Merging of ventral fibers at adhesions drives the remodeling of cellular contractile systems in fibroblasts.

Abstract

Ventral stress fibers (VSFs) are contractile actin fibers present in the ventral plane of the cell and existing in a dynamic attachment with cell-matrix focal adhesions. VSFs are critical in cellular mechanobiological functions such as traction force production, cell polarization, and migration. VSF within their intracellular network vary from short, thinner fibers that are randomly oriented to long, thick fibers that span along the whole long axis of a cell. De novo VSF formation was shown to occur by condensation from the cortical actin mesh or by crosslinking of other stress fiber subtypes (dorsal stress fibers and transverse arcs) at the cell front. However, formation of long VSFs that extend across the whole cell axis is not well understood. Here, we report a novel phenomenon of VSF merging in migratory fibroblast cells, which is guided by mechanical force balance and contributes to VSF alignment along the long cell axis. The mechanism of VSF merging involves two steps: connection of two ventral fibers by an emerging myosin II bridge at an intervening adhesion and intervening adhesion dissolution to form a cohesive, contractile VSF. Our data indicate that these two steps are interdependent, since under conditions where adhesion disassembly is slowed, formation of the myosin bridge is slowed as well. Cellular data and computational modeling show that the angle of contact between merging fibers decides successful merging, with angles closer to 180 yielding merging events and shallower angles leading to merge failure. Our data and modeling further show that merging increases the share of uniformly aligned long VSFs, which would contribute to directional traction force production. Thus, we thoroughly characterize merging as process for dynamic reorganization of VSFs in steady state, investigating the steps and variants of the process as well as its functional significance in migratory cells.