Enhanced binding of the neural siglecs, myelin-associated glycoprotein and Schwann cell myelin protein, to Chol-1 (α-series) gangliosides and novel sulfated Chol-1 analogs

Extended glycoconjugate binding specificities of three sialic acid- dependent immunoglobulin-like family member lectins (siglecs), myelin- associated glycoprotein (MAG), Schwann cell myelin protein (SMP), and sialoadhesin, were compared by measuring siglec-mediated cell adhesion to immobilized gangliosides. Synthetic gangliosides bearing the α-series determinant (NeuAc α2,6-linked to GalNAc on a gangliotetraose core) were tested, including GD1α (IV³NeuAc, III⁶NeuAc-Gg₄OseCer), GD1α with modified sialic acid residues at the III⁶-position, and the 'Chol-1' gangliosides GT1aα (IV³NeuAc, III⁶NeuAc, II³NeuAc-Gg₄OseCer) and GQ1bα (IV³NeuAc, III⁶NeuAc, II³(NeuAc)₂-Gg₄OseCer). The α-series gangliosides displayed enhanced potency for MAG- and SMP-mediated cell adhesion (GQ1ba > GT1aα, GD1α > GT1b, GD1a >> GM1 (nonbinding)), whereas sialoadhesin- mediated adhesion was comparable with α-series and non-α-series gangliosides. GD1α derivatives with modified sialic acids (7-, 8-, or 9- deoxy) or sulfate (instead of sialic acid) at the III⁶-position supported adhesion comparable with that of GD1α. Notably, a novel GT1aα analog with sulfates at two internal sites of sialylation (NeuAcα2,3Galβ1,4GalNAc-6- sulfateβ1, 4Gal3-sulfateβ1,4Glcβ1,1'ceramide) was the most potent siglec- binding structure tested to date (10-fold more potent than GT1aα in supporting MAG and SMP binding). Together with prior studies, these data indicate that MAG and SMP display an extended structural specificity with a requirement for a terminal α2,3-linked NeuAc and great enhancement by nearby precisely spaced anionic charges.