生物谷报道：美国约翰·霍普金斯大学的研究者发现一种短链核酸序列“跑”到细胞中原本不该出现的地方，而且这些指令却是来自核酸序列内部。该研究成果在了1月5日发表的《Science》杂志上。
MicroRNA，或称微小RNA，与癌症、正常发育过程都有着紧密联系。它们往往会“缠住”长链RNA序列，“肆意”结束长链RNA的原定计划——合成体现细胞功能的蛋白质。霍普金斯大学McKusick-Nathans遗传医学研究所的副教授Joshua Mendell介绍说，MicroRNA就像是“分子变阻器，精调着每个基因的蛋白合成。”这就解释了为什么microRNA总是出现在合成蛋白的分子机器周围。
现今，科学家们已知的人类microRNA总共有500多个，Mendell研究小组从中挑选了200多个仔细加以研究。但是出乎意料的事情发生了，有一个microRNA分子像是有意避开其他的microRNA分子，径直进入核内。 “很明显，这个microRNA分子选择了错误的时间和错误的地点，没去干我们预料它会干的事。我们对此进行了深入研究。” Hun-Way Hwang说道，一位人类遗传学方向的研究生，此项研究的参与者。 对于这个“任性”的microRNA分子，研究者们给了它一个特别的名字miR-29b。与其他的microRNA分子一样，miR-29b也基本由20-25个核苷酸组成（与有着数千核苷酸的RNA链相比显得非常短小精悍）；但它也有别出心裁的地方，末端的6个核苷酸与其他的microRNA分子截然不同。
研究者怀疑这六核苷酸当中蕴藏着miR-29b独特细胞定位的秘密。将这六核苷酸切下并粘合在其他的microRNA分子上，研究者们发现新组装的microRNA的行踪与miR-29b一样诡异，不“理睬”细胞中蛋白合成的分子机器，径直进入细胞核这个细胞遗传物质存储的地方。接着，研究者将同样的六核苷酸接合到小干涉RNA或称siRNA——一种可以抑制基因表达的RNA分子。同样的现象再次出现：六核苷酸序列将siRNA分子“领”入核中。
对于这些实验结果，Mendell的解释是microRNA分子虽然短小，但是它们依然包含着一些重要元件——由简单的几个核苷酸组成即可。这些元件可以调控microRNA在细胞中的一举一动。可以想象本实验中，miR-29b的六核苷酸序列就像是邮政编码，指导着miR-29b在细胞中的定位。Mendell希望能将这套细胞内固有的“邮编系统”转化为一种有效的实验手段。例如，他打算借此将一些microRNA和siRNA分子引入核中，降低目标基因的表达。
Mendell研究队伍正积极地搜寻其他影响microRNA在细胞中行为的元件。同时，他们对miR-29b在细胞核中的行为也非常感兴趣。已经知道microRNA与癌症和正常发育过程密切相关，Mendell希望对miR-29b的深入研究可以揭示microRNA的其他未知功能。
Fig. 1. miR-29b is degraded rapidly in cycling cells but is stable in mitotic cells.
(A) Northern blot demonstrating discordant expression of miR-29a and miR-29b during the cell cycle. miR-29c (not shown) was not detectable. miR-30, a constitutively expressed miRNA, served as a loading control. Relative expression levels are indicated below the two panels. (B) The miR-29b-1/miR-29a cluster but not the miR-29b-2/miR-29c cluster is expressed in HeLa cells. The genomic organization of each cluster is shown on the left (arrows represent primers). Genomic DNA (gDNA) or RNA from cells treated with siRNA directed against Drosha or Luc was amplified. 7SK RNA served as a positive control. (C) RT-PCR assay for Drosha processing. Numbers in the schematic indicate amplicons. Primer pairs, indicated by arrows in the schematic and by numbers to the left of the gels, were used to amplify gDNA or RNA from cells treated with siRNA directed against Drosha or Luc. (D) Pulse-chase assay to measure synthetic miRNA stability. Northern blotting was used to measure the abundance of si–miR-29a and si–miR-29b at the indicated time points after transfection of RNA duplexes. The mean and SD of calculated half-lives (t1/2) from three experiments are shown on the right of each blot. The mitotic half-lives were longer than the time course and are therefore reported as >12 hours.
原文出处：
A Hexanucleotide Element Directs MicroRNA Nuclear Import
Hun-Way Hwang, Erik A. Wentzel, and Joshua T. Mendell
Science 5 January 2007: 97-100.
A six-nucleotide sequence near one end of a small noncoding RNA determines its location in the cell nucleus.
Abstract »| Full Text »| PDF »| Supporting Online Material »|
作者简介：
Joshua Mendell, M.D., PH.D.
Assistant Professor, Pediatrics, Molecular Biology and Genetics
Education:
1996, B.A., Cornell University 2001, Ph.D., Johns Hopkins University 2003, M.D., Johns Hopkins University
Main interests:
Our research focuses on a recently described class of small regulatory RNAs known as microRNAs. These ~18-25 nucleotide long RNA molecules regulate the stability or translational efficiency of target mRNAs. It is estimated that at least 30% of human transcripts are regulated by microRNAs. Accumulating evidence demonstrates that these molecules play essential roles in normal physiology and are commonly dysregulated in human disease states such as cancer. We have developed tools that allow the analysis of expression of all known microRNAs simultaneously and we are applying these methodologies to study microRNA expression and function in physiologic and pathophysiologic states.
Recently, we identified a group of microRNAs that are directly regulated by the oncogenic transcription factor c-Myc. This was the first mammalian transcription factor shown to control microRNA expression. Furthermore, our studies suggested that these microRNAs play an important role in tumorigenesis. We also recently discovered a microRNA that exhibits regulated expression during the cell-cycle. Ongoing studies in my laboratory aim to further elucidate how these microRNAs are regulated, functionally dissect the specific pathways regulated by these microRNAs, and characterize additional microRNAs that act as oncogenes and tumor suppressors.
Research Interests:
Post-transcriptional regulation of gene expression microRNA biogenesis and function Characterization of microRNA dysfunction in pathologic states
Educational Activities:
Preceptor, Human Genetics and Molecular Biology Graduate Program Co-organizer, Molecular Mechanisims of Disease course Lecturer, Molecular Biology and Genomics Course
Selected Publications:
Hwang HW, Wentzel EA, Mendell JT. (2007) A Hexanucleotide element directs microRNA nuclear import. Science, 315:97-100.
Kent OA, Mendell JT. (2006). A small piece of the cancer puzzle: microRNAs as oncogenes and tumor suppressors. Oncogene, 25(46):6188-96.
Dews M, Homayouni A, Yu D, Murphy D, Sevignani C, Wentzel E, Furth EE, Lee WM, Enders GH, Mendell JT, Thomas-Tikhonenko A. (2006). Augmentation of tumor angiogenesis by a Myc-activated microRNA cluster. Nature Genetics, 38(9):1060-5.
Hwang, HW and Mendell JT. (2006). MicroRNAs in cell proliferation, cell death, and tumorigenesis. British Journal of Cancer, 94(6):776-80.
O’Donnell KA, Wentzel EA, Zeller KI, Dang CV, and Mendell JT. (2005). c-Myc-regulated microRNAs modulate E2F1 expression. Nature 435: 839-843 [Featured in a News and Views by Meltzer, PS (2005) Nature 435:745-746]
Mendell JT, Sharifi NA, Meyers JL, Martinez-Murrillo F, Dietz HC. (2004). Nonsense surveillance regulates expression of diverse classes of mammalian transcripts and mutes genomic noise. Nature Genetics 36(10):1073-1078. [Featured in Research Highlights, Nature Reviews Genetics, November 2004]
Mendell JT, ap Rhys CMJ, Dietz HC. (2002). Separable Roles for rent1/hUpf1 in Altered Splicing and Decay of Nonsense Transcripts. Science 298: 419-422. [Featured in a Perspective piece by Moore, MJ (2002) Science 298:370-371]
Mendell JT, Dietz HC. (2001). When the Message Goes Awry: Disease-Producing Mutations that Influence mRNA Content and Performance. Cell 107: 411-414.
Mendell JT, Medghalchi SM, Lake RG, Noensie EN, Dietz HC. (2000). Novel Upf2p Orthologues Suggest a Functional Link Between the Translation Initiation and Nonsense Surveillance Complexes. Molecular and Cellular Biology 20(23):8944-8957.
Mendell JT, Paniker SG, Tsao CY, Feng B, Sahenk Z, Marzluf GA, Mendell JR. (1998). Novel compound heterozygous laminin α2-chain gene (LAMA2) mutations in congenital muscular dystrophy. Human Mutation 12(2):135.
