Osteocyte State Transitions Modulate Bone Remodeling During Early Skeletal Aging
Osteocytes are long-lived cells that play a central role in bone homeostasis, yet age-related changes in their functional states remain poorly understood, particularly because skeletal aging involves multiple processes beyond cellular senescence. We generated an osteocyte-specific MepeCre mouse line and combined osteocyte ablation in young and middle-aged mice with skeletal phenotyping, single-cell transcriptomics, and senolytic treatment. MepeCre-driven recombination was largely confined to osteocytes, with minimal off-target activity. Osteocyte ablation increased bone mass at both ages, indicating that osteocytes constrain bone accrual as part of their role in skeletal homeostasis. However, the accompanying remodeling changes differed with age: enhanced osteoblast activity predominated in young mice, whereas reduced osteoclast-mediated bone resorption predominated in middle-aged mice. Single-cell transcriptomics revealed distinct osteocyte subpopulations whose relative abundance shifted with age, from a predominantly matrix-enriched state in young mice to an expanded aging-transitional state in middle-aged mice. Although this state showed partial enrichment of senescence-associated transcriptional signatures, senolytic treatment failed to recapitulate the increase in bone mass induced by osteocyte ablation. Osteocyte therefore regulate bone mass through age-dependent mechanisms that coincide with shifts in osteocyte-state composition. These changes emerge by middle age and may contribute to early remodeling imbalance before overt cellular senescence during skeletal aging. Graphical AbstractGraphical summary of the findings of this study. AA, amino acids; NA, nucleic acid; UA, uric acid; TCA, tricarboxylic acid.