Dynamic Compression and Exogenous Fibronectin Regulates Cell-Matrix Adhesions and Intracellular Signaling Proteins of Human Mesenchymal Stem Cells in 3D Collagen Environment

This dissertation, "Dynamic Compression and Exogenous Fibronectin Regulates Cell-matrix Adhesions and Intracellular Signaling Proteins of Human Mesenchymal Stem Cells in 3D Collagen Environment" by Chuen-wai, Li, 李鑽偉, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: The fundamental principle of tissue engineering is to use appropriate cell source, combined with scaffolds and bioactive factors to develop tissue constructs which restore, maintain or improve tissue function. There is increasing data emphasizing the importance of mechanical signals and extracellular matrix (ECM) proteins presented by the scaffold in determining stem cell fate/functions which are critical to tissue construct maturation and success of stem cell-based therapies. Cell-matrix adhesions are one of the major mechanosensing machineries cells use to convert information provided by ECM ligands and mechanical signals presented by scaffolds into intracellular biochemical signaling cascades which lead to particular functional responses. Therefore, understanding how ECM ligands and mechanical signals regulate cell-matrix adhesion formation and activation of associated intracellular signaling proteins is fundamental to rational design of biomaterial and loading protocol for optimal cell functional responses in tissue constructs. In this study, we attempted to understand the regulatory effects of external mechanical signal and exogenous ECM protein on cell-matrix adhesion formation and associated intracellular signaling proteins of human mesenhymal stem cells, and in particular, to test the hypothesis that mechanical stimulation or exogenous ECM protein can lead to adhesion maturation into 3D-matrix adhesions in 3D collagen environment. We used microencapsulation technique to embed cells in 3D collagen environment, forming disc-shaped hMSC-collagen constructs. By immunofluorescent staining and confocal microscopy, we visualized changes in size, morphologies and molecular composition of the adhesions. First of all, 2D adhesions of hMSCs were characterized. We showed that hMSCs form well-organized αv integrin-based focal adhesions and fibrillar adhesions in 2D culture. To investigate the regulatory effects of mechanical signals on adhesion signaling and maturation, we used micromanipulator-based loading device to impose dynamic compression to hMSC-collagen constructs. We found that dynamic compression lead to enlargement of integrin αv adhesions which recruit focal adhesion kinase (FAK), vinculin and extracellular signal-regulated kinase (ERK). In addition, FAK was activated at enlarged integrin αv adhesions and translocated to peri-nuclear region after compression, suggesting that loading induces activation of FAK signaling pathways through increased integrin αv clustering. Moreover, we demonstrated that dynamic compression can induce 3D-matrix adhesion formation, indicating the role of external force in integrin α5-based adhesion maturation in 3D collagen environment. We explored the effect of exogenous ECM proteins on adhesion maturation of hMSCs by adding fibronectin into cell-collagen mixture during fabrication of collagen constructs. Our results demonstrated that the exogenous fibronectin can induce α5 integrin-based adhesion maturation into 3D-matrix adhesions in our collagen constructs in a dose-dependent manner. This study demonstrated that the effect of external mechanical signals and exogenous ECM ligands on adhesion signaling and maturation of hMSCs in 3D collagen environment. Our findings contribute towards mechanobiology of hMSCs in 3D context. I