# Intermitt Dynamic Compression Conters Anabolic Effects in Articular Cartilage
## 4 Discussion
TGF-\(\beta\) is secreted by chondrocytes in a latent form that associates with pericellular matrix proteins such as fibrillin, proteoglycans and fibronectin [74]. Latency-associated protein (LAP), part of the inactive TGF-\(\beta\) complex, interacts directly with \(\alpha\)v-integrins [74]. Integrins contribute to the activation of latent TGF-\(\beta\) by two proposed mechanisms: (1) a protease-dependent mechanism, in which \(\alpha\)V-integrins bind to the latent TGF-\(\beta\) complex and proteinases simultaneously, facilitating enzymatic cleavage and TGF-\(\beta\) release; (2) a protease-independent mechanism, in which integrins change the conformation of latent TGF-\(\beta\) by transmitting cell traction forces [50]. The latter mechanism requires the association of the latent complex with mechanically resistant ECM. Proof of mechanically driven TGF-\(\beta\) activation by contractile forces on the bond between LAP and \(\alpha\)V-integrins has recently been demonstrated to occur in mechanically stressed cartilage [75]. [PERSON] et al. demonstrated that mechanical stress leads to a talin-centered cytoskeletal reorganization in chondrocytes, causing a stiffening that transmits to \(\alpha\)v integrins in the release of TGF-\(\beta\)[75]. [PERSON] et al. similarly demonstrated that both physiological and excessive compression of explants activates Smad2/3 signaling, linking compressive loading with TGF-\(\beta\) activation [42]. Fibronectin is a mechanosensitive protein that becomes unfolded upon cell traction, and it has been suggested that ECM protein unfolding may reveal specific-integrin binding sites [50]. Fibronectin-knockout fibroblasts have been reported to poorly incorporate latent TGF-\(\beta\) into their ECM and exhibit deficient mechanical activation by the \(\alpha\)v\(\beta\)6-integrin [76]. In addition, fibroblasts with knockout of the \(\alpha\)5\(\beta\)1 integrin, an integrin that is not directly involved in TGF-\(\beta\)1 activation but central for the assembly of fibronectin fibrils, are similarly incapable of activating latent TGF-\(\beta\)1 by \(\alpha\)v\(\beta\)6-integrin-mediated cell traction [76]. Thus, this suggests a dependency for fibronectin as an ECM scaffold in the mechanical activation of TGF-\(\beta\). We found that fibronectin degradation was increased by intermittent dynamic compression, supporting a hypothesis of mechanical-driven TGF-\(\beta\) release in these studies. The fact that TGF-\(\beta\) signaling increases FBN-C could be indicative of a feedback loop, in which mechanically released TGF-\(\beta\) restores its own latent ECM pool, requiring sequestration by fibronectin for later mechanical activations. A recurring concept in cartilage biology is that TGF-\(\beta\) activity must be maintained within an appropriate range for the maintenance of homeostasis and structural integrity [43]. The dysregulation of TGF-\(\beta\) activity is detrimental to the chondrocyte metabolism and contributes to the OA pathogenesis. [PERSON] et al. demonstrated that subchondral bone sclerosis occurring during OA progression causes excessive regional stress in the overlying articular cartilage, altering the distribution of TGF-\(\beta\) activity, being relatively elevated in high-stress regions and decreased in low-stress regions compared to healthy cartilage [75]. Furthermore, anterior-cruciate ligament transection (ACLT) mice with \(\alpha\)V-integrin knockout were protected against cartilage damage, suggesting that abolishment of excessive TGF-\(\beta\) activation in articular cartilage attenuates OA progression [75]. We acknowledge that the 50 ng/mL recombinant active TGF-\(\beta\)1 concentration used in these studies can be considered excessive; however, we did not observe metabolic abnormalities using this dose, as determined by [PERSON]. It has been demonstrated that fibronectin fragments (FN-fs) promote pro-inflammatory pathways in chondrocytes, and that especially the amino-terminal FN-fs induces potent catabolic effects [77].