A Force to be Reckoned with
AO Foundation
Carolina Maria Cordeiro
Joint biomechanics directing cartilage homeostasis
Weight-bearing joints, such as the knees, are subjected to continuous loading, combining compression, shear, and various degrees of hydrostatic pressure. During foetal development, mechanical stress induces cartilage formation. In adulthood, appropriate mechanical stimuli are crucial for maintaining stable cartilage homeostasis. In case of abnormal loading (e.g., derived from obesity and joint trauma), an imbalance is generated that will then potentiate the development of age-related degenerative conditions such as osteoarthritis[1]. Apart from these undesired mechanical stimuli, the use of clinically relevant mechanical stimuli together with cell therapy, also called regenerative rehabilitation therapies, has been emerging as a novel approach that might facilitate proper integration and help treat conditions such as osteoarthritis[2]. Although the regenerative rehabilitation field has been significantly progressing, there is still a lack of understanding of how mechanical forces (like those from physiotherapy) trigger molecular changes and how cells respond to them.
How can cells sense the mechanical stimuli?
Human cells, such as chondrocytes (i.e., the main cell type comprising cartilage tissue) and mesenchymal stem cells or MSCs (i.e., stem cells with the ability to turn into a variety of tissue cells, such as chondrocytes) can sense and respond to mechanical stimuli of their surroundings. Those physical stimuli can come from the mechanical characteristics of their microenvironment (e.g. natural network in which cells are embedded, also called extracellular matrix (ECM)), as well as from mechanical forces that surround them (e.g., compression and shear). In this blog, we will focus on this final mechanical source. In this “Outside-in” mechanical-related signalling, the cells sense those mechanical stimuli via cell membrane-related mechanosensitive microstructures such as focal adhesion complexes (i.e., multi-protein assembly responsible for cell-ECM interaction) that then transmit that external signal to the cytoskeleton (i.e., intra cellular network present in the cell cytoplasm that serves as a mechanical support structure to maintain the cell shape and resist deformation) [3].
From mechanical to molecular signal
Cell transmission of external cues to the cytoskeleton triggers cytoskeletal changes that then convert mechanical stimuli into biochemical signals. Ultimately, biochemical signals translocate into the nucleus and alter gene expression, that then lead to changes in the cell behaviour. One example of mechanically driven molecular pathway is the RhoA/ROCK pathway. This pathway is known to regulate the formation and contraction of actomyosin structures (i.e., crucial cytoskeleton elements responsible for cell movement and shape). The generated intracellular tension can then influence gene expression via YAP/TAZ activity [4]. Another mechanical-related signalling pathway, is TGF-β pathway . TGF-β is a powerful signalling molecule involved in cartilage formation, maintenance, and disease. This protein is secreted by the cells and stored in an inactive complex known as latent TGF-β. Its active form can be released from this latent complex, for example, via ECM-cytoskeleton interactions. Next, active TGF-β binds to its cell surface receptor and induces SMAD pathways, thereby mediating the expression of genes related to cartilage formation [5].
In vitro Mechanoresponsive culture system
To generate an in vitro cartilage model, the scientists have relied primarily on biochemical stimulation by supplementing TGF-β in the culture medium. This approach overlooks the important role of mechanical stimulation, which is crucial. This holds particular significance as in vitro cartilage culture systems are increasingly adopting three-dimensional configurations facilitated by biomaterials. In these 3D systems, biomaterial mechanical properties such as stiffness, surface topography, matrix pore size, and architecture need to be considered. Moreover, mechanical forces, similar to those found in human articulating joints, also merit attention. Therefore, it is crucial to take the mechanical factor into account when developing either an in vitro model or therapy (e.g., TGF-β releasing constructs). The drawback of using TGF-β supplementation strategies lies in its dosage. Usually, the dose applied is excessive, which can promote detrimental cartilaginous features such as fibrocartilage deposition, dense chondrocyte clusters, and swelling of the construct[6]. Because of this, dose and timing are crucial when biochemically delivering TGF-β1. To overcome the limitations of this strategy, an alternative could involve activating the latent form of TGF-β, produced by the cells, using a specific in vitro mechanical protocol using a bioreactor or a refined physiotherapeutic routine.
Studies have shown that by applying shear flow to synovial fluid, they were able to activate the TGF-β present in the fluid[7]. The same phenomenon occurred when mechanical shear was applied to biomaterial scaffolds in the presence or absence (i.e. latent TGF-β loaded into the scaffold) of cells [8]. Furthermore, it has been shown that a combination of compression and shear also induces human bone marrow -derived MSCs to turn into chondrocytes in the absence of TGF-β supplementation[9]. Beside the effects that mechanical forces have on TGFβ-induced cartilage formation, a study has shown that dynamic compression tiggers cartilage formation and promotes RhoA, ROCK and YAP activity of human chondrocytes embedded in a hydrogel construct [10]. Another study on shear stress revealed that fluid flow-induced shear stress activates and inhibits RhoA activity selectively, depending on the magnitude of the stress [11].
Future Perspectives
In the pursuit of emerging clinically relevant therapy for cartilage regeneration of patient with osteoarthritis, it is important to consider: i) studying the molecular pathways to strengthen translation; ii) developing an in vitro model that more closely mimics the biological and mechanical features of native cartilage, as well as its surroundings, including mechanical forces; iii) ultimately, creating a regenerative rehabilitation treatment that integrates a cell-based biomaterial, considering both the mechanical characteristics of the biomaterial, alongside a rehabilitation routine. This approach aims to improve cartilage tissue functionality and enhance patient outcomes.
