Department of Biochemistry, University of California at Riverside, Riverside, CA 92521, USA.
BMC Genomics. 2013 May 2;14:298. doi: 10.1186/1471-2164-14-298.
Small RNAs complex with proteins to mediate a variety of functions in animals and plants. Some small RNAs, particularly miRNAs, circulate in mammalian blood and may carry out a signaling function by entering target cells and modulating gene expression. The subject of this study is a set of circulating 30-33 nt RNAs that are processed derivatives of the 5' ends of a small subset of tRNA genes, and closely resemble cellular tRNA derivatives (tRFs, tiRNAs, half-tRNAs, 5' tRNA halves) previously shown to inhibit translation initiation in response to stress in cultured cells.
In sequencing small RNAs extracted from mouse serum, we identified abundant 5' tRNA halves derived from a small subset of tRNAs, implying that they are produced by tRNA type-specific biogenesis and/or release. The 5' tRNA halves are not in exosomes or microvesicles, but circulate as particles of 100-300 kDa. The size of these particles suggest that the 5' tRNA halves are a component of a macromolecular complex; this is supported by the loss of 5' tRNA halves from serum or plasma treated with EDTA, a chelating agent, but their retention in plasma anticoagulated with heparin or citrate. A survey of somatic tissues reveals that 5' tRNA halves are concentrated within blood cells and hematopoietic tissues, but scant in other tissues, suggesting that they may be produced by blood cells. Serum levels of specific subtypes of 5' tRNA halves change markedly with age, either up or down, and these changes can be prevented by calorie restriction.
We demonstrate that 5' tRNA halves circulate in the blood in a stable form, most likely as part of a nucleoprotein complex, and their serum levels are subject to regulation by age and calorie restriction. They may be produced by blood cells, but their cellular targets are not yet known. The characteristics of these circulating molecules, and their known function in suppression of translation initiation, suggest that they are a novel form of signaling molecule.
小 RNA 与蛋白质结合，在动植物中介导多种功能。一些小 RNA，特别是 microRNA，在哺乳动物血液中循环，并通过进入靶细胞和调节基因表达来发挥信号功能。本研究的对象是一组 30-33 个核苷酸的循环 RNA，它们是一小部分 tRNA 基因 5' 端的加工衍生物，与先前在培养细胞中显示出响应应激抑制翻译起始的细胞内 tRNA 衍生物（tRFs、tiRNAs、half-tRNAs、5' tRNA 一半）非常相似。
在从小鼠血清中提取的小 RNA 测序中，我们鉴定出大量源自一小部分 tRNA 的 5' tRNA 一半，这表明它们是由 tRNA 类型特异性生物发生和/或释放产生的。5' tRNA 一半不在外泌体或微泡中，而是作为 100-300 kDa 的颗粒循环。这些颗粒的大小表明 5' tRNA 一半是一种大分子复合物的组成部分；这得到了 EDTA（一种螯合剂）处理的血清或血浆中 5' tRNA 一半的丢失但肝素或柠檬酸盐抗凝的血浆中保留的支持。对体细胞组织的调查显示，5' tRNA 一半集中在血细胞和造血组织中，但在其他组织中很少，这表明它们可能由血细胞产生。特定 5' tRNA 一半亚型的血清水平随年龄显著变化，无论是升高还是降低，并且这些变化可以通过热量限制来预防。
我们证明 5' tRNA 一半以稳定的形式在血液中循环，很可能作为核蛋白复合物的一部分，其血清水平受年龄和热量限制的调节。它们可能由血细胞产生，但它们的细胞靶标尚不清楚。这些循环分子的特征及其在抑制翻译起始中的已知功能表明它们是一种新型的信号分子。
