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

Abstract

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 Tfrc (encoding Tfr1)-floxed mice with Lyz2 (LysM)-Cre and Cathepsin K (Ctsk)-Cre mice to generate Tfrc conditional knockout mice in myeloid osteoclast precursors (Tfr1ΔLysM) or differentiated osteoclasts (Tfr1ΔCtsk), 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 Tfr1ΔLysM mice. Although high trabecular bone volume in long bones was observed in both male and female Tfr1ΔCtsk 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 Tfr1ΔLysM 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 (WRC) 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.

Multinucleation resets human macrophages for specialized functions at the expense of their identity

“…MGCs have been described having increased numbers of lysosomes decades ago (Sutton & Weiss, 1966 ) and several lines of evidence link v‐ATPase‐dependent lysosomal function to osteoclast multinucleation and function (Lee et al , 2006 ; Duan et al , 2016 ). Similarly, TFRC‐mediated iron uptake induces mitochondrial respiration regulating osteoclast differentiation, mature osteoclast function and bone mass (Ishii et al , 2009 ; Das et al , 2022 ). Hence, lysosomal biogenesis coupled with iron homeostasis is likely to be a ‘metabolic booster’ for specialized functions in multinucleated macrophages.…”

Multinucleation resets human macrophages for specialized functions at the expense of mononuclear phagocyte identity

“…MGCs have been described having increased numbers of lysosomes decades ago [75] and several lines of evidence link v-ATPase-dependent lysosomal function to osteoclast multinucleation and function [76, 77]. Similarly, TFRC-mediated iron uptake induces mitochondrial respiration regulating osteoclast differentiation, mature osteoclast function and bone mass [78, 79]. Hence lysosomal biogenesis coupled with iron homeostasis is likely to be a ‘metabolic booster’ for specialized functions in multinucleated macrophages.…”

Fighting age-related orthopedic diseases: focusing on ferroptosis

“…During osteoclast differentiation, key genes in heme-dependent and TF-dependent iron intake pathways are highly upregulated, and the expression of SLC39A14 (an NTBI iron transporter) and Scara-5 and Tim-2 (ferritin transporters) are either downregulated or undetectable in osteoclast lineage cells. 80 Therefore, osteoclasts primarily obtain iron via TFR1-mediated iron uptake; furthermore, osteoclast differentiation has been demonstrated to be tightly linked to TFR1 activation and enhanced iron uptake. 61 , 62 The RANKL/OPG ratio increases when iron levels are excessive, and osteoclast differentiation and bone resorption are promoted by TFR1-mediated iron absorption.…”

“… 81 , 82 Loss of TFR1 in mature osteoclasts results in a significant reduction in total intracellular iron content (approximately 50%), whereas iron levels in monocytes and osteoclast precursor cells are affected to a lesser degree by TFR1 deficiency. 80 Mechanistically, by activating the Src-Rac1-WAVE regulatory complex axis and inhibiting mitochondrial respiration, TFR1 depletion attenuates the cytoskeletal organization and mitochondrial metabolism of mature osteoclasts in vitro and reduced bone resorption. 80 Several studies have shown that excessive iron-induced bone loss is due to an increased number and heightened activity of mature osteoclasts 83 , 84 Together, these findings suggest that excessive iron might promote osteoclast development and activity, and both can contribute to bone resorption.…”

“…80 Mechanistically, by activating the Src-Rac1-WAVE regulatory complex axis and inhibiting mitochondrial respiration, TFR1 depletion attenuates the cytoskeletal organization and mitochondrial metabolism of mature osteoclasts in vitro and reduced bone resorption. 80 Several studies have shown that excessive iron-induced bone loss is due to an increased number and heightened activity of mature osteoclasts 83,84 Together, these findings suggest that excessive iron might promote osteoclast development and activity, and both can contribute to bone resorption. Osteoblasts differentiate from bone marrow-derived mesenchymal stem cells (BM-MSCs).…”