Engineered 3D Periodontal Ligament Model with Magnetic Tensile Loading

In vitro models are invaluable tools for deconstructing the biological complexity of the periodontal ligament (PDL). Model systems that closely reproduce the 3-dimensional (3D) configuration of cell-cell and cell-matrix interactions in native tissue can deliver physiologically relevant insights. However, 3D models of the PDL that incorporate mechanical loading are currently lacking. Hence, we developed a model where periodontal tissue constructs (PTCs) are made by casting PDL cells in a collagen gel suspended between a pair of slender, silicone posts for magnetic tensile loading. Specifically, one of the posts was rigid and the other was flexible with a magnet embedded in its tip so that PTCs could be subjected to tensile loading with an external magnet. Additionally, the deflection of the flexible post could be used to measure the contractile force of PDL cells in the PTCs. Prior to tensile loading, second harmonics generation analysis of collagen fibers in PTCs revealed that incorporation of PDL cells resulted in collagen remodeling. Biomechanical testing of PTCs by tensile loading revealed an elastic response at 4 h, permanent deformation by 1 d, and creep elongation by 1 wk. Subsequently, contractile forces of PDL cells were substantially lower for PTCs under tensile loading. Immunofluorescence analysis revealed that tensile loading caused PDL cells to increase in number, express higher levels of F-actin and α-smooth muscle actin, and become aligned to the tensile axis. Second harmonics generation analysis indicated that collagen fibers in PTCs progressively remodeled over time with tensile loading. Gene expression analysis also confirmed tension-mediated upregulation of the F-actin/Rho pathway and osteogenic genes. Our model is novel in demonstrating the mechanobiological behavior that results in cell-mediated remodeling of the PDL tissue in a 3D context. Hence, it can be a valuable tool to develop therapeutics for periodontitis, periodontal regeneration, and orthodontics.