gms | German Medical Science

Objectives: The immune system plays a crucial role in fracture (Fx) healing and traumatic brain injury (TBI). In order to understand the pathomechanism underlying the phenomenon of improved fracture healing following TBI, we analysed injury-dependent immune modulations. The identification of the key mediators involved holds great therapeutic potential, as treatment options for patients with impaired bone regeneration or neurological deficits following TBI are still limited. Calcitonin gene-related peptide alpha (aCGRP), a neuropeptide released from sensory nerve fibres, might be such a mediator linking fracture healing with TBI, as it innervates bone tissue and shows powerful immunomodulatory effects in the central nervous system.

Methods: In order to study the immune regulations during the positive effect of TBI on callus formation in vivo, we used a standardised murine trauma model. Wildtype C57BL/6J (WT, n=72) and aCGRP deficient (n=39) mice were randomly grouped in control, TBI (controlled impact injury to the parietal cortex), fracture (femoral osteotomy) and combined trauma. After surgical intervention, major organ systems were retained for mRNA and protein analyses at all stages of fracture healing. For the following expression analysis, we applied established immune markers. Based on the in vivo results, we conducted in vitro experiments using murine microglial cells (BV2) and conditioned medium (CM) from bone marrow-derived osteoblasts. LPS-activated BV2 cells were stimulated with recombinant aCGRP as well as CM from differentiating osteoblasts and subsequently characterised through gene expression analyses.

Results and conclusion: The in vivo experiments performed in WT mice only revealed minor immune regulations in the callus, the intact bone, as well as other immunologically and metabolically active organs such as kidney, liver, lung, pancreas, spleen, white and brown fat. However, a significant inflammatory response was observed in the injured hypothalamus, where the TBI-induced neuroinflammation was modulated by a concomitant fracture depending on the stage of bone healing. This was accompanied by an increased expression of aCGRP in the callus following TBI in vivo. In accordance with our in vivo results, differentiating osteobasts were shown to express aCGRP in vitro, which exerted an immunomodulatory effect in activated BV2 cells. Furthermore, CM from osteoblasts dynamically altered the immune response in activated microglial cells, depending on the stage of osteoblast differentiation. In conclusion, our data indicate that the process of bone regeneration regulates the course of TBI-induced neuroinflammation, which might be mediated by aCGRP. However, further research is required in order to understand this crosstalk in detail and its relation to the phenomenon of improved fracture healing following TBI.