《Cell》：microRNAs自我调节机制新进展

【文章出处】

TUT4 in concert with Lin28 suppresses microRNA biogenesis through pre-microRNA uridylation.

Heo I, Joo C, Kim YK, Ha M, Yoon MJ, Cho J, Yeom KH, Han J, Kim VN

Cell 2009 Aug 21 138（4）：696-708

【概要评述】

这篇精彩之作，出自V. Narry Kim实验室。他们的研究揭示了TUTase4 （TUT4）这种酶，通过对miR-let-7的前体进行尿苷化，而下调miR-let-7的水平。这项研究揭示了一种新的microRNA调节机制——自我调节。

现在人们已经知道，miRNA可以对上千种基因进行调节。但是它们自己是怎样受到调节的呢？作者的前期研究结果表明，Lin28介导Let-7前体的末端尿苷化，从而阻断了Dicer的处理过程及let-7的成熟。但是这个过程是什么分子在“操纵”的，尚不清楚。在本研究中，作者通过RNA亲和纯化和免疫共沉淀这些普通的实验室技术， 阐明了一种不常见的Poly（A）多聚酶-TUT4，该酶在Lin-28的参与下，结合到 pre-let-7而阻断其成熟。体外试验发现，TUT4在Lin28帮助下，可以对 pre-let-7进行尿苷化。敲除了TUT4的鼠ES细胞失去了多能性，且let-7水平上升。此外作者发现这种交互作用发生于GGAG基序，该基序存在于pre-let-7的环部。其它带有此基序的miRNA前体，也可经 Lin28/TUT4 途径而得以尿苷化。这个研究结果，对干细胞/肿瘤生物学研究有很重要的意义。

【PI主页】 http://www.narrykim.org/

【相关评论】

The biogenesis of mature microRNAs （miRNAs） involves the cleavage of primary miRNAs by Drosha and the cleavage of the resulting precursor （pre）-miRNAs by Dicer. Previous work from Narry Kim's laboratory has shown that the pluripotency factor LIN28 can induce the terminal uridylation of pre-let-7 miRNA in embryonic stem （ES） cells to block Dicer cleavage and let-7 maturation. This has important consequences as mature let-7 promotes the differentiation of ES cells and development. Because LIN28 does not posses uridylyl transferase activity, how it is involved in inducing pre-let-7 uridylation was unknown. Three studies now reveal that the mammalian terminal uridylyl transferase （TUTase） zinc finger CCHC domain-containing protein 11 （ZCCHC11; also known as TUT4）， and its Caenorhabditis elegans orthologue poly（U） polymerase 2 （PUP-2）， is the TUTase that regulates LIN28-mediated pre-let-7 uridylation.

Studies by Heo et al. and Hagan et al. identify ZCCHC11 as the only one of seven mammalian TUTases that can uridylate pre-let-7 in mouse ES cells. Heo et al. show that small interfering RNA-mediated knockdown of ZCCHC11 or LIN28 reduces the level of pluripotency markers in mouse ES cells, which is consistent with the role of mature let-7 in promoting ES cell differentiation. They also present data suggesting that LIN28 recruits ZCCHC11 to pre-let-7 by recognizing a GGAG motif in the terminal loop of this pre-miRNA. Several additional miRNAs that contain the GGAG motif, including other let-7 family members and miR-107, miR-143 and miR-200c, also undergo uridylation in a LIN28-dependent manner. This suggests that the ability of LIN28 and ZCCHC11 to repress pre-miRNA processing by inducing uridylation may not be specific to let-7. Hagan et al. show that knockdown of ZCCHC11 or LIN28, using small interfering RNA, stops the inhibition of let-7 processing, leads to the accumulation of mature let-7 miRNA and results in the repression of reporter genes that are inhibited by mature let-7.

Finally, Lehrbach et al. examined the post-transcriptional regulation of let-7 in C. elegans. They found that LIN-28 also binds to pre-let-7 and prevents its processing by Dicer,and that thisis dependent on the ZCCHC11 orthologue, PUP-2. Furthermore, LIN-28 stimulates the uridylation of pre-let-7 by PUP-2 in vitro.

Together, these studies show that the newly defined let-7–LIN28–TUTase pathway is evolutionarily conserved to control the processing of let-7. However, as the terminal loop of pre-let-7 is not conserved in C. elegans, the exact mechanistic details might differ between organisms. Future studies are likely to confirm whether this pathway regulates the processing of other miRNAs in C. elegans and mouse ES cells.