MicroRNAs (miRNAs) are approximately 22 nucleotide RNAs that negatively regulate the expression of protein-coding genes. In a present model of miRNA function in animals, miRNAs that form imperfect duplexes with their targets inhibit protein expression without affecting mRNA levels. Here, we report that in C. elegans, regulation by the let-7 miRNA results in degradation of its lin-41 target mRNA, despite the fact that its 3'UTR regulatory sequences can only partially base-pair with the miRNA. Furthermore, lin-14 and lin-28 are targets of the lin-4 miRNA, and we show that the mRNA levels for these protein-coding genes significantly decrease in response to lin-4 expression. This study reveals that mRNAs containing partial miRNA complementary sites can be targeted for degradation in vivo, raising the possibility that regulation at the level of mRNA stability may be more common than previously appreciated for the miRNA pathway.
Alternative splicing (AS) enables programmed diversity of gene expression across tissues and development. We show here that binding in distal intronic regions (>500 nucleotides (nt) from any exon) by Rbfox splicing factors important in development is extensive and is an active mode of splicing regulation. Similarly to exon-proximal sites, distal sites contain evolutionarily conserved GCATG sequences and are associated with AS activation and repression upon modulation of Rbfox abundance in human and mouse experimental systems. As a proof of principle, we validated the activity of two specific Rbfox enhancers in KIF21A and ENAH distal introns and showed that a conserved long-range RNA-RNA base-pairing interaction (an RNA bridge) is necessary for Rbfox-mediated exon inclusion in the ENAH gene. Thus we demonstrate a previously unknown RNA-mediated mechanism for AS control by distally bound RNA-binding proteins.
SUMMARY
LIN28 is a conserved RNA binding protein implicated in pluripotency, reprogramming and oncogenesis. Previously shown to act primarily by blocking let-7 microRNA (miRNA) biogenesis, here we elucidate distinct roles of LIN28 regulation via its direct messenger RNA (mRNA) targets. Through cross-linking and immunoprecipitation coupled with high-throughput sequencing (CLIP-seq) in human embryonic stem cells and somatic cells expressing exogenous LIN28, we have defined discrete LIN28 binding sites in a quarter of human transcripts. These sites revealed that LIN28 binds to GGAGA sequences enriched within loop structures in mRNAs, reminiscent of its interaction with let-7 miRNA precursors. Among LIN28 mRNA targets, we found evidence for LIN28 autoregulation and also direct but differing effects on the protein abundance of splicing regulators in somatic and pluripotent stem cells. Splicing-sensitive microarrays demonstrated that exogenous LIN28 expression causes widespread downstream alternative splicing changes. These findings identify important regulatory functions of LIN28 via direct mRNA interactions.
The highly conserved let-7 microRNA (miRNA) regulates developmental pathways across animal phyla. Mis-expression of let-7 causes lethality in Caenorhabditis elegans and has been associated with several human diseases. We show that timing of let-7 expression in developing worms is under complex transcriptional and post-transcriptional control. Expression of let-7 primary transcripts oscillates during each larval stage but precursor and mature let-7 miRNAs do not accumulate until later in development after lin-28 activity has diminished. We demonstrate that LIN-28 binds endogenous primary let-7 transcripts co-transcriptionally. We further show that LIN-28 binds endogenous primary let-7 transcripts in the nuclear compartment of human ES cells, suggesting that this LIN-28 activity is conserved across species. We conclude that co-transcriptional interaction of LIN-28 with let-7 primary transcripts blocks Drosha processing and, thus, precocious expression of mature let-7 during early development.
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