Patterns of microfilament organization in animal cells

Cytoskeletal changes induced in HEp-2 cells by the cytotoxic necrotizing factor of Escherichia coli

The effect of the cytotoxic necrotizing factor of Escherichia coli on HEp-2 cells was studied by fluorescence and scanning electron microscopy. This cytotoxin, known for inducing the formation of giant multinucleated cells in several cell lines, caused changes in actin and tubulin organization. The presence of membrane ruffles at the cell border and of numerous thick bundles of actin crossing the cell body, suggests that the factor promotes cell spreading; this probably interferes with cytokinesis, ultimately leading to the formation of very large flattened multinucleated cells. Moreover, the nuclear segmentation observed in treated cells seems to be associated with a rearrangement of actin in the perinuclear region and with the presence of tubulin bundles in proximity to nuclear clefts. Although the primary target is still unknown, these findings suggest that the cytoskeleton is affected accounting for the multinucleation process induced by the factor.

Modulation of fibronectin gene expression in chondrocytes by viral transformation and substrate attachment

Chicken vertebral chondrocytes, which normally grow in suspension, synthesize large amounts of cartilage extracellular matrix proteins, but little fibronectin. We have analyzed the effects of both substrate attachment and transformation with a temperature-sensitive mutant of Rous sarcoma virus on fibronectin gene expression in these cells. Our experiments show that viral transformation increases fibronectin synthesis to a greater extent than substrate attachment. Furthermore, transformed chondrocytes have lost the ability to decrease fibronectin synthesis in response to suspension culture, suggesting that transformation alters the normal attachment-responsive control of fibronectin gene expression. Finally, infected substrate-attached chondrocytes shifted to the nonpermissive temperature for transformation use fibronectin RNA more efficiently in protein synthesis than cells grown under the other conditions, suggesting for the first time a role for translational control of fibronectin gene expression.

Human endothelial cell cultures: phenotypic modulation by leukocyte interleukins

We report here that soluble factors from activated mononuclear leukocytes have a dramatic effect on cultured endothelial cells. While human umbilical vein endothelial cells grown under standard conditions show a polygonal, epithelial-like morphology, cells exposed to culture media conditioned by lectin-activated human mononuclear leukocytes become extremely elongated and/or send out numerous cytoplasmic processes, assuming a dendritic configuration. This effect cannot be mimicked by exogenous cyclic AMP, is reversible upon interruption of the treatment, and appears specific for endothelial cells, since it has not been observed so far with other cell types. The shape changes are accompanied by a reorganization of the endothelial cell cytoskeleton: actin microfilament bundles tend to be disposed in parallel arrays, while intermediate filaments and microtubules penetrate up to the extremity of the cytoplasmic processes. Colchicine prevents endothelial cell elongation but only slightly impairs the formation of lateral cell processes ("dendritic configuration"). Purified interleukins were tested for their ability to induce these changes of cell shape. Escherichia coli-recombinant human interleukin 2 had no effect, and gamma-interferon only a slight effect on endothelial cell morphology. Interleukin 1 induced moderate cell elongation, while combined treatment with both interleukin 1 and gamma-interferon resulted in shape changes indistinguishable from those elicited by supernatants of activated mononuclear leukocytes. The possible relevance of the observed endothelial cell changes to the reported angiogenic activity of mononuclear cell products is discussed.

Disruption of microfilament organization after injection of F-actin capping proteins into living tissue culture cells

Capping proteins are F-actin binding proteins which interfere with the in vitro growth of an actin filament by blocking one of its ends (for recent reviews see refs 1-3). The majority of such proteins described so far "cap' the fast-growing (positive) end of the polar filament, thus reducing the velocity of filament growth while increasing the number of filaments being formed de novo from a monomer pool. We have studied the effects of capping proteins on the organization of actin filaments in living tissue culture cells by microinjection in conjunction with fluorescence, reflection contrast and electron microscopy. Our results, reported here, indicate that capping proteins from different sources disrupt microfilament bundles in a variety of cell types causing their disintegration from the distal end towards the centre of the cell.