Heme metabolism mediates RANKL-induced osteoclastogenesis via mitochondrial oxidative phosphorylation

Bone undergoes life-long remodelling, in which disorders of bone remodelling could occur in many pathological conditions including osteoporosis. Understanding the cellular metabolism of osteoclasts is key to developing new treatments for osteoporosis, a disease that affects over 200 million women worldwide per annum. We found that human osteoclast differentiation from peripheral blood mononuclear cells (PBMCs) derived from 8 female patients is featured with a distinct gene expression profile of mitochondrial biogenesis. Elevated mitochondrial membrane potential (MMP, Δψm) was also observed in RANKL-induced osteoclasts. Interestingly, the gene pathways of heme synthesis and metabolism were activated upon RANKL stimulation, featured by a transcriptomic profiling in murine cells at a single-cell resolution, which revealed a stepwise expression pattern of heme-related genes. The real-world human data also divulges potential links between heme-related genes and bone mineral density. Heme is known to have a role in the formation of functional mitochondrial complexes that regulate MMP. Disruption of heme biosynthesis via genetically silencing Ferrochelatase or a selective inhibitor, N-methyl Protoporphyrin IX (NMPP), demonstrated potent inhibition of osteoclast differentiation, with a dose-dependent effect observed in NMPP treatment and a substantial efficacy even at a single dose. In vivo study further showed the protective effect of NMPP on ovariectomy-induced bone loss in female mice. Collectively, we found that RANKL-mediated signaling regulated mitochondrial formation and heme metabolism to synergistically support osteoclastogenesis. Inhibition of heme synthesis impaired osteoclast formation and reversed excessive bone loss, representing a new therapeutic target for metabolic skeletal disorders.