The Anaphylatoxin Receptor C5aR Is Present During Fracture Healing in Rats and Mediates Osteoblast Migration In Vitro

Abstract

Background There is evidence that complement components regulate cytokine production in osteoblastic cells, induce cell migration in mesenchymal stem cells, and play a regulatory role in normal enchondral bone formation. We proved the hypothesis that complement might be involved in bone healing after fracture. Methods We investigated the expression of the key anaphylatoxin receptor C5aR during fracture healing in rats by immunostaining after 1, 3, 7, 14, and 28 days. C5aR expression was additionally analyzed in human mesenchymal stem cells (hMSC) during osteogenic differentiation, in human primary osteoblasts, and osteoclasts by reverse transcriptase polymerase chain reaction and immunostaining. Receptor functionality was proven by the migratory response of cells to C5a in a Boyden chamber. Results C5aR was expressed in a distinct spatial and temporal pattern in the fracture callus by differentiated osteoblast, chondroblast-like cells in cartilaginous regions, and osteoclasts. In vitro C5aR was expressed by osteoblasts, osteoclasts, and hMSC undergoing osteogenic differentiation. C5aR was barely expressed by undifferentiated hMSC but was significantly induced after osteogenic differentiation. C5aR activation by C5a induced strong chemotactic activity in osteoblasts, and in hMSC, which had undergone osteogenic differentiation, being abolished by a specific C5aR antagonist. In hMSC, C5a induced less migration reflecting their low level of C5aR expression. Conclusions Out in vitro and in vivo results demonstrated the presence of C5aR in bone forming osteoblasts and bone rcsorbing osteoclasts. It is suggested that C5aR might play a regulatory role in fracture healing in intramembranous and in enchondral ossification, one possible function being the regulation of cell recruitment.

Neutrophils orchestrate their own recruitment in murine arthritis through C5aR and FcγR signaling

“…In patients with RA, C5aR expression is enhanced on resident synovial cells (44), and C5aR + subtypes of mast cells are enriched in the synovium of these patients (45). Additionally, C5aR expression has also been described in osteoblasts, osteoclasts, and chondroblasts and may play a significant role in bone remodeling and inflammation (30,46). Our data, however, demonstrate that C5aR expression on radiosensitive cells alone is necessary and sufficient in autoantibody-induced arthritis, whereas C5aR on radioresistant cells is not required.…”

Experimental blunt chest trauma impairs fracture healing in rats

“…Recently, we could demonstrate that a key receptor, C5aR, is expressed by inflammatory cells, osteoblasts and chondrocytes in the fracture callus, indicating that these cells are target cells for activated complement during the healing process. 36 Even though systemic inflammation was transiently present during the very early phase of fracture healing it was able to disturb the complex fracture repair sequence resulting in impaired healing after 35 days. Which molecular and cellular mechanisms were responsible for the observed impairment of bone regeneration remain to be clarified and need to be further investigated by the evaluation of earlier healing time points.…”

International Union of Basic and Clinical Pharmacology. LXXXVII. Complement Peptide C5a, C4a, and C3a Receptors

“…Complement C3a and C5a can modulate bone biology in inflammation (Ignatius et al, 2011b). C5a 1 receptor has been shown to control osteoblast migration during fracture healing (Ignatius et al, 2011a), and efficient osteoclast differentiation requires local complement activation (Ignatius et al, 2011b). Normal heart function appears to depend on the C5a/C5a 1 receptor axis, with C5 deficiency and receptor knockout or blockade causing a "state of distress" (Mullick et al, 2011).…”