Bone Signaling Pathways (Wnt, RANK-RANKL-OPG, BMP/TGF-β, Hedgehog, Notch)

A handful of molecular pathways control every bone cell, and the exam distils them to two axes and two drugs. RANK-RANKL-OPG is the master switch for resorption (target: denosumab). Wnt/β-catenin is the switch for formation, restrained by the osteocyte brake sclerostin (target: romosozumab). Around these sit BMP/TGF-β (driving stem cells to osteoblasts; the basis of BMP-2), Indian hedgehog and PTHrP at the growth plate, and Notch as a contact-dependent modulator. Critically, the system is coupled: resorption itself releases the matrix-bound TGF-β and IGF-1 that recruit the osteoblasts to rebuild - which is why these are not independent switches but one integrated remodelling unit.

Osteoclasts are formed and activated by RANKL (Receptor Activator of Nuclear factor-κB Ligand), produced by osteoblast-lineage cells and osteocytes (and, in disease, by activated T cells). RANKL binds its receptor RANK on osteoclast precursors, which — together with M-CSF — drives their fusion, differentiation and bone-resorbing activity. The body's restraint is osteoprotegerin (OPG), a soluble decoy receptor secreted by osteoblasts that binds RANKL and prevents it reaching RANK. The RANKL:OPG ratio therefore sets the level of bone resorption: a high ratio means more osteoclasts and bone loss; a low ratio protects bone.

• Denosumab = a human monoclonal antibody to RANKL (mimics OPG) — a potent antiresorptive for osteoporosis and skeletal metastases.
• Oestrogen deficiency raises the RANKL:OPG ratio and increases IL-1/IL-6/TNF, accelerating postmenopausal bone loss.
• Many pathologies of bone loss (myeloma, inflammatory arthritis, metastasis) converge on up-regulated RANKL.
• Stopping denosumab causes a rebound in resorption with risk of multiple vertebral fractures — plan sequential therapy.

In the canonical Wnt pathway, a Wnt ligand binds the Frizzled (FZD) receptor together with the co-receptor LRP5/6. This inhibits the destruction complex so that β-catenin is stabilised, enters the nucleus and partners with TCF/LEF to switch on osteoblastogenic genes — increasing osteoblast number and bone formation (and indirectly raising OPG, lowering resorption). The pathway is held in check by inhibitors: sclerostin (made by osteocytes from the SOST gene) and Dkk1, which bind LRP4/5/6 and switch Wnt off.

• LRP5 gain-of-function → high bone mass phenotype; LRP5 loss-of-function → osteoporosis-pseudoglioma syndrome (low bone mass). These human "experiments of nature" proved Wnt's central role.
• Romosozumab is a monoclonal antibody against sclerostin — it releases the Wnt brake and is a potent anabolic osteoporosis drug (dual effect: increases formation and decreases resorption).
• Mechanical loading lowers sclerostin (bone responds to load by building) — the molecular basis of Wolff's law.

The two axes and two antibodies are not the only drug-bearing pathway. PTH and PTHrP act on the same receptor, PTH1R, on osteoblast-lineage cells - a Gs-coupled receptor signalling through cAMP/PKA. The examinable point is the paradox of timing: intermittent PTH is ANABOLIC - it increases osteoblast number and activity, reduces osteoblast apoptosis, and lowers sclerostin (de-repressing Wnt) - whereas continuous elevation (as in primary hyperparathyroidism, or a continuous infusion) is net CATABOLIC, because sustained signalling upregulates RANKL on osteoblasts and drives osteoclastic resorption. This is the basis of the anabolic drugs that complete the picture: teriparatide (recombinant PTH 1-34) and abaloparatide (a PTHrP analogue), given as a once-daily injection to exploit the intermittent-anabolic window. It also explains why hyperparathyroidism causes bone loss while a daily PTH injection builds it.