Autoregulation oflin-4microRNA transcription by RNA activation (RNAa) inC. elegans

Abstract

The conserved lin-4 microRNA (miRNA) regulates the proper timing of stem cell fate decisions in C. elegans by regulating stemness genes such as lin-14 and lin-28. (1)(-) (3) While lin-4 is upregulated toward the end of the first larval stage and functions as an essential developmental timing "switch", little is known about how lin-4 expression is regulated. (4) Here we show that in C. elegans hypodermal seam cells, transcription of lin-4 is positively regulated by lin-4 itself. In these cells, lin-4 activates its own transcription through a conserved lin-4-complementary element (LCE) in its promoter. We further show that lin-4 is required to recruit RNA polymerase II to its own promoter, and that lin-4 overexpression is sufficient for autoactivation. Finally, we show that a protein complex specifically binds the LCE in vitro, and that mutations that abolish this binding also reduce the in vivo expression of a plin-4:GFP reporter. Thus, we describe the first in vivo evidence of RNA activation (RNAa) by an endogenous miRNA, and provide new insights into an elegant autoregulatory mechanism that ensures the proper timing of stem cell fate decisions in development.

Identification of Small Activating RNAs that Enhance Endogenous OCT4 Expression in Human Mesenchymal Stem Cells

“…Apart from silencing gene expression through the well-known RNAi and related mechanisms, the small RNA machinery has been found to positively affect gene expression at the epigenetic/ transcriptional level [1,2]. This gene activation mechanism has been termed RNAa [3,4].…”

“…RNAa can be triggered by both artificially designed small activating RNAs (saRNAs) and endogenous small RNAs that target gene regulatory sequences or coding regions [2][3][4][5][6][7][8]. RNAa appears to be conserved in evolution, being present in animals ranging from Caenorhabditis elegans (C. elegans) to human [1,2,5]. Despite that the detailed mechanism for RNAa awaits to be elucidated, this unparalleled approach of sequence-specific activation of endogenous gene expression can obviously be harnessed for a number of purposes such as disease treatment [9,10] and cell fate reprogramming.…”

RNAa in action: From the exception to the norm

“…[25][26][27] Turner et al now showed that lin-4, the first miRNA discovered in C. elegans and important for the timing of stem cell fate decisions, 28 can activate its own transcription by binding to its own promoter. 11 By analyzing the promoter sequence of lin-4, the authors found a putative lin-4 complementary element (LCE) conserved in the lin-4 promoters of several nematode species. Deletion of the LCE in the lin-4-GFP reporter transgenic animals resulted in decreased transcriptional activity of the lin-4 promoter suggesting that the LCE is a functional cis-regulatory element potentially transregulated by lin-4.…”

“…5 The underlying mechanism of RNAa remained elusive however, leading to concerns that the observed gene induction was caused by secondary effects of canonical RNA interference (RNAi). New findings from a series of recent studies in C. elegans reveal once again an activating side of the small RNA-Argonaute pathways [6][7][8][9][10][11] and establish RNAa as a regulatory mechanism of endogenous gene expression.…”

Thelin-4microRNA

“…Mounting evidence supports the canonical view that miRNAs function to block expression of their targets by associating in a sequence-specific manner to messenger RNA (mRNA) transcripts, causing mRNA degradation and/or translational repression. A recent report by Turner et al offers an expanded function for miRNAs in regulating gene expression 2 . Their work, focusing on lin-4 , suggests that miRNAs can bind to their own promoters to induce their transcription—a process referred to as RNA activation (RNAa).…”