Inhibiting METTL3 in macrophages alleviates colitis by enhancing M2 polarization via upregulating IRAKM

N6-methyladenosine (m⁶A), the most prevalent RNA modification, plays a crucial role in regulating macrophage homeostasis and intestinal immunity, although its mechanism remains largely unknown. A link between increased methyltransferase-like 3 (METTL3) expression in macrophages and intestinal inflammation has been confirmed. Herein, we sought to determine the role of METTL3-mediated macrophage activation in colitis.

The dextran sulfate sodium (DSS)-induced experimental colitis model was established. Conditional knockout of METTL3 in myeloid cells mice (Mettl3^fl/flLyz2^Cre) and myeloid cells-specific deletion of IRAKM mice (Irakm^fl/flLyz2^Cre) were generated, respectively. The severity of colitis was measured using the disease activity index, colon length, and histopathological staining. Various techniques such as flow cytometry, western blot, quantitative PCR, and RNA-seq analysis were employed to assess polarization and the expression of inflammatory cytokines.

Conditional knockout of Mettl3 in myeloid cells attenuated intestinal inflammation in experimental colitis. In vivo and in vitro studies confirmed that Mettl3 deletion skewed macrophages towards M2 activation. Mechanistically, Irakm, a negative regulator of TLR4 signaling, was identified as a target of METTL3-mediated m⁶A modification. METTL3 deficiency led to a higher level of IRAKM, which ultimately suppressed TLR signaling-mediated macrophage activation. Myeloid cells-specific deletion of Irakm mice were more susceptible to DSS-induced colitis than were wild-type mice. Colon-infiltrating M1 macrophages from Irakm^fl/flLyz2^Cre mice dramatically increased compared with those from their counterpart Irakm^fl/fl mice. Additionally, deletion of IRAKM in bone marrow-derived macrophages (BMDMs) induced NF-κB activation and facilitated M1 polarization.

Our study highlights the role of METTL3-IRAKM signaling in macrophage polarization and intestinal inflammation, providing a potential therapeutic target for the treatment of colitis.