Lineage tracing of RGS5-CreER labeled cells in long bones during homeostasis and injury.

RGS5 (Regulator of G Protein Signaling 5) is a GTPase activator for heterotrimeric G-protein α-subunits, shown to be a marker of pericytes. Bone marrow stromal cell population (BMSCs) is very heterogeneous. Populations of mesenchymal progenitors, cells supportive of hematopoiesis and stromal cells regulating bone remodeling have been recently identified. Periosteal and bone marrow mesenchymal stem cells (MSCs) are participating in fracture healing, but it is hard to distiguish the soruce of cells within the callus. Considering that perivascular cells exert osteoprogenitor potential, we generated an RGS5 transgenic mouse model (Rgs5-CreER) which when crossed with Ai9 reporter animals (Rgs5/Tomato), is suitable for lineage tracing during growth and post injury. Flow cytometry analysis and histology confirmed the presence of Rgs5/Tomato+ cells within CD31+ endothelial, CD45+ hematopoietic and CD31-CD45- mesenchymal/perivascular cells. A tamoxifen chase showed expansion of Rgs5/Tomato+ cells expressing osterix within the trabeculae positioned between mineralized matrix and vasculature. Long term chase showed proportion of Rgs5/Tomato+ cells contributes to mature osteoblasts expressing osteocalcin. Following femoral fracture, Rgs5/Tomato+ cells are observed around newly formed bone within the BM cavity and expressed osterix and osteocalcin, while contribution within periosteum was low and limited to fibroblastic callus with very few positive chondrocytes. In addition, BM injury model confirmed that RGS5-Cre labels population of BMSCs expands during injury and participate in osteogenesis. Under homeostatic conditions, lineage-traced RGS5 cells within the trabecular area demonstrate osteoprogenitor capacity that in an injury model contributes to new bone formation primarily within the BM niche.