Bone-resorbing enzyme
You want it restrained: people born without working cathepsin K have denser bone, and blocking it raised bone mineral density and lowered fracture risk in trials. The caution is honest — the drug built to block it was withdrawn over a stroke signal, so the target is proven and the molecule was not.
In depth
The protease osteoclasts use to actually cut bone apart. Cathepsin K is the principal enzyme responsible for degrading bone collagen, and it degrades both of the collagens that matter here — type I in bone and type II in cartilage.
What drives it
Bone breakdown signal increases it.
RANKL matures the osteoclast; cathepsin K is the enzyme that osteoclast then secretes. Verbatim: 'Cathepsin K (CatK) is a cysteine protease abundantly expressed by osteoclasts and localized in the lysosomes and resorption lacunae of these cells' (PMID 30663085). Confirmed independently: CatK inhibition 'has no effect on osteoclastogenesis or survival of OCs' (PMID 21718816) - the signal makes the cell, the enzyme does the cutting, and they are separable steps. PMID 30663085
What it drives
It reduces Bone health.
Cathepsin K is the protease osteoclasts use to dissolve bone matrix. Verbatim: 'CatK is the principal enzyme responsible for the degradation of bone collagen' (Br J Clin Pharmacol, PMID 30663085). Human genetic confirmation: patients with pycnodysostosis carry loss-of-function mutations in the CatK gene, and CatK-null mice are osteopetrotic. Blocking it produced 'sustained reductions in resorption markers, increases in bone mineral density, and demonstrated fracture risk reduction'. Limit recorded: the drug built to block it, odanacatib, was withdrawn from FDA consideration over an increased cerebrovascular-accident signal. The target is proven; that molecule was not. PMID 30663085
It reduces Joint comfort.
The same enzyme reaches cartilage. Verbatim: 'It degrades the two main types of collagen, I and II and is predominantly expressed in osteoclasts' (PMC2951121). Type I is the bone collagen, type II the cartilage collagen, so one protease acts on both center points. PMC2951121
Blends that modulate it
Each blend page names this pathway and carries the study behind its claim.
See it on the map
Open the causal map to follow this pathway upstream and downstream, arrow by arrow, each with its study.