Transcriptome-wide identification and validation of interactions between the miRNA machinery and HuR on mRNA targets [Ago2]

The 3’ UTR of messenger RNAs serves as the regulatory region that mediates post-transcriptional control by microRNAs and RNA-binding proteins (RBPs). Aside from individual sequence-specific binding and regulation, examples of interaction between these factors at particular 3’ UTR sites have emerged in recent studies. However, the whole picture of such higher-order regulatory modules across the transcriptome is lacking. Here, we investigate the interactions between HuR, a ubiquitous RBP, and Ago2, a core effector of the miRNA pathway, at the transcriptome-wide level. Using HITS-CLIP, we map HuR and miRNA binding sites on human 3’UTRs and assess their co-occurrence. Additionally, we demonstrate global effects of HuR knockdown on Ago2 occupancy, suggesting a co-regulatory relationship. Focusing on sites of Ago2-HuR overlap, 13 candidates were screened in luciferase reporter assays, compared to miRNA site mutant controls. Eleven of the sites showed a repressive activity, which displayed significant de-repression upon subsequent testing of the reporters in Dicer-null cells, substantiating miRNA dependence. To experimentally test for HuR’s role in co-regulation, we tested the reporters in CRISPR-generated HuR KO cells. Three of the miRNA sites demonstrated altered activities, indicating that HuR has an effect on miRNA repression at those sites. Our study presents an efficient search and validation system for studying miRNA-HuR interactions, which expands our understanding of the combinatorial post-transcriptional control of gene expression at the 3’ UTR. Determination of Ago2 binding by CLIP-seq in T-REx-293 cells with and without HuR knockdown