RANKL
RANKL is the on switch for bone demolition. It is a signaling molecule that osteoblasts and other cells display to tell immature precursor cells: become an osteoclast and start dissolving bone. Without this signal, the body simply cannot make working bone-resorbing cells.
RANKL binds a receptor called RANK on osteoclast precursors, driving them to mature, fuse, and switch on resorption. The system has a built-in counterweight: osteoblasts also secrete a decoy protein, osteoprotegerin, that mops up RANKL before it can act. The ratio of RANKL to its decoy sets how much bone is broken down, and many hormones work through this dial: PTH and calcitriol raise RANKL to free calcium, while estrogen raises the decoy to protect bone.
Because RANKL sits at the final common step of osteoclast formation, it is a prized drug target. The antibody denosumab binds RANKL like an artificial decoy, sharply cutting bone resorption to treat osteoporosis and cancer that has spread to bone. The same pathway also links the immune system to the skeleton, helping explain why inflammatory diseases like rheumatoid arthritis erode bone.
Denosumab, an antibody that grabs RANKL before it can reach its receptor, is injected twice a year to suppress osteoclasts in osteoporosis.
Denosumab is an engineered RANKL decoy.
PTH does not reach osteoclasts directly; it raises RANKL on osteoblasts, and that RANKL then activates the osteoclasts. RANKL is the messenger that links the two cell types.