M2 Macrophage-Derived Osteopontin Promotes Osteogenesis of Fibro-Adipogenic Progenitors After Spinal Cord Injury

Neurogenic heterotopic ossification (NHO), which occurs within the periarticular musculature subsequent to a significant spinal cord injury (SCI) and traumatic brain injury (TBI), remains poorly understood with no effective preventative measures. Multiple studies have demonstrated that dysregulated inflammatory environments caused by macrophage activation and aberrant differentiation of FAPs are critical factors contributing to disease progression. However, the specific mechanisms mediating functional changes between these two cell types during NHO remain unclear. This study found that FAPs aggregate, proliferate, and undergo osteogenic differentiation in the pathological bone regions of NHO. Differential gene expression analysis in an NHO mouse model has shown that the OPN expression is elevated in muscles which develop NHO. Using techniques such as flow cytometry sorting with immunofluorescence staining, it has been shown that the primary cellular source of osteopontin (OPN) is the M2 macrophage (M2). The up-regulation of OPN expression is primarily controlled by the transcription factor NFKB2 within the macrophage. We also demonstrate that OPN induces the increased expression of the osteogenesis-related transcription factor RUNX2 in FAPs via the P38-MAPK signaling pathway. Together, these data show that M2-derived OPN promotes pathological bone formation after SCI, thus providing useful information relevant to a therapeutic strategy for treatment of NHO.