Watching osteogenesis: life monitoring of osteogenic differentiation using an osteocalcin reporter.

Human bone marrow-derived mesenchymal stem cells have the potential to differentiate into several cell types such as osteoblasts, chondrocytes, and adipocytes. When cultured under appropriate medium conditions stem cells can be directed toward the osteoblast lineage in vitro. Progression of osteogenic differentiation is accompanied by changes in the expression pattern of several marker proteins including bone-specific alkaline phosphatase (bALP), collagen I (Col I), and osteocalcin (OC) and can be analyzed by well-established methods like immunohistochemical staining and quantitative RT-PCR. Furthermore, expression of fluorescent protein driven by an osteogenesis promoter facilitates online monitoring of proceeding osteogenic differentiation in transiently transfected human bone marrow-derived cells. In the present study we established a new double reporter gene construct comprising OC promoter-driven expression of green fluorescent protein and constitutive expression of red fluorescent protein-tagged histone H2B for transient transfection of primary human bone cells (HBCs). Osteogenic differentiation of transiently transfected cells was visualized by fluorescence microscopy. Immunohistochemical analysis and RT-PCR confirmed the progression into the osteo-specific lineage of transfected cells. Transfection efficiency was determined by fluorescence-activated cell sorting (FACS).