Bone Anabolic Effects of Tibial Loading in Combination with Dual Sclerostin and Dkk1 Antibodies in Aged Mice

Abstract Aging is associated with decreased bone formation and bone mass and increased fracture risk. Wnt pathway activation by mechanical loading is a potent strategy to improve bone mass, however, load-induced bone formation is diminished with aging. Neutralizing antibody (Ab) therapies targeting Wnt pathway inhibitors Sclerostin (Scl) and Dickkopf-related protein 1 (Dkk1) have proven successful in preclinical and clinical osteoporotic conditions. We asked whether treatment combining Scl-Ab and Dkk1-Ab can increase load-induced bone formation in a preclinical model of osteoporosis. Aged (22-month) C57BL/6N female mice underwent combination Scl-Ab plus Dkk1-Ab therapy (15 mg/kg each, subcutaneously; 2x/wk; saline control) for 2 weeks, concomitant with tibial loading (-2200 με, 1200 cycles/day, 5 day/wk). Based on serial microCT, dual antibody treatment had a significant, positive effect on bone morphology, independent of loading. For example, from day 0 to day 15 cancellous BV/TV increased by 50% and cortical area by 25% in non-loaded limbs of antibody-treated mice, while vehicle-treated mice had no change. However, antibody treatment did not enhance the effects of tibial loading on bone morphology, which was not significant. On the other hand, antibody treatment and loading each increased periosteal bone formation indices and together had a synergistic effect. For example, periosteal BFR/BS was 10-fold higher in loaded limbs of antibody- versus vehicle-treated mice. Gene expression analysis showed that antibody treatment and loading synergistically upregulated Wnt1 expression in bone, which may have contributed to the synergistic effect on bone formation. These results confirm the potent anabolic effect of combination Scl plus Dkk1 antibody treatment. Moreover, they show that antibody treatment and skeletal loading may be more effective at increasing periosteal bone formation in aged mice than either treatment alone. Longer term studies are needed to determine if combination treatment leads to a similar synergistic effect on bone morphology and bone strength.