Relaxin 2 carried by magnetically directed liposomes accelerates rat midpalatal suture expansion and subsequent new bone formation

Relaxin (RLN) is an insulin-like peptide hormone that enables softening and lengthening of the pubic symphysis and uterine cervix. Here, we analyzed the effects of RLN2 on the expansion of rat midpalatal suture (MPS) using a magnetically directed liposome-based drug delivery system. Thirty-six male rats were divided into three groups: control (MPS was not expanded), lipo (expanded for 1 week with vehicle liposomes encapsulating ferric oxide and Cy5.5), and RLN-lipo (expanded for 1 week with the liposomes coated with RLN2). Rats were sacrificed after 1 week of expansion or after 2 weeks of retention. To accumulate RLN2-liposomes, a magnetic sheet was fixed to the palatal mucosa of the MPS. In vivo imaging showed magnetically controlled accumulation of liposomes in the MPS for 72 h. Immunohistochemistry revealed RLN2 expression in the MPS after expansion and relaxin receptor (RXFP) 2 expression at the osteogenic front (OF) in the RLN-lipo group; all groups expressed RXFP1 in the MPS. MPS expansion and bone formation were significantly accelerated at the OF in RLN-lipo group compared with the other groups. In the RLN-lipo group, significantly accelerated serrate bone deposition and elevated periostin (POSTN), iNOS, and MMP-1 levels were observed in the MPS. Sclerostin (SOST) expression was significantly reduced in newly formed bone in the RLN-lipo group. Our data revealed that RLN2 enhanced suture expansion via MMP-1 and iNOS secretion in the sutural fibroblasts and new bone formation via POSTN expression in osteoblasts at the OF. These properties may be useful for developing a new less-invasive orthopedic treatment aiming at sutural modification of cranio- and maxillofacial deformity patients.