Objective: The cellular mechanisms responsible for regulating bone formation during mandibular distraction osteogenesis (DO) are unknown. Tensile forces associated with DO differ from forces associated with rigid fixation of a bony defect. Previous studies in our lab indicate that these forces may regulate bone formation via a mechanically-induced signal transduction pathway that is not involved in fracture healing. Extracellular signal-related kinases (ERK1, ERK2) are members of the MAPK family of signal proteins thought to play a key role in the coordination of a cell’s response to mechanical stress in–vitro. To investigate the potential role of MAPK in-vivo, we compared activated ERK in distraction gap tissue to activated ERK in fracture healing and nonunion of a critical size defect using a rat model of mandibular DO.
Methods: Sprague-Dawley rats underwent unilateral mandibular osteotomy and placement of a rigid distraction device. Distraction began on post-operative day 5 at a rate of 0.3 mm twice per day, up to 5.1 mm. Animals were sacrificed on post-operative days 4, 6, 9 and 12. Control animals underwent osteotomy with fixation at 2 mm to simulate fracture healing, or at 5.1 mm to simulate nonunion of a critical size defect. Immunohistochemical (IHC) evaluation of activated ERK protein was performed using a polyclonal anti-p-ERK antibody specific for phosphorylated ERK1 and ERK2.
Results: Histologic analysis showed nonunion at the 5.1 mm gap, and various stages of fracture healing at the 2 mm gap and the distraction gap. IHC studies showed a marked increase in activated ERK in the distraction gap tissue at multiple time points compared to control specimens. Activated ERK co-localized with markers of new bone formation.
Conclusion: Activated ERK1 and ERK2, MAPKs thought to play a key role in mechanotransduction in-vitro, are markedly increased in-vivo during mandibular distraction osteogenesis. A similar increase does not occur with fracture healing or nonunion. Our results support the hypothesis that mechanical force may regulate bone formation via an ERK1/ERK2-mediated mechanotransduction pathway.
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