Development process of traumatic heterotopic ossification of the temporomandibular joint in mice

Activation of BMP4/SMAD pathway by HIF-1α in hypoxic environment promotes osteogenic differentiation of BMSCs and leads to ectopic bone formation

OBJECTIVE

Heterotopic ossification (HO), also known as ossifying myositis, is a condition that produces abnormal bone and cartilage tissue in the soft tissues. Hypoxia inducible factor lα (HIF-lα) regulates the expression of various genes, which is closely related to the promotion of bone formation, and Drosophila mothers against decapentaplegic protein (SMAD) mediates the signal transduction in the Bone morphogenetic protein (BMP) signaling pathway, which affects the function of osteoblasts and osteoclasts, and thus plays a key role in the regulation of bone remodeling. We aimed to investigate the mechanism by which HIF-1α induces osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) in a hypoxic environment.

METHODS

A cellular hypoxia model was constructed to verify the expression of HIF-1α, while alizarin red staining was performed to observe the osteogenic differentiation ability of bone marrow mesenchymal stem cells (BMSCs). Alizarin red staining was used to analyze the late mineralization ability of the cells. Western blot analysis was performed to analyze the expression levels of osteogenesis-related factors OCN, OPN proteins as well as the pathway proteins BMP4, p-Smad1/5/8, and Smad1. We also constructed a rat model of ectopic bone formation, observed ectopic ossification by X-ray, and verified the success of the rat model by ELISA of HIF-1α. HE staining was used to observe the matrix and trabecular structure of bone, and Masson staining was used to observe the collagen and trabecular structure of bone. Immunohistochemistry analyzed the expression of OCN and OPN in ectopic bone tissues, and WB analyzed the expression of pathway proteins BMP4, p-Smad1/5/8 and Smad1 in ectopic bone tissues to verify the signaling pathway of ectopic bone formation.

RESULTS

Our results indicate that hypoxic environment upregulates HIF-1a expression and activates BMP4/SMAD signaling pathway. This led to an increase in ALP content and enhanced expression of the osteogenesis-related factors OCN and OPN, resulting in enhanced osteogenic differentiation of BMSCs. The results of our in vivo experiments showed that rats inoculated with BMSCs overexpressing HIF-1α showed bony structures in tendon tissues, enhanced expression of the bone signaling pathways BMP4 and p-Smad1/5/8, and enhanced expression levels of the osteogenic-related factors OCN and OPN, resulting in the formation of ectopic bone.

CONCLUSIONS

These data further suggest a novel mechanistic view that hypoxic bone marrow BMSCs activate the BMP4/SMAD pathway by up-regulating the expression level of HIF-1α, thereby promoting the secretion of osteogenic factors leading to ectopic bone formation.

Stimulating Factors and Origins of Precursor Cells in Traumatic Heterotopic Ossification Around the Temporomandibular Joint in Mice

The contributing factors and the origins of precursor cells in traumatic heterotopic ossification around the temporomandibular joint (THO-TMJ), which causes obvious restriction of mouth opening and maxillofacial malformation, remain unclear. In this study, our findings demonstrated that injured chondrocytes in the condylar cartilage, but not osteoblasts in the injured subchondral bone, played definite roles in the development of THO-TMJ in mice. Injured condylar chondrocytes without articular disc reserves might secrete growth factors, such as IGF1 and TGFβ2, that stimulate precursor cells, such as endothelial cells and muscle-derived cells, to differentiate into chondrocytes or osteoblasts and induce THO-TMJ. Preserved articular discs can alleviate the pressure on the injured cartilage and inhibit the development of THO-TMJ by inhibiting the secretion of these growth factors from injured chondrocytes. However, the exact molecular relationships among trauma, the injured condylar cartilage, growth factors such as TGFβ2, and pressure need to be explored in detail in the future.

The effect of celecoxib in traumatic heterotopic ossification around temporomandibular joint in mice

OBJECTIVE

In this study, the role of inflammation in traumatic heterotopic ossification around temporomandibular joint (THO-TMJ), as well as the preventive and treatment effect of celecoxib in THO-TMJ both in vivo and in vitro were explored.

DESIGN

A surgically-induced THO-TMJ mouse model and a co-culture model of ATDC-5 or MC3T3-E1 and RAW-264.7 cells were used in this study for in vivo and in vitro research.

RESULTS

A series of inflammatory factors, such as CD3, CD68, CD20, IL-10, IL-6 and TNF-α, were activated 48 h after trauma in a THO-TMJ model. Local trauma initiated systemic inflammatory responses as well as T cell- and macrophage-mediated local inflammatory responses around TMJ. In addition, expression of COX-2 was significantly elevated. The findings also showed that local injection of celecoxib could effectively alleviate the inflammatory response around TMJ at the early stage of trauma and inhibit the formation of THO-TMJ in vivo. Meanwhile, celecoxib could inhibit chondrogenic differentiation of ATDC-5 and osteogenic differentiation of MC3T3-E1 under inflammatory condition in vitro. Furthermore, celecoxib could inhibit the expression of Bmpr1b in the injured condylar cartilage at the initiation stage of THO-TMJ, which implied that Bmpr1b expressed by the residual condylar cartilage might be related to the pathogenesis of THO-TMJ.

CONCLUSIONS

Inflammation played a crucial role in the pathogenesis of THO-TMJ, and anti-inflammation might be a possible choice to inhibit THO-TMJ, which provided scientific clues for the mechanisms, pharmacotherapy and molecular intervention of THO-TMJ.