Regulatory mechanism of miR-722 on C5aR1 and its functions against bacterial inflammation in half-smooth tongue sole (Cynoglossus semilaevis)

MicroRNAs (miRNAs) are small non-coding RNAs involved in various biological processes, including immunity. Previously, we investigated the miRNAs of half-smooth tongue sole (Cynoglossus semilaevis) and found that miR-722 (designated Cse-miR-722) was significantly differentially expressed after infection with Vibrio anguillarum, reflecting its importance in immune response. Our preliminary bioinformatic analysis suggested that Cse-miR-722 could target C5aR1 (designated CsC5aR1), which was known to play crucial roles in complement activation and inflammatory response, as a receptor of C5a. However, the underlying mechanisms of their interactions and specific functions in inflammatory and immune response are still enigmas. In this study, we successfully cloned the precursor sequence of Cse-miR-722 (94 bp) and the full length of CsC5aR1 (1541 bp, protein molecular weight 39 kDa). The target gene of Cse-miR-722 was verified as CsC5aR1 by a dual luciferase reporter assay, and Cse-miR-722 was confirmed to regulate CsC5aR1 at the protein level using quantitative real-time polymerase chain reaction (qRT-PCR) and Western blotting. The expression of CsC5aR1 and Cse-miR-722 in liver cells and four immune tissues of half-smooth tongue sole changed significantly after LPS stimulation and infection with V. anguillarum. To explore the functional role of Cse-miR-722 in half-smooth tongue sole, we performed both in vitro and in vivo experiments. Cse-miR-722 was observed to affect phagocytosis and respiratory burst activity of macrophages by regulating CsC5aR1 in half-smooth tongue sole. Furthermore, we found that Cse-miR-722 regulated the expression of CsC5aR1, CsC5a, and the inflammatory factors CsIL1-β, CsIL6, CsIL8, and CsTNF-α both in vitro and in vivo. In addition, Cse-miR-722 reduced mortality and pathological damage. This study clarified the regulatory mechanism of Cse-miR-722 on CsC5aR1 and provided insight into the regulatory roles of Cse-miR-722 in immune responses, laying a theoretical foundation for the feasibility of using miR-722 to prevent and control bacterial diseases in teleost.