Allometric cell spreading and the geometrical control of focal adhesion collective organization.

Focal adhesions are protein complexes that transmit actin cytoskeleton forces to the extracellular matrix and serve as signaling hubs that regulate cell physiology. While their growth is achieved through a local force-dependent process, the requirement of sustaining stress at the cell scale suggests a global regulation of the collective organization of focal adhesions. To investigate evidence of such large-scale regulation, we compared changes in cell shape and the organization of focal adhesion-like structures during the early spreading of fibroblasts either on a two-dimensional substrate or confined between two parallel plates, and for cells of different volumes. In this way, we reveal that the areal density of focal adhesions is conserved regardless of cell size or third-dimension confinement, despite different absolute values of the surface covered by adhesion clusters. In particular, the width of the focal adhesions ring, which fills the flat lamella at the cell front, adapts to cell size and third-dimension confinement and scales with cell-substrate contact radius. We find that this contact radius also adapts in the parallel plate geometry so that the cumulated area of cell-substrate contact is conserved at the cell scale. We suggest that this behavior is the result of three-dimensional cell shape changes, which govern spreading transitions. Indeed, because of volume conservation constraints, the evolution of cell body contact angle, adjusts according to cell size and confinement, whereas the rate of early spreading at the cell-substrate contact is not affected by three-dimensional geometry. Overall, our data suggest that a coordination between global and local scales mediates the adaptation of cell-substrate contacts and focal adhesions distribution to large-scale geometrical constraints, which allows an invariant cell-substrate adhesive energy.