Binding and localization of recombinant lubricin to articular cartilage surfaces

Degradation of lubricating molecules in synovial fluid alters chondrocyte sensitivity to shear strain

Articular joints facilitate motion and transfer loads to underlying bone through a combination of cartilage tissue and synovial fluid, which together generate a low-friction contact surface. Traumatic injury delivered to cartilage and the surrounding joint capsule causes secretion of proinflammatory cytokines by chondrocytes and the synovium, triggering cartilage matrix breakdown and impairing the ability of synovial fluid to lubricate the joint. Once these inflammatory processes become chronic, posttraumatic osteoarthritis (PTOA) development begins. However, the exact mechanism by which negative alterations to synovial fluid leads to PTOA pathogenesis is not fully understood. We hypothesize that removing the lubricating macromolecules from synovial fluid alters the relationship between mechanical loads and subsequent chondrocyte behavior in injured cartilage. To test this hypothesis, we utilized an ex vivo model of PTOA that involves subjecting cartilage explants to a single rapid impact followed by continuous articulation within a lubricating bath of either healthy synovial fluid, phosphate-buffered saline (PBS), synovial fluid treated with hyaluronidase, or synovial fluid treated with trypsin. These treatments degrade the main macromolecules attributed with providing synovial fluid with its lubricating properties; hyaluronic acid and lubricin. Explants were then bisected and fluorescently stained to assess global and depth-dependent cell death, caspase activity, and mitochondrial depolarization. Explants were tested via confocal elastography to determine the local shear strain profile generated in each lubricant. These results show that degrading hyaluronic acid or lubricin in synovial fluid significantly increases middle zone chondrocyte damage and shear strain loading magnitudes, while also altering chondrocyte sensitivity to loading.

O-glycans Expand Lubricin and Attenuate Its Viscosity and Shear Thinning

Lubricin, an intrinsically disordered glycoprotein, plays a pivotal role in facilitating smooth movement and ensuring the enduring functionality of synovial joints. The central domain of this protein serves as a source of this excellent lubrication and is characterized by its highly glycosylated, negatively charged, and disordered structure. However, the influence of O-glycans on the viscosity of lubricin remains unclear. In this study, we employ molecular dynamics simulations in the absence and presence of shear, along with continuum simulations, to elucidate the intricate interplay between O-glycans and lubricin and the impact of O-glycans on lubricin's conformational properties and viscosity. We found the presence of O-glycans to induce a more extended conformation in fragments of the disordered region of lubricin. These O-glycans contribute to a reduction in solution viscosity but at the same time weaken shear thinning at high shear rates, compared to nonglycosylated systems with the same density. This effect is attributed to the steric and electrostatic repulsion between the fragments, which prevents their conglomeration and structuring. Our computational study yields a mechanistic mechanism underlying previous experimental observations of lubricin and paves the way to a more rational understanding of its function in the synovial fluid.

Recombinant manufacturing of multispecies biolubricants

Lubricin, a lubricating glycoprotein abundant in synovial fluid, forms a low-friction brush polymer interface in tissues exposed to sliding motion including joints, tendon sheaths, and the surface of the eye. Despite its therapeutic potential in diseases such as osteoarthritis and dry eye disease, there are few sources available. Through rational design, we developed a series of recombinant lubricin analogs that utilize the species-specific tissue-binding domains at the N- and C-termini to increase biocompatibility while replacing the central mucin domain with an engineered variant that retains the lubricating properties of native lubricin. In this study, we demonstrate the tissue binding capacity of our engineered lubricin product and its retention in the joint space of rats. Next, we present a new bioprocess chain that utilizes a human-derived cell line to produce O-glycosylation consistent with that of native lubricin and a purification strategy that capitalizes on the positively charged, hydrophobic N- and C-terminal domains. The bioprocess chain is demonstrated at 10 L scale in industry-standard equipment utilizing commonly available ion exchange, hydrophobic interaction and size exclusion chromatography resins. Finally, we confirmed the purity and lubricating properties of the recombinant biolubricant. The biomolecular engineering and bioprocessing strategies presented here are an effective means of lubricin production and could have broad applications to the study of mucins in general.

Advanced bioactive glue tethering Lubricin/PRG4 to promote integrated healing of avascular meniscus tears

Meniscus injuries are extremely common with approximately one million patients undergoing surgical treatment annually in the U.S. alone, but no regenerative therapy exist. Previously, we showed that controlled applications of connective tissue growth factor (CTGF) and transforming growth factor beta 3 (TGFβ3) via fibrin-based bio-glue facilitate meniscus healing by inducing recruitment and stepwise differentiation of synovial mesenchymal stem/progenitor cells. Here, we first explored the potential of genipin, a natural crosslinker, to enhance fibrin-based glue's mechanical and degradation properties. In parallel, we identified the harmful effects of lubricin on meniscus healing and investigated the mechanism of lubricin deposition on the injured meniscus surface. We found that the pre-deposition of hyaluronic acid (HA) on the torn meniscus surface mediates lubricin deposition. Then we implemented chemical modifications with heparin conjugation and CD44 on our bioactive glue to achieve strong initial bonding and integration of lubricin pre-coated meniscal tissues. Our data suggested that heparin conjugation significantly enhances lubricin-coated meniscal tissues. Similarly, CD44, exhibiting a strong binding affinity to lubricin and hyaluronic acid (HA), further improved the integrated healing of HA/lubricin pre-coated meniscus injuries. These findings may represent an important foundation for developing a translational bio-active glue guiding the regenerative healing of meniscus injuries.

The Role of Lubricin, Irisin and Exercise in the Prevention and Treatment of Osteoarthritis

Osteoarthritis is a chronic degenerative musculoskeletal disease that worsens with age and is defined by pathological alterations in joint components. All clinical treatment recommendations for osteoarthritis promote exercise, although precise molecular pathways are unclear. The purpose of this study was to critically analyze the research on lubricin and irisin and how they relate to healthy and diseased joint tissue. Our research focused specifically on exercise strategies and offered new perspectives for future potential osteoarthritis treatment plans. Although lubricin and irisin have only recently been discovered, there is evidence that they have an impact on cartilage homeostasis. A crucial component of cartilage lubrication and integrity, lubricin is a surface-active mucinous glycoprotein released by the synovial joint. Its expression increases with joint movement. In healthy joints, lubricin molecules cover the cartilage surface to lubricate the boundary of the joint and inhibit protein and cell attachment. Patients with joint trauma, inflammatory arthritis, or genetically mediated lubricin deficiency, who do not produce enough lubricin to protect the articular cartilage, develop arthropathy. Irisin, sometimes known as the "sports hormone", is a myokine secreted primarily by skeletal muscle. It is a physiologically active protein that can enter the circulation as an endocrine factor, and its synthesis and secretion are primarily triggered by exercise-induced muscle contraction. We searched PubMed, Web of Science, Google Scholar, and Scopus using the appropriate keywords to identify the most recent research. The studies considered advance our knowledge of the role that exercise plays in the fight against osteoarthritis, serve as a valuable resource, and support the advancement of osteoarthritis prevention and therapy.

Recombinant mucin biotechnology and engineering

Mucins represent a largely untapped class of polymeric building block for biomaterials, therapeutics, and other biotechnology. Because the mucin polymer backbone is genetically encoded, sequence-specific mucins with defined physical and biochemical properties can be fabricated using recombinant technologies. The pendent O-glycans of mucins are increasingly implicated in immunomodulation, suppression of pathogen virulence, and other biochemical activities. Recent advances in engineered cell production systems are enabling the scalable synthesis of recombinant mucins with precisely tuned glycan side chains, offering exciting possibilities to tune the biological functionality of mucin-based products. New metabolic and chemoenzymatic strategies enable further tuning and functionalization of mucin O-glycans, opening new possibilities to expand the chemical diversity and functionality of mucin building blocks. In this review, we discuss these advances, and the opportunities for engineered mucins in biomedical applications ranging from in vitro models to therapeutics.

A Mucin-Deficient Ocular Surface Mimetic Platform for Interrogating Drug Effects on Biolubrication, Antiadhesion Properties, and Barrier Functionality

Dry eye disease (DED) affects more than 100 million people worldwide, causing significant patient discomfort and imposing a multi-billion-dollar burden on global health care systems. In DED patients, the natural biolubrication process that facilitates pain-free blinking goes awry due to an imbalance of lipids, aqueous medium, and mucins in the tear film, resulting in ocular surface damage. Identifying strategies to reduce adhesion and shear stresses between the ocular surface and the conjunctival cells lining the inside of the eyelid during blink cycles is a promising approach to improve the signs and symptoms of DED. However, current preclinical models for screening ocular lubricants rely on scarce, heterogeneous tissue samples or model substrates that do not capture the complex biochemical and biophysical cues present at the ocular surface. To recapitulate the hierarchical architecture and phenotype of the ocular interface for preclinical drug screening, we developed an in vitro mucin-deficient DED model platform that mimics the complexity of the ocular interface and investigated its utility in biolubrication, antiadhesion, and barrier protection studies using recombinant human lubricin, a promising investigational therapy for DED. The biomimetic platform recapitulated the pathological changes in biolubrication, adhesion, and barrier functionality often observed in mucin-deficient DED patients and demonstrated that recombinant human lubricin can reverse the damage induced by mucin loss in a dose- and conformation-dependent manner. Taken together, these results highlight the potential of the platform─and recombinant human lubricin─in advancing the standard of care for mucin-deficient DED patients.

Intra-Articular Adeno-Associated Virus-Mediated Proteoglycan 4 Gene Therapy for Preventing Post-Traumatic Osteoarthritis

OBJECTIVE

Lubricin, a glycoprotein encoded by the proteoglycan 4 (PRG4) gene, is an essential boundary lubricant that reduces friction between articular cartilage surfaces. The loss of lubricin subsequent to joint injury plays a role in the pathogenesis of post-traumatic osteoarthritis (PTOA). Here we describe the development and evaluation of an adeno-associated virus (AAV)-based PRG4 gene therapy intended to restore lubricin in injured joints. The green fluorescent protein (GFP) gene was inserted the PRG4 gene to facilitate tracing the distribution of the transgene product (AAV-PRG4-GFP) in vivo.

METHODS

Transduction efficiency of AAV-PRG4-GFP was evaluated in joint cells, and the conditioned medium containing secreted PRG4-GFP was used for shear loading/friction and viability tests. In vivo transduction of joint tissues following intra-articular injection of AAV-PRG4-GFP was confirmed in the mouse stifle joint in a surgical model of destabilization of the medial meniscus (DMM), and chondroprotective activity was tested in a rabbit anterior cruciate ligament transection (ACLT) model.

RESULTS

In vitro studies showed that PRG4-GFP has lubricin-like cartilage binding and anti-friction properties. Significant cytoprotective effects were seen when cartilage was soaked in PRG4-GFP prior to cyclic shear loading (n = 3). Polymerase chain reaction and confocal microscopy confirmed the presence of PRG4-GFP DNA and protein, respectively, in a mouse DMM (n = 3 per group). In the rabbit ACLT model, AAV-PRG4-GFP gene therapy enhanced lubricin expression (p = 0.001 versus AAV-GFP: n = 7-14) and protected the cartilage from degeneration (p = 0.014 versus AAV-GFP: n = 9-10) when treatments were administered immediately post-operation, but efficacy was lost when treatment was delayed for 2 weeks.

CONCLUSION

AAV-PRG4-GFP gene therapy protected cartilage from degeneration in a rabbit ACLT model; however, data from the ACLT model suggest that early intervention is essential for efficacy.

Sliding contact accelerates solute transport into the cartilage surface compared to axial loading

OBJECTIVE

The objectives of this study were: first, to compare solute uptake driven by sliding to cyclic uniaxial compression. And secondly, to evaluate the role of the superficial region on passive diffusion to determine if mechanical action is merely overcoming the low permeability of the superficial region or exceeding equilibrium capacity of the tissue.

DESIGN

Tests were performed on osteochondral plugs under two types of conditions: cyclic loading (sliding vs axial compression) and unloaded passive diffusion (intact vs superficial zone removed). The articular surfaces were exposed to a fluorescent bath and uptake was quantified from the surface to the subchondral bone using fluorescent microscopy. Primary outcome measures were total mass transfer, mass transfer rate, and surface partition factor.

RESULTS

Mass transfer was 2.1-fold higher at 0.5 h for sliding compared to uniaxial compression (p = 0.004). This increased to 4.4-fold at 2 h (p = 0.002). Solute transport for both loading conditions at 2 h had reached or exceeded intact passive diffusion at 12 h. Total mass transport and mass transport per hour was higher in samples without the superficial region compared to intact samples at equilibrium. Rate of mass transfer was not declining for samples subject to sliding indicating solute uptake induced by sliding would exceed passive tissue capacity.

CONCLUSIONS

These results are the first to quantify solute uptake between two components of joint articulation. The study demonstrates that sliding is a larger driver of solute transport compared to cyclic uniaxial compression. This has implications for cell nutrition, tissue engineering and biochemical signaling.

Lubricin as a tool for controlling adhesion in vivo and ex vivo

The ability to prevent or minimize the accumulation of unwanted biological materials on implantable medical devices is important in maintaining the long-term function of implants. To address this issue, there has been a focus on materials, both biological and synthetic, that have the potential to prevent device fouling. In this review, we introduce a glycoprotein called lubricin and report on its emergence as an effective antifouling coating material. We outline the versatility of lubricin coatings on different surfaces, describe the physical properties of its monolayer structures, and highlight its antifouling properties in improving implant compatibility as well as its use in treatment of ocular diseases and arthritis. This review further describes synthetic polymers mimicking the lubricin structure and function. We also discuss the potential future use of lubricin and its synthetic mimetics as antiadhesive biomaterials for therapeutic applications.

The role of synovial fluid constituents in the lubrication of collagen-glycosaminoglycan scaffolds for cartilage repair

Extracellular matrix (ECM)-derived scaffolds have shown promise as tissue-engineered grafts for promoting cartilage repair. However, there has been a lack of focus on fine-tuning the frictional properties of scaffolds for cartilage tissue engineering as well as understanding their interactions with synovial fluid constituents. Proteoglycan-4 (PRG4) and hyaluronan (HA) are macromolecules within synovial fluid that play key roles as boundary mode lubricants during cartilage surface interactions. The overall objective of this study was to characterize the role PRG4 and HA play in the lubricating function of collagen-glycosaminoglycan (GAG) scaffolds for cartilage repair. As a first step towards this goal, we aimed to develop a suitable in vitro friction test to establish the boundary mode lubrication parameters for collagen-GAG scaffolds articulated against glass in a phosphate buffered saline (PBS) bath. Subsequently, we sought to leverage this system to determine the effect of physiological synovial fluid lubricants, PRG4 and HA, on the frictional properties of collagen-GAG scaffolds, with scaffolds hydrated in PBS and bovine synovial fluid (bSF) serving as negative and positive controls, respectively. At all compressive strains examined (ε = 0.1-0.5), fluid depressurization within hydrated collagen-GAG scaffolds was >99% complete at ½ minute. The coefficient of friction was stable at all compressive strains (ranging from a low 0.103 ± 0.010 at ε = 0.3 up to 0.121 ± 0.015 at ε = 0.4) and indicative of boundary-mode conditions. Immunohistochemistry demonstrated that PRG4 from recombinant human (rh) and bovine sources adsorbed to collagen-GAG scaffolds and the coefficient of friction for scaffolds immersed in rhPRG4 (0.067 ± 0.027) and normal bSF (0.056 ± 0.020) solution decreased compared to PBS (0.118 ± 0.21, both p < 0.05, at ε = 0.2). The ability of the adsorbed rhPRG4 to reduce friction on the scaffolds indicates that its incorporation within collagen-GAG biomaterials may enhance their lubricating ability as potential tissue-engineered cartilage replacements. To conclude, this study reports the development of an in vitro friction test capable of characterizing the coefficient of friction of ECM-derived scaffolds tested in a range of synovial fluid lubricants and demonstrates frictional properties as a potential design parameter for implants and materials for soft tissue replacement.

Rheological properties of synovial fluid due to viscosupplements: A review for osteoarthritis remedy

The synovial fluid is a transparent electrolyte solution included in joints to provide lubrication helping the proper movement. It exhibits complex rheological properties due to the interaction among its constituents i.e. hyaluronic acid, albumin, lubricin and phospholipids. In degenerative osteoarthritis and inflammatory rheumatoid arthritis diseases, the quantity of synovial fluid and lubrication efficiency significantly deteriorates. In that case, viscosupplementation with intra-articular hyaluronic acid may be prescribed to replenish the concentration, the molecular weight and the rheological properties of natural synovial fluid. The present review concentrates on the recent advancements in viscosupplementation with emphasis into their rheological properties, its effects on the rheological behavior of synovial fluid, and finally its clinical effectiveness. Initially, the properties of synovial fluid are summarized, and then a discussion on commercial viscosupplements, the role of polymeric properties and their rheological properties are reviewed. Moreover, a detailed discussion on the clinical effectiveness and challenges of viscosupplements are provided.

A Century of Cartilage Tribology Research Is Informing Lubrication Therapies

Articular cartilage is one of the most unique materials found in nature. This tissue's ability to provide low friction and low wear over decades of constant use is not surpassed, as of yet, by any synthetic materials. Lubrication of the body's joints is essential to mammalian locomotion, but breakdown and degeneration of cartilage is the leading cause of severe disability in the industrialized world. In this paper, we review how theories of cartilage lubrication have evolved over the past decades and connect how theories of cartilage lubrication have been translated to lubrication-based therapies. Here, we call upon these historical perspectives and highlight the open questions in cartilage lubrication research. Additionally, these open questions within the field's understanding of natural lubrication mechanisms reveal strategic directions for lubrication therapy.

Effect of Lubricin Mimetics on the Inhibition of Osteoarthritis in a Rat Anterior Cruciate Ligament Transection Model

BACKGROUND

Lubricin, a mucinous glycoprotein, plays a chondroprotective role as a constituent of synovial fluid. Structural analogs have been synthesized to mimic the structure and function of native lubricin in an effort to recapitulate this effect with the goal of delaying progression of osteoarthritis (OA).

PURPOSE

To investigate the efficacy of intra-articular injections of lubricin mimetics in slowing or preventing the progression of posttraumatic OA by using a rat anterior cruciate ligament transection model.

STUDY DESIGN

Controlled laboratory design.

METHODS

Four lubricin mimetics were investigated, differing from one another in their binding orientations and steric interactions. Eighty skeletally mature Sprague-Dawley rats underwent bilateral anterior cruciate ligament transections and were randomly allocated to receive intra-articular injections (50 µL/injection) of 1 of the 4 mimetics in the right knee and equal volumes of saline injection in the contralateral knee (control). All rats were euthanized 8 weeks postoperatively and assessed via biomechanical analysis, which evaluated comparative friction coefficients across the 4 groups, and histological evaluation of articular cartilage, osteophytes, and synovitis. The Osteoarthritis Research Society International (OARSI) histopathological assessment system was used to evaluate the degree of articular cartilage degeneration and osteophytes, while synovitis was assessed through a semiquantitative scoring system. Binding efficacy of the 4 mimetics was assessed in vitro and in vivo through the immunohistochemical localization of polyethylene glycol. Articular cartilage degeneration and synovitis scoring data analyses were performed with generalized estimating equation modeling.

RESULTS

Injection of the group 3 mimetic (random 24 + 400 + 30) directly correlated with improved OARSI scores for femoral articular cartilage degeneration when compared with saline-injected contralateral control knees (P = .0410). No lubricin mimetic group demonstrated statistically significant differences in OARSI scores for tibial articular cartilage degeneration. Injection of the group 4 mimetic (AB 24 + 400 + 30) led to a statistically significant difference in osteophyte OARSI score (P = .0019). None of the 4 lubricin mimetics injections incited an additive synovial inflammatory response. Immunohistochemical staining substantiated the binding capacity of all 4 mimetics, while in vivo experimentation revealed that the group 1 and 3 mimetics were still retained within the joint 4 weeks after injection. There were no differences in friction coefficients between any pair of groups and no significant trends based on lubricin mimetic structure.

CONCLUSION

We demonstrated that the tribosupplementation of a traumatically injured knee with a specific lubricin structural analog may attenuate the natural progression of OA.

CLINICAL RELEVANCE

The current lack of efficacious clinical options to counter the onset and subsequent development of OA suggests that further investigation into the synthesis and behavior of lubricin analogs could yield novel translational applications.

Influence of Block Length on Articular Cartilage Lubrication with a Diblock Bottle-Brush Copolymer

We report how the tribological properties of a class of diblock copolymers with architecture and function inspired by the lubricating glycoprotein lubricin correlate to chemical composition. This class of diblock copolymers, consisting of a cationic cartilage-binding block and a brush-lubricating block, demonstrates that boundary lubrication of articular cartilage more strongly depends on the cartilage-binding block than the lubrication block. Specifically, the cartilage-binding functional groups (tertiary or quaternary amines) and cartilage-binding block length significantly influence the degree of lubrication under boundary mode experimental conditions. An optimal number (∼24 in this case) of cartilage-binding groups led to the lowest coefficient of friction, and an increase or decrease in the number of cations in the binding block led to partial (>24, and between 12 and 24) or complete (=12) loss of lubricating ability. The length of the lubricating block (DP = 200 or 400) chosen in this study had no effect on the degree of lubrication. These results are put into context in terms of binding affinity to the cartilage and the spatial packing density of the polymer on the cartilage surface and can serve as a useful guide for future designs of synthetic lubricants that rival the efficacy of natural lubricants.

Dynamics of Synovial Fluid Aggregation under Shear

The synovial fluid (SF) that lubricates articular joints exhibits complex rheological and tribological properties due to the interactions and behaviors of its various molecular components. Under shear, SF films abruptly thicken by more than 300% and large, dense aggregates form within the fluid. In this study, we used the Surface Force Apparatus to elucidate which SF components are involved in this shear-induced transformation by (i) determining which (if any) of all major SF components replicate the behavior of SF under shear and (ii) observing the effect of removing implicated components from SF by enzymatic digestion. While most previous studies of SF have focused on the tribological roles of lubricin or hyaluronic acid, our results indicate that albumin is a key contributor to the formation of aggregates in SF under shear. Our results also suggest that SF aggregation is associated with efficient surface protection against wear. As our findings are based on experiments involving rigid, nonporous surfaces, they may be used to investigate shear-mediated aggregation mechanisms occurring during the lubrication of artificial joints, ultimately advancing our current vision of implant design.

Tribological Behavior of Surface-Immobilized Novel Biomimicking Multihierarchical Polymers: The Role of Structure and Surface Attachment

The tribological properties of two novel biomimetic multihierarchical polymers, synthesized by covalently linking single bottlebrush polymers onto a hyaluronic acid (HA) backbone, were investigated in the boundary lubrication regime using the surface forces apparatus. The polymers were immobilized on flat substrates, and their lubrication properties and wear resistance were investigated in aqueous media in the absence of a polymer reservoir (i.e., no free polymer chains in the surrounding medium) in order to better reveal the underlying mechanism of surface-attached biomimetic polymers. The effects of composition, structure, and, more particularly, surface attachment (physisorbed vs chemisorbed) on the tribological properties were investigated and compared with other biomimicking systems reported in the literature. The covalently surface attached bottlebrushes allowed wear resistance between sliding surfaces to be significantly improved, compared to physisorbed bottlebrushes, with a constant coefficient of friction (10^-1) of up to few tens of MPa. The results confirm that surface-attached bottlebrushes on their own are not responsible for the extremely low friction often reported in the literature or found in articular joints. Moreover, the study confirmed that the irreversible attachment of bottlebrushes, or multihierarchical polymer layers, to surfaces is crucial to improving wear resistance between sliding surfaces in aqueous media.

Deciphering Isomers with a Multiple Reaction Monitoring Method for the Complete Detectable O-Glycan Repertoire of the Candidate Therapeutic, Lubricin

Glycosylation is a fundamental post-translational modification, occurring on half of all proteins. Despite its significance, our understanding is limited, in part due to the inherent difficulty in studying these branched, multi-isomer structures. Accessible, detailed, and quantifiable methods for studying glycans, particularly O-glycans, are needed. Here we take a multiple reaction monitoring (MRM) approach to differentiate and relatively quantify all detectable glycans, including isomers, on the heavily O-glycosylated protein lubricin. Lubricin (proteoglycan 4) is essential for lubrication of the joint and eye. Given the therapeutic potential of lubricin, it is essential to understand its O-glycan repertoire in biological and recombinantly produced samples. O-Glycans were released by reductive β-elimination and defined, showing a range of 26 neutral, sulfated, sialylated, and both sulfated and sialylated core 1 (Galβ1-3GalNAcα1-) and core 2 (Galβ1-3(GlcNAcβ1-6)GalNAcα1-) structures. Isomer-specific MRM transitions allowed effective differentiation of neutral glycan isomers as well as sulfated isomeric structures, where the sulfate was retained on the fragment ions. This strategy was not as effective with labile sialylated structures; instead, it was observed that the optimal collision energy for the m/z 290.1 sialic acid B-fragment differed consistently between sialic acid isomers, allowing differentiation between isomers when fragmentation spectra were insufficient. This approach was also effective for purchased Neu5Acα2-3Galβ1-4Glc and Neu5Acα2-6Galβ1-4Glc and for Neu5Acα2-3Galβ1-4GlcNAc and Neu5Acα2-6Galβ1-4GlcNAc linkage isomers with the Neu5Acα2-6 consistently requiring more energy for optimal generation of the m/z 290.1 fragment. Overall, this method provides an effective and easily accessible approach for the quantification and annotation of complex released O-glycan samples.

Boundary mode lubrication of articular cartilage with a biomimetic diblock copolymer

We report the design of a diblock copolymer with architecture and function inspired by the lubricating glycoprotein lubricin. This diblock copolymer, synthesized by sequential reversible addition-fragmentation chain-transfer polymerization, consists of a cationic cartilage-binding domain and a brush-lubricating domain. It reduces the coefficient of friction of articular cartilage under boundary mode conditions (0.088 ± 0.039) to a level equivalent to that provided by lubricin (0.093 ± 0.011). Additionally, both the EC₅₀ (0.404 mg/mL) and cartilage-binding time constant (7.19 min) of the polymer are comparable to purified human and recombinant lubricin. Like lubricin, the tribological properties of this polymer are dependent on molecular architecture. When the same monomer composition was evaluated either as an AB diblock copolymer or as a random copolymer, the diblock effectively lubricated cartilage under boundary mode conditions whereas the random copolymer did not. Additionally, the individual polymer blocks did not lubricate independently, and lubrication could be competitively inhibited with an excess of binding domain. This diblock copolymer is an example of a synthetic polymer with lubrication properties equal to lubricin under boundary mode conditions, suggesting its potential utility as a therapy for joint pathologies like osteoarthritis.

Stable recombinant production of codon-scrambled lubricin and mucin in human cells

Widespread therapeutic and commercial interest in recombinant mucin technology has emerged due to the unique ability of mucin glycoproteins to hydrate, protect, and lubricate biological surfaces. However, recombinant production of the large, highly repetitive domains that are characteristic of mucins remains a challenge in biomanufacturing likely due, at least in part, to the inherent instability of DNA repeats in the cellular genome. To overcome this challenge, we exploit codon redundancy to encode desired mucin polypeptides with minimal nucleotide repetition. The codon-scrambling strategy was applied to generate synonymous genes, or "synDNAs," for two mucins of commercial interest: lubricin and mucin 1. Stable, long-term recombinant production in suspension-adapted human 293-F cells was demonstrated for the synonymous lubricin complementary DNA (cDNA), which we refer to as SynLubricin. Under optimal conditions, a 293-F subpopulation produced recombinant SynLubricin at more than 200 mg/L of media and was stable throughout 2 months of continuous culture. Functionality tests confirmed that the recombinant lubricin could effectively inhibit cell adhesion and lubricate cartilage explants. Together, our work provides a viable workflow for cDNA design and stable mucin production in mammalian host production systems.

Investigating the effect of proteoglycan 4 on hyaluronan solution properties using confocal fluorescence recovery after photobleaching

Background

The objective of this study was to use confocal fluorescence recovery after photobleaching (FRAP) to examine the specific and dose-dependent effect of proteoglycan 4 (PRG4) on hyaluronan (HA) solutions of different molecular weight; and assess the effect of reduction and alkylation (R/A) of PRG4 on its effects on HA solutions.

Methods

Confocal FRAP was used to determine the diffusion coefficient of fluorescein isothiocyanate (FITC)-dextran tracer (D_t) through 1500 kDa and 500 kDa HA solutions (0–3.3 mg/ml) ± PRG4 or a control protein, bovine serum albumin (BSA), at physiological (450 μg/ml) or pathophysiological (45 μg/ml) concentrations. The effect of PRG4 or R/A PRG4 on 1500 kDa HA solutions was also investigated. Empirical constants obtained from fitting data to the universal scaling equation were used to calculate the average distribution of apparent mesh sizes.

Results

PRG4 at both 45 and 450 μg/ml slowed the diffusion of the FITC-dextran tracer for all concentrations of HA and caused a decrease in the apparent mesh size within the HA solution. This effect was specific to PRG4, not observed with BSA, but not dependent on its tertiary/quaternary structure as the effect remained after R/A of PRG4.

Conclusions

These results demonstrate that PRG4 can significantly alter the solution properties of HA; PRG4 essentially reduced the permeability of the HA network. This effect may be due to PRG4 entangling HA molecules through binding and/or HA crowding PRG4 molecules into a self-assembled network. Collectively these findings contribute to the understanding of PRG4 and HA interaction(s) in solution and therefore the function of SF in diarthroidal joints.

Recombinant human proteoglycan-4 reduces phagocytosis of urate crystals and downstream nuclear factor kappa B and inflammasome activation and production of cytokines and chemokines in human and murine macrophages

Background

Gout is an inflammatory arthritis caused by monosodium urate monohydrate (MSU) crystals’ joint deposition. MSU phagocytosis by resident macrophages is a key step in gout pathogenesis. MSU phagocytosis triggers nuclear factor kappa B (NFκB) activation and production of cytokines and chemokines. Proteoglycan-4 (PRG4) is a glycoprotein produced by synovial fibroblasts and exerts an anti-inflammatory effect in the joint mediated by its interaction with cell surface receptor CD44. PRG4 also binds and antagonizes TLR2 and TLR4. The objective of this study is to evaluate the efficacy of recombinant human PRG4 (rhPRG4) in suppressing MSU-induced inflammation and mechanical allodynia in vitro and in vivo.

Methods

THP-1 macrophages were incubated with MSU crystals ± rhPRG4 or bovine submaxillary mucin (BSM), and crystal phagocytosis, cytokines and chemokines expression and production were determined. NFκB p65 subunit nuclear translocation, NLRP3 induction, caspase-1 activation and conversion of proIL-1β to mature IL-1β were studied. MSU phagocytosis by Prg4^+/+ and Prg4^−/− peritoneal macrophages was determined in the absence or presence of rhPRG4, BSM, anti-CD44, anti-TLR2, anti-TLR4 and isotype control antibodies. Rhodamine-labeled rhPRG4 was incubated with murine macrophages and receptor colocalization studies were performed. Lewis rats underwent intra-articular injection of MSU crystals followed by intra-articular treatment with PBS or rhPRG4. Weight bearing and SF myeloperoxidase activities were determined.

Results

rhPRG4 reduced MSU crystal phagocytosis at 4 h (p < 0.01) and IL-1β, TNF-α, IL-8 and MCP-1 expression and production at 6 h (p < 0.05). BSM did not alter MSU phagocytosis or IL-1β production in human and murine macrophages. rhPRG4 treatment reduced NFκB nuclear translocation, NLRP3 expression, caspase-1 activation and generation of mature IL-1β (p < 0.05). MSU-stimulated IL-1β production was higher in Prg4^−/− macrophages compared to Prg4^+/+ macrophages (p < 0.001). rhPRG4, anti-CD44, anti-TLR2 and anti-TLR4 antibody treatments reduced MSU phagocytosis and IL-1β production in murine macrophages (p < 0.05). rhPRG4 preferentially colocalized with CD44 on Prg4^−/− peritoneal macrophages compared to TLR2 or TLR4 (p < 0.01). rhPRG4 normalized weight bearing and reduced SF myeloperoxidase activity compared to PBS in vivo.

Conclusion

rhPRG4 inhibits MSU crystal phagocytosis and exhibits an anti-inflammatory and anti-nociceptive activity in vitro and in vivo. rhPRG4’s anti-inflammatory mechanism may be due to targeting CD44 on macrophages.

Molecularly Engineered Biolubricants for Articular Cartilage

Lubrication within articular joints plays a crucial role in daily life, providing an extremely low coefficient of friction and preventing wear at the surface of the articular cartilage. Natural biomacromolecules responsible for lubrication are part of the synovial fluid and their degradation is associated with the onset of degenerative diseases, such as osteoarthritis (OA). The current absence of effective treatments for OA has captured the attention of chemists and material scientists over the last two decades, triggering the development of partially or fully synthetic biolubricants aimed to reduce friction within the joints and restore cartilage functions. Although there is still a long way to go before synthetic replacements of natural biolubricants can be applied clinically, this review highlights those formulations that meet the fundamental requirements for being efficient lubricants for articular cartilage.

Physiology and Engineering of the Graded Interfaces of Musculoskeletal Junctions

The connective tissues of the musculoskeletal system can be grouped into fibrous, cartilaginous, and calcified tissues. While each tissue type has a distinct composition and function, the intersections between these tissues result in the formation of complex, composite, and graded junctions. The complexity of these interfaces is a critical aspect of their healthy function, but poses a significant challenge for their repair. In this review, we describe the organization and structure of complex musculoskeletal interfaces, identify emerging technologies for engineering such structures, and outline the requirements for assessing the complex nature of these tissues in the context of recapitulating their function through tissue engineering.

Degradation alters the lubrication of articular cartilage by high viscosity, hyaluronic acid-based lubricants

Hyaluronic acid (HA) is widely injected as a viscosupplement in the treatment of osteoarthritis. Despite its extensive use, it is not currently known if cartilage degradation alters how HA-based solutions lubricate the articular surface. In this study we utilized a model of cartilage degradation by IL-1β along with a recently developed framework to study role of cartilage degradation on lubrication by clinically-approved HA-based lubricants with high viscosities. Cartilage explants were cultured up to 8 days with 10 ng/ml IL-1β. After culture, samples were examined histologically, immunohistochemically, biochemically, mechanically, topographically, and tribologically. The tribological testing analyzed both boundary and mixed lubrication modes to assess individual effects of viscosity and boundary lubricating ability. Friction testing was carried out using PBS and two clinically approved HA-based viscosupplements in a cartilage-glass configuration. After culture with IL-1β, boundary mode friction was elevated after both 4 and 8 days. Additionally, friction in mixed mode lubrication, where HA is most effective as a lubricant, was significantly elevated after 8 days of culture. As cartilage became rougher, softer, and more permeable after culture, the boundary mode plateau was extended, and as a result, significantly increased lubricant viscosities or sliding speeds were necessary to achieve effective mixed lubrication. Overall, this study revealed that lubrication of cartilage by HA is degradation-dependent and coincides with changes in mechanics and roughness. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1456-1464, 2018.

Lubrication of Articular Cartilage

The major synovial joints such as hips and knees are uniquely efficient tribological systems, able to articulate over a wide range of shear rates with a friction coefficient between the sliding cartilage surfaces as low as 0.001 up to pressures of more than 100 atm. No human-made material can match this. The means by which such surfaces maintain their very low friction has been intensively studied for decades and has been attributed to fluid-film and boundary lubrication. Here, we focus especially on the latter: the reduction of friction by molecular layers at the sliding cartilage surfaces. In particular, we discuss such lubrication in the light of very recent advances in our understanding of boundary effects in aqueous media based on the paradigms of hydration lubrication and of the synergism between different molecular components of the synovial joints (namely hyaluronan, lubricin, and phospholipids) in enabling this lubrication.

Lubricin binds cartilage proteins, cartilage oligomeric matrix protein, fibronectin and collagen II at the cartilage surface

Lubricin, a heavily O-glycosylated protein, is essential for boundary lubrication of articular cartilage. Strong surface adherence of lubricin is required given the extreme force it must withstand. Disulfide bound complexes of lubricin and cartilage oligomeric matrix protein (COMP) have recently been identified in arthritic synovial fluid suggesting they may be lost from the cartilage surface in osteoarthritis and inflammatory arthritis. This investigation was undertaken to localise COMP-lubricin complexes within cartilage and investigate if other cartilage proteins are involved in anchoring lubricin to the joint. Immunohistochemical analysis of human cartilage biopsies showed lubricin and COMP co-localise to the cartilage surface. COMP knockout mice, however, presented with a lubricin layer on the articular cartilage leading to the further investigation of additional lubricin binding mechanisms. Proximity ligation assays (PLA) on human cartilage biopsies was used to localise additional lubricin binding partners and demonstrated that lubricin bound COMP, but also fibronectin and collagen II on the cartilage surface. Fibronectin and collagen II binding to lubricin was confirmed and characterised by solid phase binding assays with recombinant lubricin fragments. Overall, COMP, fibronectin and collagen II bind lubricin, exposed on the articular cartilage surface suggesting they may be involved in maintaining essential boundary lubrication.

Do model polymer therapeutics sufficiently diffuse through articular cartilage to be a viable therapeutic route?

The ability of a polymer therapeutic to access the appropriate subcellular location is crucial to its efficacy and is defined to a large part by the many and complex cellular biological and biochemical barriers such that a construct must traverse. It is shown here that model dextrin conjugates are able to pass through a cartilaginous extracellular matrix into chondrocytes, with little perturbation of the matrix structure, indicating that targeting of potential therapeutics through a cartilaginous extracellular matrix should be proven possible. Rapid chondrocytic targeting of drugs which require intra cellularisation for their activity and uniform extracellular concentrations of drugs with an extracellular target, is thus enabled though polymer conjugation.

Cartilage oligomeric matrix protein forms protein complexes with synovial lubricin via non-covalent and covalent interactions

OBJECTIVE

Understanding the cartilage surface structure, lost in arthritic disease, is essential for developing strategies to effectively restore it. Given that adherence of the lubricating protein, lubricin, to the cartilage surface is critical for boundary lubrication, an interaction with cartilage oligomeric matrix protein (COMP) was investigated. COMP, an abundant cartilage protein, is known to be important for matrix formation.

DESIGN

Synovial fluid (SF) from arthritic patients was used to detect possible COMP-lubricin complexes by immunological methods. Recombinant (RC) COMP and lubricin fragments were expressed to characterize this bonding and mass spectrometry employed to specifically identify the cysteines involved in inter-protein disulfide bonds.

RESULTS

COMP-lubricin complexes were identified in the SF of arthritic patients by Western blot, co-immunoprecipitation and sandwich ELISA. RC fragment solid-phase binding assays showed that the C-terminal (amino acids (AA) 518-757) of COMP bound non-covalently to the N-terminal of lubricin (AA 105-202). Mass spectrometry determined that although cysteines throughout COMP were involved in binding with lubricin, the cysteines in lubricin were primarily focused to an N-terminal region (AA 64-86). The close proximity of the non-covalent and disulfide binding domains on lubricin suggest a two-step mechanism to strongly bind lubricin to COMP.

CONCLUSION

These data demonstrate that lubricin forms a complex network with COMP involving both non-covalent and covalent bonds. This complex between lubricin and the cartilage protein COMP can be identified in the SF of patients with arthritis conditions including osteoarthritis (OA) and rheumatoid arthritis (RA).

Synergistic Interactions of a Synthetic Lubricin-Mimetic with Fibronectin for Enhanced Wear Protection

Lubricin (LUB), a major mucinous glycoprotein of mammalian synovial fluids, is believed to provide excellent lubrication to cartilage surfaces. Consequently, when joint disease or replacement leads to increased friction and surface damage in the joint, robust synthetic LUB alternatives that could be used therapeutically to improve lubrication and surface protection are needed. Here, we report the characterization of a lubricating multiblock bottlebrush polymer whose architecture was inspired by LUB, and we investigate the role of fibronectin (FN), a glycoprotein found in the superficial zone of cartilage, in mediating the tribological properties of the polymer upon shear between mica surfaces. Our surface forces apparatus (SFA) normal force measurements indicate that the lubricin-mimetic (mimLUB) could be kept anchored between mica surfaces, even under high contact pressures, when an intermediate layer of FN was present. Additional SFA friction measurements show that FN would also extend the wearless friction regime of the polymer up to pressures of 3.4 MPa while ensuring stable friction coefficients (μ ≈ 0.28). These results demonstrate synergistic interactions between mimLUB and FN in assisting the lubrication and wear protection of ideal (mica) substrates upon shear. Collectively, these findings suggest that our proposed mimLUB might be a promising alternative to LUB, as similar mechanisms could potentially facilitate the interaction between the polymer and cartilage surfaces in articular joints and prosthetic implants in vivo.

Nanoassemblies of Tissue-Reactive, Polyoxazoline Graft-Copolymers Restore the Lubrication Properties of Degraded Cartilage

Osteoarthritis leads to an alteration in the composition of the synovial fluid, which is associated with an increase in friction and the progressive and irreversible destruction of the articular cartilage. In order to tackle this degenerative disease, there has been a growing interest in the medical field to establish effective, long-term treatments to restore cartilage lubrication after damage. Here we develop a series of graft-copolymers capable of assembling selectively on the degraded cartilage, resurfacing it, and restoring the lubricating properties of the native tissue. These comprise a polyglutamic acid backbone (PGA) coupled to brush-forming, poly-2-methyl-2-oxazoline (PMOXA) side chains, which provide biopassivity and lubricity to the surface, and to aldehyde-bearing tissue-reactive groups, for the anchoring on the degenerated cartilage via Schiff bases. Optimization of the graft-copolymer architecture (i.e., density and length of side chains and amount of tissue-reactive functions) allowed a uniform passivation of the degraded cartilage surface. Graft-copolymer-treated cartilage showed very low coefficients of friction within synovial fluid, reestablishing and in some cases improving the lubricating properties of the natural cartilage. Due to these distinctive properties and their high biocompatibility and stability under physiological conditions, cartilage-reactive graft-copolymers emerge as promising injectable formulations to slow down the progression of cartilage degradation, which characterizes the early stages of osteoarthritis.

Structure and Property Changes in Self-Assembled Lubricin Layers Induced by Calcium Ion Interactions

Lubricin (LUB) is a "mucin-like" glycoprotein found in synovial fluids and coating the cartilage surfaces of articular joints, which is now generally accepted as one of the body's primary boundary lubricants and antiadhesive agents. LUB's superior lubrication and antiadhesion are believed to derive from its unique interfacial properties by which LUB molecules adhere to surfaces (and biomolecules, such as hyaluronic acid and collagen) through discrete interactions localized to its two terminal end domains. These regionally specific interactions lead to self-assembly behavior and the formation of a well-ordered "telechelic" polymer brush structure on most substrates. Despite its importance to biological lubrication, detailed knowledge on the LUB's self-assembled brush structure is insufficient and derived mostly from indirect and circumstantial evidence. Neutron reflectometry (NR) was used to directly probe the self-assembled LUB layers, confirming the polymer brush architecture and resolving the degree of hydration and level of surface coverage. While attempting to improve the LUB contrast in the NR measurements, the LUB layers were exposed to a 20 mM solution of CaCl₂, which resulted in a significant change in the polymer brush structural parameters consisting of a partial denaturation of the surface-binding end-domain regions, partial dehydration of the internal mucin-domain "loop", and collapse of the outer mucin-domain surface region. A series of atomic force microscopy measurements investigating the LUB layer surface morphology, mechanical properties, and adhesion forces in phosphate-buffered saline and CaCl₂ solutions reveal that the structural changes induced by calcium ion interactions also significantly alter key properties, which may have implications to LUB's efficacy as a boundary lubricant and wear protector in the presence of elevated calcium ion concentrations.

Implications of trauma and subsequent articulation on the release of Proteoglycan-4 and tissue response in adult human ankle cartilage

The purpose of this study was to investigate the effects of trauma and subsequent articulation on adult human ankle cartilage subjected to an injurious impact. Trauma was initiated through impaction on talar cartilage explants. Articulation and loading were applied in a joint bioreactor over 5 consecutive days. The early (24 h) effects of impaction included a reduced chondrocytes viability (51% vs. 81% for non-impacted; p = 0.03), increased levels of apoptosis (43% vs. 27%; p = 0.03), and an increase in the histopathology score (4.4 vs. 1.7; p = 0.02) as compared to non-impacted cartilage explants. One of the key findings was that damage also stimulated the PRG4 release (2.2 vs. 1.5 μg/ml). Subsequent articulation for 5 days did not lead to further changes in tissue histopathology and cell viability, neither for injured nor non-injured samples. However, articulation led to an increased apoptosis in the injured samples (p = 0.03 for the interaction term). Articulation also caused a significant increase of PG/GAG release into the culture medium (p = 0.04) for both injured and non-injured samples; however, the synthesis of PG was not affected by articulation (p = 0.45) though the PG synthesis was higher in injured samples (p < 0.01). With regard to the PRG4 release, impacted samples continued to show higher amounts (p = 0.01), adding articulation led to a reduction (p = 0.02). The current study demonstrated that adult human talar cartilage increases both the PRG4 release and biosynthetic activity as an immediate cellular response to injury. Articulation played a less contributing role to biosynthesis and remodeling, behaving mostly neutral, in that no further damage emerged. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:667-676, 2017.

Synergies in lubrication

In living organisms the aqueous medium is used for providing low friction forces. This is achieved by synergistic actions of different biomolecules that together accomplish a high load bearing capacity and sustain an easily sheared water layer.

Superficial Zone Extracellular Matrix Extracts Enhance Boundary Lubrication of Self-Assembled Articular Cartilage

OBJECTIVE

Previous work has shown that increasing the production of boundary lubricant, superficial zone protein (SZP), did not reduce the friction coefficient of self-assembled articular cartilage constructs and was possibly due to poor retention of the lubricant. The aim of this investigation was to reduce the friction coefficient of self-assembled articular cartilage constructs through enhancing SZP retention by the exogenous addition of extracellular matrix (ECM) extracted from the superficial zone of native articular cartilage.

DESIGN

Superficial zone cartilage was shaved from juvenile bovine femoral condyles using a dermatome, minced finely with razor blades, extracted with 4 M guanidine-hydrochloride, buffer exchanged with culture medium, and added directly to the culture medium of self-assembled articular cartilage constructs at low (10 µg/mL) and high (100 µg/mL) concentrations for 4 weeks. Biochemical and biomechanical properties were determined at the conclusion of 4 weeks culture.

RESULTS

ECM treatment increased compressive and tensile stiffness of self-assembled articular cartilage constructs and decreased the friction coefficient. Glycosaminoglycan content decreased and collagen content increased significantly in self-assembled constructs by the ECM treatment.

CONCLUSIONS

Friction coefficients of self-assembled articular cartilage constructs were reduced by adding extracted superficial zone ECM into the culture medium of self-assembled articular cartilage constructs.

Galectin-3 Binds to Lubricin and Reinforces the Lubricating Boundary Layer of Articular Cartilage

Lubricin is a mucinous, synovial fluid glycoprotein that enables near frictionless joint motion via adsorption to the surface of articular cartilage and its lubricating properties in solution. Extensive O-linked glycosylation within lubricin’s mucin-rich domain is critical for its boundary lubricating function; however, it is unknown exactly how glycosylation facilitates cartilage lubrication. Here, we find that the lubricin glycome is enriched with terminal β-galactosides, known binding partners for a family of multivalent lectins called galectins. Of the galectin family members present in synovial fluid, we find that galectin-3 is a specific, high-affinity binding partner for lubricin. Considering the known ability of galectin-3 to crosslink glycoproteins, we hypothesized that galectins could augment lubrication via biomechanical stabilization of the lubricin boundary layer. We find that competitive inhibition of galectin binding results in lubricin loss from the cartilage surface, and addition of multimeric galectin-3 enhances cartilage lubrication. We also find that galectin-3 has low affinity for the surface layer of osteoarthritic cartilage and has reduced affinity for sialylated O-glycans, a glycophenotype associated with inflammatory conditions. Together, our results suggest that galectin-3 reinforces the lubricin boundary layer; which, in turn, enhances cartilage lubrication and may delay the onset and progression of arthritis.

Comparative analysis of gene expression profiles in normal hip human cartilage and cartilage from patients with necrosis of the femoral head

Background

The pathogenesis of necrosis of the femoral head (NFH) remains elusive. Limited studies were conducted to investigate the molecular mechanism of hip articular cartilage damage in NFH. We conducted genome-wide gene expression profiling of hip articular cartilage with NFH.

Methods

Hip articular cartilage specimens were collected from 18 NFH patients and 18 healthy controls. Gene expression profiling of NFH articular cartilage was carried out by Agilent Human 4x44K Gene Expression Microarray chip. Differently expressed genes were identified using the significance analysis of microarrays (SAM) software. Gene Ontology (GO) enrichment analysis of differently expressed genes was performed using the Database for Annotation, Visualization and Integrated Discovery (DAVID). Significantly differently expressed genes in the microarray experiment were selected for quantitative real-time PCR (qRT-PCR) and immunohistochemical validation.

Results

SAM identified 27 differently expressed genes in NFH articular cartilage, functionally involved in extracellular matrix, cytokines, growth factors, cell cycle and apoptosis. The expression patterns of the nine validation genes in qRT-PCR were consistent with that in proteinaceous extracellular matrix (false discovery rate (FDR) = 3.22 × 10^-5), extracellular matrix (FDR = 5.78 × 10^-5), extracellular region part (FDR = 1.28 × 10^-4), collagen (FDR = 3.22 × 10^-4), extracellular region (FDR = 4.78 × 10^-4) and platelet-derived growth factor binding (FDR = 5.23 × 10^-4).

Conclusions

This study identified a set of differently expressed genes, implicated in articular cartilage damage in NFH. Our study results may provide novel insight into the pathogenesis and rationale of therapies for NFH.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-016-0991-4) 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.

Structure of DPPC-hyaluronan interfacial layers - effects of molecular weight and ion composition

Hyaluronan and phospholipids play an important role in lubrication in articular joints and provide in combination with glycoproteins exceptionally low friction coefficients. We have investigated the structural organization of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) Langmuir layers at the solution-air interface at different length scales with respect to the adsorption of hyaluronan (HA). This allows us to assemble a comprehensive picture of the adsorption and the resulting structures, and how they are affected by the molecular weight of HA and the presence of calcium ions. Brewster angle microscopy and grazing incident diffraction were used to determine the lateral structure at the micro- and macro scale. The data reveals an influence of HA on both the macro and micro structure of the DPPC Langmuir layer, and that the strength of this effect increases with decreasing molecular weight of HA and in presence of calcium ions. Furthermore, from X-ray reflectivity measurements we conclude that HA adsorbs to the hydrophilic part of DPPC, but data also suggest that two types of interfacial structures are formed at the interface. We argue that hydrophobic forces and electrostatic interactions play important rules for the association between DPPC and HA. Surface pressure area isotherms were used to determine the influence of HA on the phase behavior of DPPC while electrophoretic mobility measurements were used to gain insight into the binding of calcium ions to DPPC vesicles and hyaluronan.

The interaction of lubricin/proteoglycan 4 (PRG4) with toll-like receptors 2 and 4: an anti-inflammatory role of PRG4 in synovial fluid

Background

Lubricin/proteoglycan-4 (PRG4) is a mucinous glycoprotein secreted by synovial fibroblasts and superficial zone chondrocytes. PRG4 has a homeostatic multifaceted role in the joint. PRG4 intra-articular treatment retards progression of cartilage degeneration in pre-clinical posttraumatic osteoarthritis models. The objective of this study is to evaluate the binding of recombinant human PRG4 (rhPRG4) and native human PRG4 (nhPRG4) to toll-like receptors 2 and 4 (TLR2 and TLR4) and whether this interaction underpins a PRG4 anti-inflammatory role in synovial fluid (SF) from patients with osteoarthritis (OA) and rheumatoid arthritis (RA).

Methods

rhPRG4 and nhPRG4 binding to TLR2 and TLR4 was evaluated using a direct enzyme linked immunosorbent assay (ELISA). Association of rhPRG4 with TLR2 and TLR4 overexpressing human embryonic kidney (HEK) cells was studied by flow cytometry. Activation of TLR2 and TLR4 on HEK cells by agonists Pam3CSK4 and lipopolysaccharide (LPS) was studied in the absence or presence of nhPRG4 at 50, 100 and 150 μg/ml. Activation of TLR2 and TLR4 by OA SF and RA SF and the effect of nhPRG4 SF treatment on receptor activation was assessed. PRG4 was immunoprecipitated from pooled OA and RA SF. TLR2 and TLR4 activation by pooled OA and RA SF with or without PRG4 immunoprecipitation was compared.

Results

rhPRG4 and nhPRG4 exhibited concentration-dependent binding to TLR2 and TLR4. rhPRG4 associated with TLR2- and TLR4-HEK cells in a time-dependent manner. Co-incubation of nhPRG4 (50, 100 and 150 μg/ml) and Pam3CSK4 or LPS reduced TLR2 or TLR4 activation compared to Pam3CSK4 or LPS alone (p <0.05). OA SF and RA SF activated TLR2 and TLR4 and nhPRG4 treatment reduced SF-induced receptor activation (p <0.001). PRG4 depletion by immunoprecipitation significantly increased TLR2 activation by OA SF and RA SF (p <0.001).

Conclusion

PRG4 binds to TLR2 and TLR4 and this binding mediates a novel anti-inflammatory role for PRG4.

Elastoviscous Transitions of Articular Cartilage Reveal a Mechanism of Synergy between Lubricin and Hyaluronic Acid

When lubricated by synovial fluid, articular cartilage provides some of the lowest friction coefficients found in nature. While it is known that macromolecular constituents of synovial fluid provide it with its lubricating ability, it is not fully understood how two of the main molecules, lubricin and hyaluronic acid, lubricate and interact with one another. Here, we develop a novel framework for cartilage lubrication based on the elastoviscous transition to show that lubricin and hyaluronic acid lubricate by distinct mechanisms. Such analysis revealed nonspecific interactions between these molecules in which lubricin acts to concentrate hyaluronic acid near the tissue surface and promotes a transition to a low friction regime consistent with the theory of viscous boundary lubrication. Understanding the mechanics of synovial fluid not only provides insight into the progression of diseases such as arthritis, but also may be applicable to the development of new biomimetic lubricants.

Surface zone articular chondrocytes modulate the bulk and surface mechanical properties of the tissue-engineered cartilage

The central hypothesis of functional tissue engineering is that an engineered construct can serve as a viable replacement tissue in vivo by replicating the structure and function of native tissue. In the case of articular cartilage, this requires the reproduction of the bulk mechanical and surface lubrication properties of native hyaline cartilage. Cartilage tissue engineering has primarily focused on achieving the bulk mechanical properties of native cartilage such as the compressive aggregate modulus and tensile strength. A scaffold-free self-assembling process has been developed that produces engineered cartilage with compressive properties approaching native tissue levels. Thus, the next step in this process is to begin addressing the friction coefficient and wear properties of these engineered constructs. The superficial zone protein (SZP), also known as lubricin or PRG4, is a boundary mode lubricant that is synthesized by surface zone (SZ) articular chondrocytes. Under conditions of high loading and low sliding speeds, SZP reduces friction and wear at the articular surface. The objective of this investigation was to determine whether increasing the proportion of SZ chondrocytes in cartilage constructs, in the absence of external stimuli such as growth factors and mechanical loading, would enhance the secretion of SZP and improve their frictional properties. In this study, cartilage constructs were engineered through a self-assembling process with varying ratios of SZ and middle zone (MZ) chondrocytes (SZ:MZ): 0:100, 25:75, 50:50, 75:25, and 100:0. Constructs containing different ratios of SZ and MZ chondrocytes did not significantly differ in the glycosaminoglycan composition or compressive aggregate modulus. In contrast, tensile properties and collagen content were enhanced in nearly all constructs containing greater amounts of SZ chondrocytes. Increasing the proportion of SZ chondrocytes had the hypothesized effect of improving the synthesis and secretion of SZP. However, increasing the SZ chondrocyte fraction did not significantly reduce the friction coefficient. These results demonstrate that additional factors, such as SZP-binding macromolecules, surface roughness, and adhesion, need to be examined to modulate the lubrication properties of engineered cartilage.