Osteoarthritis is a painful joint disease characterized by progressive degeneration of the articular cartilage as well as associated changes to the subchondral bone, synovium, and surrounding joint tissues. While the effects of osteoarthritis on the cartilage extracellular matrix (ECM) have been well recognized, it is now becoming apparent that in many cases, the onset of the disease may be initially reflected in the matrix region immediately surrounding the chondrocytes, termed the pericellular matrix (PCM). Growing evidence suggests that the PCM - which along with the enclosed chondrocytes are termed the "chondron" - acts as a critical transducer or "filter" of biochemical and biomechanical signals for the chondrocyte, serving to help regulate the homeostatic balance of chondrocyte metabolic activity in response to environmental signals. Indeed, it appears that alterations in PCM properties and cell-matrix interactions, secondary to genetic, epigenetic, metabolic, or biomechanical stimuli, could in fact serve as initiating or progressive factors for osteoarthritis. Here, we discuss recent advances in the understanding of the role of the PCM, with an emphasis on the reciprocity of changes that occur in this matrix region with disease, as well as how alterations in PCM properties could serve as a driver of ECM-based diseases such as osteoarthritis. Further study of the structure, function, and composition of the PCM in normal and diseased conditions may provide new insights into the understanding of the pathogenesis of osteoarthritis, and presumably new therapeutic approaches for this disease.

Osteoarthritis (OA) is a painful and debilitating condition of synovial joints without any disease-modifying therapies [A. M. Valdes, T. D. Spector, Nat. Rev. Rheumatol. 7, 23–32 (2011)]. We previously identified mechanosensitive PIEZO channels, PIEZO1 and PIEZO2, both expressed in articular cartilage, to function in chondrocyte mechanotransduction in response to injury [W. Lee et al., Proc. Natl. Acad. Sci. U.S.A. 111, E5114–E5122 (2014); W. Lee, F. Guilak, W. Liedtke, Curr. Top. Membr. 79, 263–273 (2017)]. We therefore asked whether interleukin-1–mediated inflammatory signaling, as occurs in OA, influences Piezo gene expression and channel function, thus indicative of maladaptive reprogramming that can be rationally targeted. Primary porcine chondrocyte culture and human osteoarthritic cartilage tissue were studied. We found that interleukin-1α (IL-1α) up-regulated Piezo1 in porcine chondrocytes. Piezo1 expression was significantly increased in human osteoarthritic cartilage. Increased Piezo1 expression in chondrocytes resulted in a feed-forward pathomechanism whereby increased function of Piezo1 induced excess intracellular Ca2+ at baseline and in response to mechanical deformation. Elevated resting state Ca2+ in turn rarefied the F-actin cytoskeleton and amplified mechanically induced deformation microtrauma. As intracellular substrates of this OA-related inflammatory pathomechanism, in porcine articular chondrocytes exposed to IL-1α, we discovered that enhanced Piezo1 expression depended on p38 MAP-kinase and transcription factors HNF4 and ATF2/CREBP1. CREBP1 directly bound to the proximal PIEZO1 gene promoter. Taken together, these signaling and genetic reprogramming events represent a detrimental Ca2+-driven feed-forward mechanism that can be rationally targeted to stem the progression of OA.

Objective TGF-β is synthesized in an inactive latent complex that is unable to bind to membrane receptors, thus unable to induce a cellular biological response until it has been activated. In addition to activation by chemical mediators, recent studies have demonstrated that mechanical forces may activate latent TGF-β via integrin-mediated cellular contractions, or mechanical shearing of blood serum. Since TGF-β is present in synovial fluid in latent form, and since normal diarthrodial joint function produces fluid shear, this study tested the hypothesis that the native latent TGF-β1 of synovial fluid can be activated by shearing. Design Synovial fluid from 26 bovine joints and 3 adult human joints was sheared at mean shear rates up to 4000 s−1 for up to 15 hours. Results Unsheared synovial fluid was found to contain high levels of latent TGF-β1 (4.35±2.02 ng/mL bovine, 1.84±0.89 ng/mL human; mean±radius of 95% confidence interval) and low amounts (<0.05 ng/mL) of the active peptide. Synovial fluid concentrations of active TGF-β1 increased monotonically with shear rate and shearing duration, reaching levels of 2.64±1.22 ng/mL for bovine and 0.60±0.39 ng/mL for human synovial fluid. Following termination of shearing, there was no statistical change in these active levels over the next 8 hours for either species, demonstrating long-term stability of the activated peptide. The unsheared control group continued to exhibit negligible levels of active TGF-β1 at all times. Conclusions Results confirmed the hypothesis of this study and suggest that shearing of synovial fluid might contribute an additional biosynthetic effect of mechanical loading of diarthrodial joints.

Transforming growth factor beta (TGF-β) has become one of the most widely utilized mediators of engineered cartilage growth. It is typically exogenously supplemented in the culture medium in its active form, with the expectation that it will readily transport into tissue constructs through passive diffusion and influence cellular biosynthesis uniformly. The results of this investigation advance three novel concepts regarding the role of TGF-β in cartilage tissue engineering that have important implications for tissue development. First, through the experimental and computational analysis of TGF-β concentration distributions, we demonstrate that, contrary to conventional expectations, media-supplemented exogenous active TGF-β exhibits a pronounced concentration gradient in tissue constructs, resulting from a combination of high-affinity binding interactions and a high cellular internalization rate. These gradients are sustained throughout the entire culture duration, leading to highly heterogeneous tissue growth; biochemical and histological measurements support that while biochemical content is enhanced up to 4-fold at the construct periphery, enhancements are entirely absent beyond 1 mm from the construct surface. Second, construct-encapsulated chondrocytes continuously secrete large amounts of endogenous TGF-β in its latent form, a portion of which undergoes cell-mediated activation and enhances biosynthesis uniformly throughout the tissue. Finally, motivated by these prior insights, we demonstrate that the alternative supplementation of additional exogenous latent TGF-β enhances biosynthesis uniformly throughout tissue constructs, leading to enhanced but homogeneous tissue growth. This novel demonstration suggests that latent TGF-β supplementation may be utilized as an important tool for the translational engineering of large cartilage constructs that will be required to repair the large osteoarthritic defects observed clinically.

It was recently demonstrated that mechanical shearing of synovial fluid (SF), induced during joint motion, rapidly activates latent transforming growth factor β (TGF-β). This discovery raised the possibility of a physiological process consisting of latent TGF-β supply to SF, activation via shearing, and transport of TGF-β into the cartilage matrix. Therefore, the two primary objectives of this investigation were to characterize the secretion rate of latent TGF-β into SF, and the transport of active TGF-β across the articular surface and into the cartilage layer. Experiments on tissue explants demonstrate that high levels of latent TGF-β1 are secreted from both the synovium and all three articular cartilage zones (superficial, middle, and deep), suggesting that these tissues are capable of continuously replenishing latent TGF-β to SF. Furthermore, upon exposure of cartilage to active TGF-β1, the peptide accumulates in the superficial zone (SZ) due to the presence of an overwhelming concentration of nonspecific TGF-β binding sites in the extracellular matrix. Although this response leads to high levels of active TGF-β in the SZ, the active peptide is unable to penetrate deeper into the middle and deep zones of cartilage. These results provide strong evidence for a sequential physiologic mechanism through which SZ chondrocytes gain access to active TGF-β: the synovium and articular cartilage secrete latent TGF-β into the SF and, upon activation, TGF-β transports back into the cartilage layer, binding exclusively to the SZ.

Animal cells have served as highly controllable model systems for furthering cartilage tissue engineering practices in pursuit of treating osteoarthritis. Although successful strategies for animal cells must ultimately be adapted to human cells to be clinically relevant, human chondrocytes are rarely employed in such studies. In this study, we evaluated the applicability of culture techniques established for juvenile bovine and adult canine chondrocytes to human chondrocytes obtained from fresh or expired osteochondral allografts. Human chondrocytes were expanded and encapsulated in 2% agarose scaffolds measuring Ø3–4 mm × 2.3 mm, with cell seeding densities ranging from 15 to 90 × 106 cells/mL. Subsets of constructs were subjected to transient or sustained TGF-β treatment, or provided channels to enhance nutrient transport. Human cartilaginous constructs physically resembled native human cartilage, and reached compressive Young’s moduli of up to ~250 kPa (corresponding to the low end of ranges reported for native knee cartilage), dynamic moduli of ~950 kPa (0.01 Hz), and contained 5.7 percent wet weight (%/ww) of glycosaminoglycans (≥ native levels) and 1.5 %/ww collagen. We found that the initial seeding density had pronounced effects on tissue outcomes, with high cell seeding densities significantly increasing nearly all measured properties. Transient TGF-β treatment was ineffective for adult human cells, and tissue construct properties plateaued or declined beyond 28 days of culture. Finally, nutrient channels improved construct mechanical properties, presumably due to enhanced rates of mass transport. These results demonstrate that our previously established culture system can be successfully translated to human chondrocytes.

Mechanobiologic signals regulate cellular responses under physiologic and pathologic conditions. Using synthetic biology and tissue engineering, we developed a mechanically responsive bioartificial tissue that responds to mechanical loading to produce a preprogrammed therapeutic biologic drug. By deconstructing the signaling networks induced by activation of the mechanically sensitive ion channel transient receptor potential vanilloid 4 (TRPV4), we created synthetic TRPV4-responsive genetic circuits in chondrocytes. We engineered these cells into living tissues that respond to mechanical loading by producing the anti-inflammatory biologic drug interleukin-1 receptor antagonist. Chondrocyte TRPV4 is activated by osmotic loading and not by direct cellular deformation, suggesting that tissue loading is transduced into an osmotic signal that activates TRPV4. Either osmotic or mechanical loading of tissues transduced with TRPV4-responsive circuits protected constructs from inflammatory degradation by interleukin-1α. This synthetic mechanobiology approach was used to develop a mechanogenetic system to enable long-term, autonomously regulated drug delivery driven by physiologically relevant loading.

Large-sized cartilage constructs suffer from inhomogeneous extracellular matrix deposition due to insufficient nutrient availability. Computational models of nutrient consumption and tissue growth can be utilized as an efficient alternative to experimental trials to optimize the culture of large constructs; models require system-specific growth and consumption parameters. To inform models of the [bovine chondrocyte]-[agarose gel] system, total synthesis rate (matrix accumulation rate + matrix release rate) and matrix retention fractions of glycosaminoglycans (GAG), collagen, and cartilage oligomeric matrix protein (COMP) were measured either in the presence (continuous or transient) or absence of TGF-β3 supplementation. TGF-β3’s influence on pyridinoline content and mechanical properties was also measured. Reversible binding kinetic parameters were characterized using computational models. Based on our recent nutrient supplementation work, we measured glucose consumption and critical glucose concentration for tissue growth to computationally simulate the culture of a human patella-sized tissue construct, reproducing the experiment of Hung et al., (2003). Transient TGF-β3 produced the highest GAG synthesis rate, highest GAG retention ratio, and highest binding affinity; collagen synthesis was elevated in TGF-β3 supplementation groups over control, with the highest binding affinity observed in the transient supplementation group; both COMP synthesis and retention were lower than those for GAG and collagen. These results informed the modeling of GAG deposition within a large patella construct; this computational example was similar to previous experimental results without further adjustments to modeling parameters. These results suggest that these nutrient consumption and matrix synthesis models are an attractive alternative for optimizing the culture of large-sized constructs.