Bone remodeling: new aspects of a key process that controls skeletal maintenance and repair

Bone remodeling is the concerted interplay of two cellular activities: osteoclastic bone resorption and osteoblastic bone formation. Bone remodeling is the physiologic process that maintains bone mass, skeletal integrity and skeletal function. A molecular understanding of this process is therefore of paramount importance for almost all aspects of skeletal physiology and many facets of bone diseases. Based on the morphological observation of the BMU-"bone multicellular unit" or "bone metabolic unit"-and a wide body of in vitro data, bone remodeling was thought to be controlled locally through functional coupling of resorption and formation and vice versa. However, recent genetic studies have shown that there is no obligatory tight cross-control of bone formation and bone resorption in vivo and that there is also a central axis controlling bone formation, one aspect of bone remodeling. The molecule that inhibits bone formation through a hypothalamic relay is leptin. Following binding to its receptor located on the ventromedial nuclei of the hypothalamus, leptin's action on bone formation is mediated via a neuronal signaling cascade that involves the ss-adrenergic system. The overall goal of this review is to show how the dialogue between clinical medicine and mouse genetics helped to uncover a new concept in skeletal physiology.