Heterotopic ossification (HO) consists of ectopic bone formation within soft tissues following surgery or trauma and can have debilitating consequences, but no definitive cure is available. Here we show that HO was essentially prevented in mice receiving nuclear retinoic acid receptor γ (RARγ) agonists. Side effects were minimal, and there was no significant rebound effect. To uncover mechanisms, mesenchymal stem cells were treated with RARγ agonist and transplanted into nude mice. Whereas control cells formed ectopic bone masses, the RARγ agonist-pretreated cells did not, suggesting that they had lost their skeletogenic potentials. Indeed, the cells became unresponsive to rBMP-2 and exhibited reduction of Smad1/5/8 phosphorylation and overall Smad levels. As importantly, the RARγ agonists blocked HO in transgenic mice expressing constitutive-active ALK2Q207D mutant that is related to ALK2R206H found in Fibrodysplasia Ossificans Progressiva patients. The data indicate that the RARγ agonists are potent inhibitors of HO and could also be as effective against congenital HO.

IntroductionBone marrow transplantation (BMT) is an established therapeutic modality for both malignant and nonmalignant disorders of hematopoietic stem cells. After wide recognition that bone marrow also contains progenitors of bone, [1][2][3][4] we postulated that BMT should be applicable to the treatment of osteopoietic as well as hematopoietic disorders. 5 Nilsson et al demonstrated that transplantation of whole bone marrow leads to donor-derived osteopoiesis in mice, 6 while Pereira et al showed that systemically infused murine mesenchymal stromal cells (MSCs), which are plastic adherent in vitro, 7 engrafted in bone. 3 We showed that BMT in children with osteogenesis imperfecta (OI), a genetic disorder of collagen type I, the major structural protein in bone, leads to donor-derived osteopoiesis and consequent improvement in the microscopic structure of bone 5 and in the clinical manifestations of OI. 8 Recently, BMT in a murine model of OI has corroborated our early human studies. 9 Taken together, these data validate the functional competence of donor-derived osteopoietic cells, providing the necessary proof to move forward with the development of marrow cell-based treatments for disorders of bone.Despite this progress, the cellular mechanism(s) by which BMT gives rise to robust osteopoietic activity remains unproven. Pereira et al reported that systemically infused murine MSCs engrafted in the bone of a murine model of OI, and generated a small but statistically significant increase in collagen, 10 supporting the prevailing view that BMT-associated donor-derived osteopoiesis was attributable to the engraftment and differentiation of MSCs. Thus, we reasoned that a decrease in the rate of clinical improvement in our OI patients after BMT 8 might be corrected with a boost of donor-derived MSCs, which in fact led to a second wave of accelerated growth velocity in all 5 evaluable patients. 11 This result suggested that MSCs isolated on the basis of their adherence to plastic may provide adequate therapy for patients with OI or other bone disorders. However, the issue is complicated by work showing that so-called nonadherent bone marrow cells (NABMCs) have measurable osteoprogenitor activity, 12-14 raising questions as to the developmental origin of the transplantable marrow osteoprogenitors that give rise to donor-derived osteopoiesis and hence to the marrow population most likely to yield clinical improvement in patients.Here we show that NABMCs are significantly more robust transplantable osteoprogenitors than MSCs in mice, suggesting NABMC would be effective cell therapy for bone disorders. Translating this laboratory observation to a pilot clinical trial, T cell-depleted marrow mononuclear cells, comprising Ͻ 0.01% MSCs, engraft in bone after intravenous infusion and lead to a remarkable acceleration of growth in some OI patients, suggesting vigorous osteoprogenitor activity in humans as predicted by our animal model. Finally, we demonstrate that NABMCs produce their clinical activity by engrafting in bon...

Wnt/␤-catenin signaling is required for skeletal development and organization and for function of the growth plate and articular cartilage. To further clarify these roles and their possible pathophysiological importance, we created a new transgenic mouse model in which Wnt/␤-catenin signaling can be activated in cartilage for specific periods of time. These transgenic mice expressed a constitutive active form of ␤-catenin fused to a modified estrogen receptor ligand-binding domain under the control of cartilage-specific collagen 11␣2 promoter/enhancer. Transient Wnt/␤-catenin signaling activation in young adult mice by tamoxifen injections induced growth retardation and severe deformities in knee joints. Tibial and femoral growth plates displayed an excessive number of apoptotic cells and eventually underwent abnormal regression. Articular cartilage exhibited an initial acute loss of proteoglycan matrix that was followed by increases in thickness, cell density, and cell proliferation. In reciprocal studies, we found that conditional ablation of ␤-catenin in postnatal mice using a Col2-CreER strategy led to hypocellularity in articular cartilage, growth plate disorganization, and a severe reduction in bone volume. Together, these data provide evidence that Wnt/␤-catenin signaling has important and distinct roles in growth plate and articular cartilage and that postnatal dysregulation of this signaling pathway causes diverse structural and functional changes in the two cartilaginous structures. (Am J