No abstract

Acquired heterotopic ossification (HO) is a painful and debilitating disease characterized by extraskeletal bone formation after injury. The exact pathogenesis of HO remains unknown. Here we show that TGF-β initiates and promotes HO in mice. We find that calcified cartilage and newly formed bone resorb osteoclasts after onset of HO, which leads to high levels of active TGF-β that recruit mesenchymal stromal/progenitor cells (MSPCs) in the HO microenvironment. Transgenic expression of active TGF-β in tendon induces spontaneous HO, whereas systemic injection of a TGF-β neutralizing antibody attenuates ectopic bone formation in traumatic and BMP-induced mouse HO models, and in a fibrodysplasia ossificans progressive mouse model. Moreover, inducible knockout of the TGF-β type II receptor in MSPCs inhibits HO progression in HO mouse models. Our study points toward elevated levels of active TGF-β as inducers and promoters of ectopic bone formation, and suggest that TGF-β might be a therapeutic target in HO.

and systemically in spondyloarthropathy patients with enthesopathy (37,38). In this study, we determined the role of TGF-β in Achilles tendon enthesopathy using 3 different mouse models. We found that the changes of calcaneal bone microstructure and composition were associated with the onset of Achilles tendon enthesopathy. Inhibition of TGF-β activity, using either a TGF-β-neutralizing antibody (1D11) or a conditional knockout mouse model, attenuated calcaneal and fibrocartilage pathological changes. ResultsUpregulated TGF-β activities in posterior calcaneal tuberosity in Achilles tendon enthesopathy mouse models. To examine the changes at the disease has drawn increasing attention in recent years. During tissue injury or remodeling, TGF-β is activated to recruit stem/ progenitor cells to maintain tissue homeostasis (27)(28)(29)(30)(31). We have previously shown that TGF-β is released from the bone matrix and activated during osteoclastic bone resorption (31). Active TGF-β induces mesenchymal stromal stem cell (MSC) migration to the bone-resorptive pits for new bone formation (31). However, excessive active TGF-β leads to uncoupled bone remodeling, contributing to the pathogenesis of rare genetic skeletal diseases such as Camurati-Engelmann disease (CED), osteogenesis imperfecta, and more common musculoskeletal disorders, such as osteoarthritis (31-36). High levels of TGF-β have been observed locally lower in relation to sham-operated controls ( Figure 1B). Moreover, a dramatically higher trabecular pattern factor (Tb.Pf), a quantitative parameter of the connectiveness and microarchitecture of trabecular bone (40), was noted in the SMTS mice compared with sham-operated controls, indicating uncoupled bone remodeling ( Figure 1B). Evident bony projections at the PCT were present in 4 of 10 mice 8 weeks after SMTS ( Figure 1A). The BV/TV, Tb.N, and Tb.Th of vertebral bodies were not significantly changed in the SMTS group compared with controls at week 8 (Supplemental Figure 2). Tartrate-resistant acid phosphatase (TRAP) staining showed that the osteoclast surface per bone surface (OCS/BS) increased in PCT trabecular bone 1 and 2 weeks after surgery and was back to normal levels on week 4 after surgery (Figure 1, C and D) while leaving large bone marrow cavities ( Figure 1C). A significantly higher active TGF-β concentration in serum was seen in the SMTS mice relative to sham-operated mice from 1 to 8 weeks after onset of enthesopathy, we generated these 2 models based on prior theories that enthesopathy results from unbalanced mechanical loading (2, 39). The first model we generated decreased mechanical load by partially transecting Achilles tendons in the middle of tendon body in 3-month-old male mice, termed semi-Achilles tendon transection (SMTS) (Supplemental Figure 1; supplemental material available online with this article; https://doi.org/10.1172/ JCI96186DS1). The bone volume over total tissue volume (BV/ TV) in the posterior calcaneal tuberosity (PCT), the site of Achilles tendon attachment to calcane...