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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. 

Measuring LTGF-beta activation in live tissues in situ is a major challenge due to the short half-life of activated TGF-beta in cartilage (due to rapid receptor internalization/degradation). As such, activation assessments typically require analysis of downstream events. However, assessments of intracellular TGF-beta signaling molecules (Smad2/3 phosphorylation) yield mostly qualitative measures and reporter cell assays are not compatible with intact cartilage tissues. Alternatively, in the current project, we proposed quantifying LTGF-beta activation in situ through a novel assay that capitalizes on TGF-beta’s robust autoinduction behavior; active TGF-beta activity induces a predictable increase in synthesis of soluble LTGF-beta. The dominant fraction of newly synthesized LTGF-beta is secreted from the tissue (not retained in ECM) and stable. Accordingly, measurements of LTGF-beta secretion into culture medium allows for quantifications of TGF-beta activity in cartilage. In order to confirm that LTGF-beta secretion enhancements result from TGF-beta activity (and not other load-initiated signaling cascades), a control group can readily be utilized, consisting of TGF-beta activity inhibition from a TGF-beta-receptor specific kinase inhibitor. Using this platform, we performed the first-ever measurement of the activity of TGF-beta in cartilage explants from load-induced activation. Results demonstrate that LTGF-beta secretion rates do indeed increase with cartilage mechanical loading. Upon exposure to a TGF-beta inhibitor, LTGF-beta secretion rates return to basal control levels, thus confirming that LTGF-beta secretion enhancements can be predominantly attributed to TGF-beta activity in the tissue. Upon standard curve conversion, autoinduction assay results demonstrate that mechanical load-induced activation of ECM-bound LTGF-beta gives rise to ~0.15ng/mL of TGF-beta activity in cartilage. Importantly, this measure represents the first quantitative assessment of TGF-beta activity in articular cartilage. While these levels represent the activation of only a small fraction of the total LTGF-beta stores in the cartilage ECM (~300ng/mL), they are indeed capable of giving rise to considerable chondrocyte biosynthesis enhancements in the tissue. As such, these measurements support the mechanobiological role of load-induced LTGF-beta activation in maintaining articular cartilage integrity. The assay platform advanced in this study sets the foundation for considerable advances in our understanding of the mechanistic details and physiologic importance of load-induced LTGF-beta activation in cartilage. In the future, we plan to use this quantitative platform to assess: 1) the influence of varying loading regimens on LTGF-beta activation rates (e.g., physiologic exercise, elevated stresses, high-impact trauma), and 2) changes to load-induced LTGF-beta activation with aging or joint degeneration. An abstract on this work was presented at the 2020 ASME SB3C Conference (virtual meeting) and a full-length manuscript is currently in preparation.