Chronic Toxicity And Mechanisms Of Depleted Uranium On Caenorhabditis Elegans

Uranium（U）is a natural component of the earth crust,which can be used as a raw material for nuclear reactions and is of vital importance in industrial and military applications.Depleted uranium（DU）is a by-product of the enrichment of naturally occurring uranium for its most radioactive isotope,²³⁵U.The isotopic composition of DU is typically ²³⁴U,²³⁵U and ²³⁸U.As a heavy metal,it is known to exert both radioactive and chemical toxicity.Meanwhile,compared with natural uranium,DU has low radioactivity.However,its chemical toxicity should not be ignored.With the wide application of DU in industry and military,it has become a non-negligible heavy metal pollutant,and its long-term exposure to environmental pollution and the threat to human health cannot be ignored.Nevertheless,studies of chronic toxicity of DU are still limited and not clear.Chronic effects of DU on the nervous system and their mechanisms have not been fully elucidated.Caenorhabditis elegans（C.elegans）is widely used in study of toxicology given its short life cycle,high homology with human genes and easy preservation.In this study,taking C.elegans as a model organism,we used uranyl（Ⅵ）nitrate hexahydrate to prepare DU,investigate the neurotoxicity of DU and elucidate its mechanism.In this study,we first examined the effect of DU on the lifespan of wild type（N2）and Parkinson’s disease（PD）-NL5901（unc-54::alpha synuclein::YFP+unc-119）nematodes.Secondly,we used the ending points method in C.elegans,including the record of its head swing,pharyngeal pumping,number of offspring and cell apoptosis to observe the effect of locomotive,reproductions and stress on the chronic toxicity of DU.Finally,transgenic strains of BZ555（dat-1::GFP）and NL5901 were used to assess the effect of DU on neurotoxicity,at the meantime,we measured the expression levels of antioxidant genes.The main results were as follows:1.DU had little effect on C.elegans’response to locomotive,reproduction and stress:chronic exposure of DU did not affect reproduction of the worm.Meanwhile,we examined the effect of DU on the apoptosis of germ cell,which was measured through acridine orange（AO）staining.The result showed that DU did not induce germ cell apoptosis.Head swing and pharyngeal pumping of the N2 C.elegans were measured at day 3,5,7,9,respectively.The results showed that Chronic exposure to DU did not affect the locomotive activity.Additionally,compared with the untreated control,DU treatment did not affect the survival curves of the N2 C.elegans upon exposure to UV,heat stress or oxidative stress.2.DU had little effect on the lifespan of N2,but it shortened the lifespan of PD model C.elegans:chronic exposure of DU did not affect the lifespan of the N2 C.elegans（P=0.2160）but shortened the lifespan of PD worms（P<0.01）when compared to the untreated control.Therefore,we speculated that DU may have neurotoxicity.3.DU induced the dopaminergic（DAergic）neuron degeneration:Through the detection of protein level in transgenic strains-NL5901 and BZ555,the result showed that chronic exposure to DU did affect the aggregation ofα-synuclein（P<0.001）and shrinkage of the soma of DAergic neuron（P<0.001）when compared to the untreated group.Therefore,we speculated that chronic exposure of DU induced the DAergic neuron degeneration and promoted PD development.4.Expression of antioxidant genes in nematodes:the results of q-PCR（quantitative-Polymerase Chain Reaction）showed that chronic exposure of DU decreased the mRNAs expression of cat-1,cat-2,cat-3,gst-7 and gst-10.However,DU did not affect the expression of gst-1,gst-4,gst-20,sod-1,sod-2,sod-3,sod-4and sod-5 genes.Therefore,we speculated that DU decreased the in vivo ROS scavenging ability through inhibiting the expression of antioxidant genes cat-1,cat-2,cat-3,gst-7 and gst-10.In a word,chronic exposure of DU had little effect on locomotive,reproduction and stress but can shorten the lifespan of PD model worms,and induce the aggregation ofα-synuclein and degeneration of the DAergic neuron,which implied that DU had potential neurotoxicity.The possible mechanism of its chronic toxicity may function through inhibiting the expression of antioxidant genes cat-1,cat-2,cat-3,et al,decreasing the in vivo ROS scavenging ability and further causing nerve damage.These findings might raise the public concerns of regarding DU as an etiologic agent of PD and underline its potential neurotoxicity.