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Periodontal Cell Mechanobiology
The periodontal ligament, a fibrous, soft connective tissue that attaches the tooth root to the inner wall of the alveolar bone, is a unique system in which the nature and magnitude of mechanical force controls its bone regulatory functions. These forces include those related to chewing and tongue motions. Compression related orthodontic forces cause an upregulation of the osteoclastogenic receptor activator NF kappa B ligand (RANKL) in periodontal ligament fibroblasts (PDLFs) in vivo. The periodontal ligament fibroblast (PDLF) is the main cell type in the PDL, and it responds to mechanical forces by up- or down-regulating the expression of bone regulatory proteins which promote alveolar bone remodeling. When some bacteria become dominant, chronic periodontal disease results leading to alveolar bone resorption and loss of teeth. Yet, how bacteria induce the expression of bone resorptive proteins in PDLFs has remained poorly understood. Our research objective is to understand how bacterial insult alters PDLF mechanotransduction to promote alveolar bone resorption. PDLFs function in a threedimensional collagen matrix that is under stretch and compression. However, the vast majority of studies of PDLFs in the have involved culture on two-dimensional tissue culture plastic. The function of adherent cells in three-dimensional extracellular matrix is significantly different from cells cultured on 2D tissue culture dishes. Cells in 3D culture have different shapes, cytoskeletal structure and focal adhesion assembly. Cells grown in monolayers exhibit gene expression profiles that do not correlate with those of cells grown in 3-D culture. 3D culture of cells mimics in vivo phenotypes more accurately than 2-D culture. For example, 3-D cultures of human mammary epithelial cells form acinar structures similar to breast tissue with characteristic lumens. These acini are functional because they secrete proteins characteristic of in vivo tissue. Three-dimensional culture of fibroblasts from a variety of tissue origins are far more realistic models of cell function than 2-D culture. Therefore, we propose to culture PDLFs in three dimensional bioartificial periodontal ligaments which allows computer-controlled application of mechanical stresses of controlled amplitude and frequency.
In periodontal disease, bacterial infection of periodontium results in alveolar bone resorption and loss of teeth. We have shown in vivo that metabolically active bacteria can spread from gingival tissue to connective tissue. Similar to the effects of compressive stresses on osteoclastic gene expression in PDLFs discussed above, incubation of PDLFs with bacteria upregulates osteoclastic molecules like RANKL. PDLFs from periodontal tissue infected with bacteria display pathologic responses to external mechanical stresses by expressing inflammatory cytokines, matrix metalloproteinases and osteoclastic molecules. Despite the significant body of evidence that bacteria invade periodontal cells, there are very few studies on the impact of bacteria on mechanosensitive bone regulatory gene expression in periodontal cells. . Hence we propose to focus on the mechanisms by which externally applied mechanical stresses alter expression of bone regulatory proteins in PDLFs, and how bacterial insult alters these relationships.