On the horizon: Hedgehog signaling to heal broken bones.

Uncovering the molecular pathways that drive skeletal repair has been an ongoing challenge. Initial efforts have relied on in vitro assays to identify the key signaling pathways that drive cartilage and bone differentiation. While these assays can provide some clues, assessing specific pathways in animal models is critical. Furthermore, definitive proof that a pathway is required for skeletal repair is best provided using genetic tests. Stimulating the Hh (Hedgehog) pathway can promote cartilage and bone differentiation in cell culture assays. In addition, the application of HH protein or various pathway agonists in vivo has a positive influence on bone healing. Until recently, however, genetic proof that the Hh pathway is involved in bone repair has been lacking. Here, we consider both in vitro and in vivo studies that examine the role of Hh in repair and discuss some of the challenges inherent in their interpretation. We also identify needed areas of study considering a new appreciation for the role of cartilage during repair, the variety of cell types that may have differing roles in repair, and the recent availability of powerful lineage tracing techniques. We are optimistic that emerging genetic tools will make it possible to precisely define when and in which cells promoting Hh signaling can best promote skeletal repair, and thus, the clinical potential for targeting the Hh pathway can be realized.