Long term effects of superoxide and DNA repair on lizard telomeres

Telomeres are the non-coding protein-nucleotide ‘caps’ at chromosome ends that contribute to chromosomal stability by protecting the coding parts of the linear DNA from shortening at cell division, and from erosion by reactive molecules. Recently, there has been some controversy between molecular and cell biologists, on the one hand, and evolutionary ecologists on the other, regarding whether reactive molecules erode telomeres during oxidative stress. Many studies of biochemistry and medicine have verified these relationships in cell culture, but other researchers have failed to find such effects in free-living vertebrates. Here we use a novel approach to measure free radicals (superoxide), mitochondrial ‘content’ (a combined measure of mitochondrial number and size in cells), telomere length and DNA damage at two primary time points during the mating season of an annual lizard species (Ctenophorus pictus). Superoxide levels early in the mating season vary widely and elevated levels predict shorter telomeres both at that time as well as several months later. These effects are likely driven by mitochondrial content, which significantly impacts late season superoxide (cells with more mitochondria have more superoxide), but superoxide effects on telomeres are counteracted by DNA repair as revealed by 8-hydroxy-2’-deoxyguanosine assays. We conclude that reactive oxygen species and DNA repair are fundamental for both short- and long-term regulation of lizard telomere length with pronounced effects of early-season cellular stress detectable on telomere length near lizard death.

端粒（Telomeres）是位于染色体末端的非编码蛋白质-核苷酸帽状结构，可通过保护线性DNA的编码区域免受细胞分裂时的缩短以及活性分子的侵蚀，从而维持染色体稳定性。近年来，分子细胞生物学家与进化生态学家之间就氧化应激期间活性分子是否会侵蚀端粒这一问题存在争议。诸多生物化学与医学研究已在细胞培养体系中验证了这一关联，但另有研究者在野生脊椎动物中未能观测到此类效应。本研究采用一种新颖的实验方法，在一年生蜥蜴（Ctenophorus pictus）的繁殖季的两个关键时间点，分别检测了自由基（超氧阴离子，superoxide）、线粒体含量（mitochondrial content，即细胞内线粒体数量与体积的综合衡量指标）、端粒长度以及DNA损伤水平。繁殖季早期的超氧阴离子水平差异显著，且高水平的超氧阴离子不仅可预测该时期的端粒缩短，还能预示数月后的端粒长度变短。此类效应可能由线粒体含量介导：线粒体含量显著影响繁殖季后期的超氧阴离子水平（线粒体数量更多的细胞会产生更多超氧阴离子）；而8-羟基-2’-脱氧鸟苷（8-hydroxy-2’-deoxyguanosine）检测结果显示，超氧阴离子对端粒的负面影响可被DNA修复机制抵消。综上，活性氧物种与DNA修复机制是蜥蜴端粒长度短期与长期调控的核心基础，且繁殖季早期的细胞应激所产生的显著影响，可在蜥蜴临终前的端粒长度上体现出来。