Osteoarthritis has been believed to be caused by improper mechanical function of articular joints. Xu Cao and his colleagues now show that this mechanical process leads to upregulation of transforming growth factor β1 activity in mesenchymal stem cells of subchondral bone, resulting in aberrant bone formation, further destabilization of the joint and ultimately the onset of the disease. Osteoarthritis is a highly prevalent and debilitating joint disorder. There is no effective medical therapy for the condition because of limited understanding of its pathogenesis. We show that transforming growth factor β1 (TGF-β1) is activated in subchondral bone in response to altered mechanical loading in an anterior cruciate ligament transection (ACLT) mouse model of osteoarthritis. TGF-β1 concentrations are also high in subchondral bone from humans with osteoarthritis. High concentrations of TGF-β1 induced formation of nestin-positive mesenchymal stem cell (MSC) clusters, leading to formation of marrow osteoid islets accompanied by high levels of angiogenesis. We found that transgenic expression of active TGF-β1 in osteoblastic cells induced osteoarthritis, whereas inhibition of TGF-β activity in subchondral bone attenuated the degeneration of articular cartilage. In particular, knockout of the TGF-β type II receptor (TβRII) in nestin-positive MSCs led to less development of osteoarthritis relative to wild-type mice after ACLT. Thus, high concentrations of active TGF-β1 in subchondral bone seem to initiate the pathological changes of osteoarthritis, and inhibition of this process could be a potential therapeutic approach to treating this disease.

Bone remodeling involves a coupled balance between bone resorption and bone formation. Xu Cao and his colleagues have shown before that mesenchymal stem cells (MSCs) are recruited to the surface of the bone during this process. They now show that insulin-like growth factor 1 (IGF-1) is released from the bone surface during bone resorption, where it signals the recruited MSCs to differentiate into osteoblasts. In this way, bone resorption is linked to bone formation, and IGF-1 as a target of bone therapy is suggested. Insulin-like growth factor 1 (IGF-1), the most abundant growth factor in the bone matrix, maintains bone mass in adulthood. We now report that IGF-1 released from the bone matrix during bone remodeling stimulates osteoblastic differentiation of recruited mesenchymal stem cells (MSCs) by activation of mammalian target of rapamycin (mTOR), thus maintaining proper bone microarchitecture and mass. Mice with knockout of the IGF-1 receptor (Igf1r) in their pre-osteoblastic cells showed lower bone mass and mineral deposition rates than wild-type mice. Further, MSCs from Igf1rflox/flox mice with Igf1r deleted by a Cre adenovirus in vitro, although recruited to the bone surface after implantation, were unable to differentiate into osteoblasts. We also found that the concentrations of IGF-1 in the bone matrix and marrow of aged rats were lower than in those of young rats and directly correlated with the age-related decrease in bone mass. Likewise, in age-related osteoporosis in humans, we found that bone marrow IGF-1 concentrations were 40% lower in individuals with osteoporosis than in individuals without osteoporosis. Notably, injection of IGF-1 plus IGF binding protein 3 (IGFBP3), but not injection of IGF-1 alone, increased the concentration of IGF-1 in the bone matrix and stimulated new bone formation in aged rats. Together, these results provide mechanistic insight into how IGF-1 maintains adult bone mass, while also providing a further rationale for its therapeutic targeting to treat age-related osteoporosis.