Cytosine arabinoside induces apoptosis in cerebellar neurons in culture

AraC interacts with p75NTR transmembrane domain to induce cell death of mature neurons

Cytosine arabinoside (AraC) is one of the main therapeutic treatments for several types of cancer, including acute myeloid leukaemia. However, after a high-dose AraC chemotherapy regime, patients develop severe neurotoxicity and cell death in the central nervous system leading to cerebellar ataxia, dysarthria, nystagmus, somnolence and drowsiness. AraC induces apoptosis in dividing cells. However, the mechanism by which it leads to neurite degeneration and cell death in mature neurons remains unclear. We hypothesise that the upregulation of the death receptor p75^NTR is responsible for AraC-mediated neurodegeneration and cell death in leukaemia patients undergoing AraC treatment. To determine the role of AraC-p75^NTR signalling in the cell death of mature neurons, we used mature cerebellar granule neurons' primary cultures from p75^NTR knockout and p75^NTRCys259 mice. Evaluation of neurite degeneration, cell death and p75^NTR signalling was done by immunohistochemistry and immunoblotting. To assess the interaction between AraC and p75^NTR, we performed cellular thermal shift and AraTM assays as well as Homo-FRET anisotropy imaging. We show that AraC induces neurite degeneration and programmed cell death of mature cerebellar granule neurons in a p75^NTR-dependent manner. Mechanistically, Proline 252 and Cysteine 256 residues facilitate AraC interaction with the transmembrane domain of p75^NTR resulting in uncoupling of p75^NTR from the NFκB survival pathway. This, in turn, exacerbates the activation of the cell death/JNK pathway by recruitment of TRAF6 to p75^NTR. Our findings identify p75^NTR as a novel molecular target to develop treatments for counteract AraC-mediated cell death of mature neurons.

Adjusting the neuron to astrocyte ratio with cytostatics in hippocampal cell cultures from postnatal rats: A comparison of cytarabino furanoside (AraC) and 5-fluoro-2'-deoxyuridine (FUdR)

Cell culture studies offer the unique possibility to investigate the influence of pharmacological treatments with quantified dosages applied for defined time durations on survival, morphological maturation, protein expression and function as well as the mutual interaction of various cell types. Cultures obtained from postnatal rat brain contain a substantial number of glial cells that further proliferate with time in culture leading to an overgrowth of neurons with glia, especially astrocytes and microglia. A well-established method to decrease glial proliferation in vitro is to apply low concentrations of cytosine arabinoside (AraC). While AraC primarily effects dividing cells, it has been reported repeatedly that it is also neurotoxic, which is the reason why most protocols limit its application to concentrations of up to 5 μM for a duration of 24 h. Here, we investigated 5-fluoro-2'-deoxyuridine (FUdR) as a possible substitute for AraC. We applied concentrations of both cytostatics ranging from 4 μM to 75 μM and compared cell composition and cell viability in cultures prepared from 0-2- and 3-4-day old rat pups. Using FUdR as proliferation inhibitor, higher ratios of neurons to glia cells were obtained with a maximal neuron to astrocyte ratio of up to 10:1, which could not be obtained using AraC in postnatal cultures. Patch-clamp recordings revealed no difference in the amplitudes of voltage-gated Na+ currents in neurons treated with FUdR compared with untreated control cells suggesting replacement of AraC by FUdR as glia proliferation inhibitor if highly neuron-enriched postnatal cultures are desired.

Microglia in Glia-Neuron Co-cultures Exhibit Robust Phagocytic Activity Without Concomitant Inflammation or Cytotoxicity

A simple method to co-culture granule neurons and glia from a single brain region is described, and microglia activation profiles are assessed in response to naturally occurring neuronal apoptosis, excitotoxin-induced neuronal death, and lipopolysaccharide (LPS) addition. Using neonatal rat cerebellar cortex as a tissue source, glial proliferation is regulated by omission or addition of the mitotic inhibitor cytosine arabinoside (AraC). After 7-8 days in vitro, microglia in AraC(-) cultures are abundant and activated based on their amoeboid morphology, expressions of ED1 and Iba1, and ability to phagocytose polystyrene beads and the majority of neurons undergoing spontaneous apoptosis. Microglia and phagocytic activities are sparse in AraC(+) cultures. Following exposure to excitotoxic kainate concentrations, microglia in AraC(-) cultures phagocytose most dead neurons within 24 h without exacerbating neuronal loss or mounting a strong or sustained inflammatory response. LPS addition induces a robust inflammatory response, based on microglial expressions of TNF-α, COX-2 and iNOS proteins, and mRNAs, whereas these markers are essentially undetectable in control cultures. Thus, the functional effector state of microglia is primed for phagocytosis but not inflammation or cytotoxicity even after kainate exposure that triggers death in the majority of neurons. This model should prove useful in studying the progressive activation states of microglia and factors that promote their conversion to inflammatory and cytotoxic phenotypes.

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.

Assessment of Developmental Toxicants using Human Embryonic Stem Cells

Embryonic stem (ES) cells have potential for use in evaluation of developmental toxicity because they are generated in large numbers and differentiate into three germ layers following formation of embryoid bodies (EBs). In earlier study, embryonic stem cell test (EST) was established for assessment of the embryotoxic potential of compounds. Using EBs indicating the onset of differentiation of mouse ES cells, many toxicologists have refined the developmental toxicity of a variety of compounds. However, due to some limitation of the EST method resulting from species-specific differences between humans and mouse, it is an incomplete approach. In this regard, we examined the effects of several developmental toxic chemicals on formation of EBs using human ES cells. Although human ES cells are fastidious in culture and differentiation, we concluded that the relevancy of our experimental method is more accurate than that of EST using mouse ES cells. These types of studies could extend our understanding of how human ES cells could be used for monitoring developmental toxicity and its relevance in relation to its differentiation progress. In addition, this concept will be used as a model system for screening for developmental toxicity of various chemicals. This article might update new information about the usage of embryonic stem cells in the context of their possible ability in the toxicological fields.

Isolation of cerebellar granule cells from neonatal rats

Cerebellar Granule Cells in Neurotoxicology (Jan Oberdoerster, Aventis Corporation, Research Triangle Park, North Carolina). Cultured neurons allow the researcher to investigate mechanisms of toxicity on a relatively uniform population of cells. Primary cultures of cerebellar granule cells are post-mitotic neurons that are readily isolated and may be used for experimental procedures including electrophysiology, neuronal maturation, and various biochemical and molecular analyses.

Deconstructing the perineuronal net: cellular contributions and molecular composition of the neuronal extracellular matrix

Perineuronal nets (PNNs) are lattice-like substructures of the neural extracellular matrix that enwrap particular populations of neurons throughout the central nervous system. Previous work suggests that this structure plays a major role in modulating developmental neural plasticity and brain maturation. Understanding the precise role of these structures has been hampered by incomplete comprehension of their molecular composition and cellular contributions to their formation, which is studied herein using primary cortical cell cultures. By defining culture conditions to reduce (cytosine-β-d-arabinofuranoside/AraC addition) or virtually eliminate (elevated potassium chloride (KCl) and AraC application) glia, PNN components impacted by this cell type were identified. Effects of depolarizing KCl concentrations alone were also assessed. Our work identified aggrecan as the primary neuronal component of the PNN and its expression was dramatically up-regulated by both depolarization and glial cell inhibition and additionally, the development of aggrecan-positive PNNs was accelerated. Surprisingly, most of the other PNN components tested were made in a glial-dependent manner in our culture system. Interestingly, in the absence of these glial-derived components, an aggrecan- and hyaluronan-reactive PNN developed, demonstrating that these two components are sufficient for base PNN assembly. Other components were expressed in a glial-dependent manner. Overall, this work provides deeper insight into the complex interplay between neurons and glia in the formation of the PNN and improves our understanding of the molecular composition of these structures.

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.

Apoptosis- and necrosis-induced changes in light attenuation measured by optical coherence tomography

Optical coherence tomography (OCT) was used to determine optical properties of pelleted human fibroblasts in which necrosis or apoptosis had been induced. We analysed the OCT data, including both the scattering properties of the medium and the axial point spread function of the OCT system. The optical attenuation coefficient in necrotic cells decreased from 2.2 ± 0.3 mm⁻¹ to 1.3 ± 0.6 mm⁻¹, whereas, in the apoptotic cells, an increase to 6.4 ± 1.7 mm⁻¹ was observed. The results from cultured cells, as presented in this study, indicate the ability of OCT to detect and differentiate between viable, apoptotic, and necrotic cells, based on their attenuation coefficient. This functional supplement to high-resolution OCT imaging can be of great clinical benefit, enabling on-line monitoring of tissues, e.g. for feedback in cancer treatment.

Neurofilament isoform alterations in the rat cerebellum following cytosine arabinoside administration

A number of neurotoxic agents could potentially exert their action by degrading or modifying cytoskeleton components like neurofilaments (NF). Cytosine arabinoside (AraC) is an anticancer drug commonly used in leukemia treatment. Its side effects include neuronal cell death in the cerebellum and severe motor coordination deficits. We have previously shown that AraC administration (400mg/kg bw) in adult rats reduced NF immunostaining in cerebellar neurons. To further delineate the susceptibility of individual NF isoforms (NF-H, NF-M, NF-L) to AraC, in the present study we used Western blot analysis to quantify their level. A significant and selective reduction of NF-H isoform was observed in the cerebellum of AraC-treated animals, compared to the controls. Administration of the antioxidant N-acetylcysteine (NAC) for a period of 14 days (prior to and during AraC treatment), which was previously shown to ameliorate the AraC-induced motor deficits in these animals, largely prevented the reduction in NF-H isoform. Given the significant role of NF proteins and particularly NF-H in maintaining structural integrity and synaptic transport, the observed loss of this isoform may be a key-target of AraC action in cerebellar neurons. Moreover, this study provides further data on the neuroprophylactic role of NAC in vivo against chemotherapy-induced toxicity.

Unique aspects of transcriptional regulation in neurons--nuances in NFkappaB and Sp1-related factors

The unique physiology and function of neurons create differences in their cellular physiology, including their regulation of gene expression. We began several years ago exploring the relationships between the NFκB transcription factor, neuronal survival, and glutamate receptor activation in telencephalic neurons. These studies led us to conclude that this population of cells is nearly incapable of activating the NFκB that is nonetheless expressed at reasonable levels. A subset of the κB cis elements are instead bound by members of the Sp1 family in neurons. Also surprising was our discovery that Sp1 itself, typically described as ubiquitous, is severely restricted in expression within forebrain neurons; Sp4 seems to be substituted during neuronal differentiation. These findings and their implications for neuronal differentiation – as well as potential dedifferentiation during degenerative processes – are discussed here.

CNS progenitor cells and oligodendrocytes are targets of chemotherapeutic agents in vitro and in vivo

Background

Chemotherapy in cancer patients can be associated with serious short- and long-term adverse neurological effects, such as leukoencephalopathy and cognitive impairment, even when therapy is delivered systemically. The underlying cellular basis for these adverse effects is poorly understood.

Results

We found that three mainstream chemotherapeutic agents – carmustine (BCNU), cisplatin, and cytosine arabinoside (cytarabine), representing two DNA cross-linking agents and an antimetabolite, respectively – applied at clinically relevant exposure levels to cultured cells are more toxic for the progenitor cells of the CNS and for nondividing oligodendrocytes than they are for multiple cancer cell lines. Enhancement of cell death and suppression of cell division were seen in vitro and in vivo. When administered systemically in mice, these chemotherapeutic agents were associated with increased cell death and decreased cell division in the subventricular zone, in the dentate gyrus of the hippocampus and in the corpus callosum of the CNS. In some cases, cell division was reduced, and cell death increased, for weeks after drug administration ended.

Conclusion

Identifying neural populations at risk during any cancer treatment is of great importance in developing means of reducing neurotoxicity and preserving quality of life in long-term survivors. Thus, as well as providing possible explanations for the adverse neurological effects of systemic chemotherapy, the strong correlations between our in vitro and in vivo analyses indicate that the same approaches we used to identify the reported toxicities can also provide rapid in vitro screens for analyzing new therapies and discovering means of achieving selective protection or targeted killing.

Workgroup report: incorporating in vitro alternative methods for developmental neurotoxicity into international hazard and risk assessment strategies

This is the report of the first workshop on Incorporating In Vitro Alternative Methods for Developmental Neurotoxicity (DNT) Testing into International Hazard and Risk Assessment Strategies, held in Ispra, Italy, on 19-21 April 2005. The workshop was hosted by the European Centre for the Validation of Alternative Methods (ECVAM) and jointly organized by ECVAM, the European Chemical Industry Council, and the Johns Hopkins University Center for Alternatives to Animal Testing. The primary aim of the workshop was to identify and catalog potential methods that could be used to assess how data from in vitro alternative methods could help to predict and identify DNT hazards. Working groups focused on two different aspects: a) details on the science available in the field of DNT, including discussions on the models available to capture the critical DNT mechanisms and processes, and b) policy and strategy aspects to assess the integration of alternative methods in a regulatory framework. This report summarizes these discussions and details the recommendations and priorities for future work.

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.

Concentration of nucleosides and related compounds in cerebral and cerebellar cortical areas and white matter of the human brain

1. Nucleosides potentially participate in the neuronal functions of the brain. However, their distribution and changes in their concentrations in the human brain is not known. For better understanding of nucleoside functions, changes of nucleoside concentrations by age and a complete map of nucleoside levels in the human brain are actual requirements. 2. We used post mortem human brain samples in the experiments and applied a recently modified HPLC method for the measurement of nucleosides. To estimate concentrations and patterns of nucleosides in alive human brain we used a recently developed reverse extrapolation method and multivariate statistical analyses. 3. We analyzed four nucleosides and three nucleobases in human cerebellar, cerebral cortices and in white matter in young and old adults. Average concentrations of the 308 samples investigated (mean+/-SEM) were the following (pmol/mg wet tissue weight): adenosine 10.3+/-0.6, inosine 69.5+/-1.7, guanosine 13.5+/-0.4, uridine 52.4+/-1.2, uracil 8.4+/-0.3, hypoxanthine 108.6+/-2.0 and xanthine 54.8+/-1.3. We also demonstrated that concentrations of inosine and adenosine in the cerebral cortex and guanosine in the cerebral white matter are age-dependent. 4. Using multivariate statistical analyses and degradation coefficients, we present an uneven regional distribution of nucleosides in the human brain. The methods presented here allow to creation of a nucleoside map of the human brain by measuring the concentration of nucleosides in microdissected tissue samples. Our data support a functional role for nucleosides in the brain.

An inhibitor of DNA recombination blocks memory consolidation, but not reconsolidation, in context fear conditioning

Genomic recombination requires cutting, processing, and rejoining of DNA by endonucleases, polymerases, and ligases, among other factors. We have proposed that DNA recombination mechanisms may contribute to long-term memory (LTM) formation in the brain. Our previous studies with the nucleoside analog 1-beta-D-arabinofuranosylcytosine triphosphate (ara-CTP), a known inhibitor of DNA ligases and polymerases, showed that this agent blocked consolidation of conditioned taste aversion without interfering with short-term memory (STM). However, because polymerases and ligases are also essential for DNA replication, it remained unclear whether the effects of this drug on consolidation were attributable to interference with DNA recombination or neurogenesis. Here we show, using C57BL/6 mice, that ara-CTP specifically blocks consolidation but not STM of context fear conditioning, a task previously shown not to require neurogenesis. The effects of a single systemic dose of cytosine arabinoside (ara-C) on LTM were evident as early as 6 h after training. In addition, although ara-C impaired LTM, it did not impair general locomotor activity nor induce brain neurotoxicity. Importantly, hippocampal, but not insular cortex, infusions of ara-C also blocked consolidation of context fear conditioning. Separate studies revealed that context fear conditioning training significantly induced nonhomologous DNA end joining activity indicative of DNA ligase-dependent recombination in hippocampal, but not cortex, protein extracts. Finally, unlike inhibition of protein synthesis, systemic ara-C did not block reconsolidation of context fear conditioning. Our results support the idea that DNA recombination is a process specific to consolidation that is not involved in the postreactivation editing of memories.

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.

Optimized protocols for the simultaneous preparation of primary neuronal cultures of the neocortex, hippocampus and cerebellum from individual newborn (P0.5) C57Bl/6J mice

Knockout mouse models allow preparation of primary neuronal cultures from distinct brain regions in order to investigate the underlying neuronal pathomechanisms of human metabolic diseases associated with severe, regionally distinct brain pathologies (e.g. Zellweger syndrome, the most severe form of a peroxisomal biogenesis disorder). However, homozygous mouse pups with Zellweger syndrome usually die shortly after birth. Therefore, in this study, we established optimized protocols for the simultaneous preparation and cultivation of serum-free primary neuronal cultures from distinct brain regions (medial neocortex, hippocampus and cerebellum) from individual newborn (P0.5) C57Bl/6J mice. For each of the three types of neuronal cultures, we have optimized the isolation procedures and cultivation conditions including coating substrates, enzyme digestion, mode of trituration, seeding density and composition of the culture medium. As indicated by indirect immunofluorescence using antibodies against NeuN, GFAP and CNPase, the purity of the distinct neuronal cultures was high. The percentage of oligodendrocytes was less than 1% in all neuronal cultures. Only 5% astrocytes were present in cortical, 7% in hippocampal and 10% in cerebellar cultures. Cytosine arabinofuranoside (AraC) treatment reduced the percentage of astrocytes only significantly in hippocampal cultures, however, increased the percentage of apoptotic neurons in hippocampal and cortical cultures.

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.

Patterned Purkinje cell death in the cerebellum

The object of this review is to assemble much of the literature concerning Purkinje cell death in cerebellar pathology and to relate this to what is now known about the complex topography of the cerebellar cortex. A brief introduction to Purkinje cells, and their regionalization is provided, and then the data on Purkinje cell death in mouse models and, where appropriate, their human counterparts, have been arranged according to several broad categories--naturally-occurring and targeted mutations leading to Purkinje cell death, Purkinje cell death due to toxins, Purkinje cell death in ischemia, Purkinje cell death in infection and in inherited disorders, etc. The data reveal that cerebellar Purkinje cell death is much more topographically complex than is usually appreciated.

Overexpression of the chromosome 21 transcription factor Ets2 induces neuronal apoptosis

Down syndrome (trisomy 21) neurons display an increased rate of apoptosis in vitro. The genes on chromosome 21 that mediate this increased cell death remain to be elucidated. Here we show that the chromosome 21 transcription factor Ets2, a gene that is overexpressed in Down syndrome, is expressed in neurons, and that moderate overexpression of Ets2 leads to increased apoptosis of primary neuronal cultures from Ets2 tg mice that involves activation of caspase-3. Our data therefore suggest that overexpression of ETS2 may contribute to the increased rate of apoptosis of neurons in Down syndrome.

Inhibiting cell proliferation during formation of the glial scar: effects on axon regeneration in the CNS

Following a CNS lesion many glial cell types proliferate and/or migrate to the lesion site, forming the glial scar. The majority of these cells express chondroitin sulphate proteoglycans (CS-PGs), previously shown to inhibit axonal growth. In this study, in an attempt to diminish glial scar formation and improve axonal regeneration, proliferating cells were eliminated from the lesion site. Adult rats received a continuous infusion of 2% cytosine-D-arabinofuranoside (araC) or saline for 7 days over the lesion site, immediately following a unilateral transection of the right medial forebrain bundle. Additional groups of rats that received subdural infusions prior to the lesion, and lesioned rats which received no infusion, were also compared in the analyses. Animals were killed at 4, 7, 12 or 18 days post-lesion (dpl) and immunohistochemistry was used to determine the effects of these treatments on tyrosine hydroxylase (TH)-lesioned axons, and on the injury response of glial cells. Almost complete elimination of NG2 oligodendrocyte progenitor cells from the lesion site was seen up to 7 dpl in araC-infused animals; reduced numbers of reactive CD11b microglia were also seen but no effects were seen on the injury response of GFAP astrocytes. Significantly more TH axons were seen distal to the lesion in araC-treated brains, but these numbers dwindled by 18 dpl.

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.

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.

An age-related increase in resistance to DNA damage-induced apoptotic cell death is associated with development of DNA repair mechanisms

Neurons in the developing brain die via apoptosis after DNA damage, while neurons in the adult brain are generally resistant to these insults. The basis for this resistance is a matter of conjecture. We report here that cerebellar granule neurons (CGNs) in culture lose their competence to die in response to DNA damage as a function of time in culture. CGNs at either 1 day in vitro (DIV) or 7 DIV were treated with the DNA damaging agents camptothecin, UV or gamma-irradiation and neuronal survival measured. The younger neurons were effectively killed by these agents, while the older neurons displayed a significant resistance to killing. Neuronal survival did not change with time in culture when cells were treated with C2-ceramide or staurosporine, agents which do not target DNA. The resistance to UV irradiation developed over time in culture and was not due to changes in mitotic rate. Increases in DNA strand breakage, up-regulation of the levels of both p53 and its phosphorylated form and nuclear translocation of p53 were equivalent in both older and younger neurons, indicating a comparable p53 stress response. In addition, we show that treatment of older neurons with pharmacological inhibitors of distinct components of the DNA repair machinery promotes the accumulation of DNA damage and sensitizes these cells to the toxic effects of UV exposure. These data demonstrate that older neurons appear to be more proficient in DNA repair in comparison to their younger counterparts, and that this leads to increased survival after DNA damage.

Increase in glutamate-induced neurotoxicity by activated astrocytes involves stimulation of protein kinase C

Activation of astrocytes is a common feature of neurological disorders, but the importance of this phenomenon for neuronal outcome is not fully understood. Treatment of mixed hippocampal cultures of neurones and astrocytes from day 2-4 in vitro (DIV 2-4) with 1 micro m cytosine arabinofuranoside (AraC) caused an activation of astrocytes as detected by a stellate morphology and a 10-fold increase in glial fibrillary acidic protein (GFAP) level compared with vehicle-treated cultures. After DIV 12, we determined 43% and 97% damaged neurones 18 h after the exposure to glutamate (1 mm, 1 h) in cultures treated with vehicle and AraC, respectively. Dose-response curves were different with a higher sensitivity to glutamate in cultures treated with AraC (EC50 = 0.01 mm) than with vehicle (EC50 = 0.12 mm). The susceptibility of neurones to 1 mm glutamate did not correlate with the percentage of astrocytes and was insensitive to an inhibition of glutamate uptake. In cultures treated with vehicle and AraC, glutamate-induced neurotoxicity was mediated through stimulation of the NR1-NR2B subtype of NMDA receptors, because it was blocked by the NMDA receptor antagonist MK-801 and the NR1-NR2B selective receptor antagonist ifenprodil. Protein levels of the NR2A and NR2B subunits of NMDA receptor were similar in cultures treated with vehicle or AraC. AraC-induced changes in glutamate-induced neurotoxicity were mimicked by activation of protein kinase C (PKC), whereas neuronal susceptibility to glutamate was reduced in cultures depleted of PKC and treated with AraC suggesting that the increase in glutamate toxicity by activated astrocytes involves activation of PKC.

High extracellular potassium protects against the toxicity of cytosine arabinoside but is not required for the survival of cerebellar granule cells in vitro

Depolarization of cerebellar granule cells with elevated potassium has been described as essential to maintain their survival in culture. There are several reports that this is only specific for rat cerebellar granule cells and not those of mouse. We reinvestigated this issue and found that although high potassium enhanced the survival of cerebellar granule cells from both rat and mouse it was not essential for the survival of those cultures. Further analysis of the culture system indicated that high potassium offered protection against the toxicity of glutamate and cytosine arabinose (Ara C), a standard antimitotic additive to cultures of granule cells. Ara C was found to be toxic to cerebellar cells after potassium withdrawal at concentrations standardly used in culturing these cells (10 microM). High potassium was found to diminish the expression of p53. Ara C toxicity is known to utilize the p53-dependent signaling pathway to initiate apoptosis. Another depolarizing agent, veratridine, offers no protection against Ara C but we provide evidence that the protective effect of high potassium against Ara C is mediated through calcium balance within the cells. We suggest that there is no requirement for high potassium in terms of cerebellar granule cell survival. The previously proposed role for high potassium in the survival cerebellar granule cells is rather a protective effect against toxic substances in serum such as glutamate or against agents such as Ara C.

A dominant negative inhibitor of the Egr family of transcription regulatory factors suppresses cerebellar granule cell apoptosis by blocking c-Jun activation

To investigate the role of the Egr family of transcription regulatory factors in neuronal apoptosis, we examined the effect of a dominant negative Egr inhibitor construct in a well characterized in vitro paradigm, cerebellar granule cell death induced by withdrawal of depolarizing concentrations of extracellular potassium. We found that this apoptotic stimulus increases the activity of a reporter gene driven by the Egr response element and that a dominant negative inhibitor of Egr-mediated transcription blocks granule cell apoptosis. In contrast, apoptosis of immature granule cells induced by cytosine arabinoside is not inhibited by the Egr inhibitor construct. Because activation of c-Jun is an essential step in granule cell death induced by potassium deprivation, but not cytosine arabinoside, we asked whether the Egr inhibitor acts by influencing c-Jun activation or its ability to induce apoptosis. We found that the Egr inhibitor does not block the ability of a constitutively active c-Jun construct to induce apoptosis in these cells but does suppress activation of c-Jun-mediated transcription induced by lowering extracellular potassium concentration. Furthermore, the Egr inhibitor blocks the ability of MEKK1 [mitogen-activated protein kinase (MAPK) kinase kinase 1], an upstream kinase capable of stimulating the JNK (c-Jun N-terminal protein kinase)-c-Jun pathway, to induce apoptosis and activate c-Jun. Together, these studies indicate that the Egr family of transcription factors plays a critical role in neuronal apoptosis and identify c-Jun activation as an important downstream target of the Egr family in this process.

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.

Sequential treatment of SH-SY5Y cells with retinoic acid and brain-derived neurotrophic factor gives rise to fully differentiated, neurotrophic factor-dependent, human neuron-like cells

A rapid and simple procedure is presented to obtain nearly pure populations of human neuron-like cells from the SH-SY5Y neuroblastoma cell line. Sequential exposure of SH-SY5Y cells to retinoic acid and brain-derived neurotrophic factor in serum-free medium yields homogeneous populations of cells with neuronal morphology, avoiding the presence of other neural crest derivatives that would normally arise from those cells. Cells are withdrawn from the cell cycle, as shown by 5-bromo-2'-deoxyuridine uptake and retinoblastoma hypophosphorylation. Cell survival is dependent on the continuous presence of brain-derived neurotrophic factor, and removal of this neurotrophin causes apoptotic cell death accompanied by an attempt to reenter the cell cycle. Differentiated cells express neuronal markers, including neurofilaments, neuron-specific enolase, and growth-associated protein-43 as well as neuronal polarity markers such as tau and microtubule-associated protein 2. Moreover, differentiated cultures do not contain glial cells, as could be evidenced after the negative staining for glial fibrillary acidic protein. In conclusion, the protocol presented herein yields homogeneous populations of human neuronal differentiated cells that present many of the characteristics of primary cultures of neurons. This model may be useful to perform large-scale biochemical and molecular studies due to its susceptibility to genetic manipulation and the availability of an unlimited amount of cells.

Geranylgeranyl-pyrophosphate, an isoprenoid of mevalonate cascade, is a critical compound for rat primary cultured cortical neurons to protect the cell death induced by 3-hydroxy-3-methylglutaryl-CoA reductase inhibition

We investigated the role of the intrinsic mevalonate cascade in the neuronal cell death (NCD) induced by the inhibition of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase in rat primary cortical neurons cultured from the brains of 17-d-old fetal SD rats. HMG-CoA reductase inhibitors induced NCD [HMG-CoA reductase inhibitor-induced NCD (H-NCD)] in time- and dose-dependent manners. The apoptotic characteristics were revealed by the formation of the DNA ladder and by the electron microscopical observation. During the progression of H-NCD, p53 was induced followed by the expression of Bax. Although the mevalonate completely inhibited H-NCD, the cholesterol did not. Thus, we examined two major metabolites of mevalonate, geranylgeranyl-pyrophosphate (GGPP) and farnesyl-pyrophosphate (FPP), using a novel liposome system for uptake into the cells. GGPP, not FPP, prohibited H-NCD with inhibition of the induction of p53 and Bax. The inhibition of HMG-CoA reductase decreased the amount of membrane-associated Rho small GTPase families, but not Ras small GTPase, and GGPP restored the blockage by HMG-CoA reductase inhibitor in the translocation or redistribution of Rho small GTPase families to membrane. These data indicated that (1) the inhibition of the intrinsic mevalonate cascade induces the apoptotic NCD with the induction of p53 followed by that of Bax, (2) the inhibition of HMG-CoA reductase concomitantly causes blockage of the translocation or redistribution of Rho small GTPase families, not Ras small GTPase, to membrane, and (3) GGPP, not FPP, is one of the essential metabolites in the mevalonate cascade for protecting neurons from H-NCD.

Involvement of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and p53 in neuronal apoptosis: evidence that GAPDH is upregulated by p53

We recently reported that cytosine arabinoside (AraC)-induced apoptosis of cerebellar neurons involves the overexpression of glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The present study was undertaken to investigate whether p53 and/or Bax overexpression participates in the AraC-induced apoptosis of cerebellar granule cells and, if so, the relationship between p53 induction and GAPDH overexpression in these cells. AraC-induced apoptosis of cerebellar granule cells was preceded by an increase in levels of p53 mRNA and protein detected between 1 and 8 hr after treatment. The mRNA level for a p53 target gene, Bax, was also increased. The increase in GAPDH mRNA lasted longer than that of either p53 or Bax, and the level of GAPDH protein in the particulate fraction increased after induction of GAPDH mRNA. The antisense oligonucleotide to p53 protected granule cells from AraC-induced chromatin condensation, internucleosomal cleavage, and apoptotic death. The inhibition of p53 expression by the p53 antisense oligonucleotide not only blocked the expression of Bax but also partially suppressed the increased GAPDH mRNA and protein levels. Conversely, the suppression of GAPDH expression and subsequent attenuation of apoptosis of granule cells by GAPDH antisense oligonucleotide did not influence the expression of p53 or Bax. Cerebellar granule cells prepared from p53 knock-out mice were resistant to AraC toxicity, and the p53 gene knock-out suppressed AraC-upregulated GAPDH expression. Moreover, infection of PC12 cells with an adenoviral vector containing p53 gene dramatically increased GAPDH expression and triggered cell apoptosis. These results suggest that AraC-induced apoptosis of cerebellar granule cells involves the expression of both GAPDH and p53 and that, similar to Bax, GAPDH is upregulated by p53 after exposure to the apoptotic insult.

Glutamate and antimitotic agents induce differentiation, p53 activation, and apoptosis in rodent neostriatal cell lines immortalized with the tsA58 allele of SV40 large T antigen

The tsA58 allele of SV40 large T antigen has the ability to immortalize cells, which is thought to be due, in part, to binding of p53 protein by T antigen at 33 degrees C. At the nonpermissive temperature (39.5 degrees C), it is thought that p53 is released, inducing growth arrest, vulnerability to apoptosis, and loss of the immortal phenotype. In cell lines derived from the rat neostriatum immortalized with tsA58, the toxic agents Adriamycin, cytosine arabinoside, and glutamate induced apoptosis and increased p53 activity and differentiation. The apoptosis and p53-inducing effects of the drugs were not greater at 39.5 degrees C compared to 33 degrees C, suggesting that p53 is not effectively blocked even at 33 degrees C. Growth arrest was not induced under most treatment conditions despite p53 induction. On the other hand, process extension was enhanced at 39.5 degrees C compared to 33 degrees C. Therefore, these cell lines are temperature sensitive with respect to differentiation, but not growth regulation or apoptosis.

The mechanism of Ara-C-induced apoptosis of differentiating cerebellar granule neurons

Neurotoxicity is one of the side-effects of the therapeutically useful antitumour agent, Ara-C (or 1-beta-d-arabinofuranosyl-cytosine, cytarabine). This agent is also reported to induce cell death of cultured neurons. In this study, we show that Ara-C-induced death of differentiating rat cerebellar granule neurons is prevented by cycloheximide at concentrations corresponding to its action in preventing protein synthesis. The death is accompanied by cleavage of the caspase substrate poly ADP ribose polymerase (PARP) and c-Abl-dependent activation of the stress-activated protein kinases c-Jun N-terminal kinase and p38. However, c-Jun levels do not rise and the activation of the stress-activated protein kinases is not required for this form of neuronal death. Cyclin-dependent kinase (cdk) activity and inappropriate cell-cycle re-entry have been implicated in some forms of death in differentiated neurons. Here we show that Ara-C-induced death of cerebellar granule neurons is prevented by an inhibitor of cdk4, whereas inhibition of cdk1, -2 and -5 mimics the death, and non-cdk4/6 cdks are inhibited by Ara-C treatment. Cdk1 and -2 are dramatically down-regulated during neuronal differentiation, and neither Ara-C nor inhibition of these cdks induces death in mature neurons. This mechanism could also play a significant role in the neurotoxicity associated with the therapeutic use of Ara-C, as cdk levels can be upregulated in stressed neurons of adult brain. We propose that the balance between cdk4/6 and cdk1/2/5 activity may determine the survival of early differentiating neurons, and that DNA-damaging agents may induce neuronal death by inhibiting cdk1/2/5 under conditions which require these activities for survival.

A role for MAPK/ERK in sympathetic neuron survival: protection against a p53-dependent, JNK-independent induction of apoptosis by cytosine arabinoside

The antimitotic nucleoside cytosine arabinoside (araC) causes apoptosis in postmitotic neurons for which two mechanisms have been suggested: (1) araC directly inhibits a trophic factor-maintained signaling pathway required for survival, effectively mimicking trophic factor withdrawal; and (2) araC induces apoptosis by a p53-dependent mechanism distinct from trophic factor withdrawal. In rat sympathetic neurons, we found that araC treatment for 12 hr induced approximately 25% apoptosis without affecting NGF-maintained signaling; there was neither reduction in the activity of mitogen activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) or protein kinase B/Akt, a kinase implicated in NGF-mediated survival, nor was there c-Jun N-terminal kinase (JNK) activation or c-Jun N-terminal phosphorylation, events implicated in apoptosis induced by NGF withdrawal. However, araC treatment, but not NGF-withdrawal, elevated expression of p53 protein before and during apoptosis. Additionally, araC-induced apoptosis was suppressed in sympathetic neurons from p53 null mice. Although MAPK/ERK activity is not necessary for NGF-induced survival, it protected against toxicity by araC, because inhibition of the MAPK pathway by PD98059 resulted in a significant increase in the rate of apoptosis induced by araC in the presence of NGF. Consistent with this finding, ciliary neurotrophic factor, which does not cause sustained activation of MAPK/ERK, did not protect against araC toxicity. Our data show that, in contrast to NGF deprivation, araC induces apoptosis via a p53-dependent, JNK-independent mechanism, against which MAPK/ERK plays a substantial protective role. Thus, NGF can suppress apoptotic mechanisms in addition to those caused by its own deprivation.

N8-acetyl spermidine protects rat cerebellar granule cells from low K+-induced apoptosis

The endogenous polyamines have been extensively studied with respect to their role in cellular death mechanisms, although the results are contradictory. In contrast, their primary metabolites, the N-acetyl polyamines, have not been much studied. It has been hypothesized that the N-acetyl metabolites may play a role in cellular death mechanisms, and some of the variability between different reports may be due to altered polyamine metabolic capacities. Using primary cultures of rat cerebellar granule cells, the effects of N-acetyl metabolites have been examined on basal, cytosine beta-D-arabinofuranoside (Ara-C)-induced and low K+-induced apoptosis. None of the compounds affected either basal or Ara-C-induced apoptosis at low doses. At higher doses, all compounds were toxic. Two compounds, N8-acetyl spermidine and N1-acetyl spermine, were found to protect cells from low K+-induced apoptosis, which has been shown to be p53-independent. In contrast, the parent polyamines were devoid of protective activity at subtoxic doses. This represents the first time that an antiapoptotic effect of N-acetyl polyamines has been demonstrated. These results raise the possibility that these compounds may act as endogenous neuroprotectants. The lack of effect on basal apoptosis provides evidence of at least two forms of p53-independent apoptosis that can be regulated independently.

Investigations of non-NMDA receptor-induced toxicity in serum-free antioxidant-rich primary cultures of murine cerebellar granule cells

A culture system was developed whereby murine cerebellar granule cells were grown under serum-free conditions in chemically defined B27-supplemented neurobasal medium plus depolarizing K+ levels, to allow the investigation of the role of agonists at the kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors in glutamate-mediated neurotoxicity. Neurones were killed in a concentration-dependent manner by L-glutamate, kainate and its analogues, domoate and 4-(2-methoxyphenyl)-2-carboxy-3-pyrrolidineacetic acid, but not by (S)-AMPA or (S)-5-fluorowillardiine. Kainate (60% maximal cell death at 1mM) was markedly more toxic than NMDA (40% maximal cell death at 1mM) and was shown to be the predominant cause of excitatory amino acid-induced toxicity in these cells as the neuronal death induced by KA was attenuated by the non-NMDA antagonist CNQX, but not the AMPA antagonist LY293558. This study suggests that serum-free cultures of cerebellar granule cells in B27-supplemented neurobasal medium provide a valuable model system for investigations of the role of the kainate receptor in excitatory amino acid-induced neurodegeneration.

Apoptosis of cerebellar granule cells induced by serum withdrawal, glutamate or beta-amyloid, is independent of Jun kinase or p38 mitogen activated protein kinase activation

It has been reported that in differentiated PC12 cells and neurons from the superior cervical ganglion and hippocampus, that the activation of the stress-activated protein kinases jun-N-terminal kinase (JNK) and/or p38 mitogen-activated protein (p38MAP) kinase is central to the induction of apoptosis by serum or neurotrophic factor withdrawal. Here we demonstrate that in cerebellar granule cells, withdrawal of serum does not result in the activation of JNK or p38MAP kinase, under conditions where profound apoptosis was observed. In addition, these protein kinases were not activated during the induction of apoptosis caused by addition of excitotoxic levels of glutamate or of beta-amyloid (25-35) peptide. BDNF and insulin can prevent apoptosis induced by serum withdrawal or the addition of glutamate or beta-amyloid peptide. EGF on the other can prevent apoptosis induced by glutamate and beta-amyloid peptide, but not that caused by serum withdrawal. We conclude that the induction of apoptosis of cerebellar granule cells is independent of JNK or p38MAP kinase activation and that the mechanism by which serum withdrawal promotes apoptosis of these neurons may differ from that caused by glutamate and beta-amyloid peptide.

Apoptosis, excitotoxicity, and neuropathology

While a high rate of cell loss is tolerated and even required to model the developing nervous system, an increased rate of cell death in the adult nervous system underlies neurodegenerative disease. Evolutionarily conserved mechanisms involving proteases, Bcl-2-related proteins, p53, and mitochondrial factors participate in the modulation and execution of cell death. In addition, specific death mechanisms, based on specific neuronal characteristics such as excitability and the presence of specific channels or enzymes, have been unraveled in the brain. Particularly important for various human diseases are excessive nitric oxide (NO) production and excitotoxicity. These two pathological mechanisms are closely linked, since excitotoxic stimulation of neurons may trigger enhanced NO production and exposure of neurons to NO may trigger the release of excitotoxins. Depending on the experimental situation and cell type, excitotoxic neuronal death may either be apoptotic or necrotic.

Response of postmitotic neurons to X-irradiation: implications for the role of DNA damage in neuronal apoptosis

The molecular changes responsible for inducing neuronal apoptosis are unknown. Rat cortical neurons were treated with x-irradiation 7 d after isolation to test for the role of DNA damage in neuronal death. The response of neurons to x-irradiation was compared with that of astrocytes that had been isolated 3 weeks earlier from newborn rats. At the time of irradiation, the neurons appeared well differentiated morphologically and were predominantly (90-95%) noncycling, based on flow cytometric analysis. There was a similar, linear increase in DNA double-strand breaks with increasing radiation dose in neurons and astrocytes. However, whereas doses as low as 2 Gy induced typical apoptotic changes in neurons, including nuclear fragmentation and/or internucleosomal DNA fragmentation, doses as high as 32 Gy caused little or no apoptosis in astrocytes. Radiation-induced apoptosis of neurons started 4-8 hr after irradiation, was maximal at 12 hr, and was dependent on dose up to 16 Gy. It was prevented when cycloheximide, a protein synthesis inhibitor, was added up to 6 hr after irradiation. In addition to their distinct apoptotic response, neurons rejoined radiation-induced DNA double-strand breaks more slowly than astrocytes. Treatment with benzamide to inhibit ADP-ribosylation and strand break repair increased apoptosis; splitting the dose of radiation to allow increased time for DNA repair decreased apoptosis. These data suggest that DNA damage may induce neuronal apoptosis, that the extent of damage may determine the degree of apoptosis induced, and that slow repair of damage may play a role in the susceptibility of neurons to apoptosis.

Glutamate-treated rat cortical neuronal cultures die in a way different from the classical apoptosis induced by staurosporine

The alkaloid protein kinase inhibitor staurosporine induced neuronal cell death with both the morphological and the biochemical characteristics of apoptosis. The punctate chromatin associated with apoptosis with retention of plasma membrane integrity was observed in neurons identified by colocalization of NeuN staining. Such cells had DNA fragmentation visualized by in situ end-labeling which was seen as a laddered pattern upon gel electrophoresis. In contrast cells treated with glutamate did not exhibit either of these morphological or biochemical hallmarks of apoptosis. Instead a much smaller and more compact pyknotic structure was observed associated with smeared DNA fragmentation patterns. A confocal time-lapse study of the appearance of the morphological changes in individual nuclei after staurosporine treatment showed collapse into punctate chromatin over a period of 10 min. In contrast, the collapse into small pyknotic nuclei after glutamate treatment was at least 10 times slower. It is concluded that excitotoxicity produced by glutamate did not induce cell death by an apoptotic mechanism in cultured cortical neurons.

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.

Apoptosis in neurodegenerative disorders: potential for therapy by modifying gene transcription

Apoptotic, rather than necrotic, nerve cell death now appears as likely to underlie a number of common neurological conditions including stroke, Alzheimer's disease, Parkinson's disease, hereditary retinal dystrophies and Amyotrophic Lateral Sclerosis. Apoptotic neuronal death is a delayed, multistep process and therefore offers a therapeutic opportunity if one or more of these steps can be interrupted or reversed. Research is beginning to show how specific macromolecules play a role in determining the apoptotic death process. We are particularly interested in the critical nature of gradual mitochondrial failure in the apoptotic process and propose that a maintenance of mitochondrial function through the pharmacological modulation of gene expression offers an opportunity for the effective treatment of some types of neurological dysfunction. Our research into the development of small diffusible molecules that reduce apoptosis has grown from studies of the irreversible MAO-B inhibitor (-)-deprenyl. (-)-Deprenyl can reduce neuronal death independently of MAO-B inhibition even after neurons have sustained seemingly lethal damage. (-)-Deprenyl can also influence the process outgrowth of some glial and neuronal populations and can reduce the concentrations of oxidative radicals in damaged cells at concentrations too small to inhibit MAO. In accord with earlier work of others, we showed that (-)-deprenyl alters the expression of a number of mRNAs or of proteins in nerve and glial cells and that the alterations in gene expression/protein synthesis are the result of a selective action on transcription. The alterations in gene expression/protein synthesis are accompanied by a decrease in DNA fragmentation characteristic of apoptosis and the death of responsive cells. The onco-proteins Bcl-2 and Bax and the scavenger proteins Cu/Zn superoxide dismutase (SOD1) and Mn superoxide dismutase (SOD-2) are among the 40-50 proteins whose synthesis is altered by (-)-deprenyl. Since mitochondrial membrane potential correlates with mitochondrial ATP production, we have used confocal laser imaging techniques in living cells to show that the transcriptional changes induced by (-)-deprenyl result in a maintenance of mitochondrial membrane potential, a decrease in intramitochondrial calcium and a decrease in cytoplasmic oxidative radical levels. We therefore propose that (-)-deprenyl acts on gene expression to maintain mitochondrial function and decrease cytoplasmic oxidative radical levels and thereby reduces apoptosis. An understanding of the molecular steps by which (-)-deprenyl selectively alters transcription may lead to the development of new therapies for neurodegenerative diseases.