Objective: Skeletal stromal progenitor cells in the metaphysis (met-SSPCs) contribute to trabecular bone (Tb) formation by supplying osteoblast lineages during postnatal (PN) growth.1,2 Although metaphyseal precursors derived from the hypertrophic chondrocyte or Sox-9 lineage are essential for Tb formation2, the underlying mechanism regulating Tb homeostasis remains unclear. Using inducible Cre-LoxP system, we aimed to identify the metaphyseal SSC population and their functional characteristics during bone growth and metabolism.
Methods and Results: Lineage tracing and Single-cell RNA-seq analysis of mice at PN day 28 showed that met-SSPCs originate from Asporin+ periosteal cells. Genetic ablation of the met-SSPCs lineage impairs Tb formation, reduces cross-sectional area, and shortens bone length. Largely, the met-SSPC subpopulation expressing Leptin receptor (LepR+) and matrix metalloproteinase (MMP9+) contributed to bone growth via differentiation into osteoblasts and LepR+/CXCL12-abundant reticular (CAR) cells. Their lineage expansion correlates with increased bone turnover and stemness gene expression. Notably, osteoclast differentiation factors RANKL and M-CSF, were highly expressed in LepR+MMP9+ and LepR+/CAR cells than in osteoblasts and osteocytes. Micro-CT examination revealed that conditional deletion of Rankl in LepR+ cells, but not in osteoblast-lineage cells, increased the Tb mass. Tartrate-resistant acid phosphatase staining of LepR-CreER mice femur was in congruence with the micro-CT data, thus indicating that LepR+ derived RANKL is essential for osteoclast activation and function.
Conclusion: These findings suggest that a novel functional subset of LepR⁺ progenitors is essential for Tb growth and homeostasis. We put forth that the LepR+MMP9+ progenitors are vital for both osteogenesis and osteoclast activation.