Neuregulin 4 attenuates osteoarthritis by decreasing macrophage M1 polarization through PI3K/AKT signaling

Altered polarization of synovial macrophages has been identified as a key pathogenic factor in sustaining synovial inflammation and driving osteoarthritis(OA) progression.Neuregulin 4 (Nrg4) is widely involved in inflammatory diseases, such as hepatic inflammation, Crohn's disease, and atherosclerosis.In this study, we aimed to investigate the effects of Nrg4 on macrophages and synovitis and to elucidate the underlying mechanisms in the development of OA.We first evaluated the expression of Nrg4 and ErbB4 in OA patients and mouse model. The adeno-associated virus 5 vector carrying the Nrg4 gene (AAV5-Nrg4) was injected into the knee joints to overexpress Nrg4 in two OA models.In vitro, RAW264.7 macrophages and mouse bone marrow-derived macrophages (BMDMs) were cultured, induced to M1 macrophages, and then treated with Nrg4. RNA interference (RNAi) technique was used to inhibit the expression of the Nrg4 receptor ErbB4.The results demonstrated that Nrg4-ErbB4 signaling was decreased during OA. In vitro experiments showed that Nrg4 treatment significantly inhibited the M1 polarization of RAW264.7 cells and BMDMs and down-regulated the expression of pro-inflammatory genes.RNA sequencing (RNA-seq) analysis and related experiments revealed that Nrg4 regulated macrophage polarization mainly by inhibiting the PI3K/AKT signaling pathway.Intra-articular injection of AAV5-Nrg4 effectively alleviated joint damage and synovitis in collagenase-induced OA (CIOA) and destabilization of the medial meniscus(DMM)-induced OA models. Nrg4-mediated suppression of M1 macrophage polarization in vivo was evidenced by attenuated iNOS concomitant with upregulated CD206 expression.In conclusion, our findings demonstrated that targeting Nrg4-ErbB4 axis may be a promising way to treat OA by reducing M1 macrophage polarization in synovial tissues.