Evidence for cell-replacement repair of X-ray-induced teratogenic damage in male genital imaginal discs of Drosophila melanogaster

Model systems for regeneration: Drosophila

Drosophila melanogaster has historically been a workhorse model organism for studying developmental biology. In addition, Drosophila is an excellent model for studying how damaged tissues and organs can regenerate. Recently, new precision approaches that enable both highly targeted injury and genetic manipulation have accelerated progress in this field. Here, we highlight these techniques and review examples of recently discovered mechanisms that regulate regeneration in Drosophila larval and adult tissues. We also discuss how, by applying these powerful approaches, studies of Drosophila can continue to guide the future of regeneration research.

Apoptotic repair of genotoxic tissue damage and the role of p53 gene

The spontaneous mutation rate per replication per genome is nearly invariant in microbes; however, the rate of spontaneous genomic mutations in higher eukaryotes is much higher. Furthermore, the mutation rates per locus per generation among Drosophila, mice and humans are similar, despite the large differences in generation time. A simple explanation for these findings is that mice and humans have a specific antimutagenic mechanism that is lacking in Drosophila. I propose that apoptotic repair-deletion of genotoxic damage-bearing cells-operates in mammalian germ cells and that it works more accurately in humans than in mice because of a slower rate of cell turnover and a longer generation time. It has been a long-standing puzzle that germline mutation frequencies decrease markedly as the dose-rate of radiation is lowered in mice but not in Drosophila. This can be readily explained by p53-dependent apoptotic repair, because the p53 gene is absent from the genome of Drosophila. Fetuses of p53+/+ mice have proficient apoptotic repair capacity for X-ray-induced teratogenic damage, but p53-null fetuses completely lack this capacity. Further, I propose that the primary role of the p53 gene is to guard germ cells and embryos from genotoxic damage. This implies that the tumour suppressor function of the p53 gene results from p53-dependent apoptotic deletion of cells with genotoxic damage. The reasoning behind this proposal is given by reviewing reports that Drosophila larvae are insensitive to tumour formation after irradiation. Finally, I discuss the genetic effects of radiation in humans.

Cytogenotoxicities of sublimed urethane gas to the mouse embryo

Urethane (ethyl carbamate) which has long been used for commonly used drugs and has proven to be useful in the formation of products in every-day use, is volatile, and small amounts sublime spontaneously. Pregnant ICR mice were maintained in the vinyl chamber (45 liter) which was ventilated 4 times per hour. To inhale urethane gas, air was passed first through a glass bottle containing 500 g of crystalline urethane and then into the vinyl chamber. Concentration of the sublimed urethane gas in the chamber was 1.28 +/- 0.08 mg/l, and sublimed urethane gas produced significantly high incidence of chromosomal aberrations in the cells of whole embryo, when mice inhaled it for 48 h from day 9 to day 11 of pregnancy. High and significant incidence of chromosomal aberrations (36.0%) was detected in the embryo 3 h after urethane gas inhalation, but decreased to 5.3% at 24 h after exposure and showed no significant differences from controls after 48 h, while the incidence in bone marrow cells from the adult (pregnant) mice was lower (21.5%) at 3 h after exposure but a significant increase remained until 72 h after exposure. A majority of chromosomal aberrations was chromatid types. As a consequence of cellular damages by urethane gas inhalation during pregnancy, significantly high incidence of fetal deaths and congenital malformations (cleft palate, polydactyly, tail anomaly etc.) was induced in the offspring. Thus, we must be aware of the risk of volatile chemicals, because it is difficult to perceive and avoid hazardous exposure via respiration.

p53-dependent apoptosis suppresses radiation-induced teratogenesis

About half of human conceptions are estimated not to be implanted in the uterus, resulting in unrecognizable spontaneous abortions, and about 5% of human births have a recognizable malformation. In order to find clues to the mechanisms of malformation and abortion, we compared the incidences of radiation-induced malformations and abortions in p53 null (p53-/-) and wild-type (p53+/+) mice. After X-irradiation with 2 Gy on day 9.5 of gestation, p53-/- mice showed a 70% incidence of anomalies and a 7% incidence of deaths, whereas p53+/+ mice had a 20% incidence of anomalies and a 60% incidence of deaths. Similar results were obtained after irradiation on day 3.5 of gestation. This reciprocal relationship of radiosensitivity to anomalies and to embryonic or fetal lethality supports the notion that embryonic or fetal tissues have a p53-dependent "guardian" of the tissue that aborts cells bearing radiation-induced teratogenic DNA damage. In fact, after X-irradiation, the number of cells with apoptotic DNA fragments was greatly increased in tissues of the p53+/+ fetuses but not in those of the p53-/- fetuses.

Altruistic cell suicide in relation to radiation hormesis

The high radiosensitivity to killing of undifferentiated primordial cells (Bergonié and Tribondeau 1906) can be described as a manifestation of the suicide of injured cells for the benefit of an organism as a whole if their suicide stimulates proliferation of healthy cells to replace them, resulting in complete elimination of injury. This process is called cell-replacement repair, to distinguish it from DNA repair which is rarely complete. 'Cell suicide', 'programmed death' and 'apoptosis' are terms used for the same type of active cell death. Cell suicide is not always altruistic. Altruistic suicide in Drosophila, mice, humans, plants, and E. coli is reviewed in this paper to illustrate its widely different facets. The hypothesis that in animals, radiation hormesis results from altruistic cell suicide is proposed. This hypothesis can explain the hormetic effect of low doses of radiation on the immune system in mice. In contrast, in plants, radiation hormesis seems to be mainly due to non-altruistic cell death. HORMESIS--'the stimulating effect of small doses of substances which in larger doses are inhibitory' (British Medical Dictionary, Caxton Publ. Co., 1961).