Background and objectives Increased activity of bone-resorbing osteoclasts (OC) activity contributes to bone resorption during osteoporosis and rheumatoid arthritis (RA). Negative regulation of osteoclastogenesis is thus critical to restore bone homeostasis and to prevent pathological bone erosion. Among regulatory molecules, microRNAs play important roles under physiological and pathological conditions and represent innovative therapeutic targets, but remain poorly studied in the context of osteoclastogenesis. Considering that monocytes exist in two main subsets committed to different functions, and that the classical Ly6Chigh monocyte subset represents main OC precursors (OCP), we investigated whether miRNAs differentially expressed between monocyte subsets regulate osteoclastogenesis in vivo and could represent potential target to intefere with bone loss in RA.
Materials and methods Genome-wide miRNA expression study identified miRNAs differentially expressed between classical and non-classical monocyte subsets sorted from mouse (Ly6Chigh and LyC6low) and human (CD14+CD16- and CD14dimCD16+) blood. OCP sorted from miR-146a deficient mice or control littermates were used for in vitro differentiation assays. Two mouse models of pathological bone erosion were used: K/BxN serum transfer arthritis and ovariectomy. Disease severity was assessed clinically and histologically, and in vivo bone histophotometry analysed. Intravenous injection of miR-146a mimic-containing lipoplexes was used to rescue miR-146a expression on Ly6Chigh monocyte in the mouse model of collagen-induced arthritis. Disease severity, inflammation, bone erosion and efficiency of miRNA mimics delivery to cell subset were monitored.
Results We identified miR-146a as the most differentially expressed miRNA between classical and non-classical monocytes. MiR-146a was down-regulated during OC differentiation and in OCP isolated from arthritic individuals as compared to healthy controls. Knockdown of miR-146a in OCP increased osteoclast differentiation in vitro. While no bone phenotype was evidenced in miR-146a deficient mice, neither under steady state nor under ovariectomized-induced osteoporosis conditions, arthritis-induced bone erosion was increased in miR-146a knockout mice. Finally, delivery of miR-146a mimics to Ly6Chigh monocytes efficiently interferes with pathological bone loss in inflammatory arthritis, reducing OC counts and erosion in inflamed joints as well as ex vivo osteoclastogenic differentiation capacity of OCP. This clinical benefit was associated an upregulation of the miR-146a/RelB axis in Ly6Chigh monocytes.
Conclusions Overall, the present work suggests that miR-146a is a sensor that negatively regulates osteoclastogenesis in OCP during inflammatory arthritis. We showed that reduced expression of miR-146a in Ly6Chigh monocytes is responsible for increased osteoclastogenesis in inflammatory arthritis and that delivery of miR-146a mimics to Ly6Chigh monocytes may offer valuable therapeutic strategy to interfere with pathological bone loss.