Rescuing Loading Induced Bone Formation at Senescence

The increasing incidence of osteoporosis worldwide requires anabolic treatments that are safe, effective, and, critically, inexpensive given the prevailing overburdened health care systems. While vigorous skeletal loading is anabolic and holds promise, deficits in mechanotransduction accrued with age markedly diminish the efficacy of readily complied, exercise-based strategies to combat osteoporosis in the elderly. Our approach to explore and counteract these age-related deficits was guided by cellular signaling patterns across hierarchical scales and by the insight that cell responses initiated during transient, rare events hold potential to exert high-fidelity control over temporally and spatially distant tissue adaptation. Here, we present an agent-based model of real-time Ca2+/NFAT signaling amongst bone cells that fully described periosteal bone formation induced by a wide variety of loading stimuli in young and aged animals. The model predicted age-related pathway alterations underlying the diminished bone formation at senescence, and hence identified critical deficits that were promising targets for therapy. Based upon model predictions, we implemented an in vivo intervention and show for the first time that supplementing mechanical stimuli with low-dose Cyclosporin A can completely rescue loading induced bone formation in the senescent skeleton. These pre-clinical data provide the rationale to consider this approved pharmaceutical alongside mild physical exercise as an inexpensive, yet potent therapy to augment bone mass in the elderly. Our analyses suggested that real-time cellular signaling strongly influences downstream bone adaptation to mechanical stimuli, and quantification of these otherwise inaccessible, transient events in silico yielded a novel intervention with clinical potential.Author Summary: Post-menopausal and age-related osteoporosis afflicts large segments of the population and can markedly increase skeletal fragility. Bone fractures that occur as a consequence substantially increase health care expenditures and raise levels of morbidity. While strategies that prevent further loss of bone exist, options that compensate for bone loss accrued over age are less numerous. Physical exercise holds promise in this realm. However, in part due to deficits in how cells within bone respond to skeletal loading, readily complied exercise has proved ineffective in enhancing bone mass in the elderly. In this study, we examined whether the ability of physical exercise to increase bone mass can be restored at advanced age. To this end, we developed a computational model describing how a specific aspect (or pathway) activated in bone cells by skeletal loading may be altered with age. Our model proved successful in describing age-related pathway alterations and identified specific deficits that were amenable to therapeutic manipulation. We subsequently discovered that when an extremely inexpensive, currently approved pharmaceutical is used as a supplement, bone response to skeletal loading was completely rescued in aged animals. We believe that this result provides the rationale to consider this approach as a means to increasing bone mass in the elderly.