MAGP2 promotes osteogenic differentiation during fracture healing through its crosstalk with the ß-catenin pathway.

Osteogenic differentiation is important for fracture healing. Microfibrial-associated glycoprotein 2 (MAGP2) is found to function as a proangiogenic regulator in bone formation; however, its role in osteogenic differentiation during bone repair is not clear. Here, a mouse model of critical-sized femur fracture was constructed, and the adenovirus expressing MAGP2 was delivered into the fracture site. Mice with MAGP2 overexpression exhibited increased bone mineral density and bone volume fraction (BV/TV) at Day 14 postfracture. Within 7 days postfracture, overexpression of MAGP2 increased collagen I and II expression at the fracture callus, with increasing chondrogenesis. MAGP2 inhibited collagen II level but elevated collagen I by 14 days following fracture, accompanied by increased endochondral bone formation. In mouse osteoblast precursor MC3T3-E1 cells, MAGP2 treatment elevated the expression of osteoblastic factors (osterix, BGLAP and collagen I) and enhanced ALP activity and mineralization through activating ß-catenin signaling after osteogenic induction. Besides, MAGP2 could interact with lipoprotein receptor-related protein 5 (LRP5) and upregulated its expression. Promotion of osteogenic differentiation and ß-catenin activation mediated by MAGP2 was partially reversed by LRP5 knockdown. Interestingly, ß-catenin/transcription factor 4 (TCF4) increased MAGP2 expression probably by binding to MAGP2 promoter. These findings suggest that MAGP2 may interact with ß-catenin/TCF4 to enhance ß-catenin/TCF4's function and activate LRP5-activated ß-catenin signaling pathway, thus promoting osteogenic differentiation for fracture repair. mRNA sequencing identified the potential targets of MAGP2, providing novel insights into MAGP2 function and the directions for future research.

Direct Knockdown of PDZ and LIM Domain 1 Using an Adenoviral Delivery System Accelerates Osteogenesis and Fracture Healing in Mice.

Substantial advances have been made in understanding the role of partial PDZ and LIM domain family's proteins in skeletal-related diseases. Yet, little is known about the effect of PDZ and LIM Domain 1 (Pdlim1) on osteogenesis and fracture repair. This study aimed to investigate whether direct gene delivery using an adenovirus vector carrying Pdlim1 (Ad-oePdlim1) or encoding shRNA-Pdlim1 (Ad-shPdlim1) could affect the osteogenic activity of preosteoblastic MC3T3-E1 cells in vitro, and influence the fracture healing of mice in vivo. We found that Ad-shPdlim1 transfection contributed to the calcified nodule formation in MC3T3-E1 cells. Downregulation of Pdlim1 enhanced the alkaline phosphatase activity and increased the expression of osteogenic markers (Runt-related transcription factor 2 [Runx2], collagen type I alpha 1 chain [Col1A1], osteocalcin [OCN], and osteopontin [OPN]). Further analysis indicated that Pdlim1 knockdown could activate ß-catenin signaling, as evidenced by the accumulation of ß-catenin in the nucleus and the increase levels of downstream regulators such as Lef1/Tcf7, axis inhibition protein 2, cyclin D1, and SRY-box transcription factor 9. By contrast, Pdlim1 overexpression resulted in inhibition of the osteogenic activity of MC3T3-E1 cells. In vivo, at day 3 after fracture,Ad-shPdlim1 adenovirus particles were injected into the fracture site of the femur of mice, and the process of fracture healing was evaluated by X-ray, micro-computed tomography and histological examination. Local injection of Ad-shPdlim1 promoted the early cartilage callus formation, restored bone mineral density, and accelerated cartilaginous ossification, with the upregulation of osteogenic gene (Runx2, Col1A1, OCN, and OPN) expression and activation of ß-catenin signaling. Thus, we concluded that inhibition of Pdlim1 contributed to osteogenesis and fracture healing by activating the ß-catenin signaling pathway.