PTN and osteoarthritis: RPTPζ and its ligand pleiotrophin, a heparin-binding growth factor, are reported to be elevated in both the cartilage and subchondral bone of osteoarthritis (OA) patients211,212 and in the intervertebral disc and endplate chondrocytes of mice with spinal deformities caused by static axial and asymmetrical mechanical loading.213 Although both RPTP ζ and pleiotrophin play a vital role in OA pathophysiology and intervertebral disc homeostasis, the mechanism underlying these processes remains incompletely understood.