Focal Adhesions are Composed of Filamentous Subunits Whose Length and Dynamics Depend on the Cell Spreading Area

Abstract

The cells ability to adhere to the extracellular matrix (ECM) is a fundamental feature of many higher eukaryotic cells and is required for cell migration, proliferation, and differentiation. At the scale of single cells, the primary patterns of adhesion to the ECM are called focal adhesions (FAs). They are thought to serve as mechano-sensor units. Recent progress in super resolution techniques opened the door to study ultrastructure of focal adhesions. However the dynamics of their spatial structure has not been studied yet. Here, we combine structured illumination microscopy (SIM) with total internal reflection fluorescence microscopy (TIRF) to study focal adhesions at high spatial resolution in live cells. We studied REF52 fibroblasts spread on fibronectin disks to avoid the variability of focal adhesions properties due to variable cell morphology and migratory status. We observed the formation of many focal adhesions localized at the lamellipodium-lamellum interface. We further show that (i) focal adhesions are formed by filaments subunits which grow retrograde and then shrink in ∼ 20 mins, (ii) that their formation depends on the spreading area of the cell and (iii) that each filament connects to a single actin cable, therefore linking the internal structure of focal adhesions to the maturation process through actin pulling on the focal complex. Thus our study reveals the ultrastructure and dynamics of focal adhesion and is a first step towards a better understanding of the formation of FAs with respect to the cell mechanical state.Keywords: focal adhesions, filamentous unit, SIM, retrograde flow, spreading area, paxillin.