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Every cell in nature carries a rich surface coat of glycans, its glycocalyx, which constitutes the cell's interface with its environment. In eukaryotes, the glycocalyx is composed of glycolipids, glycoproteins, and proteoglycans, the compositions of which vary among different tissues and cell types. Many of the linear and branched glycans on cell surface glycoproteins and glycolipids of vertebrates are terminated with sialic acids, nine-carbon sugars with a carboxylic acid, a glycerol side-chain, and an N-acyl group that, along with their display at the outmost end of cell surface glycans, provide for varied molecular interactions. Among their functions, sialic acids regulate cell-cell interactions, modulate the activities of their glycoprotein and glycolipid scaffolds as well as other cell surface molecules, and are receptors for pathogens and toxins. In the brain, two families of sialoglycans are of particular interest: gangliosides and polysialic acid. Gangliosides, sialylated glycosphingolipids, are the most abundant sialoglycans of nerve cells. Mouse genetic studies and human disorders of ganglioside metabolism implicate gangliosides in axon-myelin interactions, axon stability, axon regeneration, and the modulation of nerve cell excitability. Polysialic acid is a unique homopolymer that reaches >90 sialic acid residues attached to select glycoproteins, especially the neural cell adhesion molecule in the brain. Molecular, cellular, and genetic studies implicate polysialic acid in the control of cell-cell and cell-matrix interactions, intermolecular interactions at cell surfaces, and interactions with other molecules in the cellular environment. Polysialic acid is essential for appropriate brain development, and polymorphisms in the human genes responsible for polysialic acid biosynthesis are associated with psychiatric disorders including schizophrenia, autism, and bipolar disorder. Polysialic acid also appears to play a role in adult brain plasticity, including regeneration. Together, vertebrate brain sialoglycans are key regulatory components that contribute to proper development, maintenance, and health of the nervous system.

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Gangliosides are a family of sialic acidcontaining glycosphingolipids highly enriched in the mammalian nervous system. Although they are the major sialoglycoconjugates in the brain, their neurobiological functions remain poorly defined. By disrupting the gene for a key enzyme in complex ganglioside biosynthesis (GM2͞GD2 synthase; EC 2.4.1.92) we generated mice that express only simple gangliosides (GM3͞GD3) and examined their central and peripheral nervous systems. The complex ganglioside knockout mice display decreased central myelination, axonal degeneration in both the central and peripheral nervous systems, and demyelination in peripheral nerves. The pathological features of their nervous system closely resemble those reported in mice with a disrupted gene for myelin-associated glycoprotein (MAG), a myelin receptor that binds to complex brain gangliosides in vitro. Furthermore, GM2͞GD2 synthase knockout mice have reduced MAG expression in the central nervous system. These results indicate that complex gangliosides function in central myelination and maintaining the integrity of axons and myelin. They also support the theory that complex gangliosides are endogenous ligands for MAG. The data extend and clarify prior observations on a similar mouse model, which reported only subtle conduction defects in their nervous system

Myelin-associated glycoprotein (MAG) binds to the nerve cell surface and inhibits nerve regeneration. The nerve cell surface ligand(s) for MAG are not established, although sialic acid-bearing glycans have been implicated. We identify the nerve cell surface gangliosides GD1a and GT1b as specific functional ligands for MAG-mediated inhibition of neurite outgrowth from primary rat cerebellar granule neurons. MAG-mediated neurite outgrowth inhibition is attenuated by (i) neuraminidase treatment of the neurons; (ii) blocking neuronal ganglioside biosynthesis; (iii) genetically modifying the terminal structures of nerve cell surface gangliosides; and (iv) adding highly specific IgG-class antiganglioside mAbs. Furthermore, neurite outgrowth inhibition is mimicked by highly multivalent clustering of GD1a or GT1b by using precomplexed antiganglioside Abs. These data implicate the nerve cell surface gangliosides GD1a and GT1b as functional MAG ligands and suggest that the first step in MAG inhibition is multivalent ganglioside clustering.

The initial selectin-dependent events that mediate tumor cell tethering to platelets, leukocytes, and vascular endothelium can regulate the extravasation and colonization of metastatic cells into distant tissues. Little is known, however, about the identity of selectin counter-receptors on tumor cells, which facilitate the metastatic process. To address this issue, we performed SDS-PAGE analysis of membrane proteins, metabolic inhibition studies, blot rolling assays, and cell-free flow-based adhesion experiments using microbeads coated with CD44 immunoprecipitated from carcinomas and purified selectins as substrate. Here, we demonstrate that variant isoforms of CD44 (CD44v) on LS174T colon carcinoma cells possess P-/L-/E-selectin binding activity, in contrast to the standard isoform of CD44 (CD44s) on hematopoietic-progenitor cells (HPCs), which is primarily an L-/E-selectin ligand. Moreover, the selectin-binding determinants on CD44v from LS174T cells are sialofucosylated structures displayed on O-linked glycans, akin to those on P-selectin glycoprotein ligand-1, but distinct from the HECA-452-reactive N-glycans on CD44s expressed on HPCs. Using flow-based adhesion assays, we systematically characterize shear-dependent LS174T CD44 vs. HL60 CD44s adhesion to E-/P-/L-selectin. The novel finding that CD44v are selectin ligands offers a unifying perspective on the apparent enhanced metastatic potential associated with tumor cell CD44v overexpression and the critical role of selectins in metastasis.

Summary Gangliosides, sialic acid-bearing glycosphingolipids, are expressed on all vertebrate cells, and are the major glycans on nerve cells. They are anchored to the plasma membrane through their ceramide lipids with their varied glycans extending into the extracellular space. Through sugar-specific interactions with glycan binding proteins on apposing cells, gangliosides function as receptors in cell-cell recognition, regulating natural killer cell cytotoxicity via Siglec-7 binding, myelin-axon interactions via Siglec-4 (myelin-associated glycoprotein) binding, and inflammation via E-selectin binding. Gangliosides also interact laterally in their own membranes, regulating the responsiveness of signaling proteins including the insulin, epidermal growth factor and vascular endothelial growth factor receptors. In these ways, gangliosides act as regulatory elements in the immune system, in the nervous system, in metabolic regulation and in cancer progression.