Disintegration of adhesion plaques in chicken embryo fibroblasts upon Rous sarcoma virus-induced transformation: different dissociation rates for talin and vinculin

v-src-induced cell shape changes in rat fibroblasts require new gene transcription and precede loss of focal adhesions

The mechanism of v-src-induced morphological transformation is still obscure. We compared LA29 rat fibroblasts, which express a temperature-sensitive (ts) v-src mutant, with D1025 rat fibroblasts, transfected with a ts mutant of v-fps. Upon transformation, LA29 cells adopted an elongated shape with reduced focal adhesions and loss of actin stress fibers. In contrast, activation of v-fps in D1025 cells had little effect on morphology. In both cells, paxillin was strongly tyrosine phosphorylated upon activation of the kinases. This indicates that paxillin phosphorylation is not required, or not sufficient, for the v-src-induced disruption of focal adhesions. As previously described by others, v-src activated the ras-MAP kinase (MAPK) pathway, as indicated by tyrosine phosphorylation of the rasGAP-associated proteins p62 and p190 and MAPK phosphorylation. Since MAPK affects transcription, this suggested that novel gene transcription was required. This notion was confirmed using actinomycin D and cycloheximide, which did not impair activation of v-src kinase activity, but completely blocked v-src-induced morphological changes, as demonstrated using image analysis. Furthermore, we observed that v-src-induced changes in cell shape occurred before the reduction in number and size of focal adhesions. We conclude that v-src-induced transformation of rat fibroblasts depends on synthesis of a protein, which induces rapid changes in cell shape that precede the loss of focal adhesions.

Talin requires beta-integrin, but not vinculin, for its assembly into focal adhesion-like structures in the nematode Caenorhabditis elegans

In cultured cells, the 230-kDa protein talin is found at discrete plasma membrane foci known as focal adhesions, sites that anchor the intracellular actin cytoskeleton to the extracellular matrix. The regulated assembly of focal adhesions influences the direction of cell migrations or the reorientation of cell shapes. Biochemical studies of talin have shown that it binds to the proteins integrin, vinculin, and actin in vitro. To understand the function of talin in vivo and to correlate its in vitro and in vivo biochemical properties, various genetic approaches have been adopted. With the intention of using genetics in the study of talin, we identified a homologue to mouse talin in a genetic model system, the nematode Caenorhabditis elegans. C. elegans talin is 39% identical and 59% similar to mouse talin. In wild-type adult C. elegans, talin colocalizes with integrin, vinculin, and alpha-actinin in the focal adhesion-like structures found in the body-wall muscle. By examining the organization of talin in two different C. elegans mutant strains that do not make either beta-integrin or vinculin, we were able to determine that talin does not require vinculin for its initial organization at the membrane, but that it depends critically on the presence of integrin for its initial assembly at membrane foci.

Mapping of adherens junction components using microscopic resonance energy transfer imaging

Quantitative microscopic imaging of resonance energy transfer (RET) was applied for immunological high resolution proximity mapping of several cytoskeletal components of cell adhesions. To conduct this analysis, a microscopic system was developed, consisting of a highly stable field illuminator, computer-controlled filter wheels for rapid multiple-color imaging and a sensitive, high resolution CCD camera, enabling quantitative data recording and processing. Using this system, we have investigated the spatial inter-relationships and organization of four adhesion-associated proteins, namely vinculin, talin, alpha-actinin and actin. Cultured chick lens cells were double labeled for each of the junctional molecules, using fluorescein- and rhodamine-conjugated antibodies or phalloidin. RET images were acquired with fluorescein excitation and rhodamine emission filter setting, corrected for fluorescein and rhodamine fluorescence, and normalized to the fluorescein image. The results pointed to high local densities of vinculin, talin and F-actin in focal adhesions, manifested by mean RET values of 15%, 12% and 10%, respectively. On the other hand, relatively low values (less than 1%) were observed following double immunofluorescence labeling of the same cells for alpha-actinin. Double indirect labeling for pairs of these four proteins (using fluorophore-conjugated antibodies or phalloidin) resulted in RET values of 5% or lower, except for the pair alpha-actinin and actin, which yielded significantly higher values (13-15%). These results suggest that despite their overlapping staining patterns, at the level of resolution of the light microscope, the plaque proteins vinculin and talin are not homogeneously interspersed at the molecular level but form segregated clusters. alpha-Actinin, on the other hand, does not appear to form such clusters but, rather, closely interacts with actin. We discuss here the conceptual and applicative aspects of RET measurements and the implications of the results on the subcellular molecular organization of adherens-type junctions.

Immuno-EM localization of the beta 1 integrin subunit in wet-cleaved fibronectin-adherent fibroblasts

Using immuno-EM, we have studied the distribution of the beta 1 integrin subunit in chicken embryo fibroblasts allowed to adhere and spread for 3 hours on a fibronectin-coated surface in serum-free medium. The cells were wet-cleaved, which removed most of the cell body, yielding ventral plasma membranes with little, and sometimes virtually no, associated cytoskeleton. The beta 1 integrin subunit was detected with antibodies against the cytoplasmic domain. In immune fluorescence, it colocalized with adhesion plaques, in a punctate staining pattern, and often seemed to be at the periphery of the plaque. By immuno-EM, beta 1 was in fact found in discrete clusters, not throughout the plaque. In deep-cleaved cells from which virtually all cytoskeleton was removed, clusters could often be seen to be located on fibronectin fibrils. Furthermore, beta 1 was present in clusters at the cell margins, and isolated or in small groups at the very edge of the cell. When fibronectin synthesis, and consequently fibril formation, was inhibited by cycloheximide, large adhesion plaque-like structures were formed at the cell margin. This phenotype was reversed by addition of soluble fibronectin, which was incorporated into fibrils. As in normal plaques, talin and vinculin were present, the plasma membrane was very close (10-20 nm) to the substratum and the fibronectin layer underneath was removed. These plaques did contain beta 1 integrins but they were not in clusters. These observations indicate that the talin-vinculin network of an adhesion plaque is normally anchored to the substratum at discrete beta 1 integrin clusters that may be located on fibronectin fibrils, and that elsewhere the plaque is not necessarily attached to the substratum by interaction of integrins with matrix proteins. In the absence of fibronectin fibrils, adhesion plaque-like structures can be formed, but these are aberrant in size, location and fine structure.

Structures linking microfilament bundles to the membrane at focal contacts

We used quick-freeze, deep-etch, rotary replication and immunogold cytochemistry to identify a new structure at focal contacts. In Xenopus fibroblasts, elongated aggregates of particles project from the membrane to contact bundles of actin microfilaments. Before terminating, a single bundle of microfilaments interacts with several aggregates that appear intermittently over a distance of several microns. Aggregates are enriched in proteins believed to mediate actin-membrane interactions at focal contacts, including beta 1-integrin, vinculin, and talin, but they appear to contain less alpha-actinin and filamin. We also identified a second, smaller class of aggregates of membrane particles that contained beta 1-integrin but not vinculin or talin and that were not associated with actin microfilaments. Our results indicate that vinculin, talin, and beta 1-integrin are assembled into distinctive structures that mediate multiple lateral interactions between microfilaments and the membrane at focal contacts.

Transmembrane molecular assemblies in cell-extracellular matrix interactions

Several new interactions have been identified and proteins characterized in focal adhesions. Together they suggest alternative or parallel linkages between actin filaments and members of the integrin family of extracellular receptors. Transformed cells continue to serve as models for studying the assembly and disassembly of these adhesions.

Cell/substratum adhesions in RSV-transformed rat fibroblasts

Cell/substratum adhesions have been studied in rat fibroblasts transformed by a ts-mutant of Rous sarcoma virus (LA-29) using light and electron microscopy and a variety of preparative methods including immunolabeling. Cells were studied both during the process of transformation, i.e., shifting from 39 degrees to 35 degrees C, and in a fully transformed state (passaged at 35 degrees C continuously). The typical focal contacts observed at 39 degrees C (restrictive temperature) were replaced by "point-contacts" (100-200 per cell) which were classified by immunolabeling as podosome-like adhesions containing actin, beta 1 integrin subunit, vinculin, talin, alpha-actinin, and small membrane patches containing clathrin and integrin. Tyrosine-phosphorylated proteins and pp60src were found in association with groups of small particles on the protoplasmic surface of ventral membranes by gold immunolabeling. Both types of point-contacts were visualized by electron microscopy of ultrathin sections and shadowed replicas and characterized by gold immunolabeling wherever possible. The overall composition of podosome-like adhesions is similar to focal contacts but there are differences in the three-dimensional organization of the microfilaments and in the topography of vinculin which is associated more with actin filaments than with the plasma membrane. The presence of talin and extracellular matrix receptor in podosomes together with the adhesive properties of these actin-containing structures argues against the hypothesis that pp60src affects the interaction of actin with the plasma membrane by phosphorylating the fibronectin receptor and/or other associated proteins.