PRG4 and osteoarthritis: In mouse articular cartilage, EGFR signaling may have a protective role against osteoarthritis pain by maintaining the number and properties of superficial chondrocytes, promoting chondrogenic proteoglycan 4 (Prg4) expression, and stimulating the lubrication function of the cartilage surface, which is otherwise diminished in osteoarthritis [77,78,83].