Cytosine arabinoside (AraC) is a nucleoside analog that produces signi®cant 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-Lcysteine, 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¯uorescence of the dye 5-(6)-carboxy-2 H ,7 H -dichlorodihydro¯uorescein 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.

The siglecs (sialic acid‐binding Ig‐like lectins) are a distinct subset of the Ig superfamily with adhesion‐molecule‐like structure. We describe here a novel member of the siglec protein family that shares a similar structure including five Ig‐like domains, a transmembrane domain, and a cytoplasmic tail containing two ITIM‐signaling motifs. Siglec‐10 was identified through database mining of an asthmatic eosinophil EST library. Using the Stanford G3 radiation hybrid panel we were able to localize the genomic sequence of siglec‐10 within the cluster of genes on chromosome 19q13.3‐4 that encode other siglec family members. We have demonstrated that siglec‐10 is an immune system‐restricted membrane‐bound protein that is highly expressed in peripheral blood leukocytes as demonstrated by Northern, RT‐PCR and flow cytometry. Binding assays determined that the extracellular domain of siglec‐10 was capable of binding to peripheral blood leukocytes. The cytoplasmic tail of siglec‐10 contains four tyrosines, two of which are embedded in ITIM‐signaling motifs (Y597 and Y667) and are likely involved in intracellular signaling. The ability of tyrosine kinases to phosphorylate the cytoplasmic tyrosines was evaluated by kinase assay using wild‐type siglec‐10 cytoplasmic domain and Y→F mutants. The majority of the phosphorylation could be attributed to Y597 andY667. Further experiments with cell extracts suggest that SHP‐1 interacts with Y667 and SHP‐2 interacts with Y667 in addition to another tyrosine. This is very similar to CD33, which also binds the phosphatases SHP‐1 and SHP‐2, therefore siglec‐10, as CD33, may be characterized as an inhibitory receptor.

Neuronal death can be induced by DNAdamaging agents and occurs by apoptosis involving a specific signal-transduction pathway. However, to our knowledge, methods for the quantitative determination of DNA damage in individual neurons have not yet been described. Here we optimize the single-cell gel electrophoresis (SCGE) or “comet”-assay to measure DNA damage within individual neurons growing in dissociated cell culture. In addition, we have written a macro for the NIH Image program to determine the tail moment of individual comets. We have calibrated this method using γ-irradiated (0–16 Gy) cerebral cortical neurons from the rat central nervous system. Neuronal DNA damage (in the form of DNA strand breaks) occurs in a linear, dose-dependent manner, which can be quantitatively determined in vitro using the SCGE assay. These data demonstrate that the SCGE assay is an effective method to measure DNA damage in individual neurons and may be highly useful for the study of neuronal DNA damage formation, repair and apoptosis.

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 c-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.

Specific ablation of central nervous system (CNS) progenitor cells in the brain of live animals is a powerful method to determine the functions of these cells and to reveal novel avenues for the treatment of several CNS-related disorders. To achieve this goal, we generated a line of transgenic rats expressing a bacterial enzyme, Escherichia coli nitroreductase gene (NTR), under control of the nestin promoter. In this system, NTR(+) cells are selectively eliminated upon application of prodrug CB1954, through activation of programmed cell death machineries. At 5 days of age, which is a time when cerebellar development is occurring, transgenic rats bearing the nestin-NTR/green fluorescent protein (GFP) gene are overtly normal and express NTR/GFP in neuronal stem cells, without any toxicity in these cells. The functional consequence of progenitor cell ablation was demonstrated by administering prodrug CB1954 into the cerebellum at this 5-day time point. Stem cell ablation in these neonates resulted in sensorimotor abnormalities, cerebellar degeneration, overall reduction in cerebellar seize, and manifestation of ataxia. In adult rats, GFP expression was not seen in the hippocampal progenitor cells and seen only at very low levels in the lateral ventricles, indicating a different NTR/GFP expression pattern between neonates and adults. In addition, application of CB1954 by intraventricular delivery reduced the number of 5-bromo-2'-deoxyuridine-labeled proliferating cells in the lateral ventricle but not hippocampus of NTR/GFP rats. These findings shows that targeted expression of NTR under a specific promoter might be of significant value in addressing the function of distinct cell population in vivo.