Transferrin receptor 1-mediated iron uptake regulates bone mass in mice via osteoclast mitochondria and cytoskeleton

<jats:p>Increased intracellular iron spurs mitochondrial biogenesis and respiration to satisfy high-energy demand during osteoclast differentiation and bone-resorbing activities. Transferrin receptor 1 (Tfr1) mediates cellular iron uptake through endocytosis of iron-loaded transferrin, and its expression increases during osteoclast differentiation. Nonetheless, the precise functions of Tfr1 and Tfr1-mediated iron uptake in osteoclast biology and skeletal homeostasis remain incompletely understood. To investigate the role of Tfr1 in osteoclast lineage cells in vivo and in vitro, we crossed <jats:italic>Tfrc</jats:italic> (encoding Tfr1)-floxed mice with <jats:italic>Lyz2 (LysM)-</jats:italic>Cre and <jats:italic>Cathepsin K</jats:italic> (<jats:italic>Ctsk</jats:italic>)-Cre mice to generate <jats:italic>Tfrc</jats:italic> conditional knockout mice in myeloid osteoclast precursors (Tfr1<jats:sup>ΔLysM</jats:sup>) or differentiated osteoclasts (Tfr1<jats:sup>ΔCtsk</jats:sup>), respectively. Skeletal phenotyping by µCT and histology unveiled a significant increase in trabecular bone mass with normal osteoclast number in long bones of 10-week-old young and 6-month-old adult female but not male Tfr1<jats:sup>ΔLysM</jats:sup> mice. Although high trabecular bone volume in long bones was observed in both male and female Tfr1<jats:sup>ΔCtsk</jats:sup> mice, this phenotype was more pronounced in female knockout mice. Consistent with this gender-dependent phenomena, estrogen deficiency induced by ovariectomy decreased trabecular bone mass in Tfr1<jats:sup>ΔLysM</jats:sup> mice. Mechanistically, disruption of Tfr1 expression attenuated mitochondrial metabolism and cytoskeletal organization in mature osteoclasts in vitro by attenuating mitochondrial respiration and activation of the Src-Rac1-WAVE regulatory complex axis, respectively, leading to decreased bone resorption with little impact on osteoclast differentiation. These results indicate that Tfr1-mediated iron uptake is specifically required for osteoclast function and is indispensable for bone remodeling in a gender-dependent manner.</jats:p>