AFM-based experimental investigation, numerical simulation and theoretical modeling of mechanics of cell adhesion

Abstract

Cell-extracellular matrix and cell-cell adhesion are essential for biological processes such as cell motility, signaling, proliferation, cytoskeletal organization and gene expression. For this reason, extensive effort has been devoted in the past few decades to measure cell adhesion as well as identify key molecules involved. This thesis focuses on two outstanding problems in this area, namely, how to quantitatively characterize the adhesion between neural cells and the substrate and how to model the turnover of adhesions in the intriguing phenomenon of stretch-induced cell realignment. First of all, using a combined atomic force (AFM) and total internal reflection fluorescence microscope (TIRFM) system a novel method was developed to systematically and quantitatively examine the adhesion between neurite branches and the extracellular matrix. Specifically, a tipless AFM cantilever was used to penetrate between a well-developed neurite and the functionalized substrate and then gradually peel the neurite from the surface. At the same time, a laser TIRFM was utilized to monitor the activities of different adhesion molecules during the detaching process. This approach provides a solution to the long-standing problem of how to quantitatively measure neuron-extracellular matrix interactions while, simultaneously, identify the roles of various adhesion proteins in the process. Besides heathy neurons, testes have also been conducted on cells affected by the Alzheimer's disease (AD) where the influence of such disease on the mechanical response of neural cells was demonstrated. Secondly, to better understand the observed peeling response of the neurite, as well as extract key information from it, finite element (FEM) simulation was carried out using ABAQUS. It was shown that a good fit between the simulation results and experimental data can be achieved by representing the adhesion between two surfaces with simple cohesive interactions. In particular, it was found that the apparent adhesion energy density, a quantity of central interest in cell adhesion studies, is in the range of 0.2-0.8 mj/m^2. Last but not the least, a mechanochemical modeling framework was developed to investigate the mechanism of cell reorientation induced by cyclic stretching on the substrate. It was shown that the final alignment of cells reflects the competition between stress fiber assembly or disassembly, focal adhesion growth or disruption, substrate stiffening and whole-cell rotation. Predictions from the model are consistent with a variety of experimental observations, suggesting that the main physics of this intriguing phenomenon may have been well captured.