Response of Postmitotic Neurons to X-Irradiation: Implications for the Role of DNA Damage in Neuronal Apoptosis

Abstract

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.

Genetic Regulation of Behavioral and Neuronal Responses to Fluoxetine

“…While control experiments suggested that irradiation did not disrupt hippocampal function beyond limiting proliferation, the possibility remains that unknown molecular factors important to antidepressant efficacy and/or hippocampal function were affected by the procedure. X-ray irradiation of the hippocampus has been shown to cause cognitive deficits in mice (Rola et al, 2004), and may increase apoptosis, produce changes in genes associated with DNA damage and stress response, alter the morphology or functionality of mature neurons, and alter blood flow to irradiated regions (Gobbel et al, 1998;Shirai et al, 2006). Therefore, there remains a need for less invasive methods to test the link between neurogenesis and antidepressant efficacy.…”

Radiation induced CNS toxicity – molecular and cellular mechanisms

“…Recent studies have shown that radiation can induce apoptosis in neurons in vivo in newborn animals (Gobbel et al, 1998). Since no such event was found in the CNS of adult rats a direct contribution to the radiation induced CNS toxicity is unlikely.…”

Evidence that the bifunctional redox factor / AP endonuclease Ref-1 is an anti-apoptotic protein associated with differentiation in the developing retina

“…33 In addition, astrocytes express higher amounts of Ref-1 than neurons in the brain. 24 These data together are consistent with an antiapoptotic role for Ref-1 through its DNA repair activity.…”