Synaptic cell adhesion molecule SynCAM 1 is a target for polysialylation in postnatal mouse brain

Abstract

Among the large set of cell surface glycan structures, the carbohydrate polymer polysialic acid (polySia) plays an important role in vertebrate brain development and synaptic plasticity. The main carrier of polySia in the nervous system is the neural cell adhesion molecule NCAM. As polySia with chain lengths of more than 40 sialic acid residues was still observed in brain of newborn Ncam −/− mice, we performed a glycoproteomics approach to identify the underlying protein scaffolds. Affinity purification of polysialylated molecules from Ncam −/− brain followed by peptide mass fingerprinting led to the identification of the synaptic cell adhesion molecule SynCAM 1 as a so far unknown polySia carrier. SynCAM 1 belongs to the Ig superfamily and is a powerful inducer of synapse formation. Importantly, the appearance of polysialylated SynCAM 1 was not restricted to the Ncam −/− background but was found to the same extent in perinatal brain of WT mice. PolySia was located on N-glycans of the first Ig domain, which is known to be involved in homo- and heterophilic SynCAM 1 interactions. Both polysialyltransferases, ST8SiaII and ST8SiaIV, were able to polysialylate SynCAM 1 in vitro, and polysialylation of SynCAM 1 completely abolished homophilic binding. Analysis of serial sections of perinatal Ncam −/− brain revealed that polySia-SynCAM 1 is expressed exclusively by NG2 cells, a multifunctional glia population that can receive glutamatergic input via unique neuron-NG2 cell synapses. Our findings sug-gest that polySia may act as a dynamic modulator of SynCAM 1 functions during integration of NG2 cells into neural networks.

Sequences from the First Fibronectin Type III Repeat of the Neural Cell Adhesion Molecule Allow O-Glycan Polysialylation of an Adhesion Molecule Chimera

“…These are the ␣ subunit of the voltage-dependent sodium channel (28), the scavenger receptor CD36 found in milk (29), neuropilin-2 expressed on dendritic cells (30), and the polySTs themselves (autopolysialylation) (31,32). Recently, polysialic acid was reported to be present on another member of the immunoglobulin superfamily of proteins, the synaptic cell adhesion molecule SynCAM 1 (33). The very small number of polysialylated proteins, and the fact that the polySTs add polysialic acid to N-glycans associated with NCAM much more efficiently than to free N-glycans or other typically unpolysialylated proteins in vitro (34,35), led to the hypothesis that polysialylation may be a protein-specific modification, requiring an initial interaction between the polyST and protein substrate.…”

Sequences Prior to Conserved Catalytic Motifs of Polysialyltransferase ST8Sia IV Are Required for Substrate Recognition

“…SynCAM 1 is a synaptic adhesion molecule that forms homo-and heterophilic adhesive complexes and aids in the formation and maintenance of synapses (65). SynCAM 1 has been found to be polysialylated on a single N-glycan in a small subset of NG2 glial cells in perinatal brain (7). PolySia has been shown to decrease SynCAM 1 homophilic adhesion in vitro, and is postulated to regulate the formation of a neuronglial synapse (7).…”

Site-specific enzymatic polysialylation of therapeutic proteins using bacterial enzymes

“…In mammals, PSA is found mostly on the brain-specific neural cell adhesion molecule (NCAM) protein (7), on CD36, and on the recently described synaptic cell adhesion molecule (SynCAM) (8,9). PSA modification of therapeutic proteins is an attractive alternative to PEGylation, given that PSA is biodegradable and nonimmunogenic.…”