MicroRNA-21 is a central regulator of cardiac inflammation and fibrosis and its inhibition using small inhibitors prevented myocardial dysfunction in small and large animal models of heart failure. Genetic deconvolution of the pig transcriptome further revealed that miR-21 inhibitor attenuated cardiac remodelling and dysfunction primarily through modifying myocardial macrophage and fibroblast numbers. Here, we sought to determine the in vivo targetome of miR-21 in macrophages and fibroblasts using Argonaute2-ribonucleoprotein immunoprecipitation (AGO-RIP) in the presence of an antimiR directed against this microRNA.

Wild type mice were subjected to myocardial infarction (MI) by ligation of the left anterior descending artery and a synthetic inhibitor of miR-21 (antimiR-21) or control was administered intraperitoneally on days 7,8 and 9 post-MI. Two days after the last antimiR injection (day 11), we isolated macrophages and fibroblasts from the infarcted hearts. AGO-RIP followed by next generation sequencing was carried out to identify miR-21 targets related to ischemia-induced heart failure by means of their de-enrichment in the AGO2-bound transcriptome and de-repression in the mRNA transcriptome. Echocardiography was carried out to monitor cardiac function before surgery and at days 7 and 11 after surgery.

Mice administered with antimiR-21 were protected from cardiac dysfunction induced by MI. Bioinformatic analysis of the transcriptomes and the corresponding targetomes revealed a strong de-enrichment (AGO2-RIP) and de-repression (input) of miR-21 targets in macrophages isolated from antimiR-21-treated hearts as compared to antimiR-control-treated hearts. Gene ontology analysis indicated a strong repression of genes associated with extracellular matrix organization in line with the recently established pro-fibrotic role of cardiac macrophages in cardiac remodelling. 

Taken together, our data indicate macrophage miR-21 as an important regulator of ischemia-induced cardiac dysfunction and a potential target for therapeutic intervention in cardiac ischemia.