Targeting the Alk4 pathway protects against age-related bone loss

Osteoporosis is a chronic age-related condition in which imbalanced activities between bone-forming osteoblasts and bone-resorbing osteoclasts lead to the progressive loss of bone volume and quality. While drugs that target osteoclastic activity have been developed, there remains a lack of efficient therapies that increase osteoblast number and function in aging bones. Here, we investigated if Activin, known to increase in the circulation with age, plays a primary role in bone loss associated with aging. We showed that, in mouse femurs, levels of Activin signaling progressively increased with age and strongly correlated with the loss of trabecular bone. Furthermore, mice lacking the type I receptor for Activin, namely Alk4, in osteoblast progenitors (Alk4 cKO mice) had increased trabecular bone acquisition, osteoblast number, and bone formation rate. In addition, Alk4 cKO male mice were protected against early age-related trabecular bone loss observed at 1 year of age. These results indicate that Activin signaling inhibits bone formation and osteoblast activity and is likely associated with osteoporosis. To further test this, we injected 2-year-old male mice with a ligand trap (Alk4-Fc) to capture circulating Activin. Alk4-Fc protected against loss of trabecular bone in femurs and L5 vertebrae. Interestingly, Alk4-Fc also prevented a decrease in muscle mass in gastrocnemius, quadriceps, and triceps suggesting that circulating Activins also play a role in sarcopenia. In summary, our preclinical mouse models reveal that circulating Activins play a primary role in age-related bone loss and can be efficiently targeted to alleviate osteoporosis and sarcopenia in aging mice.