone H2AX (γ-H2AX), a marker for DNA double-strand breaks (DSBs), in hippocampal neurons cultured for 48 h with Ara-C. There was a time-dependent increase in the percentage of cells accumulating γ-H2AX, but TUNEL staining did not indicate the formation of DSBs. The nuclear spread of γ-H2AX remained after Ara-C was withdrawn. These features of Ara-C-induced γ-H2AX formation were quite distinct from those observed in proliferating pheochromocytoma cells. Furthermore, Ara-C-induced γ-H2AX formation appeared to utilize cyclin-dependent kinase 7, but not ataxia telangiectasia mutated (ATM) or ATM and Rad3 related, which are well-known kinases in γ-H2AX formation. Taken together, our findings indicated that Ara-C stimulated γ-H2AX formation in neurons without DSB formation and utilization of canonical kinases, leading to neuronal cell death.

Mitoxantrone is More Toxic than Doxorubicin in SH-SY5Y Human Cells: A 'Chemobrain' In Vitro Study

The potential neurotoxic effects of anticancer drugs, like doxorubicin (DOX) and mitoxantrone (MTX; also used in multiple sclerosis), are presently important reasons for concern, following epidemiological data indicating that cancer survivors submitted to chemotherapy may suffer cognitive deficits. We evaluated the in vitro neurotoxicity of two commonly used chemotherapeutic drugs, DOX and MTX, and study their underlying mechanisms in the SH-SY5Y human neuronal cell model. Undifferentiated human SH-SY5Y cells were exposed to DOX or MTX (0.13, 0.2 and 0.5 μM) for 48 h and two cytotoxicity assays were performed, the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium (MTT) reduction and the neutral red (NR) incorporation assays. Phase contrast microphotographs, Hoechst, and acridine orange/ethidium bromide stains were performed. Mitochondrial membrane potential was also assessed. Moreover, putative protective drugs, namely the antioxidants N-acetyl-l-cysteine (NAC; 1 mM) and 100 μM tiron, the inhibitor of caspase-3/7, Ac-DEVD-CHO (100 μM), and a protein synthesis inhibitor, cycloheximide (CHX; 10 nM), were tested to prevent DOX- or MTX-induced toxicity. The MTT reduction assay was also done in differentiated SH-SY5Y cells following exposure to 0.2 μM DOX or MTX. MTX was more toxic than DOX in both cytotoxicity assays and according to the morphological analyses. MTX also evoked a higher number of apoptotic nuclei than DOX. Both drugs, at the 0.13 μM concentration, caused mitochondrial membrane potential depolarization after a 48-h exposure. Regarding the putative neuroprotectors, 1 mM NAC was not able to prevent the cytotoxicity caused by either drug. Notwithstanding, 100 μM tiron was capable of partially reverting MTX-induced cytotoxicity in the NR uptake assay. One hundred μM Ac-DEVD-CHO and 10 nM cycloheximide (CHX) also partially prevented the toxicity induced by DOX in the NR uptake assay. MTX was more toxic than DOX in differentiated SH-SY5Y cells, while MTX had similar toxicity in differentiated and undifferentiated SH-SY5Y cells. In fact, MTX was the most neurotoxic drug tested and the mechanisms involved seem dissimilar among drugs. Thus, its toxicity mechanisms need to be further investigated as to determine the putative neurotoxicity for multiple sclerosis and cancer patients.

Electrical stimulation inhibits cytosine arabinoside-induced neuronal death by preventing apoptosis in dorsal root ganglion neurons

The current study aimed to investigate whether electrical stimulation could prevent apoptotic neuronal cell death during treatment with cytosine arabinoside (ara-C). From in-vitro experiments, the effects of electrical stimulation were assessed on neurite fragmentation and neuronal cell death in ara-C-treated dorsal root ganglion (DRG) explants. Ara-C treatment increased neurite fragmentation and neuronal cell death in DRG explants and activated caspase-3 by cleaving it, which could induce apoptosis. Electrical stimulation can significantly reduce neurite fragmentation and neuronal cell death compared with nonelectrically stimulated groups. Furthermore, electrical stimulation inhibited caspase-3 activation and reduced apoptotic neuronal death in DRG explants. It was suggested that the neuroprotective effect of electrical stimulation is likely mediated by the inhibition of caspase-3 activation and therefore the inhibition of apoptosis following ara-C treatment.

Long-term survival and regeneration of neuronal and vasculature cells inside the core region after ischemic stroke in adult mice

Focal cerebral ischemia results in an ischemic core surrounded by the peri-infarct region (penumbra). Most research attention has been focused on penumbra while the pattern of cell fates inside the ischemic core is poorly defined. In the present investigation, we tested the hypothesis that, inside the ischemic core, some neuronal and vascular cells could survive the initial ischemic insult while regenerative niches might exist many days after stroke in the adult brain. Adult mice were subjected to focal cerebral ischemia induced by permanent occlusion of distal branches of the middle cerebral artery (MCA) plus transient ligations of bilateral common carotid artery (CCA). The ischemic insult uniformly reduced the local cerebral blood flow (LCBF) by 90%. Massive cell death occurred due to multiple mechanisms and a significant infarction was cultivated in the ischemic cortex 24 h later. Nevertheless, normal or even higher levels of brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF) persistently remained in the core tissue, some NeuN-positive and Glut-1/College IV-positive cells with intact ultrastructural features resided in the core 7-14 days post stroke. BrdU-positive but TUNEL-negative neuronal and endothelial cells were detected in the core where extensive extracellular matrix infrastructure developed. Meanwhile, GFAP-positive astrocytes accumulated in the penumbra and Iba-1-positive microglial/macrophages invaded the core several days after stroke. The long term survival of neuronal and vascular cells inside the ischemic core was also seen after a severe ischemic stroke induced by permanent embolic occlusion of the MCA. We demonstrate that a therapeutic intervention of pharmacological hypothermia could save neurons/endothelial cells inside the core. These data suggest that the ischemic core is an actively regulated brain region with residual and newly formed viable neuronal and vascular cells acutely and chronically after at least some types of ischemic strokes.

Redox biology in normal cells and cancer: restoring function of the redox/Fyn/c-Cbl pathway in cancer cells offers new approaches to cancer treatment

This review discusses a unique discovery path starting with novel findings on redox regulation of precursor cell and signaling pathway function and identification of a new mechanism by which relatively small changes in redox status can control entire signaling networks that regulate self-renewal, differentiation, and survival. The pathway central to this work, the redox/Fyn/c-Cbl (RFC) pathway, converts small increases in oxidative status to pan-activation of the c-Cbl ubiquitin ligase, which controls multiple receptors and other proteins of central importance in precursor cell and cancer cell function. Integration of work on the RFC pathway with attempts to understand how treatment with systemic chemotherapy causes neurological problems led to the discovery that glioblastomas (GBMs) and basal-like breast cancers (BLBCs) inhibit c-Cbl function through altered utilization of the cytoskeletal regulators Cool-1/βpix and Cdc42, respectively. Inhibition of these proteins to restore normal c-Cbl function suppresses cancer cell division, increases sensitivity to chemotherapy, disrupts tumor-initiating cell (TIC) activity in GBMs and BLBCs, controls multiple critical TIC regulators, and also allows targeting of non-TICs. Moreover, these manipulations do not increase chemosensitivity or suppress division of nontransformed cells. Restoration of normal c-Cbl function also allows more effective harnessing of estrogen receptor-α (ERα)-independent activities of tamoxifen to activate the RFC pathway and target ERα-negative cancer cells. Our work thus provides a discovery strategy that reveals mechanisms and therapeutic targets that cannot be deduced by standard genetics analyses, which fail to reveal the metabolic information, isoform shifts, protein activation, protein complexes, and protein degradation critical to our discoveries.

Nucleolar control of p53: a cellular Achilles' heel and a target for cancer therapy

Nucleoli perform a crucial cell function, ribosome biogenesis, and of critical relevance to the subject of this review, they are also extremely sensitive to cellular stresses, which can cause loss of function and/or associated structural disruption. In recent years, we have learned that cells take advantage of this stress sensitivity of nucleoli, using them as stress sensors. One major protein regulated by this role of nucleoli is the tumor suppressor p53, which is activated in response to diverse cellular injuries in order to exert its onco-protective effects. Here we discuss a model of nucleolar regulation of p53, which proposes that key steps in the promotion of p53 degradation by the ubiquitin ligase MDM2 occur in nucleoli, thus providing an explanation for the observed link between nucleolar disruption and p53 stability. We review current evidence for this compartmentalization in p53 homeostasis and highlight current limitations of the model. Interestingly, a number of current chemotherapeutic agents capable of inducing a p53 response are likely to do so by targeting nucleolar functions and these compounds may serve to inform further improved therapeutic targeting of nucleoli.

Effect of cytosine arabinoside on cerebellar neurofilaments during development: A sexual dimorphism

Previous reports suggest that the resistance of neuronal cytoskeleton to drug toxicity may vary with age and gender. The aim of the present study was to assess the impact of cytosine arabinoside (AraC) treatment on neurofilament (NF) levels and phosphorylation status in the developing cerebellum of male, female and testosterone propionate (1.25 mg/rat)-androgenized female rats. AraC (200 mg/kg bw) was administered from postnatal day (PND) 14–16 and changes in the level and phosphorylation of NFs were detected at PND 16 by Western blot analysis. The drug had no effect in male pups, while it increased the non-phosphorylated NF subunits of medium and low molecular weight in females. Androgenization of females prevented the AraC-induced increase in NF subunits. The levels of estrogen receptor beta (ER-β), known to mediate neuroprotective actions of estrogens in the brain, were significantly higher in the developing female cerebellum, as compared to males and androgenized females.

These data show that the neurofilament cytoskeleton in the developing rat cerebellum exhibits resistance to AraC that appears sexually dimorphic. In young males the resistance is exemplified by a lack of responsiveness, whereas in juvenile females it is presented by an androgenization-sensitive NF upregulation.

Whole-cell recording in isolated primary sensory neurons

The isolated sensory neuron in vitro is a powerful model with which to address a number of important neurobiological questions. Isolated neurons are relatively easy to prepare from both neonatal and adult animals and can be studied both acutely and after considerable time on culture. Intracellular recording is one of the most powerful ways to study these neurons. Methods are described for both the preparation of isolated sensory neurons in vitro as well as for recording major classes of ionic currents (Na(+), K(+), and Ca(2+)) from these neurons with whole cell voltage-clamp techniques. Methods are also provided for an initial characterization of active and passive electrophysiological properties of these neurons in current clamp as well as the use of perforated patch recording as a means to mitigate some of the limitations associated with conventional whole cell patch recording. The reader should be aware that the regulation of ion channels in sensory neurons may very subtle, requiring considerably more sophisticated protocols than have been provided here. The reader should also be aware that there is a tremendous heterogeneity among sensory neurons, which is both a curse and a blessing for those who wish to study them. Thus, the methods provided here should only be considered the starting point for a more detailed analysis of sensory neurons.

Time controlled release of arabinofuranosylcytosine (Ara-C) from agarose hydrogels using layer-by-layer assembly: an in vitro study

Experimentally induced axonal regeneration is compromised by glial scar formation arising from leptomeningeal fibroblasts cells in and around the hydrogel scaffold implanted for nerve repair. Strategies are needed to prevent such fibroblastic reactive cell layer formation for enhanced axonal regeneration. Here, we implement the technique of layer-by-layer assembled degradable, hydrogen bonded multilayers on agarose hydrogels to incorporate an anti-mitotic drug (1-β-D-arabinofuranosylcytosine (Ara-C)) within the agarose hydrogels. We show controlled release of Ara-C under physiological conditions over a period of days. The concentrations of Ara-C released from agarose at the different time points were sufficient to inhibit fibroblast growth in vitro, while not adversely affecting the viability of the neuronal cells.

S phase entry causes homocysteine-induced death while ataxia telangiectasia and Rad3 related protein functions anti-apoptotically to protect neurons

A major phenotype seen in neurodegenerative disorders is the selective loss of neurons due to apoptotic death and evidence suggests that inappropriate re-activation of cell cycle proteins in post-mitotic neurons may be responsible. To investigate whether reactivation of the G1 cell cycle proteins and S phase entry was linked with apoptosis, we examined homocysteine-induced neuronal cell death in a rat cortical neuron tissue culture system. Hyperhomocysteinaemia is a physiological risk factor for a variety of neurodegenerative diseases, including Alzheimer's disease. We found that in response to homocysteine treatment, cyclin D1, and cyclin-dependent kinases 4 and 2 translocated to the nucleus, and p27 levels decreased. Both cyclin-dependent kinases 4 and 2 regained catalytic activity, the G1 gatekeeper retinoblastoma protein was phosphorylated and DNA synthesis was detected, suggesting transit into S phase. Double-labelling immunofluorescence showed a 95% co-localization of anti-bromodeoxyuridine labelling with apoptotic markers, demonstrating that those cells that entered S phase eventually died. Neurons could be protected from homocysteine-induced death by methods that inhibited G1 phase progression, including down-regulation of cyclin D1 expression, inhibition of cyclin-dependent kinases 4 or 2 activity by small molecule inhibitors, or use of the c-Abl kinase inhibitor, Gleevec, which blocked cyclin D and cyclin-dependent kinase 4 nuclear translocation. However, blocking cell cycle progression post G1, using DNA replication inhibitors, did not prevent apoptosis, suggesting that death was not preventable post the G1-S phase checkpoint. While homocysteine treatment caused DNA damage and activated the DNA damage response, its mechanism of action was distinct from that of more traditional DNA damaging agents, such as camptothecin, as it was p53-independent. Likewise, inhibition of the DNA damage sensors, ataxia-telangiectasia mutant and ataxia telangiectasia and Rad3 related proteins, did not rescue apoptosis and in fact exacerbated death, suggesting that the DNA damage response might normally function neuroprotectively to block S phase-dependent apoptosis induction. As cell cycle events appear to be maintained in vivo in affected neurons for weeks to years before apoptosis is observed, activation of the DNA damage response might be able to hold cell cycle-induced death in check.

Different viabilities and toxicity types after 6-OHDA and Ara-C exposure evaluated by four assays in five cell lines

Cell viability studies are useful when screening novel drugs for the diseases that are related to either increased cell death or enhanced cell survival. There are numerous assays but the results that they produce are rarely unanimous. Here we compared the performance of (1) morphological microscopic assay with double DNA staining, (2) propidium iodide-digitonin assay, (3) MTT-assay, and (4) ATP-assay in human neuroblastoma (SH-SY5Y), rat glioma (C6), rabbit smooth muscle (SMC), Chinese hamster ovary (CHO) and monkey fibroblast cells (CV1-P) exposed to cytosine arabinoside (Ara-C) and 6-hydroxydopamine (6-OHDA). We found that neuronal SH-SY5Y cells were most sensitive to both toxins and the results in all viability tests correlated well. All the other cell lines were much more resistant, particularly to Ara-C but also to 6-OHDA. Toxicity of the compounds was best revealed by MTT and ATP assays, measuring the metabolic activity of the cells, and only occasionally by morphological observations or with the propidium iodide-digitonin assay which is based on the cell membrane integrity. In this research, Ara-C induced pure apoptosis whereas the toxicity type of 6-OHDA was dose-dependent. The use of several viability tests and cell lines is recommended when studying cell death, particularly apoptosis, and performance of antiapoptotic compounds.

Effects of AraC treatment on motor coordination and cerebellar cytoarchitecture in the adult rat

Intact cerebellum cytoarchitecture and cellular communication are indispensable for successful motor coordination and certain forms of memory. Cytosine arabinoside (AraC), often used as an anti-neoplastic agent in humans, can have cerebellum-targeting adverse effects. In order to characterize the nature of AraC-induced cerebellar lesions in an adult rodent model, we have administered AraC (400 mg/kg b.w., i.p.) in adult male Wistar rats for 5 days. The animals' walking pattern, motor coordination, locomotion, spatial navigation and cognition were evaluated, along with neurofilament- and calbindin-like distribution in the cerebellum. AraC-treated rats demonstrated a disturbed walking pattern and a reduced ability of motor learning and coordination, indicative of a mild cerebellar deficit. Although the general locomotion and spatial cognition of AraC-treated rats was not significantly altered, their navigation into the water, in terms of swimming velocity, was irregular, compared to vehicle-treated animals. Neurofilament-like immunostaining was reduced in the molecular cerebellar layer, while calbindin D 28 kDa levels were increased in Purkinje neurons, following AraC treatment. Administration of the antioxidant N-acetylcysteine (NAC) (200 mg/kg b.w., p.o.), for 14 days (prior to and during AraC treatment) largely prevented the AraC-induced behavioral deficits. Our in vivo model of neurotoxicity provides data on the AraC-induced behavioral and cellular alterations concerning the adult rat cerebellum. Furthermore, it provides evidence of a possible neuroprophylactic role of the antioxidant N-acetylcysteine in this model of chemotherapy-induced toxicity.

Cytosine beta-D-arabinofuranoside used as a paradigm modifier to increase production of tau aggregates in a cellular model of tauopathy

Intraneuronal deposition of filamentous tau is a hallmark of Alzheimer's disease (AD) and related tauopathies. We developed previously a cellular model recapitulating such tau anomaly and demonstrated therein consistent production of 70-kD tau. Importantly, the 70-kD species appears to derive from tau fragments with carboxy-terminal truncation and is larger than intact tau in size, suggesting the oligomeric nature in its assembly from tau. To generate the 70-kD tau in sufficient quantity for its characterization at the molecular level, we explored and demonstrated herein that cytosine beta-D-arabinofuranoside is a useful paradigm modifier to increase production of the 70-kD tau. Such oligomeric tau was enriched thereafter by immunoprecipitation to remove tau species with intact carboxy-terminus. Two-dimensional gel electrophoresis revealed that the 70-kD tau has an isoelectric point of 5.8-6.0. Future elucidation of key aggregates will provide valuable insights into the natural history of neurofibrillary degeneration and identify novel targets to develop therapeutic interventions.

Neuronal apoptosis and gray matter heterotopia in microcephaly produced by cytosine arabinoside in mice

Primary microcephaly can be accompanied by numerous migration anomalies. This experiment was undertaken to examine the pathogenesis of gray matter heterotopia and microcephaly that is produced after administering cytosine arabinoside (Ara-C) to mice. Pregnant mice were intraperitoneally injected with Ara-C at 30 mg/kg body weight on days 13.5 and 14.5 of gestation, and then their offspring were examined. On embryonic day 15.5, in the ventricular zone of the cingulate cortex, the neuroepithelial cells lacked BrdU immunoreactivity. Nestin-immunoreactive radial glial fibers and calretinin-positive subplate fibers were disrupted. TUNEL reaction was remarkable throughout the cerebral hemisphere. Subcortical heterotopia in the cingulate cortex and subependymal nodular heterotopia in the dorsolateral part of the lateral ventricles became detectable by the first day after birth. Thirty-two days after birth, microcephaly was apparent; subcortical heterotopia was observed to have increased in size while it was still located in the frontal and cingulate cortices. This experiment demonstrated that Ara-C induces neuronal apoptosis throughout the cerebral hemisphere. The immunohistochemical characteristics in the gray matter heterotopia suggest that both the subcortical and the subependymal heterotopias were formed by neurons originally committed to the neocortex. We conclude that the gray matter heterotopia that accompanies the microcephaly was produced by a disturbance of radial, tangential, and interkinetic neuronal migrations due to the toxicity of Ara-C in the immature developing brain.

The BH3-only protein Puma is both necessary and sufficient for neuronal apoptosis induced by DNA damage in sympathetic neurons

DNA damage activates apoptosis in several neuronal populations and is an important component of neuropathological conditions. While it is well established that neuronal apoptosis, induced by DNA damage, is dependent on the key cell death regulators p53 and Bax, it is unknown which proteins link the p53 signal to Bax. Using rat sympathetic neurons as an in vitro model of neuronal apoptosis, we show that cytosine arabinoside is a DNA damaging drug that induces the expression of the BH3-only pro-apoptotic genes Noxa, Puma and Bim. Increased expression occurred after p53 activation, measured by its phosphorylation at serine 15, but prior to the conformational change of Bax at the mitochondria, cytochrome c (cyt c) release and apoptosis. Hence Noxa, Puma and Bim could potentially link p53 to Bax. We directly tested this hypothesis by the use of nullizygous mice. We show that Puma, but not Bim or Noxa, is a crucial mediator of DNA damage-induced neuronal apoptosis. Despite the powerful pro-apoptotic effects of overexpressed Puma in Bax-expressing neurons, Bax nullizygous neurons were resistant to Puma-induced death. Therefore, Puma provides the critical link between p53 and Bax, and is both necessary and sufficient to mediate DNA damage-induced apoptosis of sympathetic neurons.

Delayed neurotoxicity associated with therapy for children with acute lymphoblastic leukemia

Most children diagnosed today with acute lymphoblastic leukemia (ALL) will be cured. However, treatment entails risk of neurotoxicity, causing deficits in neurocognitive function that can persist in the years after treatment is completed. Many of the components of leukemia therapy can contribute to adverse neurologic sequelae, including craniospinal irradiation, nucleoside analogs, corticosteroids, and antifolates. In this review, we describe the characteristic radiographic findings and neurocognitive deficits seen among survivors of childhood ALL. We summarize what is known about the pathophysiology of delayed treatment-related neurotoxicity, with a focus on the toxicity resulting from pharmacologic disruption of folate physiology within the central nervous system. Finally, we suggest testable strategies to ameliorate the symptoms of treatment-related neurotoxicity or decrease its incidence.

Mutually exclusive subsets of BH3-only proteins are activated by the p53 and c-Jun N-terminal kinase/c-Jun signaling pathways during cortical neuron apoptosis induced by arsenite

The c-Jun N-terminal protein kinase (JNK)/c-Jun and p53 pathways form distinct death-signaling modules in neurons that culminate in Bax-dependent apoptosis. To investigate whether this signaling autonomy is due to recruitment of particular BH3-only proteins, we searched for a toxic signal that would activate both pathways in the same set of neurons. We show that arsenite activates both the JNK/c-Jun and p53 pathways in cortical neurons, which together account for >95% of apoptosis, as determined by using the mixed-lineage kinase (JNK/c-Jun) pathway inhibitor CEP11004 and p53-null mice. Despite the coexistence of both pathways in at least 30% of the population, Bim mRNA and protein expression was increased only by the JNK/c-Jun signaling pathway, whereas Noxa and Puma mRNA and Puma protein expression was entirely JNK/c-Jun independent. About 50% of Puma/Noxa expression was p53 dependent, with the remaining signal being independent of both pathways and possibly facilitated by arsenite-induced reduction in P-Akt. However, functionally, Puma was predominant in mediating Bax-dependent apoptosis, as evidenced by the fact that more than 90% of apoptosis was prevented in Puma-null neurons, although Bim was still upregulated, while Bim- and Noxa-null neurons died similarly to wild-type neurons. Thus, the p53 and JNK/c-Jun pathways can activate mutually exclusive subclasses of BH3-only proteins in the same set of neurons. However, other factors besides expression may determine which BH3-only proteins mediate apoptosis.

Short-term serum supplementation improves glucose-oxygen deprivation in primary cortical cultures grown under serum-free conditions

Brain ischemia can be studied in vitro by depriving primary neurons of oxygen and glucose by replacing oxygen with argon and glucose with its antimetabolite 2-deoxy-D-glucose. In this contribution, we explain how to construct a reliably functioning ischemia chamber and use it to study neuronal cell death in neuron-enriched fetal primary cortical cultures grown under serum-free conditions. We observed that these cultures exhibited a significant cell death even during exposure to oxygenated balanced salt solution used as control for oxygen-glucose deprivation. We show that addition of only 2% fetal calf serum 24 h prior, during, and after treatment almost abolished this undesirable cell loss and proportionally increased cell death induced by oxygen-glucose deprivation. Western blots and immunocytochemistry showed that these effects were mainly due to an increase in neuronal viability under control conditions accompanied by a limited glial proliferation independent of the treatment condition. Under these modified conditions, the cultures could also still be effectively preconditioned by a short-term oxygen-glucose deprivation. In summary, this modified protocol combines the advantages of serum-free neuronal culture, where potentially toxic antimitotic substances can be omitted, with a serum-mediated protection of neurons against unspecific factors and concomitant sensitization for oxygen-glucose deprivation.

Cisatracurium, but not mivacurium, inhibits survival and axonal growth of neonatal and adult rat peripheral neurons in vitro

Cisatracurium and mivacurium are widely used neuromuscular blocking drugs. Previous reports have indicated growth-inhibitory effects of cisatracurium, but not mivacurium, on two human cell lines in vitro. These effects were ascribed to oxidative stress elicited by acrylate esters formed during cisatracurium breakdown. The aim of the present study was to investigate whether these agents would possibly interfere with the peripheral nervous system. Survival and axonal growth of rat primary neurons obtained from the superior cervical sympathetic ganglion (SCG) or from the adult dorsal root ganglion (DRG) were investigated after treatment with cisatracurium or mivacurium at concentrations from 1 to 10 microM for 24 h. Cisatracurium, but not mivacurium, significantly decreased neuronal survival in a dose-dependent manner, and axonal length was considerably reduced by cisatracurium as compared to controls. It is concluded that high concentrations of cisatracurium are potentially neurotoxic.

The cytotoxicity and mechanisms of 1,2-naphthoquinone thiosemicarbazone and its metal derivatives against MCF-7 human breast cancer cells

We have investigated the antitumor functions and mechanisms of 1,2-naphthoquinone-2-thiosemicarbazone (NQTS) and its metal complexes (Cu(2+), Pd(2+), and Ni(2+)) against MCF-7 human breast cancer cells. The cells were dosed with these complexes at varying concentrations, and cell viability was measured by a sulforhodamine B (SRB) method. To study mechanisms of action, the complexes were incubated with topoisomerase II (topo II) and supercoiled DNA, linear DNA, nicked open DNA, and relaxed DNA were detected by agarose gel electrophoresis. The results revealed that these complexes are effective antitumor chemicals in inhibiting MCF-7 cell growth, with Ni-NQTS being the most effective among the complexes studied. Our data also indicated that Ni-NQTS is more effective than the commercial antitumor drug, etoposide, based on IC(50) values. The mechanistic study of action showed that metal complexes of NQTS, NQ, and NQTS can only stabilize the single-strand nicked DNA, but not double-strand breakage intermediates. In addition, metal derivatives of these ligands, but not the parent NQ and NQTS, exerted an antagonizing effect on topoisomerase II activity. In summary, chemicals with or without metal derivatives might possess different chemical-topoisomerase II-DNA interactions.

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

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

Calpains mediate p53 activation and neuronal death evoked by DNA damage

DNA damage is an initiator of neuronal death implicated in neuropathological conditions such as stroke. Previous evidence has shown that apoptotic death of embryonic cortical neurons treated with the DNA damaging agent camptothecin is dependent upon the tumor suppressor p53, an upstream death mediator, and more distal death effectors such as caspases. We show here that the calcium-regulated cysteine proteases, calpains, are activated during DNA damage induced by camptothecin treatment. Moreover, calpain deficiency, calpastatin expression, or pharmacological calpain inhibitors prevent the death of embryonic cortical neurons, indicating the important role of calpain in DNA damage-induced death. Calpain inhibition also significantly reduced and delayed the induction of p53. Consistent with the actions of calpains upstream of p53 and the proximal nature of p53 death signaling, calpain inhibition inhibited cytochrome c release and DEVD-AFC cleavage activity. Taken together, our results indicate that calpains are a key mediator of p53 induction and consequent caspase-dependent neuronal death due to DNA damage.

JNK-independent activation of c-Jun during neuronal apoptosis induced by multiple DNA-damaging agents

Activation of the JNK pathway and induction of the AP-1 transcription factor c-Jun are critical for neuronal apoptosis caused by a variety of insults. Ara-C-induced DNA damage caused rapid sympathetic neuronal death that was associated with an increase of c-jun expression. In addition, c-Jun was phosphorylated in its N-terminal transactivation domain, which is important for c-Jun-mediated gene transcription. Blocking c-Jun activation by JNK pathway inhibition prevented neuronal death after stress. In contrast, neither the JNK inhibitor SP600125 nor the mixed lineage kinase inhibitor CEP-1347 prevented cytosine arabinoside-induced neuronal death, demonstrating that the JNK pathway was not necessary for DNA damage-induced neuronal apoptosis. Surprisingly, SP600125 or CEP-1347 could not block c-Jun induction or phosphorylation after DNA damage. Pharmacological inhibitors of cyclin-dependent kinase (CDK) activity completely prevented c-Jun phosphorylation after DNA damage. These results demonstrate that c-Jun activation during DNA damage-induced neuronal apoptosis was independent of the classical JNK pathway and was mediated by a novel c-Jun kinase. Based on pharmacological criteria, DNA damage-induced neuronal c-Jun kinase may be a member of the CDK family or be activated by a CDK-like kinase. Activation of this novel kinase and subsequent phosphorylation of c-Jun may be important in neuronal death after DNA damage.

Ara-C induces apoptosis in monkey fibroblast cells

The effect of cytosine arabinoside (Ara-C) on cell viability has been studied in African green monkey kidney fibroblasts (CV1-P). It has been shown previously that Ara-C- induced cell death in neurons is mediated by apoptosis. We investigated whether Ara-C can induce apoptosis also in CV1-P cells, and if the apoptosis is p53-associated. For comparison, human neuroblastoma cells (SH-SY5Y) were studied as a model of human neuronal cells. SYTO13/propidium iodide staining revealed condensed and fragmented nuclei in both cell lines. Ara-C treatment for 48 h induced approximately 24% apoptosis in CV1-P cells whereas approximately 55% of SH-SY5Y cells were apoptotic. Ara-C increased the level of p53 in both CV1-P and SH-SY5Y cells compared to control. The maximum level of p53 in SH-SY5Y cells was reached at 12 h and this then rapidly faded whereas CV1-P cells p53 levels remained elevated after reaching their maximum. Caspase-3 activity was 5-fold higher in human neuroblastoma cells than in monkey fibroblasts, this reflected the decreased cell viability. Our results prove that Ara-C- induced apoptosis in CV1-P cells is associated with an increase of p53 and activation of caspase-3. Ara-C-induced toxicity in CV1-P cells is modest compared to that seen in neuronal cells.

Oxidative stress mediates neuronal DNA damage and apoptosis in response to cytosine arabinoside

Cytosine arabinoside (AraC) is a nucleoside analog that produces significant neurotoxicity in cancer patients. The mechanism by which AraC causes neuronal death is a matter of some debate because the conventional understanding of AraC toxicity requires incorporation into newly synthesized DNA. Here we demonstrate that AraC-induced apoptosis of cultured cerebral cortical neurons is mediated by oxidative stress. AraC-induced cell death was reduced by treatment with several different free-radical scavengers (N-acetyl-L-cysteine, dipyridamole, uric acid, and vitamin E) and was increased following depletion of cellular glutathione stores. AraC induced the formation of reactive oxygen species in neurons as measured by an increase in the fluorescence of the dye 5-(6)-carboxy-2',7'-dichlorodihydrofluorescein diacetate. AraC produced DNA single-strand breaks as measured by single-cell gel electrophoresis and the level of DNA strand breakage was reduced by treatment with the free radical scavengers. These data support a model in which AraC induces neuronal apoptosis by provoking the generation of reactive oxygen species, causing oxidative DNA damage and initiating the p53-dependent apoptotic program. These observations suggest the use of antioxidant therapies to reduce neurotoxicity in AraC chemotherapeutic regimens.

Neuronal life and death: an essential role for the p53 family

Recent evidence indicates that the p53 tumor suppressor protein, and its related family member, p73, play an essential role in regulating neuronal apoptosis in both the developing and injured, mature nervous system. In the developing nervous system, they do so by regulating naturally-occurring cell death in neural progenitor cells and in postmitotic neurons, acting to ensure the apoptosis of cells that either do not appropriately undergo the progenitor to postmitotic neuron transition, or that fail to compete for sufficient quantities of trophic support. Somewhat surprisingly, in developing postmitotic neurons, p53 plays a proapoptotic role, while a naturally-occurring, truncated form of p73, DeltaNp73, antagonizes p53 and plays an anti-apoptotic role. In the mature nervous system, numerous studies indicate that p53 is essential for the neuronal death in response to a variety of insults, including DNA damage, ischemia and excitotoxicity. It is likely that all of these insults culminate in DNA damage, which may well be a common trigger for neuronal apoptosis. In this regard, the signaling pathways that are responsible for triggering p53-dependent neuronal apoptosis are starting to be elucidated, and involve cell cycle deregulation and activation of the JNK pathway. Finally, accumulating evidence indicates that p53 is perturbed in the CNS in a number of neurodegenerative disorders, leading to the hypothesis that longterm oxidative damage and/or excitotoxicity ultimately trigger p53-dependent apoptosis in the chronically degenerating nervous system.

The Ras/phosphatidylinositol 3-kinase and Ras/ERK pathways function as independent survival modules each of which inhibits a distinct apoptotic signaling pathway in sympathetic neurons

Ras promotes robust survival of many cell systems by activating the phosphatidylinositol 3-kinase (PI3-kinase)/Akt pathway, but little is understood about the survival functions of the Ras/ERK pathway. We have used three different effector-loop mutant forms of Ras, each of which activates a single downstream effector pathway, to dissect their individual contributions to survival of nerve growth factor (NGF)-dependent sympathetic neurons. The PI3-kinase pathway-selective protein Ras(Val-12)Y40C was as powerful as oncogenic Ras(Val-12) in preventing apoptosis induced by NGF deprivation but conferred no protection against apoptosis induced by cytosine arabinoside. Identical results were obtained with transfected Akt. In contrast, the ERK pathway-selective protein Ras(Val-12)T35S had no protective effects on NGF-deprived neurons but was almost as strongly protective as Ras(Val-12) against cytosine arabinoside-induced apoptosis. The protective effects of Ras(Val-12)T35S against cytosine arabinoside were completely abolished by the ERK pathway inhibitor PD98059. Ras(Val-12)E37G, an activator of RalGDS, had no survival effect on either death pathway, similar to RasS17N, the full survival antagonist. Thus, Ras provides two independent survival pathways each of which inhibits a distinct apoptotic mechanism. Our study presents one of the few clear-cut cases where only the Ras/ERK, but not the Ras/PI3K/Akt pathway, plays a dominant survival signaling role.

Despite the internucleosomal cleavage of DNA, reactive oxygen species do not produce other markers of apoptosis in cultured neurons

The cell death induced by hydroxyl radicals generated by Cu-phenanthroline and peroxynitrite generated by 3-morpholinosydnonimine hydrochloride (SIN-1) in rat primary cortical neuronal cultures was compared with the apoptotic death induced by staurosporine and the necrotic death induced by glutamate. Both SIN-1 and Cu-phenanthroline were capable of generating internucleosomal cleavage of DNA-a hallmark of apoptosis. Other characteristics of this cell death, such as nuclear morphology by light microscopy; DNA breaks by single-cell gel electrophoresis; the effects of the apoptotic inhibitors cycloheximide, aurintricarboxylic acid, and tosyl-l-lysine chloromethyl ketone; the measurement of caspase activity; and the effects of antioxidants, were then analyzed. The conclusion from these hallmarks of apoptosis is that the cell death induced by these reactive oxygen species is not apoptosis.

Apoptosis and the nervous system

Apoptosis is now recognized as a normal feature in the development of the nervous system and may also play a role in neurodegenerative diseases and aging. This phenomenon has been investigated intensively during the last 6-7 years, and the progress made in this field is reviewed here. Besides a few in vivo studies, a variety of neuronal preparations from various parts of the brain, the majority of which were primary cultures, and some cell lines have been investigated. Several apoptosis-inducing agents have been identified, and these include lack of neurotrophic support, neurotransmitters, neurotoxicants, modulators of protein phosphorylation and calcium homeostasis, DNA-damaging agents, oxidative stress, nitric oxide, and ceramides. The precise signaling cascade is not well established, and there are lacunae in many suggested pathways. However, it appears certain that the Bcl family of proteins is involved in the apoptotic pathway, and these proteins in turn affect the processing of interleukin-1beta converting enzyme (ICE)/caspases. The available evidence suggests that there may be several apoptotic pathways that may depend on the cell type and the inducing agent, and most of the pathways may converge at the ICE/caspases step.

Autophagy is activated by apoptotic signalling in sympathetic neurons: an alternative mechanism of death execution

Autophagy is a mechanism whereby cells digest themselves from within and so may be used in lieu of apoptosis to execute cell death. Little is known about its role in neurons. In newly isolated sympathetic neurons, two independent apoptotic stimuli, NGF-deprivation or cytosine arabinoside added in the presence of NGF, caused a 30-fold increase in autophagic particle numbers, many autophagosomes appearing before any signs of DNA-fragmentation. The anti-autophagic drug 3-methyladenine also delayed apoptosis, its neuroprotection correlating with inhibition of cytochrome c release from mitochondria and prevention of caspase activation. In contrast, autophagic activity remained elevated in neurons treated with the pan-caspase inhibitor Boc-Asp(OMe)fmk, which inhibited morphological apoptosis but did not inhibit cytochrome c release nor prevent cell death. We propose that the same apoptotic signals that cause mitochondrial dysfunction also activate autophagy. Once activated, autophagy may mediate caspase-independent neuronal death.

Distinct mode of apoptosis induced by genotoxic agent etoposide and serum withdrawal in neuroblastoma cells

Here we compared the features of apoptosis induced by DNA-damaging agent, etoposide, and by withdrawal of the growth factors in NB 2a neuroblastoma cells. We showed that serum deprivation and etoposide induced a distinct pattern of regulation of c-Fos, c-Jun and p53 protein levels, as well as the differential changes in DNA-binding activity of AP-1 and NF-kappaB transcription factors. The late phase of apoptesis induced by serum withdrawal was associated with disintegration of nuclear DNA both into high molecular weight (HMW) and oligonucleosomal DNA fragments, whereas etoposide induced the formation of HMW-DNA fragments without internucleosomal DNA cleavage. Incubation of etoposide-treated cells without serum resulted in an additive effect on the pattern of DNA fragmentation. Differences in DNA fragmentation profiles induced by serum withdrawal and etoposide in NB 2a cells were reproducible in nonproliferating cerebellar granule cells and also in a cell free system assay after treatment of isolated normal nuclei with cytosolic extracts prepared from serum-deprived or etoposide-treated cells. Both HMW and oligonucleosomal DNA fragmentation in serum-deprived cells was inhibited by aurintricarboxylic acid and was completely abrogated by cycloheximide. In contrast, DNA fragmentation in etoposide-treated cells was insensitive to the inhibitory effect of aurintricarboxylic acid, and was not prevented by cycloheximide. Our results indicate that in NB 2a neuroblastoma cells etoposide and serum withdrawal induce a distinct mode of apoptosis which is associated with a distinct pattern of regulation of immediately early response genes in the early phase, and with recruitment of different mechanisms for DNA disintegration in the late phase of apoptosis.

Cyclin-dependent kinases participate in death of neurons evoked by DNA-damaging agents

Previous reports have indicated that DNA-damaging treatments including certain anticancer therapeutics cause death of postmitotic nerve cells both in vitro and in vivo. Accordingly, it has become important to understand the signaling events that control this process. We recently hypothesized that certain cell cycle molecules may play an important role in neuronal death signaling evoked by DNA damage. Consequently, we examined whether cyclin-dependent kinase inhibitors (CKIs) and dominant-negative (DN) cyclin-dependent kinases (CDK) protect sympathetic and cortical neurons against DNA-damaging conditions. We show that Sindbis virus-induced expression of CKIs p16(ink4), p21(waf/cip1), and p27(kip1), as well as DN-Cdk4 and 6, but not DN-Cdk2 or 3, protect sympathetic neurons against UV irradiation- and AraC-induced death. We also demonstrate that the CKIs p16 and p27 as well as DN-Cdk4 and 6 but not DN-Cdk2 or 3 protect cortical neurons from the DNA damaging agent camptothecin. Finally, in consonance with our hypothesis and these results, cyclin D1-associated kinase activity is rapidly and highly elevated in cortical neurons upon camptothecin treatment. These results suggest that postmitotic neurons may utilize Cdk4 and 6, signals that normally control proliferation, to mediate death signaling resulting from DNA-damaging conditions.

Cold jet: a method to obtain pure Schwann cell cultures without the need for cytotoxic, apoptosis-inducing drug treatment

This paper presents a new and gentle method to separate Schwann cells from fibroblasts obtained from foetal rat dorsal root ganglia (DRG). The method exploits the different growth and adhesion characteristics of fibroblasts and Schwann cells under different experimental conditions such that antiproliferative (cytotoxic) drugs or time-consuming centrifugation is not needed. Standard procedures were used to obtain mixed cultures of Schwann cells, fibroblasts and neurons. After about 5 days further purification of the cells was achieved by exploiting the different responses of Schwann cells and fibroblasts to a temperature shock. Cooling the cells with cold phosphate-buffered saline (PBS), followed by pipetting cold medium directly on top of the cells ('cold jet'), resulted in specific detachment of Schwann cells and neurons, whereas fibroblasts remained securely attached. Schwann cells attached to the surface of new, uncoated culture dishes whereas neurons did not. Two cycles of the cold jet procedure resulted in nearly pure (98-100%) cultures of Schwann cells. Besides being gentle, this method is easy and fast, and because cytotoxic drugs are not used, it does not affect cell survival negatively.

5-Azacytidine induces toxicity in PC12 cells by apoptosis

5-Azacytidine (5 Az)is a potent inhibitor of DNA methylation, and it may allow inactive genes to become expressed. In a previous study, we demonstrated that 5 Az administered to the dam induced apoptosis in the brains of fetal mice. In this study, the 5 Az-induced apoptosis was further characterized in differentiated PC 12 cells as a model for neuronal apoptosis. Cell death, determined by the activity of released lactate dehydrogenase (LDH) into the medium, occurred from 24 to 48 hrs after 5 Az treatment. Toxicity for differentiated PC 12 cells was observed on treatment with more than 10(-1) micrograms/ml of 5 Az, and it reached the maximal level at 10 micrograms/ml. Cycloheximide, an inhibitor of protein synthesis, prevented 5 Az toxicity, suggesting that this cell death required protein synthesis which could be related to the activation of a dormant gene(s). Electrophoresis of DNA from 5 Az-treated cells evoked ladder formation, indicating the cleavage of DNA into nucleosomes. Scanning electron microscopy demonstrated bleb formation, the so-called apoptotic bodies on the cell surface. The biochemical and morphological findings indicated that 5 Az-induced cell death occurred in the form of apoptosis. 5 Az-induced cell death was prevented by treatment with cAMP but not by treatment with high K+ or deoxycytidine. These results suggest that a cAMP-sensitive mechanism is involved in 5 Az-induced cell death. PC 12 cells should be of value in elucidating the molecular mechanism of 5 Az-induced neuronal apoptosis.

Cytosine arabinoside is neurotoxic to chick embryo spinal cord motoneurons in culture

Cytosine arabinoside (1-beta-D-arabinofuranosylcytosine, AraC) is a commonly used antimitotic agent that kills proliferating cells by inhibiting DNA synthesis. We report that AraC is toxic to cultured chick embryo spinal cord motoneurons (MTNs) in a concentration-dependent fashion with an EC50 of about 2 microM. Interestingly, this type of MTN death is specific, resembles that occurring upon muscle extract (MEX) trophic deprivation regarding its morphological and temporal characteristics, and has apoptotic features, as judged by observation of nuclear morphology. The death of AraC-treated MTNs can be blocked by 2'-deoxycytidine (dC), a pyrimidine metabolite AraC is structurally related to. Overall, these findings suggest that dC may participate in a pathway, different from inhibition of DNA synthesis, that is necessary for cultured MTNs to respond to the trophic activities present in MEX.

Use of two-dimensional gel electrophoresis to characterize protein synthesis during neuronal death in cerebellar culture

Two-dimensional gel electrophoresis was performed to further investigate the biochemical changes in protein synthesis observed in two neuronal death models, induced respectively by cytosine arabinoside and glutamate. These drugs induced, respectively, apoptotic and necrotic types of cell death in cerebellar cultures, as previously reported. Most of the proteins showed decreased labeling after toxic exposure, as expected, but some polypeptides showed increased labeling or appeared to be newly synthesized. The identification of these polypeptides and their implication in neuronal death are discussed.

Involvement of p53 in DNA strand break-induced apoptosis in postmitotic CNS neurons

The tumour suppressor p53 gene serves as a critical regulator of the cell cycle and of apoptosis following the exposure of normal cells to DNA damage. To examine the role of p53 in postmitotic CNS neurons, we cultured cerebellar neurons from normal wild-type mice and mutant p53-null mice under various conditions inducing neuronal death. When cerebellar neurons from 15- to 16-day postnatal wild-type mice were treated with ionizing radiation or DNA-damaging agents, massive neuron death occurred after 24-72 h. In contrast, neurons from p53-/- mice evidently resisted gamma-irradiation and some DNA-damaging agents, such as etoposide and bleomycin. On the other hand, low-K+ medium-induced apoptosis of cerebellar neurons was not affected by p53 status. Neither cell cycle progression nor DNA synthesis occurred during cell death induced by gamma-irradiation and low-K+ medium, as well as in normal cultures of p53+/+ and p53-/- neurons. These results suggest that p53 is required for the apoptotic death of postmitotic cerebellar neurons induced by DNA strand breaks.

p53 involves cytosine arabinoside-induced apoptosis in cultured cerebellar granule neurons

The tumor suppressor p53 gene plays a key role in controlling the cell cycle checkpoint and in apoptosis following the exposure of normal cells to DNA damage. To investigate the role of p53 in cytosine arabinoside (Ara C)-induced cell death of CNS neurons, we examined the effect of Ara C on the survival of cultured cerebellar granule neurons from normal wild-type and p53 null mutant mice. When the neurons from wild-type mice were cultured with Ara C, they gradually died after 24 h in culture. In contrast, the neurons from p53 null mutant mice showed a resistance to the Ara C neurotoxicity. These results indicate that p53 involves Ara C-induced apoptosis in cultured cerebellar granule neurons, in which DNA damage may initiate the apoptotic death program of the neurons.

Activation of p44 and p42 MAP kinases is not essential for the survival of rat sympathetic neurons

We have examined whether activation of MAP kinases [or extracellular signal-regulated kinases (ERKs)] is required for the survival of rat sympathetic neurons by comparing the actions of three survival factors whose survival-promoting actions can be blocked by neutralizing Fab fragments to p21 ras (Nobes and Tolkovsky, 1995, Eur. J. Neurosci., 7, 344-350), nerve growth factor (NGF), the cytokines ciliary neurotrophic factor (CNTF) and leukaemia inhibitory factor (LIF), and the cyclic AMP analogue 4-(8-chlorophenylthio)cAMP (CPTcAMP). NGF-induced survival was accompanied by an intense (15- to 30-fold) and steady (> 24 h) activation of p44 and p42 ERKs which waned rapidly (t1/2 approximately 30 min) upon NGF withdrawal. However, concentrations of NGF that induced a weak (4- to 5-fold) stimulation of the ERKs were not sufficient to maintain long-term survival. Moreover, prolonged and intense stimulation of the ERKs by NGF for up to 15.5 h was unable to confer long-term survival, since withdrawal of NGF after this time resulted in neuronal death that was kinetically indistinguishable from the death of neurons that had not been exposed to NGF. By contrast, CNTF and LIF continued to support survival for up to 3 days after eliciting only transient (< 30 min and 1 h respectively) activation of p44 and p42 ERKs, while CPTcAMP induced survival for several days without any measurable activation of the ERKs. Taken together, these data suggest that ERK activation per se is neither necessary nor sufficient for survival and that alternative pathways exist for effecting long-term survival of rat sympathetic neurons.

Deoxynucleoside induces neuronal apoptosis independent of neurotrophic factors

Postmitotic sympathetic neurons are known to undergo a programmed cell death (apoptosis) when they are deprived of nerve growth factor (NGF) or treated with arabinofuranosyl nucleoside antimetabolites. Here we report the existence of a biochemical mechanism for the induction of neuronal death by an endogenous nucleoside in the presence of NGF. In support of such a mechanism we show that 2-deoxyadenosine (dAdo) induces apoptosis in chick embryonic sympathetic neurons supported in culture by NGF, excess K+, phorbol 12,13-dibutyrate, or forskolin. Neuronal death was related to a dramatic increase in the dATP content of sympathetic neurons exposed to dAdo (34.96 +/- 5.98 versus 0.75 +/- 0.16 pmol/micrograms protein in untreated controls, n = 9), implicating dATP in the toxicity. Supportive evidence for a central role of dATP was gained by inhibition of kinases necessary for phosphorylation of dAdo. 5'-Iodotubercidin in nanomolar concentrations completely and dose-dependently inhibited formation of dATP and also protected against toxicity of submillimolar concentrations of dAdo in sympathetic neurons. Although some of these actions of dAdo were remarkably similar to those reported for human lymphoid cells, several were uniquely different. For example, [3H]dAdo was not transported into neurons by the nucleoside transporter, and therefore inhibition of the transporter (dilazep, nitrobenzylthioinosine) did not prevent neurotoxicity by dAdo. Precursors of pyrimidine synthesis (2'-deoxycytidine, uridine) or NAD+ synthesis (nicotinamide) were ineffective in protecting sympathetic neurons against dAdo toxicity. Finally, inhibition of adenosine deaminase by deoxycoformycin or erythro-9-(2-hydroxy-3-nonyl) adenine did not potentiate the toxic effects of dAdo. Our results provide evidence for the first time that neuronal cells are as susceptible to nucleoside lethality as human lymphocytes are, and provide a new model to study the salvage pathway of deoxyribonucleosides in controlling neuronal populations through programmed cell death.

Neutralizing anti-p21ras Fabs suppress rat sympathetic neuron survival induced by NGF, LIF, CNTF and cAMP

In purified cultures of newly isolated rat sympathetic neurons plated on laminin, apoptosis is suppressed by the cytokines leukaemia inhibitory factor (LIF) and ciliary neurotrophic factor (CNTF), by the permeant cAMP analogue 8-(4-chlorophenylthio)cAMP, and by nerve growth factor. Whilst nerve growth factor, 8-(4-chlorophenylthio)cAMP and LIF/CNTF initiate survival by using different kinases, in each case survival is inhibited by a Fab fragment of Y13-259, a neutralizing antibody to p21ras proteins, but not by rat IgG Fab. The inhibitory effect of Y13-259 could be partially attenuated by cotrituration of the Fab with T'24(inactive)ras. Thus, prevention of apoptosis in rat sympathetic neurons by several different survival factors appears to be critically dependent on p21ras protein activity.