Role of a 16S glycoprotein complex in cellular adhesion

A Brief History of Adherons: The Discovery of Brain Exosomes

Although exosomes were first described in reticulocytes in 1983, many people do not realize that similar vesicles had been studied in the context of muscle and nerve, beginning in 1980. At the time of their discovery, these vesicles were named adherons, and they were found to play an important role in both cell–substrate and cell–cell adhesion. My laboratory described several molecules that are present in adherons, including heparan sulfate proteoglycans (HSPGs) and purpurin. HSPGs have since been shown to play a variety of key roles in brain physiology. Purpurin has a number of important functions in the retina, including a role in nerve cell differentiation and regeneration. In this review, I discuss the discovery of adherons and how that led to continuing studies on their role in the brain with a particular focus on HSPGs.

Cell Biology of Cardiac Cushion Development

The valves of the heart develop in the embryo from precursor structures called endocardial cushions. After cardiac looping, endocardial cushion swellings form and become populated by valve precursor cells formed by an epithelial-mesenchymal transition (EMT). Endocardial cushions subsequently undergo directed growth and remodeling to form the valvular structures and the membranous septa of the mature heart. The developmental processes that mediate cushion formation include many prototypic cellular actions including adhesion, signaling, migration, secretion, replication, differentiation, and apoptosis. Cushion morphogenesis is unique in that these cellular possesses occur in a functioning organ where the cushions act as valves even while developing into definitive valvular structures. Cardiovascular defects are the most common congenital defects, and one of the most common causes of death during infancy. Thus, there is significant interest in understanding the mechanisms that underlie this complex developmental process. In this regard, substantial progress has been made by incorporating an understanding of cardiac morphology and cell biology with the rapidly expanding repertoire of molecular mechanisms gained through human genetics and research using animal models. This article reviews cardiac morphogenesis as it relates to heart valve formation and highlights selected growth factors, intracellular signaling mediators, and extracellular matrix components involved in the creation and remodeling of endocardial cushions into mature cardiac structures.

Partial purification of HLAMP-1 provides direct evidence for the multicomponent nature of the particulate matrix associated with cardiac mesenchyme formation

H-LAMP-1 is a 283 kDa protein that is involved in the transformation of endothelial cells into mesenchyme within the AV canal and proximal outflow tract of the heart. This protein is part of the particulate matrix that has been suggested to be composed of multicomponent complexes that have been termed cardiac adherons. However, to date no direct evidence has been provided that these proteins are complexed into an adheron-like particle. This report provides the first such evidence by showing that purification of hLAMP-1, under gentle conditions, results in the isolation of multiple bands of similar molecular weight within the fractions that contain anti-hLAMP-1 activity.

NCAM-mediated adhesion of transfected cells to agrin

The vertebrate neural cell adhesion molecule NCAM mediates heterophilic adhesion to heparan sulfate proteoglycans in embryonic chick brain membranes. In this study, mouse L cells transfected with chicken NCAM were used to identify two of these ligands as agrin and the target of the 6C4 monoclonal antibody. A third heparan sulfate proteoglycan, perlecan, appeared not to support NCAM-mediated adhesion. Enzymatic degradation of chondroitin sulfates decreased adhesion in agrin-containing membrane fractions but increased adhesion if the agrin had previously been removed by immunoprecipitation, suggesting that interactions between heparan sulfate and chondroitin sulfate proteoglycans have important influences on adhesion. Our experiments support the view that NCAM can interact with multiple, but not with all, heparan sulfate and chondroitin sulfate proteoglycans in chick brain membranes in both positive and negative ways to influence cell adhesion.

Multiple glycoproteins localize to a particulate form of extracellular matrix in regions of the embryonic heart where endothelial cells transform into mesenchyme

Cells derived from an epithelial-mesenchymal transformation within the atrioventricular canal and outflow tract are involved in the partitioning of the early embryonic heart into a four-chambered organ. This transformation process has been shown to proceed from an inductive interaction between the myocardium and competent, target endothelial cells within these regions of the heart. Interestingly, immunohistochemistry revealed the presence of fibronectin-positive particulates within the matrix of mesenchyme-forming regions (Mjaatvedt et al., 1987). This particulate matrix is extractable by EDTA and can elicit the epithelial-mesenchymal transformation in culture (Mjaatvedt and Markwald, 1989). Analysis of EDTA extracts of embryonic heart tissue revealed the presence of fibronectin and about 40 unidentified proteins, 6 of which appeared to be enriched in the biologically active 100,000g pellet fraction (Mjaatvedt and Markwald, 1989). Based on these and other data we have proposed that the particulate matrix is composed of a multicomponent complex of fibronectin and one or more of the low-molecular-weight proteins in this pellet. The purpose of the present study was to begin a biochemical characterization of the nonfibronectin proteins thought to be present in the matrix particulates. Given that many matrix constituents are glycoproteins, lectins were used to initially characterize the particulate constituents. Of the lectins tested, soybean agglutinin (SBA) was found to be specific only for matrix particulates. Histochemical analyses showed that SBA and antibodies against fibronectin colocalized regionally and temporally to the same matrix particulates in embryonic heart tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific configurations of fibronectin-containing particles correlate with pathways taken by neural crest cells at two axial levels

Although neural crest (NC) cells can potentially enter a number of intertissue spaces, they select a particular pathway that varies depending on the axial level. In the cranial region, NC cells enter the dorsal-lateral pathway (i.e., immediately subjacent to the ectoderm) and avoid the ventral pathway (i.e., pathway between the mesoderm and neural tube and within the mesodermal cell population), whereas in the trunk region, the majority of the NC cells enter the ventral pathway (i.e., between the somite and neural tube) and not the dorsal-lateral pathway. Our working hypothesis is that one determining factor in directing NC cell migration is the composition and/or intermolecular associations of the extracellular matrix (ECM) in these pathways. Histochemical staining, immunostaining, and lectin-binding studies on cryofixed and conventionally fixed tissue were conducted to initially characterize the ECM found in potential NC cell pathways prior to and during initial NC cell migration at two different axial levels. We found that, regardless of the axial level, the pathways into which NC cells eventually enter possessed a characteristic ECM arrangement. This arrangement included: 1) the presence of multicomponent, glycoprotein-containing spherical particles (0.1-0.5 micron in diameter); and 2) a low-sulfated ECM content. Although all particles contained fibronectin, only those in specific regions were able to bind to a monoclonal antibody directed to the cell-binding domain of fibronectin, suggesting that the conformation of fibronectin may be important in the expression of any in situ function of the molecule.

Isolation and partial characterization of endothelial cell extracellular complexes

Human endothelial cells release components into the growth medium that stimulate cell-substratum adhesion. Several macromolecular components were isolated by ultracentrifugation of the endothelial cell conditioned medium. The components were heterogeneous, consisting of several sizes when examined by sedimentation velocity and gel filtration. When the extracellular components were evaluated by electron microscopy, structurally discrete particles were observed. The extracellular components and the complexes mediated cell-substratum adhesion to both human umbilical and arterial endothelial cells. The majority of the extracellular components that promote endothelial cell adhesion were pelleted by ultracentrifugation. Although the complexes contained fibronectin, antibodies to fibronectin did not inhibit cell adhesion to the complexes. Significant inhibition of endothelial cell adhesion was observed in the presence of heparin and heparan sulfate. The supernatant fraction following ultracentrifugation of the growth medium contained a component that suppressed endothelial cell adhesion to culture dishes coated with fibronectin, type I collagen, and endothelial cell complexes. SDS-polyacrylamide gel electrophoresis indicated that the complexes contained several components, and the majority of the large-molecular-weight components were pelleted by ultracentrifugation. The conditioned medium from human endothelial cells contains specific complexes that promote cell-substratum adhesion and components that suppress cell-substratum adhesion.

Cell-substratum adhesion in chick neural retina depends upon protein-heparan sulfate interactions

Embryonic chick neural retina cells in culture release complexes of proteins and glycosaminoglycans, termed adherons, which stimulate cell-substratum adhesion when adsorbed to nonadhesive surfaces. Two distinct retinal cell surface macromolecules, a 170,000-mol-wt glycoprotein and a heparan sulfate proteoglycan; are components of adherons that can independently promote adhesion when coated on inert surfaces. The 170,000-mol-wt polypeptide contains a heparin-binding domain, as indicated by its retention on heparin-agarose columns and its ability to bind [3H]heparin in solution. The attachment of embryonic chick retinal cells to the 170,000-mol-wt protein also depends upon interactions between the protein and the heparan sulfate proteoglycan, since heparan sulfate in solution disrupts adhesion of chick neural retina cells to glass surfaces coated with the 170,000-mol-wt protein. This adhesion is not impaired by chondroitin sulfate or hyaluronic acid, which indicates that inhibition by heparan sulfate is specific. Polyclonal antisera directed against the cell surface heparan sulfate proteoglycan also inhibit attachment of retinal cells to the 170,000-mol-wt protein, which suggests that cell-adheron binding is mediated in part by interactions between cell surface heparan sulfate proteoglycan and 170,000-mol-wt protein contained in the adheron particles. Previous studies have indicated that this type of cell-substratum adhesion is tissue-specific since retina cells do not attach to muscle adherons. Schubert D., M. LaCorbiere, F. G. Klier, and C. Birdwell, 1983, J. Cell Biol. 96:990-998.

Cell-substratum adhesion in embryonic chick central nervous system is mediated by a 170,000-mol-wt neural-specific polypeptide

Embryonal chick neural retina cells release into the culture medium a complex of proteins and glycosaminoglycans, termed adherons, that promote cell to substratum adhesion. A monoclonal antibody (C1H3) blocks adheron-mediated cell to substratum adhesion and specifically binds to a 170,000-mol-wt protein present in retinal adherons (Cole, G.J., and L. Glaser, 1984, J. Biol. Chem., 259:4031-4034). The 170,000-mol-wt protein also can be identified in embryonic chick brain and peripheral nervous tissue. In the neural retina, C1H3 also binds to a second antigen with a molecular weight of 140,000 that is absent in the brain. Embryonic brain, therefore, provides a source for the immunopurification of the 170,000-mol-wt protein. Brain adherons also contain the 170,000-mol-wt protein, and cell to substratum adhesion mediated by these adherons is blocked by the C1H3 monoclonal antibody. The 170,000-mol-wt protein in the brain is therefore functionally identical to that in the retina. To demonstrate that adheron-mediated cell to substratum adhesion is caused by cell binding to the 170,000-mol-wt protein, we showed that (a) protease digestion, but not glycosaminoglycan hydrolase digestion of adherons, blocked their ability to bind cells to substratum; (b) the immunopurified 170,000-mol-wt protein blocks adheron-mediated cell to substratum adhesion; and (c) cells can bind to immunopurified 170,000-mol-wt protein bound to glass surfaces.

A role for adherons in neural retina cell adhesion

Embryonic chick neural retina cells release glycoprotein complexes, termed adherons, into their culture medium. When absorbed onto the surface of petri dishes, neural retina adherons increase the initial rate of neural retina cell adhesion; they also stimulate the rate of cell-cell aggregation. Adheron-stimulated adhesion is tissue specific, and the spontaneous aggregation of neural retina cells is inhibited by monovalent Fab' fragments prepared from an antiserum against neural retina adherons. Therefore cell surface antigenic determinants shared with adherons are involved in normal cell-cell adhesions. The particles from the heterogeneous neural retina population contain many proteins and several glycosaminoglycans. The adherons migrate as a symmetrical 12S peak on sucrose gradients and are predominantly 15-nm spheres when examined by electron microscopy. Finally, the specific activity of neural retina adherons increases from embryonic days 7 through 12 and then declines. These results suggest that glycoprotein particles may be involved in some of the adhesive interactions between neural retina cells and between the cells and their environment.

Properties of extracellular adhesion-mediating particles in myoblast clone and its adhesion-deficient variant

Both the skeletal muscle myoblast cell line L6 and an adhesion-deficient variant of L6 released glycoprotein complexes, termed adherons, into their culture medium. The adherons from the variant, M3A, differed from those of L6 in a number of properties. M3A adherons were much less effective in promoting the cell-substratum and cell-cell adhesion of myoblasts than L6 particles. The adherons from the two cell lines also differed in their relative sedimentation velocities in sucrose gradients and had different chemical compositions. The M3A particle lacked chondroitin and contained relatively less collagen and fibronectin than the L6 adheron. Both L6 and M3A particles adhered to plastic surfaces and cells equally well in the absence of calcium ions. Neither cell-cell adhesion nor particle aggregation occurred in calcium-free medium. However, in the presence of calcium, the L6 adherons aggregated completely and M3A particles aggregated poorly. These data suggest that at least two sets of interactions are required for adheron-mediated adhesion: a calcium-independent binding of the adheron to the cell, and a calcium-dependent interaction between particles that is directly responsible for adhesion. The M3A variant is blocked at the calcium-dependent step, resulting in an adhesion deficiency.