Wnt/ß-catenin signaling mediates osteoblast differentiation triggered by peptide-induced α5ß1 integrin priming in mesenchymal skeletal cells.

The α5ß1 integrin is a key fibronectin (FN) receptor that binds to RGD-containing peptides to mediate cell adhesion. We previously reported that α5ß1 integrin promotes osteogenic differentiation in mesenchymal skeletal cells (MSCs), but the underlying mechanisms are not fully understood. In this study, we determined the signaling mechanisms induced by α5ß1 integrin interaction with its high-affinity ligand CRRETAWAC in murine and human MSCs and in vivo. We show that cyclized CRRETAWAC fully displaced MSC adhesion to FN, whereas related peptides lacking the full RRET sequence produced a partial displacement, indicating that RRET acts as an RGD-like sequence that is required to antagonize FN-mediated cell adhesion. However, all peptides increased focal adhesion kinase phosphorylation, OSE2 transcriptional activity, osteoblast gene expression, and matrix mineralization in MSCs, indicating that peptide-induced α5ß1 integrin priming can promote osteogenic differentiation independently of the RRET sequence. Biochemical analyses showed that peptide-induced α5ß1 integrin priming transiently increased PI3K/Akt phosphorylation and promoted Wnt/ß-catenin transcriptional activity independently of RRET. Consistently, pharmacological inhibition of PI3K activity reduced osteoblast differentiation and abolished Wnt regulatory gene expression induced by α5ß1 integrin priming. In vivo, systemic delivery of cyclized GACRETAWACGA linked to (DSS)6 to allow delivery to bone-forming sites for 6 weeks increased serum osteocalcin levels and improved long bone mass and microarchitecture in SAMP-6 senescent osteopenic mice. The results support a mechanism whereby α5ß1 integrin priming by high-affinity ligands integrates Wnt/ß-catenin signaling to promote osteoblast differentiation independently of cell adhesion, which could be used to improve bone mass and microarchitecture in the aging skeleton.

Low-dose PTH increases osteoblast activity via decreased Mef2c/Sost in senescent osteopenic mice.

Intermittent administration of parathyroid hormone (PTH) 1-34 at a standard dose has been shown to induce anabolic effects in bone. However, whether low-dose PTH promotes bone formation during senescence is unknown. To address this issue, we determined the effects of low-dose PTH and analysed the underlying mechanisms in prematurely senescent mice that display osteopenia. Treatment of 9-week-old Samp6 mice for 6 weeks with PTH at a standard dose (100 µg/kg per day) increased vertebral and femoral bone mass and improved bone microarchitecture as a result of increased bone-forming surfaces and mineral apposition rate (MAR). At a tenfold lower dose (10 µg/kg per day), PTH increased axial bone volume and trabecular thickness, as detected by bone histomorphometry but not by micro-computed tomography analysis. This anabolic effect resulted from increased osteoblast activity, as reflected by increased serum N-terminal propeptide of type 1 procollagen (P1NP) levels and MAR, with unchanged bone-forming surface or osteoblast surface. Mechanistically, low-dose PTH increased the expression of osteoblast markers in bone marrow stromal cells and mature osteoblasts, which was associated with increased expression of the Wnt effector Wisp1. Moreover, low-dose PTH decreased the expression of the Mef2c transcription factor, resulting in decreased Sost expression in osteoblasts/osteocytes. These results indicate that PTH at a low dose is effective at promoting bone formation and increased bone volume in senescent osteopenic mice through increased osteoblast activity and modulation of specific Wnt effectors, which raises the potential therapeutic use of intermittent PTH at low dose to increase bone forming activity and bone mass in skeletal senescence.