生物谷報(bào)道：美國約翰·霍普金斯大學(xué)的研究者發(fā)現(xiàn)一種短鏈核酸序列“跑”到細(xì)胞中原本不該出現(xiàn)的地方,，而且這些指令卻是來自核酸序列內(nèi)部,。該研究成果在了1月5日發(fā)表的《Science》雜志上,。
MicroRNA，或稱微小RNA,，與癌癥,、正常發(fā)育過程都有著緊密聯(lián)系。它們往往會(huì)“纏住”長鏈RNA序列,，“肆意”結(jié)束長鏈RNA的原定計(jì)劃——合成體現(xiàn)細(xì)胞功能的蛋白質(zhì),。霍普金斯大學(xué)McKusick-Nathans遺傳醫(yī)學(xué)研究所的副教授Joshua Mendell介紹說，MicroRNA就像是“分子變阻器,，精調(diào)著每個(gè)基因的蛋白合成,。”這就解釋了為什么microRNA總是出現(xiàn)在合成蛋白的分子機(jī)器周圍。
現(xiàn)今,，科學(xué)家們已知的人類microRNA總共有500多個(gè),，Mendell研究小組從中挑選了200多個(gè)仔細(xì)加以研究。但是出乎意料的事情發(fā)生了,，有一個(gè)microRNA分子像是有意避開其他的microRNA分子,，徑直進(jìn)入核內(nèi),。 “很明顯,，這個(gè)microRNA分子選擇了錯(cuò)誤的時(shí)間和錯(cuò)誤的地點(diǎn)，沒去干我們預(yù)料它會(huì)干的事,。我們對(duì)此進(jìn)行了深入研究,。” Hun-Way Hwang說道，一位人類遺傳學(xué)方向的研究生,，此項(xiàng)研究的參與者,。 對(duì)于這個(gè)“任性”的microRNA分子，研究者們給了它一個(gè)特別的名字miR-29b,。與其他的microRNA分子一樣,，miR-29b也基本由20-25個(gè)核苷酸組成（與有著數(shù)千核苷酸的RNA鏈相比顯得非常短小精悍）；但它也有別出心裁的地方,，末端的6個(gè)核苷酸與其他的microRNA分子截然不同,。
研究者懷疑這六核苷酸當(dāng)中蘊(yùn)藏著miR-29b獨(dú)特細(xì)胞定位的秘密。將這六核苷酸切下并粘合在其他的microRNA分子上,，研究者們發(fā)現(xiàn)新組裝的microRNA的行蹤與miR-29b一樣詭異,，不“理睬”細(xì)胞中蛋白合成的分子機(jī)器，徑直進(jìn)入細(xì)胞核這個(gè)細(xì)胞遺傳物質(zhì)存儲(chǔ)的地方,。接著,，研究者將同樣的六核苷酸接合到小干涉RNA或稱siRNA——一種可以抑制基因表達(dá)的RNA分子。同樣的現(xiàn)象再次出現(xiàn)：六核苷酸序列將siRNA分子“領(lǐng)”入核中,。
對(duì)于這些實(shí)驗(yàn)結(jié)果,，Mendell的解釋是microRNA分子雖然短小，但是它們依然包含著一些重要元件——由簡單的幾個(gè)核苷酸組成即可,。這些元件可以調(diào)控microRNA在細(xì)胞中的一舉一動(dòng),。可以想象本實(shí)驗(yàn)中,，miR-29b的六核苷酸序列就像是郵政編碼,，指導(dǎo)著miR-29b在細(xì)胞中的定位。Mendell希望能將這套細(xì)胞內(nèi)固有的“郵編系統(tǒng)”轉(zhuǎn)化為一種有效的實(shí)驗(yàn)手段。例如,，他打算借此將一些microRNA和siRNA分子引入核中,，降低目標(biāo)基因的表達(dá)。
Mendell研究隊(duì)伍正積極地搜尋其他影響microRNA在細(xì)胞中行為的元件,。同時(shí),，他們對(duì)miR-29b在細(xì)胞核中的行為也非常感興趣。已經(jīng)知道m(xù)icroRNA與癌癥和正常發(fā)育過程密切相關(guān),，Mendell希望對(duì)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.
原文出處：
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 »|
作者簡介：
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.
