Gangliosides indirectly inhibit the binding of laminin to sulfatides

Mercury leads to abnormal red blood cell adhesion to laminin mediated by membrane sulfatides

Exposure to mercury is associated with numerous health problems, affecting different parts of the human body, including the nervous and cardiovascular systems in adults and children; however, the underlying mechanisms are yet to be fully elucidated. We investigated the role of membrane sulfatide on mercuric ion (Hg²⁺) mediated red blood cell (RBC) adhesion to a sub-endothelial matrix protein, laminin, using a microfluidic system that mimics microphysiological flow conditions. We exposed whole blood to mercury (HgCl₂), at a range of concentrations to mimic acute (high dose) and chronic (low dose) exposure, and examined RBC adhesion to immobilized laminin in microchannels at physiological flow conditions. Exposure of RBCs to both acute and chronic levels of Hg²⁺ resulted in elevated adhesive interactions between RBCs and laminin depending on the concentration of HgCl₂ and exposure duration. BCAM-Lu chimer significantly inhibited the adhesion of RBCs that had been treated with 50 μM of HgCl₂ solution for 1 h at 37 °C, while it did not prevent the adhesion of 3 h and 24 h Hg²⁺-treated RBCs. Sulfatide significantly inhibited the adhesion of RBC that had been treated with 50 μM of HgCl₂ solution for 1 h at 37 °C and 0.5 μM of HgCl₂ solution for 24 h at room temperature (RT). We demonstrated that RBC BCAM-Lu and RBC sulfatides bind to immobilized laminin, following exposure of RBCs to mercuric ions. The results of this study are significant considering the potential associations between sulfatides, red blood cells, mercury exposure, and cardiovascular diseases.

Calcium indirectly regulates immunochemical reactivity and functional activities of the N-domain of thrombospondin-1

Conformational changes induced in thrombospondin-1 by removal of calcium regulate interactions with some ligands of its N-modules. Because calcium binds primarily to elements of the C-terminal signature domain of thrombospondin-1, which are distant from the N-modules, such regulation was unexpected. To clarify the mechanism for this regulation, we compared ligand binding to the N-modules of thrombospondin-1 in the full-length protein and recombinant trimeric thrombospondin-1 truncated prior to the signature domain. Three monoclonal antibodies were identified that recognize the N-modules, two of which exhibit calcium-dependent binding to native thrombospondin-1 but not to the truncated trimeric protein. These antibodies or calcium selectively modulate interactions of fibronectin, heparin, sulfatide, alpha3beta1 integrin, tumor necrosis factor-alpha-stimulated gene-6 protein, and, to a lesser extent, alpha4beta1 integrin with native thrombospondin-1 but not with the truncated protein. These results indicate connectivity between calcium binding sites in the C-terminal signature domain and the N-modules of thrombospondin-1 that regulates ligand binding and functional activities of the N-modules.

Spatiotemporal changes in HNK-1/L2 glycoconjugates on avian embryo somite and neural crest cells

Neural crest cell migration was studied in trunks of quail and chick embryos using HNK-1 and L2 antibodies. At the intersegmental cleft, labeled crest cells were associated with the rostral wall of the somite rather than blood vessels. Migration into and through the rostral part of the sclerotomes was more rapid (40-70 microns/hr; quail) and the onset of localization was earlier (after 13-16 hr; quail) than previously supposed. Crest cells here were initially mono- to multipolar, scattered, and inconsistently oriented and formed numerous close (about 20 nm) homo- and heterotypic cell-cell contacts. In vitro models suggested that significant numbers of crest cells, however, could be unlabeled at this early phase. Somitic properties covarying with the hemisegmental pattern of crest cell immigration were investigated. Laminin distribution, although asymmetric in the somites, was not closely related to that of crest cells. Tenascin distribution matched that of crest cells, but only at the localization stage. Earlier, maximal tenascin expression occurred in the somite's caudal wall, a region avoided by crest cells. Chondroitin 6-sulfate proteoglycan expression was elevated in the caudal somite-half at the localization phase and also, at lumbar levels, at the immigration stage. This is consistent with tenascin and proteoglycan having a negative role in crest cell migration. The rostral somite-half was also labeled by HNK-1 and L2, but only in quails. This was associated with the cell surface, was transient, was stable to mild proteolysis, and was labile to cryoprocessing and lipophilic solvents. The spatial and temporal congruence with crest migration suggests that the HNK/L2 adhesion-related carbohydrate epitope on the somites indicates a molecule (possibly glycolipid) which acts via heterotypic cell-cell contacts to provide one cue in the patterned distribution of crest cells in the somites.

Sulfated glycolipids and cell adhesion

The adhesive glycoproteins laminin, thrombospondin, and von Willebrand factor bind specifically and with high affinity to sulfatides, and it is this binding that probably accounts for their ability to agglutinate glutaraldehyde-fixed erythrocytes. The three proteins differ, however, in the inhibition of their binding to sulfatides by sulfated polysaccharides. Fucoidan strongly inhibits binding of both laminin and thrombospondin, but not of von Willebrand factor, suggesting the involvement of laminin or thrombospondin, or other unknown sulfatide-binding proteins in specific cell interactions that are also inhibited by fucoidan. Thrombospondin adsorbed on plastic promotes the attachment and spreading of some melanoma cells. Interestingly, fucoidan and an antibody against the sulfatide-binding domain of thrombospondin selectively inhibit spreading but not attachment to thrombospondin-coated surfaces. Sulfatides, but not neutral glycolipids or gangliosides, when adsorbed on plastic also promote attachment and spreading of some cultured cell lines. Direct adhesion of melanoma cells requires high densities of adsorbed sulfatide. In the presence of laminin, however, specific adhesion of some cell types to sulfatide is strongly stimulated and requires only low densities of adsorbed lipid, suggesting that laminin is mediating adhesion by crosslinking receptors on the cell surface to sulfatide adsorbed on the plastic. Although thrombospondin also binds to sulfatides and to melanoma cells, it does not enhance but rather inhibits direct and laminin-dependent melanoma cell adhesion to sulfatide, presumably because it is unable to bind simultaneously to ligands on opposing surfaces. Thus, sulfated glycolipids can participate in both laminin- and thrombospondin-mediated cell adhesion, but their mechanisms of interaction are different.

Glycoconjugates and cell adhesion: the adhesive proteins laminin, thrombospondin and von Willebrand's factor bind specifically to sulfated glycolipids

The adhesive glycoproteins laminin, thrombospondin and von Willebrand's factor bind specifically and with high affinity to sulfated glycolipids, and it is this binding that probably accounts for their ability to agglutinate glutaraldehyde-fixed erythrocytes. The 3 proteins differ, however, in the effect of sulfated polysaccharides on their binding to sulfatides. Fucoidan strongly inhibits binding of both laminin and thrombospondin, but not of von Willebrand's factor, suggesting the involvement of laminin or thrombospondin or other unknown sulfatide-binding proteins in specific cell interactions that are also inhibited by fucoidan. Thrombospondin adsorbed onto plastic promotes the attachment and spreading of G361 melanoma cells. Interestingly, fucoidan and an antibody directed against the sulfatide-binding domain of thrombospondin selectively inhibit spreading but not attachment. Sulfatides, but not neutral glycolipids or gangliosides, when adsorbed onto plastic also promote attachment and spreading of G361 melanoma cells. Direct adhesion of G361 cells requires high densities of sulfatide. In the presence of laminin, however, specific adhesion of G361 cells to sulfatide is strongly stimulated and requires only low densities of adsorbed lipid, suggesting that laminin mediates adhesion by cross-linking receptors on the melanoma cell surface to sulfatide adsorbed onto the plastic. Although thrombospondin binds to sulfatide and to G361 cells, it does not enhance but rather inhibits direct and laminin-dependent G361 cell adhesion to sulfatide, presumably because it is unable to bind simultaneously to ligands on opposing surfaces. Thus, sulfated glycoconjugates participate in both laminin- and thrombospondin-mediated cell adhesion, but their mechanisms of interaction are different.

Laminin: molecular organization and biological function

Laminin, the most abundant glycoprotein molecule found in basement membrane, has multiple functions in eukaryotic tissues. It serves to attach epithelial cells to basement membrane, aids development and migration of specific cell types in growth and maturation, and has been implicated in tumor metastasis and some types of infection. Current concepts of the molecular organization and myriad functions of the laminin molecule are reviewed.

Molecules that make axons grow

The study of neurite growth in tissue culture has been a productive way to identify substances that may control the behavior of axons in vivo. Molecules that promote the outgrowth of neurites include nerve growth factor, laminin, fibronectin, and a protease inhibitor derived from glia. Evidence that these molecules may influence axon growth and guidance in vivo is discussed. The effects these molecules have at the cellular level are compared, in an attempt to identify common mechanisms of action. Several less well-characterized molecules that influence the behavior of neurites are also discussed.

Binding of laminin to brain gangliosides and inhibition of laminin-neuron interaction by the gangliosides

Binding of laminin to glycolipids of neuronal membranes was studied with a thin-layer chromatography overlay assay. The major brain ganglioside GD1A was the main binding component, when chromatograms containing the same molar amount of the different brain gangliosides and the brain sulfatide were incubated with laminin at physiological ionic strength. The possible role of laminin binding to brain gangliosides in laminin-neuron interactions was studied with adhesion assays. It was found that binding of rat brain neurons to laminin is blocked by 10-40 microM brain gangliosides but not by sulfatide. The inhibition by the gangliosides is suggested to be due to competition with the cell surface interaction sites of laminin and not to binding of the gangliosides to the cells. Our findings support the idea that the adhesive and neurite-promoting effect of laminin is dependent on its interaction with gangliosides at the neuronal cell surfaces.