Overexpression of microRNA-722 fine-tunes neutrophilic inflammation through inhibiting Rac2 in zebrafish

Neutrophilic inflammation is essential for defending against invading pathogens, but can also be detrimental in many clinical settings. The hematopoietic-specific small Rho-GTPase Rac2 regulates multiple pathways that are essential for neutrophil activation, including adhesion, migration, degranulation and production of reactive oxygen species. This study tested the hypothesis that partially suppressing rac2 in neutrophils with a microRNA would inhibit neutrophil migration and activation, which will reduce the immunological damage caused by systemic inflammation. We have generated a transgenic zebrafish line that over-expresses microRNA-722 (miR-722) in neutrophils. Neutrophil motility and chemotaxis to tissue injury or infection are significantly reduced in this line. MiR-722 downregulates the transcript level of rac2 through binding to seed match in the rac2 3’UTR. Furthermore, miR-722 overexpressing larvae display improved outcomes in both sterile and bacterial systemic models, which correlates with a robust upregulation of the anti-inflammatory cytokines in the whole larvae and isolated neutrophils. Finally, the miR-722 mimics protect zebrafish from lethal LPS challenge. Together, we provide evidence and the mechanism of an anti-inflammatory microRNA that restrains detrimental systemic