Lubricin is a surface-active mucinous glycoprotein secreted in the synovial joint that plays an important role in cartilage integrity. In healthy joints, lubricin molecules coat the cartilage surface, providing boundary lubrication and preventing cell and protein adhesion. Arthropathy occurring in patients with joint trauma, inflammatory arthritis or genetically mediated lubricin deficiencies have insufficient lubricin to prevent damage to articular cartilage. Recent studies in lubricin null joints indicate that lubricin (Prg4) plays a role in preventing damage to the superficial zone and preservation of chondrocytes. Progress in the production of recombinant forms of lubricin and the successes of lubricin supplementation in small animal models identify rhPRG4 as a potential therapeutic for patients with transient lubricin deficiency in the setting of trauma or autoimmune arthritis.

Osteoarthritis is a complex disease involving the mechanical breakdown of articular cartilage in the presence of altered joint mechanics and chondrocyte death, but the connection between these factors is not well established. Lubricin, a mucinous glycoprotein encoded by the PRG4 gene, provides boundary lubrication in articular joints. Joint friction is elevated and accompanied by accelerated cartilage damage in humans and mice that have genetic deficiency of lubricin. Here, we investigated the relationship between coefficient of friction and chondrocyte death using ex vivo and in vitro measurements of friction and apoptosis. We observed increases in whole-joint friction and cellular apoptosis in lubricin knockout mice compared with wild-type mice. When we used an in vitro bovine explant cartilage-on-cartilage bearing system, we observed a direct correlation between coefficient of friction and chondrocyte apoptosis in the superficial layers of cartilage. In the bovine explant system, the addition of lubricin as a test lubricant significantly lowered the static coefficient of friction and number of apoptotic chondrocytes. These results demonstrate a direct connection between lubricin, boundary lubrication, and cell survival and suggest that supplementation of synovial fluid with lubricin may be an effective treatment to prevent cartilage deterioration in patients with genetic or acquired deficiency of lubricin.camptodactyly-arthropathy-coxa vara-pericarditis | shear strain | antiadhesion | tribology

Objective. To investigate whether cartilage degeneration is prevented or minimized following intraarticular injections of lubricin derived from human synoviocytes in culture, recombinant human PRG4 (rhPRG4), or human synovial fluid (SF) in a rat model of anterior cruciate ligament (ACL) injury.Methods. Unilateral ACL transection (ACLT) was performed in Lewis rats (n ‫؍‬ 45). Nine animals were left untreated. The remaining rats were given intraarticular injections (50 l/injection) of either phosphate buffered saline (PBS) (n ‫؍‬ 9), human synoviocyte lubricin (200 g/ml; n ‫؍‬ 9), rhPRG4 (200 g/ml; n ‫؍‬ 9), or human SF lubricin (200 g/ml; n ‫؍‬ 9) twice weekly beginning on day 7 after injury. Joints were harvested on day 32 after injury. Histologic analysis was performed using Safranin O-fast green staining, and articular cartilage degeneration was graded using the Osteoarthritis Research Society International (OARSI)-modified Mankin criteria. Histologic specimens were immunoprobed for lubricin and sulfated glycosaminoglycans. A 24-hour urine collection was performed on days 17 and 29 postinjury, and urinary C-terminal telopeptide of type II collagen (CTX-II) levels were measured.Results. Treatment with human synoviocyte lubricin resulted in significantly lower OARSI scores for cartilage degeneration compared with no treatment or PBS treatment (P < 0.05). Increased immunostaining for lubricin in the superficial zone chondrocytes and on the surface of cartilage was observed in lubricin-treated, but not untreated or PBS-treated, joints. On day 17, urinary CTX-II levels in human synoviocyte lubricinand human SF lubricin-treated animals were significantly lower than those in untreated animals (P ‫؍‬ 0.005 and P ‫؍‬ 0.002, respectively) and in PBS-treated animals (P ‫؍‬ 0.002 and P < 0.001, respectively).Conclusion. After treatment with any of the 3 types of lubricin evaluated in this study, a reduction in cartilage damage following ACLT was evident, combined with a reduction in type II collagen degradation. Our findings indicate that intraarticular lubricin injection following an ACL injury may be beneficial in retarding the degeneration of cartilage and the development of posttraumatic OA.

Objective. To examine the effects of anterior cruciate ligament transection (ACLT) in a rat model on lubricin metabolism and its relationship to markers of inflammation and cartilage damage, and to determine whether blocking the metabolic effects of tumor necrosis factor ␣ (TNF␣) by etanercept increases the chondroprotection provided by lubricin.Methods. Unilateral ACLT was performed in Lewis rats. Levels of lubricin, TNF␣, interleukin-1␤ (IL-1␤), and sulfated glycosaminoglycans (sGAG) in synovial fluid (SF) lavage specimens and synovial tissue lubricin gene expression were evaluated at 1 week and 4 weeks following ACLT. Histologic evaluation of articular cartilage included staining with lubricin-specific monoclonal antibody 9G3 and Safranin O. The percentage of lubricin staining on the surface of articular cartilage in weight-bearing areas was estimated by digital imaging. Blocking of TNF␣ was performed using etanercept, which was administered subcutaneously at a dose of 0.5 mg/kg around the ACL-transected joints, using different dosing strategies. The ACL-transected and contralateral joints of these rats were harvested 4 weeks following surgery.Results. Four weeks following ACLT, SF lubricin concentrations and the percentage of cartilage surface lubricin staining were significantly lower in the injured joints compared with the contralateral joints. A significant decrease in synovial tissue lubricin gene expression was associated with elevated TNF␣ and IL-1␤ concentrations in SF lavage samples. With all of the etanercept treatment strategies, blocking of TNF␣ significantly increased the amount of lubricin bound to cartilage, coupled with a significant decrease in sGAG release. However, changes in the concentrations of lubricin in SF were variable.Conclusion. Blocking TNF␣ resulted in a chondroprotective effect, exemplified by increased lubricin deposition on articular cartilage and a decrease in sGAG release from articular cartilage in an animal model of posttraumatic arthritis.

Objective To investigate whether cartilage degeneration is prevented or minimized in an anterior cruciate ligament (ACL) injury rat model following a single dose-escalated intra-articular injection of lubricin derived from human synoviocytes in culture (HSL). Methods Unilateral ACL transection (ACLT) of the right hindlimb was performed in Lewis rats (N = 56). Control animals underwent a capsulotomy alone leaving the ACL intact (N = 11). Intra-articular injections (50μl/injection) of PBS (N = 14) and HSL (N = 14; 1600μg/ml) were performed on day 7 post-surgery. Animals were euthanized on day 70 post-surgery. Histological specimens were immunoprobed for lubricin, and sulfated glycosaminoglycans. Urinary CTX-II (uCTX-II) levels were measured on day 35 and 70 post-surgery. Hindlimb maximum applied force was determined using a variable resistor walkway to monitor quadruped gait asymmetries. Results Increased immunostaining for lubricin in the superficial zone and on the surface of cartilage was observed in lubricin-treated and control animals but not the PBS-treated nor the untreated ACLT animals. On post-operative day 35 and 70, uCTXII levels of HSL-treated animals were lower than corresponding untreated and PBS-treated (p=0.005; p<0.001 respectively) animals. ACLT animals treated with HSL and control animals distributed their weight equally between hindlimbs compared to PBS treated or untreated animals (p<0.01). Conclusion A single intra-articular injection of concentrated lubricin, following ACLT, reduced collagen type II degradation and improved weight bearing in the affected joint. This study supports the practice of tribosupplementation with lubricin in retarding cartilage degeneration and possibly the development of post-traumatic OA.

Joint exercise resulted in decreased lubricin cartilage expression, increased cartilage degeneration and reduced superficial zone chondrocyte viability in the ACLT joint. Intra-articular lubricin administration ameliorated cartilage damage due to exercise and preserved superficial zone chondrocytes' viability.

Background Lubricin, or proteoglycan 4 (PRG4), is a glycoprotein responsible for joint boundary lubrication. PRG4 has been previously shown to be down-regulated following traumatic joint injury such as a meniscal tear. There is preliminary evidence suggesting that intra-articular injection of PRG4 post-injury will reduce cartilage damage in rat models of surgically-induced post-traumatic osteoarthritis. Objective To determine the efficacy of intra-articular injection of full length recombinant human lubricin (rhPRG4) for reducing cartilage damage after medial meniscus destabilization (DMM) in a pre-clinical large animal model. Study Design Controlled laboratory study Methods Unilateral DMM was performed in 29 Yucutan minipigs. One week post-DMM, animals received 3 weekly intra-articular injections (3cc/injection): 1) rhPRG4 [1.3mg/ml; n=10], 2) rhPRG4+hyaluronan [1.3mg/ml rhPRG4 and 3mg/ml hyaluronan (~950 kDA); n=10], and 3) phosphate buffered saline [PBS; n=9]. Hind limbs were harvested 26 weeks post-surgery. Cartilage integrity was evaluated using macroscopic (India Ink) and microscopic (Safranin O-fast green and hematoxylin & eosin) scoring systems. Secondary outcomes evaluated using ELISA included PRG4 levels in synovial fluid, CTX-II concentrations in urine and serum, and IL-1β levels in synovial fluid and serum. Results The rhPRG4 group had significantly less macroscopic cartilage damage in the medial tibial plateau compared to the PBS group (p=.002). No difference was found between the rhPRG4+hyaluronan and PBS groups (p=.23). However, no differences in microscopic damage scores were observed between the three groups (p=.70). PRG4 production was elevated in the rhPRG4 group synovial fluid compared to the PBS group (p=.033). The rhPRG4 group presented significantly lower urinary CTX-II levels, but not serum levels, when compared to the PBS (p=.013) and rhPRG4+hyaluronan (p=.011) groups. In serum and synovial fluid, both rhPRG4 (p=.006; p=.017) and rhPRG4+hyaluronan groups (p=.009; p=.03) presented decreased IL-1β levels. Conclusion All groups exhibited significant cartilage degeneration following DMM surgery. However, animals treated with rhPRG4 had the least amount of cartilage damage and less inflammation, providing evidence that intra-articular injections of rhPRG4 may slow the progression of post-traumatic osteoarthritis. Clinical Relevance Patients with meniscal trauma are at high risk for post-traumatic osteoarthritis. This study demonstrates that an intra-articular injection regimen of rhPRG4 may attenuate cartilage damage following meniscal injury.

To identify the molecular pathophysiology present in early post-traumatic osteoarthritis (PTOA), the transcriptional profile of articular cartilage and its response to surgical PTOA induction were determined. Thirty six Yucatan minipigs underwent anterior cruciate ligament (ACL) transection and were randomly assigned in equal numbers to no further treatment, reconstruction or ligament repair. Cartilage was harvested at 1 and 4 weeks post-operatively and histology and RNA-sequencing were performed and compared to controls. Microscopic cartilage scores significantly worsened at 1 (p = 0.028) and 4 weeks (p = 0.001) post-surgery relative to controls, but did not differ between untreated, reconstruction or repair groups. Gene expression after ACL reconstruction and ACL transection were similar, with only 0.03% (including SERPINB7 and CR2) and 0.2% of transcripts (including INHBA) differentially expressed at 1 and 4 weeks respectively. COL2A1, COMP, SPARC, CHAD, and EF1ALPHA were the most highly expressed non ribosomal, non mitochondrial genes in the controls and remained abundant after surgery. A total of 1,275 genes were differentially expressed between 1 and 4 weeks post-surgery. With the treatment groups pooled, 682 genes were differentially expressed at both time-points, with the most significant changes observed in MMP1, COCH, POSTN, CYTL1, and PTGFR. This study confirmed the development of a microscopic PTOA stage after ACL surgery in the porcine model. Upregulation of multiple proteases (including MMP1 and ADAMTS4) were found; however, the level of expression remained orders of magnitude below that of extracellular matrix protein-coding genes (including COL2A1 and ACAN). In summary, genes with established roles in PTOA as well as novel targets for specific intervention were identified. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:318-329, 2018.