The role of substance P and noradrenaline in callus differentiation

During the progress of fracture healing, bone and fracture callus become innervated by substance P (SP) and noradrenaline (NA) containing sensory and sympathetic nerve fibers. Aim of this research is to analyze the impact of SP and NA on callus differentiation and biomechanical bone parameters in a murine fracture model and an ex vivo fracture explant model. We studied unstabilized tibia fractures and stabilized femora fractures in wild type (WT), tachykinin 1-deficient (Tac1−/−) and sympathectomized mice. We further applied callus measurement, biomechanical testing (torque/angle of failure/stiffness), callus explants cultures, and quantitative RT-PCR. Fracture calli of Tac1−/− mice exhibited a lower volume and a higher physical density compared to WT. Fractured femora of WT mice resisted stronger bending forces compared to Tac1−/− and resisted to greater torque than femora of Tac1−/− and sympathectomized mice. Control legs of WT mice resisted to greater torque and were more stiff compared to Tac1−/− and sympathectomized mice. Biomechanical parameters of newly formed bone after fracture healing were not different to existing bone of control legs. Gene expression of tissue inhibitors of matrix metalloproteinases (timp)-1, −2, −3 and neurokinin-1 receptor were upregulated after stimulation with SP (10 −8 M) and IL-1 β (0.5 ng/ml) and downregulated without IL-1 β (SP 10 −8 / −10 M). In callus chondrocytes of Tac1−/− mice, gene expression of timp - 2 , - 3 , matrix metalloproteinases 3 and 14 ( mmp - 3 , - 14 ) and cox2 is higher than in WT mice. SP regulates the expression of genes that play a role in matrix composition in the inflammatory phase and alters callus size and density. NA and SP affect the mechanical stability of bone in general but not of newly formed bone after fracture healing.