Consequences of Disease-causing Mutations on Lubricin Protein Synthesis, Secretion, and Post-translational Processing

Scale-Dependent Rheology of Synovial Fluid Lubricating Macromolecules

Synovial fluid (SF) is the complex biofluid that facilitates the exceptional lubrication of articular cartilage in joints. Its primary lubricating macromolecules, the linear polysaccharide hyaluronic acid (HA) and the mucin-like glycoprotein proteoglycan 4 (PRG4 or lubricin), interact synergistically to reduce boundary friction. However, the precise manner in which these molecules influence the rheological properties of SF remains unclear. This study aimed to elucidate this by employing confocal microscopy and multiscale rheometry to examine the microstructure and rheology of solutions containing recombinant human PRG4 (rhPRG4) and HA. Contrary to previous assumptions of an extensive HA-rhPRG4 network, it is discovered that rhPRG4 primarily forms stiff, gel-like aggregates. The properties of these aggregates, including their size and stiffness, are found to be influenced by the viscoelastic characteristics of the surrounding HA matrix. Consequently, the rheology of this system is not governed by a single length scale, but instead responds as a disordered, hierarchical network with solid-like rhPRG4 aggregates distributed throughout the continuous HA phase. These findings provide new insights into the biomechanical function of PRG4 in cartilage lubrication and may have implications in the development of HA-based therapies for joint diseases like osteoarthritis.

Proteoglycan 4 (PRG4) treatment improves skin wound healing in a porcine model

Proteoglycan 4 (PRG4) is a boundary lubricant originally identified in articular cartilage and has been since shown to have immunomodulation and antifibrotic properties. Previously, we have demonstrated that recombinant human (rh)PRG4 treatment accelerates auricular cartilage injury closure through an inhibition of the fibrotic response, and promotion of tissue regeneration in mice. The purpose of the current study was to examine the effects of rhPRG4 treatment (vs. a DMSO carried control) on full-thickness skin wound healing in a preclinical porcine model. Our findings suggest that while rhPRG4 did not significantly accelerate nor impede full-thickness skin wound closure, it did improve repair quality by decreasing molecular markers of fibrosis and increasing re-vascularization. We also demonstrated that rhPRG4 treatment increased dermal adipose tissue during the healing process specifically by retaining adipocytes in the wound area but did not inhibit lipolysis. Overall, the results of the current study have demonstrated that rhPRG4 acts as antifibrotic agent and regulates dermal adipose tissue during the healing processes resulting in a tissue with a trajectory that more resembles the native skin vs. a fibrotic patch. This study provides strong rationale to examine if rhPRG4 can improve regeneration in human wounds.

Quadruped Gait and Regulation of Apoptotic Factors in Tibiofemoral Joints following Intra-Articular rhPRG4 Injection in Prg4 Null Mice

Camptodactyly-arthropathy-coxa vara-pericarditis (CACP) syndrome leads to diarthrodial joint arthropathy and is caused by the absence of lubricin (proteoglycan 4—PRG4), a surface-active mucinous glycoprotein responsible for lubricating articular cartilage. In this study, mice lacking the orthologous gene Prg4 served as a model that recapitulates the destructive arthrosis that involves biofouling of cartilage by serum proteins in lieu of Prg4. This study hypothesized that Prg4-deficient mice would demonstrate a quadruped gait change and decreased markers of mitochondrial dyscrasia, following intra-articular injection of both hindlimbs with recombinant human PRG4 (rhPRG4). Prg4^−/− (N = 44) mice of both sexes were injected with rhPRG4 and gait alterations were studied at post-injection day 3 and 6, before joints were harvested for immunohistochemistry for caspase-3 activation. Increased stance and propulsion was shown at 3 days post-injection in male mice. There were significantly fewer caspase-3-positive chondrocytes in tibiofemoral cartilage from rhPRG4-injected mice. The mitochondrial gene Mt-tn, and myosin heavy (Myh7) and light chains (Myl2 and Myl3), known to play a cytoskeletal stabilizing role, were significantly upregulated in both sexes (RNA-Seq) following IA rhPRG4. Chondrocyte mitochondrial dyscrasias attributable to the arthrosis in CACP may be mitigated by IA rhPRG4. In a supporting in vitro crystal microbalance experiment, molecular fouling by albumin did not block the surface activity of rhPRG4.

Lubricin Contributes to Homeostasis of Articular Cartilage by Modulating Differentiation of Superficial Zone Cells

Lubricin encoded by the proteoglycan 4 (Prg4) gene is produced from superficial zone (SFZ) cells of articular cartilage and synoviocytes, which is indispensable for lubrication of joint surfaces. Loss-of-function of human and mouse Prg4 results in early-onset arthropathy accompanied by lost SFZ cells and hyperplastic synovium. Here, we focused on increases in the thickness of articular cartilage in Prg4-knockout joints and analyzed the underlying mechanisms. In the late stage of articular cartilage development, the articular cartilage was thickened at 2 to 4 weeks and the SFZ disappeared at 8 weeks in Prg4-knockout mice. Similar changes were observed in cultured Prg4-knockout femoral heads. Cell tracking showed that Prg4-knockout SFZ cells at 1 week of age expanded to deep layers after 1 week. In in vitro experiments, overexpression of Prg4 lacking a mucin-like domain suppressed differentiation of ATDC5 cells markedly, whereas pellets of Prg4-knockout SFZ cells showed enhanced differentiation. RNA sequencing identified matrix metalloproteinase 9 (Mmp9) as the top upregulated gene by Prg4 knockout. Mmp9 expressed in the SFZ was further induced in Prg4-knockout mice. The increased expression of Mmp9 by Prg4 knockout was canceled by IκB kinase (IKK) inhibitor treatment. Phosphorylation of Smad2 was also enhanced in Prg4-knockout cell pellets, which was canceled by the IKK inhibitor. Expression of Mmp9 and phosphorylated Smad2 during articular cartilage development was enhanced in Prg4-knockout joints. Lubricin contributes to homeostasis of articular cartilage by suppressing differentiation of SFZ cells, and the nuclear factor-kappa B-Mmp9-TGF-β pathway is probably responsible for the downstream action of lubricin. © 2020 American Society for Bone and Mineral Research (ASBMR).

Lessons from joint development for cartilage repair in the clinic

More than 250 years ago, William Hunter stated that when cartilage is destroyed it never recovers. In the last 20 years, the understanding of the mechanisms that lead to joint formation and the knowledge that some of these mechanisms are reactivated in the homeostatic responses of cartilage to injury has offered an unprecedented therapeutic opportunity to achieve cartilage regeneration. Very large investments in ambitious clinical trials are finally revealing that, although we do not have perfect medicines yet, disease modification is a feasible possibility for human osteoarthritis.

Synovial fluid lubricin and hyaluronan are altered in equine osteochondral fragmentation, cartilage impact injury, and full-thickness cartilage defect models

The objectives of this study were to evaluate temporal changes in lubricin, hyaluronan (HA), and HA molecular weight (MW) distributions in three distinct models of equine joint injury affecting the carpal (wrist), tarsal (ankle), and femoropatellar (knee) joints. To establish ranges for lubricin, HA, and HA MW distributions across multiple joints, we first evaluated clinically healthy, high-motion equine joints. Synovial fluid was collected from high-motion joints in horses without clinical signs of joint disease (n = 11 horses, 102 joints) and from research horses undergoing carpal osteochondral fragmentation (n = 8), talar cartilage impact injury (n = 7), and femoral trochlear ridge full-thickness cartilage injury (n = 22) prior to and following arthroscopically induced joint injury. Lubricin and HA concentrations were measured via enzyme-linked immunosorbent assays, and gel electrophoresis was performed to evaluate HA MW distributions. Synovial fluid parameters were analyzed via linear regression models, revealing that lubricin and HA concentrations were conserved across healthy, high-motion joints. Lubricin concentrations increased post-injury in all osteoarthritis models (carpal fragmentation P = .001; talar impact P < .001; femoral trochlear ridge cartilage defect P = .03). Sustained loss of HA was noted post-arthroscopy following carpal osteochondral fragmentation (P < .0001) and talar impact injury (P < .001). Lubricin may be elevated to compensate for the loss of HA and to protect cartilage post-injury. Further investigation into the mechanisms regulating lubricin and HA following joint injury and their effects on joint homeostasis is warranted, including whether lubricin has value as a biomarker for post-traumatic osteoarthritis.

Investigating lubricin and known cartilage-based biomarkers of osteoarthritis

Introduction: Osteoarthritis (OA) is a degenerative disease which primarily affects hyaline cartilage, leading to pain, stiffness and loss of mobility of the entire articulation. Diagnosis is commonly based on symptoms and radiographs, but there is a growing interest in detecting novel biomarkers, in serum, urine and synovial fluid, which can be predictors of disease onset and progression.Areas covered: This review provides an overview of the main biomarkers currently used in OA clinical practice, with a focus on lubricin, a surface glycoprotein secreted in the synovial fluid that lubricates the cartilage and reduces the coefficient of friction within the joint. Key findings of the last years are presented throughout the article.Expert opinion: Analysis of biomarkers might suggest personalized protocols of treatment, guide the classification of OA phenotypes, contribute to precision medicine, avoid further unnecessary exams, facilitate drug discovery or refine treatment guidelines. For all these reasons, the investigation of novel cartilage-based biomarker of osteoarthritis needs to be promoted and improved.

Genotype-phenotype investigation of 35 patients from 11 unrelated families with camptodactyly-arthropathy-coxa vara-pericarditis (CACP) syndrome

Background

The camptodactyly–arthropathy–coxa vara–pericarditis syndrome (CACP) is a rare autosomal recessive condition characterized by camptodactyly, noninflammatory arthropathy, coxa vara, and pericarditis. CACP is caused by mutations in the proteoglycan 4 (PRG4) gene, which encodes a lubricating glycoprotein present in the synovial fluid and at the surface of articular cartilage.

Methods

In the present study, we compared the clinical and molecular findings of CACP syndrome in 35 patients from 11 unrelated families. In 28 patients, whole exome sequencing was used to investigate genomic variations.

Results

We found that camptodactyly of hands was the first symptom presented by most patients. Swelling of wrists, knees, and elbows began before 4 years of age, while the age of joint involvement was variable. Patients reported an increased pain level after the age of 10, and severe hip involvement developed after 20 years old. All patients presented developmental coxa vara and seven patients (~22%) had pleural effusion, pericarditis, and/or ascites. We identified nine novel genomic alterations, including the first case of homozygous complete deletion of exon 1 in the PRG4 gene.

Conclusion

With this study, we contribute to the catalog of CACP causing variants. We confirm that the skeletal component of this disease worsens with age, and presents the potential mechanisms for interfamily variability, by discussing the influence of a modifier gene and escape from nonsense‐mediated mRNA decay. We believe that this report will increase awareness of this familial arthropathic condition and the characteristic clinical and radiological findings will facilitate the differentiation from the common childhood rheumatic diseases such as juvenile idiopathic arthritis.

Experimental evaluation of multiscale tendon mechanics

Tendon's primary function is a mechanical link between muscle and bone. The hierarchical structure of tendon and specific compositional constituents are believed to be critical for proper mechanical function. With increased appreciation for tendon importance and the development of various technological advances, this review paper summarizes recent experimental approaches that have been used to study multiscale tendon mechanics, includes an overview of studies that have evaluated the role of specific tissue constituents, and also proposes challenges/opportunities facing tendon study. Tendon has been demonstrated to have specific structural characteristics (e.g., multi-level hierarchy, crimp pattern, helix) and complex mechanical properties (e.g., non-linearity, anisotropy, viscoelasticity). Physical mechanisms including uncrimping, fiber sliding, and collagen reorganization have been shown to govern tendon mechanical responses under both static and dynamic loading. Several tendon constituents with relatively small quantities have been suggested to play a role in its mechanics, although some results are conflicting. Further research should be performed to understand the interplay and communication of tendon mechanical properties across levels of the hierarchical structure, and further show how each of these components contribute to tendon mechanics. The studies summarized and discussed in this review have helped elucidate important aspects of multiscale tendon mechanics, which is a prerequisite for analyzing stress/strain transfer between multiple scales and identifying key principles of mechanotransduction. This information could further facilitate interpreting the functional diversity of tendons from different species, different locations, and even different developmental stages, and then better understand and identify fundamental concepts related to tendon degeneration, disease, and healing. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1353-1365, 2017.

The autocrine role of proteoglycan-4 (PRG4) in modulating osteoarthritic synoviocyte proliferation and expression of matrix degrading enzymes

BACKGROUND

Lubricin/proteoglycan 4 (PRG4) is a mucinous glycoprotein secreted by synovial fibroblasts and superficial zone chondrocytes. Recently, we showed that recombinant human PRG4 (rhPRG4) is a putative ligand for CD44 receptor. rhPRG4-CD44 interaction inhibits cytokine-induced rheumatoid arthritis synoviocyte proliferation. The objective of this study is to decipher the autocrine function of PRG4 in regulating osteoarthritic synoviocyte proliferation and expression of catabolic and pro-inflammatory mediators under basal and interleukin-1 beta (IL-1β)-stimulated conditions.

METHODS

Cytosolic and nuclear levels of nuclear factor kappa B (NFκB) p50 and p65 subunits in Prg4 ^+/+ and Prg4 ^-/- synoviocytes were studied using western blot. Nuclear translocation of p50 and p65 proteins in osteoarthritis (OA) fibroblast-like synoviocytes (FLS) in response to IL-1β stimulation in the absence or presence of rhPRG4 was studied using DNA binding assays. OA synoviocyte (5000 cells per well) proliferation following IL-1β (20 ng/ml) treatment in the absence or presence of rhPRG4 (50-200 μg/ml) over 48 hours was determined using a colorimetric assay. Gene expression of matrix metalloproteinases (MMPs), tissue inhibitor of metallproteinases-1 (TIMP-1), TIMP-2, IL-1β, IL-6, IL-8, TNF-α, cycloxygenae-2 (COX2) and PRG4 in unstimulated and IL-1β (1 ng/ml)-stimulated OA synoviocytes, in the presence or absence of rhPRG4 (100 and 200 μg/ml), was studied following incubation for 24 hours.

RESULTS

Prg4 ^-/- synoviocytes contained higher nuclear p50 and p65 levels compared to Prg4 ^+/+ synoviocytes (p < 0.05). rhPRG4 (100 μg/ml) reduced p50 and p65 nuclear levels in Prg4 ^+/+ and Prg4 ^-/- synoviocytes (p < 0.001). Similarly, rhPRG4 (200 μg/ml) inhibited NFκB translocation and cell proliferation in OA synoviocytes in a CD44-dependent manner (p < 0.001) via inhibition of IκBα phosphorylation. IL-1β reduced PRG4 expression in OA synoviocytes and rhPRG4 (100 μg/ml) treatment reversed this effect (p < 0.001). rhPRG4 (200 μg/ml) reduced basal gene expression of MMP-1, MMP-3, MMP-13, IL-6, IL-8, and PRG4 in OA synoviocytes, while increasing TIMP-2 and cycloxygenase-2 (COX2) expression (p < 0.001). rhPRG4 (200 μg/ml) reduced IL-1β induction of MMP-1, MMP-3, MMP-9, MMP-13, IL-6, IL-8, and COX2 expression in a CD44-dependent manner (p < 0.001).

CONCLUSION

PRG4 plays an important anti-inflammatory role in regulating OA synoviocyte proliferation and reduces basal and IL-1β-stimulated expression of catabolic mediators. Exogenous rhPRG4 autoregulates native PRG4 expression in OA synoviocytes.

The Efficacy of Yttrium-90 Radiosynovectomy in Patients with Camptodactyly-Arthropathy-Coxa Vara-Pericarditis Syndrome

OBJECTIVE

Camptodactyly-arthropathy-coxa-vara-pericarditis (CACP) syndrome is an autosomal recessive disorder caused by mutations in PRG4 gene that encodes for proteoglycan 4, the main lubricant for joints and tendon surfaces. It is a non-inflammatory arthropathy, characterized by joint effusions and synovial hypertrophy. So far, there is no effective treatment for this disorder. To evaluate the effectiveness of yttrium-90 radiosynovectomy in arthropathy of patients with CACP syndrome.

METHODS

Consecutive patients with CACP syndrome were prospectively evaluated at the enrollment and 3 months after the right knee injection with yttrium-90. The outcome variables were patient/parent and physician's global assessment measured by a 3-point scale, right knee swelling and range of motion on a 3-point scale, in addition to magnetic resonance imaging (MRI) assessment of the right knee for bone, cartilage, fluid, synovial hypertrophy and soft tissue changes.

RESULTS

Six (three boys, three girls) patients with a mean age of 12 years and mean follow-up duration of 8.5 years completed a single right knee intra-articular yttrium-90 injection with 5 mCi. The procedure was well tolerated without adverse events apart from mild and transient joint pain in two patients. There was a minimal radioisotope leakage to soft tissue in two patients. During the 3-month follow-up interval, there was no improvement in the outcome variables. Patients and parents did not notice favorable therapeutic effects and global physician assessment was unsatisfactory. There was no difference in knee joint swelling or range of motion. Furthermore, MRI findings were unchanged. However, there was a minimal increase in synovial fluid post injection.

CONCLUSION

Yttrium-90 radiosynovectomy seems to be a safe and well tolerated procedure, however, it did not show a beneficial therapeutic effect in arthropathy of CACP syndrome with the given dosage and interval. Studies including a larger number of patients and probably repeated injections are needed to derive satisfactory results about the effectiveness of yttrium-90 in CACP syndrome patients.

Superficial zone cellularity is deficient in mice lacking lubricin: a stereoscopic analysis

Background

Lubricin, a mucinous glycoprotein secreted by synoviocytes and chondrocytes plays an important role in reducing the coefficient of friction in mammalian joints. Elevated cartilage surface friction is thought to cause chondrocyte loss; however, its quantification and methodological approaches have not been reported. We adapted a stereological method and incorporated vital cell staining to assess cellular loss in superficial and upper intermediate zones in lubricin deficient mouse cartilage.

Methods

The femoral condyle cartilage of the intact knees from lubricin wild type (Prg4^+/+), heterozygote (Prg4^+/-), and knockout (Prg4^-/-) mice was imaged using fluorescein diacetate (FDA), propidium iodide (PI), and Hoechst staining, and confocal microscopy. Three dimensional reconstructions of confocal images to a depth of 14 μm were analyzed using Matlab to determine the volume fraction occupied by chondrocytes in cartilage of both medial and lateral femoral condyles. Living chondrocyte volume fraction was defined as FDA stained chondrocyte volume/total volume of superficial + upper intermediate zone. Living and dead (total) chondrocyte volume fraction was defined as FDA + PI stained chondrocyte volume/total volume of superficial + upper intermediate zone. MicroCT provided an orthogonal measure of cartilage thickness. Immunohistology for activated caspase-3 and TUNEL staining were performed to evaluate the presence of apoptotic chondrocytes in Prg4 mutant mice.

Results

Living chondrocyte volume fraction of the medial femoral condyle was significantly lower in Prg4^-/- mice compared to Prg4^+/+ (p = 0.002) and Prg4^+/- (p = 0.002) littermates. There was no significant difference in medial condyle chondrocyte volume fraction between Prg4^+/+ and Prg4^+/- mice (p = 0.82). No significant differences were observed for the chondrocyte volume fraction for the lateral condyle (p > 0.26). Cartilage thickness increased in the medial condyle for Prg4^-/- mice compared to Prg4^+/+ (p = 0.02) and Prg4^+/- (p = 0.03) littermates, and the lateral condyle for Prg4^-/- mice compared to Prg4^+/+ (p < 0.0001) and Prg4^+/- (p < 0.0001) littermates, indicating that a multi-dimensional increase in cartilage volume did not artifactually lower the chondrocyte volume fraction in the medial condyle. Significantly higher number of caspase-3 positive cells were observed in the superficial and upper intermediate zone cartilage of the medial femoral condyle of Prg4^-/- mice compared to Prg4^+/+ (p = 0.01) and Prg4^+/- (p = 0.04) littermates, and the lateral femoral condyle of Prg4^-/- mice compared to Prg4^+/+ (p = 0.02) and Prg4^+/- (p = 0.02) littermates. There were no significant differences in TUNEL staining among different Prg4 genotypes in both condyles (p > 0.05 for all comparisons).

Conclusions

Increased Caspase-3 activation is observed in Prg4 deficient mice compared to Prg4 sufficient littermates. Absence of Prg4 induces loss of chondrocytes in the superficial and upper intermediate zone of mouse cartilage that is quantifiable by a novel image processing technique.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-016-0967-4) contains supplementary material, which is available to authorized users.

Effect of disulfide bonding and multimerization on proteoglycan 4's cartilage boundary lubricating ability and adsorption

PURPOSE

The objectives of this study were to assess the cartilage boundary lubricating ability of (1) nonreduced (NR) disulfide-bonded proteoglycan 4 (PRG4) multimers versus PRG4 monomers and (2) NR versus reduced and alkylated (R/A) PRG4 monomers and to assess (3) the ability of NR PRG4 multimers versus monomers to adsorb to an articular cartilage surface.

MATERIALS AND METHODS

PRG4 was separated into two preparations, PRG4 multimer enriched (PRG4Multi+) and PRG4 multimer deficient (PRG4Multi-), using size exclusion chromatography (SEC) and characterized by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The cartilage boundary lubricating ability of PRG4Multi+ and PRG4Multi- was compared at a physiological concentration (450 μg/mL) and assessed over a range of concentrations (45, 150, and 450 μg/mL). R/A and NR PRG4Multi- were evaluated at 450 μg/mL. Immunohistochemistry with anti-PRG4 antibody 4D6 was performed to visualize the adsorption of PRG4 preparations to the surface of articular cartilage explants.

RESULTS

Separation into enriched populations of PRG4Multi+ and PRG4Multi- was achieved using SEC and was confirmed by SDS-PAGE. PRG4Multi+ and PRG4Multi- both functioned as effective friction-reducing cartilage boundary lubricants at 450 μg/mL, with PRG4Multi+ being more effective than PRG4Multi-. PRG4Multi+ lubricated in a dose-dependent manner, however, PRG4Multi- did not. R/A PRG4Multi- lubricated similar to NR PRG4Multi-. PRG4-containing solutions showed 4D6 immunoreactivity at the articular surface; the immunoreactive intensity of PRG4Multi+ appeared to be similar to SF, whereas PRG4Multi- appeared to have less intensity.

CONCLUSIONS

These results demonstrate that the intermolecular disulfide-bonded multimeric structure of PRG4 is important for its ability to adsorb to a cartilage surface and function as a boundary lubricant. These findings contribute to a greater understanding of the molecular basis of cartilage boundary lubrication of PRG4. Elucidating the PRG4 structure-lubrication function relationship will further contribute to the understanding of PRG4's role in diarthrodial joint homeostasis and disease.

Characterisation of lubricin in synovial fluid from horses with osteoarthritis

REASON FOR PERFORMING STUDY

The glycoprotein lubricin contributes to the boundary lubrication of the articular cartilage surface. The early events of osteoarthritis involve the superficial layer where lubricin is synthesised.

OBJECTIVES

To characterise the glycosylation profile of lubricin in synovial fluid from horses with osteoarthritis and study secretion and degradation of lubricin in an in vitro inflammation cartilage model.

STUDY DESIGN

In vitro study.

METHODS

Synovial fluid samples collected from horses with joints with normal articular cartilage and structural osteoarthritic lesions; with and without osteochondral fragments, were analysed for the lubricin glycosylation profiles. Articular cartilage explants were stimulated with or without interleukin-1β for 25 days. Media samples collected at 3-day intervals were analysed by quantitative proteomics, western blot and enzyme-linked immunosorbent assay.

RESULTS

O-glycosylation profiles in synovial fluid revealed both Core 1 and 2 O-glycans, with Core 1 O-glycans predominating. Synovial fluid from normal joints (49.5 ± 1.9%) contained significantly lower amounts of monosialylated Core 1 O-glycans compared with joints with osteoarthritis (53.8 ± 7.8%, P = 0.03) or joints with osteochondral fragments (57.3 ± 8.8%, P = 0.001). Additionally, synovial fluid from normal joints (26.7 ± 6.7%) showed higher amounts of disialylated Core 1 O-glycan than from joints with osteochondral fragments (21.2 ± 4.9%, P = 0.03). A C-terminal proteolytic cleavage site in lubricin was found in synovial fluid from normal and osteochondral fragment joints and in media from interleukin-1β stimulated and unstimulated articular cartilage explants.

CONCLUSIONS

This is the first demonstration of a change in the glycosylation profile of lubricin in synovial fluid from diseased equine joints compared with that from normal joints. We demonstrate an identical proteolytic cleavage site of lubricin both in vitro and in vivo. The reduced sialation of lubricin in synovial fluid from diseased joints may affect the boundary lubricating ability of the superficial layer of articular cartilage and could be one of the early events in the progression of osteoarthritis.

Synthesis and characterization of a lubricin mimic (mLub) to reduce friction and adhesion on the articular cartilage surface

The lubricating proteoglycan, lubricin, facilitates the remarkable low friction and wear properties of articular cartilage in the synovial joints of the body. Lubricin lines the joint surfaces and plays a protective role as a boundary lubricant in sliding contact; decreased expression of lubricin is associated with cartilage degradation and the pathogenesis of osteoarthritis. An unmet need for early osteoarthritis treatment is the development of therapeutic molecules that mimic lubricin function and yet are also resistant to enzymatic degradation common in the damaged joint. Here, we engineered a lubricin mimic (mLub) that is less susceptible to enzymatic degradation and binds to the articular surface to reduce friction. mLub was synthesized using a chondroitin sulfate backbone with type II collagen and hyaluronic acid (HA) binding peptides to promote interaction with the articular surface and synovial fluid constituents. In vitro and in vivo characterization confirmed the binding ability of mLub to isolated type II collagen and HA, and to the cartilage surface. Following trypsin treatment to the cartilage surface, application of mLub, in combination with purified or commercially available hyaluronan, reduced the coefficient of friction, and adhesion, to control levels as assessed over macro-to micro-scales by rheometry and atomic force microscopy. In vivo studies demonstrate an mLub residency time of less than 1 week. Enhanced lubrication by mLub reduces surface friction and adhesion, which may suppress the progression of degradation and cartilage loss in the joint. mLub therefore shows potential for treatment in early osteoarthritis following injury.

Lessons from rare diseases of cartilage and bone

Studying severe phenotypes of rare syndromes can elucidate disease mechanisms of more common disorders and identify potential therapeutic targets. Lessons from rare bone diseases contributed to the development of the most successful class of bone active agents, the bisphosphonates. More recent research on rare bone diseases has helped elucidate key pathways and identify new targets in bone resorption and bone formation including cathepsin K and sclerostin, for which drugs are now in clinical trials. By contrast, there has been much less focus on rare cartilage diseases and osteoarthritis (OA) remains a common disease with no effective therapy. Investigation of rare cartilage syndromes is identifying new potential targets in OA including GDF5 and lubricin. Research on the arthropathy of the ultra-rare disease alkaptonuria has identified several new features of the OA phenotype, including high density mineralized protrusions (HDMPs) which constitute a newly identified mechanism of joint destruction.

Anti-lubricin monoclonal antibodies created using lubricin-knockout mice immunodetect lubricin in several species and in patients with healthy and diseased joints

Lubricin, encoded by the gene PRG4, is the principal lubricant in articulating joints. We immunized mice genetically deficient for lubricin (Prg4-/-) with purified human lubricin, and generated several mAbs. We determined each mAb’s binding epitope, sensitivity, and specificity using biologic samples and recombinant lubricin sub-domains, and we also developed a competition ELISA assay to measure lubricin in synovial fluid and blood. We found the mAbs all recognized epitopes containing O-linked oligosaccharides conjugated to the peptide motif KEPAPTTT. By western blot, the mAbs detected lubricin in 1 μl of synovial fluid from several animal species, including human. The mAbs were specific for lubricin since they did not cross-react with other synovial fluid constituents from patients with camptodactyly-arthropathy-coxa vara-pericarditis syndrome (CACP), who genetically lack this protein. The competition ELISA detected lubricin in blood samples from healthy individuals but not from patients with CACP, indicating blood can be used in a diagnostic test for patients suspected of having CACP. Lubricin epitopes in blood do not represent degradation fragments from synovial fluid. Therefore, although blood lubricin levels did not differentiate patients with inflammatory joint disease from healthy controls, epitope-specific anti-lubricin mAbs could be useful for monitoring disease activity in synovial fluid.

Transcriptional regulation of proteoglycan 4 by 17β-estradiol in immortalized baboon temporomandibular joint disc cells

Temporomandibular joint disorders (TMDs) affect a significant portion of the population of the USA, with the majority of those seeking treatment being women of childbearing age. Owing to this striking sexual dimorphism it has been postulated that sex hormones play a role in the maintenance of normal temporomandibular joint (TMJ) function. Proteoglycan 4 (PRG4) is a secreted lubricating molecule required for maintaining low frictional levels within articular joints; however, its role in the TMJ is not well characterized. In this study we describe the development of immortalized baboon cells isolated from specific regions of the TMJ disc and their use in the investigation of PRG4 expression and localization patterns in the TMJ. We identified conserved estrogen response elements within the 5' flanking region of the PRG4 gene of several species, and found that treatment of baboon TMJ disc cells with estrogen led to reduced PRG4 promoter activity and reduced expression of PRG4 mRNA in vitro. The observed negative regulation of PRG4 by estrogen could lead to increased friction and degradation of joint components over time. This study, for the first time, provides evidence of the regulatory potential of estrogen on PRG4 gene expression and suggests a novel etiology for the gender disparity observed among TMD patients.

Defects in tendon, ligament, and enthesis in response to genetic alterations in key proteoglycans and glycoproteins: a review

This review summarizes the genetic alterations and knockdown approaches published in the literature to assess the role of key proteoglycans and glycoproteins in the structural development, function, and repair of tendon, ligament, and enthesis. The information was collected from (i) genetically altered mice, (ii) in vitro knockdown studies, (iii) genetic variants predisposition to injury, and (iv) human genetic diseases. The genes reviewed are for small leucine-rich proteoglycans (lumican, fibromodulin, biglycan, decorin, and asporin); dermatan sulfate epimerase (Dse) that alters structure of glycosaminoglycan and hence the function of small leucine-rich proteoglycans by converting glucuronic to iduronic acid; matricellular proteins (thrombospondin 2, secreted phosphoprotein 1 (Spp1), secreted protein acidic and rich in cysteine (Sparc), periostin, and tenascin X) including human tenascin C variants; and others, such as tenomodulin, leukocyte cell derived chemotaxin 1 (chondromodulin-I, ChM-I), CD44 antigen (Cd44), lubricin (Prg4), and aggrecan degrading gene, a disintegrin-like and metallopeptidase (reprolysin type) with thrombospondin type 1 motif, 5 (Adamts5). Understanding these genes represents drug targets for disrupting pathological mechanisms that lead to tendinopathy, ligamentopathy, enthesopathy, enthesitis and tendon/ligament injury, that is, osteoarthritis and ankylosing spondylitis.

CACP syndrome: identification of five novel mutations and of the first case of UPD in the largest European cohort

Camptodactyly-Arthropathy-Coxa vara-Pericarditis (CACP) syndrome is a rare autosomal recessive disorder caused by mutations in PRG4 gene that encodes for proteoglycan 4, a mucin-like glycoprotein that is the major lubricant for joints and tendon surfaces. The molecular studies reported so far have described the identification of 15 mutations associated with this syndrome and the majority of them were found in families of Arabian origin. Here we report the molecular investigation of the largest European cohort that comprises 13 patients, and allowed the identification of 5 novel mutations and of the first case of CACP syndrome resulting from uniparental disomy of chromosome 1.

Molecular weight characterization of PRG4 proteins using multi-angle laser light scattering (MALLS)

OBJECTIVES

Alternative splicing and variable post-translational modifications result in proteoglycan 4 (PRG4) proteins with historically reported apparent molecular weights (Ma) ranging from 150 to 400 kDa. The objectives of this study were to (1) identify and determine the weight averaged molecular weights (M(W)'s) of PRG4 proteins purified from medium with transforming growth factor-beta 1 (TGF-β1) conditioned by mature bovine articular cartilage explants and (2) to examine the effect of reduction and alkylation (RA) on PRG4.

METHODS

Non-reduced (NR) and RA preparations of PRG4 were separated using high performance liquid chromatography-size-exclusion chromatography with an in-line multi-angle laser light scattering (MALLS) detector, which was used for absolute determination of PRG4 M(W). Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), immunoblotting, and tandem mass spectrometry (MS/MS) analysis were used to confirm the identity of separated proteins.

RESULTS

Three putative PRG4 monomers, one with previously uncharacterized M(W), were identified in NR and RA PRG4 preparations of 239 (223,255), 379 (369,389), and 467 (433,501) kDa. Additionally ∼1 MDa putative PRG4 dimer was identified. Release of a ∼90 kDa PRG4 fragment was also observed on SDS-PAGE after RA. Western Blotting with anti-PRG4 antibodies detected immunoreactive bands with Ma similar to M(W) for all species and excised bands were confirmed to be PRG4 by MS/MS.

CONCLUSIONS

A variety of monomeric PRG4 proteins and a disulfide-bonded dimer/multimer are secreted by chondrocytes in bovine cartilage explants. The observed decrease in M(W)'s of monomeric PRG4 species upon RA may be due to the release of post-translationally cleaved fragments. Further study of these species will provide insight into the PRG4 molecular structure and function relationship.

Total hip arthroplasty in adolescents with severe hip arthropathy and dysplasia associated with camptodactyly-arthropathy-coxa vara-pericarditis syndrome

Camptodactyly-arthropathy-coxa vara-pericarditis (CACP) syndrome is a rare disorder. Patients with this syndrome experience early symptomatic arthropathy of the hips. We report a case of adolescent siblings with bilateral arthropathy associated with CACP syndrome in which total hip arthroplasty was performed as treatment of severe associated disability. Postoperative Harris Hip Scores for patient 1 were 86 for the right at 18 months and 96 for the left at 12 months. Postoperative Harris Hip Score at 6 months for patient 2 was 53; however, he had good range of motion and lacked deformity. Based on our limited experience and the limited available clinical data, we feel that total hip arthroplasty is a reasonable treatment option for adolescents with debilitating hip arthropathy associated with CACP syndrome.

Dose-dependent and synergistic effects of proteoglycan 4 on boundary lubrication at a human cornea-polydimethylsiloxane biointerface

OBJECTIVES

Proteoglycan 4 (PRG4), also known as lubricin, is a boundary lubricating mucin-like glycoprotein present on several tissue surfaces in the body. The objectives of this study were to (1) implement and characterize an in vitro boundary lubrication test at a human cornea-polydimethylsiloxane (PDMS) biointerface and (2) determine the dose-dependent and synergistic effects of PRG4, with hyaluronan (HA), on ocular surface boundary lubrication using this test.

METHODS

Human corneas and model PDMS material were articulated against each other, at effective sliding velocities v(eff) between 0.3 and 30 mm/sec under physiologic loads of approximately 8 to 25 kPa. Samples were tested serially in (1) saline, PRG4 at 30, 100, 300 μg/mL resuspended in saline, then saline again or (2) saline, AQuify Comfort Eye Drops (containing 0.1% HA), 300 μg/mL PRG4 in saline, 300 μg/mL PRG4 in AQuify, then saline again. Both static and kinetic friction coefficients were calculated.

RESULTS

PRG4 effectively lowered friction at the cornea-PDMS biointerface, both alone in a dose-dependent manner and in combination with HA. PRG4 reduced kinetic friction coefficients, <μ(kinetic, Neq)>, from approximately 0.30 in saline, to approximately 0.30, 0.24, and 0.17 in 30, 100, and 300 μg/mL PRG4, respectively. Values of <μ(kinetic, Neq)> in AQuify, approximately 0.32, were similar to those in saline; however, when combined with 300 μg/mL PRG4, values of <μ(kinetic, Neq)> were reduced to approximately 0.15.

CONCLUSIONS

PRG4 functions as an effective ocular surface boundary lubricant, both alone in a dose-dependent manner and in combination with HA.

Teriparatide as a chondroregenerative therapy for injury-induced osteoarthritis

There is no disease-modifying therapy for osteoarthritis, a degenerative joint disease that is projected to afflict more than 67 million individuals in the United States alone by 2030. Because disease pathogenesis is associated with inappropriate articular chondrocyte maturation resembling that seen during normal endochondral ossification, pathways that govern the maturation of articular chondrocytes are candidate therapeutic targets. It is well established that parathyroid hormone (PTH) acting via the type 1 PTH receptor induces matrix synthesis and suppresses maturation of chondrocytes. We report that the PTH receptor is up-regulated in articular chondrocytes after meniscal injury and in osteoarthritis in humans and in a mouse model of injury-induced knee osteoarthritis. To test whether recombinant human PTH(1-34) (teriparatide) would inhibit aberrant chondrocyte maturation and associated articular cartilage degeneration, we administered systemic teriparatide (Forteo), a Food and Drug Administration-approved treatment for osteoporosis, either immediately after or 8 weeks after meniscal/ligamentous injury in mice. Knee joints were harvested at 4, 8, or 12 weeks after injury to examine the effects of teriparatide on cartilage degeneration and articular chondrocyte maturation. Microcomputed tomography revealed increased bone volume within joints from teriparatide-treated mice compared to saline-treated control animals. Immediate systemic administration of teriparatide increased proteoglycan content and inhibited articular cartilage degeneration, whereas delayed treatment beginning 8 weeks after injury induced a regenerative effect. The chondroprotective and chondroregenerative effects of teriparatide correlated with decreased expression of type X collagen, RUNX2 (runt-related transcription factor 2), matrix metalloproteinase 13, and the carboxyl-terminal aggrecan cleavage product NITEGE. These preclinical findings provide proof of concept that Forteo may be useful for decelerating cartilage degeneration and inducing matrix regeneration in patients with osteoarthritis.

Lubricin in human temporomandibular joint disc: an immunohistochemical study

AIMS

To evaluate, immunohistochemically, the presence and distribution of lubricin in human temporomandibular joint (TMJ) discs without any degenerative changes, obtained from autopsies, in order to elucidate the TMJ lubrication system and disc tribology.

METHODS

Immunohistochemistry for lubricin detection was carried out on 34 TMJ discs. Any disc had signs of degenerative or inflammatory joint disease nor disc were displaced. Sections were incubated with diluted rabbit polyclonal anti-lubricin antibody and scored according to the percentage of lubricin immunopositive cells. Three different TMJ disc tissue compartments taken from the intermediate zone were analysed, namely: the central region as well as the temporal (superior) and condylar (inferior) disc surfaces. The Friedman test, was used to compare lubricin at a protein level expression, amongst the regions of disc specimens.

RESULTS

Staining was noted within the TMJ disc cell populations in every disc tissue sample, however, the number of disc cells immunolabelled varied according to disc tissue regions. The percentage of immunostained cells, was statistically significant lower in the central region than in each disc surface (p<0.0001), whilst any statistically significant difference was found when comparing the two surfaces one another.

CONCLUSIONS

Lubricin is present in several location of TMJ disc being significantly more expressed at disc surfaces than in the central part.

Engineering lubrication in articular cartilage

Despite continuous progress toward tissue engineering of functional articular cartilage, significant challenges still remain. Advances in morphogens, stem cells, and scaffolds have resulted in enhancement of the bulk mechanical properties of engineered constructs, but little attention has been paid to the surface mechanical properties. In the near future, engineered tissues will be able to withstand and support the physiological compressive and tensile forces in weight-bearing synovial joints such as the knee. However, there is an increasing realization that these tissue-engineered cartilage constructs will fail without the optimal frictional and wear properties present in native articular cartilage. These characteristics are critical to smooth, pain-free joint articulation and a long-lasting, durable cartilage surface. To achieve optimal tribological properties, engineered cartilage therapies will need to incorporate approaches and methods for functional lubrication. Steady progress in cartilage lubrication in native tissues has pushed the pendulum and warranted a shift in the articular cartilage tissue-engineering paradigm. Engineered tissues should be designed and developed to possess both tribological and mechanical properties mirroring natural cartilage. In this article, an overview of the biology and engineering of articular cartilage structure and cartilage lubrication will be presented. Salient progress in lubrication treatments such as tribosupplementation, pharmacological, and cell-based therapies will be covered. Finally, frictional assays such as the pin-on-disk tribometer will be addressed. Knowledge related to the elements of cartilage lubrication has progressed and, thus, an opportune moment is provided to leverage these advances at a critical step in the development of mechanically and tribologically robust, biomimetic tissue-engineered cartilage. This article is intended to serve as the first stepping stone toward future studies in functional tissue engineering of articular cartilage that begins to explore and incorporate methods of lubrication.

Immunolocalization and expression of lubricin in the bilaminar zone of the human temporomandibular joint disc

Lubricin, which is a boundary joint lubricant, was investigated immunohistochemically in the bilaminar zone (BZ) of the human temporomandibular joint (TMJ), without any degenerative changes. Immunohistochemistry for lubricin detection was carried out on 33 TMJ discs obtained from 17 cadavers. Sections were incubated with diluted rabbit polyclonal anti-lubricin antibody and scored according to the percentage of lubricin immunopositive cells. Three different TMJ disc tissue compartments were analyzed, namely: the upper lamina, the inferior lamina and the loose connective tissue in the space between the laminae. The Mann-Whitney U test was used to compare protein expression (lubricin) among disc specimens' regions. Staining was noted within the TMJ disc cell populations in every disc tissue sample, with almost every cell immunolabeled by the lubricin antibody. The number of disc cells immunolabeled was almost the same in the 3 bilaminar zone regions. Positive extracellular matrix staining was also seen. The results of the present study suggest that lubricin is expressed in the TMJ disc bilaminar zone. Lubricin may have a role in normal disc posterior attachment physiology through the prevention of cellular adhesion as well as providing lubrication during normal bilaminar zone function.

Synovial fibroblasts self-direct multicellular lining architecture and synthetic function in three-dimensional organ culture

OBJECTIVE

To define the intrinsic capacity of fibroblast-like synoviocytes (FLS) to establish a 3-dimensional (3-D) complex synovial lining architecture characterized by the multicellular organization of the compacted synovial lining and the elaboration of synovial fluid constituents.

METHODS

FLS were cultured in spherical extracellular matrix (ECM) micromasses for 3 weeks. The FLS micromass architecture was assessed histologically and compared with that of dermal fibroblast controls. Lubricin synthesis was measured via immunodetection. Basement membrane matrix and reticular fiber stains were performed to examine ECM organization. Primary human and mouse monocytes were prepared and cocultured with FLS in micromass to investigate cocompaction in the lining architecture. Cytokine stimuli were applied to determine the capacity for inflammatory architecture rearrangement.

RESULTS

FLS, but not dermal fibroblasts, spontaneously formed a compacted lining architecture over 3 weeks in the 3-D ECM micromass organ cultures. These lining cells produced lubricin. FLS rearranged their surrounding ECM into a complex architecture resembling the synovial lining and supported the survival and cocompaction of monocyte/macrophages in the neo-lining structure. Furthermore, when stimulated by cytokines, FLS lining structures displayed features of the hyperplastic rheumatoid arthritis synovial lining.

CONCLUSION

This 3-D micromass organ culture method demonstrates that many of the phenotypic characteristics of the normal and the hyperplastic synovial lining in vivo are intrinsic functions of FLS. Moreover, FLS promote survival and cocompaction of primary monocytes in a manner remarkably similar to that of synovial lining macrophages. These findings provide new insight into inherent functions of the FLS lineage and establish a powerful in vitro method for further investigation of this lineage.

Differential regulation of proteoglycan 4 metabolism in cartilage by IL-1alpha, IGF-I, and TGF-beta1

OBJECTIVES

To determine (1) if interleukin-1 alpha (IL-1alpha), insulin like growth factor I (IGF-I), and transforming growth factor-beta 1 (TGF-beta1) regulate proteoglycan 4 (PRG4) metabolism in articular cartilage, in terms of chondrocytes expressing PRG4 and PRG4 bound at the articular surface, and (2) if these features of cartilage PRG4 metabolism correlate with its secretion.

METHODS

Articular cartilage explants were harvested and cultured for 6 days with or without 10% fetal bovine serum (FBS), alone, or with the addition of 10ng/ml IL-1alpha, 300ng/ml IGF-I, or 10ng/ml TGF-beta1. PRG4 expression by chondrocytes in the cartilage disks was assessed by immunohistochemistry (IHC). PRG4 bound to the articular surface of disks was quantified by extraction and enzyme-linked immunosorbent assay (ELISA). PRG4 secreted into culture medium was quantified by ELISA and characterized by Western Blot.

RESULTS

PRG4 expression by chondrocytes near the articular surface was markedly decreased by IL-1alpha, stimulated by TGF-beta1, and not affected by IGF-I. The level of PRG4 accumulation in the culture medium was correlated with the number of chondrocytes expressing PRG4. The amount of PRG4 bound at the articular surface was modulated by incubation in medium including FBS, but did not correlate with levels of PRG4 secretion.

CONCLUSIONS

Cartilage secretion of PRG4 is highly regulated by certain cytokines and growth factors, in part through alteration of the number of PRG4-secreting chondrocytes near the articular surface. The biochemical milieu may regulate the PRG4 content of synovial fluid during cartilage injury or repair.

High abundance synovial fluid proteome: distinct profiles in health and osteoarthritis

The development of increasingly high-throughput and sensitive mass spectroscopy-based proteomic techniques provides new opportunities to examine the physiology and pathophysiology of many biologic fluids and tissues. The purpose of this study was to determine protein expression profiles of high-abundance synovial fluid (SF) proteins in health and in the prevalent joint disease osteoarthritis (OA). A cross-sectional study of 62 patients with early OA (n = 21), patients with late OA (n = 21), and control individuals (n = 20) was conducted. SF proteins were separated by using one-dimensional PAGE, and the in-gel digested proteins were analyzed by electrospray ionization tandem mass spectrometry. A total of 362 spots were examined and 135 high-abundance SF proteins were identified as being expressed across all three study cohorts. A total of 135 SF proteins were identified. Eighteen proteins were found to be significantly differentially expressed between control individuals and OA patients. Two subsets of OA that are not dependent on disease duration were identified using unsupervised analysis of the data. Several novel SF proteins were also identified. Our analyses demonstrate no disease duration-dependent differences in abundant protein composition of SF in OA, and we clearly identified two previously unappreciated yet distinct subsets of protein profiles in this disease cohort. Additionally, our findings reveal novel abundant protein species in healthy SF whose functional contribution to SF physiology was not previously recognized. Finally, our studies identify candidate biomarkers for OA with potential for use as highly sensitive and specific tests for diagnostic purposes or for evaluating therapeutic response.

The role of lubricin in the mechanical behavior of synovial fluid

Synovial fluid is a semidilute hyaluronate (HA) polymer solution, the rheology of which depends on HA-protein interactions, and lubricin is a HA-binding protein found in synovial fluid and at cartilage surfaces, where it contributes to boundary lubrication under load. Individuals with genetic deficiency of lubricin develop precocious joint failure. The role of lubricin in synovial fluid rheology is not known. We used a multiple-particle-tracking microrheology technique to study the molecular interactions between lubricin and HA in synovial fluid. Particles (200 nm mean diameter) embedded in normal and lubricin-deficient synovial fluid samples were tracked separately by using multiple-particle-tracking microrheology. The time-dependent ensemble-averaged mean-squared displacements of all of the particles were measured over a range of physiologically relevant frequencies. The mean-squared displacement correlation with time lag had slopes with values of unity for simple HA solutions and for synovial fluid from an individual who genetically lacked lubricin, in contrast to slopes with values less than unity (alpha approximately 0.6) for normal synovial fluid. These data correlated with bulk rheology studies of the same samples. We found that the subdiffusive and elastic behavior of synovial fluid, at physiological shear rates, was absent in fluid from a patient who lacks lubricin. We conclude that lubricin provides synovial fluid with an ability to dissipate strain energy induced by mammalian locomotion, which is a chondroprotective feature that is distinct from boundary lubrication.

Binding and localization of recombinant lubricin to articular cartilage surfaces

Lubricin is a secreted, cytoprotective glycoprotein that contributes to the essential boundary lubrication mechanisms necessary for maintaining low friction levels at articular cartilage surfaces. Diminishment of lubricin function is thereby implicated as an adverse contributing factor in degenerative joint diseases such as osteoarthritis. Lubricin occurs as a soluble component of synovial fluid, and is synthesized and localized in the superficial layer of articular cartilage (and thus has also been described as "superficial zone protein", or SZP); however, defined interactions responsible for lubricin retention at this site are not well characterized. In the current studies, we identified molecular determinants that enable lubricin to effectively bind to articular cartilage surfaces. Efficient and specific binding to the superficial zone was observed for synovial lubricin, as well as for recombinant full-length lubricin and a protein construct comprising the lubricin C-terminal (hemopexin-like) domain (LUB-C, encoded by exons 7-12). A construct representing the N-terminal region of lubricin (LUB-N, encoded by exons 2-5) exhibited no appreciable cartilage-binding ability, but displayed the capacity to dimerize, and thus potentially influence lubricin aggregation. Disulfide bond disruption significantly attenuated recombinant lubricin and LUB-C binding to cartilage surfaces, demonstrating a requirement for protein secondary structure in facilitating the appropriate localization of lubricin at relevant tissue interfaces. These findings help identify additional key attributes contributing to lubricin functionality, which would be expected to be instrumental in maintaining joint homeostasis.

Boundary lubrication of articular cartilage: Role of synovial fluid constituents

OBJECTIVE

To determine whether the synovial fluid (SF) constituents hyaluronan (HA), proteoglycan 4 (PRG4), and surface-active phospholipids (SAPL) contribute to boundary lubrication, either independently or additively, at an articular cartilage-cartilage interface.

METHODS

Cartilage boundary lubrication tests were performed with fresh bovine osteochondral samples. Tests were performed using graded concentrations of SF, HA, and PRG4 alone, a physiologic concentration of SAPL, and various combinations of HA, PRG4, and SAPL at physiologic concentrations. Static (mu(static, Neq)) and kinetic (<mu(kinetic, Neq)>) friction coefficients were calculated.

RESULTS

Normal SF functioned as an effective boundary lubricant both at a concentration of 100% (<mu(kinetic, Neq)> = 0.025) and at a 3-fold dilution (<mu(kinetic, Neq)> = 0.029). Both HA and PRG4 contributed independently to a low mu in a dose-dependent manner. Values of <mu(kinetic, Neq)> decreased from approximately 0.24 in phosphate buffered saline to 0.12 in 3,300 mug/ml HA and 0.11 in 450 mug/ml PRG4. HA and PRG4 in combination lowered mu further at the high concentrations, attaining a <mu(kinetic, Neq)> value of 0.066. SAPL at 200 mug/ml did not significantly lower mu, either independently or in combination with HA and PRG4.

CONCLUSION

The results described here indicate that SF constituents contribute, individually and in combination, both at physiologic and pathophysiologic concentrations, to the boundary lubrication of apposing articular cartilage surfaces. These results provide insight into the nature of the boundary lubrication of articular cartilage by SF and its constituents. They therefore provide insight regarding both the homeostatic maintenance of healthy joints and pathogenic processes in arthritic disease.

Hemopexin domains as multifunctional liganding modules in matrix metalloproteinases and other proteins

The heme-binding hemopexin consists of two, four-bladed propeller domains connected by a linker region. Hemopexin domains are found in different species on the phylogenetic tree and in the human species represented in hemopexin, matrix metalloproteinases (MMPs), vitronectin, and products of the proteoglycan 4 gene. Hemopexin and hemopexin domains of human proteins fulfill functions in activation of MMPs, inhibition of MMPs, dimerization, binding of substrates or ligands, cleavage of substrates, and endocytosis by low-density lipoprotein receptor-related protein-1 (LRP-1; CD91) and LRP-2 (megalin, GP330). Insights into the structures and functions of hemopexin (domains) form the basis for positive or negative interference with the formation of molecular complexes and hence, might be exploited therapeutically in inflammation, cancer, and wound healing.

Mapping lubricin in canine musculoskeletal tissues

Lubricin, also known as superficial zone protein or PRG4, has many distinct biological functions, including lubrication, antiadhesion, and as a regulator of cell growth. This study investigated lubricin in canine musculoskeletal tissues using RT-PCR and immunohistochemistry. One or more variants were noted in canine flexor digitorum profundus (FDP) tendon, Achilles tendon, patellar tendon, A2 pulley, anterior cruciate ligament (ACL), knee lateral collateral ligament (LCL), articular cartilage, meniscus, muscle, and skin. We found 6 N-terminal lubricin splicing variants. The variants with larger sizes were identified in FDP tendon, ACL, LCL, A2 pulley, and cartilage. Lubricin was distributed both on the tissue surfaces and at the interface of fiber bundles within tissues, but this distribution varied by tissue type. We conclude that lubricin is present in many tissues; variations in splicing and physical distribution suggest that the variants of lubricin may play different roles in different locations.

The proprotein convertases and their implication in sterol and/or lipid metabolism

The proprotein convertases represent a family of nine proteinases, comprising seven basic amino acid-specific subtilisin-like serine proteinases related to yeast kexin, known as PC1/3, PC2, furin, PC4, PC5/6, PACE4 and PC7, and two other subtilases that cleave at non-basic residues, called SKI-1/S1P and NARC-1/PCSK9. The present review concentrates on the regulatory role played by some of these convertases in cholesterol and lipid metabolism. Thus, PC5/6, PACE4 and Furin upregulate high-density lipoprotein (HDL) levels via the inactivation of endothelial and lipoprotein lipases. The SKI-1/S1P-directed cleavage of membrane-bound transcription factors known as sterol regulatory element binding proteins (SREBP-1 and SREBP-2) results in upregulation of the synthesis of sterols, lipids and the LDL receptor (LDLR). Finally, PCSK9 downregulates the protein levels of the LDLR by enhancement of its intracellular metabolic pathway in subcellular acidic compartments.

Dynamic shear stimulation of bovine cartilage biosynthesis of proteoglycan 4

OBJECTIVE

The boundary lubrication function of articular cartilage is mediated in part by proteoglycan 4 (PRG4) molecules at the articular surface and in synovial fluid. The objective of this study was to determine the effects of dynamic shear stimulation on PRG4 biosynthesis by bovine cartilage explants.

METHODS

Cartilage disks with intact articular surfaces were harvested from immature bovines. Some disks were subjected to 24 hours (day 1) of loading, consisting of a step load of 20% static compression either alone or with superimposed dynamic shear (3% amplitude and 0.1 Hz), while other disks were cultured free-swelling as controls. After the 24-hour loading period, disks were terminated or were further incubated for up to 72 hours (days 2-4) in free-swelling culture to assess chondrocyte responses to, and following, unloading. PRG4 products secreted into culture medium were quantified by enzyme-linked immunosorbent assay and characterized by Western blotting. Chondrocytes expressing PRG4 were localized by immunohistochemistry, and depth-associated variations in chondrocyte PRG4 expression were quantified by image analysis.

RESULTS

Dynamic shear stimulation increased PRG4 secretion to 3-4 times that of unloaded controls and statically compressed samples. Sheared cartilage secreted more PRG4 of 345 kd relative to smaller molecular weight species, as compared with unloaded controls. Immunohistochemistry revealed that shear stimulation also increased the total number of cells expressing PRG4 by inducing expression by cells at a depth of 200-400 microm.

CONCLUSION

The paradigm that certain mechanical stimuli up-regulate biosynthesis in cartilage appears operative not only for load-bearing matrix constituents, but also for PRG4 molecules that mediate lubrication.

The cysteine-rich domain of the secreted proprotein convertases PC5A and PACE4 functions as a cell surface anchor and interacts with tissue inhibitors of metalloproteinases

The proprotein convertases PC5, PACE4 and furin contain a C-terminal cysteine-rich domain (CRD) of unknown function. We demonstrate that the CRD confers to PC5A and PACE4 properties to bind tissue inhibitors of metalloproteinases (TIMPs) and the cell surface. Confocal microscopy and biochemical analyses revealed that the CRD is essential for cell surface tethering of PC5A and PACE4 and that it colocalizes and coimmunoprecipitates with the full-length and C-terminal domain of TIMP-2. Surface-bound PC5A in TIMP-2 null fibroblasts was only observed upon coexpression with TIMP-2. In COS-1 cells, plasma membrane-associated PC5A can be displaced by heparin, suramin, or heparinases I and III and by competition with excess exogenous TIMP-2. Furthermore, PC5A and TIMP-2 are shown to be colocalized over the surface of enterocytes in the mouse duodenum and jejunum, as well as in liver sinusoids. In conclusion, the CRD of PC5A and PACE4 functions as a cell surface anchor favoring the processing of their cognate surface-anchored substrates, including endothelial lipase.