Cells from individuals with the recessive cancerprone disorder ataxia telangiectasia (A-T) are hypersensitive to ionizing radiation (I-R). ATM (mutated in A-T)is a protein kinase whose activity is stimulated by I-R. c-Abl, a nonreceptor tyrosine kinase, interacts with ATM and is activated by ATM following I-R. Rad51 is a homologue of bacterial RecA protein required for DNA recombination and repair. Here we demonstrate that there is an I-R-induced Rad51 tyrosine phosphorylation, and this induction is dependent on both ATM and c-Abl. ATM, c-Abl, and Rad51 can be co-immunoprecipitated from cell extracts. Consistent with the physical interaction, c-Abl phosphorylates Rad51 in vitro and in vivo. In assays using purified components, phosphorylation of Rad51 by c-Abl enhances complex formation between Rad51 and Rad52, which cooperates with Rad51 in recombination and repair. After I-R, an increase in association between Rad51 and Rad52 occurs in wild-type cells but not in cells with mutations that compromise ATM or c-Abl. Our data suggest signaling mediated through ATM, and c-Abl is required for the correct posttranslational modification of Rad51, which is critical for the assembly of Rad51 repair protein complex following I-R. Ataxia telangiectasia (A-T)1 is an autosomal recessive genetic disease characterized by diverse clinical symptoms that include neuronal degeneration, immune deficiency, gonadal abnormalities, cancer predisposition, premature aging, and oculocutaneous telangiectasias (1, 2). Cells from individuals with A-T are hypersensitive to ionizing radiation (I-R) and chemicals that cause DNA double-strand breaks (DSB). Upon I-R, A-T cells exhibit defects in multiple cell cycle checkpoint functions, including an aberrant p53 response and DNA DSB repair (1, 3). The DSB repair defect in A-T cells is subtle, but nevertheless contributes significantly to the radiosensitivity and chromosomal instability phenotype of these cells (4 -6). At a time when rejoining of I-R-induced DSB is completed in normal mitotic cells, A-T cells still contain a significant number of DSBs, which may be converted into chromosomal breaks (7,8). Additionally, the DSB repair process in A-T cells is errorprone (9, 10). A-T cells also possess an elevated level of spontaneous intrachromosomal recombination (11). In meiosis of A-T cells, the synaptonemal complexes fail to form at the pachytene stage, thus aborting gametogenesis (12)(13)(14). Little is known about the molecular basis for the meiotic and DNA DSB repair defects in A-T cells.In eukaryotes, DNA DSB can be repaired by either homologous recombination or nonhomologous end-joining pathways (15). Homologous recombination is achieved through multiple enzyme-catalyzed steps that include processing and resection of the DSB, searching for and pairing of DNA homologs, strand invasion, DNA synthesis, and finally, resolution of recombination intermediates (16,17). Rad51, a eukaryotic homolog of the bacterial recombinase RecA, is required for recombinational repair of DSB. In in vitro recombin...

Stressful life events are important contributors to relapse in recovering cocaine addicts, but the mechanisms by which they influence motivational systems are poorly understood. Studies suggest that stress may "set the stage" for relapse by increasing the sensitivity of brain reward circuits to drug-associated stimuli. We examined the effects of stress and corticosterone on behavioral and neurochemical responses of rats to a cocaine prime after cocaine self-administration and extinction. Exposure of rats to acute electric footshock stress did not by itself reinstate drug-seeking behavior but potentiated reinstatement in response to a subthreshold dose of cocaine. This effect of stress was not observed in adrenalectomized animals, and was reproduced in nonstressed animals by administration of corticosterone at a dose that reproduced stress-induced plasma levels. Pretreatment with the glucocorticoid receptor antagonist RU38486 did not block the corticosterone effect. Corticosterone potentiated cocaine-induced increases in extracellular dopamine in the nucleus accumbens (NAc), and pharmacological blockade of NAc dopamine receptors blocked corticosterone-induced potentiation of reinstatement. Intraaccumbens administration of corticosterone reproduced the behavioral effects of stress and systemic corticosterone. Corticosterone treatment acutely decreased NAc dopamine clearance measured by fast-scan cyclic voltammetry, suggesting that inhibition of uptake 2 -mediated dopamine clearance may underlie corticosterone effects. Consistent with this hypothesis, intra-accumbens administration of the uptake 2 inhibitor normetanephrine potentiated cocaine-induced reinstatement. Expression of organic cation transporter 3, a corticosterone-sensitive uptake 2 transporter, was detected on NAc neurons. These findings reveal a novel mechanism by which stress hormones can rapidly regulate dopamine signaling and contribute to the impact of stress on drug intake.

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