An official website of the United States government
ABSTRACT The articular cartilage extracellular matrix (ECM) is a complex network of biomolecules that includes fibronectin (FN). FN acts as an extracellular glue, controlling the assembly of other macromolecular constituents to the ECM. However, how FN participates in the binding and retention of synovial fluid components, the natural lubricant of articulated joints, to form a wear-protecting and lubricating film has not been established. This study reports on the role of FN and its molecular conformation in mediating macromolecular assembly of synovial fluid ad-layers. FN films as precursor films on functionalized surfaces, a model of FN’s articular cartilage surface, adsorbed and retained different amounts of synovial fluid (SF). FN conformational changes were induced by depositing FN at pH 7 (extended state) or at pH 4 (unfolded state) on self-assembled monolayers on gold-coated quartz crystals, followed by adsorption of diluted SF (25%) onto FN precursor films. Mass density, thin film compliance, surface morphologies, and adsorbed FN films’ secondary and tertiary structures reveal pH-induced differences. FN films deposited at pH 4 were thicker, more rigid, showed a more homogeneous morphology, and had alteredα-helix andβ-sheet content, compared to FN films deposited at pH 7. FN precursor films deposited at pH 7 adsorbed and retained more synovial fluid than those at pH 4, revealing the importance of FN conformation at the articular cartilage surface to bind and maintain a thin lubricating and wear protective layer of synovial fluid constituents. This knowledge will enable a better understanding of the molecular regulation of articular cartilage-SF interface homeostasis and joint pathophysiology and identify molecular interactions and synergies between the articular cartilage ECM and SF to reveal the complexity of joint biotribology.
more »
« less
Loss of effective lubricating viscosity is the primary mechanical marker of joint inflammation in equine synovitis
Abstract Inflammation of the synovium, known as synovitis, plays an important role in the pathogenesis of osteoarthritis (OA). Synovitis involves the release of a wide variety of pro‐inflammatory mediators in synovial fluid (SF) that damage the articular cartilage extracellular matrix and induce death and apoptosis in chondrocytes. The composition of synovial fluid is dramatically altered by inflammation in OA, with changes to both hyaluronic acid and lubricin, the primary lubricating molecules in SF. However, the relationship between key biochemical markers of joint inflammation and mechanical function of SF is not well understood. Here, we demonstrate the application of a novel analytical framework to measure the effective viscosity for SF lubrication of cartilage, which is distinct from conventional rheological viscosity. Notably, in a well‐established equine model of synovitis, this effective lubricating viscosity decreased by up to 10,000‐fold for synovitis SF compared to a ~4 fold change in conventional viscosity measurements. Further, the effective lubricating viscosity was strongly inversely correlated (r = −0.6 to −0.8) to multiple established biochemical markers of SF inflammation, including white blood cell count, prostaglandin E2(PGE2), and chemokine ligand (CCLs) concentrations, while conventional measurements of viscosity were poorly correlated to these markers. These findings demonstrate the importance of experimental and analytical approaches to characterize functional lubricating properties of synovial fluid and their relationships to soluble biomarkers to better understand the progression of OA.
more »
« less
- Award ID(s):
- 2245367
- PAR ID:
- 10571905
- Publisher / Repository:
- Wiley-Liss
- Date Published:
- Journal Name:
- Journal of Orthopaedic Research
- Volume:
- 42
- Issue:
- 7
- ISSN:
- 0736-0266
- Page Range / eLocation ID:
- 1438 to 1447
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract 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.more » « less
-
Abstract Intra‐articular injections of hyaluronic acid (HA) are the cornerstone of osteoarthritis (OA) treatments. However, the mechanism of action and efficacy of HA viscosupplementation are debated. As such, there has been recent interest in developing synthetic viscosupplements. Recently, a synthetic 4 wt% polyacrylamide (pAAm) hydrogel was shown to effectively lubricate and bind to the surface of cartilage in vitro. However, its ability to localize to cartilage and alter the tribological properties of the tissue in a live articulating large animal joint is not known. The goal of this study was to quantify the distribution and extent of localization of pAAm in the equine metacarpophalangeal or metatarsophalangeal joint (fetlock joint), and determine whether preferential localization of pAAm influences the tribological properties of the tissue. An established planar fluorescence imaging technique was used to visualize and quantify the distribution of fluorescently labeled pAAm within the joint. While the pAAm hydrogel was present on all surfaces, it was not uniformly distributed, with more material present near the site of the injection. The lubricating ability of the cartilage in the joint was then assessed using a custom tribometer across two orders of magnitude of sliding speed in healthy synovial fluid. Cartilage regions with a greater coverage of pAAm, that is, higher fluorescent intensities, exhibited friction coefficients nearly 2‐fold lower than regions with lesser pAAm (Rrm = −0.59,p < 0.001). Collectively, the findings from this study indicate that intra‐articular viscosupplement injections are not evenly distributed inside a joint, and the tribological outcomes of these materials is strongly determined by the ability of the material to localize to the articulating surfaces in the joint.more » « less
-
TGF-β is a prominent anabolic signaling molecule associated with synovial joint health. Recent work has uncovered mechanochemical mechanisms that activate the latent form of TGF-β (LTGF-β) in the synovial joint-synovial fluid (SF) shearing or cartilage compression-pointing to mechanobiological phenomena, whereby enhanced TGF-β activity occurs during joint stimulation. Here, we implement computational and experimental models to better understand the role of mechanochemical-activated TGF-β (aTGF-β) in regulating the functional biosynthetic activities of synovial joint tissues. Reaction-diffusion models describe the pronounced role of extracellular chemical reactions-load-induced activation, reversible ECM-binding, and cell-mediated internalization-in modulating the spatiotemporal distribution of aTGF-β in joint tissues. Of note, aTGF-β from SF shearing predominantly acts on cells in peripheral tissue regions (superficial zone [SZ] chondrocytes and synoviocytes) and aTGF-β from cartilage compression acts on chondrocytes through all cartilage layers. Further, ECM reversible binding sites in cartilage act to modulate the temporal delivery of aTGF-β to cells, creating a dynamic where short durations of joint activity give rise to extended periods of aTGF-β exposure at moderated doses. Ex vivo tissue models were subsequently utilized to characterize the influence of physiologic aTGF-β activity regimens in regulating functional biosynthetic activities. Physiologic exposure regimens of aTGF-β in SF induce strong 4-fold to 9-fold enhancements in the secretion rate of the synovial biolubricant, PRG4, from SZ cartilage and synovium explants. Further, aTGF-β inhibition in cartilage over 1-month culture leads to a pronounced loss of GAG content (30-35% decrease) and tissue softening (60-65% EY reduction). Overall, this work advances a novel perspective on the regulation of TGF-β in the synovial joint and its role in maintaining synovial joint health.more » « less
-
ABSTRACT Osteoarthritis (OA) is a prevalent joint disorder that is characterized by the degeneration of articular cartilage in synovial joints. Most of the current treatment options for this disorder tend to focus on symptom management rather than addressing the underlying progression of the disease. Cartilage tissue engineering has emerged as a promising approach to address the limitations of current OA treatments, aiming to regenerate cartilage and restore the natural function of affected joints. Like any other tissue engineering field, cartilage tissue engineering uses different fabrication techniques and biomaterials to develop the constructs. Numerous studies over the last few years have demonstrated the preclinical efficacy of tissue‐engineered constructs in promoting cartilage regeneration and highlight the potential of tissue‐engineered constructs as a viable therapeutic approach for OA. This paper aims to provide a focused review of advancements in tissue‐engineered constructs over the past decade. Specifically, we highlight the constructs based on natural, synthetic, and composite biomaterials and the varying conventional and advanced fabrication techniques. We also highlight the challenges instate‐of‐the‐artcartilage tissue engineering that must be overcome in the upcoming years to fully replicate the complex anatomy of the native cartilage. We believe that continued collaborative research efforts among researchers from various facets of engineering and clinicians are required to advance the field of cartilage tissue engineering and become a viable OA therapy.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1002/jor.25793
