On the mechanisms of cortical actin flow and its role in cytoskeletal organisation of fibroblasts.

In this review we discuss the organization of F-actin in motile fibroblasts in relation to the phenomenon of cortical actin flow; we review some of the mechanisms proposed to drive this process and then relate some of our new findings on the interactions of cortical flow and substratum adhesions in fibroblasts. It is our thesis that the centripetal flow of F-actin through the lamellipodium and leading lamella is the major determining factor organising the polarized stress fiber system and associated cell-substratum adhesions in crawling fibroblastic cells. The broad flattened region of cytoplasm anterior to the nucleus, called the leading lamella, is fringed with much thinner structures, the lamellipodia, which are the primary protrusive organelles of motile cells. Lamellipodia are filled with a criss-crossed network of actin filaments interspersed with small bundles or ribs. In the leading lamella, the actin cytoskeleton is largely confined to two cortical layers beneath the dorsal and ventral cell surfaces. The ventral cortex is engaged in cell adhesion; the dorsal cortex is made up of a circumferentially orientated sheet of actin filaments and bundles which we team the dorsal cortical microfilament sheath (DCMS). Stress fibers insert into the ventral adhesions and pass back through the lamella rising up to meet the DCMS. Cortical flow appears to be a constitutive process in most types of cells when they become motile. In fibroblasts a continuous centripetal flux of structure is seen flowing through the lamellipodium and into the more central regions of the lamella at approximately 0.1 micron per second. Some of the structures engaged in the flux pass back and merge into the DCMS which is also moving rearward at a slower rate of 1 to 5 microns per minute. We find that the formation of a stress fibre is a direct consequence of cortical flow. Initially, stress fibre formation involves the establishment of a focal adhesion between an F-actin bundle in the lamellipodium and the substratum. Subsequently, a fibre grows centripetally from the adhesion and elongates coordinately with the rearward flowing cortex. Cortical flow is restrained locally at the distal end of the nascent fibre leading to indentation and folding of the sheet of filaments. This fold develops into an arc. We demonstrate that mechanical linkages exist between the lamellipodium and the DCMS. Cytochalasin induces a sudden and massive centripetal collapse of the DCMS which drags the lamellipodia with it.(ABSTRACT TRUNCATED AT 400 WORDS)