Chemiluminescence detection of gangliosides by thin-layer chromatography

Ganglioside-binding specificities of E. coli enterotoxin LT-IIc: Importance of long-chain fatty acyl ceramide

Bacterial heat-labile (LT) enterotoxins signal through tightly regulated interactions with host cell gangliosides. LT-IIa and LT-IIb of Escherichia coli bind preferentially to gangliosides with a NeuAcα2-3Galβ1-3GalNAc terminus, with key distinctions in specificity. LT-IIc, a newly discovered E. coli LT, is comprised of an A polypeptide with high homology, and a B polypeptide with moderate homology, to LT-IIa and LT-IIb. LT-IIc is less cytotoxic than LT-IIa and LT-IIb. We theorized that LT-IIc-host cell interaction is regulated by specific structural attributes of immune cell ganglioside receptors and designed experiments to test this hypothesis. Overlay immunoblotting to a diverse array of neural and macrophage gangliosides indicated that LT-IIc bound to a restrictive range of gangliosides, each possessing a NeuAcα2-3Galβ1-3GalNAc with a requisite terminal sialic acid. LT-IIc did not bind to GM1a with short-chain fatty acyl ceramides. Affinity overlay immunoblots, constructed to a diverse array of known ganglioside structures of murine peritoneal macrophages, established that LT-IIc bound to GM1a comprised of long-chain fatty acyl ceramides. Findings were confirmed with LT-IIc also binding to GM1a of RAW264.7 cells, comprised of a long-chain fatty acyl ceramide. Thus, LT-IIc-ganglioside binding differs distinctly from that of LT-IIa and LT-IIb. LT-IIc binding is not just dependent on carbohydrate composition, but also upon the orientation of the oligosaccharide portion of GM1a by the ceramide moiety. These studies are the first demonstration of LT-ganglioside dependence upon ceramide composition and underscore the contribution of long-chain fatty acyl ceramides to host cell interactions.

Mammalian cell ganglioside-binding specificities of E. coli enterotoxins LT-IIb and variant LT-IIb(T13I)

LT-IIb, a type II heat-labile enterotoxin of Escherichia coli, is a potent immunologic adjuvant with high affinity binding for ganglioside GD1a. Earlier study suggested that LT-IIb bound preferentially to the terminal sugar sequence NeuAcalpha2-3Galbeta1-3GalNAc. However, studies in our laboratory suggested a less restrictive binding epitope. LT-IIb(T13I), an LT-IIb variant, engineered by a single isoleucine-threonine substitution, retains biological activity, but with less robust inflammatory effects. We theorized that LT-IIb has a less restrictive binding epitope than previously proposed and that immunologic differences between LT-IIb and LT-IIb (T13I) correlate with subtle ganglioside binding differences. Ganglioside binding epitopes, determined by affinity overlay immunoblotting and enzymatic degradation of ganglioside components of RAW264.7 macrophages, indicated that LT-IIb bound to a broader array of gangliosides than previously recognized. Each possessed NeuAcalpha2-3Galbeta1-3GalNAc, although not necessarily as a terminal sequence. Rather, each had a requisite terminal or penultimate single sialic acid and binding was independent of ceramide composition. RAW264.7 enterotoxin-binding and non-binding ganglioside epitopes were definitively identified as GD1a and GM1a, respectively, by enzymatic degradation and mass spectroscopy. Affinity overlay immunoblots, constructed to the diverse array of known ganglioside structures of murine peritoneal macrophages, established that LT-IIb bound NeuAc- and NeuGc-gangliosides with nearly equal affinity. However, LT-IIb(T13I) exhibited enhanced affinity for NeuGc-gangliosides and more restrictive binding. These studies further elucidate the binding epitope for LT-IIb and suggest that the diminished inflammatory activity of LT-IIb(T13I) is mediated by a subtle shift in ganglioside binding. These studies underscore the high degree of specificity required for ganglioside-protein interactions.

Determination of glycolipid-protein interaction specificity

Glycolipids are found on all eukaryotic cells. Their expression varies among tissues, with the highest density found in the brain, where glycolipids are the most abundant of all glycoconjugate classes. In addition to playing roles in membrane structure, glycolipids also act as cell surface recognition molecules, mediating cell-cell interactions, as well as binding certain pathogens and toxins. Because of their amphipathic nature, underivatized glycolipids are amenable to immobilization on hydrophobic surfaces, where they can be probed with lectins, antibodies, pathogens, toxins, and intact cells to reveal their binding specificities and affinities. Three particularly useful methods to probe specific glycolipid-mediated recognition events are microwell adsorption (ELISA), thin layer chromatography overlay, and surface plasmon resonance (SPR) spectroscopy.

Ganglioside GM3 inhibits matrix metalloproteinase-9 activation and disrupts its association with integrin

Gangliosides GM3 and GT1b both inhibit epithelial cell adhesion and migration on fibronectin. GT1b binds to integrin alpha5beta1 and blocks the integrin-fibronectin interaction; GM3 does not interact with integrin, and its effect is poorly understood. We evaluated the effects of endogenous modulation of GM3 expression on epithelial cell motility on several matrices and the mechanism of these effects. Endogenous accumulation of GM3 decreased cell migration on fibronectin, types I, IV, and VII collagen matrices; depletion of GM3 dramatically increased cell migration, regardless of matrix. GM3 overexpression and depletion in vitro correlated inversely with the expression and activity of matrix metalloproteinase-9; consistently, the cell migration stimulated by GM3 depletion is reversed by inhibition of matrix metalloproteinase-9 activity. Accumulation and depletion of GM3 in epithelial cells grown on fibronectin also correlated inversely with epidermal growth factor receptor and mitogen activated protein kinase phosphorylation and with Jun expression. Ganglioside depletion facilitated the co-immunoprecipitation of matrix metal-loproteinase-9 and integrin alpha5beta1, while endogenous accumulation of GM3, but not GT1b, blocked the co-immunoprecipitation. These data suggest modulation of epidermal growth factor receptor signaling and dissociation of integrin/matrix metalloproteinase-9 as mechanisms for the GM3-induced effects on matrix metalloproteinase-9 function.

Epidermal growth factor receptor glycosylation is required for ganglioside GM3 binding and GM3-mediated suppression [correction of suppresion] of activation

Gangliosides are able to bind to the epidermal growth factor receptor and inhibit its activation, but the mechanism of this inhibition is unknown. To address the role of receptor carbohydrates in facilitating interaction with gangliosides, we examined the ability of GM3 to bind the deglycosylated receptor and inhibit its autophosphorylation. Flow cytometry studies demonstrated that deglycosylation of the receptor did not affect its ability to be transported to the cell membrane. In contrast with the native (fully glycosylated) receptor, GM3 did not coimmunoprecipitate with the deglycosylated receptor. Using a novel colorimetric bead binding assay, GM3 was shown to bind well to the immunoprecipitated native receptor but not at all to the deglycosylated receptor. Finally, the addition of GM3 to cells with deglycosylated epidermal growth factor receptors did not result in significant further inhibition of autophosphorylation of the receptor, despite a 10-fold decrease in phosphorylation of the native epidermal growth factor receptor by 200 microM GM3. These studies suggest that ganglioside affects epidermal growth factor receptor activity through a direct interaction that requires receptor glycosylation, and contribute to our understanding of the role of gangliosides in cell membrane function.

Carbohydrate-carbohydrate binding of ganglioside to integrin alpha(5) modulates alpha(5)beta(1) function

Gangliosides GT1b and GD3, components of keratinocyte membranes, inhibit keratinocyte adhesion to fibronectin. Although ganglioside sialylation is known to be important, the mechanism of inhibition is unknown. Using purified insect recombinant alpha(5) and beta(1) proteins and alpha(5)beta(1) integrin from lysed keratinocyte-derived SCC12 cells, we have shown that GT1b and GD3 inhibit the binding of alpha(5)beta(1) to fibronectin. Co-immunoprecipitation of GT1b and alpha(5)beta(1) from SCC12 cells and direct binding of GT1b and GD3 to affinity-purified alpha(5)beta(1) from SCC12 cells and insect recombinant alpha(5)beta(1), particularly the alpha(5) subunit, further suggest interaction between ganglioside and alpha(5)beta(1). The carbohydrate moieties of integrin appear to be critical since gangliosides are unable to bind deglycosylated forms of alpha(5)beta(1) from SCC12 and insect cells or poorly glycosylated recombinant alpha(5)beta(1) from Escherichia coli cells. The GT1b-alpha(5)beta(1) interaction is inhibited by concanavalin A, suggesting that GT1b binds to mannose structures in alpha(5)beta(1). The preferential binding of GT1b to high mannose rather than reduced mannose ovalbumin further implicates the binding of GT1b to mannose structures. These data provide evidence that highly sialylated gangliosides regulate alpha(5)beta(1)-mediated adhesion of epithelial cells to fibronectin through carbohydrate-carbohydrate interactions between GT1b and the alpha(5) subunit of alpha(5)beta(1) integrin.