Setd7 and its contribution to Boron-induced bone regeneration in Boron-mesoporous bioactive glass scaffolds

Boron (B), a trace element found in the human body, plays an important role for health of bone by promoting the proliferation and differentiation of osteoblasts. Our research group previously fabricated B-mesoporous bioactive glass (MBG) scaffolds, which successfully promoted osteogenic differentiation of osteoblasts when compared to pure MBG scaffolds without boron. However, the mechanisms of the positive effect of B-MBG scaffolds on osteogenesis remain unknown. Therefore, we performed in-vivo experiments in OVX rat models with pure MBG scaffolds and compared them to B-MBG scaffold. As a result, we found that B-MBG scaffold induced more new bone regeneration compared to pure MBG scaffold and examined genes related to bone regeneration induced by B-MBG scaffold through RNA-seq to obtain target genes and epigenetic mechanisms. The results demonstrated an increased expression and affiliation of Setd7 in the B-MBG group when compared to the MBG group. Immunofluorescent staining from our in vivo samples further demonstrated a higher localization of Setd7 and H3K4me3 in Runx2-positive cells in defects treated with B-MBG scaffolds. KEGG results suggested that specifically Wnt/β-catenin signaling pathway was highly activated in new bone area associated with B-MBG scaffolds. Thereafter, in vitro studies with human bone marrow stem cells (hBMSCs) stimulated by extracted liquid of B-MBG scaffolds was associated with significantly elevated levels of Setd7, as well as H3K4me3 when compared to MBG scaffolds alone. To verify the role of Setd7 in new bone formation in the presence of Boron, Setd7 was knocked down in hBMSCs with stimulation of the extracted liquids of B-MBG or MBG scaffolds. The result showed that osteoblast differentiation of hBMSCs was inhibited when Setd7 was knocked down, which could not be rescued by the extracted liquids of B-MBG scaffolds confirming its role in osteoblast differentiation and bone regeneration. As a histone methylase, Setd7 may be expected to be a potential epigenetic target for new treatment schemes of osteoporosis.

Boron-containing MBG scaffold has already been proved to promote bone regeneration in femoral defects of OVX rats by our research group, however, the epigenetic mechanism of Boron's positive effects on bone generation remains ill-informed. In our present study, we found an increased expression and affiliation of Setd7 and H3K4me3 in Runx2-positive osteoblasts in vivo. And in vitro, the higher expression of Setd7 enhanced osteogenic differentiation of human BMSCs stimulated by extracted liquids of B-MBG scaffold compared to MBG scaffold, which was associated with the activation of Wnt/β-catenin signaling pathway. Above all, it suggests that Setd7 plays an positive role in osteogenic differentiation and it may become a potential epigenetic target for new schemes for osteoporosis.