Overexpression of the PLAP-1 gene inhibits the differentiation of BMSCs into osteoblast-like cells

Periodontal ligament-associated protein-1 (PLAP-1) is a newly discovered member of the extracellular matrix family of proteins known as proteoglycans and is a negative regulator that plays a crucial role in the homeostasis of periodontal tissues. It can protect the periodontal ligament from excessive osteogenesis. However, the molecular mechanisms of PLAP-1 during osteogenic differentiation and osteogenesis remain unclear. In this study, we constructed a PLAP-1 recombinant retroviral plasmid vector named pBABE-hygro-PLAP-1. We transfected this plasmid into rat bone marrow stromal cells (rBMSCs) to obtain a stable cell line with overexpression of PLAP-1 to verify whether PLAP-1 also acts as an inhibitory factor in rBMSCs during bone mineralization. A rBMSC line stably overexpressing PLAP-1 was established successfully as determined by the mRNA levels of PLAP-1, which were measured by real time-qPCR (RT-qPCR), and protein expression, which was measured by immunocytochemistry and western blot analysis. At the same time, a Cell Counting Kit-8 assay did not reveal any statistically significant changes in the transfected cells (P > 0.05). Then, mineral-inducing cultures were performed, and mineralized nodules were observed at weeks 2, 3 and 4 under a microscope. Alizarin Red (Sigma) staining was performed at 4 week to illustrate calcium accumulation. The mineralized nodules in the PLAP-1-transfected rBMSC group were fewer than those in the control groups. The time span of the formation of the mineralized nodules was prolonged. Meanwhile, osteogenic genes were also detected in the mineral-inducing cells by RT-qPCR. An RT-qPCR analysis demonstrated that the levels of the osteoblast markers of rBMSCs that were transfected with pBABE-hygro-PLAP-1, including Runx2, Osterix, alkaline phosphatase, bone sialoprotein and osteocalcin, were lower than those in the non-transfected rBMSCs and rBMSCs that were transfected with empty vector (P < 0.01). These results suggest that PLAP-1 has an inhibitory function in rBMSCs when they differentiate into osteoblast-like cells.