Cell adhesion and spreading on fluid membranes through microtubules-dependent mechanotransduction

During cell adhesion, integrins form clusters that transmit mechanical forces to the substrate (mechanotransduction) and regulate biochemical signaling depending on substrate stiffness. Studies on mechanotransduction significantly advanced our understanding of cell adhesion and were mostly performed on rigid substrates. In contrast to rigid substrates, integrins ligands on fluid supported lipid bilayers (SLBs) are mobile and adhesive complexes cannot serve as anchoring points promoting cell spreading. Here, we demonstrate that cells spread on SLBs coated with Invasin, a high-affinity integrin ligand. We show that in contrast to SLBs functionalized with RGD peptides, integrin clusters grow in size and complexity on Invasin-SLBs to a similar extent as on glass. While actomyosin contraction dominates adhesion maturation on stiff substrates, we find that integrin mechanotransduction and cell spreading on fluid SLBs rely on dynein pulling forces along microtubules perpendicular to membranes and microtubules pushing on adhesive complexes, respectively. These forces that may also occur on non-deformable surfaces are revealed in fluid substrate set ups. Our findings, supported by a theoretical model, demonstrate a new mechanical role for microtubules in integrin clustering.