High Resolution Force Distribution Of Migrating And Spreading Cells

Cell motility is crucial for the immune system and its misregulation is associated with cancer and other diseases. Migration is a complex cellular activity with many proteins coordinating within a subcellular motility machine. The machine's parts conspire to propel the cell forward by exerting forces on the environment with spatial variations down to the nanoscale. In this work we measured and statistically characterized the force distributions with the aim of illuminating underlying mechanisms. We cultured mouse embryonic fibroblast cells expressing GFP-actin on dense arrays of flexible nanoengineered pillars. During cell spreading pillar deflections quantitatively revealed cell traction patterns on the substrate. Simultaneous actin imaging enabled the cellular force map to be visualized within the evolving cell contour. The greatest forces were exerted in a ∼10-20 micron strip behind the leading edge and are directed inward. We analyzed time profiles of forces exerted on single pillars as the cell passed over the pillar location. Forces were relatively short lived and showed more fluctuation for pillars with smaller diameter than for larger ones. When the protruding cell edge reaches a pillar a period of large traction force begins, typically lasting several minutes, with maximum stress on the order of several nN/μm2. Stiffer substrates provoked larger tractions than more compliant substrates. Once the front of the cell has passed by, a drastic reduction in traction occurs. Spatial force distribution and its evolution were characterized by correlation functions relative to the leading edge. By varying the geometric parameters of the pillars, we probed a range of effective substrate rigidities. In summary, we combined high resolution measurements, statistical analysis and quantitative modeling to characterize and interpret signature features of migrating and spreading cell force maps.