Regulation of plasma membrane blebbing by the cytoskeleton

When neuroblastoma cells are exposed to lysophosphatidic acid (LPA), they undergo a vigorous, but transient blebbing phase. The effect is sensitive to inhibition by staurosporine, KT 5926 (an inhibitor of myosin light chain kinase), and cytochalasin B, suggesting that LPA activates the phosphorylation of myosin light chain and increases the contractile activity of the actomyosin network. Cell contractions increase the intracellular pressure driving bleb formation. Calyculin, an inhibitor of protein phosphatase2A, also causes blebbing which continues as long as the drug is present, presumably by keeping myosin light chain in the phosphorylated state. Blebbing of neuroblastoma cells is regulated by the status of all three cytoskeletal systems: disassembly of microtubules by nocodazole and of intermediate filaments by acrylamide increased the number of blebbing cells. Cytochalasin B, on the other hand, prevents bleb retraction and, after prolonged incubation, bleb formation. These results are discussed in terms of a model viewing the cytoskeleton as an integrated network transmitting force throughout the cell. Bleb retraction was studied by transfecting neuroblastoma cells with a vector containing the gene for gamma-cytoplasmic actin fused to the green fluorescent protein EGFP (EGFP-actin). EGFP-actin was not detected on the membranes of extending blebs, but started accumulating along the cytoplasmic surface of blebs as soon as the extension phase came to an end and retraction set in. These results confirm earlier suggestions that actin polymerization is required for bleb retraction and for the first time directly relate the two events.