The α-smooth muscle actin promoter defines osteoprogenitors in the periodontium

OBJECTIVES: The osteoprogenitor population residing in the bone marrow stroma and adipose tissue exhibits expression of pericyte/myofibroblast markers. Osteoprogenitors are present within the periodontium during development and in postnatal life. Previous studies revealed the presence of common markers for mesenchymal progenitor cells within the bone marrow stromal population and in the periodontal area. The purpose of this study is to define if the α-SMA promoter would direct GFP expression to osteoprogenitor cells in dental follicle and periodontal ligament.

METHODS: We have utilized a transgenic model in which GFP expression is driven by an α-SMA promoter. Epifluorescence was observed in histological sections of mandibles derived from neonatal and 4-6 week old mice. Transgene expression was evaluated in primary cultures derived from dental follicle (mDF) and periodontal ligament (mPDL). Observation of GFP expression was complemented with analysis of osteogenic differentiation. In addition to RNA expression of bone markers, histochemical staining for alkaline phosphatase and detection of mineralized nodules was utilized. Flow cytometry (FACS) was used to determine the cellular profile of freshly isolated or cultured α-SMA positive cells.

RESULTS: α-SMAGFP expression was observed in the dental follicle, but not in mature bone. GFP+ cells were observed in apical regions of the root, areas rich with vascularization. mDF and mPDL cultures exhibited an intense ALP activity and a lot of von Kossa stained mineralized nodules. Strong expression of α-SMAGFP was observed during early stages of the culture and diminished as mineralization progressed. FACS analysis revealed that freshly digested mDF cells expressed the mesenchymal markers Thy-1 (40%) and Sca-1 (8.79%). In vitro expansion enriched for α-SMAGFP+ population of which 60% of cells were Thy-1 and Sca-1+.

CONCLUSION: α-SMA promoter has the potential to identify a population of mesenchymal progenitor cells residing within the dental follicle and periodontal ligament. Supported by NIH grant U24 DE06495.