Fibrosis is the default response to myocardial injury, where a fibrotic scar replaces the infarct tissue and later progresses into heart failure. MicroRNA-21 (miR-21) has been previously reported as a key regulator of myocardial fibrosis. When we sequenced small-RNA fractions from the major cardiac cell types, we observed that miR-21 was upregulated in murine cardiac cells seven days after myocardial infarction (MI). Previous studies with small and large animal models of heart failure revealed that miR-21 inhibition prevented cardiac dysfunction and myocardial fibrosis. Here, we aim to identify the target cell population through which miR-21 inhibition treatment prevents cardiac disease.

Myocardial infarction was induced in wild-type mice, and antimiR-21 or control was injected for three consecutive days starting seven days after the injury. The first group of hearts was harvested two days after the last injection (day 11) to assess miR-21 activity through its targetome deregulation in macrophages and fibroblasts using Argonaute2-ribonucleoprotein immunoprecipitation (AGO-RIP). The second group of hearts was harvested on day 21 to evaluate molecular changes associated with the treatment at the border zone and infarct region. Cardiac function was measured by echocardiography throughout the study.

Unlike controls, mice treated with antimiR-21 showed improved cardiac function both shortly after treatment and on day 21. The treatment improved cardiac function parameters, such as ejection fraction and longitudinal strain.Bioinformatic analysis of the transcriptome and the corresponding targetome revealed a strong de-enrichment (AGO2-RIP) and de-repression (input) of miR-21 targets in macrophages after treatment. Additionally, gene ontology analysis indicated downregulation of genes associated with inflammation and fibrosis. For the hearts harvested on day 21, a stronger effect of antimiR-21 was observed at the border zone than at the infarct region. Moreover, an anti-fibrotic phenotype was found for the significant-deregulated-abundant genes of the border zone, indicating conservation of the phenotype observed on day 11 in fibroblasts. 

Altogether, our data suggest that macrophages from the border zone are the main target cell population of antimiR-21 treatment, as they promote an anti-fibrotic phenotype that translates into an immediate and sustained improvement in heart function.