Physical understanding of cells and cell-ECM interactions

Abstract

Various kinds of signals are constantly transmitted into and out of cells to regulate their activities such as migration, apoptosis, and proliferation. Among these signals, the roles of physical cues like the applied force or deformation in processes such as mechano-transduction and mechano-sensing have received increasing attention. In particular, intense efforts have been spent to understand how different physical signals are detected by and transmitted into cells as well as how they eventually influence the behavior of cells, which are also of the main objectives of this thesis. Firstly, we used the atomic force microscopy (AFM) as the platform to quantitatively measure the mechanical response of cytoskeleton and cell-extracellular matrix (ECM) adhesion, two key components of the force transmission system in cells. Specifically, a novel peeling method was developed to systematically measure the viscoelasticity of neurite and neurite-substrate adhesions. Subsequent cohesive zone based finite element modeling was conducted which allows us to estimate that the initial and long-term elastic moduli of neurite are around 7.5 kPa and 2.8 kPa respectively, with a characteristic time of relaxation around 2 s, while the neurite-substrate adhesion energy density should be around"0.07 m J/" "m" ^"2" . In addition, to understand how disorders like Alzheimer's disease (AD) affect neural cells from a mechanics point of view, we applied AFM rheology test to measure the dynamic response of neurons treated with Amyloid beta (Aβ) peptides to mimic the progression of AD. Interestingly, it was found that both the storage and loss moduli of neural cells increase gradually within the first 6 hours of Aβ treatment before steady-state values are reached, with a higher dosage of Aβ leading to larger changes in cell properties. Therefore, a quantitative link between Aβ accumulation and the physical characteristics of neurons was established. Then, our research goes into more details on focal adhesions (FAs), which adhere cells to the ECM and serve as hubs for the exchange of information. Two approaches for describing the formation and functioning of FAs are considered, one is the motor-clutch model with discrete and stochastic binding and breakage while the other is based on free energy-driven assembly/disassembly of FA. Specifically, by examining the dynamics of individual molecular clutches, we investigated how cell adhesion and spreading are influenced by the interplay of different cellular (like the clutch binding and FA lifetime) and material (such as the relaxation of ECM) time scales. On the other hand, continuum level simulation by examining the distribution of strain energy along the FA boundary free energy was also conducted to reveal the effects of external force on the shape evolution of FAs. Finally, a theoretical model was developed to explain cell filopodia recognize the surface characteristics of the surrounding environment, which suggests that the contact between the disordered topography and the filopodial tip plays a key role in altering filopodial growth dynamics. It was found that large surface asperities can block the movement of filopodial tip, delay its extension and cause the bending/kinking of the structure, in quantitative agreement with experimental observations.