Directing mesenchymal stem cells to bone to augment bone formation and increase bone mass

Targeting mesenchymal stem cells (MSCs), progenitors of osteoblasts, to bone has been a long-standing goal but has had limited success so far. Here, Min Guan and her colleagues deliver a peptidomimetic integrin ligand against integrin α4β1 conjugated to the bone-seeking agent bisphosphonate alendronate as a means of attracting infused and/or endogenous MSCs to the bone surface to stimulate bone formation. The approach was tested in both xenotransplantation and immunocompetent mice, as well as in mouse models of trabecular bone loss induced by aging and estrogen deficiency (ovariectomy). Aging reduces the number of mesenchymal stem cells (MSCs) that can differentiate into osteoblasts in the bone marrow, which leads to impairment of osteogenesis. However, if MSCs could be directed toward osteogenic differentiation, they could be a viable therapeutic option for bone regeneration. We have developed a method to direct MSCs to the bone surface by attaching a synthetic high-affinity and specific peptidomimetic ligand (LLP2A) against integrin α4β1 on the MSC surface to a bisphosphonate (alendronate, Ale) that has a high affinity for bone. LLP2A-Ale induced MSC migration and osteogenic differentiation in vitro. A single intravenous injection of LLP2A-Ale increased trabecular bone formation and bone mass in both xenotransplantation studies and in immunocompetent mice. Additionally, LLP2A-Ale prevented trabecular bone loss after peak bone acquisition was achieved or as a result of estrogen deficiency. These results provide proof of principle that LLP2A-Ale can direct MSCs to the bone to form new bone and increase bone strength.