In vivo detection of ultraviolet photoproducts and their repair in purkinje cells

Neuronal responses to physiological stress

Physiological stress can be defined as any external or internal condition that challenges the homeostasis of a cell or an organism. It can be divided into three different aspects: environmental stress, intrinsic developmental stress, and aging. Throughout life all living organisms are challenged by changes in the environment. Fluctuations in oxygen levels, temperature, and redox state for example, trigger molecular events that enable an organism to adapt, survive, and reproduce. In addition to external stressors, organisms experience stress associated with morphogenesis and changes in inner chemistry during normal development. For example, conditions such as intrinsic hypoxia and oxidative stress, due to an increase in tissue mass, have to be confronted by developing embryos in order to complete their development. Finally, organisms face the challenge of stochastic accumulation of molecular damage during aging that results in decline and eventual death. Studies have shown that the nervous system plays a pivotal role in responding to stress. Neurons not only receive and process information from the environment but also actively respond to various stresses to promote survival. These responses include changes in the expression of molecules such as transcription factors and microRNAs that regulate stress resistance and adaptation. Moreover, both intrinsic and extrinsic stresses have a tremendous impact on neuronal development and maintenance with implications in many diseases. Here, we review the responses of neurons to various physiological stressors at the molecular and cellular level.

Role of nucleotide- and base-excision repair in genotoxin-induced neuronal cell death

Base-excision (BER) and nucleotide-excision (NER) repair play pivotal roles in protecting the genomes of dividing cells from damage by endogenous and exogenous agents (i.e. environmental genotoxins). However, their role in protecting the genome of post-mitotic neuronal cells from genotoxin-induced damage is less clear. The present study examines the role of the BER enzyme 3-alkyladenine DNA glycosylase (AAG) and the NER protein xeroderma pigmentosum group A (XPA) in protecting cerebellar neurons and astrocytes from chloroacetaldehyde (CAA) or the alkylating agent 3-methyllexitropsin (Me-Lex), which produce ethenobases or 3-methyladenine (3-MeA), respectively. Neuronal and astrocyte cell cultures prepared from the cerebellum of wild type (C57BL/6) mice or Aag(-/-) or Xpa(-/-) mice were treated with 0.1-50 microM CAA for 24h to 7 days and examined for cell viability, DNA fragmentation (TUNEL labeling), nuclear changes, and glutathione levels. Aag(-/-) neurons were more sensitive to the acute (>20 microM) and long-term (>5 microM) effects of CAA than comparably treated wild type neurons and this sensitivity correlated with the extent of DNA fragmentation and nuclear changes. Aag(-/-) neurons were also sensitive to Me-Lex at comparable concentrations of CAA. In contrast, Xpa(-/-) neurons were more sensitive than either wild type or Aag(-/-) neurons to CAA (>10 microM), but less sensitive than Aag(-/-) neurons to Me-Lex. Astrocytes from the cerebellum of wild type, Aag(-/-) or Xpa(-/-) mice were essentially insensitive to CAA at the concentrations tested. These studies demonstrate that BER and NER are required to protect neurons from genotoxin-induced cell death.

Quantification of DNA adducts in individual cells by immunofluorescence: effects of variation in DNA conformation

Previously reported detection of melphalan-DNA adducts by immunofluorescent staining indicated considerable intercell variation in fluorescence levels. Investigations were undertaken to determine whether this variation reflected actual intercell differences in adduct levels. Melphalan-treated CCRF-CEM leukaemia cells were analysed by the trapped-in-agarose DNA immunostaining (TARDIS) method using fluorescein immunofluorescence and Hoechst dye-DNA fluorescence. Increasing the time of DNA denaturation in alkali affected the staining intensity, in agreement with known adduct properties, but failed to reduce intercell heterogeneity. To test the hypothesis that heterogeneity resulted from variation in levels of DNA strand breaks, drug-treated cells were exposed to ionising radiation. An increase in level and reduction in heterogeneity of immunofluorescence were observed, optimal at 10 Gy. When samples were irradiated after lysis, 1 Gy was optimal. At the optimal doses, irradiation before or after lysis resulted in similar levels of DNA strand breaks. Our conclusions are as follows: (a) There was no major intercell variation in the number of adducts other than from variation in DNA content. (b) Detection of melphalan, and possibly other adducts, by immunofluorescence can be markedly influenced by the level of strand breaks present in the DNA. (c) Samples analysed for melphalan adducts by immunofluorescence should be irradiated to minimise errors due to this factor.