Inflammasome expression patterns during bone regeneration: predictive biomarkers and potential therapeutic targets in non-union formation
Abstract
Background Non-union formation is one of the major complications in trauma and orthopedic surgery. The inflammatory response at the fracture site is crucial for determining the healing outcome. However, there is limited information on the expression profiles of inflammasomes during both successful fracture repair and non-union formation. Methods Hence, we analyzed the cellular inflammatory response as well as inflammasome expression during successful bone regeneration and fracture healing failure in a standardized murine fracture and non-union model over 70 days. Bone healing was analyzed by means of X-ray, biomechanics, micro-computed tomography (µCT), histology as well as immunohistochemistry and Western blotting. Results The analysis of the callus tissue in non-unions showed a slightly increased number of MPO-positive cells and a significantly increased number of macrophages during the early healing phase. The number of macrophages remained significantly elevated until 70 days after surgery, indicating an overshooting and prolonged inflammatory response when compared to unions. This was associated with an enhanced number of p16-positive cells. Additional Western blot analyses revealed distinct differences in the inflammasome expression profiles of unions and non-unions. In particular, non-unions presented with a higher expression of the nucleotide-binding oligomerization domain (NOD)-like receptor protein (NLRP)3 at 10 days. Furthermore, at 14 days non-unions showed a higher expression of NLRP1 whereas the expression of absent in melanoma (AIM)2 was significantly reduced. Conclusion In a clinical setting, these distinct expression profiles may serve as predictive markers for fracture healing outcome. Moreover, in the future novel treatment approaches modulating inflammasome expression may be used to stimulate bone regeneration and eventually prevent non-union formation.