The serine proteinase HtrA1 is ubiquitous and abundant in osteoarthritic joints, but what is it doing?

The proteolytic degradation of articular cartilage is a central characteristic of osteoarthritis (OA). While generally accepted that metalloproteinases such as MMP-13 and ADAMTS-5 are critical players in the pathological cleavage of collagen and aggrecan, respectively, it is now clear that proteolysis in OA is more complex than one or two proteinases acting in isolation. An interesting example is high-temperature requirement serine protease A1 (HtrA1) — a serine proteinase that many studies have associated with OA.