Relaxin Accelerates Rat Mid-palatal Suture Expansion and Subsequent Bone Formation

Objectives: Relaxin (RLN) is an insulin-like peptide hormone known to soften and lengthen the pubic symphysis and cervix. This study analyzed the effect of RLN2 on expansion of rat MPS utilizing a unique magnetically localized liposome-based drug delivery system.
Methods: Thirty-six male SD rats were divided into three groups: the control (MPS was not expanded); lipo (expanded with liposomes only); and RLN-lipo groups (expanded with liposomes encapsulating RLN2 [100 ng/kg], ferric oxide, and fluorescent Cy5.5). The MPS expanded for 1 week with a 0.014-inch spring. To localize RLN, a magnetic sheet was fixed to the palatal mucosa just above the MPS. Liposomes or RLN-lipo were injected into the MPS every 48 hours. Bone labeling was performed by injection of calcein and tetracycline. Liposome localization was tracked using an IVIS imaging system. Morphological analysis of skeletal tissue was performed by micro-CT, HE, and alkaline phosphatase (ALP) staining. RLN, relaxin receptor (Rxfp), periostin (Postn), and matrix metalloproteinase 1 (Mmp1) expression were analyzed by immunohistochemical (IHC) staining.
Results: In vivo imaging showed that magnetic sheets localized liposomes to the MPS for 72 hours. IHC staining showed RLN2 in the MPS after 1 week of expansion, and Rxfp2 at the osteogenic front in RLN-lipo only. Micro-CT imaging, bone labeling analysis, and ALP staining showed significant acceleration of expansion and bone formation at the osteogenic front in RLN-lipo compared with that in the other groups. HE staining showed sinuous bone deposition at the osteogenic front only in RLN-lipo. Finally, IHC staining showed higher expression of Mmp1 in the mid-sutural area and Postn at the osteogenic front in RLN-lipo.
Conclusions: These results suggested that RLN promotes MPS expansion through Mmp1 secretion at fibroblasts in the mid-sutural area, and could enhance bone formation by Postn at the osteogenic front through Rxfp2 during the MPS expansion period.