Expression of mRNA for osteoprotegerin and receptor activator of nuclear factor kappa β ligand (RANKL) during root resorption induced by the application of heavy orthodontic forces on rat molars

Introduction: Receptor activator of nuclear factor kappa P ligand (RANKL) activates osteoclast differentiation, whereas this activity is blocked by osteoprotegrin (OPG), so that the relative expression of these 2 proteins might contribute to bone and root resorption during orthodontic tooth movement. We describe experiments with RANKL and OPG mRNA expression in rats subjected to orthodontic forces. It was hypothesized that the ratios of RANKL to OPG expression would increase during root resorption processes. Methods: Fixed Sentalloy (GAC, Bohemia, NY) closed-coil springs capable of delivering approximately 100 g of force were applied for mesial movement of the mandibular left first molar in 9 male, 7-week-old Wistar rats; the right mandibular molar was used as an internal control for each animal. After 14 days, the rats were killed; tissues from 2 rats were examined by paraffin histology, and high-quality messenger ribonucleic acid (mRNA) was extracted from 4-mm widths of the mesial bony tissues in the remaining animals. Results: Paraffin sections showed osteoclastic resorption of roots on the mesial surfaces of teeth subjected to orthodontic forces. The integrity of mRNA was confirmed by reverse transcriptase polymerase chain reaction (RT-PCR) for the housekeeping gene GAPDH, and that of primers specific for OPG and RANKL was determined by RT-PCR for these genes in material isolated from the UM106 rat cell line known to express both proteins. Densitometric analysis of the RT-PCR OPG product showed an increase in background levels of OPG mRNA in bony tissues subjected to orthodontic forces in all animals studied (P < .05). In contrast, low levels of mRNA for RANKL were detected in only 5 animals and only in association with orthodontic forces. Conclusions: Data are consistent with changes in levels of OPG and RANKL in tissues subjected to orthodontic forces and experiencing root resorption.