ABSTRACT
Protrusions traditionally linked with the leading edge of a persistently migrating cell are well understood; whereas, those formed laterally and away from the leading edge remain poorly described. Here, using aligned fiber networks that also serve as force sensors, we quantitate the role of lateral nano-projections (twines) that mature into broad force-exerting structures through the formation of twine-bridges, thus allowing anisotropic cells to spread laterally. Using quantitative microscopy at high spatiotemporal resolution, we show that twines of varying lengths can originate from stratification of cyclic actin waves traversing along the entire length of the cell, and not just at the leading edge. Primary twines can swing freely in 3D and engage with neighboring fibers in interaction times of seconds. Once engaged, the actin lamellum grows along the length of primary twine and re-stratifies to form a secondary twine. Engagement of secondary twine with the neighboring fiber leads to the formation of a suspended primary-secondary twine-bridge; a critical step in providing a conduit for actin to advance along and populate (mature) the twine-bridge. Using force vectors that originate from adhesion sites and directed along f-actin stress fibers, we show that twine-bridges arising mostly perpendicular to the anisotropic cell body (perpendicular lateral protrusions PLPs) broaden to apply tens of nanoNewton contractile forces, thus allowing cells to spread onto multiple fibers resulting in higher contractility and lower migration rates. Our identification of similarities in the utility of PLPs across multiple cell lines generalizes the role of twine-bridges in persistent cell migration and fibroblastic cell contraction.