Accelerated calvarial healing in mice lacking Toll-like receptor 4.
The bone and immune systems are closely interconnected. The immediate inflammatory response after fracture is known to trigger a healing cascade which plays an important role in bone repair. Toll-like receptor 4 (TLR4) is a member of a highly conserved receptor family and is a critical activator of the innate immune response after tissue injury. TLR4 signaling has been shown to regulate the systemic inflammatory response induced by exposed bone components during long-bone fracture. Here we tested the hypothesis that TLR4 activation affects the healing of calvarial defects. A 1.8 mm diameter calvarial defect was created in wild-type (WT) and TLR4 knockout (TLR4(-/-)) mice. Bone healing was tested using radiographic, histologic and gene expression analyses. Radiographic and histomorphometric analyses revealed that calvarial healing was accelerated in TLR4(-/-) mice. More bone was observed in TLR4(-/-) mice compared to WT mice at postoperative days 7 and 14, although comparable healing was achieved in both groups by day 21. Bone remodeling was detected in both groups on postoperative day 28. In TLR4(-/-) mice compared to WT mice, gene expression analysis revealed that higher expression levels of IL-1β, IL-6, TNF-α,TGF-β1, TGF-β3, PDGF and RANKL and lower expression level of RANK were detected at earlier time points (≤ postoperative 4 days); while higher expression levels of IL-1β and lower expression levels of VEGF, RANK, RANKL and OPG were detected at late time points (> postoperative 4 days). This study provides evidence of accelerated bone healing in TLR4(-/-) mice with earlier and higher expression of inflammatory cytokines and with increased osteoclastic activity. Further work is required to determine if this is due to inflammation driven by TLR4 activation.
Department of Plastic Surgery, University of Pittsburgh, Pennsylvania, United States of America.
Dan
D
Wang
James R
JR
Gilbert
James J
JJ
Cray
Adam A
AA
Kubala
Melissa A
MA
Shaw
Timothy R
TR
Billiar
Gregory M
GM
Cooper
D000818:Animals; D016723:Bone Remodeling / Q000235:genetics / Q000276:immunology; D001842:Bone and Bones / Q000000981:diagnostic imaging / Q000276:immunology / Q000378:metabolism; D020869:Gene Expression Profiling; D007150:Immunohistochemistry; D053583:Interleukin-1beta / Q000235:genetics / Q000378:metabolism; D015850:Interleukin-6 / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D053244:Osteoprotegerin / Q000235:genetics / Q000378:metabolism; D010982:Platelet-Derived Growth Factor / Q000235:genetics / Q000378:metabolism; D053245:RANK Ligand / Q000235:genetics / Q000378:metabolism; D011859:Radiography; D053246:Receptor Activator of Nuclear Factor-kappa B / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012886:Skull / Q000276:immunology / Q000293:injuries / Q000378:metabolism*; D013997:Time Factors; D051197:Toll-Like Receptor 4 / Q000235:genetics* / Q000276:immunology / Q000378:metabolism; D053773:Transforming Growth Factor beta1 / Q000235:genetics / Q000378:metabolism; D053782:Transforming Growth Factor beta3 / Q000235:genetics / Q000378:metabolism; D014409:Tumor Necrosis Factor-alpha / Q000235:genetics / Q000378:metabolism; D014945:Wound Healing / Q000235:genetics* / Q000276:immunology
D053583:Interleukin-1beta; D015850:Interleukin-6; D053244:Osteoprotegerin; D010982:Platelet-Derived Growth Factor; D053245:RANK Ligand; D053246:Receptor Activator of Nuclear Factor-kappa B; C506031:Tnfrsf11b protein, mouse; C505689:Tnfsf11 protein, mouse; D051197:Toll-Like Receptor 4; D053773:Transforming Growth Factor beta1; D053782:Transforming Growth Factor beta3; D014409:Tumor Necrosis Factor-alpha