Advanced imaging of biopolymer networks and filaments to resolve structure, filament-level mechanics, and cell-matrix interactions




Lissek, Emanuel Norbert

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The extracellular matrix (ECM) makes up the environment of many cells in our body and gives tissues their stability. It is comprised of highly non-linear fibrous biopolymer networks which display unique mechanical properties, such as stiffening by several orders of magnitude under strain. Cells interact with individual filaments of these networks, exploiting their properties to communicate and orient themselves, while migrating through the extracellular space. Its mechanical properties in turn profoundly influence the fate of cells. The cell-matrix interaction, however, is not well understood due to a lack of experimental techniques to both study the mechanical interplay between cells and their local environment on the single filament length scale, and quantify the contribution of single filaments to the large-scale network properties. To address this need, two novel microscopy techniques are described. First, quantitative Thermal Noise Imaging (TNI), a three-dimensional scanning probe technique which relies on a trapped nanoparticle as the probe. TNI is capable of imaging soft, optically heterogeneous and porous matter, with submicroscopic spatial resolution in aqueous solution. TNI images of both collagen fibrils in a network and grafted microtubules are shown, and it is demonstrated that structures can be localized with a precision of ~ 10nm. As a direct consequence of the work done with TNI, Activity Microscopy (aMic), a new way to visualize local network mechanics with single filament resolution is also introduced. Fibril positions in large two-dimensional slices through a collagen network with nanometer precision are localized, and fibrils' transverse thermal fluctuations with megahertz bandwidth along their contour are quantified. The fibrils' thermal fluctuations are then used as an indicator for their tension. The network displays a heterogeneous stress distribution, where “cold" fibrils with small thermal fluctuations surround regions of highly fluctuating “hot" fibrils. Finally, HeLa cells are seeded into the collagen network and the anisotropy in the propagation of their forces is quantified. While the data shown is limited to collagen, aMic will be of significant use when studying the mechanics of other fibrous networks and their application to artificial tissue and organ growth



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