一种叫做真涡虫（Planarian）的扁形蠕虫有着惊人的组织修复能力，把它们切成两段后，两边能再生出新的肌肉、皮肤、肠道甚至完整的大脑，而且这种再生好像能无限地进行下去。美国物理学家组织网近日报道称，英国诺丁汉大学研究人员揭示了它们这种无限再生寿命背后的支持机制，为研究人类细胞克服老化，延缓衰老特征的出现带来了希望。相关论文发表在美国《国家科学院学报》上。
细胞老化的原因之一与端粒长度有关。端粒是DNA链末端的“保护帽”，就像鞋带末端防止开线的小胶棒。细胞每分裂一次，端粒就会变短一点，端粒变得太短时，细胞就丧失了分裂的能力。如果一种动物能无限地保持其端粒的长度，就可能永远分裂下去，它就可能永生不死。此前的研究显示，保持端粒长度靠的是一种端粒酶的活性。在大部分有性繁殖生物中，这种酶只在早期发育阶段表现出最大活性，随着年龄增长活性丧失，端粒就开始变短。
诺丁汉大学生物学院的阿齐兹·阿博贝克说，真涡虫的干细胞却能以某种方式避免老化过程，保持自身细胞一直分裂下去。在真涡虫的成熟干细胞中，染色体能主动保持着端粒的长度，理论上它们是永生的。他们用两种真涡虫做了一系列实验，一种是有性繁殖，另一种是无性繁殖，即简单地分成两段。
他们识别出一段端粒酶基因，关闭了该基因活性后，真涡虫端粒开始变短。然后他们检测了该基因活性和端粒长度之间的关系，发现无性繁殖的真涡虫再生时，端粒酶基因的活性大大增加，使干细胞能在分裂中保持端粒长度；而有性繁殖的真涡虫却没显示出这样的情况。研究人员对这二者间的差异非常吃惊，因为两种真涡虫都有无限的再生能力。他们推测，要么有性繁殖真涡虫最终会出现端粒变短，要么它们是通过一种不需要端粒酶的机制来保持端粒长度。
阿博贝克说，他们的下一个目标是详细研究这种机制，这对理解某些与老化有关的基本过程具有重要意义。无性繁殖真涡虫能在再生过程中保持端粒长度，从这里有望找出让一个动物永生不死的原因。(生物谷：Bioon.com)
doi:10.1073/pnas.1118885109
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Telomere maintenance and telomerase activity are differentially regulated in asexual and sexual worms
Thomas C. J. Tan, Ruman Rahman, Farah Jaber-Hijazi, Daniel A. Felix, Chen Chen, Edward J. Louis, and Aziz Aboobaker
In most sexually reproducing animals, replication and maintenance of telomeres occurs in the germ line and during early development in embryogenesis through the use of telomerase. Somatic cells generally do not maintain telomere sequences, and these cells become senescent in adults as telomeres shorten to a critical length. Some animals reproduce clonally and must therefore require adult somatic mechanisms for maintaining their chromosome ends. Here we study the telomere biology of planarian flatworms with apparently limitless regenerative capacity fueled by a population of highly proliferative adult stem cells. We show that somatic telomere maintenance is different in asexual and sexual animals. Asexual animals maintain telomere length somatically during reproduction by fission or when regeneration is induced by amputation, whereas sexual animals only achieve telomere elongation through sexual reproduction. We demonstrate that this difference is reflected in the expression and alternate splicing of the protein subunit of the telomerase enzyme. Asexual adult planarian stem cells appear to maintain telomere length over evolutionary timescales without passage through a germ-line stage. The adaptations we observe demonstrate indefinite somatic telomerase activity in proliferating stem cells during regeneration or reproduction by fission, and establish planarians as a pertinent model for studying telomere structure, function, and maintenance.
