Formation of focal adhesion-stress fibre complexes coordinated by adhesive and non-adhesive surface domains

Cell motility consists of repeating cycles of protrusion of a leading edge in the direction of migration, attachment of the advancing membrane to the matrix, and pulling of the trailing edge forward. In this dynamic process there is a major role for the cytoskeleton, which drives the protrusive events via polymerisation of actin in the lamellipodium, followed by actomyosin contractility. To study the transition of the actin cytoskeleton from a ´protrusive´ to ´retractive´ form, we have monitored the formation of focal adhesions and stress fibres during cell migration on a micro-patterned surface. This surface consisted of parallel arrays of 2 μm-wide, fibronectin-coated gold stripes, separated by non-adhesive (poly(ethylene glycol)-coated) glass areas with variable width, ranging from 4-12 μm. Monitoring the spreading of motile cells indicated that cell spreading was equally effective along and across the adhesive stripes, as long as the non-adhesive spaces between them did not exceed 6 μm. When the width of the PEG region was 8 μm or more, cells became highly polarised upon spreading, and failed to reach the neighboring adhesive stripes. It was also noted that as soon as the protruding lamella successfully crossed the PEG-coated area and reached an adhesive region, the organisation of actin in that area was transformed from a diffuse meshwork into a bundle, oriented perpendicularly to the stripes and anchored at its ends in focal adhesions. This transition depends on actomyosin-based contractility and is apparently triggered by the adhesion to the rigid fibronectin surface.