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

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.

Dissolvable Immunomodulatory Microneedles for Treatment of Skin Wounds

Sustained inflammation can halt or delay wound healing, and macrophages play a central role in wound healing. Inflammatory macrophages are responsible for the removal of pathogens, debris, and neutrophils, while anti-inflammatory macrophages stimulate various regenerative processes. Recombinant human Proteoglycan 4 (rhPRG4) is shown to modulate macrophage polarization and to prevent fibrosis and scarring in ear wound healing. Here, dissolvable microneedle arrays (MNAs) carrying rhPRG4 are engineered for the treatment of skin wounds. The in vitro experiments suggest that rhPRG4 modulates the inflammatory function of bone marrow-derived macrophages. Degradable and detachable microneedles are developed from gelatin methacryloyl (GelMA) attach to a dissolvable gelatin backing. The developed MNAs are able to deliver a high dose of rhPRG4 through the dissolution of the gelatin backing post-injury, while the GelMA microneedles sustain rhPRG4 bioavailability over the course of treatment. In vivo results in a murine model of full-thickness wounds with impaired healing confirm a decrease in inflammatory biomarkers such as TNF-α and IL-6, and an increase in angiogenesis and collagen deposition. Collectively, these results demonstrate rhPRG4-incorporating MNA is a promising platform in skin wound healing applications.

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.

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.

Proteomics Analysis of Tears and Saliva From Sjogren's Syndrome Patients

Sjogren's syndrome (SS) is characterized by dysfunctional mucous membranes and dysregulated moisture-secreting glands resulting in various symptoms, including dry mouth and dry eyes. Here, we wanted to profile and compare the tear and saliva proteomes of SS patients to healthy controls. Tear and saliva samples were collected and subjected to an isotopic dimethylation labeling shotgun proteomics workflow to identify alterations in protein levels. In tear samples, we identified 83 upregulated and 112 downregulated proteins. Pathway enrichment analysis of the changing proteins by Metascape identified leukocyte transendothelial migration, neutrophil degranulation, and post-translation protein phosphorylation in tears of SS patients. In healthy controls' tears, an enrichment for proteins related to glycolysis, amino acid metabolism and apoptotic signaling pathway were identified. In saliva, we identified 108 upregulated and 45 downregulated proteins. Altered pathways in SS patients' saliva included cornification, sensory perception to taste and neutrophil degranulation. In healthy controls' saliva, an enrichment for proteins related to JAK-STAT signaling after interleukin-12 stimulation, phagocytosis and glycolysis in senescence were identified. Dysregulated protease activity is implicated in the initiation of inflammation and immune cell recruitment in SS. We identified 20 proteases and protease inhibitors in tears and 18 in saliva which are differentially expressed between SS patients and healthy controls. Next, we quantified endogenous proteoglycan 4 (PRG4), a mucin-like glycoprotein, in tear wash and saliva samples via a bead-based immune assay. We identified decreased levels of PRG4 in SS patients' tear wash compared to normal samples. Conversely, in saliva, we found elevated levels of PRG4 concentration and visualized PRG4 expression in human parotid gland via immunohistological staining. These findings will improve our mechanistic understanding of the disease and changes in SS patients' protein expression will help identify new potential drug targets. PRG4 is among the promising targets, which we identified here, in saliva, for the first time.

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.

A histological study of the medial meniscus posterior root tibial insertion

Purpose/Aim of the study: Posterior root injury of the medial meniscus often leads to articular cartilage degeneration due to altered biomechanics. To avoid dysfunction, the attachment must be repaired using the transtibial pullout technique. To guide appropriate placement of the tibial tunnel, additional details on the normal anatomy of the meniscus insertion are needed. Therefore, we performed a histological analysis of a tibial bone slice with the medial meniscus posterior insertion obtained during total knee arthroplasty surgery. Materials and methods: Horizontal slices of the proximal tibia were obtained from 7 patients with osteoarthritis who underwent total knee arthroplasty. After decalcification, the region of the posterior horn was cut out and segmented into four pieces (2.0 mm thickness; medial to lateral). Sagittal sections were evaluated by safranin O staining or immunohistochemistry with anti-type collagen antibody. Results: Safranin O staining showed that the insertion of the posterior root consisted primarily of fibrocartilaginous layers in segment 2. Anatomically, segment 2 corresponded to the sagittal plane passing through the peak of the medial intercondylar tubercle. In this section, safranin O staining and immunohistochemistry revealed that the anterior one-third of the posterior root insertion was richer in proteoglycans and type II collagen than the central and posterior one-third. Conclusions: Anatomical insertion of the posterior root of the medial meniscus was located at the sagittal plane passing through the peak of the medial intercondylar tubercle. The structure of the medial meniscus posterior insertion was mainly localized in the anterior one-third.

Recombinant human PRG4 (rhPRG4) suppresses breast cancer cell invasion by inhibiting TGFβ-Hyaluronan-CD44 signalling pathway

Metastasis is the major cause of cancer-related morbidity and mortality. The ability of cancer cells to become invasive and migratory contribute significantly to metastatic growth, which necessitates the identification of novel anti-migratory and anti-invasive therapeutic approaches. Proteoglycan 4 (PRG4), a mucin-like glycoprotein, contributes to joint synovial homeostasis through its friction-reducing and anti-adhesive properties. Adhesion to surrounding extracellular matrix (ECM) components is critical for cancer cells to invade the ECM and eventually become metastatic, raising the question whether PRG4 has an anti-invasive effect on cancer cells. Here, we report that a full-length recombinant human PRG4 (rhPRG4) suppresses the ability of the secreted protein transforming growth factor beta (TGFβ) to induce phenotypic disruption of three-dimensional human breast cancer cell-derived organoids by reducing ligand-induced cell invasion. In mechanistic studies, we find that rhPRG4 suppresses TGFβ-induced invasiveness of cancer cells by inhibiting the downstream hyaluronan (HA)-cell surface cluster of differentiation 44 (CD44) signalling axis. Furthermore, we find that rhPRG4 represses TGFβ-dependent increase in the protein abundance of CD44 and of the enzyme HAS2, which is involved in HA biosynthesis. It is widely accepted that TGFβ has both tumor suppressing and tumor promoting roles in cancer. The novel finding that rhPRG4 opposes HAS2 and CD44 induction by TGFβ has implications for downregulating the tumor promoting roles, while maintaining the tumor suppressive aspects of TGFβ actions. Finally, these findings point to rhPRG4's potential clinical utility as a therapeutic treatment for invasive and metastatic breast cancer.

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.

Probing the Molecular Interactions and Lubrication Mechanisms of Purified Full-Length Recombinant Human Proteoglycan 4 (rhPRG4) and Hyaluronic Acid (HA)

Probing the adsorption and lubrication behavior of lubricin, also known as proteoglycan 4 (PRG4), is important for understanding the ultralow friction of cartilage lubrication. Most previous research has focused on native lubricin either purified from synovial fluid or articular cartilage explant culture media. In this work, the adsorption behavior and lubrication mechanism of full-length recombinant human PRG4 (rhPRG4) on mica as well as the effect of adding hyaluronic acid (HA, a polysaccharide) were systematically investigated using a surface forces apparatus (SFA) technique. A low friction coefficient (μ ∼ 0.04) was measured when multilayer rhPRG4 (∼31 nm) was confined in between mica surfaces, even when the load increased to ∼1.2 MPa. Intriguingly, a previously unreported ultralow friction coefficient (μ < 0.005) was observed at a low sliding velocity ( v = 0.14 μm/s) with the applied load P reaching ∼3.6 MPa when a diluted rhPRG4 solution (∼90 μg/mL) was used. The distinct friction behavior is likely due to the smooth and more close-packed lubricin coating, as made evident by the atomic force microscope imaging. Adding HA onto multilayer rhPRG4-coated mica increased the friction coefficient μ to ∼0.1; however, the load bearing property increased, indicating potential synergistic effect between rhPRG4 and HA, which was further demonstrated by the weak adhesion observed when separating rhPRG4-coated mica and HA-coated aminopropyltriethoxysilane-mica (APTES-mica). Alternatively, adding premixed rhPRG4-HA on mica had a friction coefficient (μ ∼ 0.1) close to that of injecting concentrated rhPRG4 (∼450 μg/mL) with lower load sustainability. Our results provide fundamental insights into the adsorption and lubrication behavior of lubricin and its interaction with HA, with useful implications for the underlying mechanism of ultralow friction provided by synovial fluid.

Lubricin/proteoglycan 4 detected in vocal folds of humans and five other mammals

OBJECTIVES/HYPOTHESIS

Lubricin/proteoglycan-4 (PRG4) lubricates connective tissues such as joints and tendon sheaths, enabling them to better withstand shearing and frictional forces during motion. We wondered whether PRG4 might play a role in phonation, as normal vocal folds withstand repetitive, high-velocity deformations remarkably well. As a first step, we tested whether PRG4 is expressed in vocal folds.

STUDY DESIGN

Laboratory study.

METHODS

Anatomical and molecular methods were applied to 47 larynges from humans, macaque (Macaca fascicularis), canines, pigs, calves, and rats. Immunohistochemistry (IHC), Western blot, and quantitative real-time polymerase chain reaction (qRT-PCR) methods were used to test for the presence of PRG4.

RESULTS

In all species, the true vocal fold lamina propria (TVF-LP) was positive for PRG4 by IHC, whereas immunoreactivity of the false vocal fold was weak or absent, depending on the species. Human TVF-LP was strongly stained across all layers. Immunoreactivity was seen variably on the vocal fold surface and within the vocal fold epithelium, in the conus elasticus and thyroglottic ligament, and at the tip of vocal process. Western blots of four humans and six pigs demonstrated immunoreactivity at appropriate molecular weight. qRT-PCR of pig tissues confirmed PRG4 mRNA expression, which was highest in the TVF-LP.

CONCLUSIONS

PRG4 was found in phonatory tissues of six mammals. We suggest it might act as a lubricant within the lamina propria and possibly on the vocal fold surface, limiting phonation-related damage to vocal fold extracellular matrix and epithelium, and enhancing vocal efficiency by reducing internal friction (viscosity) within the vocal fold.

LEVEL OF EVIDENCE

NA Laryngoscope, 129:E229-E237, 2019.

Cartilage boundary lubrication synergism is mediated by hyaluronan concentration and PRG4 concentration and structure

Background

Proteoglycan 4 (PRG4) and hyaluronan (HA) are key synovial fluid constituents that contribute synergistically to cartilage boundary lubrication; however, the effects of their concentrations as well as their structure, both of which can be altered in osteoarthritis, on this functional synergism are unknown. The objectives of this study were to evaluate cartilage boundary lubricating ability of 1) PRG4 + HA in solution at constant HA concentration in a range of PRG4 concentrations, 2) constant PRG4 concentration in a range of HA concentrations, 3) HA + reduced/alkylated (R/A) PRG4, and 4) hylan G-F 20 + PRG4.

Methods

Static and kinetic friction coefficients (μ_static,Neq, <μ_kinetic,Neq>) were measured using a previously characterized cartilage-cartilage boundary mode friction test for the following concentrations of purified PRG4 and HA: Test 1: HA (1.5 MDa, 3.3 mg/mL) + PRG4 from 4.5 – 1500 μg/mL; Test 2: PRG4 (450, 150, 45 μg/mL) + HA (1.5 MDa) from 0.3 – 3.3 mg/mL. Test 3: hylan G-F 20 (3. 3 mg/mL) + PRG4 (450 μg/mL). Test 4: HA (3.3 mg/mL) + R/A PRG4 (450 μg/mL). ANOVA was used to compare lubricants within (comparing 6 lubricants of interest) and between (comparing 3 lubricants of interest) test sequences, with Tukey and Fishers post-hoc testing respectively.

Results

This study demonstrates that both PRG4 and HA concentration, as well as PRG4 disulfide-bonded structure, can alter the cartilage boundary lubricating ability of PRG4 + HA solutions. The boundary lubricating ability of high MW HA + PRG4 solutions was limited by very low concentrations of PRG4. Decreased concentrations of high MW HA also limited the cartilage boundary lubricating ability of HA + PRG4 solutions, with the effect exacerbated by low PRG4 concentrations. The reduction of friction by addition of PRG4 to a cross-linked HA viscosupplement product, but not with addition of R/A PRG4 to HA, is consistent with a non-covalent mechanism of interaction where tertiary and quaternary PRG4 structure are important.

Conclusions

Collectively, these results demonstrate that deficiency of either or both PRG4 and HA, or alterations in PRG4 structure, may be detrimental to SF cartilage boundary lubricating function. This study provides further insight into the nature of cartilage boundary lubrication and advancement towards potential formulation of new intra-articular biotherapeutic treatments for osteoarthritis using PRG4 ± HA.