Abstract
The mechanics of living materials, alongside their chemical and biological properties, are essential for life to construct basic structures, regulate biological activities and maintain the homeostasis of life. Compared to traditional soft matters, living materials exhibit emergent properties across different spatial and temporal scales, which arise from their diverse constituents and complicated molecular interactions. Due to these complexities, our fundamental understanding of their mechanical properties and their relations to physiological functions and diseases remains limited. In this thesis, we will explore the mechanics and its biological significance of two representative living materials, including protein condensates and living cells, which focus on the mechanics of living materials at both subcellular and whole-cell levels. In the first part, we demonstrated the emergent mechanics of a functional multivalent condensate reconstituted with six postsynaptic density proteins (6xPSD) using atomicforce-microscopy-based mesoscale rheology and quantitative fluorescence measurements. The measured relaxation modulus E(t) and protein compositions reveal that the majority of proteins (80%) are mobile and diffuse in a partitioned space made by the dynamicallycross-linked network, consisting of the rest minority (20%). The hierarchical structures bring about two relaxation modes in E(t), including an initial exponential decay and a power law decay, which deviate significantly from Maxwell fluids. The power law rheology with an exponent α ≃ 0.5 is the hallmark of binding/unbinding dynamics of the weak bonds in the network, which allows the network to flow at long times longer than the network unbinding time τoff. Our coarse-grained mechanical profiling together with the fluorescence measurements, thus provides a reliable experimental framework that can be used to quantitatively evaluate the mechanical state of the protein condensates and study their physiological functions and connections with diseases.
In the second part, we studied how the identified percolated network emerges in the condensates and how its properties change with protein interaction by using a PrLDSAM model condensate system comprising an intrinsically disordered domain PrLD and a folded domain SAM. By using single amino acid mutations, we generate a series of PrLD-SAM mutants with a varying SAM-SAM binding strength ranging from weak to strong. As the protein interaction increases, the PrLD-SAM condensates undergo a rigidity percolation transition, after which their modulus and viscosity are enhanced by more than 100-fold. The drastic increase in condensate viscoelasticity is attributable to the formation of the percolating network, which only occurs when the protein interaction is elevated above a critical threshold. Below the threshold, the condensates behave like Newtonian or Maxwell-like fluids with their mechanics primarily determined by a surface tension and viscosity. After the percolation transition, a condensate-spanning network is formed and gives rise to a power law rheology in relaxation modulus E(t) (or complex shear modulus G∗(ω)) with exponent α ≃ 0.59. Notably, the pathological PrLD-SAM condensates carrying mutations found in patients with neurological disorders exhibit mechanical features akin to fluid-like condensates that lack a network. Our study highlights the network’s essential role within the condensate for normal neuronal function, stabilizing PSD structure, and localizing receptors, offering profound insights into the structural basis for synapse plasticity and advanced brain functions.
In the third part, we explored the mechanical properties of living cells and their relations to cellular states and activities using the multi-scale relaxation modulus E(t), which includes an exponential decay, a power law decay and a persistent component. These three modes, along with relaxation parameters E1, τ1, E2, τ2, alpha, E∞, characterize the mechanical response of cytosol, cytoskeleton and cellular active stress, respectively. In a human skin cell line, we found that E1, τ1 and alpha are correlated with the hydration state of the skin cells under osmotic challenges, which can be used to evaluate the moisturizing effects of aesthetic substances. In a retinal epithelial cell line, we demonstrated the mechanical role of a cytoskeleton binding protein GAS2L1 on cytoskeletal dynamics, which promotes actin stress fiber formation and stabilizes the cytoskeleton by increasing τ2. Additionally, we derived a quantitative mechanical spectrum for normal and cancerous breast cell lines. It was found that the breast cancer cells are generally softer than normal cells but possess a larger persistent modulus E∞, implying higher mechanical adaptability and cellular stress that facilitate cancer metastasis. These three examples on cell mechanics demonstrate that the relaxation modulus E(t) is an effective measure of cellular states and their implications for biological functions and pathological processes.