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 oc series determinant (NeuAc alpha 2,6-linked to GalNAc on a gangliotetraose core) were tested, including GD1 alpha (IV(3)NeuAc, III(6)NeuAc-Gg(4)OseCer), GD1 alpha with modified sialic acid residues at the III6-position, and the "Chol-1" gangliosides GT1a alpha (IV(3)NeuAc, III(6)NeuAc, II(3)NeuAc-Gg(4)OseCer) and GQ1b alpha (IV(3)NeuAc, III(6)NeuAc, II3(NeuAc)(2)-Gg(4)OseCer). The a-series gangliosides displayed enhanced potency for MAG- and SMP-mediated cell adhesion (GQ1b alpha > GT1a alpha, GD1 alpha > GT1b, GD1a >> GM1 (nonbinding)), whereas sialoadhesin-mediated adhesion was comparable with cu-series and non-a-series gangliosides, GD1 alpha derivatives with modified sialic acids (7-, 8-, or g-deoxy) or sulfate (instead of sialic acid) at the III6-position supported adhesion comparable with that of GD1 alpha. Notably, a novel GT1a alpha analog with sulfates at two internal sites of sialylation (NeuAc alpha 2,3Gal beta 1,4GalNAc-6-sulfate beta 1, 4Gal3-sulfate beta 1,4Glc beta 1,1'ceramide) was the most potent siglec-binding structure tested to date (10-fold more potent than GT1a alpha 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 alpha 2,3-linked NeuAc and great enhancement by nearby precisely spaced anionic charges.