A new role for OPG: Putting RANKL in its place

The receptor activator of NF-kB (RANK), its cognate ligand (RANKL), and the secreted decoy receptor osteoprotegrin (OPG) are critical molecules that regulate bone remodeling through modulation of osteoclast differentiation and function. (1) The former is expressed on the surface of the myeloid precursors of osteoclasts as well as on mature osteoclasts, whereas the latter two are expressed by osteoblasts. Genetic evidence obtained from both mouse knockout models and human studies demonstrate that the genes encoding these proteins are required for normal bone modeling and remodeling. OPG forms a complex with RANKL that prevents the RANKL-RANK interaction necessary for downstream signal activation in osteoclast precursors. It is generally accepted that this system is balanced by the ratio of RANKL to secreted OPG. This ratio is believed to be critical for maintaining bone homeostasis and is disrupted in bone disease states, for example, in postmenopausalosteoporosis and Pagetdisease. However, thedogma that the balance between membrane-bound RANKL and secreted OPG decoy receptor as the mechanism underlying control of osteoclast differentiation and function is challenged by the elegant work of Masashi Honma, Hiroshi Suzuki, and colleagues presented in the current issue. (2) This work builds on previous studies by this same group demonstrating that the majority of newly synthesized RANKL in osteoblasts is not transported to the cell surface but shuttled from the Golgi apparatus to secretory lysosomes, where it is stored. (3) A small portion of RANKL is transported directly from the Golgi apparatus to the cell surface by what is referred by these authors to as the ‘‘alternative pathway.’’ When this cell surface RANKL makes contact with beads coated with RANK, the RANKLinthelysosomalpooltravelstothecellsurface. (3) Thenew work starts with the premise that OPG may be involved in RANKL transport to the secretory lysosome. Cell biologic approaches are used to demonstrate that in OPG / osteoblasts RANKL accumulates in the Golgi apparatus, but reintroduction of OPG results in accumulation in the lysosomal compartment. This type of assay was used to demonstrate that the three known domains of OPG, the N-terminal cysteine-rich domain (CRD) that mediates binding to RANKL, the death domain homologous domain, and the C-terminal heparin-binding domain are all required and act in concert to promote lysosomal trafficking of RANKL. The mechanism of OPG action seems to involve enhancing the interaction of RANKL with components of the machinery for lysosomal transport such as Vps33a. The most convincing evidence for the biologic significance of RANKL lysosomal trafficking comes from experiments with a chimeric OPG protein in which the CRD of RANK is substituted for the OPG CRD domain. In a coculture assay with the stromal ST2 cell line, addition of the chimeric RANK-CRD-OPG protein to the cell medium inhibits the osteoclastogenic activity of the ST2 cells; that is, the chimeric protein acts as an effective decoy receptor. In contrast, when coexpressed in ST2 cells with green fluorescent protein (GFP)-RANKL, the RANK-CRD-OPG protein