Activity Microscopy: Extracting filament mechanical properties from thermal noise

Synopsis

Both within and outside of the cell, molecular scaffolds perform vital roles in organization and determining mechanical properties. Inside the cell, microtubules are responsible for organelles organization, cell division and active transport. Outside the cell, collagen fibrils in the extracellular matrix provide rigidity to tissue and mediate mechanical signals between cells in a process known as mechanotransduction. In one example of mechanotransduction, sensing local network stiffness results in tissue specific stem cell differentiation. Studying local mechanical properties allows us to understand the forces that can be transmitted by these structures and get information about the internal force response of the fibers. Activity microscopy is a new optical-tweezer-based technique developed by our lab that allows one to study the mechanics of filaments using the naturally occurring thermal fluctuations. We are able to image the collagen fibrils to extract the fibril’s thickness profile while simultaneously measuring the fibril's fluctuation amplitude. By comparing these measurements to the worm-like chain model based simulations, we are able to measure the Young's modulus of individual collagen fibrils. With the help of the high sampling rates, we can analyze the motion of the fluctuating fibrils and compare the power spectral densities of the filament transverse motion to the predictions of the worm-like chain model. As we are not limited to measurements on the coverslip surface, we hope to expand our technique to imaging local filament fluctuations in networks and analyze the local response of the network to external force.