title: Lubricin/proteoglycan 4 increases in both experimental and naturally occurring equine osteoarthritis.
abstract: The goals of this study were (1) to quantify proteoglycan 4 (PRG4) gene expression; (2) to assess lubricin immunostaining; and (3) to measure synovial fluid lubricin concentrations in clinical and experimental models of equine carpal osteoarthritis (OA).
  
  
  
  Lubricin synovial fluid concentrations and cartilage and synovial membrane PRG4 expression were analyzed in research horses undergoing experimental OA induction (n = 8) and in equine clinical patients with carpal OA (n = 58). Lubricin concentrations were measured using a custom sandwich enzyme-linked immunosorbent assay, and PRG4 expression was quantified using qRT-PCR. Lubricin immunostaining was assessed in synovial membrane and osteochondral sections in the experimental model.
  
  
  
  Lubricin concentrations increased in synovial fluid following induction of OA, peaking at 21 days post-operatively in OA joints vs sham-operated controls (331 ± 69 μg/mL vs 110 ± 19 μg/mL, P = 0.001). Lubricin concentrations also increased in horses with naturally occurring OA as compared to control joints (152 ± 32 μg/mL vs 68 ± 4 μg/mL, P = 0.003). Synovial membrane PRG4 expression increased nearly 2-fold in naturally occurring OA (P = 0.003), whereas cartilage PRG4 expression decreased 2.5-fold (P = 0.025). Lubricin immunostaining was more pronounced in synovial membrane from OA joints as compared to controls, with intense lubricin localization to sites of cartilage damage.
  
  
  
  Although PRG4 gene expression decreases in OA cartilage, synovial membrane PRG4 expression, synovial fluid lubricin concentrations and lubricin immunostaining all increase in an equine OA model. Lubricin may be elevated to protect joints from post-traumatic OA.
authors: Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853, USA. Electronic address: hlr42@cornell.edu.
authors: H L
authors: HL
authors: Reesink
authors: Department of Large Animal Clinical Sciences, Texas A&M University, College Station, TX 77843, USA. Electronic address: Awatts@cvm.tamu.edu.
authors: A E
authors: AE
authors: Watts
authors: Department of Population Medicine and Diagnostic Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853, USA. Electronic address: hom1@cornell.edu.
authors: H O
authors: HO
authors: Mohammed
authors: Department of Emergency Medicine, Warren Alpert Medical School of Brown University, Providence, RI 02903, USA; Department of Engineering, Brown University, Providence, RI 12903, USA. Electronic address: gregory_jay_md@brown.edu.
authors: G D
authors: GD
authors: Jay
authors: Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853, USA. Electronic address: ajn1@cornell.edu.
authors: A J
authors: AJ
authors: Nixon
mesh_terms: D000818:Animals; D004797:Enzyme-Linked Immunosorbent Assay / Q000662:veterinary; D005260:Female; D006023:Glycoproteins / Q000032:analysis / Q000378:metabolism*; D006734:Horse Diseases / Q000378:metabolism*; D006736:Horses; D008297:Male; D010003:Osteoarthritis / Q000378:metabolism / Q000662:veterinary*; D011509:Proteoglycans / Q000032:analysis / Q000378:metabolism*; D060888:Real-Time Polymerase Chain Reaction / Q000662:veterinary; D013582:Synovial Fluid / Q000737:chemistry; D000099158:Proteoglycan 4
chemical_list: D006023:Glycoproteins; D011509:Proteoglycans; D000099158:Proteoglycan 4
keywords: Cartilage; Lubricin; Osteoarthritis; Proteoglycan 4; Superficial zone protein; Synovium