Potent Glycan Inhibitors of Myelin-associated Glycoprotein Enhance Axon Outgrowth in Vitro*

The injured adult mammalian central nervous system (CNS)1 is a highly inhibitory environment for axon regeneration, in large part due to specific axon regeneration inhibitors expressed on residual myelin that persists at sites of CNS injury and on astrocytes recruited to injury sites (1–3). The inhibitors include three myelin proteins, myelin-associated glycoprotein (MAG), Nogo, and oligodendrocyte-myelin glycoprotein (OMgp), and chondroitin sulfate proteoglycans, which are found on the astrocytic scar and on residual myelin. Axon regeneration inhibitors, on myelin and astrocytes, bind specifically to targets on the axon surface, initiating a signaling cascade that ultimately activates RhoA and halts axon outgrowth by modulating actin dynamics. Detailed knowledge of axon regeneration inhibitors, their axonal ligands, and the signaling pathways leading to changes in the actin cytoskeleton promises to provide opportunities to enhance axon regeneration after CNS injury (e.g. spinal cord injury) and in diseases involving axon loss, such as multiple sclerosis, Parkinson disease, and Alzheimer disease. MAG has two classes of well-defined axonal targets: sialylated glycans, specifically gangliosides GD1a and GT1b (4), and the glycosylphosphatidylinositol-linked proteins of the NgR family, NgR1 (5, 6) and NgR2 (7), which may also be axonal ligands for Nogo and OMgp (8). There are conflicting data concerning the sialidase sensitivity of MAG-NgR binding (5–7). Although the relative roles of gangliosides and NgRs as MAG ligands have yet to be resolved, in some systems MAG inhibition is completely reversed by sialidase, suggesting that, at least in those systems, MAG uses sialylated glycans as its major axonal ligands. In those systems, or in combination with blockers of NgR (9), it is anticipated that potent glycan inhibitors of MAG may be valuable tools to enhance axon regeneration. MAG (Siglec-4), a member of the Siglec (sialic acid-binding immunoglobulin-like lectin) family (10), binds preferentially to the terminal sequence “NeuAcα2–3Galβ1–3GalNAc” (11), especially in the context of larger glycan structures bearing additional anionic groups (see Fig. 1). In the brain, this structure is most abundant as the terminus of gangliosides GD1a and GT1b, which are high-affinity MAG ligands and mediate MAG inhibition of axon outgrowth in vitro (4, 12). Glycan structures with an additional sialic acid 6-linked to the GalNAc residue, “NeuAcα2–3Galβ1–3(NeuAcα2–6)GalNAc,” bind to MAG with an order of magnitude higher affinity than the corresponding structure without the 6-linked NeuAc (13–15). This terminus occurs on the quantitatively minor “α-series” gangliosides (16–18) and on O-linked glycoprotein glycans (19). Because they were discovered as extensions of the disaccharide “Galβ1–3GalNAc,” which is called core 1 or the T antigen on glycoproteins, we denote the above structures as 3-sialyl T and disialyl T, respectively (see Fig. 1). Ganglioside nomenclature is that of Svennerholm (52). Fig. 1 Sialosides used in this study and related ganglioside structures A screen of synthetic sialoglycans for their ability to inhibit MAG-sialic acid binding in vitro (15) revealed that threonine methyl ester glycosides of disialyl T, 3-sialyl T, and 6-sialyl T (Galβ1–3(NeuAcα2–6)GalNAc) were potent inhibitors of MAG-sialoside binding, with IC50 values of 0.3, 1.6, and 10 μM, respectively. We report here that these glycans, when added to live neurons in vitro, effectively reverse MAG-mediated inhibition of axon outgrowth with the same relative potencies.