Heterotopic ossification following musculoskeletal trauma: modeling stem and progenitor cells in their microenvironment.

Heterotopic ossification (HO), characterized by the formation of mature bone in the soft tissues, is a complication that can accompany musculoskeletal injury, and it is a frequent occurrence within the military population that has experienced orthopaedic combat trauma. The etiology of this disease is largely unknown. Our laboratory has developed strategies to investigate the cellular and molecular events leading to HO using clinical specimens that were obtained during irrigation and debridement of musculoskeletal injuries. Our approach enables to study (1) the cell types that are responsible for pathological transformation and ossification, (2) the cell- and tissue-level signaling that induces the pathologic transformation, and (3) the effect of extracellular matrix topography and force transduction on HO progression. In this review, we will report on our findings in each of these aspects of HO etiology and describe our efforts to recapitulate our findings in an animal model for traumatic HO.

Fibroregulation of mesenchymal progenitor cells by BMP-4 after traumatic muscle injury.

OBJECTIVES: Traumatized muscle is a complex healing environment containing cells with robust reparative and regenerative potential interacting in a cytokine milieu that influences the function and differentiation of these cells, leading to a spectrum of healing responses. In particular, bone morphogenetic protein-4 (BMP-4) is of interest as a potential modulator of healing because its dysregulation has been associated with fibrosis and heterotopic ossification formation. We propose a descriptive study of altered BMP-4 expression in traumatized muscle tissue and to evaluate its role in the fibroregulatory function of resident mesenchymal progenitor cells (MPCs) at the protein- and gene-expression levels. METHODS: Protein-level expression of BMP-4 from cells resident in traumatized muscle specimens was evaluated using ELISA and also using sodium dodecyl sulfate-polyacrylamide gel electrophoresis to compare BMP-4 in homogenized muscle tissue specimens. BMP-4, cartilage oligomeric matrix protein (COMP), and osteocalcin expression localization was analyzed via immunohistochemistry. Reverse transcription-polymerase chain reaction was performed to evaluate fibroregulatory gene expression in MPCs after treatment with BMP-4. RESULTS: BMP-4 was present in all traumatized muscle tissue specimens. Immunohistochemistry demonstrated that traumatized muscle fibers contained greater number of cells expressing BMP-4 in a more disorganized fashion compared with control samples. Reverse transcription-polymerase chain reaction demonstrated that COMP, growth and differentiation factor-10, and integrin beta-2 were up-regulated, whereas tumor necrosis factor-alpha was significantly down-regulated. COMP expression was colocalized in the traumatized muscle tissue with osteocalcin. CONCLUSIONS: BMP-4 has an effect on MPCs that seems to promote fibrotic tissue formation. These findings suggest that BMP-4, while promoting osteoinduction, may also act on MPCs to promote formation of a fibrotic osteoinductive matrix. Thus, this signaling axis might be a potential target for heterotopic ossification prevention.