Poly-␣2,8-sialic acid (polySia) is a unique modification of the neural cell adhesion molecule, NCAM, tightly associated with neural development and plasticity. However, the vital role attributed to this carbohydrate polymer has been challenged by the mild phenotype of mice lacking polySia due to NCAM-deficiency. To dissect polySia and NCAM functions, we generated polySia-negative but NCAM-positive mice by simultaneous deletion of the two polysialyltransferase genes, St8sia-II and St8sia-IV. Beyond features shared with NCAM-null animals, a severe phenotype with specific brain wiring defects, progressive hydrocephalus, postnatal growth retardation, and precocious death was observed. These drastic defects were selectively rescued by additional deletion of NCAM, demonstrating that they originate from a gain of NCAM functions because of polySia deficiency. The data presented in this study reveal that the essential role of polySia resides in the control and coordination of NCAM interactions during mouse brain development. Moreover, this first demonstration in vivo that a highly specific glycan structure is more important than the glycoconjugate as a whole provides a novel view on the relevance of protein glycosylation for the complex process of building the vertebrate brain.The cellular glycosylation machinery is a most impressive example of how cells enhance structural and functional complexity by use of only limited parts (Ͻ10%) of the genome. Glycans conjugated to lipids and proteins form the glycocalyx, the outer rim and prominent communication structure of animal cells (1, 2). Their paramount importance is emphasized by the growing group of congenital disorders of glycosylation, which manifest as severe multisystemic diseases including neuropathological symptoms (3).Poly-␣2,8-linked sialic acid (polySia) 3 is a unique glycan added to the neural cell adhesion molecule, NCAM, and is known to exert an important influence on the development and function of the nervous system (4 -6). The intriguing role assigned to polySia in promoting neurogenesis, migration, axon outgrowth, and synaptic plasticity has been explained predominantly by a negative regulation of cell-cell interactions due to the stereochemical properties of the large polyanion (shown schematically in Fig. 1A) (4, 7). Recent x-ray and neutron reflectivity data as well as direct force measurements confirm that an increased intermembrane repulsion in the presence of polySia overcomes homophilic NCAM binding and attenuates cadherin-dependent adhesion (8, 9). On the other hand, polySia is known to exert highly specific functions. For instance, NCAM trans-interactions with heparan sulfate proteoglycans involved in the formation and remodeling of hippocampal synapses depend on the presence of polySia (10, 11). Finally, competition experiments carried out with exogenously added polySia indicate that the carbohydrate polymer mediates autonomous, NCAM-independent functions. These concern the development of commissural axons in zebrafish (12), the strengthening of ...

Polysialic acid (PSA), a carbohydrate polymer attached to the neural cell adhesion molecule (NCAM), promotes neural plasticity and tumor malignancy, but its mode of action is controversial. Here we establish that PSA controls tumor cell growth and differentiation by interfering with NCAM signaling at cell-cell contacts. Interactions between cells with different PSA and NCAM expression profiles were initiated by enzymatic removal of PSA and by ectopic expression of NCAM or PSA-NCAM. Removal of PSA from the cell surface led to reduced proliferation and activated extracellular signal-regulated kinase (ERK), inducing enhanced survival and neuronal differentiation of neuroblastoma cells. Blocking with an NCAM-specific peptide prevented these effects. Combinatorial transinteraction studies with cells and membranes with different PSA and NCAM phenotypes revealed that heterophilic NCAM binding mimics the cellular responses to PSA removal. In conclusion, our data demonstrate that PSA masks heterophilic NCAM signals, having a direct impact on tumor cell growth. This provides a mechanism for how PSA may promote the genesis and progression of highly aggressive PSA-NCAM-positive tumors