Nanonet Force Microscopy
Cells migrate through extracellular matrices, often through formation of integrin-mediated focal adhesions. This allows the cells to sense the external environment by exerting forces and thus allowing cells to achieve tensional homeostasis. Mechanical forces are important for biological processes ranging from migration to remodeling of ECM in tumor environment to maintaining tissue homeostasis and organ development. Measurement and calibration of these forces can provide insight in cell mechanobiology and pathology of diseases.
At STEP lab, we have devised a suspended fiber-based force measurement platform (Nanonet Force Microscopy, NFM) using ECM-mimicking anisotropic fibers. NFM estimates cell forces from deflection of fibers (inwards or outwards) as cells tug or push on them. We deposit large diameter fibers (μm) that act as supporting structures (‘base fibers’). Smaller diameter fibers are deposited orthogonal to base fibers at desired spacing and fused at the intersections to achieve fixed-fixed boundary conditions. Our method establishes force vectors that originate from the focal adhesion sites and are directed along the major actin stress fibers (force bearing elements). A combination of beam mechanics and optimization techniques allows us to estimate forces by minimizing the error between finite element model predictions and measured experimental fiber deflections. NFM can be used to study the two-broad classification of forces: innate contractility inside-out (IO) forces and externally applied forces outside-In (OI). The unique fiber based platform has allowed us to demonstrate the following specific applications:
Force-exerting perpendicular lateral protrusions in fibroblastic cell contraction
Aligned extracellular matrix fibers enable fibroblasts to undergo myofibroblastic activation and achieve elongated shapes. Activated fibroblasts adhere to the extracellular fibers and contract, perpetuating the alignment of these fibers. This poorly understood feedback process is critical in chronic fibrosis conditions, including cancer. Here, using fiber networks that serve as force sensors, we identify “3D perpendicular lateral protrusions” (3D-PLPs) that evolve from lateral cell extensions named twines. The specific morphology of PLPs enables them to exert force on parallel neighboring fibers, causing cells to increase their contractility. Twines originate from stratification of cyclic actin-waves traversing the cell and swing freely in 3D to engage neighboring fibers. Once engaged, a lamellum forms and extends multiple secondary twines, which fill in to form a sheet-like PLP, in a force- entailing process that transitions focal adhesions to elongated 3D-adhesions. Controlling the geometry of extracellular networks confirms that anisotropic fibrous environments support 3D-PLP formation and function, suggesting an explanation for cancer-associated desmoplastic expansion at single-cell resolution.