Cell adhesion and migration in the early vertebrate embryo: location and possible role of the putative fibronectin receptor complex

Localization of cell surface sites involved in fibronectin fibrillogenesis

Fibronectin binding sites on cultured human fibroblasts were localized by high voltage electron microscopy using either 5- or 18-nm colloidal gold beads (Au5 or Au18) bound to intact fibronectin, the 70-kD amino-terminal fragment of fibronectin that blocks incorporation of exogenous fibronectin into extracellular matrix, or 160-180-kD fragments of fibronectin with cell adhesion and heparin-binding activities. Binding sites for Au18-fibronectin on the cell surface were localized to specific regions along the edge of the fibroblast and on retraction fibers. Au18-fibronectin complexes at these sites were initially localized in clusters that co-aligned with intracellular microfilament bundles. With longer incubations, Au18-fibronectin complexes were arranged into long fibrillar networks on the cell surface and in the extracellular space. The appearance of Au18-fibronectin in these fibrillar networks and disappearance of clusters of Au18-fibronectin suggest that Au18-fibronectin complexes are arranged into matrix at specific regions of the cell surface. Au18-70-kD fragment complexes initially had a similar distribution to Au18-fibronectin complexes. With longer incubations, Au18-70-kD fragment complexes were found in long linear arrangements on the cell surface. Double labeling experiments using Au18-70-kD fragment and Au5-160-180-kD fragments showed that the 70-kD fragment and the 160-180-kD fragments bind to different regions of the cell.

The survival of adult mouse sensory neurons in vitro is enhanced by natural and synthetic substrata, particularly fibronectin

Primary cultures derived from adult mouse dorsal root ganglia have been maintained in the presence or absence of 5 X 10(-6)M cytosine arabinoside for periods of up to 4 weeks. In cultures in which cytosine arabinoside is present, the non-neuronal cell population is effectively reduced. When uncoated plastic substrata are used there is also a concurrent decrease in the number of neurons if the medium is supplemented with cytosine arabinoside. The effects on neuron survival of substrata coated with fibronectin, polyornithine, polylysine, and exudates prepared from mouse liver cells were studied. It was shown that neuronal densities similar to those with uninhibited media may be retained in the presence of cytosine arabinoside if fibronectin-coated substrata are prepared. With the other coating agents neuronal survival was also enhanced but to a lesser extent. The study offers a means therefore of producing purer cultures of dorsal root ganglia neurons than has previously been possible from adult mammalian sources.

Fibronectin and collagens modulate the proliferation and morphology of astroglial cells in culture

The proliferation and morphology of astroglia derived from neonatal rat cortex and cultured in serum-free medium on either untreated, or fibronectin-, or collagen I-, or collagen IV-treated substrates were investigated using tritiated thymidine autoradiography and immunocytochemical staining of glial fibrillary acidic protein (GFAP) and actin. Modification of culture substratum with fibronectin enhanced the rate of proliferation of astroglial cells and increased the proportion of process-bearing astroglial cells. The distribution of actin and patterns of adhesion observed were typical for motile cells. Both types of collagen decreased the proportion of astroglial cells undergoing mitosis. Many of the astroglial cells exhibited a flat morphology and displayed prominent stress fibres in the cell body and processes. The data suggest that specific interactions with the substratum modulate the proliferation and morphological behaviour of astroglial cells.

Homing of hemopoietic precursor cells to the embryonic thymus: characterization of an invasive mechanism induced by chemotactic peptides

During embryonic development, T cell precursors migrate to the thymus, where immunocompetency is acquired. Our previous studies have shown that avian hemopoietic precursor cells are recruited to the thymus by chemotactic peptides secreted by thymic epithelial cells (Champion, S., B. A. Imhof, P. Savagner, and J. P. Thiery, 1986, Cell, 44:781-790). In this study, we have characterized the homing of these precursor cells to the thymus in vivo by electron and light microscopy. Hemopoietic precursors could be seen to extravasate from blood or lymphatic vessels, migrate in the mesenchyme, traverse the perithymic basement membrane, and finally intercalate into the thymic epithelium. Labeled hemopoietic precursors injected into the blood circulation also followed the same pathway. Migrating hemopoietic precursor cells were found to express the fibronectin receptor complex. In the presence of thymic chemotactic peptides, hemopoietic precursors traverse a human amniotic basement membrane. This invasive process was inhibited by antibodies to laminin or to fibronectin, two major glycoproteins of the amniotic membrane, by monovalent Fab' fragments of antibodies to the fibronectin receptor, and, finally by synthetic peptides that contain the cell-binding sequence Arg-Gly-Asp-Ser of fibronectin. These results indicate that hemopoietic precursors respond to thymic chemotactic peptides by invasive behavior. Direct interactions between basement membrane components and fibronectin receptors appear to be required for this developmentally regulated invasion process.

Regulation of fibronectin receptor distribution by transformation, exogenous fibronectin, and synthetic peptides

Recent studies have shown that fibronectin and its 140K membrane receptor complex are spatially associated with microfilaments to form cell surface linkage complexes which are thought to mediate adhesive interactions between fibroblasts and their substrata. We examined the regulation of the organization of these cell surface structures in transformed and fibronectin-reconstituted cells as well as in cells treated with a competitive synthetic peptide inhibitor of fibronectin binding to its receptor. Correlative localization experiments with interference reflection microscopy and double-label or triple-label immunofluorescence revealed a concomitant loss of fibronectin, 140K receptor, and alpha-actinin colocalization at cell substratum extracellular matrix contact sites after transformation of chick fibroblasts by wild-type or temperature-sensitive Rous sarcoma viruses (RSV). Western and dot immunoblot analyses established that although similar total quantities of intact 140K molecules were present in the transformed cell cultures, significantly more was released into the culture medium of transformed cells. The 140K molecules on transformed cells were available for interaction with exogenously added fibronectin, which could reconstitute fibronectin-140K linkage complexes. In such fibronectin reconstitution experiments, many cells expressed both fibronectin-140K-actin linkage complexes and RSV pp60src, indicating that the morphological reversion could occur even in the continued presence of RSV transformation. The synthetic peptide Gly-Arg-Gly-Asp-Ser derived from the sequence of the cell-binding region of fibronectin could also prevent the organization of fibronectin-140K linkage complexes. Our results suggest that fibronectin interaction with cells regulates the organization of fibronectin receptor complexes and cytoskeletal components at the cell surface.

An antibody to a receptor for fibronectin and laminin perturbs cranial neural crest development in vivo

Previous studies from this laboratory (M. Bronner-Fraser (1985). J. Cell Biol. 101, 610) have demonstrated that an antibody to a cell surface receptor complex caused alterations in avian neural crest cell migration. Here, these observations are extended to examine the distribution and persistency of injected antibody, the dose dependency of the effect, and the long-term influences of antibody injection. The CSAT antibody, which recognizes a cell surface receptor for fibronectin and laminin, was injected lateral to the mesencephalic neural tube at the onset of cranial neural crest migration. Injected antibody molecules did not cross the midline, but appeared to diffuse throughout the injected half of the mesencephalon, where they remained detectable by immunocytochemistry for about 22 hr. Embryos were examined either during neural crest migration (up to 24 hr after injection) or after formation of neural crest-derived structures (36-48 hr after injection). In those embryo fixed within the first 24 hr, the major defects were a reduction in the neural crest cell number on the injected side, a buildup of neural crest cells within the lumen of the neural tube, and ectopically localized neural crest cells. In embryos allowed to survive for 36 to 48 hr after injection, the neural crest derivatives appeared normal on both the injected and control side, suggesting that the embryos compensated for the reduction in neural crest cell number on the injected side. However, the embryos often had severely deformed neural tubes and ectopic aggregates of neural crest cells. In contrast, several control antibodies had no effect. These findings suggest that the CSAT receptor complex is important in the normal development of the neural crest and neural tube.

Distribution of a putative cell surface receptor for fibronectin and laminin in the avian embryo

The cell substratum attachment (CSAT) antibody recognizes a 140-kD cell surface receptor complex involved in adhesion to fibronectin (FN) and laminin (LM) (Horwitz, A., K. Duggan, R. Greggs, C. Decker, and C. Buck, 1985, J. Cell Biol., 101:2134-2144). Here, we describe the distribution of the CSAT antigen along with FN and LM in the early avian embryo. At the light microscopic level, the staining patterns for the CSAT receptor and the extracellular matrix molecules to which it binds were largely codistributed. The CSAT antigen was observed on numerous tissues during gastrulation, neurulation, and neural crest migration: for example, the surface of neural crest cells and the basal surface of epithelial tissues such as the ectoderm, neural tube, notochord, and dermomyotome. FN and LM immunoreactivity was observed in the basement membranes surrounding many of these epithelial tissues, as well as around the otic and optic vesicles. In addition, the pathways followed by cranial neural crest cells were lined with FN and LM. In the trunk region, FN and LM were observed surrounding a subpopulation of neural crest cells. However, neither molecule exhibited the selective distribution pattern necessary for a guiding role in trunk neural crest migration. The levels of CSAT, FN, and LM are dynamic in the embryo, perhaps reflecting that the balance of surface-substratum adhesions contributes to initiation, migration, and localization of some neural crest cell populations.

Coupled expression and colocalization of 140K cell adhesion molecules, fibronectin, and laminin during morphogenesis and cytodifferentiation of chick lung cells

We have analyzed the expression and distribution of fibronectin, laminin, and the 140K cell adhesion molecules (140K complex) in embryonic chick lung cells by a combination of biochemical and immunofluorescent approaches. The 140K complex was identified by monoclonal antibody JG22E as a complex of glycoproteins averaging 140,000 Mr and has been implicated in vitro as a receptor for fibronectin and laminin. Our studies provide the first description that the 140K complex is developmentally regulated, and that the 140K complex appears to be involved in adhesion of epithelial and endothelial cells during morphogenesis. We have shown that the 140K complex is expressed in high quantity in embryonic lung cell types, but is markedly reduced in all of the differentiated cell types except smooth muscle. Embryonic lung cells are enriched in 140K complex on portions of cells in close proximity to areas rich in fibronectin. For example, during the formation of airways and alveolar tissues, 140K complex is concentrated at the basal surfaces of epithelial cells adjacent to fibronectin. Likewise, during the angiogenic invasion of capillaries into lung mesenchyme, the 140K complex becomes localized at sites on the basal surfaces of endothelial cells in close contact with fibronectin. Finally, cytodifferentiating lung smooth muscle cells show unusually high levels of 140K complex, fibronectin, and laminin that persist into the adult. In contrast to fibronectin, laminin is found to be uniformly distributed in the basement membranes of differentiating epithelial cells. It becomes prominent in adult alveolar epithelium and airway epithelium concomitant with a reduction or loss of 140K complex and fibronectin at cell-basement membrane attachment sites. Surprisingly, laminin is also present in a punctate pattern in the mesenchyme of early lung buds, however, laminin, fibronectin, and 140K complex are greatly reduced or lost during mesenchymal maturation. Our results are consistent with the active participation of the 140K complex in cell-to-matrix adhesion during morphogenesis of alveolar walls and cytodifferentiation of mesenchymal and smooth muscle cells.

Phosphorylation of the fibronectin receptor complex in cells transformed by oncogenes that encode tyrosine kinases

The fibronectin (FN) receptor in avian cells has been characterized previously as a complex of three membrane glycoproteins of about Mr 160,000, Mr 140,000, and Mr 120,000 (simply termed protein band 1, band 2, and band 3, respectively). Monoclonal antibodies to the band 3 protein of the complex prevent FN and laminin binding both in vivo and in vitro and enable the detection of the receptor proteins in the plasma membrane and in adhesion plaques. Association of the FN receptor proteins with the adhesion-plaque protein talin also has been reported. We now find that the band 2 and band 3 proteins in the complex are phosphorylated in Rous sarcoma virus-transformed chicken cells but not in normal chicken cells. Phosphorylation occurs predominantly on tyrosine and is accompanied by a reorganization of the receptor complex in the membrane of the transformed cells. Whereas normal cells contain the FN receptor in focal contacts and cellular processes between cells, v-src-transformed cells exhibit a more diffuse distribution of this receptor. In addition to the viral v-src oncogene, cells transformed by other avian oncogenes that also encode tyrosine kinases (v-fps, v-erbB, and v-yes) also express the receptor complex proteins in the phosphorylated state regardless of whether the transforming protein is detectable in adhesion plaques. These results suggest that the altered FN and laminin receptor proteins may contribute to the transformed phenotype, but their significance and role in the transformed state remain to be established.

Pathways of avian neural crest cell migration in the developing gut

The NC-1 and E/C8 monoclonal antibodies recognize a similar population of neural crest cells as they migrate from vagal levels of the neural tube and colonize the branchial arch region of 2- to 3-day-old chicken embryos. Some of these immunoreactive cells then appear to enter the gut mesenchyme on the third day of incubation just caudal to the third branchial cleft. After entering the gut, these cells migrate in a rostral-caudal direction, using primarily the superficial splanchnic mesodermal epithelium of the gut as a substratum. The antigen-positive cells remain preferentially associated with the splanchnopleure. Few antigenic cells enter the mesenchyme surrounding the endoderm at anterior levels whereas they are found throughout the mesenchyme when nearing the umbilicus. At postumbilical levels, immunoreactive cells are distributed on both sides of the differentiating muscle layer but not within it. Although fibronectin immunoreactivity can be found throughout the wall of the gut, there is no apparent relationship between the distribution of fibronectin and the location of the immunoreactive cells. These results suggest that a mechanism more complex than a mere interaction with fibronectin may account for migration of crest-derived cells in the gut.