Targeting the Alk4 pathway protects against age-related bone loss.

Osteoporosis is a chronic age-related condition where imbalanced activities between bone-forming osteoblasts and bone-resorbing osteoclasts result in the progressive loss of bone volume and reduced bone quality. While drugs that target osteoclast activity have been developed, there remains a lack of efficient therapies that increase osteoblast function in aging bones. Here, we showed that in mouse femurs, levels of Activin signaling progressively increased with age and strongly correlated with the loss of trabecular bone. When we removed Alk4, the type I receptor for Activin in osteoblast progenitors, Alk4 cKO mice had increased numbers of osteoblasts, an increased rate of bone formation, and increased trabecular bone acquisition. These results indicate that Activin through Alk4 signaling inhibits bone formation and osteoblast activity and is likely associated with osteoporosis. Furthermore, when we injected 2-year-old male mice with Alk4-Fc to capture circulating Activin, aged mice were protected against loss of trabecular bone in both femurs and L5 vertebrae. Remarkably, Alk4-Fc also prevented the age-related decrease in muscle mass that normally accompanies age-related bone loss, indicating 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 and muscle loss and can be efficiently targeted to alleviate osteoporosis and sarcopenia in aging mice.