Protrusive activity, cytoplasmic compartmentalization, and restriction rings in locomoting blebbing Walker carcinosarcoma cells are related to detachment of cortical actin from the plasma membrane.
The dynamic events at the front of locomoting blebbing Walker carcinosarcoma cells [Keller and Bebie, Cell Motil. Cytoskeleton 33:241-251, 1996] are interpreted on the basis of an analysis of the actin cytoskeleton and its relationship to the plasma membrane in fixed cells using a novel double-staining procedure. The data show that blebs are formed where cortical actin is locally depolymerized and/or by detachment of the plasma membrane from more or less intact cortical actin layers. Dissociation between the cortical actin layer and the plasma membrane, which is stimulated by microtubule disassembly, is achieved by forward movement of the plasma membrane, rather than by retraction of the actin layer. Therefore, the detached actin layers form a boundary between the newly forming protrusions and the rest of the cell. They can be associated with "constriction rings," which we have termed "restriction rings." Detached actin layers can impede entry of organelles and the nucleus into the protrusions and thereby compartmentalize the cytoplasm. Later, detached cortical actin layers depolymerize, allowing for relaxation of the restriction rings and for forward movement of cytoplasmic organelles and the nucleus. Actin may repolymerize along the detached plasma membrane allowing for a new cycle to occur. Estimates indicate that the actin polymerization/depolymerization cycles may be largely confined to the front of blebbing cells. The findings suggest that the dynamic events at the front of blebbing metazoan cells are similar to those previously found in Amoeba proteus [Grebecki, Protoplasma, 154:98-111, 1990] but different from those found in lamellipodia.