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. 2014 Nov 6;56(3):347-359.
doi: 10.1016/j.molcel.2014.09.018. Epub 2014 Oct 23.

Endogenous miRNA and target concentrations determine susceptibility to potential ceRNA competition

Andrew D Bosson  1 , Jesse R Zamudio  2 , Phillip A Sharp  3
Affiliations

Endogenous miRNA and target concentrations determine susceptibility to potential ceRNA competition

Andrew D Bosson et al. Mol Cell. .

Abstract

Target competition (ceRNA crosstalk) within miRNA-regulated gene networks has been proposed to influence biological systems. To assess target competition, we characterize and quantitate miRNA networks in two cell types. Argonaute iCLIP reveals that hierarchical binding of high- to low-affinity miRNA targets is a key characteristic of in vivo activity. Quantification of cellular miRNA and mRNA/ncRNA target pool levels indicates that miRNA:target pool ratios and an affinity partitioned target pool accurately predict in vivo Ago binding profiles and miRNA susceptibility to target competition. Using single-cell reporters, we directly test predictions and estimate that VSports手机版. 3,000 additional high-affinity target sites can affect active miRNA families with low endogenous miRNA:target ratios, such as miR-92/25. In contrast, the highly expressed miR-294 and let-7 families are not susceptible to increases of nearly 10,000 sites. These results show differential susceptibility based on endogenous miRNA:target pool ratios and provide a physiological context for ceRNA competition in vivo. .

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V体育ios版 - Figures

Figure 1
Figure 1. In Vivo miRNA and Target Pool Quantitation for Prediction of ceRNA Effects
(A) Steady-state equation governing miRNA interactions (top) and illustration of a potential ceRNA gene in context of a miRNA's total target pool (bottom). Nodes depict target genes and size represents different expression levels. (B) Experimental system for quantifying miRNA regulation networks. TT-FHAgo2 and TT-Ago2 ESCs lack endogenous Ago1-4 and express only FLAG-HA-hAgo2 or untagged hAgo2 under a doxycycline inducible promoter. (C) iCLIP read coverage at p21 and Casp2 genes from TT-FHAgo2 or TT-Ago2 ESCs. iCLIP read 5′ ends predict the exact crosslink site and are indicated in red. Coverage is normalized to mean values of background RNA to allow comparison between data sets, as described in the Experimental Procedures. Scale represents 0–20 for TT-FHAgo2 coverage and 0–2 for TT-Ago2 coverage. (D) iCLIP read or crosslink (Xlink) density across all miR-294 seed match sites (GCACTT) in expressed 3′ UTRs. Xlink sites represent 5′ end of sequenced reads. iCLIP density represents read coverage or number of Xlinks at a given nt position/total read coverage or number of Xlinks within the ± 200 nt window surrounding the target sites; 0 in the x axis corresponds to the first nucleotide of the 7-mer seed match (i.e., across from position 8 of the miRNA). (E) Correlation between log2 iCLIP coverage per gene and log2 gene repression, for each gene containing at least one 3′ UTR iCLIP cluster. iCLIP coverage represents the number of reads across all 3′ UTR clusters normalized to wild-type (WT) gene expression (Supplemental Experimental Procedures). Gene repression is the log2 Ago+/Ago− fold change/median Ago+/Ago− value of a random control set (Supplemental Experimental Procedures). The least-squares linear regression best-fit line is depicted in gray with the coefficient of determination noted (R2).
Figure 2
Figure 2. Quantitative iCLIP Coverage across Entire Target Pool Identifies Active ESC miRNAs
(A) Proportion of miRNA expression, bound target sites (≥1 iCLIP read), or total target sites for the indicated miRNA seed families. (B) Histogram of the average iCLIP coverage per expressed 3′ UTR 8-mer site for each of the top 30 expressed ESC miRNA families (green). miRNA family name and 7-mer seed match is indicated above corresponding value for families with signal over background control (p < 0.01). The histogram for 998 random seed “families” used to estimate background iCLIP coverage levels is in blue. (C) Heatmap of average crosslinks (Xlinks) per nucleotide across all 7/8-mer sites in expressed 3′ UTRs (≥1 iCLIP read within 10 nt of the seed match) for each of the top 30 expressed miRNA families in ESCs. These are ranked by Xlink signal. The three miRNA families with seed matches similar to highly expressed miRNAs are moved to the bottom; 0 in the x axis corresponds to the first nucleotide of the 7-mer seed match. Average Xlinks color scale is indicated to the right. Asterisks mark significantly active families identified in Figure 3B. (D) Plot of average iCLIP coverage per conserved 3′ UTR 7/8-mer site versus Ago-dependent expression change of the corresponding gene set, calculated as the area under the curve (AUC) between the cumulative distribution of log2 Ago+/Ago− FPKM test set values and matched control gene values. Only significantly active ESC miRNA families are plotted. Genes considered have a bound 3′ UTR 7/8-mer site with PhastCons score > 0.8. The least-squares linear regression best-fit line is depicted in gray with the coefficient of determination noted (R2).
Figure 3
Figure 3. Relative Target Pool Abundance Influences Site Type Binding Pattern
(A) Cumulative distribution of log2 iCLIP coverage at each 8-mer target site for the active ESC miRNA families and the nonexpressed let-7 miRNA family (included as an additional estimate of background signal in this analysis). Only sites with ≥1 iCLIP read were included. Number of sites in each set is indicated in legend parentheses. Colored dots at bottom represent mean log2 iCLIP coverage. 3′ UTR matched control set is included (Experimental Procedures). (B) Same as (A) for 7-mer sites. (C) Same as (A) for 6-mer sites. (D) Measured copies per cell (cpc) values for miRNA and corresponding 3′ UTR target pools of indicated site type for significantly active ESC miRNAs. y axis is log scale. (E) Average crosslinks (Xlinks) per nucleotide across all target sites in expressed 3′ UTRs (≥1 iCLIP read within 10 nt), for highly expressed miR-294 family (top) and lowly expressed miR-92/25 (bottom). Distribution across each site type is plotted individually; 0 in the x axis corresponds to the first nucleotide of the 7-mer seed match.
Figure 4
Figure 4. Total Target Pools Are Dominated by 3′ UTR Binding and Above the Concentration of Most miRNAs
(A) Coverage plot of FHAgo2 iCLIP read (black) and crosslink (Xlink) (red) coverage at the indicated non-3′ UTR ESC targets. (B) Proportion of iCLIP reads at 7/8-mer target sites coming from indicated genic categories for each of the top 30 expressed ESC miRNAs. iCLIP reads at sites with ambiguous genic annotation are counted in both categories. Asterisks mark significantly active families identified in Figure 3B. (C) Measured copies per cell (cpc) values for miRNA and corresponding total target abundance (6/7/8-mer sites from all genic categories). Top 30 expressed miRNAs are plotted. miRNA family cpc is a sum of all miRNAs sharing the same 6-mer seed, thus miR-294 and miR-292/467 have identical values. The y axis is log scale.
Figure 5
Figure 5. Mathematical Model of miRNA Binding Predicts ceRNA Effects Based on Intratarget Pool Competition
(A) miRNA and target stoichiometries per site type are plotted against total binding estimates from iCLIP, for each active ESC miRNA. miRNA:target ratio represents miRNA cpc/ site type target pool cpc. Total Ago occupancy represents iCLIP reads per million (RPM)/site type target pool cpc. The p = 0.01 is the empirical p value calculated from Total Ago Occupancy values of 1,000 random sets of control sites is indicated by dotted line. The x axis is log scale. Total Ago occupancy values for each miRNA family are listed in Table 1 and can be used to identify the corresponding point on the graph. (B) Diagram of Ago-miRNA binding competition between different affinity target sites. The total target pool is partitioned by seed match type with different KD and concentrations (indicated by color and subscript). (C) Simulated target titration curves for a representative lowly expressed miRNA (miR-92/25) showing the relationship between proportion of free targets and total 8-mer target pool concentration in cpc. Dotted black vertical line indicates endogenous 8-mer target pool concentration. Gray-black solid vertical lines indicate estimates of physiological ESC ceRNA perturbations, corresponding to loss (left of dotted line) of a highly expressed mRNA (200 cpc) containing 3 × 8-mer sites (600 site cpc) or 10- and 100-fold upregulation (right of dotted line) of an average target gene (13 cpc) containing 3 × 8-mer sites (390, 3,900 site cpc). The x axis is log scale.
Figure 6
Figure 6. Single Cell Reporter Assays Confirm Differential Susceptibility to Target Competition Based on miRNA:Target Ratio
(A) Top: log-log plot of miRNA targeted or untargeted mCherry mean fluorescence across 100 bins of eYFP signal in ESCs for miR-92/25 (purple), miR-293 (blue), or miR-294 (green) reporters. Bottom: box plots of fold repression (mCherryuntargeted/mCherry3×sites) in first ten bins (Low), middle ten bins centered around log10 value of 3.75 (Mid), and last ten bins (High). Targeted mCherry constructs contained 3 × 8-mer sites. Untargeted mCherry contained 3 × 8-mer sites for the nonexpressed let-7 miRNA. Error bars represent SEM from three biological replicates. All fluorescent values are background normalized (Experimental Procedures). (B) miRNA-mediated fold repression across a range of reporter induction as in (A) except measured in MSCs, using mCherry reporters containing 3 × 8-mer sites for miR-92/25 (purple) or let-7 (yellow). Untargeted mCherry contains a short 3′ UTR with no miRNA binding sites. Error bars represent SEM from three biological replicates. (C) Hierarchical affinity model of miRNA target competition. The endogenous miRNA target pool is indicated in a partitioned triangle illustrating the hierarchical affinity classes and their relative abundances. Ago-miRNA target pool coverage will depend on the relative miRNA:target ratio as indicated in red for exemplary low (left) or high (right) ratios and spread toward low-affinity targets. Individual miRNA target pools demonstrate differential susceptibility to an identical level of high-affinity ceRNA overexpression (black bars) as described in the Discussion.
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