Lineage tracking of mesenchymal and endothelial progenitors in BMP-induced bone formation

Abstract To better understand the relative contributions of mesenchymal and endothelial progenitor cells to rhBMP-2 induced bone formation, we examined the distribution of lineage-labeled cells in Tie2-Cre:Ai9 and αSMA-creERT2:Col2.3-GFP:Ai9 reporter mice. Established orthopedic models of ectopic bone formation in the hind limb and spine fusion were employed. Tie2 -lineage cells were found extensively in the ectopic bone and spine fusion masses, but co-staining was only seen with tartrate-resistant acid phosphatase (TRAP) activity (osteoclasts) and CD31 immunohistochemistry (vascular endothelial cells), and not alkaline phosphatase (AP) activity (osteoblasts). To further confirm the lack of a functional contribution of Tie2 -lineage cells to BMP-induced bone, we developed conditional knockout mice where Tie2 -lineage cells are rendered null for key bone transcription factor osterix ( Tie2-cre:Osxfx/fx mice). Conditional knockout mice showed no difference in BMP-induced bone formation compared to littermate controls. Pulse labeling of mesenchymal cells with Tamoxifen in mice undergoing spine fusion revealed that αSMA- lineage cells contributed to the osteoblastic lineage ( Col2.3-GFP ), but not to endothelial cells or osteoclast populations. These data indicate that the αSMA + and Tie2 + progenitor lineages make distinct cellular contributions to bone formation, angiogenesis, and resorption/remodeling.