Does activation of the hedgehog signaling pathway increase bone regeneration compared with delivering growth factors alone?
In this study of 33 female Lewis rats, activation of the hedgehog signaling pathway through the delivery of smoothened agonist to a critical-sized rat mandibular defect was associated with increases in bone regeneration compared with delivering only growth factors. The most bone regeneration occurred when the defect was treated with bone morphogenetic protein 6, vascular endothelial growth factor, and smoothened agonist tethered to an osteogenic polysaccharide-based scaffold.
This finding suggests that targeting the hedgehog signaling pathway may offer a new reconstructive option for bony craniofacial defects as well as nonunion and delayed healing fractures.
Osseous craniofacial defects are currently reconstructed with bone grafting, rigid fixation, free tissue transfer, and/or recombinant human bone morphogenetic protein 2. Although these treatment options often have good outcomes, they are associated with substantial morbidity, and many patients are not candidates for free tissue transfer.
To assess whether polysaccharide-based scaffold (PS) constructs that are cross-linked with smoothened agonist (SAG), vascular endothelial growth factor (VEGF), and bone morphogenetic protein 6 (BMP-6) would substantially increase bone regeneration.
Design, Setting, and Participants
This animal model study was conducted at the University of Virginia School of Medicine Cui Laboratory from March 1, 2017, to June 30, 2017. Thirty-three 10-week-old female Lewis rats were acquired for the study. Bilateral nonsegmental critical-sized defects were created in the angle of rat mandibles. The defects were either left untreated or filled with 1 of the 9 PSs. The rats were killed after 8 weeks, and bone regeneration was evaluated using microcomputed tomographic imaging and mechanical testing. Analysis of variance testing was used to compare the treatment groups.
Main Outcomes and Measures
Blinded analysis and computer analysis of the microcomputed tomographic images were used to assess bone regeneration.
In the 33 female Lewis rats, minimal healing was observed in the untreated mandibles. Addition of SAG was associated with increases in bone regeneration and bone density in all treatment groups, and maximum bone healing was seen in the group with BMP-6, VEGF, and SAG cross-linked to PS. For each of the 5 no scaffold group vs BMP-6, VEGF, and SAG cross-linked to PS group comparisons, mean defect bone regeneration was 4.14% (95% CI, 0.94%-7.33%) vs 66.19% (95% CI, 54.47%-77.90%); mean bone volume, 14.52 mm3 (95% CI, 13.07-15.97 mm3) vs 20.87 mm3 (95% CI, 14.73- 27.01 mm3); mean bone surface, 68.97 mm2 (95% CI, 60.08-77.85 mm2) vs 96.77 mm2 (95% CI, 76.11-117.43 mm2); mean ratio of bone volume to total volume, 0.11 (95% CI, 0.10-0.11) vs 0.15 (95% CI, 0.10-0.19); and mean connectivity density 0.03 (95% CI, 0.02-0.05) vs 0.32 (95% CI, 0.25-0.38). On mechanical testing, mandibles with untreated defects broke with less force than control mandibles in which no defect was made, although this force did not reach statistical significance. No significant difference in force to fracture was observed among the treatment groups.
Conclusions and Relevance
In this rat model study, activation of the hedgehog signaling pathway using smoothened agonist was associated with increased craniofacial bone regeneration compared with growth factors alone, including US Food and Drug Administration–approved recombinant human bone morphogenetic protein 2. Pharmaceuticals that target this pathway may offer a new reconstructive option for bony craniofacial defects as well as nonunion and delayed healing fractures.
Level of Evidence
Miller MQ, McColl L, Arul MR, et al. Assessment of Hedgehog Signaling Pathway Activation for Craniofacial Bone Regeneration in a Critical-Sized Rat Mandibular Defect. JAMA Facial Plast Surg. Published online December 06, 2018. doi:10.1001/jamafacial.2018.1508
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