Anhydrobiosis-Associated Nuclear DNA Damage and Repair in the Sleeping Chironomid: Linkage with Radioresistance

Abstract

Anhydrobiotic chironomid larvae can withstand prolonged complete desiccation as well as other external stresses including ionizing radiation. To understand the cross-tolerance mechanism, we have analyzed the structural changes in the nuclear DNA using transmission electron microscopy and DNA comet assays in relation to anhydrobiosis and radiation. We found that dehydration causes alterations in chromatin structure and a severe fragmentation of nuclear DNA in the cells of the larvae despite successful anhydrobiosis. Furthermore, while the larvae had restored physiological activity within an hour following rehydration, nuclear DNA restoration typically took 72 to 96 h. The DNA fragmentation level and the recovery of DNA integrity in the rehydrated larvae after anhydrobiosis were similar to those of hydrated larvae irradiated with 70 Gy of high-linear energy transfer (LET) ions (4He). In contrast, low-LET radiation (gamma-rays) of the same dose caused less initial damage to the larvae, and DNA was completely repaired within within 24 h. The expression of genes encoding the DNA repair enzymes occurred upon entering anhydrobiosis and exposure to high- and low-LET radiations, indicative of DNA damage that includes double-strand breaks and their subsequent repair. The expression of antioxidant enzymes-coding genes was also elevated in the anhydrobiotic and the gamma-ray-irradiated larvae that probably functions to reduce the negative effect of reactive oxygen species upon exposure to these stresses. Indeed the mature antioxidant proteins accumulated in the dry larvae and the total activity of antioxidants increased by a 3–4 fold in association with anhydrobiosis. We conclude that one of the factors explaining the relationship between radioresistance and the ability to undergo anhydrobiosis in the sleeping chironomid could be an adaptation to desiccation-inflicted nuclear DNA damage. There were also similarities in the molecular response of the larvae to damage caused by desiccation and ionizing radiation.

Sequence analysis of the DNA-repair gene rad51 in the tardigrades Milnesium cf. tardigradum, Hypsibius dujardini and Macrobiotus cf. harmsworthi

“…This strongly suggested that the homologous repair system is activated by radiation in tardigrades and supports the hypothesis that the DNA repair system may play an important role in tardigrades when recovering after radiation exposure. Rad51 was also upregulated in the radiation resistant African chironomid, P. vanderplanki (Gusev et al, 2010). However, tardigrades seem to tolerate more radiation (5000 Gy; Horikawa et al, 2006) than P. vanderplanki (2000 Gy; Watanabe et al, 2006).…”

The induction of anhydrobiosis in the sleeping chironomid: Current status of our knowledge

“…Subsequently, ROS is responsible for peroxidation of membrane lipids, carbonylation of proteins, and DNA damage (20). Analysis of the anhydrobiosis-related EST database in P. vanderplanki revealed that genes related to the oxidative stress response were strongly upregulated on the desiccation process, and the relative antioxidant activity was actually enhanced during this period (21). One of the major pathways of ROS scavenging relies on the superoxide dismutase and catalase system.…”

“…Superoxide dismutase transforms oxygen free radicals into hydrogen peroxide, which will be then reduced to water by catalase. In P. vanderplanki, catalase and several superoxide dismutase genes were identified, and their expression was upregulated during the desiccation process (19,21) (Fig. 3).…”

“…The latter strategists can survive for an extended period even after their body water content falls below a threshold, under which there is no longer any bulk cytoplasmic water present (less than 23% water on a fresh weight basis, or less than 0.3 g water g 21 dry mass) (4). An excellent example of the strategy is anhydrobiosis (life without water).…”

Element analysis of the eutardigrades Richtersius coronifer and Milnesium cf. asiaticum using Particle Induced X-ray Emission (PIXE)

“…Few studies on DNA damage and repair in tardigrades have appeared, but Neumann et al (2009) and Rebecchi et al (2009) reported evidence of DNA degradation after exposure to periods of anhydrobiosis, with an increase in damage with time spent in the dry state. Evidence that DNA repair mechanisms are an important part of the tolerance to desiccation and radiation in anhydrobiotic animals also comes from studies on insect larvae (Gusev et al, 2010) and rotifers (Gladyshev and Meselson, 2008).…”