A4.4 MIR-146A deficiency in LY6C<sup>high</sup>monocytes contributes to pathogenic bone loss during inflammatory arthritis
Authors: M Ammari, I Duroux-Richard, J Presumey, C Ponsolles, G Roussignol, C Roubert, V Escriou, K Toupet, AL Mausset-Bonnefont, P Georgel, C Jorgensen, F Apparailly
Journal: Annals of the Rheumatic Diseases (2015)

<h3>Background and objectives</h3> 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 Ly6C<sup>high</sup> monocyte subset represents main OC precursors (OCP), we investigated whether miRNAs differentially expressed between monocyte subsets regulate osteoclastogenesis <i>in vivo</i> and could represent potential target to intefere with bone loss in RA. <h3>Materials and methods</h3> Genome-wide miRNA expression study identified miRNAs differentially expressed between classical and non-classical monocyte subsets sorted from mouse (Ly6C<sup>high</sup> and LyC6<sup>low</sup>) and human (CD14<sup>+</sup>CD16<sup>-</sup> and CD14<sup>dim</sup>CD16<sup>+</sup>) blood. OCP sorted from miR-146a deficient mice or control littermates were used for <i>in vitro</i> 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 <i>in vivo</i> bone histophotometry analysed. Intravenous injection of miR-146a mimic-containing lipoplexes was used to rescue miR-146a expression on Ly6C<sup>high</sup> 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. <h3>Results</h3> 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 <i>in vitro</i>. 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 Ly6C<sup>high</sup> monocytes efficiently interferes with pathological bone loss in inflammatory arthritis, reducing OC counts and erosion in inflamed joints as well as <i>ex vivo</i> osteoclastogenic differentiation capacity of OCP. This clinical benefit was associated an upregulation of the miR-146a/RelB axis in Ly6C<sup>high</sup> monocytes. <h3>Conclusions</h3> 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 Ly6C<sup>high</sup> monocytes is responsible for increased osteoclastogenesis in inflammatory arthritis and that delivery of miR-146a mimics to Ly6C<sup>high</sup> monocytes may offer valuable therapeutic strategy to interfere with pathological bone loss.