Ultraviolet effects on killing, fruiting body formation and the spores of Dictyostelium discoideum

Differentiation of Dictyostelium discoideum vegetative cells into spores during Earth orbit in space

We reported previously that emerged amoebae of Dictyostelium (D.) discoideum grew, aggregated and differentiated to fruiting bodies with normal morphology in space. Here, we investigated the effects of space radiation and/or microgravity on the number, viability, kinetics of germination, growth rate and mutation frequency of spores formed in space in a radiation-sensitive strain, gamma s13, and the parental strain, NC4. In gamma s13, there were hardly spores in the fruiting bodies formed in space. In NC4, we found a decrease in the number of spores, a delay in germination of the spores and delayed start of cell growth of the spores formed in space when compared to the ground control. However, the mutation frequency of the NC4 spores formed in space was similar to that of the ground control. We conclude that the depression of spore formation might be induced by microgravity and/or space radiation through the depression of some stage(s) of DNA repair during cell differentiation in the slime mold.

Mutation frequency of Dictyostelium discoideum spores exposed to the space environment

Two strains of cellular slime mold Dictyostelium discoideum, a radiosensitive mutant and the parental wild-type strain, were used to investigate the effects of cosmic radiation on viability and mutation frequency at the spore stage for about 9 days in Space Shuttle of NASA. We measured little effect of space environment on viability and cell growth in the both strains as compared to ground controls. The mutation frequency of the flown spores were similar to that of ground control. These results suggest that there could be no effect of cosmic radiation, containing high linear energy transfer radiation at about 0.9 mSv/day as detected by real-time radiation monitoring device on the induction of mutation at the spore stage.

Cell growth and morphology of Dictyostelium discoideum in space environment

Two strains of cellular slime mold Dictyostelium discoideum, a radiation-sensitive mutant and the parental wild-type strain, were used to investigate the effects of microgravity and/or cosmic radiation on their morphology through the whole life span from spores to fruiting bodies for about 7 days in space shuttle of NASA. We found almost no effect of space environment on amoeba cell growth in both strains. It was also observed that almost the same number and shape of fruiting bodies in space compared to the control experiments on earth. These results suggest that there is little effect of microgravity and space radiation on germination, cell aggregation, cell differentiation and cell morphology in the cellular slime mold.

Influence of pre-treatment with DNA-damaging agents on hyperthermic potentiation in cell killing of ultraviolet light-irradiated Dictyostelium discoideum

Heat treatment at 30 degrees C for 15 min immediately after ultraviolet (UV) irradiation enhanced cell killing of Dictyostelium discoideum. However, when the cells were UV-irradiated and cultured at 23 degrees C for 30 min before giving UV irradiation and heat treatment, the enhancement of cell killing by heat treatment immediately after post-UV irradiation was depressed. Similar depressions in the heat enhancement of cell killing of UV-irradiated cells were also observed in the case of the cells cultured after pretreatment with 4-nitroquinoline 1-oxide, cis-platinum(II) diaminedichloride or 8-methoxypsoralen photoaddition, but not with N-methyl-N'-nitro-N-nitrosoguanidine or methylmethanesulphonate. These results suggest that the depression in heat enhancement in cell killing of UV-irradiated cells may be deeply related with kinds of DNA lesions or DNA repair processes for them.

Depression of hyperthermic potentiation in cell killing of ultraviolet light-irradiated Dictyostelium discoideum by pre-heat treatment

Heat treatment of 30 degrees C for 15 min immediately after ultraviolet (UV) irradiation enhanced cell killing of Dictyostelium discoideum. However, when the cells were heated at 30 degrees C for 15 min and then cultured for more than 10 min at 23 degrees C prior to UV exposure, the hyperthermic potentiation in cell killing was depressed. When these preheated cells were cultured in the presence of cycloheximide, the depression of the hyperthermic potentiation disappeared. These results suggest that the depression in hyperthermic potentiation may be the result of the induction of some heat-shock-type proteins.

Hyperthermic effects on cell killing in Dictyostelium discoideum amoebae treated with DNA-damaging agents

Hyperthermic effects on cell killing of amoeboid cells treated with DNA-damaging agents of Dictyostelium (D). discoideum were studied. Though the amoeboid cells are usually cultured at 23 degrees C, heat treatment at 30 degrees C for 15 min immediately after UV irradiation markedly enhanced cell killing, while not observed in the case of heat treatment immediately before UV irradiation. Similar effects were observed in the case of heat treatment after treatment with 4-nitroquinoline 1-oxide, cis-platinum(II) diamminedichloride or 8-methoxypsoralen photoaddition, while not observed when heat was applied before these treatments. On the contrary, little enhancement by heat treatment was observed even after treatment with N-methyl-N'-nitro-N-nitrosoguanidine or methylmethanesulphonate. The hyperthermic mechanisms in D. discoideum are discussed, based on a diverse set of DNA lesions caused by chemicals and DNA repair mechanisms for them.

Hyperthermic effects on DNA repair of UV-irradiated Dictyostelium discoideum

DNA repair of a lower eukaryote, Dictyostelium discoideum, has been investigated through the analysis of heat effects on cell mortality and DNA repair of UV-irradiated amoeboid cells. In a wild-type strain (NC4), an increase in temperature immediately after UV irradiation resulted in an increase in cell mortality, though similar heat treatment before UV irradiation had no such effect. Similar results were obtained in another wild-type strain, HPS83. In NC4, heat treatment after UV irradiation did not inhibit the nicking of DNA strands during excision repair processes, but did inhibit the rejoining of the DNA strand breaks. Removal of thymine-containing pyrimidine dimers from DNA molecules was also depressed by heat treatment after UV irradiation. In contrast, heat treatment before UV irradiation had no effect on any stage of the nicking process, the excision of the dimers or the rejoining. On the other hand, a radiation-sensitive mutant (TW8) defective in an incision step of the excision repair process did not show an increase in cell mortality in response to heat treatment administered either before or after UV irradiation. Though the optimum temperature for cell growth of the amoebae was 23 degrees C, the critical temperature for effective enhancement of cell killing was ca. 30 degrees C. Hence we assume that the excision repair of UV-damaged DNA is selectively sensitive to heat treatment.

Dictyostelium discoideum fruiting bodies observed by scanning electron microscopy

The fruiting bodies of Dictyostelium discoideum, fixed in a vapor of 25% glutaraldehyde, were observed by scanning electron microscopy. The surface of the basal disk had rootlike ripples, sometimes with concentric rings of riblike striations at its periphery. The surface of the stalk had pleated and twisted ridges along its entire length, but no lateral projections. The spores were randomly orientated and had a crinkled surface. The surface of the lower part of the papilla had flattened ridges running horizontally. A slime coat remained in the region of transition between the stalk and sporangium, but not on the spores.