Dynamics of contacts between lamellae of fibroblasts: essential role of the actin cytoskeleton

Cell volume-dependent phosphorylation of proteins of the cortical cytoskeleton and cell-cell contact sites. The role of Fyn and FER kinases

Cell volume affects diverse functions including cytoskeletal organization, but the underlying signaling pathways remained undefined. We have shown previously that shrinkage induces Fyn-dependent tyrosine phosphorylation of the cortical actin-binding protein, cortactin. Because FER kinase was implicated in the direct phosphorylation of cortactin, we investigated the osmotic responsiveness of FER and its relationship to Fyn and cortactin. Shrinkage increased FER activity and tyrosine phosphorylation. These effects were abolished by the Src family inhibitor PP2 and strongly mitigated in Fyn-deficient but not in Src-deficient cells. FER overexpression caused cortactin phosphorylation that was further enhanced by hypertonicity. Exchange of tyrosine residues 421, 466, and 482 for phenylalanine prevented cortactin phosphorylation by hypertonicity and strongly decreased it upon FER overexpression, suggesting that FER targets primarily the same osmo-sensitive tyrosines. Because constituents of the cell-cell contacts are substrates of Fyn and FER, we investigated the effect of shrinkage on the adherens junctions. Hypertonicity provoked Fyn-dependent tyrosine phosphorylation in beta-catenin, alpha-catenin, and p120(Cas) and caused the dissociation of beta-catenin from the contacts. This process was delayed in Fyn-deficient or PP2-treated cells. Thus, FER is a volume-sensitive kinase downstream from Fyn, and the Fyn/FER pathway may contribute to the cell size-dependent reorganization of the cytoskeleton and the cell-cell contacts.

Biochemical and functional characterization of intercellular adhesion and gap junctions in fibroblasts

Despite their significance in wound healing, little is known about the molecular determinants of cell-to-cell adhesion and gap junctional communication in fibroblasts. We characterized intercellular adherens junctions and gap junctions in human gingival fibroblasts (HGFs) using a novel model. Calcein-labeled donor cells in suspension were added onto an established, Texas red dextran (10 kDa)-labeled acceptor cell monolayer. Cell-to-cell adhesion required Ca(2+) and was >30-fold stronger than cell-to-fibronectin adhesion at 15 min. Electron micrographs showed rapid formation of adherens junction-like structures at approximately 15 min that matured by approximately 2-3 h; distinct gap junctional complexes were evident by approximately 3 h. Immunoblotting showed that HGF expressed beta-catenin and that cadherins and connexin43 were recruited to the Triton-insoluble cytoskeletal fraction in confluent cultures. Confocal microscopy localized the same molecules to intercellular contacts of acceptor and donor cells. There was extensive calcein dye transfer in a cohort of Texas red dextran-labeled cells, but this was almost completely abolished by the gap junction inhibitor beta-glycyrrhetinic acid and the connexin43 mimetic peptide GAP 27. This donor-acceptor cell model allows large numbers (>10(5)) of cells to form synchronous cell-to-cell contacts, thereby enabling the simultaneous functional and molecular studies of adherens junctions and gap junctions.

Lateral zone of cell-cell adhesion as the major fluid shear stress-related signal transduction site

It has been proposed previously that actin filaments and cell adhesion sites are involved in mechanosignal transduction. In this study, we present certain morphological evidence that supports this hypothesis. The 3D disposition of actin filaments and phosphotyrosine-containing proteins in endothelial cells in situ was analyzed by using confocal microscopy and image reconstruction techniques. Surgical coarctations were made in guinea pig aortas, and the same 3D studies were conducted on such areas 1 week later. Stress fibers (SFs) were present at both basal and apical regions of endothelial cells regardless of coarctation, and several phosphotyrosine-containing proteins were associated with SF ends. Apical SFs had one end attached to the apical cell membrane and the other attached to either the basal membrane or the lateral cell border. Within the coarctation area, the actin filament-containing and vinculin-containing structures became prominent, especially at the apical and the lateral regions. Substantially higher levels of anti-phosphotyrosine and anti-Src staining were detected in the constricted area, particularly at the cell-cell apposition, whereas the anti-focal adhesion kinase, anti-CT10-related kinase, anti-platelet endothelial cell adhesion molecule-l, anti-vinculin, and phalloidin staining intensities increased only slightly after coarctation. We propose that apical SFs directly transmit the mechanical force of flow from the cell apex to the lateral and/or basal SF anchoring sites and that the SF ends associated with signaling molecules are sites of signal transduction. Our results support the idea that the cell apposition area is the major fluid shear stress-dependent mechanosignal transduction site in endothelial cells.

Removal of calcium ions triggers a novel type of intercadherin interaction

Depletion of Ca(2+) ions from epithelial cell cultures has been shown to result in the rapid destruction of intercellular junctions. To understand the mechanism of this effect we have examined how removal of calcium ions from the culture medium of A-431 epithelial cells affects complexes incorporating the cell-cell adhesive receptors, E-cadherin, desmoglein or desmocollin. Sedimentation and biochemical analysis demonstrated that calcium removal triggers a rapid formation of a novel type of complex formed via direct lateral E-cadherin-desmoglein, E-cadherin-desmocollin and desmoglein-desmocollin dimerization of the extracellular cadherin regions. Replacement of Trp(156) and Val(157) of E-cadherin, that has been shown to abolish lateral and adhesive E-cadherin homodimerization in standard cultures, did not influence the formation of these ‘calcium-sensitive’ complexes. Furthermore, experiments with this mutant revealed that EGTA induced lateral Trp(156)/Val(157)-independent homodimerization of E-cadherin. Deletion mutagenesis of E-cadherin showed that these complexes are mediated by at least two extracellular cadherin domains, EC3 and EC4. Notably, protein kinase inhibitor H-7 which confers EGTA-independence of the adhesive E-cadherin complexes does not block this association. We propose that this novel type of intercadherin interaction is involved in the assembly of adherens junctions and their disassembly in low-calcium medium.

Myosin-dependent contractile activity of the actin cytoskeleton modulates the spatial organization of cell-cell contacts in cultured epitheliocytes

The spatial organization of cell-cell adherens junctions is distinct in cultured cells from two different tissue types, specifically, epitheliocytes and fibroblasts. In epitheliocytes, contacts are localized tangentially, along contacting cell edges and in association with circumferential actin bundles. Contacts between fibroblasts are radially oriented; that is, they are perpendicular to the overlapping edges of the cells and are associated with straight bundles of actin filaments. In the present study, we establish that the spatial organization of cell-cell contacts in the epithelial cell line IAR-2 can be converted from the typical tangential pattern to the radial pattern observed in fibroblasts. This transition can be induced by treatment with two agents, phorbol 12-myristate 13-acetate and nocodazole, which have different modes of action. Inhibition of myosin contractility reverses tangential-to-radial conversion of cell-cell contacts. These data suggest that formation of radially aligned contacts depends on modulation of contractility within the actin cytoskeleton through the myosin motor protein. The results open the possibility that modulation of the spatial organization of cell-cell contacts may play important roles in regulating organization and physiological functions of epithelial tissues.

Analysis of actin filament bundle dynamics during contact formation in live epithelial cells

The actin cytoskeleton is an integral component of the cell-cell adherens junction complex. We used fluorescence labeling of actin filaments and time-lapse laser scanning confocal microscopy to investigate the functional relationship between the organization of the actin cytoskeleton and formation of adherens junctions in live epithelial cells. Rhodamine-phalloidin was loaded into cultured cells by wounding epithelial monolayers in the presence of fluorescent analog. Rhodamine-phalloidin was incorporated into the actin filaments in stress fibers, circumferential bundles, and marginal bundles. Cells containing labeled actin filaments appeared physiologically normal since the rates of migration, rates of pseudopodial protrusion/retraction, ability to form contacts, and sensitivity to cytochalasin B were equivalent to non-loaded, control epithelial cells. Marginal actin bundles initially formed as bow-shaped bundles that were observed to straighten as the bundles flowed rearward and away from the free cell edge. When lamellae from adjacent cells made contact, rearward flow of marginal bundles ceased and the bundles started to disassemble with higher frequency. Next, we observed the formation of arc-like bundles at the edges of contacting cells, a position suggestive of a role in lateral expansion of the contact. During later stages of contact formation, new actin bundles assembled along the length of the expanding cell-cell boundary. These newly formed bundles are likely to participate in the establishment of the initial cadherin/actin cytoskeleton linkage and eventually form the circumferential bundles at the cell-cell adherens junction. Additionally, indirect immunolocalization studies characterized the location of myosin-II. A model is presented describing the function of the spatial and temporal dynamics of actin filament bundles and myosin-II activity in the formation of adherens junctions.

Effects of the myosin inhibitor 2,3-butanedione monoxime on the physiology of fission yeast

F-actin and associated myosins are thought to take part in a wide range of cellular processes, like motility and contraction, polarized growth, and secretion. The reagent 2,3-butanedione monoxime (BDM) is a well characterized inhibitor of the contraction of vertebrate muscle that reversibly affects myosin function and influences the intracellular concentration of Ca2+. Here we describe the influence of BDM on growth and division of the fission yeast Schizosaccharomyces pombe. At concentrations from 1-30 mM, BDM gradually inhibited formation and growth of S. pombe colonies on agar plates, with a lethal effect at > or = 15 mM. In strains of S. pombe that were blocked by elevated temperature from entry into mitosis, drug treatment reversibly decreased microtubule-independent tip growth and septation, with an IC50 value around 12 mM; nuclear division, on the other hand, was essentially unaffected by up to 15 mM BDM. At 30 mM BDM the secretion of invertase, which required both F-actin and microtubules, was decreased to the same extent as that seen when cytochalasin D was used to disrupt F-actin. However, the actin cytoskeleton was insensitive to up to 10 mM BDM, while the actin patches lost their polar distribution at 20-30 mM BDM. Cells treated with 5-20 mM BDM for 3 hours and then high pressure frozen did not show an accumulation of secretory vesicles. However, 10 mM BDM treatment disorganized the fungal cell wall, resulting in some unusually thick parts lying next to regions were the wall was almost absent. These defects could be rescued by incubating the cells in inhibitors of glucanases. Osmolytic stabilization with sorbitol rescued the effect of 15 mM BDM on colony survival, indicating that the secretion of wall components and/or wall-modifying enzymes may be the principal reason for cell death caused by BDM. Our results are consistent with the hypothesis that BDM influences actin-dependent processes in fission yeast and that actomyosin-dependent motility contributes to the secretory process of tip growth.

Cytomechanics of cadherin-mediated cell-cell adhesion

Cadherin-mediated adhesion regulates transitions from initial cell-cell recognition to loosely adherent cell clusters and ultimately, to strongly compacted groups of cells in colonies. Recent studies have described distinct roles for intermolecular clustering of cadherins as well as interactions of cadherin with the actin cytoskeleton in establishing cell-cell adhesion. Integrating cytomechanical roles of cadherin-mediated adhesion will lead to a greater understanding of how cadherins regulate tissue morphogenesis.