Biomechanical Regulation of Gene Expression in Knee Ligaments

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Abstract

Ligaments maintain joint stability by resisting tensile loads and preserving compressive loads between bones. During normal function, ligaments are subjected to mechanical loads which consequently induce the deformation in the ligament tissues. Two knee ligaments of clinical interest are the anterior cruciate ligament (ACL), which fails to heal after injury, and the medial collateral ligament (MCL), which exhibits a healing response. Characterizing the biomechanical regulation of cellular behavior relevant to healing and remodeling processes would contribute to our understanding of mechanically induced changes in ligaments. This dissertation investigates the role of mechanical loading on the expression of collagens in ligament fibroblasts through the use of an in vitro cell culture system and an ex vivo organ culture preparation

Expression of types I and HI collagens were determined for ACL and MCL fibroblast monolayers subjected to cyclic mechanical loads. ACL fibroblasts responded with an upregulation of type I collagen expression while MCL fibroblasts exhibited a striking increase in type in collagen expression. In addition, type III collagen expression patterns in ACL cells depended on the maximum strain. These results were consistent with observations in tissue healing and remodeling in vivo, and demonstrate an additional intrinsic difference that exists between ACL and MCL fibroblasts.

To preserve the anatomical cellular loading environment of ligament fibroblasts, an organ culture preparation novel for the application of mechanical loads to ligaments was designed. In particular, the ACL was chosen for further analysis. A tissue harvest procedure was developed for rat ACLs and shown to be feasible by cell viability assessment. Application of 5 N incremental loads induced ACL surface strains on the order of those previously measured in human ACLs. After initiation of incremental loading, expression of type I collagen increased significantly within 1 hour, but then decreased significantly to 46% of control values after 2 hours.

These studies contribute to our understanding of cellular responses to mechanical loading. Development of an organ culture system for the study of mechanically-induced responses in ligaments provides impetus for further research in ligament healing and remodeling as well as in relevant cellular and molecular processes.

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