lubricin

Lubricin, also known as proteoglycan 4 (PRG4), is a large glycoprotein secreted into the synovial fluid of diarthrodial joints. It is produced by synoviocytes lining the joint capsule and by superficial zone chondrocytes in articular cartilage. The molecule features a heavily glycosylated mucin-like domain and, as a proteoglycan, may carry glycosaminoglycan chains. Its primary role is to act as a boundary lubricant, adsorbing to cartilage surfaces to form a protective film that reduces friction between opposing joint surfaces during movement.

Function and significance: By forming a lubricating layer on articular cartilage, lubricin minimizes wear and tear and helps maintain smooth joint motion. It also has anti-adhesive properties that can influence protein and cell interactions at the joint surface. Adequate lubricin levels are linked to joint health, whereas reduced or defective lubricin can lead to increased cartilage erosion and degenerative changes.

Clinical relevance: Mutations in the PRG4 gene cause CACP syndrome (camptodactyly-arthropathy-coxa vara-pericarditis), a noninflammatory joint disorder presenting early in life with stiffness, effusions, and skeletal abnormalities. In animal models and human studies, strategies to restore lubricin function or supplement it are explored as potential therapies to decrease friction and protect cartilage after injury or in osteoarthritis.

Distribution and research: In addition to joints, PRG4 expression has been detected in multiple tissues and secretions, reflecting broader protective roles on mucosal surfaces. Ongoing research seeks to better understand regulation, distribution, and therapeutic potential of lubricin in joint disorders.